Comprehensive cellulose chemistry

D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Comprehensive Cellulose Chemistry Volume 2 Functionalization...

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D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht

Comprehensive Cellulose Chemistry Volume 2 Functionalization of Cellulose

WILEY-VCH Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht

Comprehensive Cellulose Chemistry Volume 2 Functionalization of Cellulose

® WILEY-VCH Weinheim · New York · Chichester · Brisbane · Singapore · Toronto

Prof. Dr. D. Klemm Friedrich-Schiller-Universität Jena Institut für Organische und Makromolekulare Chemie Humboldtstraße 10 07743 Jena Germany

Dr. T. Heinze Friedrich-Schiller-Universität Jena Institut für Organische und Makromolekulare Chemie Humboldtstraße 10 07743 Jena Germany

Prof. Dr. B. Philipp Max-Planck-Institut für Kolloidund Grenzflächenforschung Kantstraße 55 14513 Teltow-Seehof Germany

Dr. U Heinze Friedrich-Schiller-Universität Jena Institut für Organische und Makromolekulare Chemie Humboldtstraße 10 07743 Jena Germany

Dr. W. Wagenknecht Max-Planck-Institut für Kolloidund Grenzflächenforschung Kantstraße 55 14513 Teltow-Seehof Germany

This book was carefully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. applied for. A catalogue record for this book is available from the British Library Die Deutsche Bibliothek - CIP-Einheitsaufnahme applied for

© WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1998 Printed on acid-free and low chlorine paper All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine-radable language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Graphik & Textstudio, D-93164 Laaber-Waldetzenberg Printing: betz-druck, D-64291 Darmstadt Bookbinding: W Osswald, D-67433 Neustadt

Preface

Cellulose, as the most abundant organic polymer, has served mankind for thousands of years as an indispensable material for clothing and housing, and has formed a large part of human culture since the Egyptian papyri. In contrast with cellulose application as a natural product, the use of this polymer as a chemical raw material started just 150 years ago with the discovery of the first cellulose derivatives, but subsequently developed to a production volume of more than 5 million tons annually during this century. At the same time the classical areas of processing cellulose as a natural product by mechanical technologies, for example the manufacture of textile goods from cotton, received a strong impetus by combining them with chemical processes to improve product quality. This line of progress is closely related to and often originated from the development of a systematic chemistry of cellulose comprising predominantly the chemical transformation of the macromolecule. The knowledge acquired in this area was compiled during this century in a number of monographs and text books still serving as a valuable scientific background in today's cellulose research. But most of these books were published several decades ago, and thus could not take into account recent developments, for example the relevance of ecological problems in cellulose processing, discussion of the advantages and shortcomings of natural resources in general, or today's boom in synthetic organic and supramolecular chemistry. Besides this, some of these books consider only a special field or reflect a rather special point of view. In the authors' opinion, no text book or monograph on the organic chemistry of cellulose is available now, that presents in a comprehensive and still conveniently readable manner the theoretical background and the experimental state of the art at the end of this century. It is the intention to fill this gap by the two volumes of this book, centered on the routes and the mechanisms of cellulose functionalization, but covering also the close interrelation between a heterogeneous cellulose reaction and the supramolecular structure of this polymer. Special emphasis has been put on distribution of functional groups in relation to reaction conditions and on analytical techniques for their characterization. Not only recent efforts in cellulose research and development are presented and cited but also important results on the last centuries actual up to now are included in order to give a comprehensive description of the chemistry of cellulose.

VI

Preface

The authors are indebted to WILEY-VCH Verlag for agreeing to a twovolume presentation, allowing accuracy and readability of the text to be combined, and also leaving enough space for numerous experimental procedures, that are suitable for making a graduate student familiar with the practical laboratory work in cellulose chemistry. From a didactic point of view, as well as for the sake of convenient information retrieval, the authors found it appropriate to survey in the first volume some aspects of cellulose of a more general nature relevant to chemical reactions. Included are e.g. its properties and structure in relation to reactivity, the processes of swelling and dissolution, with their consequences to chemical reactions, and the pathways of cellulose degradation accompanying chemical transformations of this polymer. Special emphasis is given in this part to aspects of physical chemistry and colloid chemistry. A rather detailed presentation of cellulose analytics for characterizing the original polymer and its derivatives at the various structural levels is also included in Volume I. Volume II deals with the various classes of cellulose derivatives, with emphasis on the reaction mechanisms and distribution of functional groups, including, in addition, in each of the chapters also a brief abridgment of relevant industrial processes and an overview of properties and areas of application of the products in question. In both volumes results obtained by the authors' groups are adequately accentuated, especially with regard to Figures and Tables. It is hoped that the two volumes of this book will be accepted as a useful textbook by graduate students in science, with special interest in cellulosics, and that it will serve as a comprehensive source of information for chemists, physicists, biologists, and engineers professionally engaged with this polymer. The authors' work would find its best appreciation, if the book helps to stimulate young scientists to professional activities in cellulose chemistry, which offers a challenge to innovative ideas and new experimental pathways, also into the next century.

Contents

Volume 1: Chapters 1 to 3 1

Introduction

2

General Considerations on Structure and Reactivity of Cellulose Structure and Properties of Cellulose The molecular structure The supramolecular structure The morphological structure Pore structure and inner surface The accessibility of cellulose Alien substances associated with the cellulose matrix Macroscopic properties of cellulose General properties and gross morphology Mechanical properties of cellulose Electrical, optical and thermal properties of cellulose Chemical and environmental properties of cellulose

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.1.6 2.1.7 2.1.7M 2.1.7.2 2.1.7.3 2.1.7.4 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.3 2.2.1.4 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.3

Swelling and Dissolution of Cellulose Limited swelling of cellulose Swelling of cellulose in water Limited swelling of cellulose in some organic liquids in comparison with water Swelling of cellulose in aqueous solutions of sodium hydroxide and in related systems Interaction of cellulose in media in the transition range between solvent and swelling agent Dissolution of cellulose Some general comments on cellulose dissolution Systematic description of important classes of cellulose solvent systems Structure formation of cellulose and cellulose derivatives Concluding remarks

1

9 9 9 15 22 25 29 32 33 33 35 37 39 43 44 45 51 56 58 60 60 62 73 79

VIII

2.3 2.3.1 2.3.1.1 2.3.1.2 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.4 2.4.1 2.4.2 2.4.2.1 2.4.2.2 2.4.2.3 2.4.3

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

Contents

Degradation of Cellulose Hydrolytic degradation of cellulose Acid hydrolysis of cellulose Enzymatic hydrolysis Degradation of cellulose by aqueous alkali Oxidative degradation of cellulose Mechanical degradation of cellulose Thermal degradation of cellulose and cellulose derivatives Radiation degradation of cellulose Consequences of degradation of cellulose on its chemical processing

83 84 85 93 99 101 104 107 118

Principles of Cellulose Reactions Some principles of polymer reactions Survey of important reaction types of cellulose Principles and characteristics of cellulose reactions under homogeneous conditions Principles and characteristics of cellulose reactions under heterogeneous conditions Activation of cellulose Advantages and limitations of cellulose reactions in DMA/LiCl solution

130 130 135

Analytical Methods in Cellulose Chemistry Determination of the Degree of Polymerization of Cellulose and its Derivatives Chemical Analysis (Elemental Analysis and Functional Group Analysis) of Cellulose and Cellulose Derivatives Application of Instrumental Analysis in Cellulose Chemistry Techniques of Polymer Fractionation and Chromatographie Separation in Cellulose Analysis Summary of Analytical Routes to Total DS and Substituent Distribution Characterization of the Structure of Cellulosics in the Solid State Characterization of Cellulose-Liquid Interaction on Swelling and Dissolution Outlook for the Future Development of Cellulose Analysis

124

141 145 150 155 167 168 173 181 195 202 204 213 217

Contents

Appendix I Experimental Protocols for the Analysis of Cellulose Fractionation of cellulose nitrate Preparation of: level-off DP cellulose decrystallized cellulose cellulose tricarbanilate Determination of: DP of cellulose DS of cellulose acetate carbonyl group content of cellulose carboxyl group content water retention value of cellulose DS of cellulose xanthogenate DS of carboxymethylcellulose DS of trity!cellulose Structure analysis of thexyldimethylsily!celluloses by NMR spectroscopy and HPLC Alkali resistance of cellulosic materials Alkali solubility of cellulose materials Subject index

IX

223 227 232 232 233 234 235 236 236 237 238 240 241 241 243 247 253

X

Contents

Volume 2; Chapters 4 and 5 4 4.1 4.1.1 4.1.2 4.1.2.1 4.1.2.2 4.1.2.3 4.1.2.4 4.1.2.5 4.1.3 4.1.3.1 4.1.3.2 4.1.3.3 4.1.3.4 4.1.4 4.2 4.2.1 4.2.2 4.2.2.1 4.2.2.2 4.2.2.3 4.2.2.4 4.2.2.5 4.2.2.6 4.2.2.7 4.2.3 4.2.3.1 4.2.3.2 4.2.3.3

Systematics of Cellulose Derivatization Formation and Modification of the Polymer Skeleton of Cellulose Synthesis of the polymer skeleton of cellulose Covalent crosslinking of cellulose Principles of cellulose crosslinking Chemical routes to crosslinking of cellulose Role of supramolecular and morphological structure in cellulose crosslinking Material properties of crosslinked cellulose Applications of cellulose crosslinking Grafting onto cellulose chains Relevance of grafting Chemistry of cellulose graft copolymer formation Some effects of supramolecular and morphological structure Properties and applications of graft copolymers of cellulose Synthesis of cellulose block copolymers

1 1 2 6 6 6 14 15 16 17 17 17 22 24 27

Interaction of Cellulose with Basic Compounds 31 Preparation and properties of alkali cellulosates 32 Interaction of cellulose with aqueous and alcoholic solutions of alkali hydroxides 33 General comments on the process of interaction and on product properties 33 Swelling and dissolution of cellulose in alkali hydroxide solutions 34 Chemical processes of interaction between cellulose and alkali hydroxide solutions 35 Role of cellulose physical structure in cellulose-alkali hydroxide interaction 40 Cocepts for understanding cellulose-alkali hydroxide interaction .... 46 Survey of commercisl processes based on cellulose-alkali hydroxide interaction 49 Properties and application of alkali cellulose 50 Interaction of cellulose with tetraalkylammonium hydroxides 51 Swelling and dissolution of cellulose in solutions of tetraalkylammonium hydroxides 52 Chemical interaction between cellulose and tetraalkylammonium hydroxides 52 Changes in cellulose structure and 54

Contents

XI

4.2.4 4.2.5 4.2.6 4.2.7

Interaction of cellulose with guanidinium hydroxide Interaction of cellulose with ammonia and hydrazine Interaction of cellulose with aliphatic mono- and diamines Concluding remarks

54 57 62 66

4.3 4.3.1 4.3.2 4.3.2.1 4.3.2.2

Metal Complexes of Cellulose General routes of cellulose-metal atom interaction Chemistry of cellulose-metal complex formation Copper complexes of cellulose with N-containing ligands Other aqueous cellulose solvents based on transition metalamine complexes Transition metal-alkali-tartaric acid complexes of cellulose Interaction of cellulose with metal hydroxo compounds Interaction of cellulose with some inorganic salts Supramolecular and morphological aspects of cellulose-metalcomplex formation Properties of cellulose-metal complexes Application of cellulose-metal complexes Filament and film formation from cellulose-metal-complex solutions Covalent functionalization of cellulose dissolved in metalcomplex systems Characterization of cellulose in metal-complex systems Determination of foreign substances in cellulosic products by means of metal-complex solvents Future problems of cellulose-metal complex research

71 71 73 74

Esterification of Cellulose Esters of cellulose with inorganic acids Cellulose nitrate Cellulose nitrite Cellulose sulfates Cellulose phosphate and other phosphorus-containing cellulose derivatives Cellulose borates Desoxycelluloses Cellulose esters with reagents derived from carbonic acid (H2CO3) Cellulose esters of monothiocarbonic acid (H2CSO2) Cellulose dithiocarbonate esters Cellulose carbamate Esterification with organic acids General remarks

99 100 101 112 115

4.3.2.3 4.3.2.4 4.3.2.5 4.3.3 4.3.4 4.3.5 4.3.5.1 4.3.5.2 4.3.5.3 4.3.5.4 4.3.6 4.4 4.4.1 4.4.1.1 4.4.1.2 4.4.1.3 4.4.1.4 4.4.1.5 4.4.1.6 4.4.2 4.4.2.1 4.4.2.2 4.4.2.3 4.4.3 4.4.3.1

78 82 85 86 90 92 93 93 94 95 95 96

133 140 142 145 145 147 161 164 164

XII

Contents

4.4.3.2 4.4.3.3 4.4.3.4 4.4.3.5

166 168 182

4.4.3.9 4.4.4

Cellulose formate Cellulose acetate Cellulose esters of higher aliphatic acids Esters of cellulose with substituted monocarboxylic aliphatic acids Esters of cellulose with di- and tricarboxylic aliphatic acids and their derivatives Cellulose esters with aromatic acids Esters of cellulose with organic acids carrying sulfonic or phosphonic acid groups Phenylcarbamates of cellulose Concluding remarks on cellulose esterification

4.5 4.5.1 4.5.2 4.5.2.1 4.5.2.2 4.5.2.3 4.5.3 4.5.3.1 4.5.3.2 4.5.3.3 4.5.3.4 4.5.4 4.5.4.1 4.5.4.2 4.5.4.3 4.5.5 4.5.5.1 4.5.5.2 4.5.5.3 4.5.5.4 4.5.5.5 4.5.6

Etherification of Cellulose General remarks on etherification Aliphatic ethers of cellulose Alkyl ethers of cellulose Carboxymethylcellulose and related anionic cellulose ethers Hydroxyalkyl ethers of cellulose Various functionalized alkyl ethers of cellulose Cyanoethylcellulose and related cellulose ethers Functionalized cellulose ethers with basic N-functions Sulfoalkyl ethers of cellulose Miscellaneous functionalized alkyl ethers of cellulose Aralkylethers and arylethers Arylmethyl ethers TriphenylmethylCtrityl') and related ethers Arylethers Silyl ethers of cellulose Heterogeneous silylation of cellulose Homogeneous silylation of cellulose Properties and structure characterization Subsequent reactions of silylcelluloses Formation of supramolecular structures using silylcelluloses Summary and outlook

207 207 210 210 221 234 249 250 255 260 261 262 262 263 273 274 278 279 280 285 290 294

4.6 4.6.1 4.6.2

Oxidation of Cellulose Oxidation of primary hydroxy groups Oxidation of secondary hydroxy groups

302 304 308

5 5.1 5.2

Outlook onto Future Developments in Cellulose Chemistry Cellulose as a Raw Material for Chemical Conversion The Relevance of Intermolecular Interactions

315 316 318

4.4.3.6 4.4.3.7 4.4.3.8

186 189 190 194 196 197

Contents

5.3 5.4 5.5

New Cellulosic Compounds Commercial Processes of Chemical Conversion of Cellulose Supramolecular Architectures

XIII

319 321 322

Appendix II Experimental Procedures for the Functionalization of Cellulose 327 Preparation of FeTNa solvent for cellulose 331 Dissolution of cellulose in TV^-dimethylacetamde (DMA)TLiCl 331 Preparation of a cellulose trinitrate without significant chain degradation 332 Sulfation of cellulose with SO3-DMF 332 Cellulose sulfate, synthesis via cellulose trifluoroacetate in DMF 334 Cellulose sulfate, synthesis via trimethylsilylcellulose in THF 335 Preferentially C-6-substituted cellulose sulfate via an acetate sulfate mixed ester 336 Predominantly C-2/C-3-substituted cellulose sulfates 337 Cellulose phosphate from a partially substituted cellulose acetate 338 Preparation of a cellulose fiber xanthogenate and a cellulose xanthogenate solution 339 Cellulose tricarbanilate 340 Cellulose phenylcarbamate, synthesis via cellulose trifluoroacetate inpyridine 341 Cellulose formate, synthesis in HCOOHTPOCl3 342 Laboratory procedure for the preparation of cellulose triacetate by fiber acetylation 343 Acetylation of bacterial cellulose 344 Site-selective deacetylation of cellulose triacetate 344 Cellulose dichloroacetate, synthesis with dichloroacetic acid/POC!3 345 Cellulose trifluoroacetate (DS = 1.5), synthesis with TFA/TFAA ........Ϊ.'.'.'.'.'.'.'! 346 Cellulose methoxyacetates, synthesis in DMA/LiCl 347 Cellulose-4-nitrobenzoate, synthesis via cellulose trifluoroacetate catalyzed with p-tosyl chloride 348 Cellulose-4-nitrobenzoate, synthesis via cellulose trifluoroacetate with 4-nitro-benzoic acid imidazolide 349 Cellulose tosylate, homogeneous synthesis in DMA/LiCl 350 2,3-Di-O-methylcellulose 352 Carboxymethylcellulose, heterogeneous synthesis in isopropanol/water 353 Carboxymethylcellulose, synthesis in DMA/LiCl 355 Carboxymethylcellulose, synthesis via cellulose trifluoroacetate in DMSO 357 6-O-Tripheny!methyl (trityl) cellulose, homogeneous synthesis in DMA/LiCl 359 2,3-O-Carboxymethyl-6-O-triphenylmethylcellulose, synthesis via 6-O-tritylcellulose in DMSO 361

XIV

Contents

Detritylation of 2,3-O-carboxymethyl-6-O-triphenylmethyl cellulose Crosslinking of cellulose powder with epichlorohydrin Organosoluble cyanoethylcellulose Trimethylsilylcellulose, synthesis in pyridine/THF Trimethylsilylcellulose, synthesis in DMA/LiCl Celluloses esters, synthesis via trimethylsilylcellulose, general procedure without solvents 6-O-Thexyldimethylsilylcellulose 2,6-Di-O-thexyldimethylsilylcellulose 6-O-Thexyldimethylsilyl-2,3-di-O-methylcellulose Trimethylsilylcellulose methoxyacetate. synthesis via cellulose methoxyacetate in DMA 6-Carboxycellulose, homogeneous synthesis with phosphoric acid

362 363 364 365 367

Subject index

377

368 370 371 372 373 374

List of Abbreviations for Volumes 1 and 2

AGU Bn Cadoxen CMC COSY CP-MAS CTA CTFA Guam Cuen DMA DMAP DMF DMSO DP DPn DPV DPW DS D5Ac

D5N DSp D5S DS$i DSx DTA DVS EDA Et FeTNa FT GPC g-t GuOH

acetic acid anhydride anhydroglucopyranose unit(s) benzyl cadmiumethylenediamin chelate carboxymethylcellulose homonuclear chemical shift correlation spectroscopy cross-polarization magic angle spinning cellulose triacetate cellulose trifluoroacetate cuprammonium hydroxide [Cu(NH3)4]OH cupriethylenediamine chelate 7V,Af-dimethylacetamide AÂf-dimethylaminopyridine Af,W-dimethylformamide dimethyl sulfoxide degree of polymerization number- average degree of polymerization viscosity-average degree of polymerization weight average degree of polymerization degree of substitution degree of substitution of acetyl groups degree of substitution of chlorine atoms degree of substitution determined by means of HPLC degree of substitution of nitrogen atoms degree of substitution of phosphorus atoms degree of substitution of sulfur atoms degree of substitution of silyl groups degree of substitution of xanthogenate groups differential thermal analysis divinyl sulfone electron donor-acceptor ethyl ferric sodium tartrate Fourier transform gel-permeation chromatography gauche-trans guanidinium hydroxide

XVI

Abbreviations

H-CMC HEC HMPT HPC LB LODP LRV M.W. mesylate Me MF MS Nioxam Nioxen NMMNO NMP r.h. rt s (index) SAXS SEC SEM SERS TDMS cellulose TDMSCl TEA TEM TG t-g THE TMS TMS-Cl TPC triflat WAXS WRV (index)

the free acid of carboxymethylcellulose hydroxyethylcellulose hexamethylphosphoric acid triamide hydroxypropylcellulose Langmuir-B lodgett level-off degree of polymerization liquid retention value molecular weight methylsulfonate methyl mole fraction molar substitution nickel ammonium hydroxide nickel ethylenediamine chelate Af-methylmorpholine-TV-oxide 7V-methylpyrrolidone relative humidity room temperature substituted small-angle X-ray scattering size-exclusion chromatography scanning electron microscopy Surface enhanced Raman spectroscopy thexyldimethylsilyl cellulose thexyldimethylchlorosilane triethylamine transmission electron microscopy thermogravimetry trans-gauche tetrahydrofuran trimethylsilyl trimethylsilyl chloride triphenylcarbinol trifluoromethanesulfonate wide-angle X-ray scattering water retention value

neighbour C-atom

4 Systematics of Cellulose Functionalization

The following systematics of cellulose functionalization will be structured according to the typical reaction types of hydroxy groups, i.e. esterification, etherification and oxidation. Specific characteristics of the cellulose macromolecule will also be considered, i.e. the formation of addition compounds with basic substances and the formation of metal complexes, as well as changes of the polymer skeleton by grafting or crosslinking. Each of the chapters will be product-centered, describing primarily the chemistry of formation of the derivative, its subsequent modification and considering also properties and applications of the product formed. Special emphasis will be given to the kinetics and mechanism of the derivatization reaction and on the role of cellulose supramolecular structure. For products of commercial relevance, a brief description of the technical process is included. For selected products of scientific and/or practical interest, a laboratory procedure for synthesis, purification and characterization will be given in the Appendix of this volume.

4.1 Formation and Modification of the Polymer Skeleton of Cellulose Before turning to the functionalization of cellulose at the hydroxy groups, it is appropriate to survey briefly some routes of formation and modification of the polymer skeleton of cellulose, considering the following topics: (i) synthesis of the ß-l,4-glucan chain, (ii) covalent crosslinking between cellulose chains, (iii) combination of ß-l,4-glucan sequences with synthetic macromolecules by grafting and by synthesis of block copolymers. (iv) modification of the cellulose skeleton by formation of cyclic ethers across the AGU, and subsequent changes in the configuration of the functional groups, Most of the work published in the whole subject area is concerned with grafting and crosslinking of cellulose, the latter topic being of great practical relevance in connection with the finishing of cellulosic textiles. Results on the chemical synthesis of the cellulose molecule are still rather scarce and have met with limited success only in comparison with the perfect achievement of nature. But the first successful regio- and stereoselective synthesis of nature-identical Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

2

4.1 Formation and Modification of the Polymer Skeleton of Cellulose

cellulose without enzymes or microorganisms in 1996 (Nakatsubo et al. 1996), was an intellectually important result and a principally novel way to prepare functionalized celluloses. Changes in the configuration of the macromolecule via inner ether formation, as well as the block copolymerization of cellulose, are presently considered as areas of limited interest only, but of scientific relevance.

4.1.1 Synthesis of the polymer skeleton of cellulose Three routes of synthesis of the cellulose chain have to be considered here, i.e. (i) biosynthesis in living organisms, (ii) in vitro enzymatic synthesis, (iii)chemical synthesis by polymerization of suitable monomers. Cell wall

Cytoplasm Cy \ jeftjzs

Pore subunit

Crystallization subunit

Microfibril

Plasma membrane

Figure 4.1.1. Hypothetical model of a cellulose synthase complex in the plasma membrane (Delmer and Amor, 1995).

Until the middle of this century, cellulose was taken for granted as a polymer delivered by nature, and the research activities were centered on its chemical and physical processing and on the elucidation of its structure. But this situation has changed in more recent decades, due to the rapid developments in biochemistry. The course and the mechanism of biosynthesis of cellulose has received growing interest in academic research, as demonstrated by the rapidly growing number of relevant publications, which however up to now have remained without technological consequences. The very complex process of cellulose biosynthesis comprises not only the stepwise formation of the ß-l,4-glucan chain, but also the establishment of a

4.1.1 Synthesis of the polymer skeleton of cellulose

3

well-defined supramolecular order and fibrillar architecture in the solid polymer formed. Furthermore, different mechanisms have to be assumed for the formation of cellulose in higher plants on the one hand, and in bacteria and algae on the other. According to Brown (1996) and Delmer and Amor (1995) this is accomplished by a complex of proteins with different enzymic and other functional activities (see Fig. 4.1.1). A detailed description of cellulose biosynthesis was published by Colvin (1985) and Tarchevsky and Marchenko (1991). In the last few years, the biosynthesis of cellulose using bacteria such as Acetobacter xylinum has been extended as the synthesis of partially functionalized celluloses. According to Ogawa and Tokura (1992a, b), the copolymerization of ßD-glucose with 7V-acetylglucosamine by Acetobacter xylinum leads to the incorporation of the amino sugar into the cellulose skeleton of up to 4 mol %. The enzymatic in vitro synthesis was investigated in recent years along two routes: (i) reacting UDP(uridine-diphosphat)-glucose with purified cellulose synthase; (ii) condensation of glucose or its derivatives by cellulases. Achievements along the first route are summarized by Lin and Brown Jr. (1989), (see also Amikan and Benziman, 1989; Kudlicka et al., 1996; Blanton and Northcote, 1990). A simplified scheme of this route is shown in Fig. 4.1.2 (Kobayashi et al., 1995). The enzymatic in vitro synthesis of 'short chain' cellulose of DP 22 has been described (Kobayashi et al., 1992; 1995; 1996). ß-Cellobiosyl fluoride was condensed as the substrate in a mixed solvent of acetonitrile and an aqueous buffer (pH 5) by means of purified cellulase from Trichoderma viride, an enzyme system well known for its hydrolysis activity on the glycosidic linkages of longchain cellulose (see chapter 2.2). The reaction system changed from a homogeneous to a heterogeneous state during the 12 h of treatment, and the reaction product obtained was characterized as a linear ß-l,4-glucopyran, identical with cellulose, by 13C NMR- and IR spectroscopy, as well as by conversion to cellulose triacetate after previous deactivation of the enzyme system. With a purified cellulase, Lee et al. (1994) succeeded in assembling the ß-l,4-glucan chains during their synthesis to a defined supramolecular structure resembling cellulose I. It was assumed that a micellar aggregation of the partially purified enzyme occurs and that in the substrate, in an organic/aqueous solvent system, there is alignment of glucan chains with the same polarity and extended chain conformation favored. Included in the enzymatic in vitro synthesis is the preparation of functionalized celluloses, e.g. of the methyl ether starting from 6-0-methyl-ßcellobiosyl fluoride. Since the early attempts by Schlubach (Schlubach and Luhrs, 1941) numerous research efforts have been devoted to the chemical synthesis of the cellulose macromolecule by polycondensation or by ring opening polymerization, but all

4


of these studies had limited success, obviously due to the difficulties of obtaining a strictly linear stereoregular chain structure. So, for example, the condensation of 2,3,6-glucose tricarbanilate with ?2θ5 in a mixture of CHCl3 and DMSO resulted in a cellulose-like but branched polymer, containing about 1 % phosphorus. Also, by a cationic polymerization of l,4-anhydro-2,3,6-0-benzyl-oc-Dglucopyranose with various Lewis acids, no stereoregular 1,4-glucopyran could be obtained (Micheel et al., 1974; Micheel and Broode, 1974 and 1975; Uryu et al., 1985). Obviously, the choice of suitable protecting groups in the monomer is the decisive point, as demonstrated by Uryu et al. (1981) by the synthesis of a ß1,4-D-ribopyran by a cationic ring opening polymerization. For this topic the reader is also referred to a comprehensive review by Kotchetkov (1987) on the synthesis of polysaccharides with a regular structure. OH O

O

Il

Il

,Ps.

J\

I ^0^1 O O HO

OH

UDP-glucose

cellulose synthase

OH

HO

OH cellulose

Figure 4.1.2. Simplified scheme of enzymatic in vitro synthesis of cellulose starting from UDP-glucoe (Kobayashi et al, 1995). Recently, Nakatsubo et al. (1996) succeeded in synthesizing cellulose molecules by cationic ring-opening polymerization of 3,6-di-O-benzyl-a-D-glucose1,2,4-0-pivalate to 3,6-di-0-benzyl-2-O-pivaloyl-ß-D-glucopyran, and subsequent removal of the protecting ether and ester groups. The presence of adequate ether groups, preferably benzyl groups, in the 3-O-position is considered to be essential for achieving a stereoregular structure, and the presence of ester groups, preferably pivaloyl groups in the 2-O-position, is required for securing a ß-glucosidic linking of the monomer units. A simplified scheme of the synthesis is presented in Fig. 4.1.3.

4.1.1 Synthesis of the polymer skeleton of cellulose

1

R

5

R

-" n

Figure 4.1.3. Simplified scheme of cellulose synthesis by cationic ring-opening polymerization (Nakatsubo et al., 1996). Λ^Λ^-carbonyldiimidazole served as a dehydrating agent in ortho ester synthesis. This reagent, frequently employed in glucoside and peptide synthesis, preferentially attacks a hydroxy group that is more acidic than 4-OH to give a 1-0carbonylimidazole derivative. This is further converted to a dioxocarbenium ion intermediate, by removal of the carbonyl imidazole group and then to an orthoester by intramolecular attack of 4-OH. Polymerization of the ortho-ester can be catalyzed by BF3 · Et2O, by (phenyl)3+CSbC!6~ or most efficiently by (phenyl)3+CBF4~ in methylene chloride as the medium. The ß-l,4-glucopyran structure of the compound obtained with a DPn of about 20 was confirmed by 13 C NMR spectroscopy. The transformation of this compound to cellulose was achieved via the triacetate by converting it at first to the 2-O-acetyl derivative with MeONa in tetrahydrofuran (THF)/MeOH, and subsequently with acetanhydride in pyridine, followed by debenzylation with Pd/H2 under pressure and acetylation of the free hydroxy groups with acetic anhydride in pyridine. No depolymerization was observed during this procedure. After deacetylation with MeONa in THF, finally a cellulose showing the X-ray pattern of cellulose II was obtained. This route of synthesis described here in some detail obviously represents the present 'state of the art' and simultaneously gives an impression of the difficulties and problems to be overcome in regio- and stereoselective cellulose chemosynthesis. An interesting route to a highly branched cellulose macromolecule was recently reported by Franzier et al. (1996). An anhydrous solution of cellulose in DMA/LiCl was treated with hydrogen fluoride in pyridine at a low HF concen-

6


tration, resulting in long-chain branching of the polymer, which is obviously caused by transglycosidation via glycosyl fluoride groups as intermediates.

4.1.2 Covalent crosslinking of cellulose 4.1.2.1 Principles of cellulose crosslinking From the commercial point of view, the formation of covalent crosslinks between the cellulose chains is the most important route to modify the polymer skeleton of this polysaccharide. It is widely employed on a large, industrial scale to improve the performance of cellulosic textiles. Although structure and material properties of cellulose in the solid state are largely determined by a selfcrosslinking via intermolecular hydrogen bonds, this intermolecular interaction is partially reversible in the presence of water and is completely overcome by conventional cellulose solvents like aqueous Guam. Thus covalent crosslinking is required to avoid undesirable changes of cellulosic goods in the wet state. Since Eschalier's reported crosslinking of cellulose by the action of formaldehyde (Eschalier, 1906 and 1907) at the beginning of this century, numerous crosslinking agents and crosslinking reactions have been described, most of them being based on the formation of ether bonds by alkylation of hydroxy groups at neighboring cellulose chains. Material properties after crosslinking were found to depend on the constitution and length distribution of the crosslinks on the one hand, and on the crosslink density (average distance between two crosslink points along the cellulose chain) and the distribution of this crosslink density within the fiber structure on the other. This implies a strong influence of cellulose supramolecular and morphological structure on the effects of crosslinking with the reagent employed. In this subchapter, the chemistry of crosslinking will be considered first, turning then to the interplay between crosslinking and physical structure, and finally surveying the changes in the material properties obtained and the industrial application of covalent crosslinking.

4.1.2.2

Chemical routes to crosslinking of cellulose

There are various routes to crosslinking the polymer by covalent or ionic reactions. • Recombination of cellulose macroradicals formed chemically or by irradiation. • Reaction of anionic cellulose derivatives by at least divalent metal cations. • Oxidative crosslinking by formation of disulfide bridges from mercapto groups attached to cellulose.

4.1.2 Covalent crosslinking of cellulose

1

• Formation via urethane bridges by reaction of cellulosic hydroxy groups with isocyanates. • Crosslinking via ester groups formed by reaction with polycarboxylic acids. • Formation of ether bonds with an at least difunctional etherifying agent. Covalent or ionic reactions can take place either intermolecularly, i.e. between reactive sites of two or more different macromolecules, or intramolecularly, i.e. between suitable sites along the same polymer chain. Both processes usually occur simultaneously to a varying extent. The analytical characterization of the crosslinked products still poses serious problems: usually only an average number of crosslinks per unit chain length (crosslink density) can be estimated from the amount of heteroatoms like nitrogen or sulfur introduced, or from a determination of the gain in weight of the sample, due to addition of the crosslinking agent. Information on the distribution of the crosslinks and on details of their structure is still rather scarce and mostly obtained by indirect methods, such as for example characterization of physicochemical bulk properties of the crosslinked products, such as for example swelling or solubility. Macroradicals suitable for crosslink formation by recombination can be generated from cellulose chains either by high-energy irradiation leading to homolytic bond cleavage, or by transfer reactions from a radical source outside the macromolecules. Kriss et al. (1985) reported the photolytic generation of ligand radicals of Mn3+ complexes with acetyl acetonate, and the subsequent formation of cellulosic macroradicals by a transfer reaction, finally resulting in crosslinking, and a predominant crosslinking in comparison with cellulose chain degradation is assumed by Philipp et al. (1982) after electron-beam irradiation of cellulose at a low dose rate. Ionic crosslinking requires the presence of anionic groups, like carboxymethyl groups or sulfuric acid half-ester groups. As suitable crosslinking agents FeCl3, Al2(SO4^ or Cr2(SO4)3 are known (Heinze et al. 1990). Further details on ionic crosslinking and application of the gels obtained will be described in connection with carboxymethylcellulose (see chapter 4.5) and with carboxycellulose (see chapter 4.6). Crosslinking of cellulose by oxidative coupling of mercapto groups to disulfide bridges was studied (Sakamoto et al., 1970), comparing samples with the mercapto groups directly bound to the cellulose chain with those with the mercapto groups tethered to the polymer backbone via a long spacer. In the latter case a complete and fully reversible oxidative crosslinking could be easily achieved due to the mobility of the mercapto groups, while with these groups directly bound to the backbone only a small fraction could be converted to disulfide bridges. The reaction of cellulosic hydroxy groups with diisocyanates usually poses no problems (Sakamoto et al., 1970). This route is not practiced in textile finishing

8


due to the toxicological hazards involved. A combination of the activities of the isocyanate group and a vinylic C=C double bond in cellulose crosslinking was realized by employing acrylic isocyanate as the crosslinking agent. Crosslinking by ester bond formation occurs in the reaction of cellulose with a suitable di- and polycarboxylic acid. According to recent infrared studies (Yang and Wang, 1996) five-membered cyclic anhydrides are formed as intermediates in the thermally activated crosslinking of cotton fabrics with suitable polycarboxylic acids. Comparing the crosslinking performance of different polycarboxylic acids, those carrying their carboxyl groups at adjacent C atoms, and thus being capable of forming five-membered anhydrides, were found to be more effective in cellulose crosslinking than those carrying their carboxyl groups at alternating C atoms of the polymeric acid chain. The only six-membered cyclic anhydride formed and detected on the treated cotton was that of poly aery lie acid. Self-crosslinking via intermolecular esterification can take place with anionic cellulose derivatives, especially carboxymethylcellulose, at low pH and elevated temperature, due to reaction of acid groups with free hydroxy groups of neighboring polymer chains. The numerous routes to crosslinking cellulose via acetal resp. ether bonds will now be considered in some detail due to their scientific and commercial relevance: • • • • •

Acetalization of hydroxy groups with formaldehyde Acetalization with glyoxal or its homologs Reaction with N-derivatives of formaldehyde like dimethylol urea Michael addition of divinylic compounds with hydroxy groups Etherification of hydroxy groups by aliphatic di- or tri-halogenated compounds like dichloroethane • Alkylation by epoxides like 1,2,3,4-diepoxibutane • Etherification by epichlorohydrin Crosslinking with formaldehyde proceeds as a two-step reaction via a cellulose hemiacetal (methylolcellulose) as an intermediate according to the general scheme CeII-OH + CH2O

·

- CeII-O-CH2OH

CeII-O-CH2OH + CeII-OH

·

- CeII-O-CH2-O-CeII + H2O

In reality, the formation of acetal bridges - usually taking place in an aqueous acid medium - is considerably more complicated by the fact that:


9

(i) both steps proceed as equilibrium reactions, and the acetal bridges exhibit a limited stability only and can split-off formaldehyde under suitable conditions; (ii) crosslinking in the acid medium is inevitably accompanied by some chain degradation due to acid hydrolysis of glycosidic linkages, which becomes more pronounced with increasing reaction temperature; (iii) the kinetics of the crosslinking reaction is governed by a specific acid ca+ talysis, with the rate of formaldehyde add-on increasing with increasing H or + H3O concentration (Fig. 4.1.4). _ © fast _ ® CeII-O-CH2-OH + H ^=^ CeII-O-CH2-OH2 _ © slow _ Θ © ±: CeII-O-CH2-OH2 ^= [CeII-O-CH2 -CeII-O = CH 2 J+ H2O

H

- θ fast Ie CeII-O-CH 2 + CeII-OH ^=^ CeII-O-CH2-O-CeII H

ΙΘ CeII-O-CH 2 -O-CeII

fast Ä ^=± CeII-O-CH 2 -O-CeII + He

Figure 4.1.4. Scheme of acid catalysis in formaldehyde crosslinking of cellulose (taken from Meyer et al., 1976). Meyer et al. (1976) mentions in his detailed studies on the kinetics and the mechanism of this process that a specific catalysis by H+ or H3O+ is responsible for more than 98 % of the crosslinks formed by formaldehyde. It was assumed that added metal salts like MgCl2 co-catalyze the process by increasing the H3O+ concentration and not by a catalytic action of the metal cation itself. The overall course of the reaction was determined by one of the chemical reaction steps or by swelling and diffusion processes, depending on reaction conditions and structure of the cellulose sample. In practise, crosslinking with formaldehyde can be performed as a wet process by treating the specimen with an aqueous acidic formaldehyde solution at room temperature and subsequent curing at 100-130 0C, with the crosslinking taking place within minutes during this drying process. An alternative is the so-called dry process, with the specimen soaked at first with aqueous boric acid followed by drying and subsequently by the crosslinking action of paraformaldehyde vapor.

10


The inconvenient handling of free formaldehyde can be avoided and the structure of the crosslinks can be varied within wide limits by employing as crosslinking agents the methylol or alkoxymethyl derivatives of different N-containing compounds (urea, cyclic ureas, carbamates, acid amides or triazines) forming acetal bridges between the cellulose chains as indicated by the scheme (Fig. 4.1.5): O M

2 CeII-OH + HO-CH2-N-C-N-CH2-OH I - ι ' "" H H

H®

-2W 2 O

O Il CeII-O-CH2-N-C-N-CH2-O-CeII I I H H

O Il H® 2 CeII-OH + H3CO-CH2-N-C-N-CH2-OCH3 — I I -2 CH3OH H H

Figure 4.1.5. Crosslinking of cellulose with urea derivatives. Besides the crosslinking reaction proper, self-condensation of the agent as well as liberation of formaldehyde have to be taken into account in this usually acidcatalyzed process governed by interdependent chemical equilibria. The structural type of CH2O binding in these crosslinkers can vary widely, resulting in large differences in stability against formaldehyde liberation (Petersen and Petri, 1985): •^s

Λ N—GH,—Ο—Cell I

· CeII-O-CH2-O-CeII

ο ^N-CH,-OH

J-CH2-OR ' O

R - Alkyl O

A ^^Ν —CH -N' 2

· CeII-O-CH2-OH

· CH2O , HO-CH2-OH ,

4.1.2 Cov alent er o s slinking of cellulose

O

11

O

HOCH2-N^N-CH2OH

HOCH2-N

A NH +

CH2O

100 χ 10,-5

50

2

4.

6

8 pH

10

Figure 4.1.6. pH-dependent stability of a methylol group in a cyclic urea (Peterson and Petri, 1985).

As illustrated by the example in Fig. 4.1.6, the stability of methylol groups against acid or alkaline hydrolysis is largest near the neutral point, with the rate constant of hydrolysis increasing steeply to both sides of the pH scale. The kinetics and the mechanism of these crosslinking processes have been thoroughly studied over the last 30 years (Peterson and Petri, 1985). The reactivity of the agents and the stability of the crosslinks formed against formaldehyde liberation could be correlated to their constitution. By techniques of molecular modeling and statistical design, high-performance crosslinkers have been developed with only a minimal tendency to liberate CH2O during processing and storage of the textile goods subjected to this crosslinking treatment. While formaldehyde must be considered as difunctional in forming acetal bridges between cellulose chains, glyoxal can act as a tetrafunctional crosslinker, connecting two cellulose chains already at the hemiacetal formation stage of the reaction. Model experiments with low molecular alcohols (Sangsari et al., 1990) on competitive hemiacetal and acetal crosslinking, led to the conclusion that alcohols with two vicinal hydroxy groups are much more effective in hemiacetal formation than those with isolated hydroxy groups, while the subsequent catalyzed acetal formation proceeded preferentially with isolated alcoholic hydroxy groups. A predominant formation of dioxan bisacetal structures was reported in this study, and the reaction mechanism derived was assumed to hold true in

12


principle also for crosslinking of cellulose by glyoxal. Glycol aldehyde and glycol were reported to be effective co-reagents. In agreement herewith also, the properties of an hydroxyalkyl ether of cellulose like hydroxyethylcellulose in an aqueous medium can be efficiently changed by crosslinking with glyoxal. Crosslinking by Michael addition of cellulosic hydroxy groups onto vinylic carbon-carbon double bonds is preferably performed with divinyl sulfone (DVS) in an aqueous alkaline system according to: 2 CeII-OH + CH2 = CH-SO2-CH = CH2 CeII-O-CH2-CH2-SO2-CH2-CH2-O-CeII The formation of hydrogels from mixtures of CMC and hydroxyethylcellulose in aqueous alkaline solution (0.02 M KOH) by crosslinking with DVS may be sighted (Esposito et al., 1996). The crosslinking density, defined the ratio between the maximal number of reacted sites (based on DVS input) and the total number of reactive sites, was varied within wide limits via the molar ratio of DVS to polymer. Crosslinking densities above 1 indicate a partially monofunctional mode of reaction of the difunctional crosslinker. Formation of ether crosslinks by reaction of cellulose with alkyl halides or epoxides proceeds along the conventional routes of cellulose etherification (see chapter 4.5), as indicated by the examples in Fig. 4.1.7. 2 CeII-OH + CICH 2 -CH 2 CI

OH0 ^ - 2 HCI

CeII-O-CH 2 -CH 2 -O-CeII

2 CeII-OH + CH 2 -CH-CHp-CH

\ / O

OH0

>

\ / O

CeII-O-CH 2 -CH-CH-CH 2 -O-CeII I I OH OH Figure 4.1.7. Crosslinking of cellulose by 1,2-dichloroethane and 1,2,3,4-diepoxybutane. While the reaction between halide functions and the hydroxy groups requires a strongly alkaline medium, the ring opening and subsequent formation of ether bonds with diepoxides is catalyzed already by a low alkali concentration. Also acid catalysis of this reaction has been reported. According to Benerito et al.

4.1.2 Covalent crossünking of cellulose

13

(1961) the change of cotton properties by crosslinking with diepoxides depends largely on the ratio of Zn(B F4)2 as an acidic catalyst per mol of AGU. (OH®)

CH 2 -CH-CH 2 CI \ / O Θ CeII-OH + CHo-CH-CHoCI ,θ ΙΟΙ CeII-O-CH2-CH-CH2CI

θ CH2-CH-CH2CI 1 θ ΙΟΙ

CeII-O-CH2-CH-CH2CI OH

> CeII-O-CH2-CH-CH2 +

OH

O

Cell-O-CH2-CH-CH2+Cell-OH \ /

CeII-O-CH2-CH-CH2-O-CeII OH

Side reactions CeII-O-CH2-CH-CH2OH OH

CeII-O-CHp-CH-CHpCI I OH CH-CH-CH2CI O

CH2-CH-CH2CI O

CeII-O-CH2-CH-CH2CI Q-CH2-CH-CH2CI

H2O / ΟΗΘ

-*- CH2-CH-CH2 I l I OH OH OH

OH

Figure 4.1.8. Scheme of cellulose crosslinking with epichlorohydrin.

A combination of the halide function and the epoxide function is realized in the frequently employed crosslinking agent epichlorohydrin. According to the reaction scheme presented in Fig. 4.1.8, the epoxide ring is cleaved in the alkaline reaction medium with subsequent formation of a l-chloro-2-hydroxypropyl ether of cellulose. Then the Cl atom is split-off as a chloride anion in the presence of the strong alkali and a 1,2-epoxide is formed which, after cleavage, reacts with a second hydroxy group of cellulose to give a 2-hydroxypropyl ether crosslink. A direct reaction between the chlorine atom and a cellulosic hydroxy group (ether formation by Williamson reaction) is obviously impeded under the strongly alkaline conditions employed, favoring epoxy ring formation. As a side reaction, saponification of the l-chloro-2-hydroxypropyl ether to a 1,2dihydroxypropyl ether of cellulose can take place. Furthermore, epichlorohydrin can be saponified to glycerine, or further molecules of the crosslinker can be

14


added to the hydroxy group of the l-chloro-2-hydroxypropyl ether resulting in longer crosslinking bridges. The reactivity of cellulosic hydroxy groups in epichlorohydrin crosslinking decreases in the order OH-2 > OH-6 > OH-3 (Luby et al., 1979). The presence of a sufficient amount of water and of a NaOH concentration of at least 9 % (Dautzenberg et al., 1980) have to be considered as necessary prerequisites for successful epichlorohydrin crosslinking, which is performed usually by steeping in the epichlorohydrin-containing alkaline liquid, by spraying this liquid onto the cellulose sample, or by treatment with epichlorohydrin vapor after alkaline steeping. A reaction time of 2 h and a reaction temperature of 60 0C were found to be adequate in crosslinking of cellulose powder (Fanter, 1980). A very large amount of water present and a low temperature of reaction have been reported to favor 1,2-dihydroxypropyl ether formation and thus to decrease the reagent yield for crosslinking which can be 80-90 % under optimal reaction conditions. The degree of crosslinking can be varied within wide limits up to about 1.5 via the molar ratio of epichlorohydrin per AGU.

4.1.2.3 Role of supramolecular and morphological structure in cellulose crosslinking The number of crosslinks formed and their distribution within the cellulose sample depends largely on its structure. This holds true for acid-catalyzed formaldehyde or methylol urea crosslinking, as well as for the action of diepoxides or of epichlorohydrin in a strongly alkaline medium. An important factor controlling crosslink density and distribution, and thus also the changes in material properties, is the state of swelling of the sample prior to or during the crosslinking process. The distribution of formaldehyde crosslinks was assessed by a special dying technique with rhodamine B (Kokot et al., 1975). A different distribution of dimethylol urea derivatives in cotton was reported after previous NH3 treatment on the one hand, and mercerization with NaOH on the other, with this different distribution also being reflected in the material properties of the crosslinked samples (Zeronian et al., 1990). On crosslinking with epichlorohydrin, the crystallinity of cellulose I is affected only after previous transformation to sodium cellulose. After neutralization and drying of the crosslinked sample a rather diffuse X-ray pattern inbetween the lattice types of sodium cellulose and cellulose II was observed due to the spacing action of the ether crosslinks impeding the formation of a welldefined cellulose II lattice (Dautzenberg et al., 1980). The mode of alkali treatment and the structural changes resulting therefrom were found to influence largely the course of epichlorohydrin crosslinking. The gross morphology of


15

cellulose powder particles exhibited only minor changes after epichlorohydrin crosslinking, and the altered morphology on the fibrillar level revealed by scanning electron microscopy seemed to be caused mainly by subsequent deswelling and shrinking and not by the crosslinking reaction itself.

4.1.2.4

Material properties of crosslinked cellulose

Just as with other linear polymers, cellulose is rendered insoluble in its common solvents by crosslinking to a sufficiently high density. The solubility in Guam of epichlorohydrin-crosslinked !inters powder was found to decrease sharply, well below a degree of crosslinking of 0.1 in the case of a uniform crosslinked distribution throughout the cellulose structure. The presence of non-crosslinked regions shifted the onset of solubility decrease to a somewhat higher degree of crosslinking. 80

x102

60

a)

§200

"100 fe

ι

g

I 0.2

0.6 1.0 Degree of crosslinking

I20 0.01 0.03 0.05 Mole crosslink /mole cellulose

Figure 4.1.9. Change of WRV with degree of crosslinking: (a) crosslinking with formaldehyde (Young, 1985); (b) crosslinking with epichlorohydrin (Fanter, 1980). Water retention as an important end-use property of cellulosics is remarkably changed on crosslinking. The amount and the direction of the change depend largely on crosslinking agent and crosslink density (see Fig. 4.1.9). After crosslinking with formaldehyde via short acetal bridges, a continuous decrease in water retention value (WRV) with increasing degree of crosslinking can be observed. Crosslinking with epichlorohydrin from a swollen state, on the other hand, resulted in a cellulosic of distinct maximum WRV in dependence on de-

16


gree of crosslinking. Obviously the spacer action and the hydrophilicity of 1,2dihydroxypropyl ether chains formed dominates at low and medium crosslink density and enhances the WRV, before it decreases again at high crosslink density (see chapters 2.2 and 2.3). It is interesting to note that the susceptibility to enzymatic or acid hydrolysis of glycosidic bonds also passed a distinct maximum with increasing degree of crosslinking. Crosslinking, especially with formaldehyde or formaldehyde urea compounds, affects decisively the mechanical properties of cellulose fibers and threads. These effects are the basis of commercial application of cellulose crosslinking in the textile industry. The stiffness and wrinkle resistance of cellulosic threads are significantly enhanced by crosslinking, while strength and extensibility are diminished. According to Cowan and Hurwitz (1982) this strength loss is largely reversible after de-crosslinking by cleaving the acetal bridges with alkali, and thus is not to be traced back to the inevitable loss of DP connected with the process, but is caused by the crosslinking itself.

4.1.2.5 Applications of cellulose crosslinking The crosslinking of cellulose finds its most important commercial application in textile finishing of cellulose-based fabrics for conveying to them some end-use properties relevant for the consumer, like e.g. wrinkle resistance, permanent press and easy care properties, or a special handle. These developments started with the integration of a formaldehyde treatment into the viscose process and was later expanded to the treatment of textile goods from cotton. Today predominantly methylolated or alkoxymethylated urea compounds are employed as crosslinking agents. Usually the fabric is soaked with the aqueous crosslinking system in a continuous process at room temperature and at a speed of 60100 m/min and then continuously cured at a temperature between 100 and 130 0C. The actual development aims to have the crosslinking agents liberating a minimum of formaldehyde in processing as well as in storage and use of the fabrics, a partially methylated dimethylol urea derivative being sighted as an example (Petersen, 1990). Crosslinking by epichlorohydrin was employed to modify the pore structure and the swelling behavior of cellulose beads (Loth and Philipp, 1989). Formation of hydrogels by crosslinking water-soluble cellulose ethers with various crosslinking agents has been proposed for the preparation of Chromatographie materials. Especially the crosslinking of carboxymethylcellulose along various routes has been widely studied in order to open up new areas of application, for example as dental glue after partial self-crosslinking between hydroxy and carboxyl groups, or as a component in sanitary goods making use of the high swelling and high water-binding capacity of CMC, rendered insoluble in water by covalent crosslinking (Klemm et al., 1985; Young, 1985; Heinze et al., 1990).

4.1.3 Grafting onto cellulose chains

17

4.1.3 Grafting onto cellulose chains 4.1.3.1

Relevance of grafting

Grafting of synthetic polymers onto the macromolecule cellulose has been amply studied in the second half of this century as a scientific challenge based on principles of cellulose chemistry as well as on general polymer chemistry, and as a promising route to combine the advantages of the material properties of cellulose with those of synthetic polymers. The 'state of the art' about 10 years ago has been comprehensively described by Helbreich and Guthrie (1981). Generally all the routes of polymer synthesis known today can be employed for a covalent attachment of polymer side chains onto a cellulose backbone, but free radical polymerization of vinylic compounds initiated by a redox system or by high-energy radiation dominates by far. Mostly the grafting is performed onto cellulosic materials in the solid state applying liquid or gaseous monomers, with the consequence of a strong influence of the supramolecular and morphological structure of the cellulosic substrate on the course of the grafting reaction. Despite the remarkable and often favorable changes in the material properties of cellulosics obtainable by grafting, and despite several promising developments reaching the pilot plant level, the commercial application of cellulose grafting remained behind the optimistic expectations announced two or three decades ago, obviously mainly for economical reasons. Within this subchapter, the chemical principles of cellulose grafting will be considered first, in connection with the relevant reaction parameters and the structural parameters employed for cellulose graft copolymer characterization. Subsequently, some effects of supramolecular and morphological structure of the substrate on the course of grafting will be surveyed briefly, turning then finally to the material properties and some areas of application of cellulose graft copolymers.

4.1.3.2

Chemistry of cellulose graft copolymer formation

Ushakov (1943) first attempted to copolymerize allyl and vinyl derivatives of cellulose with acrylic acid esters, resulting in the formation of insoluble grafted polymers. Table 4.1.1 summarizes typical routes of cellulose grafting. But quite predominantly the free radical polymerization of vinylic compounds has been used in studying cellulose grafting (Berlin and Kislenko, 1992). As shown in the scheme below, cellulose graft polymerization is inevitably combined with some homopolymerization of the monomer. The analytical characterization of a cellulose graft copolymer therefore requires, besides the determination of the so-called add-on, i.e. the amount of monomer transformed to polymer, a separate assessment of the homopolymer formed via its extraction, in order to obtain the grafting efficiency. Furthermore, the length of the grafted

18


side chains and their number per backbone molecule of average chain length can vary within wide limits and can be estimated after hydrolysis of the cellulose backbone by performing a macromolecular characterization of the side chains. It must be emphasized, however, that graft copolymer analysis poses many problems and uncertainties in its practical realization. Table 4.1.1. Routes to graft copolymers of cellulose

Example Styrene after redox initiation Acrylonitrile after high-energy irradiation Anionic polymerization Acrylonitrile onto alkali cellulosate Cationic polymerization 'Cardanol' after initiation with BF3 etherate (John and Pillai, 1989) Ring opening polymerization Caprolactam Polyaddition Ethylene oxide + NaOHaq Polycondensation Amino carbonic acid chlorides Coupling of preformed macro- Polyamide, polyesters molecules onto cellulose

Route Free radical polymerization

Initiation (irradiation)

Propagation

C — C — C'

C

+ M —CM

M — M*— M*

C*

+ M* — CM* or C*+ M

CM*+ nM —CM n + 1 M*

+ nM —- MO+1

Termination Chain transfer C* + M * — CM C* + S — C + S CM*,+CM;— c 2 M m+n S* + M S + M' CM;+CM;— CM n -CM m Mn*

+ M* -Mn+1

A broad variety of cellulosic materials has in the meantime been employed as substrates for grafting. Besides cotton and other natural fibers, wood pulp and viscose filaments and fabrics, also lignocellulosic materials like straw or cellu-


19

lose derivatives like cellulose acetates have been used. Some monomers, frequently reported as suitable for cellulose grafting, are: • • • • • •

Styrene · Acrylic acid Acrylonitrile · Na-vinyl sulfonate Acrylic acid esters Methacrylic acid esters Acrylamide Fluorinated methacrylate

· Vinylpyridine · Dimethylaminoethyl methacrylate

Quite predominantly, the grafting is conducted in a heterogeneous system with the solid polymer and with the monomer being present in the liquid state, often in the presence of water or organic liquid. But also grafting under homogeneous conditions has been reported, e.g. in the DMA/LiCl system. For starting a graft side chain, a radical site at the cellulose backbone is definitely required. These radical sites can originate from the homolytic bond cleavage within the AGU, for example after high-energy irradiation, from the decomposition of a suitable functional group at the macromolecule, e.g. a peroxide group, or from a radical transfer reaction initiated by a radical formed outside the macromolecule, for example by a redox reaction. Important radical-generating systems used in cellulose grafting (Young, 1977; Krässig, 1971) are Ce(III)/ Ce(IV), Mn(II)Mn(III), and Fe(II)/H2O2/xanthogenate group. They have the advantage of being applicable in aqueous media. The rather complex action of Ce4+ on cellulose can be formulated in a highly simplified manner as

CeII-H +Ce 4+ —

Cell· + Ce 3+ + H+

(Stannett and Hopfenberg, 1971). Grafting of vinylacetate onto a sulfitedissolving pulp by means of the redox system Fe(II)/H2O2 has been recently reported (Zara et al., 1995). Mn3+ leads to the oxidation of the aldehyde groups and the 1,2-glycol moieties at the chain ends and of the 2,3-diole units of the AGU within the macromolecule (Ränby, 1981). A simplified reaction scheme for the xanthogenate redox system is presented in Fig 4.1.10. According to Krässig (1971) this method leads to grafts with numerous and rather short side chains, and the reaction can be easily controlled via the amount of xanthogenate groups previously introduced and the monomer concentration. The 'xanthogenate method' is also well suited to grafting onto lignocellulosic materials like mechanical pulp (Hornof et al., 1977).

20

4. l Formation and Modification of the Polymer Skeleton of Cellulose

For attaching various types of cationic side chains onto cellulose, a further route to free radical grafting was investigated (Bojanic, 1996). Cellulosic hydroxy groups are at first transformed to an acrylic ester by reaction with acryloyl chloride, and subsequently a conventional free radical polymerization is started at the C=C bonds introduced in the first step. I Il ^Fe I M .Fe LCH 2 -O-C-S + HO· —* hCH-O-C-S + H2O

S

S

,

Il

^CH 2 -O-C-SH +HO·

Il

—^ h C H - O - C - S · + H O

- Subsequent grafting with vinyl monomers (CH2 = CHX) e.g. styrene, acrylonitrile-

OH KC-CH 2 -CHxJcH 2 -CHXi-CH 2 -CH 2 X 1

H

IhCH I

L

"

and

Jn

r

i

2 -O-C-S-CH 2 -CHX4CH 2 -CHX + CH 2 -

"

-

J

n

Figure 4.1.10. Reaction scheme of cellulose grafting by the xanthogenate method (Krässig, 1971). Grafting of vinyl monomers as e.g. styrene onto cellulose derivatives with structopendant unsaturated ester moieties, especially onto cellulose cinnamate, has been reported (Zhang and McCormick, 1997), employing AIBN (azobisisobutyronitrile) as an initiator in this homogeneous free radical graft polymerization in DMA/LiCl. After mechanochemical treatment of cellulose, three types of radicals suitable for a subsequent graft copolymerization could be detected by a combination of scanning calorimetry and ESR spectrometry. These are alkoxy radicals formed at C-4 by glucosidic bond cleavage, carbon radicals at C-I and carbon radicals at C-2 and C-3 due to carbon bond scission between these two C atoms. The alkoxy radicals proved to be rather stable at ambient temperature and inert against oxygen, while the C radicals form peroxyradicals in the presence of oxygen.


21

Radiation grafting of cellulose is generally performed with high-energy electron-beam or γ-irradiation, although an initiation by corona discharge or by UV radiation is mentioned too in the literature. In spite of its high susceptibility to chain cleavage by high-energy radiation, cellulose is one of the most frequently radiation-grafted polymers. The grafting is performed either by a pre-irradiation technique, i.e. a two-step process consisting of irradiation of the substrate as the first step and the interaction of the pre-irradiated material with the monomer as the second. Also, the so-called simultane technique, by applying irradiation to the monomer-soaked cellulose material, was used. Fig. 4.1.11 gives an example of the increase of mass of the sample due to grafting by the two-step technique in dependency on radiation dose in the preirradiation step at otherwise constant reaction conditions. A steep increase of add-on occurs already at a rather low dose, followed by a levelling-off. This indicates the advantage of a rather low irradiation dose for an efficient grafting, while a further increase of the dose mainly promotes chain scission without improving the graft yield. In order to secure a high efficiency of grafting, the transition time between pre-irradiation and grafting must be kept short, as the add-on is proportional to the actual radical concentration and decreases steeply with increasing transition time (Fig. 4.1.12). The course of radiation grafting is strongly influenced by the moisture content of the cellulose sample, as well as by its supramolecular structure (see the following section). 16

£13

ο 10

2

6

10

U

Dose [RGy]

Figure 4.1.11. Increase in mass of sample in dependence on radiation dose in two step radiation grafting (other reaction conditions kept costant) (Rätzsch et al, 1990). In conclusion, the structure of the grafted polymer and the material properties dependent thereon are influenced by a large number of parameters, combining the degrees of freedom of the cellulose reaction with those of the free radical polymerization. So, for example, the number of side chains and their distribution

22


depends on the initiation technique and the monomer employed, as well as on cellulose supramolecular and morphological structure. The length of the side chains is mainly determined by the reaction system employed, but can additionally be controlled by the presence of a 'chain regulator' like CCl4. Side chains representing alternating copolymers can be grafted onto cellulose by a suitable choice of two monomers forming electron donator-acceptor complexes (Gailord, 1976). Monomers with two carbon-carbon double bonds can of course also be applied to cellulose grafting, but the probability of an irregular course of reaction and of crosslink formation is considerably increased here. Besides the parameters given by the reaction components, also the external reaction conditions, such as concentration ratios, reaction temperature and reaction time, are of high relevance in determining the structure of a cellulose graft copolymer. 22

E

16

ω K (Λ O

ε 12

υ _c

10

8

10 20 Transition time [min]

30

Figure 4.1.12. Decrease of add-on (increase of mass) with transition time in two-step radiation grafting of cellulose (other reaction conditions kept constant) (Rätzsch et al., 1990).

4.1.3.3

Effects of supramolecular and morphological structure on cellulose grafting

The supramolecular and morphological structure of the cellulose sample strongly influences the course of a grafting reaction, as well as the structure and properties of the graft material, via the spatial distribution, the mobility and the stability of the radicals formed, as well as via the transport rate of the monomer into the fiber wall. By an appropriate choice of the grafting system and the reaction conditions, either a rather uniform grafting throughout the cellulose fiber or a preferential surface grafting can be achieved. These general statements hold true for chemical as well as radiation-initiated grafting. A Mn3+-initiated grafting of various acrylic acid esters onto soft-wood pulp starts at the fiber surface and then proceeds gradually into the interior of the fiber (Ränby, 1981). With meth-


23

ylacrylate, the diffusion of the initiator proved to be the limiting factor, while with the more voluminous butyl acrylate an impeded monomer diffusion limited the grafting to the fiber surface. The high surface selectivity in the Ce(IV) graft copolymerization of acryl amide and a cationic monomer onto wood pulp fibers was emphasized (Gruber and Granzow, 1996). In radiation grafting the course of reaction significantly depends on the moisture content of the substrate. Radiation grafting of a completely dry preirradiated cellulose did not start until the temperature of thermal polymerization of the monomer was reached, while the starting temperature was significantly decreased by stepwise enhancement of the water content up to a level between 5 and 20 % (Plotnikov and Lesins, 1981). The mobility of the radicals formed increases with the moisture content in the less well ordered regions of a pulp or cotton fiber, resulting in an increase in polymer add-on with the moisture content in a grafting experiment employing the 'simultaneous method', and the decay rate of the radicals also increases with the content of H2O. A much higher stability of radicals trapped in the crystalline regions of the fiber as compared with those located in the amorphous regions was emphasized (Rätzsch et al., 1990). Stannett and Hopfenberg (1971) demonstrated the influence of swelling of a cellulose substrate, in connection with the gel effect of radical polymerization, by the dependency of molar mass of the graft and of polymer add-on by grafting of cellulose 2,5-acetate in styrene/pyridine mixtures of increasing swelling power (see Fig. 4.1.13).

20 40 60 Pyridine in styrene [%]

80

100

Figure 4.1.13. Effect of swelling on the yields and molecular weights of the grafted side chains for the mutual radiation grafting of styrene to cellulose acetate films · 0.0025 mm; O 0.025 mm thickness. Dose of 10 Mrad at 0.35 Mrad/h at 25 0C (Stannett and Hopfenberg, 1971).

24


A maximum in both parameters is found at a medium degree of swelling, permitting a sufficiently fast excess of the monomer entering the substrate but securing a sufficiently large gel effect to impede side chain termination. The mutual interaction between fiber morphology and course of grafting involves, however, not only the effect of fiber morphology on the grafting reaction but also the change of this morphology due to grafting. The morphological changes of a cotton fiber on radiation grafting with various vinyl monomers significantly depend on the molar volume of the monomer applied (Arthur, 1976). For example, side chains of poly (methyl methacrylate) were uniformly distributed in a collapsed fiber structure, while in the case of poly(butyl methacrylate) and higher poly(alkyl acrylates) a fiber opening and layering effect was observed. By appropriate timing of irradiation and swelling, either a uniform grafting throughout the fiber or a skin/core grafting can be achieved. A cationic graft copolymer can exhibit quite a different morphology depending on grafting technique (pre-irradiation or simultaneous method) (Rätzsch et al., 1990). The preradiation technique was recommended for surface grafting, especially of beech pulp as the substrate, while the simultane technique resulted in a more uniform grafting across the fiber.

4.1.3.4

Properties and applications of graft copolymers of cellulose

Graft copolymerization of cellulose with appropriate monomers frequently results in decisive changes of the chemical and physical properties as well as in numerous more or less qualitatively evaluated end-use properties of the polymer. The expectations promoting research in this area, i.e. an advantageous combination of properties of natural and synthetic polymers, could be widely realized at a laboratory or a small-sized technical scale. But in contrast to the large number of publications dealing with the effects of grafting on macromolecular structure (see for example Table 4.1.2; Krässig, 1971), investigations correlating, in a quantitative manner, end-use properties to grafting systems and grafting conditions and the structural changes resulting therefrom, are comparatively scarce. An example is given in Table 4.1.3 (Rogowin, 1972), regarding the glass transition temperature of styrene-grafted cotton. Most of the information available today on property changes by grafting concerns fibers, filaments and fabrics, and more recently also to some extent cellulose-based membranes. Properties of cellulose fibers affected by grafting are:


Fiber fineness Tensile strength Elongation at break Elastic modulus Water vapor uptake Water inbibition Thermoplasticity Dimensional stability Abrasion resistance

25

Degradability Permanent press behavior Wrinkle resistance Water repellency Oil repellency Soil release Microbial resistance Flame retardancy

Generally, the property changes observed can be traced back to a varying extent to changes in the chemical structure of the macromolecules by the covalently attached synthetic side chains on the one hand, and to an altered supramolecular and morphological structure on the other. In the case of water inbibition, a parameter relevant to cellulose textiles as well as to membranes, a prevailing effect of supramolecular and morphological structure has been assumed, with the constitution of the side chains playing a minor role only. Cationic side chains, however, were reported to bind less water than anionic ones under comparable conditions (Mukherjee et al., 1983). Table 4.1.2. Examples of the relation between grafting conditions and structure of cellulose graft polymers(Krassig, 1971)

Backbone polymer

Method Grafting conof initia- ditions tion (Mrad)

Post irra- 0.32 diation styrene grafting 3.24 styrene Simulta- 0.32 1 neous styrene irradia3.24 tion grafting styrene 0.02 M Cotton Redox (DP- 1200) reaction Ce(IV); acrylonitrile

Cotton (DP-900)

Add-on Homopolymer (%) (%) 24 22.6

M.W. of Side chains side per AGU chains 6 (x 10 ) 0.02 3.02

83.7

18

2.26

0.08

19.6

39

1.07

0.04

47.5

35

0.31

0.35

27.5

20

0.06

1.13

As can be expected from the broad spectrum of cellulose properties that can be changed by grafting, a host of applications for cellulose graft copolymers has

26


been proposed, especially during the 1970s. These include, besides the modification of textile yarns and fabrics of cellulose, grafting onto cellulose derivatives like cellulose acetate or onto lignocelluloses like straw, with the polymer add-on being much less at the lignin than at the cellulose component (Fanta et al., 1987). Further examples are the use of special monomers like perfluorinated compounds or various cationic acrylics, and last but not least the application of grafted products outside the textile field, for example in ion-exchange and filtering processes (Duntsch et al., 1989), in membrane separation processes for oil/water mixtures, or in soil conditioning and seed planting (Stannett, 1985). Table 4.1.3. Effect of grafting on the glass transition of cellulose (Rogowin, 1972)

Copolymer Cellulosepolystyrene

Composition of reaction products Grafted polymer Cellulose (%) (%) 60 40 60 40 74.8 25.2 72.9 27.1

M. W. of the grafted chain

158.000 74.150 7.800 4.150

Glass transition temperature 126 102 104 96

But despite all these achievements of research and development, only a few of the grafting procedures and graft product applications proposed arrived at the stage of pilot-scale production or even industrial manufacture. Obviously shortcomings in process economy, problems in subsequent processing steps and a lack of market acceptance may be the main reasons for this disappointing situation, which led to a significant decline of research activities in this area during the last 15 years. Nevertheless, some of these developments will be surveyed briefly at the end of this subchapter. Much effort has been spent on preparing cellulose-based super-absorbing materials by grafting anionic side chains onto the cellulose backbone, but at least up to now these products could not compete efficiently with the crosslinked acryl-based synthetic materials dominating the market (Stannett, 1985). An interesting combination of properties of cellulose and acrylonitrile fibers has been achieved by Rogowin (1974), who investigated the grafting of various monomers onto viscose before, during and after the spinning step and developed a technical process of grafting acrylonitrile onto freshly spun viscose fibers in aggregates, producing nearly l t of graft material per batch. This process, however, depended on temporary regional economic conditions and therefore was later abandoned. Another process developed to the pilot scale was an antimicrobial finish of cellulosic fabrics by grafting with acrylic or methacrylic acid to a grafting degree of 2-3 % and subsequent binding of copper ions to the carboxyl groups at

4.1.4 Synthesis of cellulose block copolymers

27

the side chains (Heger, 1990). The product obtained, and primarily intended for hospital laundry, exhibited a satisfactory antimicrobial behavior of good permanency, but its poor handling and color impeded acceptance in the market. Last but not least, the combination of an acid-catalyzed crosslinking of cellulosics by methylol acrylamides and a subsequent free radical grafting shall be mentioned, which was the first industrial application of radiation grafting for conveying permanent press properties, high wrinkle recovery and shrink resistance to cellulosic textiles.

4.1.4 Synthesis of cellulose block copolymers In principle, cellulose block copolymer synthesis starts from a cellulosic prepolymer of usually low DP provided with reactive end groups and with protected hydroxy groups at the C-2, C-3 and C-6 position of the AGU to avoid side chain grafting. These reactive end groups can then be used either to initiate the formation of a block of a synthetic polymer or to form a covalent linkage to a synthetic macromer. Two- and three-block copolymers, as well as star-shaped block copolymers synthesized along these routes have been described. Attempts reviewed by Rogowin and Galbraich (1983) to provide reactive radical end groups by homolytic chain scission via the input of mechanical energy (ball milling, vibration milling) succeeded in the combination of cellulosic segments with those of e.g. polyamides, but the copolymers obtained were of rather ill-defined structure. Examples of synthesis of a polymer sequence onto cellulose end groups by free radical or cationic polymerization, resulting in welldefined structures, have been described (Feger and Cantow, 1980 and 1982). A polymeric photoinitiator suitable for starting a subsequent free radical polymerization of vinylic monomers has been obtained by coupling a strictly monofunctional hydroxy-end-group-terminated sequence of a cellulose triester (acetate, propionate, butyrate) with bis-4-isocyanatophenyl disulfide. Cellulosederivative-terminated three-block copolymers of defined structure were prepared by a macroinitiator-started free radical polymerization, the latter being considered more suitable for block formation onto cellulosics than a living anionic polymerization. A route to linear or star-shaped block copolymers containing sequences of trimethylcellulose and of polyoxytetramethylene was realized via a cationic polymerization of THF (Mezger and Cantow, 1983). Trimethylcellulose was partially cleaved by acid hydrolytic scission of the glycosidic bonds to obtain chain fragments with a reactive end group, from which a cationic polymerization of THF was started with AgSbF6 as a catalyst and finally terminated by addition of KCN in methanolic KOH. Solution properties of these copolymers were governed by an incompatibility of the two kinds of blocks.

28


Coupling a synthetic prepolymer with suitable end groups to those of a cellulosic sequence protected at the C-2, C-3 and C-6 position using the highly reactive isocyanate group has been successfully employed. As an example, the combination of low DP cellulose triacetate with polypropylene glycol via an end group reaction with toluene diisocyanate in the presence of stannous octanoate as a catalyst (Amick et al., 1980) shall be cited. Just as in other routes of block copolymer synthesis, starting from cellulose triacetate sequences, the protecting groups can be subsequently removed by an appropriate saponification procedure. As rather special applications of this principle in cellulosic copolymer synthesis, the reaction of cellulose triacetate oligomers with diisocyanates to cellulose triacetate chains with urethane links at regular distances and the formation of some kind of alternating copolymer with urea and urethane linkages obtained by reacting glucosamine with a suitable diisocyanate may be mentioned. Examples of recombining the two wood components, cellulose and lignin, by simultaneous block and graft copolymerization were recently given (de Oliveira and Glasser, 1994; Demaret and Glasser, 1989). Segments of cellulose triacetate and cellulose tripropionate in the DP range between 5 and 60 after end group functionalization by isocyanate groups were reacted with hydroxypropyl lignin. A strong dependency of the shape of the macromolecules in solution, as well as of the morphology of the copolymers, on the length of the cellulosic segments was observed. As grafting, block copolymerization of cellulosics represents a route to a thorough modification of the material properties of the polymer, and several areas of application have been proposed, for example enhancement of the biodegradability of synthetic polymers (Kim et al., 1976), but so far none of these has been realized on an industrial scale.

References Amick, R., Gilbert, R.D., Stannett, V.T., Polymer 1980, 27, 648-650. Amikan, D., Benziman, M., /. Bacteriol 1989, 777, 6649-6655. Arthur, J.C., /. Macromol Sci.-Chem. 1976, AlO, 653-670. Benerito, R.R., Webre, B.C., McKelvey, J.B., Textile Res. J. 1961, 37, 757. Berlin, A.A., Kislenko, V.N., Prog. Polym. ScL 1992, 77, 765-825. Blanton, R.L., Northcote, D.H., Planta 1990, 7SO, 324-332. Bojanic, V., /. AppL Polym. ScL 1996, 60, 1719-1725. Brown Jr., R.M.,/. Macromol. ScL, Pure Appl. Chem. 1996, A33, 1345-1373. Colvin, J.R., in Encyclopedia of Polymer Science and Engineering, Vol. 3, Mark, H.F., Bikales, N.M., Overberger, G.G., Menges, G., Kroschwitz, JJ. (Eds.), New York: John Wiley & Sons, 1985, pp. 60-68.

References

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Cowan, S.L., Hurwitz, M.D., Ind. Eng. Chem. Prod. Res. Dev. 1982, 27, 629632. Dautzenberg, H., Fanter, C., Fink, H.-P., Philipp, B., Cellul Chem. Technol. 1980,14, 633-653. de Oliveira, W., Glasser, W.G., Polymer 1994, 35, 1977-1985. Delmer, D.P., Amor, Y., Plant Cell 1995, 7, 987-1000. Demaret, V., Glasser, W.G., Polymer 1989, 30, 570-575. Duntsch, L., Petzold, G., Rätzsch, M., Heger, A., Jacobasch, H.-J., Petr, A., Patent DD 269 561, 1989; Chem. Abstr. 1990, 772, 38829. Eschalier, X., British Patent 1906, 25, 647. Eschalier, X., /. Soc. Chem. Ind. 1907, 26, 821. Esposito, F., DeNobile, M.A., Mensitieri, G., Nicolais, L., /. Appl. Polym. Sei. 1996, 60, 2403-2407. Fanta, G.F., Burr, R.C., Doane, W.M., 7. Appl. Polym. Sei. 1987, 33, 899-906. Fanter, C., Ph.D. Thesis, Academy of Science (GDR) 1980. Feger, C., Cantow, HJ., Polym. Bull. 1980, 3, 407-413. Feger, C., Cantow, HJ., Polym. Bull. 1982, 6, 321-326 and 583-588. Franzier, Ch.E., Wendler, St.L., Glasser, W.G., Carbohydr. Polym. 1996, 3l, 11-18. Gailord, N.G., /. Macromol. Sci-Chem. 1976, A 10, 737-757. Gruber, E., Granzow, C., Papier (Darmstadt) 1996, 50, 293-299. Helbreich, A., Guthrie, J.T., in The Chemistry and Technology of Cellulosic Copolymer, Berlin: Springer Verlag, 1981. Heger, A., in Technologie der Strahlenchemie von Polymeren, Berlin: Akademie Verlag, 1990. Heinze, Th., Klemm, D., Loth, F., Philipp, B., Acta Polym. 1990, 41, 259-269. Hornof, V., Danesault, C., Kokta, B.V., Valade, J.L., /. Appl Polym. Sei. 1977, 27, 2991-3002. John, G., Pillai, C.K.S., Polym. Bull. 1989, 22, 89-94. Kim, S., Stannett, V.T., Gilbert, R.D., /. Macromol Sci.-Chem. 1976, AlO, 671-679. Klemm, D., Schnabelrauch, M., Geschwend, G., Wiss. Zeitschr. FriedrichSchiller-Univ. Jena, Naturwiss. R. 1985, 34, 813-820. Kobayashi, S., Kashiwa, K., Shimada, J., Kawasaki, T., Shoda, S., Makromol. Chem., Macromol Symp. 1992, 54/55, 509-518. Kobayashi, S., Shoda, S., Uyama, H., Adv. Polym. Sei. 1995, 727, 1-30. Kobayashi, S., Okamoto, E., Wen, X., Shoda, S., J. Macromol Sei., Pure Appl. Chem. 1996, A33, 1375-1384. Kokot, S., Komatsu, K., Meyer, U., Zollinger, H., Textile Res. J. 1975, 45, 673681. Kotchetkov, Tetrahedron 1987, 43, 2389-2436.

30


Krässig, H., Sven. Papperstidn. 1971, 74, 417-428. Kriss, E., Bukhtiyarov, V.K., Kryukov, A.J., Tkachenko, Z.A., Shrets, D.J., Teor. Prikl Khim. beta-Diketonatov. Met. 1985, 101-110. Kudlicka, K., Lee, J.H., Brown Jr., R.M., Am. J. Bot. 1996, 83, 274-284. Lee, J.H., Brown Jr., R.M., Kuga, S., Shoda, S.-L, Kobayashi, S., Proc. Natl Acad. ScL U.S.A. 1994, 91, 7425-7429. Lin, F.C., Brown Jr., R.M., in Cellulose and Wood-Chemistry and Technology, Schnerch, C. (Ed.), New York: John Wiley & Sons, 1989, pp. 473-492. Loth, F., Philipp, B., Makromol. Chem., Macromol. Symp. 1989, 30, 273-287. Luby, P., Kuniak, L., Fanter, C., Makromol Chem. 1979,180, 2379-2386. Meyer, U., Müller, K., Zollinger, H., Text. Res. J. 1976, 46, 756-762. Mezger, T., Cantow, H.-J., Makromol. Chem. 1983,110, 13-27. Micheel, F., Broode, O.-E., Liebigs Ann. Chem. 1974, 702. Micheel, F., Broode, O.-E., Liebigs Ann. Chem. 1975, 1107. Micheel, F., Broode, O.-E., Reinking, K., Liebigs Ann. Chem. 1974, 124. Mukherjee, A.K., Sayal, S., Siddhartha, S., Cellul Chem. Technol. 1983, 178, 141-153. Nakatsubo, F., Kamitakahara, H., Hori, M., /. Am. Chem. Soc. 1996, 118, 1677-1681. Ogawa, R., Tokura, S., Carbohydr. Polym. 1992a, 19, 171-178. Ogawa, R., Tokura, S., Int. J. Biol. Macromol. 1992b, 14, 343-347. Petersen, H., Petri, N., Melliand Textilber. 1985, 66, 217-222; 285-295; 363369. Petersen, H., Colour. Annu. 1990, 61-65. Philipp, B., Dan, D.C., Jacopian, V., Heger, Α., Acta Polym. 1982, 33, 542-545. Plotnikov, O.V., Lesins, A., Khim. Drev. 1981,1, 111-112. Ränby, B., Int. Symp. Wood Pulping Chem., Ekman.Days, 1981, Stockholm: SPCI, 1981, Vol. 4, 111-117. Rätzsch, M., Dunsch, L., Petzold, G., Petr, A., Heger, Α., Acta Polym. 1990, 41, 620-627. Rogowin, Z.A., /. Polym. ScL, Part C 1972, 37, 221-237. Rogowin, Z.A., Tappi 1974, 57, 65-68. Rogowin, Z. A., Galbraich, L. S., in Die chemische Behandlung und Modifizierung der Zellulose, Stuttgart: Thieme, 1983. Sakamoto, M., Takeda, J., Yamada, Y., Tonami, H., J. Polym. Sei. Part A-I 1970, 8, 2139-2149. Sangsari, F.H., Chastrette, F., Chastrette, M., Blanc, A., Mattioda, G., Reel. Trav. Chim. Pays-Bas 1990, 709, 419-424. Schlubach, H.M., Luhrs, L., Liebigs Ann. 1941, 547, 73.

References

31

Stannett, V.T., Hopfenberg, H.B., in Cellulose and Cellulose Derivatives, Bikales, N.N.M., Segal, L. (Eds.), New York: John Wiley & Sons, 1971, Part V, pp. 907-936. Stannett, V.T., in Cellulose and Its Derivatives, Kennedy, J.F. (Ed.), Chichester, UK: Ellis Horwood, 1985, pp. 387-399. Tarchevsky, J.A., Marchenko, G.N., Cellulose, Biosynthesis and Structure, Berlin: Springer Verlag, 1991. Uryu, T., Kitano, K., Ito, K., Yamanouchi, J., Matsuzaki, K., Macromolecules 1981,74, 1. Uryu, T., Yamanouchi, J., Kato, T., Higuchi, S., Matsuzaki, K., /. Am. Chem. Soc. 1983, 6865. Uryu, T., Yamaguchi, C., Morikawa, K., Terui, K., Kanai, T., Matsuzaki, K., Macromolecules 1985, 18, 599. Ushakov, S.N., Fiz.-Mat. Nauk (USSR) 1943, 7, 35. Yang, C.Q., Wang, X.L., /. Polym. ScL, Part A - Polym. Chem. 1996, 34, 1573-1580. Young, R.A., J.Agric. Food Chem. 1977, 25, 138. Young, R.A., in Absorbency, Chaterjce, P.K. (Ed.), Amsterdam: Eisevier Sei. Publ, 1985, pp. 217. Zara, L., Erdelyi, J., Hell, Z., Borbely, E., Rusznäk, L, Tappi J. 1995, 78, 131134. Zeronian, S.H., Bertoniere, N.R., Alger, K.W., Xie, Q., /. Text. lust. 1990, 87, 310-318. Zhang, Z.B., McCormick, C.L., J. Appl. Polym. ScL 1997, 66, 307-317.

4.2 Interaction of Cellulose with Basic Compounds This chapter will be centered on various classes of 'addition compounds' of cellulose, i.e. compounds formed without covalent derivatization of the macromolecule but nevertheless representing chemical entities by themselves, with chemical and physical properties differing often decisively from that of unmodified cellulose. Quite predominantly, processes of interaction of solid cellulose are the topic of this text. Thus the interdependency between the chemical interaction and the supramolecular and morphological structure of the cellulose sample plays a decisive role. After considering briefly the so-called alkali cellulosates this subchapter will be structured according to the reagent employed in preparing the various addition compounds with cellulose, i.e. aqueous and alcoholic solutions of alkali and tetraalkylammonium hydroxides, guanidinium hydroxide, hydrazine, ammonia and aliphatic amines.

References

31

Stannett, V.T., Hopfenberg, H.B., in Cellulose and Cellulose Derivatives, Bikales, N.N.M., Segal, L. (Eds.), New York: John Wiley & Sons, 1971, Part V, pp. 907-936. Stannett, V.T., in Cellulose and Its Derivatives, Kennedy, J.F. (Ed.), Chichester, UK: Ellis Horwood, 1985, pp. 387-399. Tarchevsky, J.A., Marchenko, G.N., Cellulose, Biosynthesis and Structure, Berlin: Springer Verlag, 1991. Uryu, T., Kitano, K., Ito, K., Yamanouchi, J., Matsuzaki, K., Macromolecules 1981,74, 1. Uryu, T., Yamanouchi, J., Kato, T., Higuchi, S., Matsuzaki, K., /. Am. Chem. Soc. 1983, 6865. Uryu, T., Yamaguchi, C., Morikawa, K., Terui, K., Kanai, T., Matsuzaki, K., Macromolecules 1985, 18, 599. Ushakov, S.N., Fiz.-Mat. Nauk (USSR) 1943, 7, 35. Yang, C.Q., Wang, X.L., /. Polym. ScL, Part A - Polym. Chem. 1996, 34, 1573-1580. Young, R.A., J.Agric. Food Chem. 1977, 25, 138. Young, R.A., in Absorbency, Chaterjce, P.K. (Ed.), Amsterdam: Eisevier Sei. Publ, 1985, pp. 217. Zara, L., Erdelyi, J., Hell, Z., Borbely, E., Rusznäk, L, Tappi J. 1995, 78, 131134. Zeronian, S.H., Bertoniere, N.R., Alger, K.W., Xie, Q., /. Text. Inst. 1990, 81, 310-318. Zhang, Z.B., McCormick, C.L., J. Appl. Polym. ScL 1997, 66, 307-317.

4.2 Interaction of Cellulose with Basic Compounds This chapter will be centered on various classes of 'addition compounds' of cellulose, i.e. compounds formed without covalent derivatization of the macromolecule but nevertheless representing chemical entities by themselves, with chemical and physical properties differing often decisively from that of unmodified cellulose. Quite predominantly, processes of interaction of solid cellulose are the topic of this text. Thus the interdependency between the chemical interaction and the supramolecular and morphological structure of the cellulose sample plays a decisive role. After considering briefly the so-called alkali cellulosates this subchapter will be structured according to the reagent employed in preparing the various addition compounds with cellulose, i.e. aqueous and alcoholic solutions of alkali and tetraalkylammonium hydroxides, guanidinium hydroxide, hydrazine, ammonia and aliphatic amines.

Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

32

4.2 Interaction of Cellulose with Basic Compounds

4.2.1 Preparation and properties of alkali cellulosates Alkali cellulosates as analogues of alkali alcoholates (alkali alkoxides) can be prepared by reacting the polymer with the alkali metals Li, Na or K in liquid ammonia, as first shown by Scherer (Scherer and Hussey, 1931) for Kcellulosate. According to Schmid and Becker (1925), Schmid et al. (1928) and Muskat (1934) the reaction proceeds at -35 to -50 0C within some hours, with the evolution of hydrogen probably via the formation of alkali amide as the reactive intermediate, and can be considerably accelerated by addition of sodium chloride. With sodium metal, a trisubstituted cellulosate was obtained, while with potassium or lithium only a DS below 3 was reached and calcium proved to be unsatisfactory as a reagent. Bredereck (Bredereck and Vlachopoulos, 198Oa) prepared a lithium cellulosate of DS = 3 by reacting an ammonia cellulose obtained from cotton - with lithium in liquid NH3. A fast cellulosate formation in the disordered regions of the ammonia cellulose was observed with all three alkali metals, potassium, sodium and lithium, but a subsequent rather fast firstorder reaction within the lattice layers of the addition compound was observed with lithium only. Sodium reacted much more slowly and potassium did not penetrate the lattice at all. The reactivity of alkali alkoxides obviously is insufficient to convert cellulose into cellulosates, while with thallium alkoxide in diethyl ether or benzene a partial introduction of cellulosate groups (DS < 3) could be achieved (Harris and Purves, 1940). On the other hand, the route to cellulosates via the corresponding alkoxides proved to be successful with tetraalkylammonium compounds: by reaction of a suspension of native cellulose I with the methoxides of the tetramethylammonium and the benzyltrimethylammonium cation in anhydrous MeOH or DMSO, the corresponding cellulosates with a DS of up to 0.7 have been prepared. The DS increased with the concentration of the methoxide and decreased with the molar volume of the tetraalkylammonium cation under given reaction conditions (Bredereck and Thi Bach Phnong Dau, 198Ob). As to be expected, all the cellulosates so far prepared exhibit a very high reactivity and can be converted to cellulose esters by reaction with acid anhydrides or acid chlorides or to cellulose ethers with alkyl halides. Xanthation with CS2 (see chapter 4.4) proceeds rapidly in the presence of a small amount of water (Scherer and Gotsch, 1939). According to Bredereck and Thi Bach Phnong Dau (198Ob) the reactivity of various cellulosates with a DS of 0.4 in a subsequent methylation increases in the order of the cations Li+ < Na+ < Me4N+ < Me3BnN+. All cellulosates are highly basic and rapidly decomposed by water or by CO2 from the air, and they can be kept for some time only with strict exclusion of moisture.

4.2.2 Aqueous and alcoholic solutions of alkali hydroxides

4.2.2

Interaction of cellulose with aqueous and alcoholic solutions of alkali hydroxides

4.2.2.1

General comments

33

Since John Mercer observed about 150 years ago the swelling of cotton fibers in aqueous sodium hydroxide and the changes in physical fiber properties after removal of the lye, the interaction between cellulose and aqueous alkali hydroxides, especially NaOH, has been one of the principal research topics in the chemistry and physics of cellulose, leading to decisive progress in understanding cellulose structure and reactivity and resulting in large-scale technical processes. The interaction between cellulose fibers and aqueous alkali hydroxides is characterized by an uptake of alkali hydroxide and water onto the fiber resulting in a decrease of lye concentration in the surrounding medium, by a strong lateral swelling of the fiber and by a change in X-ray lattice dimensions in the ordered regions above a specific lye concentration. The binding state of the alkali hydroxide onto the cellulose can still not be exactly defined: obviously some anionization of the hydroxy groups occurs without a true 'cellulosate' (alcoholate) being formed. In the interaction of cellulose with aqueous alkali hydroxide solutions the hydration shell of the alkali hydroxide ion dipoles and its change with lye concentration plays a dominant part, regarding alkali hydroxide and water uptake by the fiber. This course of alkali uptake with lye concentration depends strongly on the supramolecular structure of the sample, resembling a typical heterogeneous course of reaction with highly ordered cellulose fibers. On removal of the alkali hydroxide by washing or by neutralization, cellulose in the lattice modification of cellulose II is regenerated from all the alkali celluloses formed, with a degree of order usually lower than that of the starting material. Among the reaction products of cellulose with various aqueous alkali hydroxides, only the so-called sodium cellulose is of practical relevance as an intermediate of limited stability: it decomposes rather rapidly by sorption of CO2 from the air, and it is depolymerized by the oxidative action of air oxygen (see chapter 2.3). Two routes of application of this reactive intermediate are realized today in large-scale processes, i.e. (i) the transformation of native cellulose I to mercerized cellulose (cellulose II) with changed textile properties via sodium cellulose. (ii) the transformation of native cellulose I into sodium cellulose as the starting material for subsequent large-scale esterification or etherification of cellulose, especially xanthation and carboxymethylation.

34

4.2 Interaction of Cellulose "with Basic Compounds

4.2.2.2

Swelling and dissolution of cellulose in alkali hydroxide solutions

The most striking phenomenon in cellulose-alkali hydroxide interaction is the strong and fast lateral swelling of cellulose fibers in aqueous alkali hydroxide solutions. If performed without tension this lateral swelling is connected with a decrease in fiber length, and in any case the tensile strength of the fiber significantly decreases. The swelling takes place on a time scale of seconds to a few minutes and obviously is diffusion-controlled. As already discussed in the chapter 2.2, the swelling power of the lye passes a maximum in dependency on lye concentration, which is shifted to higher alkali hydroxide concentration with increasing atomic weight of the alkali cation, but corresponds in all cases to about the same molar alkali hydroxide concentration (Heuser and Bartunek, 1925). From LiOH to CsOH the steepness and absolute height of the maximum decrease in correspondence to a decreasing hydration shell of the alkali cation. Comprehensive work on swelling of cellulose in aqueous sodium hydroxide has shown that the increase in fiber diameter not only depends on lye concentration but also on the physical structure of the sample. A lowering of the steeping temperature generally results in a higher degree of swelling and favors the dissolution of low DP cellulose from the accessible parts of the sample (see chapter 2.2). The increase in solubility by lowering the temperature due to an exothermic heat of cellulose dissolution in aqueous NaOH has been investigated thoroughly in recent years (e.g. Yamashiki et al., 1990; Lang and Laskowski, 1991). Optimal results were obtained within 9-10 % NaOH at a temperature of about 10 0C. Rather clear solutions with a cellulose content up to 5 % could be obtained from degraded cellulose samples with a DP up to 200, while at higher DP a partial solubility only was observed (compare Fig. 4.2.1). The mode of degradation is obviously of minor influence here (Fig. 4.2.1). 100 r

__,80

^6O

I 40 ^ ω 20 200

400

DP

600

800

Figure 4.2.1. Solubility of degraded spruce sulfite pulp samples in 10% aqueous NaOH at -10 0C in dependence on DP. Mode of degradation: · thermal treatment; O acid hydrolysis; · electron beam irradiation; Δ irradiation and thermal treatment (Lang and Laskowski, 1991).


35

A problem impeding technical application for film spinning arises from the instability of these solutions, which form coherent gels on standing. According to Lang et al. (1989) these gels can be redissolved by a suitable transient elevation of temperature. A cyclic cooling and heating procedure was found to be most suitable to obtain fiber-free solutions. Completeness of dissolution as well as the stability of these solutions can be enhanced by addition of zinc oxide and/or urea. The phenomena observed are interpreted by Lang and Laskowski (1991) as being due to an interaction of NaOH with the cellulose via an incorporation of cellulosic hydroxy groups into the solvation shell of the NaOH solvates, which despite their high hydration number are stabilized at the low temperature and thus provide a spacer action to separate the cellulose chains (see also chapter 4.3). A 1H- and 13C NMR study of degraded cellulose (DP-15) dissolved in NaOD/U2O was centred on hydroxy group dissociation in dependence on NaOD concentration (4-30%). The hydroxy group at C-3 proved to be the most resistant one to dissociation. According to this study, cellulose macromolecules dissolved in NaOH behave different from those in a highly swollen state (Isogai, 1997).

4.2.2.3

Chemical processes of interaction between cellulose and alkali hydroxide solutions

As observed already at the beginning of this century (Heuser and Bartunek, 1925), all the alkali hydroxides from LiOH to CsOH are strongly chemisorbed from their aqueous solution onto cellulose, with a stepwise sorption isotherm being found with highly ordered cotton cellulose. The plateaus of these isotherms indicate a constant molar ratio of alkali sorbed per AGU over a rather wide range of lye concentration. Subsequent studies of alkali sorption were predominantly concerned with aqueous NaOH and in some cases also with KOH (Mori, 1991). Employing a more sophisticated technique (Schwarzkopf, 1932) with an inert salt of negligible sorption tendency added to the lye, the step isotherm was confirmed for NaOH and KOH. As demonstrated in Fig. 4.2.2 for the NaOH sorption from aqueous lye by spruce sulfite pulp, the alder values of the so-called apparent alkali uptake are misleading in so far as they neglect the simultaneous uptake of water by the cellulose moiety, which is adequately considered, however, by determining the so-called true alkali uptake. The plateau of true alkali uptake appearing between 15 and 20 % NaOH by weight corresponds to a NaOH sorption of 1 mol of NaOH/mol of AGU, i.e. a one-to-one addition compound, and shows a further increase above this concentration. The water sorption was found to pass a pronounced maximum corresponding to a water uptake of 4-5 mol/mol of AGU. The uptake of alkali and water proceeds very rapidly on about the same time scale as the lateral fiber swelling, and is practically «complete after 10-20 min, with the initial rate showing a

36


maximum at a lye concentration of 15-20 %. Obviously this process of alkali sorption is also diffusion-controlled.

I 01.0

5< ο

ε

"δ ·—Ό.5 χ ο

ι| 10

20 NaOH [Wt %]

30

Figure 4.2.2. Equilibrium values of NaOH uptake at room temperature (·, true uptake after pulp redrying at 20 0C; O, at 105 0C; D, apparent uptake) and water uptake (·) (Philipp, 1955). In order to understand the course of alkali uptake with lye concentration and the mechanism of cellulose-alkali hydroxide interaction, the structure of aqueous alkali hydroxide solutions as well as the physical structure of the polymer must be included in the consideration. The present structural concept for aqueous alkali hydroxide solutions is based on the assumption of a hydrogen-bonded water structure with some monomolecular H2O besides the water clusters, and a disturbance of this water structure by dissolved ions tightly associated with water molecules in their Α-shell of hydration and more loosely associated with water molecules of the B-shell. In the series of alkali hydroxides, the hydration shell of the cation decreases drastically with increasing atomic weight, i.e. from 120 mol of H2OTLi+ to 13 mol of H2OTCs+ (Dobbins, 1973); Li+ and Na+ are usually classified as structureforming ions, while K+, Rb+ and Cs+ are assumed to be structure-breaking ones. For the isolated OH~ ion, a stable hydration shell with three water molecules is described (Hinton and Amis, 1967; Eigen, 1963). At higher lye concentration, an insertion of the OH~ ion into the hydration shell of the cation is assumed, resulting in a hydrated ion dipole. In dependence on lye concentration, a rather large number of defined hydration states has been postulated for sodium hydroxide, while a much smaller one is assumed for KOH. Experimental evidence on several defined hydration states for NaOH has been obtained from measurements of the line width of the 23Na NMR signal (Kunze et al., 1985; Fig. 4.2.3). The tendency of association to an ion dipole corresponding to a decrease in degree of dissociation of the alkali hydroxide in dilute solution, increases in the order KOH < NaOH < LiOH.


10 20 30 NaOH concentration [wt%]

37

40

Figure 4.2.3. Viscosity reduced line width of the 23Na NMR signal of aqueous NaOH solutions at different temperatures (· 268 K; O 303 K; D 323 K) (Kunze et aL, 1985). From the site of the polymer, the following reasoning is based on a two-phase concept for cellulose with ordered and disordered regions (see chapter 2.1), with this supramolecular order being stabilized by intra- and intermolecular hydrogen bonds. On interaction with water, a first layer of H2O molecules is associated very tightly with cellulosic hydroxy groups in the disordered regions while further sorption occurs more loosely, comparable to the A- and B-shell of hydration in the case of ions. In connection with water, cellulose is considered to be structure-breaking. The interaction between cellulose and aqueous alkali hydroxides resulting in swelling and specific uptake of alkali and water must be considered as a very complex process comprising destruction of hydrogen bonds within the cellulose moiety as well as within the aqueous lye phase, as a decrease in supramolecular order of the polymer, changes in the structure of hydration shells as well as in the chain conformation of cellulose, and finally as a partial anionization of cellulosic hydroxy groups. Despite a host of experimental evidence obtained mainly with cotton cellulose and represented here by a few examples only, a final separate evaluation of all these factors with regard to their relevance for the whole process is not yet possible. Nevertheless, some considerations on the mechanism of cellulose-alkali interaction depending on lye concentration and type of alkali cations shall be subsequently presented, while for further experimental data the reader is referred to the comprehensive reviews of Warwicker et al. (1966), Zeronian and Cabradilla (1973), and to other publications (Philipp et al., 1983 and 1985). The following context is centered on interaction with NaOH but deals also with a comparison of NaOH and KOH and with the effect of substituting an aqueous medium by an alcoholic one. According to present concepts, free monomolecular water penetrates first into the cellulose structure, destroying intermolecular hydrogen bonds in the less

38


ordered regions. So-called s welling-active NaOH ion dipoles (Heuser and Bartunek, 1925) and/or hydroxy anions are assumed to promote the interaction in the ordered regions above an NaOH concentration of about 9 %, being partially or totally depleted of their hydration shell in this process and thus providing a further amount of monomolecular water (Bartunek, 1956). Usually the hydroxy anions are seen to be responsible for the primary interaction with the cellulosic hydroxy groups in the ordered regions of the structure, while the hydrated cation is seen to be responsible for the resulting swelling. Progressively, the original stabilization of the cellulose structure by inter- and intramolecular hydrogen bonds is thus substituted by a stabilization via addition complexes between cellulosic hydroxy groups, NaOH ion dipoles and water molecules, with cellulosic hydroxy groups being included in the hydration shell of the ion dipoles, and water molecules being released from this shell. At a lye concentration between 35 and 40 %, a stable tetra-solvate with two H2O molecules and two hydroxy groups for example, has been concluded from the experimental evidence available. On the molecular level no binding of NaOH onto the cellulose chains was detected up to a lye concentration of about 9 %, while rather dramatic changes take place in the concentration range between 9 and 15 %, characterized by the specific uptake of NaOH and water in the disordered as well as in the ordered regions, changes in chain conformation, with a preference for twisted conformations at the glycosidic bond between C-I and C-4, and a change in lattice dimensions of the ordered regions (see next section). At about 15 % NaOH, the transformation to sodium cellulose I is completed, resulting in a still rather highly ordered structure despite some loss of X-ray crystallinity in this conversion process. A rather uniform chain conformation and an overall chemical composition of 1 mol of NaOH and 4 to 5 mol of H2O/mol of AGU remains nearly constant up to a lye concentration of about 22 %. A site-preferential interaction of NaOH with the hydroxy groups at C-2 and C-3 is assumed (Fink et al., 1995). Still open remains the question of anionization of cellulosic hydroxy groups: obviously a state of binding in between an addition compound with completely intact cellulosic hydroxy groups and an anionization to an alcoholate anion has to be considered. From 23Na NMR line width measurements, after stepwise depletion of alkali cellulose samples from adhering lye by pressing, three rather well-defined states of NaOH binding can be concluded, i.e. a delocalized binding in the disordered regions, a localized binding in the disordered regions and a localized binding in the crystalline regions, with a rapid exchange obviously taking place between the tightly bound and the loosely bound Na+ ions (Kunze, 1983). At an NaOH concentration of about 25 %, with the lye being already depleted of free water, a further significant change in cellulose-alkali structure becomes visible by NMR and WAXS measurements: a still tighter interaction between Na+ and O-atoms at C-2 and C-3 takes place, the overall chain conformation being changed from a two-fold to a three-fold screw axis, with the lattice spacing re-


39

sembling that of a sodium cellulose II, and also a conformational change being observed for the primary CH2OH group. These changes are mainly coursed by additional breakage of hydrogen bonds. At still higher NaOH concentration (up to 50 %) a further decrease in supramolecular order takes place in connection with a rather wide spread of conformational states of the primary CH2OH group, which probably now also interacts more intensely with the alkali. In summary, the interaction between cellulose and aqueous NaOH up to a lye concentration of 50 % can be considered to be based on spatial and conformational changes of the polymer chains by destruction of the original hydrogen-bond pattern in connection with concentration-dependent specific interaction between cellulosic hydroxy groups and hydrated NaOH ion dipoles. NaOH uptake in the first part of the sorption isotherm (up to 15 % NaOH) in the lye is obviously governed by an incomplete accessibility of the cellulose structure to alkali cellulose formation, while the further NaOH uptake above a lye concentration of 20 % is probably connected with changes in hydration of the NaOH and further insertion of hydroxy groups into the hydration shell of the ion dipoles. The effect of temperature on cellulose interaction with aqueous NaOH as a diffusion-controlled reaction is rather small. A lowering of the temperature from the range 20-40 0C, employed for sodium cellulose formation in the viscose process, down to about O0C, results in a somewhat stronger binding of Na+ onto the polymer, as revealed by the shape of the 23Na NMR signal, besides a higher swelling due to stabilization of the NaOH hydration shell. Comparing the action of aqueous NaOH, and aqueous KOH on the other, onto cellulose, two points of difference have to be emphasized besides many similarities: KOH penetrates into the ordered regions of cellulose at a somewhat lower molar concentration than NaOH, and KOH uptake is higher than that of NaOH up to a lye concentration of about 4.4 N, while above that concentration NaOH uptake exceeds that of KOH. Probably the different behavior of KOH is caused by its somewhat higher basic strength and its lower tendency to ion dipole formation, resulting in a stronger partial anionization of cellulosic hydroxy groups, in agreement with the stronger ionic character of potassium alcoholate as compared with sodium alcoholate. It seems worth mentioning that the reactivity of potassium cellulose in a subsequent cyan ethylation exceeds that of sodium cellulose. The second point of difference between the action of KOH and of NaOH is connected with the lower hydration of KOH and its lack of swellingactive hydrates, resulting in swelling values only half as high as those obtained with NaOH and also resulting in an incomplete conversion of the ordered regions of the cellulose moiety into potassium cellulose according to Mori (1991). The high relevance of solvation in the interaction between cellulose and alkali hydroxides becomes clearly visible also by comparing aqueous and ethanolic NaOH, as in the latter case the interaction proceeds much more slowly and with much less swelling of the fibers (Philipp et al., 1987a), and the changes in eel-

40


lulose physical structure differ significantly from those observed with aqueous lye (see next section). The alkalization effect obtained with NaOH dissolved in a mixture of water and isopropanol resembles that observed with an aqueous lye of much higher concentration, obviously due to formation of a cellulose/NaOH/ water phase with a high alkali concentration at the expense of alkali and water content of the surrounding alcoholic phase. Furthermore, some competition between alcohol molecules and NaOH ion dipoles for H2O molecules can be assumed, resulting in a decrease of the NaOH hydration shell and in consequence in a mode of cellulose/NaOH interaction observed with aqueous lye of much higher concentration. 4.2.2.4 Role of cellulose physical structure in cellulose-alkali hydroxide interaction The complex interaction between cellulose and dissolved alkali hydroxides affects all the structural levels of the polymer and vice versa is influenced by changes in any of the structural levels. The subdivision employed here into 'chemical interactions' and 'role of physical structure' mainly serves the purpose of clearness without being necessarily the result of scientific reasoning. Changes in supramolecular structure on alkali treatment of cellulose have been predominantly investigated by WAXS, supplemented by solid state CPMAS 13C NMR spectroscopy and by IR spectroscopy, with the effect of NaOH concentration on degree of crystallinity, crystallite size and lattice dimensions of the ordered regions being the most frequent topic of research. With !inters as the starting material the lattice transition from cellulose I to that of Na-cellulose I, and after neutralization to cellulose II, begins at a lye concentration of about 10 % and is completed at about 14 % NaOH (see Fig. 4.2.4).

10 12 14 16 NaOH concentration [wt%]

Figure 4.2.4. Content of sodium cellulose and cellulose II, dependent on the aqueous NaOH concentration.


41

The difference observed between the percentage of sodium cellulose I formed at a given lye concentration and the amount of cellulose II obtained after neutralization indicates a partial reversibility of sodium cellulose formation, i.e. its partial retransformation to cellulose I on neutralization (Philipp et al., 1985; Hayashi, 1976). This partial reversibility is obviously caused by an incomplete transformational change of a part of the macromolecules and led to the assumption of two different sodium cellulose I modifications, i.e. Na-cellulose I1 with a bent 4Ci conformation retransformable to cellulose I, and Na-cellulose I2 with a bent and twisted 4Ci conformation yielding cellulose II on neutralization. At a sufficiently high lye concentration, all the cellulose chains have taken the bent and twisted conformation of Na-cellulose I2, and a 100 % yield of cellulose II is observed on neutralization. A detailed analysis of the X-ray patterns in dependence on lye concentration revealed a preferential conversion of the smaller and/or less well-ordered crystallites within the transition interval and definitely indicated only a moderate decrease in crystallinity on alkali treatment, with the highly swollen sodium cellulose still exhibiting a remarkable crystalline order. Substitution of NaOH by KOH proved to be of minor influence only on the lattice transition curve based on molar lye concentrations, with the beginning of the transition obviously starting at a somewhat lower molar concentration in the case of KOH. According to Zeronian and Cabradilla (1973), fiber swelling alone is not a sufficient prerequisite for lattice transformation, the start of which depends on the alkali cation, the reaction temperature and the medium, besides the lye concentration. At a lye concentration of 5 N, lattice conversion was found to be completed with LiOH, NaOH and KOH, while differences between these alkali hydroxides were observed at lower concentration. The WAXS results outlined here are corroborated by recent solid state CP-MAS 13C NMR data, indicating the beginning of conformational changes at a lye concentration of about 9 %, with the most significant changes in signal position and shape occurring at up to 15 % NaOH and indicating an increasing preference for twisted conformations (Fink et al., 1995). With NaOH of 15 % by weight, a complete lattice transformation to Nacellulose I2 is achieved at room temperature in a fast diffusion-controlled lattice layer reaction (so-called permodoid reaction), resulting in the complete accessibility of the hydroxy groups in the crystalline regions to consecutive reactions. But the conversion of the cellulose I lattice to sodium cellulose I by no means is the only one observed by WAXS, and already about 50 years ago Sobue et al. (1939) published a phase diagram of sodium cellulose modifications in dependence on steeping lye concentration and steeping temperature, together with the unit cell dimensions of the various phases (see Table 4.2.1 and Fig. 4.2.5). Although with the dependence on cellulose starting material and conditions of preparation of the alkali cellulose somewhat deviating WAXS data may be ob-

42


tained, the results of Sobue et al. (1939) can still be considered a valid basis for practical work. The lattice transition curve from cellulose I to cellulose II via sodium cellulose I (percentage of cellulose II versus lye concentration) depends significantly on the supramolecular structure of the starting material: cotton !inters require a higher lye concentration for this lattice conversion than wood pulp (see Fig. 4.2.6), and even between different spruce sulfite dissolving pulps, significant differences in the course of the curve have been reported by Philipp et al. (1959). Table 4.2.1. Unit cell dimensions of various Na-cellulose modifications (Sobue et al., 1939).

b(k) 13.2 10.00 9.17 9.98 13.95 7.84

Modification a(A) Na-cellulose I 25.6 Na-cellulose II 10.00 Na-cellulose III 22.20 Na-cellulose IV 10.03 Na-cellulose V 13.95 Cellulose I (for comparison) 8.23

C(A) 20.50 15.4 10.26 10.3 15.3 10.28

Tf 40° 60° 90° 52° 41°40' 84°

c = fiber axis. Temperature [0C] 100

10

20

30

NaOH - Concentration [Weight-%]

Figure 4.2.5. Phase diagram of the sodium cellulose compound depending on the NaOH concentration and temperature (Sobue et al., 1939, compare also Krässig, 1993).


43

100 -

50

9

11 NoOH [%]

13

Figure 4.2.6. Lattice transition curve from cellulose I to cellulose II of cotton !inters (·) and a spruce sulfite pulp (O) in dependence on steeping lye concentration at room temperature (Philipp et al., 1959). Activation of !inters cellulose with liquid NH3 prior to the alkali treatment results in a definite shift of the transition curve to lower alkali concentration (see Fig. 4.2.7), and a similar shift to lower alkali concentration is observed for the first step in the curve of alkali uptake versus lye concentration (Loth et al., 1984). According to Käufer (1984) the rate of steeping lye diffusion differs between ordered and disordered regions and depends on crystallite size, with the appropriate consequences on the kinetics of sodium cellulose formation.

r-,100 ~

3 6 9 12 NaOH concentration [%]

Figure 4.2.7. Lattice transition of cellulose I to cellulose II of a spruce sulfite pulp sample before (*) and after activation (Δ) with NH3 (Schleicher et al., 1973 and 1974). Corresponding to the changes on the supramolecular level so far considered, remarkable effects are also observed in the fibrillar morphology of cellulose samples on treatment with alkali hydroxides (see Fig. 4.2.8). Purz et al. (1995) compared in a recent morphological study the action of aqueous and ethanolic

44


Spruce sulfite pulp: (a) untreated; (b) 10 % NaOH; (c) 11 % NaOH; (d) 12 % NaOH.

Cotton !inters: (a) untreated; (b) 12 % NaOH; (c) 15 % NaOH; (d) 25 % NaOH.

Bacterial cellulose: (a) untreated; (b) 10 % NaOH; (c) 12 % NaOH; (d) 15 % NaOH.

Figure 4.2.8. Changes of the microfibril structure of cellulose treated with aqueous NaOH for l h at room temperature revealed by REM (Philipp and Purz, 1983; Purz et al., 1995).


45

NaOH on cotton !inters, spruce sulfite pulp and bacterial cellulose from Acetobacter xylinum. The microfibrillar structure of the wood pulp and the bacterial cellulose was found to be destroyed by aqueous NaOH above a concentration corresponding to an almost complete lattice transition, and after regeneration to cellulose II no fine fibrillar structure could be resolved in the electron microscopic image. The lye concentration required proved to be higher with the bacterial cellulose than with the sulfite pulp, in agreement with the higher lye concentration necessary for lattice transformation, due to the high crystallinity and larger crystallite dimensions of the bacterial cellulose. With !inters, on the other hand, a fine fibrillar structure prevailed throughout the whole process and could be definitely resolved after regeneration to cellulose II, obviously due to a higher fibrillar organization of !inters cellulose compared with wood cellulose. All these morphological changes occurred within 1 h, depending somewhat on the history of the sample, and by lowering the temperature of treatment from 20 0C to O0C the limiting concentration of NaOH required was shifted to somewhat lower values. The cellulose II recovered from alkali cellulose by washing and/or neutralization differs from the original cellulose I sample not only with regard to lattice dimensions but also with regard to degree of order, fibrillar morphology, and pore and void structure, as well as with regard to water vapor sorption and liquid water retention. The degree of crystallinity xc is generally somewhat diminished after conversion of high molecular cellulose I samples to cellulose II, but can be enhanced with low DP cellulose due to short-chain extraction from the amorphous regions by the alkali and due to excessive recrystallization on washing and drying, as observed with LODP !inters by Fink et al. (1992). The changes in fibrillar morphology already discussed find their counterpart in an altered pore and void structure: according to Fink et al. (1992) the total pore volume as well as the total inner pore surface was considerably enhanced by conversion of LODP !inters to cellulose II via sodium cellulose, while the average pore diameter was found in this SAXS study to be significantly diminished. As possible causes, a partial collapse of pores on deswelling as well as the formation of new small pores, in combination with an enlargement of already existing pores to a size outside the range of the SAXS method, have been discussed. Transformation to cellulose II via alkali cellulose generally results in a considerable reduction of the LODP after hydrolysis down to a limiting value of about 70 after thorough mercerization. This drop in LODP was observed by Zeronian and Cabradilla (1973) to increase under comparable conditions in the order of alkali hydroxides of LiOH < NaOH < KOH. Water regain (sorption of water at 65 % relative humidity) is increased by conversion to cellulose II to nearly twice the original value for high DP cotton !inters, in agreement with the changes in degree of order and pore structure, but this increase in regain obvi-

46


ously cannot be directly correlated with the previous swelling during cellulosealkali interaction. The increase in water retention value generally observed after interaction of cellulose with aqueous alkali and subsequent neutralization depends largely on steeping lye concentration and type of alkali employed (see Fig. 4.2.9), as well as on the physical structure of the original sample and the procedures of alkalization, neutralization and drying. The mechanical tension applied on the sample during alkali treatment also exerts an influence on the criteria considered here, as well as on the mechanical properties of the regenerated fibers (Warwicker et al., 1966). UO

120

|100

I 80 60

4

8 12 Να 0 H [vol %]

15

Figure 4.2.9. Change of the WRV of cotton !inters (DP = 890; means slope of the two regions of the curve) after treatment with aqueous NaOH and subsequent neutralization (Jayme and Roffael, 1970). 4.2.2.5

Concepts for understanding cellulose-alkali hydroxide interaction

The complex chemical and physical structural changes of cellulose on interaction with alkali hydroxides, and the interdependency of effects occurring at different structural levels, justify an overview of previous and present concepts and models for understanding these processes. The viewpoint of the organic chemist was represented by Z. A. Rogowin, who assumed a preferential anionization of the hydroxy group at C-2 due to its higher acidity and could explain the behavior of alkali celluloses in consecutive derivatization reactions, but neglected widely the role of supramolecular structure. The viewpoint of Neale (1929; 1930; 1931) and of Pennings (Pennings et al., 1961; Pennings and Prins, 1962), on the other hand, was determined by principles of colloid chemistry and membrane theory, assuming a Donnan equilibrium between an external phase of aqueous sodium hydroxide solution and an internal phase of the cellulose-alkali hydroxide water moiety and giving a plausible interpretation of cellulose fiber swelling on interaction with aqueous alkali. The


47

cellulose in the minor phase is considered here as a weak monobasic acid partially forming a sodium salt with NaOH according to the mass action law in a dynamic equilibrium

CeII-OH +NaOH- CeII-ONa +H 2 O To balance the nonequilibrium between inner and outer phases, water penetrates into the inner phase and swells the cellulose until a swelling pressure due to cohesive forces in the polymer structure is reached to compensate the osmotic forces. Sobue et al. (1939) founded their considerations mainly on WAXS results and emphasized the role of supramolecular structure in the alkalization process, arriving at the concept of a permotoid lattice layer reaction comprising amorphous as well as crystalline structural regions. This concept proved to be suitable for understanding the enhanced reactivity of alkali cellulose and the appearance of different WAXS phases varying in lattice dimensions and composition, but there remained some discrepancy between results on alkali uptake by chemical analysis and the X-ray data on crystalline phase composition. This discrepancy can be reconciled by the concept of the 'reactive structural fractions' (RSF concept) published by Fink et al. (Fink et al., 1986; Fig. 4.2.10), at least in the practically important region of up to 20 % aqueous NaOH. 100

80

Cell

^40

20 O

8 12 16 NaOH [wt%]

20

Figure 4.2.10. Reactive structural fractions (RSF) versus concentration of NaOH.

The concept is based on the two-phase model of the cellulose structure with crystalline and amorphous regions, and is centered on the statements backed by experimental evidence from sorption and WAXS studies that: (i) the X-ray crystalline fraction of sodium cellulose I has a constant composition of maximal 0.5 mol NaOH, to minimal 3.5 mol H2O/AGU, up to a lye concentration of about 20 %, while the water and alkali content of the amorphous fraction varies with the lye concentration and can reach a value of about 2 mol NaOH/AGU at a sufficiently high lye concentration;

48


(ii) integral sorption values of NaOH and H2O in the lye concentration range up to 15 %, i.e. in the range of lattice transition, should be replaced by a so-called specific uptake considering, besides the fully accessible amorphous regions, also part of the crystalline regions, which has already been transformed to the cellulose I lattice. Application of this concept permits a plausible interpretation of the swelling maximum of cellulose in aqueous lye and results in a good compatibility of sorption and WAXS data. Despite its qualitative and at that time rather hypothetical character, the socalled 'hydrate shell explosion' theory of Heuser and Bartunek (1925) opened up a new and very promising route to understanding cellulose-alkali hydroxide interaction, as it focused for the first time on the important role of NaOH hydration and of the so-called free water on swelling, alkali uptake and lattice transition of cellulose interacting with aqueous NaOH. A more recent concept consistent with ample experimental evidence and represented with slight variations by several groups' (Fink et al., 1995) is centered on the breaking of defined inter- and intramolecular hydrogen bonds within the solid state structure of cellulose by hydrated NaOH ion dipoles, resulting in conformational changes of the macromolecules with a preference for twisted conformations at higher lye concentration.

0(2)

Figure 4.2.11. Scheme of Na-cellulose I structure according to Fink et al. (1995).

Figure 4.2.11 shows the various possibilities of interaction, including the hydrogen bonds involved. At lower NaOH concentration, e.g. 18 % (resulting in Na-cellulose I formation), the interaction preferentially takes place at C-2 and C6, and not until arriving at a higher concentration of > 22 % NaOH does it occur at C-3. Due to cleavage of the C-3---O-5 hydrogen bond, also the two-fold screw-axis of the polymer backbone gets lost. By using this concept, accentuating the important role of defined alkali hydroxide hydrates on a more modern


49

level, the effects of lye concentration, of type of alkali as well as of the liquid medium (water or alcohol), can be understood. The concept implies a preferential cellulose NaOH interaction at the C-2 position, although other opinions (Fengel and Wegener, 1989) have been published too. As an open question remains the state of binding of NaOH at the different positions of the AGU, which cannot yet be exactly defined and probably is situated somewhere in between the borderline cases of an alcoholate and an addition compound stabilized by intermolecular forces only. 4.2.2.6

Survey of commercial processes based on cellulose-alkali hydroxide interaction

Mercerization of cotton originally consists of the transformation of the native cellulose I of cotton fabrics to cellulose II ('mercerized' cellulose) via the intermediate formation of sodium cellulose by the action of aqueous NaOH under mechanical tension. The process has the purpose of enhancing dyeability and gloss of the cotton fabric and can be conducted as a so-called 'cold mercerization' or as a 'hot mercerization'. In cold mercerization the fabric is drawn through aqueous NaOH of about 30 % concentration at a temperature of about 20 0C at a speed of 3040 m/min with a residence time of some minutes in the alkaline bath. The fabric is then washed free of alkali with water in a stepwise counter-current process still under tension, eventually with the addition of some acetic acid to neutralize the last traces of lye. In hot mercerization a temperature of 60 to 70 0C is employed at a significantly lower lye concentration of about 22-24 % NaOH, also under mechanical tension. Elimination of alkali by washing with water can be helped by addition of acetic acid in the last step here too, but this may complicate the recycling of the washing liquid. The know-how in both mercerization processes mainly consists of the optimal adaptation of mechanical tension and in the most economical use of water in the washing steps including recycling. Further development is proposed to increase the velocity of the moving fabric through the alkaline bath up to about 100 m/min in cold mercerization. A process analogous to mercerization, developed specifically for viscose rayon staple fabric in order to increase dyeability, is the treatment of this fabric with aqueous NaOH of about 6 % concentration at 70-80 0C also under mechanical tension with subsequent washing. These milder conditions take into consideration the much lower alkali resistance of the rayon staple in comparison with cotton. The increase in dyeability achieved by all three modes of the mercerization process can be traced back to the altered pore and void structure of the polymer regenerated after alkaline treatment. An alkali cellulose in the form of sodium cellulose I suitable for subsequent xanthation in the viscose process is generally obtained by the action of aqueous NaOH of about 18 % concentration at a temperature between 20 and 40 0C onto

50


a hard wood or soft wood dissolving pulp in the form of sheets, rolls or flocks. In an older mode of the process now barley practised, pulp sheets fixed between perforated iron plates were treated in a chest-like iron 'steeping press' with lye of appropriate concentration for about 1 h, then pressed to a press weight ratio of about 3.2:1 and then shredded to fibrous flakes suitable for subsequent xanthation after adequate oxidative depolymerization ('preripening'). A standard alkali cellulose from spruce sulfite pulp had a composition of 32-34 % cellulose, 15-17 % NaOH and about 50 % water, and contained less than 1 % Na2CO3 in the freshly prepared state. Today, generally a slurry steeping process is practised in the viscose plants, mainly to saving on man-power. The continuous slurry steeping process proceeds by mixing and beating the pulp with the lye usually at a temperature of about 40 0C for a maximum of l h and subsequent automated pressing to the press weight ratio required, followed by shredding and preripening. The capacity of today's slurry steeping reactors, made of stainless steel, is about 10m3. The enhancement of steeping temperature to about 40 0C in comparison with about 20 0C in the classical steeping press process has no significant bearing on the chemical reactions and structural changes in alkali cellulose formation, but mainly serves as a viscosity reduction of the lye for better handling. A so-called hot alkalization at about 100 0C has been proposed, especially for beach pulp, by Pavlov et al. (1983) in order to enhance pulp reactivity in xanthation and viscose quality for spinning, but to the authors knowledge this process is not practised in industry probably due to a high loss of polymer by degradation to soluble products and an unsatisfactory control of oxidative degradation before the scheduled preripening step. Alkali cellulose production in the viscose process is now often performed in the presence of a small amount of a nonionic or anionic surfactant, which does not significantly interfere with the course of alkali cellulose I formation (Schleicher et al., 1967), but promotes a smooth xanthation and a good filterability of the viscose solution. Alkali celluloses for subsequent manufacture of cellulose alkyl ethers or carboxymethylcellulose are in principle prepared also by a slurry steeping process, now centered in its further development on a drastic reduction of liquid-to-solid ratio in the steeping reactor for ecological reasons. In contrast with alkali cellulose production in the viscose process, the steeping is performed here with a significantly higher NaOH concentration of between 30 and 40 % NaOH depending on type of cellulose ether and procedure of etherification, and resulting in an alkali cellulose of considerably higher cellulose and NaOH contents. 4.2.2.7

Properties and applications of alkali cellulose

Alkali celluloses employed as intermediates in cellulose derivatization can be characterized as a white-to-yellowish slippery fibrous mass of highly alkaline

4.2.3 Interaction of cellulose with tetraalkylammonium hydroxides

51

nature. All alkali celluloses are unstable in so far as on residence in the open air they undergo a rather fast oxidative depolymerization, and are decomposed by the CO2 in the air finally to a degraded cellulose II and sodium carbonate. In the presence of an excess of water cellulose II is formed from alkali cellulose via intermediate, unstable addition-compound structures (Sobu et al., 1939). On heating, alkali cellulose is rapidly decomposed by alkaline degradation of the polymer to low molecular products. The products of interaction between cellulose and alkali hydroxides are employed as intermediates only, with sodium celluloses being the only products of industrial relevance. The complete solubility of low DP cellulose in aqueous NaOH under special conditions has become the basis of an alternative process for cellulose fiber spinning, which is now in development but so far has not been practised in industry (see chapters 2.2 and 4.2.2.2). The filaments obtained here without a transient covalent derivatization resemble in their structure and their textile properties more those obtained by the amine oxide process than those manufactured by the viscose process. The main problems still impeding cellulose fiber spinning from aqueous NaOH solutions are the necessity to employ a cellulose of too low a DP for achieving optimal textile properties and an uncontrollable instability of the solutions at a sufficiently high polymer content. A recent study on the supramolecular structure and the mechanical properties of filament spun from aqueous NaOH solution (Yamane et al., 1996) indicated a high crystallinity similar to amine-oxide-spun fibers and a low crystal orientation due to a low draft and stretching ratio, and more strongly developed intramolecular than intermolecular hydrogen bonds. Tensile strength and elongation were reported to be comparable to those of viscose fibers. Besides being the basis of intermediate products, cellulose-alkali hydroxide interaction is employed in cellulose analysis for determining the alkali-soluble part of pulps and for chain-length fractionation by extraction in the low DP range (see chapter 3).

4.2.3 Interaction of cellulose with tetraalkylammonium hydroxides Due to their highly basic character and the ability to form hydrated ion dipoles in aqueous solution, tetraalkylammonium hydroxides with the general formula R4NOH interact with cellulose in quite a similar manner to alkali hydroxides, with the only significant difference of being not only swelling agents, but also good solvents for cellulose on appropriate choice of the substituents R. For the overview of R4NOH-cellulose interaction it is therefore appropriate to follow the same route of presentation as that pursued with alkali hydroxides.


51

nature. All alkali celluloses are unstable in so far as on residence in the open air they undergo a rather fast oxidative depolymerization, and are decomposed by the CO2 in the air finally to a degraded cellulose II and sodium carbonate. In the presence of an excess of water cellulose II is formed from alkali cellulose via intermediate, unstable addition-compound structures (Sobu et al., 1939). On heating, alkali cellulose is rapidly decomposed by alkaline degradation of the polymer to low molecular products. The products of interaction between cellulose and alkali hydroxides are employed as intermediates only, with sodium celluloses being the only products of industrial relevance. The complete solubility of low DP cellulose in aqueous NaOH under special conditions has become the basis of an alternative process for cellulose fiber spinning, which is now in development but so far has not been practised in industry (see chapters 2.2 and 4.2.2.2). The filaments obtained here without a transient covalent derivatization resemble in their structure and their textile properties more those obtained by the amine oxide process than those manufactured by the viscose process. The main problems still impeding cellulose fiber spinning from aqueous NaOH solutions are the necessity to employ a cellulose of too low a DP for achieving optimal textile properties and an uncontrollable instability of the solutions at a sufficiently high polymer content. A recent study on the supramolecular structure and the mechanical properties of filament spun from aqueous NaOH solution (Yamane et al., 1996) indicated a high crystallinity similar to amine-oxide-spun fibers and a low crystal orientation due to a low draft and stretching ratio, and more strongly developed intramolecular than intermolecular hydrogen bonds. Tensile strength and elongation were reported to be comparable to those of viscose fibers. Besides being the basis of intermediate products, cellulose-alkali hydroxide interaction is employed in cellulose analysis for determining the alkali-soluble part of pulps and for chain-length fractionation by extraction in the low DP range (see chapter 3).

4.2.3 Interaction of cellulose with tetraalkylammonium hydroxides Due to their highly basic character and the ability to form hydrated ion dipoles in aqueous solution, tetraalkylammonium hydroxides with the general formula R4NOH interact with cellulose in quite a similar manner to alkali hydroxides, with the only significant difference of being not only swelling agents, but also good solvents for cellulose on appropriate choice of the substituents R. For the overview of R4NOH-cellulose interaction it is therefore appropriate to follow the same route of presentation as that pursued with alkali hydroxides. Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

52


4.2.3.1

Swelling and dissolution of cellulose in solutions of tetraalkylammonium hydroxides

All the compounds considered here are, in aqueous solution, strong swelling agents for cellulose, with the effect of swelling generally increasing with increasing concentration of the base, and also with the total molar volume of the substituents R at a given base concentration, due to an increasing spacer effect of the substituent groups. Swelling in aqueous tetraethylammonium hydroxide, measured by the increase in thickness of pulp sheets, reached its final value after a few minutes depending somewhat on the drying history of the sample, and followed a rate law dQ/dt = k (£L - Q)2 (Schwabe and Philipp, 1955). Aqueous solutions of tetraalkylammonium hydroxides with sufficiently large substituents act as solvents for cellulose, if the base concentration exceeds a limiting value decreasing with increasing molar mass of the substituents (Lieser, 1937). Lieser and Leckzyck (1936) mention a minimal molar mass of the tetraalkylammonium hydroxide of 150 as a prerequisite for solvent action, while Strepicheev et al. (1957) assume a somewhat higher value of molar mass, and tetraethylammonium hydroxide as well as trimethylbenzylammonium hydroxide are not classified as solvents. Triethylbenzylammonium hydroxide, as well as dimethyldibenzylammonium hydroxide, however, are explicitly recommended as good solvents for cellulose.

4.2.3.2

Chemical interaction between cellulose and tetraalkylammonium hydroxides

The quite similar uptake of base from aqueous solution of tetraethylammonium hydroxide on the one hand, and NaOH on the other, is demonstrated by Fig. 4.2.12, indicating a 'step isotherm' in both cases with a steep increase at about the same molar fraction of base and a subsequent plateau corresponding to the addition compound of 1 mol of base/mol of AGU, if the so-called 'true uptake of base' according to Schwarzkopf (1932) (see also chapter 2.1.2) is taken as the criterion. The maximal water uptake was found here (2-3 mol of H2O/mol of AGU) to be somewhat lower than with NaOH (about 4 mol of H2O/mol of AGU). Decrystallization and depolymerization of the pulp sample employed by ball milling resulted in significant changes in the curve of base uptake versus base concentration, which then resembles more a distribution curve for a solute between two liquid phases than the step isotherm that is typical for a heterogeneous reaction. The uptake of base from aqueous solution proceeds very rapidly on about the same time scale as the swelling of the sample, and is obviously diffusioncontrolled, arriving at its final value within 10 min.


53

1.5 TEOH

NaOH

1.0 0.5

0.05

0.10 MoI fraction

0.05

0.10

Figure 4.2.12. True base uptake' of NaOH and tetraethylammonium hydroxide (TEOH) by spruce sulfite pulp in dependence on mole fraction of base in aqueous solution (Schwabe and Philipp, 1955).

The interaction between cellulose and tetraalkylammonium hydroxides in aqueous solution is assumed by Strepicheev et al. (1957) to proceed via a hydrate complex between cellulosic hydroxy groups, water molecules and the polar end of the R4NW OH^~) ion dipole, with the nonpolar substituents R acting as spacers to promote the separation of polymer chains. Pasteka (1984) proposed a model for dissolution of cellulose in triethylbenzylammonium hydroxide which is based on a sheet lattice structure for the crystalline regions of native cellulose, with the hydroxy groups caring for intersheet cohesion via hydrogen bonds and nonpolar forces for cohesion of the macromolecules within the sheets, and which is centered on the idea that the polar part of the tetraalkylammonium base disrupts the intersheet hydrogen bonds, while the nonpolar parts penetrate between the cellulose chains within each sheet and separates them. As demonstrated by the results obtained by Schwabe and Philipp (1955) with solutions of tetraethylammonium hydroxide, the substitution of water by methanol or rc-pentanol as a solvent for the base decisively diminishes the equilibrium values of swelling and of base uptake and decreases the rate of both these processes by about two orders of magnitude (see Fig. 4.2.13).

100

Figure 4.2.13. Kinetics of the tetraethylammonium hydroxide uptake from methanol (TEOH = tetraethylammonium hydroxide).

54


4.2.3.3

Changes in cellulose structure and applications

As shown by Sisson and Saner (1939) for several of these compounds with different substituents R, tetraalkylammonium hydroxides in aqueous solutions penetrate at sufficiently high base concentration into the crystalline regions of cellulose to form addition compounds exhibiting crystal lattice dimensions different from those of the starting material. Simultaneously the crystalline order of the sample is significantly decreased. Depending on the route of decomposition of these compounds by washing and/or neutralization, cellulose II as well as cellulose I can be recovered: with cotton cellulose swollen in trimethylbenzylammonium hydroxide a rather well decrystallized cellulose II was recovered in the presence of organic liquids, while on recrystallization with water cellulose I was obtained (Vigo et al, 1969, 1970 and 1972). Trimethy!benzyl (Triton B) and dimethyl dibenzyl (Triton F) ammonium hydroxide were recommended in the past as solvents for viscosity determination of cellulose, but obviously are not widely used now. Cellulose solutions of higher concentration in aqueous tetraalkylammonium hydroxides have been transformed into cellulose filaments with acceptable textile properties by spinning in an acid bath, but this route cannot compete economically with other ones as an alternative for the viscose process. Of some interest in the organic chemistry of cellulose functionalization is the use of tetraalkylammonium hydroxides like dimethyldibenzylammonium hydroxide or triethylbenzylammonium hydroxide as solvents for cellulose for performing etherification reactions, especially alkylations under homogeneous conditions, taking advantage of a more uniform substituent distribution along and between the polymer chains. This route, however, is suitable on the laboratory scale only, without the chance of industrial realization due to the high price of the solvent and the problems of recycling and/or disposal. A complete derivatization of all the hydroxy groups to a trixanthogenate of DS = 3 has been reported by Lieser and Leckzyck (1936) by reacting cellulose dissolved in Et4NOH with an excess of CS2. A decrystallization of cellulose via the intermediate formation of an adduct with tetraalkylammonium hydroxide for enhancing cellulose reactivity by this activation process has also been considered and practised on a laboratory scale, but probably the effects obtainable are inferior to those of ammonia treatment.

4.2.4 Interaction of cellulose with guanidinium hydroxide Guanidinium hydroxide (GuOH) formed from guanidine in aqueous solution according to the scheme NH-C

,NH2

Ί

+ H2O

2

Cl-NH9 OH' "NH2 _|

4.2.4 Interaction of cellulose with guanidinium hydroxide

55

is a strong base, comparable to alkali hydroxides, and is additionally capable of forming hydrogen bonds or interacting with them via the amino groups of its mesomeric stabilized cation. Just as with alkali hydroxides, a formation of ion dipoles can be observed in concentrated aqueous solution. Already about 70 years ago Dehnert and König (1925) reported a mercerization-like action of aqueous solutions of guanidine on cellulose in their study of interactions between this polymer and various onium compounds. A more detailed investigation has been published (Koura et al., 1975; Philipp et al., 1987b), which covers swelling, base uptake and X-ray patterns of cellulose on interaction with aqueous GuOH solutions, as well as the LODP and the water retention value of the cellulose samples regenerated by washing and neutralization. Subsequently, a brief survey of these results will be given. Aqueous solutions of GuOH cause a strong swelling of cellulose, increasing with the base concentration up to at least 50 %, but not resulting in dissolution of the polymer even at the highest concentration investigated of 53 % GuOH. The latter fact can probably be traced back to the formation of hydrogen bonds between cellulosic hydroxy groups and the N functions of the guanidinium cation acting as a 'trifunctional hydrogen-bond crosslinker'. The curve of base uptake versus base concentration indicates a strong sorption of GuOH from the surrounding solution and confirms the adduct formation already mentioned by Dehnert and König (1925), arriving at a molar ratio of about 1.5 GuOH/AGU at the highest base concentration investigated of 53 % (ca. 8.5 mol/1). But in contrast with aqueous sodium hydroxide, this 'true base uptake' increases rather continuously with base concentration without showing a definite step in this sorption isotherm. Water sorption passes a maximum of ca. 2 H2O/mol of AGU at a base concentration of about 50 %. Interaction between cellulose and GuOH probably proceeds similarly to that of alkali hydroxides via a complex formation between cellulosic hydroxy groups and the hydrated GuOH ion dipoles, but is supplemented here by the formation of hydrogen bonds between the amino groups of the base and the hydroxy groups of the polymer. This type of hydrogen bond is known to be stronger than those formed between hydroxy groups. The peculiar course of GuOH sorption onto a sample of highly ordered !inters cellulose resembles more the distribution equilibrium of a solute between two liquid phases than the step isotherm of a heterogeneous reaction, and is similar to that encountered in NaOH sorption onto a well decrystallized cellulose. This apparent contradiction is reconciled by evaluating and comparing the appropriate WAXS patterns: while in the case of NaOH a still rather highly ordered and well-defined WAXS phase of sodium cellulose is formed, a complete loss of supramolecular order must be concluded from the pattern of an originally highly crystalline cellulose after loading with aqueous GuOH in the range of base con-

56


centration between 25 and 30 %. Already at lower base concentration a significant lowering of supramolecular structure can be observed (Fig. 4.2.14).

Figure 4.2.14. WAXS diagram of !inters cellulose treated with guanidinium hydroxide (GuOH): (a) 10 % GuOH, (b) 10 % GuOH plus regeneration (Philipp et al, 1987b)

At very high base concentrations this amorphization is not so complete, possibly due to the very high viscosity of the GuOH solution impeding a complete penetration of the crystalline regions of the cellulose moiety during the reaction time employed. In the low range of base concentration of up to about 20 % GuOH, the WAXS pattern of cellulose I is fairly well retained indicating, however, a continuous loss of supramolecular order with increasing base concentration. The disordered structure of cellulose is widely retained after decomposition of the cellulose GuOH adduct by washing with water, neutralization with acetic acid, solvent exchange and drying, and is not significantly changed even after boiling the sample with water. The X-ray patterns of the regenerated samples indicated in some cases a poorly ordered cellulose I, and in others a poorly ordered cellulose II, but so far no clear cut correlation could be derived between the conditions of regeneration and the cellulose modification obtained. Solid state CP-MAS 13C NMR spectra of the regenerated samples confirmed the low degree of order and indicated the coexistence of various chain conformations (Philipp et al., 1987b). The regenerated samples showed a decisive increase in accessibility, with the largest effects being obtained by treatment with a GuOH solution of about 2530 %, i.e. in the same range where the most pronounced amorphization of the GuOH-loaded cellulose has been observed. These regenerated samples had a water retention value of about twice the original one, and their LODP had dropped from about 160 to about 100. Remarkable is the extremely good accessibility of these samples to enzymatic degradation (Dan, 1981), which may be

4.2.5 Interaction of cellulose with ammonia and hydrazine

57

traced back to the low degree of order and the conformational nonuniformity, as well as to the morphological changes at the fibrillar level observed in electron microscopic studies: in the scanning electron microscopy (SEM) micrographs of a cotton !inters sample treated with 40 % aqueous GuOH the original regular, fine fibrillar structure of the fiber surface was widely destroyed and replaced by rather nonstructured lumps of cellulosic matter and deep holes, and only some disarranged and twisted fibril bundles remained of the original structure. The interaction between cellulose and guanidinium hydroxide so far finds no practical application. However, GuOH treatment of cellulose with subsequent regeneration of the sample might be of interest in the laboratory as an effective activation technique for enhancing the reactivity and accessibility of cellulose.

4.2.5

Interaction of cellulose with ammonia and hydrazine

Ammonia and hydrazine are much less basic than the agents considered so far in this chapter on adduct formation between cellulose and basic compounds, as their basicity constants in aqueous solutions amount to only 2 x 10~5 and 2 χ 10~ 6 , respectively. Thus the interaction between the polymer and NH3 or N2H4 can be supposed to occur predominantly by breaking O ··· H ··· O bonds and replacing them by N ··· H ··· O hydrogen bonds, and not by interaction between cellulosic hydroxy groups and hydrated ion dipoles of the base. Nevertheless, NH3 as well as N2H4 are able to penetrate even into the highly ordered regions of cellulose and to form well-defined addition compounds resulting in significant changes of cellulose structure after decomposition the addition compound and regeneration of the polymer. Subsequently, the interaction of cellulose with ammonia and its consequences on cellulose structure will be described in some detail with respect to its relevance for organic reactions and the textile processing of this polymer, followed by a brief survey of results relatetd to hydrazine. Interaction of ammonia with cellulose, resulting in structural changes of the polymer, can take place with NH3 in the liquid state or in the gaseous state at sufficiently high pressure, and also with solutions of the base in water or in polar liquids above a minimum level of NH3 concentration. Systematic studies on adduct formation and lattice transitions have predominantly been performed with liquid NH3. Liquid NH3 (boiling point -33 0C) is a moderate swelling agent for cellulose, with a swelling power in between that of water and that of aqueous NaOH of optimal swelling concentration. It can be turned into a cellulose solvent by adding suitable inorganic salts like isothiocyanates or iodides as a second solvent component.

58


The formation of at least two defined addition compounds between cellulose and liquid NH3 has been reported already about 60 years ago by Hess and coworkers (Hess and Trogus, 1935), with the results of the studies still representing the actual state of knowledge, i.e. (i) a 1 : 1 complex obtained after the evaporation of all free liquid NH3 at a temperature above -33 0C; (ii) a complex consisting of 2 mol of NH3/mol of AGU formed below -33 0C. Furthermore, a 6 : 1 complex is claimed to be formed according to Hess and Gundermann (1937). As indicated by the corresponding crystal lattice dimensions obtained by WAXS at -20 to -33 0C for ammonia cellulose I and at a temperature below -33 0C for ammonia cellulose II, the interaction and adduct formation takes place in the highly ordered regions of the cellulose structure too. The 1-0-1 lattice spacing is considerably enhanced compared with the starting material, cellulose I. On standing under anhydrous conditions, ammonia cellulose I slowly decomposes with evolution of NH3 to cellulose III, a modification resembling cellulose II, but being transformed, however, on treatment with water to cellulose I. In this way, cellulose I can be reversibly converted to cellulose III according to CeIII

·

NH3

H2O

CeIIIII

Starting from a cellulose II sample, cellulose II is regenerated via the intermediate transitions to ammonia cellulose and cellulose III, indicating some memory effect of the intermediates for the structure of the original sample. All the cellulose samples regenerated after treatment with liquid NH3 have a lower degree of order than the original one, the decrystallization effect depending widely on the procedure of ammonia treatment as well as that of regeneration. Under suitable conditions, samples exhibiting no crystalline X-ray pattern at all can be prepared from cellulose I as well as from cellulose II, which, however, are susceptible to recrystallization after a longer time of residence, especially in the presence of moisture. The fibrillar architecture is significantly damaged by an ammonia treatment, too, as can be seen by a loosening and distortion of the concentric rings of fibrils in the TEM micrograph of the fiber cross section and by the appearance of deep clefts and fissures partially covered by fibril strings in the SEM micrograph of the fiber surface. This decrease in supramolecular order and fibrillar regularity is reflected also by a significant increase in water retention value and water regain at 65 % relative humidity, as well as by a decrease of the LODP from an original value of about 160 to about 90 for cotton !inters cellulose and in a considerable increase in the initial rate of acetylation with acetanhydride. An even higher enhancement of accessibility has been reported for the synergistic action of liquid ammonia and aqueous NaOH in consecutive treatment steps (Vigo


59

et al., 1972). An activating pretreatment with liquid ammonia has also been claimed to promote the conversion of cellulose to soluble cellulose ethers (Hoechst AG, 1984). A shift of the concentration required for the Cell I -> Na-CeIl I -^ Cell II transition to lower values by pretreating the sample with liquid NH3 (Schleicher et al., 1973 and 1974) has already been mentioned. Also, dissolution of cellulose by emulsion xanthation was found to take place at lower concentration of NaOH after activation with liquid NH3. In a subsequent silylation of cellulose with trimethylsilyl chloride, a significant difference in the effect of activation with liquid ammonia has been observed by Wagenknecht et al. (1992) in dependence on activation temperature, an activation at -60 0C results in a smoother derivatization than a pretreatment at about -30 0C. Obviously it can make a difference in a subsequent derivatization reaction whether ammonia cellulose II or ammonia cellulose I is formed in the pretreatment step. Ammonia at a pressure of 0.5-0.7 MPa at room temperature shows similar effects to liquid ammonia in the low temperature range and converts cellulose I to cellulose III of a significantly lower degree of order, with the effect being enhanced by the action of CO2, SO2 or acetic anhydride in a subsequent treatment step at the same level of pressure (Prusakov, 1982). Much more convenient and less hazardous than an activation with liquid ammonia, but of comparable efficiency is the pretreatment of cellulose I with highly concentrated solutions of NH3 in suitable solvents, as shown in a comprehensive study (Koura et al., 1973; Koura and Schleicher, 1973) with !inters cellulose and by Wagenknecht et al. (1992) in connection with a subsequent silylation of cellulose. While solutions of NH3 in alcohols like ethanol or glycol proved to be ineffective over the whole range of concentrations, mixtures of NH3 with water, DMSO or formamide brought about significant structural changes and considerable activation effects at a molar ratio of NH3-to-solvent > 1, and with solvents containing amino functions, like ethanolamine or morpholine, an even smaller molar ratio of about 0.7 was required for this purpose. Within the structural criteria employed, the NH3 concentration required was lowest for an increase in WRV and successively higher for an increase in water regain and a drop in LODP or an accelerated acetylation. Noticeable is the distinct maximum in WRV of more than twice the original value obtained by treatment of !inters cellulose with a mixture of 1 mol of NH3 and 2 mol of ethanolamine (Koura et al., 1973; Koura and Schleicher, 1973). Especially for a subsequent silylation, an activation procedure employing a saturated solution of NH3 in DMF or THF at -10 to -15 0C has been reported by Wagenknecht et al. (1992). Addition of an NH3TDMF mixture instead of the two single components to the predried and precooled cellulose was found to be essential for this route of activation. An activation time of about 2 h proved to be sufficient for achieving the maximal effect, before accelerating the etherification by raising the temperature slowly to about 60 0C (see chapter 4.5). Too early an increase in temperature was observed to be detrimental to the activation intended.

60


The examples cited here demonstrate the relevance of activation by NH3 in the organic chemistry of cellulose. The structural changes resulting in the supermolecular and the fibrillar level (see Fig. 4.2.15) from interaction of NH3 with cellulose are of consequence also in the material properties of the polymer. Therefore, this interaction has also become the basis of textile processes for improving properties of cotton and viscose fabrics, especially with regard to dyeability and handling. These processes show some similarity to mercerization as both consist of a lowering of supramolecular order and a loosening of morphological structure, although differences do exist with regard to the details of these effects. In comparison with mercerization with NaOH, these treatments with liquid ammonia have the advantage of an easy elimination of the reagent, especially in the so-called dry process, but include the hazards of handling liquid ammonia. In a recent review (Brederik and Blüher, 1991) a so-called 'dry' and a so-called 'wet' process of liquid NH3/cellulose interaction for the pretreatment of cotton fabric prior to easy care treatment have been compared with regard to structural changes of the polymer: in the 'dry' process with elimination of ammonia by evaporation, cellulose III, besides cellulose I, was found in the final product, while in the 'wet' process with the NH3 being washed out by water the final product consisted exclusively of cellulose I. In both cases a decrease in degree of order and in crystallite size was observed after the NH3 treatment, and the pore and void structure proved to be more uniform than in the untreated fabric.

Figure 4.2.15. Changes in the microfibrillar structure of bacterial cellulose by treatment with liquid NH3 (-65 0C, 30 min) revealed by TEM: (a) untreated; (b) solvent exchange and treatment with liquid NH3; (c) mechanically disintegrated, solvent exchange and NH3 treatment (micrographs by HJ. Purz, Teltow-Seehof).


61

Interaction of cellulose with hydrazine exhibits similarities, but also some differences from that with ammonia. In contrast with NH3, anhydrous N2H4 was found to be a solvent for cellulose at elevated temperature, despite its still lower basicity constant of about 2 χ 10~6, acting without covalent derivatization by breaking down H ··· O ··· H bonds in the cellulose structure and replacing them by N — H — O bonds in the cellulose-solvent complex (Litt and Kumar, 1977). From this solution, cellulose II with a lamellar morphology could be regenerated (Kolpak et al., 1977), and on extruding the hot solution, cellulose threads with quite a special texture were obtained after elimination of the N2H4 (Lee and Blackwell, 1981). In a comprehensive WAXS study of ramie (cellulose I), mercerized ramie (cellulose II) and fortisan fiber (cellulose II), after soaking with nearly anhydrous N2H4 of 97 % concentration, after evaporation of excess N2H4 and after decomposition of the cellulose hydrazine complexes by water vapor, Lee and Blackwell (1981) confirmed the intracrystalline swelling on interaction of N2H4 with cellulose and arrived at different WAXS patterns for the hydrazine complexes formed with cellulose I on the one hand, and cellulose II on the other. A molar ratio of 0.5 N2H4/mol of AGU and of 1.5 mol of N2H4/mol of AGU were reported by the authors for the complexes formed with mercerized ramie and with fortisan, respectively. For native ramie, no change in X-ray crystallinity was observed along the transition route from cellulose I via the hydrazine-cellulose complex to again cellulose I. Similar to aqueous solutions of NH3, interaction of solutions of N2H4 in water of sufficiently high base concentration results in changes in supramolecular structure and a significant enhancement of accessibility: according to Trogus and Hess (1931), the action of a 60 % aqueous N2H4 solution (molar ratio, 1 N2H4 : 1.3 H2O) leads to a strong intracrystalline swelling and a change in unit cell dimensions in the crystalline regions, which again are different for cellulose I and cellulose II as the starting material in agreement with the recent observations by Blackwell employing nearly anhydrous N2H4. In a study on activation of cotton !inters and LODP !inters by aqueous solutions of N2H4 (Koura et al., 1975), a significant increase in WRV was already observed at a concentration as low as 5.9 % N2H4, corresponding to a molar ratio of 1 N2H4 : 28 H2O. At a molar ratio of about 1 N2H4 : 2 H2O, i.e. 1 H2N group to 1 H2O molecule, about the same activation effect with regard to WRV and LODP was obtained at 20 0C, as with an aqueous NH3 solution of a molar ratio of 1 : 1 at -20 0C. Lowering the temperature of treatment from 20 to -20 0C, or increasing the N2H4 concentration to a molar ratio of about 1 N2H4 : 1 H2O (hydrazine hydrate), did not significantly change the effects obtained. From a practical point of view, aqueous solutions of N2H4 present no advantages as an activating agent for cellulose in comparison with NH3 in water. The possibility of dissolving cellulose in anhydrous N2H4 and of forming threads

62


from these solutions is of scientific interest regarding correlations between solvent action and filament structure obtained from the solution, but will probably not find any practical application due to the hazards of handling anhydrous N2H4.

4.2.6 Interaction of cellulose with aliphatic mono- and diamines Just as ammonia or hydrazine, aliphatic mono- and diamines can penetrate even in the highly ordered regions of cellulose and form addition compounds, resulting in a change in crystalline lattice dimensions determined by WAXS and in an increase in accessibility and reactivity after decomposition of the complex and regeneration of the cellulose. The driving force here also consists of the replacement of O ··· H ··· O bonds between cellulosic hydroxy groups with the stronger N — H — O bonds between cellulose and amine. Research activities in this area have been centered on addition-compound structure and lattice dimensions in relation to amine structure on the one hand, and on the activation effects obtained via a transient amine adduct formation on the other. Primary aliphatic amines act as rather strong swelling agents on cellulose without being solvents by themselves or being turned into solvents by addition of salts (Wagenknecht, 1976; Davis et al., 1943; Lokhande, 1966; Howsman and Sisson, 1954). A unique case is the solvent action of binary systems of methylamine and DMSO (see also section 2.2). As demonstrated by the examples in Table 4.2.2, swelling of cellulose in primary aliphatic monoamines is favored by a low temperature and proceeds rather slowly already with ft-propylamine, while with ethylene diamine the final value is reached within 1 h. Table 4.2.2. Liquid retention value (LRV) of cotton !inters in aliphatic amines (Wagenknecht, 1976).

Amine

T( 0 C)

LRV (%) after 1 day 4 days 64 83 94 111 133 130 62 58 66 96 131 155 129 117 96 95 Ih

C3H7NH2 C4H9NH2 H2N(CH2)2NH2 H2N(CH2)2OH for comparison: HO(CH2)2OH DMF

20 O 20 O 20 20 20 20

59 45

-

60 45

4.2.6 Interaction of cellulose with aliphatic mono- and diamines

63

Regarding amine chemical structure, at least one primary amino group is required for addition-compound formation in the highly ordered regions, while secondary and tertiary amines are ineffective and also ethanolamine obviously does not penetrate into the crystallites. Polyamines containing primary as well as secondary amino groups, on the other hand, can penetrate after suitable preswelling, with the secondary amino groups obviously also being active in hydrogenbond interaction (Creely and Wade, 1978). Steric hindrance of the primary amino group in hydrogen-bond formation can impede penetration into the cellulose structure, as demonstrated by the action of isopropylamine or secondary butylamine in comparison with the corresponding η-compounds (Creely, 1971), but this obstacle can be overcome by pretreatment with a suitable swelling agent. The addition complexes with methyl-, ethyl- and rc-propylamine can be obtained by direct interaction between dry cellulose I or cellulose II and the appropriate amine, while the adducts with the higher amines from C4 to C7 require a two-step procedure, i.e. a preswelling with e.g. ethylamine and subsequent substitution of this primary swelling agent by the higher amine. As illustrated by Fig. 4.2.16, the 1-0-1 lattice spacing, indicating the distance between adjacent lattice layers in the crystalline regions, increases steadily with the number of C atoms of the linear primary aliphatic amine. 3,02,52,0-

0,50,0

3

4

No. of C-atoms

Figure 4.2.16. 1-0-1 lattice layer spacing of cellulose amine complexes dependent on the number of C-atoms of the amine (see Creely, 1971). Aliphatic diamines with terminal H2N groups at both ends of the carbon chain can accomplish intracrystalline swelling and addition-compound formation

64


throughout the cellulose structure, as experimentally studied with these compounds up to octamethylene diamine, and the action of ethylene diamine has been comprehensively investigated (Creely et al., 1959; Creely, 1977). Concerning addition complex stoichiometry, the idea of crosslinking action of one ethylene diamine molecule between two hydroxy groups in adjacent lattice layers resulting in a ratio of 1 amine/2 AGU seems logical, but most experimental evidence available today is in favor of a rather stable 1 : 1 complex, which, however, does not exclude some interlayer crosslinking. According to Howsman and Sisson (1954) some freedom of rotation of monomer chain units around the glycosidic bond, resulting in a twisted conformation, is assumed for monoamines, while in the case of diamines this rotation is restricted by the hydrogen-bond crosslinks. All the cellulose amine complexes can be decomposed by water yielding cellulose I in the case of native cellulose as the starting material, and cellulose II if a cellulose II sample had been used. On decomposition of a complex obtained from native cellulose by evaporation of the amine, cellulose III was found in the case of a monoamine, while cellulose I was obtained from the complex with a diamine (Trogus and Hess, 1931). These results comply well with the model outlined above for the mode of swelling of these two classes of compounds. Decomposition of the addition compounds by nonaqueous media may lead to an alternative lattice modification too, as shown by Lokhande et al. (1976) for ethylene diamine treated cotton, where on regeneration of the cellulose with water a mixture of cellulose I and cellulose II was obtained, while decomposition by methanol yielded cellulose III. Besides the lattice modification obtained on cellulose regeneration, the activation effect can also be influenced by the mode of decomposition of the adduct. After passing intermediate adduct formation with a mono- or diamine, the degree of order of a cellulose sample is decreased, and its accessibility and reactivity is enhanced, as demonstrated by some data on X-ray crystallinity, water vapor regain and reactivity in esterification in Table 4.2.3 after treatment with ethylamine and ethylene diamine. Table 4.2.3. Ratio of disorder after-to-before amine treatment of cotton cellulose (Venkataraman et al., 1979; Warwicker et al., 1966)

Criterion

Disorder ratio after treatment with: ethylamine ethylene diamine

l-;c c H2O regain Reactivity in esterification

2.33 1.4 ca. 2a

a

acetylation;

b

formylation.

1.94 1.3 1.3

4.2.6 Interaction of cellulose with aliphatic mono- and diamines

65

The LODP was found to decrease from 229 to 129 after ethylamine treatment and from 182 to 112 after ethylene diamine treatment. Disordering is generally favored by a low temperature of amine treatment just as is swelling, and by regeneration of the cellulose in a nonaqueous medium. The 'disordering effect' of amines on the cellulose structure is by no means limited to the supramolecular level: the SEM micrographs of !inters fiber surfaces revealed, after treatment with ethylene diamine, a severe distortion and loosening of the fibrillar architecture with long isolated fibrils on the one hand, and tide fibrillar clusters on the other, resulting finally in a significant higher degree of destruction of the original morphology than a treatment with liquid ammonia (Dan, 1981). Just as in the case of NH3, a decrease in supramolecular order corresponding to an increase in accessibility and reactivity can be achieved not only with aliphatic amines in the pure state but also with their solution in water or in a suitable organic liquid of sufficiently high base concentration. Usually a molar ratio of amine to solvent of about 1 : 1 is required for this purpose. With water as the solvent, favoring penetration into the cellulose structure, a somewhat lower content of base may be sufficient, while with alcohols a higher ratio may be required. Changes in WRV and water vapor regain are frequently observed already at a lower base concentration than changes in LODP and X-ray crystallinity. A lowering of the treatment temperature increases the activation effect also with amine solutions. With aqueous solutions of ethylene diamine, activation effects become visible above a base concentration of ca. 40 % and reach their final value at about 60 %, corresponding to a molar ratio of 1 water molecule/H2N group. The sorption isotherm (Fig. 4.2.17) shows a steep increase in base uptake in the range 2.51.5 mol of H2O/mol of base, resembling in its shape that of alkali hydroxides but with the step being situated at a much lower molar ratio of water-to-base.

1

2

3

i

H 2 O : Ethylendiomine [mol / mol]

Figure 4.2.17. Ethylene diamine uptake of !inters cellulose versus molar ratio H2O : ethylene diamine (Philipp and Brandt, 1983).

66


Also in contrast with alkali hydroxides or guanidinium hydroxide, the uptake of ethylene diamine was not accompanied by a specific water sorption at the beginning or within the plateau of base uptake corresponding to a molar ratio of 0.91.0 mol of ethylene diamine/mol of AGU. Ethanolamine on the other hand, obviously cannot penetrate the ordered regions of the cellulose structure, as confirmed by a sorption of only about 0.1 mol/mol of AGU over the whole range of concentration of the ethanolamine/water mixture. But, ethanolamine can eventually increase the activation effect of an aliphatic amine above that obtained with the pure amine, as demonstrated by the course of WRV observed after treating cotton !inters with methylamine/ethanolamine mixtures of increasing methylamine concentration (see Table 4.2.4). In a comprehensive study on the activating action of amine/solvent mixtures onto cotton !inters, from which some selected data are presented in Table 4.2.4, Koura et al. (1973) arrived at the conclusion that the intermolecular interactions in the ternary systems of cellulose aliphatic amine and solvent are governed by: (i) the competition between O ··· H ··· O and N ··· H ··· O bond formation with the latter being significantly stronger especially at low temperature; (ii) the potential hydrogen-bond density of the amine (or the amine solvent associate) besides its molecular volume and geometrical shape; (iii) the interaction of the amine with the solvent or with solvent associates, an example being the destruction of self-associates of ethanolamine molecules by hydrogenbond interaction between the hydroxy group and an amino group with the effect of enhanced swelling of the cellulose by the newly formed ethanolamine associate. Especially this last point accentuates again the relevance of active agent/solvent interaction for understanding swelling and activation of cellulose in these binary mixtures. Cellulose-amine complexes find no application as products, but are of scientific interest as intermediates in special routes of cellulose activation.

4.2.7 Concluding remarks This chapter deals with the cellulose chemistry of intermolecular interaction, resulting in addition compounds of limited stability and sometimes ill-defined composition. These compounds originate either from complex formation between a cellulosic hydroxy group and a hydrated ion dipole of an alkali or tetraalkylammonium hydroxide, or from hydrogen-bond interaction by replacing O ··· H ··· O bonds between cellulosic hydroxy groups with the stronger O ··· H ··· N bonds between cellulosic hydroxy groups and a suitable basic compound like NH3, N2H4 or an aliphatic amine. In the first case the uptake of base is combined with a specific water sorption, while in the second case obviously no bound water is included in the complex.

Molar ratio Amine: Solvent — 1:3 1:2 1:2 2:3 1:1 1:3 1:1 1:3 1:1 1:3 1:1 1:2 1:1

acetylation: 3 min at 25 0C with acetanhydride/HC!O4.

without H2N(CH2)2OH H2N(CH2)2OH H2N(CH2)2OH H2N(CH2)2OH H2N(CH2)2OH (CH3)2SO (CH3)2SO H2N(CH2)2OH H2N(CH2)2OH (CH3)2SO (CH3)2SO H2O H2O

without CH3NH2 CH3NH2 CH3NH2 CH3NH2 CH3NH2 CH3NH2 CH3NH2 H2N(CH2)2NH2 H2N(CH2)2NH2 H2N(CH2)2NH2 H2N(CH2)2NH2 H2N(CH2)2NH2 H2N(CH2)2NH2

a

Solvent

Amine

WRV (%) 52 78 100 118 112 107 70 96 78 106 73 105 89 94

Temperature/ time °C/h

0/1 0/1 0/24 0/1 0/1 0/1 0/1 20/1 20/1 20/1 20/1 20/1 20/1

H2O vapor respir. (%) 6.8 9.2 9.3 9.2 7.7 9.3 9.0 160 160 132 100 127 100 160 88 140 99 150 104 115 88

22 24 45 (47) 28 46 32 40 34 42 -

LOOP Acetyl3 (Guam) content (%)

Table 4.2.4. Increase in the accessibility of cotton !inters (DP 1300) by treatment with amine containing solvent mixtures (Koura et al., 1973).

a S-

1

OQ

S'

K) X)

68


On interaction of cellulose with guanidinium hydroxide, obviously both these principles are realized. Addition-compound formation in water as the reaction medium proceeds very rapidly and is usually diffusion-controlled, but can be delayed and slowed down in nonaqueous media, e.g. alcohols. Aqueous solutions of the compounds considered here lead to complex formation even in the highly ordered regions of the cellulose structure, above a limiting base concentration in the solution, corresponding to a change in WAXS pattern and frequently a steep increase in base uptake within a step-like sorption isotherm typical for a heterogeneous type of reaction. After decomposition of the complexes by the action of e.g. water, or by evaporation of the base in the case of volatile agents, cellulose I, II or III of lower degree of order and an enhanced accessibility and reactivity, with an altered fibrillar architecture, is obtained. The lattice modification and the magnitude of the decrystallization effect depend on the complex-forming system involved and on the procedure of regeneration of the cellulose from the complex. The complexes formed between cellulose and basic compounds find no application as products, but are of high scientific and practical relevance as intermediates in activating pretreatments of the polymer for subsequent covalent derivatization.

References Bartunek, R., Kolloid-Z. 1956,146, 35. Bredereck, K., Vlachopoulos, G.,Angew. Makromol Chem. 198Oa, 84, 81-96. Bredereck, K., Thi Bach Phnong Dau, Angew. Makromol Chem. 198Ob, 89, 167-181. Bredereck, K., Blüher, Α., Melliand Textilber. 1991, 72, 46-54. Creely, J.J., Segal, L., Loeb, L., /. Polym. ScL 1959, 36, 205-214. Creely, J.J., Text. Res. J. 1971, 41, 274-275. Creely, J.J., J. Polym. ScL9 Polym. Chem. Ed. Al 1977, 75, 521-522. Creely, JJ., Wade, R.H., J. Polym. ScI, Polym. Lett. Ed. 1978, 76, 291-295. Dan, D.C., Ph.D. Thesis, Academy of Science (GDR) 1981. Davis, W.E.A., Barry, A.J., Peterson, F.C., King, A.J., /. Am. Chem. Soc. 1943, 63, 1294-1299. Dehnert, F., König, W., Cellul-Chem. 1925, 6, 1. Dobbins, R.J., Tappi 1973, 53, 2284-2290. Eigen, M., Angew. Chem. 1963, 75, 489. Fengel, D., Wegener, G., Wood, Chemistry, Ultrastructure, Reactions, Berlin: de Gruyter & Co., 1989. Fink, H.-P., Dautzenberg, H., Kunze, J., Philipp, B., Polymer 1986, 27, 944948.

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Fink, H.-P., Philipp, B., Zschunke, C., Hayn, M., Acta Polym. 1992, 43, 266269. Fink, H.-P., Walenta, E., Kunze, J., Mann, G., in Cellulose and Cellulose Derivatives, Physico-chemical Aspects and Industrial Applications, Kennedy, J.F., Phillips, G.O., Williams, P.A., Piculell, L. (Eds.), Cambridge: Woodhead Publ. Ltd., 1995, pp. 523-528. Harris, C.A., Purves, C.B., Pap. Trade J. 1940, 770, 29. Hayashi, J., Sen'i Gakkaishi, 1976, 32, P37. Hess, K., Trogus, C., Ber. Dtsch. Chem. Ges. 1935, 68, 1986. Hess, K., Gundermann, J., Ber. Dtsch. Chem. Ges. 1937, 70, 1788. Heuser, E., Bartunek, R., Cellul.-Chem. 1925, 6, 19-26. Hinton, J.F., Amis, E.S., Chem. Rev. 1967, 67, 367. Hoechst AG, Patent DE 3241720, 1984; Chem. Abstr. 1984, 707, 74631. Howsman, J.A., Sisson, W.A., in Cellulose and Cellulose Derivatives, Ott, E., Spurlin, H.M., Graffein, M.W. (Eds.), New York: Interscience Publ. Inc., 1954, pp. 328. Isogai, A., Cellulose, 1997, 4, 99-107. Jayme, G., Roffael, E., Papier (Darmstadt) 1970, 24, 181-186. Käufer, M., Papier (Darmstadt) 1984, 38, 583-589. Kolpak, F.J., Blackwell, J., Litt, M., /. Polym. ScL, Polym. Lett. Ed. 1977, 75, 655. Koura, A., Schleicher, H., Faserforsch. Textiltech. 1973, 24, 82-86. Koura, Α., Schleicher, H., Philipp, B., Faserforsch. Textiltech. 1973, 24(5), 187-194. Koura, A., Philipp, B., Schleicher, H., Wagenknecht, W., Faserforsch. Textiltech. 1975, 26, 514-515. Krässig, H.A., in Cellulose - Structure, Accessibility and Reactivity, Krässig, W.A. (Ed.), Yverdon: Gordon and Breach Sei. Publ. S.A., 1993. Kunze, J., Ph.D. Thesis, Academy of Science (GDR) 1983. Kunze, J., Ebert, A., Lang, H., Philipp, B., Z. Phys. Chem. (Leipzig) 1985, 266, 49-58. Lang, H., Bertram, D., Loth, F., Patent DD 260190, 1988; Chem. Abstr. 1989, 770, 156358. Lang, H., Laskowski, I., Cellul Chem. Technol 1991, 25, 143-153. Lee, D.M., Blackwell, J., /. Polym. ScL, Polym. Phys. Ed. 1981, 79, 459-465. Lieser, Th., Leckzyck, E., Ann. 1936, 522, 56. Lieser, Th., Liebigs Ann. Chem. 1937, 528, 276. Litt, M., Kumar, N.G., Patent US 4 028 132, 1976; Chem. Abstr. 1977, 87, 186315. Lokhande, H.T., Bombay Technol. 1966, 76, 22-26.

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Lokhande, H.T., Shukla, S.R., Chidambareswaran, P.K., Paul, RB., /. Polym. ScL, Polym. Lett. Ed. 1976, 14, 747-749. Loth, F., Philipp, B., Dautzenberg, H., Acta Polym. 1984, 35, 483-486. Mori, U., Ph.D. Thesis, University of Jena 1991. Muskat, I.E., /. Am. Chem. Soc. 1934, 56, 693; 2449. Neale, S.M., /. Text. Inst. 1929, 20, T373. Neale, S.M., /. Text. Inst. 1930, 27, T255. Neale, S.M., /. Text. Inst. 1931, 22, T320; T349. Pasteka, M., Cellul. Chem. Technol 1984,18, 379-387. Pavlov, P., Makazchieva, V., Simeonov, N., Dimov, K., Cellul. Chem. Technol. 1983, 77, 575-583; 585-592. Pennings, A.J., Prins, W., Hale, R.D., Ränby, B.C., /. Appl. Polym. ScL 1961, 5, 676. Pennings, A.J., Prins, W., J. Polym. Sei. 1962, 58, 229-248. Philipp, B., Faserforsch. Textiltech. 1955, 6, 180-181. Philipp, B., Lehmann, R., Ruscher, Ch., Faserforsch. Textiltech. 1959, 70, 22-35. Philipp, B., Brandt, A., Cellul Chem. Technol 1983, 77, 323-332. Philipp, B., Purz, HJ., Papier (Darmstadt) 1983, 37, V1-V13. Philipp, B., Fink, H.-P., Kunze, J., Purz, HJ., Tappi Proc., Int. Dissolv. Specialty Pulps 1983, 177-183. Philipp, B., Fink, H.-P., Kunze, J., Frigge, K., Ann. Phys. (Leipzig) 1985, 42, 507-523. Philipp, B., Kunze, J., Fink, H.-P., Structures of Cellulose, ACS Symp. Ser. 1987a, 340, 178. Philipp, B., Kunze, J., Loth, F., Fink, H.-P., Acta Polym. 1987b, 38, 31-36. Prusakov, V.V., Chim. Drew. 1982, 4, 112-113. Purz, HJ., Graf, H., Fink, H.-P., Papier (Darmstadt) 1995, 49, 714-730. Scherer, P.C., Hussey, R.E., J. Am. Chem. Soc. 1931, 53, 2344. Scherer, P.C., Gotsch, L.P., Bull Va. Polytech. Inst. 1939, 32. Schleicher, H., Philipp, B., Ruscher, Ch., Faserforsch. Textiltech. 1967, 78, 1-4. Schleicher, H., Daniels, C., Philipp, B., Faserforsch. Textiltech. 1973, 24, 371376. Schleicher, H., Daniels, C., Philipp, B., J. Polym. ScL, Symp. 1974, 47, 251-260. Schmid, L., Becker, B., Berichte 1925, 585, 1966. Schmid, L., Waschkaw, A., Ludwig, E., Monatsh. 1928, 49, 107. Schwabe, K., Philipp, B., Holzforschung 1955, 9, 104-109. Schwarzkopf, O., Z Elektrochem. 1932, 38, 353-458. Sisson, W.A., Saner, W.R., /. Phys. Chem. 1939, 43, 687. Sobue, H., Kiessig, H., Hess, K., Z Phys. Chem. 1939, B43, 309-328. Strepicheev, A.A., Klunjanc, J.L., Nikolaeva, N.S., Mogilevskij, E.M., 7zv. Akad. Nauk SSSR, Otd. Chim. Nauk 1957, 6, 750-754.

4.3. l General routes of cellulose-metal atom interaction

71

Trogus, C., Hess, K., Z. Phys. Chem. (Leipzig) 1931, B14, 387. Venkataraman, A., Subramanian, D.R., Maniunath, B.R., Padkye, M.R., Indian J. Text. Res. 1979, 4, 106-110. Vigo, T.L., Wade, R.H., Mitcham, R., Welch, C.M., Text. Res. J. 1969, 39, 305316. Vigo, T.L., Mitcham, R., Welch, C.M., /. Polym. Sd. 1970, 8, 385-393. Vigo, T.L., Mitcham, R., Welch, C.M., Text. Res. J. 1972, 42, 96. Wagenknecht, W., Ph.D. Thesis, Academy of Science (GDR) 1976. Wagenknecht, W., Nehls, L, Stein, A., Klemm, D., Philipp, B., Acta Polym. 1992, 43, 266-269. Warwicker, J.O., Jeffries, R., Colbrain, R.L., Robinson, R.N., in The Cotton Silk and Man-Made Fibers Research Association, Didsburg: Shirley Institute Pamphlet, 1966, No. 93. Yamane, J., Mori, M., Saito, M., Okajima, K., Polym. J. 1996, 20, 1039-1047. Yamashiki, T., Kamide, K., Okajima, K., in Cellul. Sources Exploit., Kennedy, J.F., Phillips, G.O. (Eds.), London: Ellis Horwood, 1990, pp. 197-202. Zeronian, S.H., Cabradilla, K.E., /. Appl. Polym. ScL 1973, 77, 539-552.

4.3 Metal Complexes of Cellulose 4.3.1 General routes of cellulose-metal ion interaction Cellulose-metal ion interaction has many features, ranging from the sorption and desorption of calcium ions during wood pulp manufacture and artificial fiber spinning using cellulose cuprammonium hydroxide solutions, to the design and preparation of sophisticated cellulosic materials with e.g. catalytic properties. Two main routes to metal-ion-containing cellulose products have to be considered, i.e. (i) the use of the cellulose backbone as a polymeric carrier of functional groups deliberately introduced by covalent reaction and subsequently employed for the interaction with metal ions; (ii) the engagement of hydroxy groups of the polyhydroxy compound 'cellulose' as a polymer ligand coordinated to a metal cation acting as the center of complexation. A detailed discussion of the first one of these routes would by far surpass the scope of this book, and only two examples will therefore be mentioned briefly for illustration: the binding of Ca2+ or Fe2+ to cellulose powders containing varying numbers of carboxyl groups. The interaction was found to be governed by the concentration ratio of [M2+] to [H+] on the one hand, and the level of carboxyl content on the other, without a full saturation of all carboxylic sites by the metal cation being obtained under the conditions investigated (Jacopian et

4.3. l General routes of cellulose-metal atom interaction

71

Trogus, C., Hess, K., Z. Phys. Chem. (Leipzig) 1931, B14, 387. Venkataraman, A., Subramanian, D.R., Maniunath, B.R., Padkye, M.R., Indian J. Text. Res. 1979, 4, 106-110. Vigo, T.L., Wade, R.H., Mitcham, R., Welch, C.M., Text. Res. J. 1969, 39, 305316. Vigo, T.L., Mitcham, R., Welch, C.M., /. Polym. Sd. 1970, 8, 385-393. Vigo, T.L., Mitcham, R., Welch, C.M., Text. Res. J. 1972, 42, 96. Wagenknecht, W., Ph.D. Thesis, Academy of Science (GDR) 1976. Wagenknecht, W., Nehls, L, Stein, A., Klemm, D., Philipp, B., Acta Polym. 1992, 43, 266-269. Warwicker, J.O., Jeffries, R., Colbrain, R.L., Robinson, R.N., in The Cotton Silk and Man-Made Fibers Research Association, Didsburg: Shirley Institute Pamphlet, 1966, No. 93. Yamane, J., Mori, M., Saito, M., Okajima, K., Polym. J. 1996, 20, 1039-1047. Yamashiki, T., Kamide, K., Okajima, K., in Cellul. Sources Exploit., Kennedy, J.F., Phillips, G.O. (Eds.), London: Ellis Horwood, 1990, pp. 197-202. Zeronian, S.H., Cabradilla, K.E., /. Appl. Polym. ScL 1973, 77, 539-552.

4.3 Metal Complexes of Cellulose 4.3.1 General routes of cellulose-metal ion interaction Cellulose-metal ion interaction has many features, ranging from the sorption and desorption of calcium ions during wood pulp manufacture and artificial fiber spinning using cellulose cuprammonium hydroxide solutions, to the design and preparation of sophisticated cellulosic materials with e.g. catalytic properties. Two main routes to metal-ion-containing cellulose products have to be considered, i.e. (i) the use of the cellulose backbone as a polymeric carrier of functional groups deliberately introduced by covalent reaction and subsequently employed for the interaction with metal ions; (ii) the engagement of hydroxy groups of the polyhydroxy compound 'cellulose' as a polymer ligand coordinated to a metal cation acting as the center of complexation. A detailed discussion of the first one of these routes would by far surpass the scope of this book, and only two examples will therefore be mentioned briefly for illustration: the binding of Ca2+ or Fe2+ to cellulose powders containing varying numbers of carboxyl groups. The interaction was found to be governed by the concentration ratio of [M2+] to [H+] on the one hand, and the level of carboxyl content on the other, without a full saturation of all carboxylic sites by the metal cation being obtained under the conditions investigated (Jacopian et Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

72

4.3 Metal Complexes of Cellulose

al., 1975). Maekawa and Koshijima (1990) describe the synthesis of a cellulosebased polymer hydroxamic acid via 2,3-dicarboxycellulose, in connection with a subsequent complexation of various transition metal cations to the hydroxamic acid functions. Table 4.3.1. Survey of cellulose-metal complexes.

Type of complex

Medium

Transition metal complexes with NH3 or amine as ligands Transition metal complexes with tartaric acid as ligand Metal hydroxo complexes Neutral salts of special structure (with or without NH3)

Water Water Water Water, dipolar aprotic liquid

The second route with cellulose acting as a polymeric polyhydroxy ligand in metal complex formation will be the topic of this chapter, with the somewhat arbitrary subdivisions employed here being given in Table 4.3.1. The systems considered here are predominantly aqueous ones containing the polymer in the dissolved or highly swollen state. The first three classes of complexes and their routes of formation can be understood by the principles of complex chemistry with cellulosic hydroxy groups in the deprotonated or nondeprotonated state acting as ligands to a central atom, and with ligand-exchange processes playing a dominant role. In the last mentioned case, polymer metal cation interaction is considerably weaker and can be interpreted either as electron donor-acceptor complex formation between the salt and the cellulose, or as participation of cellulosic hydroxy groups in the solvation of the ion dipole of the salt. Subsequently, the chemistry of cellulose-metal complex formation will be presented in some detail, accentuating adequately the scientifically and practically important copper complexes and putting some emphasis on the many still open questions. Effects of supramolecular structure of the polymer on complex formation will be considered in connection with processes of cellulose swelling and dissolution, as many cellulose-metal complexes are well suited to get the polymer to form an aqueous solution. The subchapter on cellulose-metal complex application (4.3.5) is therefore also centered on their action as cellulose solvents, considering especially the commercial cuprammonium process for the manufacture of filaments, fibers and films of regenerated cellulose. This chapter is closed by an outlook on the questions still open and promising routes of future research.

4.3.2 Chemistry of cellulose-metal complex formation

73

4.3.2 Chemistry of cellulose-metal complex formation Cellulose-metal cation interaction represents a broad variety of phenomena regarding type and strength of binding between cellulose hydroxy groups and the metal moiety, type of complex-forming metal atom and its position in the periodic table, as well as the structure of ligands coordinated to the central atom. Most of the knowledge acquired up to now results from studies of cellulosecopper complexes with ammonia or amines as further ligands, which will therefore be treated in a separate section, before than comparing them with other transition metal-amine complexes. Although the complex chemistry of cellulose is primarily concerned with the AGU along the macromolecule and the three hydroxy groups in each of the repeating units, it can be understood and efficiently promoted only in connection with the intra- and intermolecular hydrogen bond system of cellulose and its changes during cellulose-metal ion interaction. Fig. 2.1.6 in chapter 2.1.2 emphasizes this point and is presented here again for the readers convenience (Fig. 4.3.1). cellulose I

cellulose

Figure 4.3.1. Most probable hydrogen bond patterns of cellulose allomorphs (Kroon-Batenburg et al., 1990).

74


4.3.2.1

Copper complexes of cellulose with N-containing ligands

As early as 1857, Schweizer described the dissolution of cellulosic materials in a solution of cupric hydroxide and aqueous ammonia, and this 'Guam-cellulose system' is still the number one among cellulose-metal complexes with regard to practical use and scientific challenge. Due to its technical relevance for artificial cellulose filament production, this system was studied in the first half of this century by numerous groups world wide. It was recognized rather early on that [Cu(NH3)4](OH)2 is the active species, which strongly interacts with the cellulosic hydroxy groups at the C-2 and C-3 positions, and that a small amount of alkali hydroxide promoted dissolution and complexation of the polymer. An excess of alkali resulted in precipitation of a swollen compound, usually described as the Normann compound, with the formula NaJC[Cu(C6H8O5)2] (χ ~ 2). Increasing the copper content in a given system, the cellulose showed at first limited swelling, then partial dissolution, and finally complete dissolution to a homogeneous medium, with the different states of dispersion depending also on NH3 concentration, temperature, polymer-to-liquid ratio, and DP of the cellulose. At a copper concentration of between 15 and 30 g/1 and an ammonia concentration not below 15 %, even high DP cellulose was found to dissolve quickly and completely. The copper atom in the dissolved complex was assumed to be coordinated to the hydroxy groups at C-2 and C-3 on the one hand and to two NH3 molecules on the other. But these early investigations performed by e.g. optical rotation measurements, dialysis, electrolysis or ion exchange, led to a controversial discussion on the real structure of the cellulose-copper complex formed, centering frequently on the question of a cationic or an anionic nature of the polymer-copper containing species. More recent work on the chemical structure of the cellulose-copper complex shall be surveyed by mentioning the ESR study of Baugh et al. (1968) regarding the spatial position of the copper atoms in relation to the polymer chain, and the circular dichroism (CD) measurements of Miyamoto et al. (1996), who related the two cotton effects observed to the state of copper binding, and assumed an equilibrium between copper-substituted monomer units, unsubstituted AGU and copper tetramine cations in the aqueous Cuam-cellulose system, with an average copper-to-AGU ratio of 0.6-0.8. Decisive progress in the elucidation of the complex structure was achieved introducing the techniques and the reasoning of modern inorganic complex chemistry into this area of cellulose chemistry (Kettenbach et al., 1997). Burchardt succeeded for the first time in performing static and dynamic light scattering measurements in the deeply colored cellulose-Cuam system (Burchardt et al., 1994). From the results obtained with low molecular polyols as models, as well as with cellulose itself, it can be concluded that:


75

(i) a very stable polyolato complex is formed by interaction of [Cu(NH3)4](OH)2 with cellulose, with two coordination sites of the copper atom being occupied by the deprotonated O atoms at C-2 and C-3 of the AGU, and the other two sites binding NH3 molecules (see Fig. 4.3.2), approaching a degree of complexation of nearly 100 % of the AGU with decreasing diol concentration in the system;

--HO m Cu(NH3Jn(OH)2 -2m H2O, -m (n-2)NH3 H3N

NH3

Figure 4.3.2. Scheme of Cuam-cellulose complex structure (Burchardt et al., 1994). (ii) even at high copper and NH3 concentration, a small amount of a copper bisdiolato complex (see Fig. 4.3.3) can be formed, resulting in intra- or intermolecular crosslinking, which is favored by a high OH~ ion concentration and can finally lead to precipitation of compounds of the Normann type; (iii) despite the breaking of the major part of the hydrogen bonds existing in the starting polymer, during complexation and dissolution, strong hydrogen bonds of the type OH--·Ο~ can be formed between the primary hydroxy group at C-6 and the deprotonated O-2 atom of an adjacent AGU without interfering with the chain conformation, thus leading to an increased chain stiffness.

76


The complexation of one copper atom and two NH3 ligands to each diol unit leads to an increase in molar mass of these AGUs from 162 to 258. The remarkable chain stiffness of the complex is reflected by a Kuhn segment length of 25.6 nm corresponding to about 50 monomer units (compared with about 2 nm and 10 monomer units in the case of a polystyrene chain). It is interesting to note that in a dilute Cuam-cellulose solution, the increase in radius of gyration and in hydrodynamic radius of the polymer coils with DP is slowed down, obviously due to some 'back-coiling', via formation of intramolecular bisdiolate (cuprate) crosslinks (Burchardt et al., 1994). The gel formation observed with celluloseCuam solutions of higher concentration after a long residence time may possibly be traced back to the same causes on the intermolecular level. Cellulose complexed and dissolved by Guam is rather susceptible to oxidative chain degradation, with nitrite ions formed from the ammonia present acting as the active intermediate in oxygen transfer.

π 2+

a)

-NH 2 O\ ^y

CH

Cu

b) CH 2 O

c)

\

^NH

-CH I -CH

*HC-°\ /°-CH' I

HC-

Cu \

cellulose chain

ι 2-

-CH

Figure 4.3.3. Structures a) ethlene diamine copper complex, Cuen, b) Cuen cellulose complex, c) cellulose cuprate (Burchardt et al., 1994). Other copper complex-based aqueous cellulose solvents can be prepared with ethylene diamine (en) or 1,3-diaminopropane (pren) as ligands to the central copper atom. These systems show similarities to the Guam system in so far as diolato copper complexes with the hydroxy groups at C-2 and C-3 are formed here too, but also some differences exist regarding complex structure and bind-


77

ing strength: the Cuen system is prepared by dissolving cupric hydroxide in just the sufficient amount of aqueous ethylene diamine to form the complex [Cu(Cn)2](OH)2 as the active species, while the Guam solution always contains a large excess of ammonia. This solvent, first described by Traube (1911), is somewhat less efficient at complexing cellulose than Guam, as a higher amount of copper per AGU is required, obviously due to the fact that the bidentate Iigand ethylene diamine forms a stable five-membered ring with the central atom and therefore exhibits some resistance to ligand exchange, with the C-2/C-3 diol structure of the AGU necessary for complexation and dissolution of the cellulose. But also here the formation of a heteroleptic copper complex with a bidentate diolato ligand on the one hand, and a bidentate ethylene diamine ligand on the other, can be assumed to be the driving force for cellulose dissolution. Also here, the formation of bisdiolato crosslinks (cuprate structures) with the elimination of ethylene diamine as a ligand must be taken into account (see Fig. 4.3.3). Cellulose degradation in Cuen was observed to be much less than in Guam. An efficient copper complex-based cellulose solvent described by Gadd (1982) is obtained by dissolving freshly precipitated Cu(OH)2 in a slight excess of 1,3-diaminopropane, resulting in a copper cation complexed by two bidentate diaminopropane units as the prevailing active solvent species. The sixmembered rings formed in this ligand coordination are less stable than the fivemembered ones in the case of ethylene diamine, and a ligand exchange with a partially deprotonated diol structure of the AGU can take place rather easily, as indicated by the scheme in Fig. 4.3.4. 2+

CH 2 OH

H2N

^ Θ

+20Η HO

OH

H2N

NH2

H2N

NH2

6 CH 2 OH 5>-0 Ο--θ

+

+ H2O

O OH \ / Cu / N H2N'

YlH 2

Figure 4.3.4. Scheme of cellulose 1,3-diaminopropane copper complex structure (Gadd, 1982).

78


A copper-to-AGU ratio of 1 : 1 was observed above a limiting copper concentration, and at a high pH formation of a bisdiolato complex without residual diamine ligands is reported here also. Gadd (1982) emphasizes the importance of an adequate stability of the primary homoleptic cationic complex, permitting a partial but not a complete ligand exchange with the diol units of cellulose, and he emphasizes further the necessity of an at least partial deprotonation of these diol units. With ethanolamine as a ligand to copper, obviously no cellulose solvent system can be realized due to the very strong binding of the deprotonated ligand to the central Cu atom, which impedes subsequent ligand exchange with the cellulosic diol moiety. Finally, the Cu(OH)2-biuret-alkali complexes shall be mentioned briefly, which have been described as solvents for cellulose already by Schiff (1898) and later were thoroughly investigated by Jayme and Lang (1957). Employing a molar ratio of 1 Cu to 2 biuret the best results on cellulose dissolution were obtained with KOH as the alkali added, and a highly viscous, clear violet solution with a concentration of up to 8 % cellulose of a DP of about 800 could be prepared. A probable formula for the active species is presented in Fig. 4.3.5. O H H Οθ \\ \ / / C-N N= C / \ / \ HN Cu NH \ / \ / C= N N—C 4θ / \ \\ . Ο H H O _

2 K®· 4 H 9 O

Figure 4.3.5. Proposed structure of the cellulose-biuret-copper complex (Jayme and Lang, 1957).

Structural details of the cellulose complex formed in the solvent have not been published so far, but a ligand exchange with formation of a diolato complex seems to be probable also here.

4.3.2.2

Other aqueous cellulose solvents based on transition metal-amine complexes

The chemistry of cellulose-metal complexes received an important impetus in the middle of this century, when Jayme and his group (Jayme, 1971) discovered numerous new cellulose solvents based on cationic complexes of zinc, cadmium, cobalt and nickel, with ethylene diamine or ammonia as ligands. An overview of these solvents and their active species is presented in Table 4.3.2.


79

The efficiency of these solvents is mostly lower than that of the copper-based ones, and a higher metal concentration is required to get e.g. a dissolving pulp into solution. So, for example, a cobalt concentration of about 70 g/1 or a zinc concentration of about 80 g/1 are mentioned as optimal in the publications of Jayme's group, compared with about 15 g copper in the case of a Guam solution. The solvent power of Nioxam was found to increase with the nickel as well as with the NH3 content of the system, ca. 1.5 % Ni (at high ammonia concentration) and 15 % NH3 (at high nickel concentration) representing the minimum values for dissolution (Jayme and Neuschäffer, 1955). From a cellulose solution in Nioxen, a precipitate with a Ni-to-AGU ratio of 0.87 and an ethylene diamine-to-Ni ratio of up to 2.7 could be isolated by precipitation with /i-propanol (Jayme and Neuschäffer, 1955), indicating a binding of about one Ni cation from the solution to each monomer unit of cellulose. According to Hoelkeskamp (1964) no real complex formation occurs with Nioxen. More detailed studies on the relation between solvent preparation and composition, and the solvent power, as well as on the mode of cellulose-solvent interaction have been performed with Cadoxen, which, as a colorless solvent of rather high solvent power, found wide application in the analytical characterization of cellulosic products. Figure 4.3.6 illustrates the dependency of solvent power on solvent composition and Cd starting compound. Table 4.3.2. Transition metal complex solvents for cellulose.

Solvent

Active species

Guam Cuen Cupren Pd-en Cooxen Zincoxen Cadoxen Nioxam Nioxen Nitren

[Cu (NH3)4](OH)2 [Cu (H2N-(CH2)2-NH2)2](OH)2 [Cu (H2N-(CH2)3-NH2)2](OH)2 [Pd (H2N-(CH2)2-NH2)](OH)2 [Co (H2N-(CH2)2-NH2)2](OH)2 [Zn (H2N-(CH2)2-NH2)2](OH)2 [Cd (H2N-(CH2)2-NH2)3](OH)2 [Ni (NH3)6](OH)2 [Ni (H2N-(CH2)2-NH2)3](OH)2 [Ni (NH2CH2CH2)3N](OH)2

Regarding the binding of Cd to cellulose, Jayme originally presumed by analogy to Cuam or Cuen, a complex interaction with the C-2 and C-3 hydroxy group of the AGU. From conductivity measurements in dependence on cadmium and cellulose concentration, a rather strong binding of Cd to cellulose was concluded with a sterically feasible sorption of 2 Cd/3 AGU (Hugglins, 1987). Dialysis experiments, on the other hand, as well as 13C and 113Cd measurements, (Bain et al., 1980) and 13C NMR studies (Nehls et al., 1995), led to the conclu-

80


sion that Cadoxen is a noncoordinating cellulose solvent forming no chelate complex with the diol moiety of the AGU, but interacting with cellulose according to an acid-base principle similar to aqueous alkali. This view is corroborated by the increase in solvent power observed on enhancing the Cd concentration and on adding some NaOH to the system. This is confirmed in a recent publication (Burger et al, 1995), who assumes for all the transition metal-amine complex solvents listed in Table 4.3.2, not a diolato complex formation with cellulose, but rather an acid-base interaction similar to alkali-cellulose formation, with an additional chain separating effect of the voluminous aminecomplex cation persisting as a homoleptic cationic complex in the system.

80

•"60

Initial cadmium compound Cellulose Cadmium Cadmium hydroxide solubility oxide low inchloride Completely soluble Partially D soluble Insoluble D

Basic cadmium chloride

Area in which lie the compositions that dissolv« cellulose at 2O 0 C

.2

l2-20 6

8 10 12 Cadmium content [wt%]

U

16

Figure 4.3.6. Solubility of cellulose in Cadoxen solutions of various compositions (Jayme, 1971).

The same publication describes two new transition metal complex solvents which really dissolve the cellulose by formation of a diolato complex: a solvent system composed of nickel nitrate with a slight molar excess of tris-2aminoethylamine ftren') and the double molar quantity of NaOH-dissolved cellulose to a blue viscous solution under formation of a diolato complex with the structure presented in Fig. 4.3.7. This heteroleptic complex with a tetradentate amine ligand, exhibited striking differences to the cellulose Nioxen system, for example the Ni-cellulose-tren complex was decomposed by addition of ethylene diamine with precipitation of cellulose due to the preference of nickel for amine ligands. The Ni-tren system proved to be an effective solvent also for other polysaccharides, e.g. amylose or chitosan.


81

NH 5

oleum > ClSO3H > SO2Cl2 > CH3-CO-SO4H > H2NSO3H. The SO3-DMF complex, oleum with 33 % SO3 or 66 % SO3, and especially a high-quality chlorosulfonic acid, act very fast in these esterifications and the reaction is nearly completed within half an hour even at O0C. This high reaction rate can cause problems with regard to product uniformity, due to an uneven reagent distribution, if larger charges of the viscous cellulose acetate solution with a polymer content between 10 and 20 % are to be processed (Wagenknecht and Schwarz, 1996) to a low degree of sulfation. Acetylsulfuric acid exhibits a more moderate reactivity but still higher than that of amidosulfonic acid. The latter proved to be a very convenient sulfating agent at a reaction temperature between 50 and 80 0C (see the DSs-time plot in Fig. 4.4.10). A conceivable amination at the cellulose chain by this reagent has never been observed in employing amidosulfonic acid for sulfation of cellulose acetates. As an essential point of cellulose sulfate synthesis with all these reagents, the requirement of a strictly anhydrous medium with a residual water content lower than 0.05 % must be emphasized. As demonstrated by Fig. 4.4.11, summarizing the results of sulfation experiments with cellulose-2,5-acetate in DMF and SO3 or ClSO3H, the acetyl group really acts as a reliable protecting group, as the DS$, with values between 0.4 and 0.5, does not exceed the amount of free hydroxy groups even at a large excess of sulfating agent.

4AJ Esters of cellulose with inorganic acids

121

(U 0.3

ω 0.2 Q

0.1

0

1 2 3 Reaction time [h]

4

5

Figure 4.4.10. Course of sulfation of cellulose-2,5-acetate with amido sulfonic acid at 50 0C with 0.5 (O) and 2,5 (D) mol/mol AGU and at 80 0C with 0.5 (·) and 2.5 (·) mol/mol AGU (Wagenknecht, 1991a). 0.5 0.3 0.1

0.2

0.6

1.2

Mol sulfoting agents / mol AGU

Figure 4.4.11. DS§ of cellulose sulfate from cellulose acetate (DSp^c = 2.4) in dependence on molar ratio of agent (SO3, ClSO3H) per AGU (Philipp et al., 1990). The effective deacetylation of the cellulose acetate sulfate obtained by a solution of NaOH in ethanol is confirmed by the data plotted in Fig. 4.4.12, demonstrating also the stability of the cellulose sulfate in this alkaline medium. In the practical procedure of deacetylation and subsequent cellulose sulfate processing, a flocculated, loose structure of the primary precipitate of cellulose acetate sulfate is essential for the following purification by washing. This is favorably achieved by a stepwise addition of aqueous sodium acetate solution or, in the case of a higher ratio of sulfate to acetyl groups, with a mixture of acetone and ethanol as the precipitant, while a direct addition of NaOH in ethanol to the reaction system leads to a hard, dense structure of Na-cellulose sulfate resisting further purification. After the heterogeneous deacetylation with NaOH in ethanol the Na-cellulose sulfate is washed free of low molecular salts with ethanol. As a further advantage of amidosulfonic acid as sulfating agent in comparison with e.g. SO3, the much higher solubility of sodium amidosulfonate in comparison with Na2SU4 shall be mentioned. These somewhat detailed considerations on precipitation, deacetylation and purification of the Na-cellulose sulfate may

122

4.4 Esterification of Cellulose

serve as an example of the great importance of a suitable processing procedure of the reaction system after a homogeneous derivatization reaction of cellulose.

O

40 80 Reaction time [min]

Figure 4.4.12. DS^C (·) and DS$ (·) dependence on time of deacetylation (4 % ethanolic NaOH, 20 0C) of cellulose acetate sulfate (Wagenknecht, 1991). Regarding the distribution of sulfuric acid half-ester groups within the AGU, a highly significant influence of the sulfating agent was observed in the low DS range up to about 0.3, if a commercial cellulose-2- or 2,5-acetate with a rather statistical substituent distribution, i.e. about equal amounts of free hydroxy groups in the positions C-2, C-3 and C-6, served as the starting material. As demonstrated by the data in Table 4.4.10, a preferential O-6 sulfation is observed with chlorosulfonic acid, acetylsulfuric acid and amidosulfonic acid, while an O-2 sulfation prevails with 803 at low DS. At a higher DS§ of about 1, sulfation with 803 results in an approximately equal distribution of sulfate groups to all three positions, starting from a cellulose acetate with a DS of 1.8. Employing cellulose triacetate samples regioselectively deacetylated in positions 2 and 3, and a sufficiently high input of sulfating agent, regioselectively functionalized cellulose-2,3-sulfate can be prepared with the C-2 position completely occupied by sulfate groups and the C-3 position sulfated to about 50 % with regard to free hydroxy groups, while the O-6 position is more or less completely protected by the acetyl groups still present (see Table 4.4.10). SO3 at 20 0C or amidosulfonic acid at 80 0C proved to be favorable sulfating agents here, avoiding a chain degradation nearly completely in the case of amidosulfonic acid.

4.4.1 Esters of cellulose with inorganic acids

123

Table 4.4.10. Sulfation of statistically (S) and regioselectively (R) deacetylated cellulose acetate samples (Philipp et al., 1995).

Cellulose acetate Type DS AC S

2.38

R R R

2.64 1.86 1.48

Sulfating agent Agent mol/ mol AGU SO3 0.4 ClSO3H 0.5 H2NSO3H 0.5 H2NSO3H 1.0 H2NSO3H 1 H2NSO3H 2 H2NSO3H 3

DS8

Partial DSS in position C-2 C-3 C-6 % in C6

0.35 0.22 0.35 0.52 0.25 0.95 1.15

0.20 0.04 0.11 0.17 0.17 0.55 0.74

0.0 0.0 0.04 0.15 0.08 0.20 0.15

0.15 0.18 0.20 0.20 0.0 0.20 0.26

43 82 57 38 O 21 23

A convenient route to regioselectively or preferentially C-6-substituted cellulose sulfates, employing also the cellulose acetate sulfate as intermediate, consists of the competitive esterification of cellulose suspended in DMF with a mixture of acetic anhydride and SOß or CISOßH, and a subsequent deacetylation with NaOH in ethanol. After preparation at room temperature, the system is heated to about 50 0C, and esterification takes place during 30 min to 4 h with gradual and finally complete dissolution of the polymer. The DS$ obtained after deacetylation depends primarily on the molar ratio of sulfating agent to acid anhydride and can reach an upper value of about 1.5. An exclusive sulfation of the C-6 position was indicated by the 13C NMR spectrum up to a DS§ of about 0.8, CISOßH showing a somewhat higher regioselectivity than 803. For a reliable control of DS§, an input of about 8 mol of acid anhydride/mol of AGU and an appropriate adjustment of the amount of sulfation agent added was found to be favorable. The results of some of these experiments are summarized in Table 4.4.11. The sodium cellulose sulfates, prepared via acetosulfation, were completely water-soluble at and above a DS of 0.3. This somewhat higher minimal DS$, as compared with samples obtained in a strictly homogeneous course of reaction from partially substituted cellulose acetates, is obviously caused by a less uniform sulfate group distribution along and between the polymer chains due to the initially heterogeneous reaction system. On the other hand, this route of acetosulfation permits the synthesis of Na-cellulose sulfates of higher solution viscosity (about 200 mPa s in 1 % aqueous solution) with a high DP cotton !inters (1400) as the starting material. The solution viscosity of samples prepared from partially substituted cellulose acetate is limited to less than 15 mPa s due to the rather low DP (about 250) of the starting material.

124


Table 4.4.11. Acetosulfation of !inters cellulose (Philipp et al, 1995).

mol of AC2Ü/ mol of AGU 16 8 8 8

mol of ClSO3H/ mol of AGU 1.4 0.7 2 3

DS5

0.20 0.50 0.75 1.30

O-6 esterification (%) 100 95 95 58

The exact mechanism of this acetosulfation with a transition from the heterogeneous to the homogeneous system is not yet clear. The mechanistic concept of dissolution acetylation with £[2804 as the catalyst and the intermediate introduction of some sulfate groups obviously cannot be transferred to all routes of acetosulfation due to the other ratio of acetyl to sulfate groups (see chapter 4.4.3). We assume in our system of acetosulfation a rather fast reaction of easily accessible C-6 hydroxy groups, combined with a gradual esterification of hydroxy groups in all the three positions by acetanhydride. As already indicated above, the protecting action of ester or ether groups already present, together with a sufficiently large number of free hydroxy groups, can be used to synthesize various sulfated ethers and esters of cellulose even with a regioselective pattern of substitution, and can thus provide routes to new doubly functionalized cellulose derivatives with interesting applicational properties. The cellulose acetosulfates frequently cited above showed a high water binding capacity and were successfully tested as sanitary supersorbers. Doubly modified derivatives with interesting surfactant properties were obtained by us by sulfating the 6-position of a 2,3-0-laurylcellulose or preferentially the 2,3position of predominantly 6-O-tosyl or -tritylcelluloses, employing 803 as the sulfating agent and DMF or pyridine as the reaction medium. The sulfation of CMC in the DS range of 0.5-2.0 in the usual manner, i.e. with SO3 in a dipolar aprotic solvent under homogeneous conditions, posed problems due to an only minimal solubility of the polymer in these solvents. These difficulties could be overcome, however, by presenting the CMC in a very fine dispersed state to the reagent, either by previous dissolution in water, subsequent precipitation with an excess of DMF and elimination of the water by azeotropic distillation, or still better by preparing a highly swollen slurry of CMC in a mixture of dimethylacetamide and /?-toluenesulfonic acid prior to sulfation with SO3 in this system (Vogt et al., 1995 and 1996). Sulfation of cellulose via displacement of labile ester or ether groups In contrast with the acetyl group with its well-established protecting action against sulfating agents in a dipolar aprotic medium, the very labile nitrite group


125

is displaced rather easily by various sulfating agents from its position in the AGU. In this way, a reaction system of cellulose dissolved with an excess of N2O4 in DMF (> 3 mol of N2O4/mol of AGU) to a cellulose trinitrite and containing an excess of N2O4 and HNO3 as further components, can be directly sulfated without isolation of the cellulose trinitrite (Schweiger, 1974; Wagenknecht et al., 1993) according to the scheme in Fig. 4.4.13 by the sulfating agents indicated. SO3

CeII-OSO3H

+ N2O4

CeII-OSO3NO SO2

NOSO4H

Cellulose/N2O4/DMF (excess of N2O4, HNO3)

CISO3H

SO2CI2

H2NSO3H

CeII-OSO3H

+ N2O3

CeII-OSO 3 NO+ HNO2 ΓΏΙΙ Ueil

OCO M U WoVJ 3

-ι- iNvJUl ΜΟΓΜ +

CeII-OSO2CI + NOCI r*aii_r>cr> u

j. M t α. H O 2

Figure 4.4.13. Scheme of possible reactions in the system cellulose/N2O4/DMF on addition of different sulfating agents (Wagenknecht et al., 1993).

SO2 here reacts via an intermediate formation of NOSO4H with the HNO3 present in the system. The DS$ obtained depends, under comparable external conditions, significantly on the sulfating agent employed and covers a range between 0.3 with NOSO4H and 1.6 with SO2Cl2, with this large difference obviously been caused by a different position of the transesterification equilibrium. With all the sulfating agents studied except H2SO4, water-soluble Na-cellulose sulfates were obtained above a DS$ of 0.2-0.25 after elimination of residual nitrite groups by hydrolysis in a protic medium and subsequent neutralization and purification of the cellulose sulfate half-ester (Wagenknecht et al., 1993). By minimizing hydrolytic chain degradation during this product processing, cellulose sulfates with very high solution viscosity, up to 2500 mPa s (1 % aqueous solution), could be synthesized from cotton !inters, DP 1400. Due to a site-selective transesterification reactivity of the nitrite groups in dependence on sulfating agents and reaction temperature, a wide variety of substitution patterns of Na-cellulose sulfates can be realized, covering the range from 100 % C-6 substitution with NOSO4H, down to < 20 % in the case of SO3 at low reaction temperature (see Table 4.4.12).

126

4Λ Esterification of Cellulose

Table 4.4.12. Regioselectivity in sulfation of cellulose trinitrite (Wagenknecht et aL 1993).

Sulfating agent

NOSO4H H2NSO3H SO2Cl2 SO3

Conditions of reaction mol agent/ Time Temp. mol of AGU (h) (0Q 4 2 20 2 3 20 2 2 20 2 3 20 2 -20 1.5

Total DS by NMR 0.35 0.40 1.00 0.92 0.55

Partial DS by NMR C-2 C-3 C-6 0.04 0.10 0.30 0.26 0.45

-

0.31 0.30 0.70 0.66 0.10

A peculiar influence of the reaction temperature on substituent distribution is observed with 803 as the sulfating agent, leading to a rather exclusive sulfation of secondary position at O-2 at low temperature, while at room temperature the O-6 position is rather strongly preferred (see Table 4.4.12). Also, an addition of small amounts of water at the end of the sulfation reaction was found to favor O-2 sulfation (Wagenknecht et al., 1993). By these results, former controversies between our findings and an earlier publication (Schweiger, 1979), who for the first time prepared cellulose sulfates via the cellulose nitrite system, could be completely reconciled. As discussed in detail in Wagenknecht et al. (1993), the site-selectivity of transesterification from cellulose nitrite to cellulose sulfate can be widely understood by the two counteracting effects of a high spatial accessibility of the C-6 nitrite group and a high intrinsic reactivity of the C-2 nitrite group. In contrast with alkyl ethers, trialkylsilyl ether groups (see chapter 4.5) are readily displaced from the cellulose chain by 803 or ClSC^H without a simultaneous sulfation of free hydroxy groups present in the AGU. The reaction is completed with trimethylsily!cellulose samples of a moderate DS^ of between 1 and 2. DMF can be used as the solvent and reaction medium, with the subsequent precipitation of the reaction product by addition of THF, elimination of residual ether groups and neutralization of the sulfuric acid half-ester groups by NaOH in EtOH and purification of the Na-cellulose sulfate by washing with EtOH. The Na-cellulose sulfates prepared along this route are completely soluble in water above a DS§ of 0.2, if the sulfation was started with a clear, gel-free trimethylsilyl (TMS)-cellulose solution, and very high solution viscosities of the end product, up to 2000 mPa s (1 % aqueous solution), can be realized due to the preservation of the high initial DP during silylation and sulfation. A modified procedure of synthesis can be performed by silylation with TMS chloride of an ammonia-activated cellulose, evaporation of the ammonia and subsequent sulfation without isolation of the TMS-cellulose in the solid state, but the benefits


127

of this simplified synthesis have to be repaid by additional efforts in processing and purifying the reaction products (Wagenknecht et al., 1992b). The DS§ of the final product depends of course on the molar ratio of sulfating agent to AGU, but is limited by the level of the previous silylation, and was not found to exceed the DS^. Figure 4.4.14 illustrates this by the results of sulfation of two TMS-cellulose samples of different 2.5

«n 1.5 ω

Q

0.5

O

2 A 6 8 10 MoI sulfcrting agents / mol AGU

Figure 4.4.14. DS$ dependence on the amount of sulfating agent (SOs ^ ·; ClSOsH ·) during homogeneous sulfation of TMS-cellulose with DS 1.5 and 2.4 at 20 0C, 3 h (Wagenknecht et al., 1992b, reprinted with permission from Elsevier Science). With highly substituted silylcelluloses, DS$ values of 2.5 and higher could be realized, but a definitely trisubstituted cellulose sulfate has not yet been prepared along this route. In the sulfation of TMS-cellulose, a remarkable preference of the C-6 position is generally observed, resulting in an exclusively C-6-substituted cellulose sulfate up to a DS of 0.95, with chlorosulfonic acid as sulfating agent. At higher DS values also the C-2 position, and to a smaller extent additionally the C-3 position, is occupied by sulfate groups. Addition of pyridine as a weak base to the reaction mixture on sulfation results in a decrease of DS§ and a somewhat more pronounced esterification of the C-2 position. Regarding the mechanism of silylcellulose sulfation, the trialkylsilyl ether group definitely acts as a leaving group. Even under rather mild conditions, i.e. on sulfation with amidosulfonic acid in the presence of an excess of TEA, no protecting action of the silyl groups could be detected, as observed in the esterification of TMS-cellulose with carbonic acid chlorides (see chapter 4.4.3 and 4.5), and again the O-6 position was preferentially esterified in 2,6-O-TMS-cellulose of DS^ of 1.5. Still an open question remains, as to why the DS$ was never found to exceed the DS$i due to sulfation of residual free hydroxy groups in moderately high substituted silylcelluloses. Some kind of 'steric shielding' of the hydroxy groups by the large voluminous reaction complex between the trialkylsilyl groups and the sulfating agent could be discussed as a possible cause. Concerning the interaction between the trialkylsilyl ether group and 803, a primary cellulose derivative con-

128


taining approximately equal molar amounts of sulfur and silicium has been isolated from the reaction mixture under anhydrous aprotic conditions. From the analytical data obtained (Wagenknecht et al, 1992b; Nehls, 1994) we assumed an insertion reaction of 803 between the cellulose chain and the trialkylsilyl group with the primary formation of the cellulose silylsulfate with a subsequent splitting off of the silyl moiety as a trialkylsilanol, rapidly forming hexaalkyldisiloxane. This route of reaction is known from low molecular analogues (Bott et al., 1965). The sulfuric acid haifester groups are rather stable in an aqueous alkaline medium, but are saponified much faster in an aqueous or alcoholic acid milieu. A preferential loss of sulfate groups at C-2 has been observed in the "methylation analysis" of cellulose sulfates (Gohdes et al., 1997). Just as with other cellulose-related polysaccharides, xylans can be converted to sulfate half-esters too. With a beech wood xylan (DP ~ 140, 80-83 % pentosan content) dissolved after appropriate activation in the ^04/DMF system, a DS§ of 0.2 was obtained with SÜ2 (via NOSC^H formed in situ) and a DS$ of 0.55 reached with 803 as the sulfating agent (Philipp et al., 1987). Summary of routes to cellulose sulfates with defined patterns of substitution The following Table 4.4.13 gives an overview of the various procedures for synthesizing cellulose sulfates with a defined pattern of functionalization within the AGU including the range of DS§ realized. Table 4.4.13. Overview of routes to regioselectively functionalized Na-cellulose sulfates.

Site of sulfation

Intermediate

Sulfating agent

C-6

Nitrite Silyl ether Cellulose Nitrite Nitrite Silyl ether Acetate

NOSO4H; SO2 ClSO3H Ac2O + ClSO3H SO3 SO3; SO2Cl2 SO3;C1SO3H SO3; H2NSO3H

C-2 C-6/C-2

C-2/C-3

Range of DS$ realized 0.3-0.6 0.3-1.0 0.3-0.8 0.3-1.0 1.0-2.0 0.5-2.0 0.3-1.4

Properties of cellulose sulfates Cellulose sulfuric acid half-esters in the acid form (H+ form) can be isolated from the appropriate reaction mixture as a white hygroscopic mass soluble in water and in rather polar organic liquids at a DS above 0.2-0.3. Due to the strongly acidic character of the SOßH groups the products are unstable in the


129

solid state as well as in solution, as they are susceptible to a fast 'autohydrolytic' chain degradation and a splitting off of the half-ester groups. The stable form of cellulose sulfuric acid half-esters generally employed in application is the sodium salt, a white, odorless and tasteless powder that can be transformed to clear films via an aqueous solution, and which exhibits a good thermal stability up to 150 0C for a short time, and up to 100 0C for a longer time, in the purified, acid-free state. In dependence on DP, Na-cellulose sulfates are completely water-soluble above a DS of 0.2-0.3, the limiting value depending on the uniformity of substituent distribution along and between the polymer chains, in consequence of the procedure of synthesis. Furthermore, a predominant C-6 substitution obviously favors solubility as compared with the positions C-2 and C-3. For water-soluble Nacellulose sulfates in the low DS range, between 0.2 and 0.45, an [i]]-Mw relationship of [77] = 1.365 x 10'2 M^Y0-94, with [77] given in ml/g, has been reported by Anger et al. (1987). Very high solution viscosities of up to about 5000 mPa s for the 1 % aqueous solution have been reported by Schweiger (1979) for Nacellulose sulfate samples in the low DS range, between 0.3 and 0.5, prepared via cellulose nitrite from a high molecular cellulose (cotton) avoiding significant degradation, while in the DS range above 1, only viscosities of about 1000 mPa s were realized by the same author. In our work, solution viscosities up to 2000 mPa s were measured with samples prepared from cotton !inters via cellulose trinitrite or TMS-cellulose, while with wood dissolving pulps as the starting material, the solution viscosity of comparable samples did not exceed a value of about 500 mPa s. Na-cellulose sulfate solutions of higher concentration exhibit pseudoplastic (thixotropic) behavior, with the thixotropic effect increasing with decreasing DS. Aqueous solutions of Na-cellulose sulfate show a remarkably good resistance against thermodegradation and shear degradation (Schweiger, 1979). A viscosity reduction of only 25 % was supported after 25 h of thermal treatment at 100 0C; and chain degradation on continuous shearing proved to be much less than with other polysaccharides, including conventional cellulose ethers. A special rheological phenomenon observed with aqueous solutions of Na-cellulose sulfates is the formation of thermoreversible gels (Holzapfel et al., 1986; Dautzenberg et al., 1994), first reported (Schweiger, 1972) with high DS samples in the presence of potassium ions. According to our results (Dawydoff et al., 1984) aqueous Nacellulose sulfate solutions with a polymer content of 1 % and a DS between 0.25 and 0.40, form stiff thermoreversible gels with a melting point of about 65 0C in the presence of 2 % KCl after isolating the Η-form of the ester by water-free methanol from the homogeneous N2O4/SO2/DMF medium. For the samples of very low DS, between 0.15 and 0.20, thermoreversible gels can be obtained with+ out addition of K by dissolving the polymer in hot water and subsequent cooling of the 1 % solution (Philipp et al., 1985).

Tertiary oil recovery Oil-well drilling Paints Paper Textiles Explosives Photography Cosmetics Toothpaste Food

+ + + + + +

+

+

+

+

+

Viscosity Pseudoplasticity and yield point

+

+ +

+ +

+

Solubility

+

+

+

+

Enzyme Shear resistance resistance

+

+

+ + + +

+ + +

+

Temp- Suspenerature sion stability stability

+ +

Film formation

+ +

Crosslinking film

Table 4.4.14. Cellulose sulfate: relationship between certain properties and applications (Schweiger, 1979).

+

+

+

+ +

+

Cross- Protein linking reacsolution tivity

+

Solvent tolerance


131

Na-cellulose sulfate behaves as a strong polyelectrolyte. The aqueous solution shows a considerable compatibility with some organic liquids, e.g. lower aliphatic alcohols, which increases somewhat with the DS in the range between 0.3 and 1.0. Na-cellulose sulfate samples with a DS between 0.3 and 1.5 are not precipitated from their aqueous solution by mono-, di- or trivalent metal cations. As an anionic polyelectrolyte, Na-cellulose sulfate forms polyelectrolyte complexes, including insoluble polysalts with cationic polyelectrolytes (Philipp et al., 1989), and also with the cationic sites of proteins. In the presence of a sufficiently large amount of hydrophobic acetyl groups, i.e. in the case of Nacellulose acetate sulfates, the water solubility of the product gets loss, but a remarkably high water-binding power of up to 1000 % and more still remains, unfortunately, however, combined with a low salt tolerance due to the ionic character of the hydrophilic sulfate groups. Na-cellulose sulfate can be degraded by cellulolytic enzymes or cellulaseproducing microorganisms up to a DS of about 1, the limiting value depending somewhat on the uniformity of substituent distribution along the polymer chains (Schweiger, 1972). High-purity Na-cellulose sulfates, free of acid residues and toxic heavy-metal ions, definitely exhibit no cytotoxicity (Dautzenberg et al., 1985a; 1996a and 1996b). In line with other sulfate-group-bearing polyelectrolytes, Na-cellulose sulfate can show biological activity on interaction with human blood (heparinoid effects) (Okajima et al., 1982). For further details on the properties of cellulose sulfates the reader is referred to the comprehensive overview given by Schweiger (1972). Application of cellulose sulfate Up to now, Na-cellulose sulfate has not been a commodity derivative of cellulose but is still a specialty despite numerous promising areas of application. This may be caused mainly by the fact that technologically, economically and ecologically feasible routes of synthesis, avoiding excessive chain degradation, have not been published before about 1980, while a broad variety of water-soluble cellulose ethers had already established its market. An overview of possible areas of application in relation to product properties has been published (Schweiger, 1972) and is presented in Table 4.4.14. The numerous areas of application already tested or proposed can be systematized according to (i) film-forming properties of Na-cellulose sulfates; (ii) special rheological effects of Na-cellulose sulfates in aqueous solution; (iii) behavior of Na-cellulose sulfates as anionic polyelectrolytes; (iv) biological activity of Na-cellulose sulfate. The film-forming properties of Na-cellulose sulfates have been proposed for application in coatings, especially in the paper industry, taking into account the possi-

132


ble modification by subsequent crosslinking of the highly accessible, free hydroxy groups by conventional crosslinking agents for cellulose, e.g. formaldehyde. The high solution viscosity of adequately synthesized Na-cellulose sulfates in water means these products are recommended as thickeners and viscosity enhancers in many industrial and domestic areas, and the high efficiency of these solutions in stabilizing suspensions of e.g. TiU2 has been emphasized. Their gelforming properties in connection with nontoxicity and good compatibility with other polysaccharides make cellulose sulfates of an appropriate DS well suited for preparing thermoreversible gels as required e.g. in microbiology, either as a single component or in a gel blend with other polysaccharides. The anionic component in polyelectrolyte complexes of Na-cellulose sulfate finds promising applications in the membrane area: pervaporation membranes composed of a low DS cellulose sulfate with a special substitution pattern and polydimethyldialylammonium chloride have combined mechanical stability, high flux rate and good selectivity in the separation of lower aliphatic alcohols from their mixture with water (Richau et al., 1996). The interface reaction between Nacellulose sulfate in aqueous solution and a solution of a suitable cationic polyelectrolyte can be used to encapsulate biological materials under quasi-physiological conditions without impeding their biological activity, as shown in comprehensive investigations (Dautzenberg et al., 1985a) with enzymes, living cells, microorganisms or cell organelles. Interaction between Na-cellulose sulfates and proteins can be employed to enhance the viscosity of these system and/or to separate special proteins from aqueous solution (Schwenke et al., 1988). Last but not least, the heparinoid action (i.e. anticlotting activity of human blood) of special Na-cellulose sulfates must be mentioned here. Heparinoid activity was observed (Okajima et al., 1982) with highly substituted products and shown to depend especially on a high degree of substitution in the C-2/C-3 position also at moderate total DS$ (Klemm et al., 1997). Table 4.4.15 demonstrates this with some results. Table 4.4.15. Anticlotting activity of Na-cellulose sulfates (NaCS) with different patterns of substitution (25 μg of NaCS/ml of blood) (Klemm et al., 1997).

Total DSS

0.95 0.95 1.14 Blank experiment

Partial C-2 0.00 0.30 0.74

TT Thrombin time. PTT Partial thromboplastin time.

D5§ at position: C-3 C-6 0.00 0.95 0.30 0.35 0.09 0.31

TT (S)

18.9 29.0 > 600.0 17.5

PTT (s) 80.80 136.5 > 600.0 35.0


133

Similar anticlotting effects are known for xylan sulfates with a DS$ of 1.5-2 (Kindness et al., 1979, 1980; Philipp et al., 1987; Stscherbina and Philipp, 1991). Also, some other polysaccharide sulfates, cellulose and xylan sulfates were reported to stimulate immunological defense, to inhibit growth of cancer and to show beneficial effects against HIV infections (Hatanaka et al., 1991).

4.4.1.4 Cellulose phosphate and other phosphorus-containing cellulose derivatives General comments on phosphorylation reactions and products obtained The element phosphorus can be covalently attached to the cellulose chain via a reaction of hydroxy groups to give: phosphate groups CeIl-O-P(O)(OH)2 phosphite groups CeIl-O-P(OH)2 phosphonic acid groups CeIl-P(O)(OH)2 Many of the reactions involved are not quite clear yet regarding their course and mechanism, as well as the pattern of substitution. The products obtained are frequently insoluble due to crosslinking and rather ill-defined, and are often characterized by their phosphorus content only. Most frequently employed are derivatives of pentavalent phosphorus, i.e. ^ΡΟφ Ρ2θ5, and POC^. Compared with the corresponding compounds of hexavalent sulfur, these phosphorylating agents, usually leading to anionic cellulose phosphates, show a lower reactivity in esterification and lead to much less chain degradation during this process. As a peculiarity of cellulose phosphorylation by the above-mentioned reagents, a tendency to form oligophosphate side chains has to be mentioned, frequently resulting in crosslinking between cellulose chains, and thus impeding product solubility. Phosphorylation of cellulose is performed either by reaction at the hydroxy groups of the original polymer, or by a second-hand derivatization of a cellulose ether or ester already formed. In the former case the reaction usually starts in a heterogeneous system or employs a cellulose solution in a nonderivatizing solvent system; in the latter case a homogeneous system is generally preferred in order to arrive at soluble products. Regioselective patterns of substitution can in principle be realized along both of these routes. As reaction products, usually anionic cellulose derivatives are obtained. Their complete solubility in water or aqueous alkali, however, is, in contrast with cellulose sulfate synthesis, rather more the exception than the rule, due to the above-mentioned crosslinking reaction, and requires special procedures for the reaction itself and for the subsequent product isolation and purification. Applications of cellulose phosphorylation already practised are the preparation of cellulose-based cation exchangers and the flame proofing of cellulosic textiles.

134


Reaction routes and systems for cellulose phosphorylation Highly concentrated or water-free orthophosphoric acid has been widely used as an effective phosphating agent, and various procedures have been reported for preparing soluble as well as insoluble cellulose phosphates with phosphorus contents of about 10 % (Nuessle et al., 1956). According to Touey (1956), water-soluble cellulose phosphates of rather high DP can be prepared with waterfree ί^ΡΟφ For enhancing phosphorylation reactivity, mixtures of HßPC^ with ^2^5 have been employed. As to be expected, the degree of substitution of phosphorus atoms (DSp) increases with the molar ratio of reagent per AGU and the time of reaction, but chain degradation is enhanced too. Water-soluble cellulose phosphates have been synthesized in ternary systems of Η^ΡΟφ ?2θ5 and DMSO, connected with severe chain degradation down to a DP of about 200, with cotton cellulose as the starting material, and also with ternary systems of Ι^ΡΟφ ^2^5 and aliphatic alcohols with 4 to 8 C-atoms, arriving at products with up to 6 % phosphorus, corresponding to a DSp of < 0.2 (Nuessle, 1956; Touey, 1956). The reaction of cellulose with a melt solution of t^PC^ and urea resulted in the formation of a soluble, but strongly degraded, cellulose monophosphate monoammonium salt. The same system was employed by Nehls and Loth (1991) at a lower temperature of 120 0C for the phosphorylation of bead cellulose and cellulose powders to highly swellable but still water-insoluble products with DSp values between 0.3 and 0.6. The nitrogen content of these cellulose phosphates was very low (0.1-0.2 %). A very preferential C-6 substitution could be concluded from the 13C NMR spectra. A significantly higher phosphorus content than that corresponding to the DSp calculated from the 13 C NMR spectrum indicates the formation of cellulose oligophosphates, which obviously form crosslinks impeding solubility. A hydrogen bond stabilized complex between Η^ΡΟφ urea and cellulose, according to the scheme in Fig. 4.4.15, is assumed as the transition state in this cellulose phosphorylation. H I .Ox CeII-CH2^ X H HOx !x A /P-OJ IxNH-C-NH2 X HO Il ^H M O O Figure 4.4.15. Scheme of reaction complex in cellulose phosphorylation with and urea (Nehls and Loth, 1991).


135

Phosphorus oxychloride (POCl3) is known as an effective phosphating agent for cellulose from numerous studies, starting from a cellulose suspension in DMF or pyridine, or from a cellulose solution in a nonderivatizing solvent system. Usually only partially soluble products are obtained by the procedures described, and phosphorylation is frequently accompanied by an excessive chlorination, i.e. formation of desoxycellulose entities. According to Vigo and Welch (1973) immidinium compounds can be formed in systems containing POCl3 or PCl3 and DMF (see scheme in Fig. 4.4.16) which promote cellulose chlorination.

PCI3

H ? ||_o-P-OH

T + 20 = C-N(CH )

3 2

Ce

Cellulosephosphite + H2O Cl I

CeII-O-P-OH + CeII-CI Chlorodesoxycellulose + CeII-OH I - DMF H I

X

θ

0-C=N(CH3)2

CI-P '

O-C=N(CH3)2 H

e

2 Cl

+ CeII-OH - HC/, - DMF

Cl

>

Ce

ι θ |,_o_p_o__Cz:N(CH3)2Cle

- HCI, - DMF

+ H2O

OH I

CeII-O-P-OH Cellulosephosphite Figure 4.4.16. Scheme of reaction of PCl3 in DMF with cellulose (Vigo and Welch, 1973; Wagenknecht et al., 1979). As shown by Wagenknecht et al. (1979) for the action of PCl5, POCl3 and PCl3 on cellulose in formamide and dimethylformamide as the medium, the phosphorylation to partially soluble, considerably degraded products with DSp values of about 0.3 is accompanied by an excessive chlorination, up to a degree of substitution of chlorine atoms (DS(^) of 0.7 in DMF, whereas the products obtained in formamide contained only very small amounts of chlorine (DSî < 0.05). The problem of simultaneous phosphorylation and chlorination of cellulose by POCl3 was comprehensively studied by Zeronian et al. (1980) in dependence on various reaction parameters. The reaction of cellulose dissolved in nonderivatizing systems like NMMNO, LiCl/HMPT or DMA/LiCl results in a spontaneous coagulation and rather inho-

136

4A Esterification of Cellulose

mogeneous reaction products containing phosphorus as well as chlorine that are only partially soluble in water and rather heavily degraded. As compared with cellulose suspensions as the starting system, these initially homogeneous systems exhibit no advantages, if the preparation of soluble high molecular cellulose phosphates is intended. Trivalent phosphorus can be introduced into the cellulose molecule by reaction with PC13 (Vigo and Welch, 1973) or by transesterification with dimethyl phosphite, arriving at hydrolysis-susceptible phosphite esters of cellulose (Yuldashev et al., 1965). Synthesis of cellulose phosphites has also been reported, employing mixed anhydrides of hydrophosphorus and acetic acid and arriving at phosphorus contents of up to 8 % (Predvoditelev et al., 1966). Experimental routes to cellulose phosphonates with the phosphorus directly bound to a C-atom of the polymer are either an esterification with methyl or phenylphosphonic anhydride (Yuldashev and Muratova, 1966; Petrov et al., 1965), or a two-step reaction consisting of chlorination of the polymer with SOC12 to give chlorodesoxycellulose with a high Cl content (up to 16 %) and the subsequent reaction of this compound with triethylphosphite to the cellulose phosphonate via an Apruzov rearrangement. Also the preparation of cellulose phosphonites has been reported (Kiselev and Danilov, 1962). Completely or partially substituted cellulose derivatives have been phosphated by various acids or acid chlorides of pentavalent phosphorus, usually starting from a homogeneous system and rather frequently arriving at soluble products. Stable ether groups like the carboxymethyl groups and also the acetyl group of cellulose acetates act as efficient protecting groups in the nonaqueous systems involved, and only free hydroxy groups are converted to phosphate groups. CMC with a DS of 0.8 was converted to an ether ester with a DSp of 0.3 in the system F^PC^/urea with the phosphate groups again preferentially located at C-6 (Nehls and Loth, 1991). With a sample of hydroxyethy!cellulose (MS ~ 2) a considerable higher DSp of 0.6 was obtained with the same system under comparable conditions of reaction, obviously due to the participation of the hydroxy end groups of the side chains in esterification. A somewhat more detailed consideration is deserved by the phosphorylation of cellulose acetates, as different patterns of substitution of cellulose phophates can be realized here after splitting off the acetate groups in aqueous alkaline medium without significantly affecting the phosphate groups. The preparation of soluble cellulose acetate phosphates by reacting the cellulose acetate after dissolution in acetone with POC^ in the presence of an aliphatic amine has been reported. Whistler and To wie (1969) used polytetraphosphoric acid in combination with tri-ft-butylamine in DMF to esterify free hydroxy groups of a low-DS cellulose acetate at 120 0C to a DSp of about 1, arriving at a water-soluble product after elimination of the acetyl groups. From a comparison of different phosphating agents, i.e. diphosphoryl tetrachloride, phosphorus oxychloride, dichlo-

4Λ.1 Esters of cellulose with inorganic acids

137

rophosphoric acid, and polytetraphosphoric acid, in phosphating partially substituted cellulose acetates in DMF in the presence of an aliphatic amine, it can be concluded that all the chlorine-containing agents, especially diphosphoryl tetrachloride, lead to an early coagulation of the initially homogeneous reaction system, resulting in cellulose phosphates of poor solubility in spite of a rather high DSp of between 0.5 and 1.0 (see Table 4.4.16). Table 4.4.16. Comparison of different phosphating agents in the presence of tn-nbutylamine, in the phosphorylation of commercial cellulose 2-acetate (reaction time 6 h; deacetylation in NaOH/EtOH) (Philipp et al., 1995). Phosphating agent (mol/mol AGU) HPO2Cl2 (2.0) P2O3Cl4 (1.5) (1.5)

Amine (mol/mol AGU) 15 15 3

T DSp (0C)

% Cl

Solubility 2 N NaOH H2O

20 20 120

0.07 0.17

Gel Insoluble Soluble

0.59 0.45 0.78

Gel Insoluble Soluble

With polytetraphosphoric acid, on the other hand, the system remained homogeneous during the whole reaction, and water- or alkali-soluble cellulose phosphates could be isolated under suitable conditions after deacetylation (Wagenknecht, 1996). The advantages of the combination polytetraphosphoric acid/tri-ft-butylamine were fully confirmed in this study, and this combination has been employed for phosphating partially substituted cellulose acetates over a wide range of DS and with different patterns of substitution. Some results obtained with statistically and with regioselectively in C-6-substituted acetates are summarized in Table 4.4.17. The pattern of substitution of the resulting cellulose phosphates resembles an inverse image of that of the original acetate, with the DSp increasing generally with decreasing DS^C. But it must be emphasized that in contrast with sulfation, not all of the free hydroxy groups could be converted to phosphate groups, the difference increasing with the increasing amount of free hydroxy groups. As can be seen also from the data in this Table, phosphate groups in the C-6 position obviously promote product solubility in aqueous media much more than an equal amount of ester groups in the C-2/C-3 position. Unstable primary substituents (ether or ester groups) can act as the leaving group in a subsequent phosphorylation with ?2θ5 or POClß in the absence of an amine, as shown by our results in the cellulose nitrite system or in case of TMScellulose. TMS-cellulose of DS 1.5 with the silyl groups predominantly in the O6 position could be reacted with an excess of phosphating agent in DMF/TEA to give an insoluble cellulose phosphate with a DSp of 0.3-0.6 (Klemm et al.,

138


1990). With POCl3 or PO(OH)Cl2 as the phosphating agent, a considerable chlorine content (up to a DS^\ of 0.3) was found in the products. A cellulose trinitrite solution in DMF, prepared by dissolving the polymer in N2U4/DMF under strictly anhydrous conditions, is susceptible to phosphorylation by P2U5 or POCl3 too, with a selective substitution at the C-6 position being observed with P205 as the phosphating agent. The products, however, proved to be insoluble, but swellable in water or aqueous alkali. Table 4.4.17. Phosphorylation of statistically (a) and regioselectively (b) substituted cellulose acetates in DMF with polytetraphosphoric acid/tri-n-butylamine (1.5mol of agent/3 mol of TBA/AGU; 6 h; 120 0C) (Wagenknecht, 1996).

;

Commercial cellulose acetate DS

DSp (NMR)

2.4a 1.9a 2.60b 1.74b

O. 25 O. 75 O .1 O. 65

Solubility of cellulose phosphates Pattern of substitution0 After Before deacetylation C-2/C-3 C-6 deacetylation NaOH H2O NaOH H2O 0.05 O. 20 soluble swelling soluble swelling 0.50 O. 25 soluble soluble soluble soluble 0.1 O soluble insoluble soluble swelling 0.55 O. 1 soluble swelling swelling insoluble

after deacetylation.

Soluble cellulose phosphates, however, can be prepared from both these systems in the presence of an excess of a tertiary amine like TEA, applying additionally a hydrolytic aftertreatment subsequent to the reaction, which is obviously necessary to cleave oligophosphate crosslinks (Wagenknecht et al., 199Ib). From TMS-cellulose (DS = 1.5), cellulose phosphates with a DSp of up to 0.7 and a preferential C-2/C-3 substitution were prepared with POCl3 or PO(OH)Cl2 in DMF as the medium, the latter being somewhat less reactive than POCl3. After desilylation, an optimum of solubility of the Na-cellulose phosphates was observed at DSp values of about 0.5. Comprehensive studies on the phosphorylation of cellulose trinitrite in DMF with POCl3 (Wagenknecht et al., 199Ib) confirmed the necessity of an excess of tertiary amine and the hydrolytic aftertreatment as prerequisites for obtaining soluble cellulose phosphates. Furthermore, a partial defunctionalization of the acid chloride by reacting it prior to use with N2U4 to give probably a phosphoryl chloride nitrate, or with H2O to PO(OH)Cl2, was found to favor the formation of soluble cellulose phosphates. The DSp increased with the molar ratio of POCl3 as well as of TEA per AGU (see Fig. 4.4.17) and reached values of up to 1.4.


139

0.8 0.6

0.6

0.2

0.2

0

2 4 6 MoI POCl 3 XmOlAGU

0 4 . 8 12 16 20 Mol TEA/molAGU

Figure 4.4.17. Effect of POC13 (a) and TEA (b) input on cellulose phosphorylation in N2O4/DMF at 20 0C (Wagenknecht et al., 199Ib).

The Cl content of the product depended significantly on the order of addition of POC13 and amine, and was much higher (DSQ up to 0.2) with the POC^ added before the amine. According to our experience, a long residence time of a strongly acidic phosphating system with an acid chloride as the agent generally favors chlorination, while the presence of the amine exerts some buffering action, besides its effect as an adjuvant base for enhancing the reactivity of the agent. From the NMR spectra of the phosphates, a preferential location of the ester groups in the C-2/C-3 position could be concluded. Optimal solubility was observed also here at a DSp level of about 0.5, this range being broadened somewhat by employing a difunctionalized POQ^. Obviously, the solubility of cellulose phosphates, prepared by this as well as by other procedures in water or aqueous alkali, is determined by two counteracting effects, increasing with DSp, i.e. an increasing hydrophilicity due to the anionic substituents, and an increasing tendency to crosslinking. Probably some kind of optimal balance is obtained in the DSp region of about 0.5. Finalizing this presentation of experimental routes to cellulose phosphates, the heterogeneous reaction of alkali cellulose with POC^ in the presence of benzene shall be mentioned as a modification of the Schotten-Baumann reaction for esterification, leading here, according to Reid and Mazzeno (1949), to a considerably degraded cellulose phosphate. Properties of cellulose phosphates The attachment of phosphorus atoms to the cellulose chain significantly decreases the inflammability of cellulose threads due to less formation of inflammable volatiles on thermal degradation. This flame retardation is still increased by the presence of chlorine atoms frequently introduced in side reactions of phosphorylation such as chlorodesoxycellulose units.

140


By introducing the anionic phosphate groups into the cellulose molecule, cation-exchange properties are conveyed to the polymer and its hydrophilicity is enhanced. The H+ form of the phosphate group shows a moderate acidity only and can be stored for some time without significant hydrolytic chain cleavage, in contrast with cellulose sulfate. At a DSp above 0.2, sodium cellulose phosphates can be, but do not necessarily have to be, water- or alkali-soluble. As demonstrated, especially by the regioselectively substituted cellulose phosphates prepared via cellulose acetates, the site of substitution is also relevant to solubility, C-6-substituted products showing a much better solubility. With soluble sodium cellulose phosphates, very high solution viscosities can be obtained if excessive chain degradation during esterification is avoided. Probably also strong intermolecular interactions via phosphate groups and/or oligophosphate side chains contribute to this high viscosity. Application of cellulose phosphates Phosphorylation of cellulose threads is employed to convey flame retardancy to cellulosic textiles for special, mostly technical, use, taking into account some deterioration of textile mechanical properties and textile handling. Cellulose particles of different sizes and shapes bearing phosphate groups, find wide application as weak cation exchangers, especially in biochemical separation processes. Soluble cellulose phosphates have been recommended as viscosity enhancers and thickeners in aqueous systems, with the nontoxicity of these products being an advantage. Regioselectively (in the C-2/C-3 position) substituted cellulose phosphates were recently observed to inhibit the activation of detrimental blood proteins in hemodialysis after incorporation in hemodialysis membranes (Wagenknecht, 1996).

4.4.1.5

Cellulose borates

Boron-containing cellulose derivatives have been studied predominantly in order to improve special applicational properties of cellulosic materials, for example flame retardancy or heat stability. Systematic chemical investigations on the course and mechanism of cellulose borylation are rather scarce and are obviously impeded by ill-defined products due to crosslinking and formation of oligo- and polyborate moieties. These tendencies being more pronounced than in the case of phosphorylation. Two main routes of synthesis have being employed rather frequently to prepare boronic acid esters of cellulose, i.e. (i) the direct esterification of cellulosic hydroxy groups with orthoboric or metaboric acid according to CeIl(OH)3 + H3BO3 -»(CeIlO)3B


141

(ii) a transesterification of cellulose with boronic acid esters of lower aliphatic alcohols (boron alkoxides) CeIl(OH)3 + B(OR)3 -> (CeIlO)3B Due to the strong crosslinking tendency of the borylation agents indicated in the borderline schemes of reaction, a meaningful assessment of the DS requires additional assumptions on reagent functionality realized in the reaction, and the products are therefore usually characterized just by their boron content. A direct borylation of cellulosic hydroxy groups has usually been performed with ortho- or metaboric acid in a melt of urea at 150-200 0C. Ermolenko (Ermolenko et al, 197Ia) reports a boron content of 1.8 % after reacting cellulose with HBO2/urea at 220 0C for 1 h. This boron content corresponds to a formal DS of about 0.7, assuming a trifunctional mode of reaction. A parallelism between this borylation reaction and a phosphorylation with HPO3/urea at 150 0C to a DSp of about 1 is emphasized in the above-mentioned publication. The preparation of a mixed borate/phosphate of cellulose by subsequently reacting the polymer with H3PO3/urea and with Η4Ρ2θ7 or HPO3/urea in the temperature range 100-200 0C has been described in Ermolenko et al. (197Ib). According to Ermolenko et al. (197Ia) cellulose acetate can been converted to an acetate borate mixed ester by treatment with H3BO3 at 260 0C, obviously via the intermediate formation of poly boric acids. The transesterification of cellulose with boron trialkoxides [B(OR)3, with R = Me, Et, Pr] can be performed at considerably lower temperature, for example in benzene as the medium (Gertsev et al., 1990). According to Arthur and Bains (1974) a boron content of 6.8 % could be obtained by this procedure, corresponding to trisubstitution of the cellulosic hydroxy groups assuming again a trifunctional reaction. Also, graft copolymers of cellulose can be borylated with boron trialkoxides, as demonstrated by Tyuganova and Butylkina (1992) or graft copolymers of cellulose with 2-methyl-5-vinylpyridine. As a rather special route to cellulose borates the reaction of cellulose as a hydroxy group containing polymer with trialkylboranes has to be mentioned, which, according to BR3 + R'-OH -» ROBR2 + RH is applied to the analytical determination of active hydrogen atoms (Koester et al., 1971). As described by Dahlhoff et al. (1988), a per-O-diethyl-borylated amylose or cellulose can be regioselectively reduced by an ethyl diborane to a boron-substituted polyanhydroglycitol.

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The formation of five-membered ring complexes between vicinal hydroxy groups of polysaccharides including cellulose with boric acid in aqueous systems has already been reported many years ago. More recently, a reversible gel formation of a well-degraded 2,3-dihydroxypropylcellulose of DP 20 with borax in aqueous solution has been studied, and the formation constants of the probable five-membered ring complexes have been determined (Sato et al., 1992). Regarding now special product properties of cellulose borates, the attachment of borate groups conveys to the cellulose chain a cation-exchange capacity and an enhanced thermal stability due to a decreased rate of thermal oxidation (Arthur and Bains, 1975). According to Ermolenko and Luneva (1977) the nontoxic borate group exhibits antibacterial and antifungal as well as hemostatic activities. Important for several areas of application is the strong crosslinking tendency during borylation of cellulose. The stability of the borate ester group to hydrolysis or alcoholysis is discussed with some degree of controversy in the literature, probably due to different amounts of crosslinking in the products investigated. Based on the above-mentioned properties, various areas of application of borylated cellulose have been proposed: the crosslinking tendency on borylation was claimed to be advantageous in packaging and micro-encapsulation. The enhanced thermal stability of borylated cellulose has been considered advantageous in the preparation of e.g. insulating paper. The broad antibacterial activity of cellulose borates was emphasized as a basis for medical use.

4.4.1.6

Desoxycelluloses

The term 'desoxycellulose' denotes cellulose derivatives resulting from the substitution of a hydroxy group by halogen, sulfur or nitrogen or even carbon, with the hetero- or carbon atom directly bound to a carbon atom of the AGU. Halo-, pseudohalo- and thiodesoxycelluloses can be formally considered as cellulose esters of the appropriate hydrogen halides, hydrogen pseudohalides or of hydrogen sulfide. Of special relevance to the organic chemistry of cellulose up to now are the chloro- and the iododesoxycelluloses. But a systematic investigation of the synthesis of desoxycelluloses is an open field of cellulose chemistry. A route to desoxycelluloses starts from the cellulose esters with ptoluenesulfonic acid (tosylcellulose) or with methanesulfonic acid (mesylcellulose), usually reacting these esters with inorganic salts containing the group to be introduced as the nucleophilic reagent in this displacement reaction. Some examples are presented in Table 4.4.18. According to Titcombe et al. (1989) the use of tetraalkylammonium fluorides proved to be successful for reaching a high degree of substitution. Chlorodesoxycellulose is most conveniently prepared by reacting cellulose with SOC^ in pyridine (Carre and Manclere, 1931), DMF (Polyakov and Rogowin, 1963),


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CC14 (Fumasoni and Schippa, 1963) or CHCl3, arriving at DS^ values of up to 1.0. But frequently also some sulfur (DSg up to 0.1) is introduced into the macromolecule, probably via cyclic sulfides (Carre and Manclere, 1931). Also, SO2C12 can be employed to prepare chlorodesoxycelluloses with DS values of 0.4-0.8 (Wagenknecht et al., 1979). A homogeneous route to chlorodesoxycellulose was described by Furuhata et al. (1992), starting from a solution of the polymer in DMA/LiCl and reacting with W-chlorosuccinimide and triphenylphosphine. A homogeneous chlorination can also be performed with methylsulfuryl chloride after dissolving the polymer in the system chloral/DMF. For preparing fluorodesoxycellulose, a treatment of mesylcellulose with an aqueous NaF solution has been described earlier by Pascu and Schwenker (1957) and Krylova (1987). But this route leads to a very low DS only, due to dissolution problems. Table 4.4.18. Preparation of desoxycelluloses via (A) tosyl- or (B) mesylcellulose.

Desoxy group FluoroChloro-

Bromo-

IodoMercapto-

CyanoThiocyanatoAzido-

Reagents and conditions B NaF in H2O A Tosyl chloride and pyridine at high temperature A LiCl in acetylacetone (2 h at 130 0C) B NaBr in H2O A NaBr in acetylacetone (2 h at 130 0C) A/B NaI in acetylacetone (2 h at 130 0C) A H2S in pyridine (8 h at 40 0C, then 70 h at room temperature) A Na2S2O3 in DMSO A KCN in DMF or methanol (100-150 0C) A NaSCN in acetonylacetone (11 h at 110 0C) A NaN3 in DMSO (110-13O0C)

^Desoxy = degree of substitution of desoxy groups.

~ 0.05 0.4-0.9 ~ 1.00 ~ 0.1 ~ 1.0 -1.0 ~ 0.28

~ 0.41 ~ 1.03 0.19 0.43

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According to Ishii et al. (1977) a fast reaction takes place at the C-6 position, followed by C-3, whereas no chlorination was observed at C-2. A complete exchange of the tosylate groups at C-6 with chlorodesoxy groups was recently reported by Rahn (1997), who reacted a cellulose tosylate (prepared under homogeneous conditions; see chapter 4.3) with LiCl in acetylacetone for 2 h at 130 0C. Bromodesoxycellulose can be obtained by analogy to the chloro compound with TV-bromosuccinimide and triphenylphosphine (Tseng et al., 1995). Also, the nucleophilic exchange of tosylate groups with bromodesoxy groups by reacting tosylcellulose with NaBr in acetylacetone can be recommended (Rahn, 1997). A tosylation and subsequent iodination to iododesoxycellulose was often formally employed to assess the amount of free hydroxy groups at the C-6 in partially substituted cellulose derivatives, because only the tosylate groups in this position were selectively replaced by iodine (Malm et al., 1948; Heuser et al., 1950). According to Rahn (1997) this procedure is somewhat questionable as a quantitative method, as deviations in the DS balance have been observed. Similar displacement reactions can be performed with cellulose nitrate, as only the nitrate groups in the C-6 position are substituted by iodine. Sulfur bound directly to this C-atom can be introduced by reacting tosylcellulose with ^28203 in DMSO to a 'Bunte-salt' of tosylcellulose, which is subsequently oxidized to a disulfide bridge with e.g. F^C^ in an alkaline medium (Camacho Gomez, 1997). Just as described for the halodesoxycelluloses, pseudohalodesoxy derivatives can be obtained, and the same holds true for nitrodesoxycellulose (CeIl-NC^) prepared by reacting tosylcellulose with NaNC^. Tosylcellulose is employed as the starting material also for preparing aminodesoxycellulose by reacting it with Nt^, aliphatic amines, or hydrazine (Teshirogi et al., 1979; Engelskirchen, 1987). An alternative route starts from a highly substituted cellulose nitrate, which is reacted with NaNH2 in liquid NF^. Products with a DS of nitrogen of up to 1 were obtained, which were soluble in FÔ and dilute aqueous acids, but not in organic liquids (Scherer and Feild, 1941). Desoxycelluloses can be considered as promising starting materials for subsequent steps of cellulose functionalization: an acidodesoxycellulose obtained by reaction of tosylcellulose with sodium, can be cleared by UV irradiation, opening a route to a selectively oxidized 6-aldehydecellulose (Clode and Horton, 1971). The binding of a rather complex functional group directly to the skeleton of cellulose was demonstrated recently by Rahn (1997) by reacting tosylcellulose for 8 h at 100 0C in a DMF/water medium with the sodium salt of iminodiacetic acid. About 50 % of the toslyate groups at C-6 were substituted by the iminodiacetic acid group attached to the polymer skeleton via a C-N bond. A transformation of 6-chlorodesoxycellulose to hydrazlnodesoxycellulose and a substituted hydrazlnodesoxycellulose was recently employed by Nakamura and Amano (1997).

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Halodesoxycelluloses, especially chlorodesoxycellulose, exhibit a rather high thermal stability, with the temperature of beginning thermal decomposition decreasing in the order chloro- > bromo- > iododesoxycellulose. Thermal decomposition takes place with a liberation of the appropriate hydrogen halide. Quite similar to phosphorylation, chlorination of cellulose results in increased char formation and decreased evolution of inflammable volatiles in thermal decomposition (Jain et al., 1987a), and therefore has found some attention in the flame proofing of cellulosic textiles. A route to the attachment of long alkyl side chain on the cellulose molecule via C-N-C bonds is the reaction of 6-chlorodesoxycellulose with n-alkylamines (CH3(CH2)nNH2; N = 5, 11, 17) yielding alkylaminodesoxycelluloses (Nakamuraetal., 1997).

4.4.2

Cellulose esters with reagents derived from carbonic acid (H2CO3)

Despite much experimental effort, cellulose esters of carbonic acid (cellulose carbonates) have not been isolated up to now, obviously due to the instability of these compounds. But cellulose esters of the thio analogue of t^CC^, i.e. of monothiocarbonic acid and dithiocarbonic acid are well known, the cellulose half-ester of dithiocarbonic acid ('cellulose xanthogenate'), as its Na salt, representing the key intermediate in artificial fiber spinning by the commercial viscose process. Furthermore, esters of cellulose with carbamic acid in recent years have been amply studied in connection with an alternative process of artificial fiber manufacture. These three classes of compound only are of interest as process intermediates and not as final products, and therefore will be subsequently considered with regard to their chemistry of formation as well as that of decomposition and subsequent reactions.

4.4.2.1

Cellulose esters of monothiocarbonic acid (H2CSO2)

Carbonyl sulfide (COS), the moderately stable anhydride of the presumably extremely unstable and not yet isolated monothiocarbonic acid P^CSC^, reacts with anionized alcoholic hydroxy groups to give alkyl monothiocarbonic acid half-ester anions COS + RO- -> ROCOSThis bimolecular reaction proceeds about three orders of magnitude faster than the corresponding one between COS and hydroxy ions leading to monothiocarbonate anions. In contrast with the esterification reactions with inorganic acid anhydrides considered so far, the esterification with COS requires an activation

4.4.2 Cellulose esters with reagents derived from carbonic acid (H2 COj)

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Halodesoxycelluloses, especially chlorodesoxycellulose, exhibit a rather high thermal stability, with the temperature of beginning thermal decomposition decreasing in the order chloro- > bromo- > iododesoxycellulose. Thermal decomposition takes place with a liberation of the appropriate hydrogen halide. Quite similar to phosphorylation, chlorination of cellulose results in increased char formation and decreased evolution of inflammable volatiles in thermal decomposition (Jain et al., 1987a), and therefore has found some attention in the flame proofing of cellulosic textiles. A route to the attachment of long alkyl side chain on the cellulose molecule via C-N-C bonds is the reaction of 6-chlorodesoxycellulose with n-alkylamines (CH3(CH2)nNH2; N = 5, 11, 17) yielding alkylaminodesoxycelluloses (Nakamuraetal., 1997).

4.4.2

Cellulose esters with reagents derived from carbonic acid (H2CO3)

Despite much experimental effort, cellulose esters of carbonic acid (cellulose carbonates) have not been isolated up to now, obviously due to the instability of these compounds. But cellulose esters of the thio analogue of t^CC^, i.e. of monothiocarbonic acid and dithiocarbonic acid are well known, the cellulose half-ester of dithiocarbonic acid ('cellulose xanthogenate'), as its Na salt, representing the key intermediate in artificial fiber spinning by the commercial viscose process. Furthermore, esters of cellulose with carbamic acid in recent years have been amply studied in connection with an alternative process of artificial fiber manufacture. These three classes of compound only are of interest as process intermediates and not as final products, and therefore will be subsequently considered with regard to their chemistry of formation as well as that of decomposition and subsequent reactions.

4.4.2.1

Cellulose esters of monothiocarbonic acid (H2CSC^)

Carbonyl sulfide (COS), the moderately stable anhydride of the presumably extremely unstable and not yet isolated monothiocarbonic acid P^CSC^, reacts with anionized alcoholic hydroxy groups to give alkyl monothiocarbonic acid half-ester anions COS + RO- -> ROCOSThis bimolecular reaction proceeds about three orders of magnitude faster than the corresponding one between COS and hydroxy ions leading to monothiocarbonate anions. In contrast with the esterification reactions with inorganic acid anhydrides considered so far, the esterification with COS requires an activation Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

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of the alcoholic component by anionization of the hydroxy groups in the manner of a Schotten-Baumann reaction. In an aqueous alkaline medium, alkyl monothiocarbonates are considerably more stable than monothiocarbonate itself, yielding SH~, CO^2~ and ROH as end products of decomposition.

0.5

I

0 5

10

15 20 NaOH [wt.%]

25

30

Figure 4.4.18. Maximal DS in the reaction of alkali cellulose with carbonyl sulfide in dependence on steeping lye concentration (Philipp, 1957a).

As first reported by Hess and Grotjahn (1952), alkali cellulose (sodium salt cellulose I) can be converted by reaction with COS at about O0C to a solid-fiber salt of cellulose monothiocarbonic acid half-ester with a limiting DS$ of about 1, which can be rather completely dissolved in dilute aqueous alkali without, however, yielding a fiber-free solution. Studies of our own (Philipp, 1957a) confirmed these findings of Hess and Grotjahn and revealed a close correlation between the so-called true alkali uptake of the alkali cellulose employed as starting material and the maximal DS$ of the greenish-gray cellulose monothiocarbonate half-ester salt, with a rather constant level of DS$ between 0.8 and 0.9 being observed in the range of alkali-cellulose steeping-lye concentrations of 14-20 % (see Fig. 4.4.18). Throughout this range of lye concentration, sodium cellulose I is formed with a nearly constant true alkali uptake of 1 mol of NaOH/mol of AGU, which in this heterogeneous reaction obviously sets an upper limit for substitution of hydroxy groups by monothiocarbonate residues. Furthermore, it could be concluded from these experiments that in the fibrous cellulose monothiocarbonate, as well as in its aqueous alkaline solution, a rather fast transesterification between cellulosic hydroxy groups via free COS has to be assumed, as expressed by the equilibrium CeII-O' + COS

CeII-O-COS'

Na cellulose monothiocarbonate and its aqueous alkaline solutions decomposed rather rapidly to sulfide, carbonate and cellulose and can be handled only at low temperatures of about O0C. Due to its high rate of formation and decomposition, and the fast transesterification mentioned above, Na cellulose mono-

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thiocarbonate ('COS xanthogenate') plays some role as an intermediate in cellulose xanthation and transxanthation during the viscose process (see section 2.3.2.2).

4.4.2.2

Cellulose dithiocarbonate esters

General comments on reaction and product properties Just as with the alkoxy anions of low molecular alcohols, carbon disulfide (CS2) reacts with anionized cellulosic hydroxy groups to give a moderately stable cellulose dithiocarbonic acid half-ester anion according to CeIl-O- Na+ + CS2 -> CeIl-O-CSS- Na+ which in principle can be subsequently reacted to a full ester with an alkyl halide. Of practical relevance, however, is the sodium salt of the half-ester only, as the introduction of a sufficient amount of anionic dithiocarbonate groups to the cellulose chain makes the polymer water- or alkali-soluble by transforming it to a poly electrolyte. Thus, therewith, is the purpose of converting the cellulose fibers to the homogeneous dissolved polymer component of an aqueous spinning solution in the manufacture of artificial cellulose fibers via the viscose process. The chemistry of this process is, however, not so simple, as indicated by the above equation, as only about 70 % of the CS2 input is converted to cellulose xanthogenate. The rest is consumed by formation of inorganic sulfidic products, and as xanthogenate formation and decomposition, taking place simultaneously in an aqueous alkaline medium, and as finally the conversion of the alkaline cellulose xanthogenate solution to a filament of cellulose II by spinning in an acid bath representing a complex chemical process too, which is largely affected by the previous steps of xanthation and xanthogenate dissolution. Subsequently, the chemistry of cellulose xanthogenate formation and decomposition will be described in some detail, together with the results of model experiments, turning then to the role of alkali-cellulose structure, and finally giving an overview of the present state of the viscose process for manufacturing artificial cellulose fibers and filaments via cellulose xanthogenate.

The chemistry of xanthogenate formation and decomposition in aqueous media As industrial cellulose xanthogenate formation and decomposition takes place in systems containing between 50 and 85 % of water, a brief survey of the general chemistry of xanthogenate and by-product formation in aqueous media, as well as on decomposition of xanthogenates in dependence on pH, with reference to homogeneous model systems, seems appropriate to make the reader familiar

148


with the complex reaction mechanism before turning to the characteristics of cellulose xanthation. As can be seen from the reaction rate constants in Table 4.4.19, the conversion of alkoxy anions to xanthogenate anions is generally highly favored in comparison with dithiocarbonate formation with hydroxy ions. Table 4.4.19. Parameters of the reactions of CS2 with various anions at 10 0C in 0.1-1.5 N aqueous NaOH.

Reaction CS2 + OHCS2 + SHCS2 + ROCS2 + CS2O2CS2 + CSO22-

Rate constant (mnrM-moH) 0.009 0.085 4.7 5.9 2.6

(kcal/mol) 20-21 21 16 15.6 -

(cal/mol-0C) 5.1 10.5 0.3 -0.5 -

= Entropy of activation.

The dithiocarbonate, as a reactive intermediate, gives rise to consecutive reactions, finally leaving to sulfide, carbonate and trithiocarbonate (CS3) as stable end products, but also involving the formation and subsequent decomposition of carbonyl sulfide. The latter reacts by analogy to €82 independently with RO~ anions, as well as with hydroxy anions, to give alkyl monothiocarbonate and thiocarbonate, respectively, with rate constants about three orders of magnitude higher than those of the corresponding reactions with CS2- The nucleophilicity of the anions in question increases in the order OH~ < SH~ < RO~ = CS2O2~. The energy of activation was found to be significantly lower for RO~ and CS2O2~ compared with the other anions, possibly due to an asymmetry of the hydration shell. The entropy of activation decreases in the order S2~ > OH~ > RO~ (R = C2H5) > CS2O2~, indicating an increasing demand for special orientation of the anion in order to form the reaction complex with CS2 (Dautzenberg and Philipp, 1969). Due to the limited solubility of CS2 in aqueous systems (1.4 g = 18 mmol/1 in pure water), the course of reaction can become diffusion controlled with an excess of CS2 present as a separate liquid phase, as shown by Philipp (1955) for the 'limiting system' H2O/NaOH/CS2, where the transport of €82 to the aqueous phase was found to be rate determining above 30 0C. In this system, sulfide was formed as the only stable end product up to about pH 10, while at higher alkalinity an increasing amount of CS32~ was observed, passing a maximum at about 5 N NaOH and then decreasing rapidly above 7.5 N NaOH due to changes in the hydration shell of the NaOH dipoles (see chapter 4.2). In the presence of air or oxidants, sulfide is oxidized to disulfide, which reacts very rapidly with CS2 to perthiocarbonate,

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rapidly with CS2 to perthiocarbonate, and also some thiosulfate is formed, all these compounds playing a role as minor by-products in the viscose process. The rate of xanthogenate formation between an alkoxy group and C$2 largely depends on the chemical constitution of the alcohol in question and can differ between low molecular aliphatic alcohols by about two orders of magnitude, taking as examples the slow reaction of isopropanol and the fast reaction of glycerine. The rate and the mechanism of xanthogenate decomposition are governed by the chemical constitution of the alcohol, but also depend decisively on the pH of the medium. In the acid region of the pH scale rapid decomposition to ROH and CS2 via free xanthogenic acid takes place with any xanthogenate, with only a small rate difference between so-called stable xanthogenates like ethylxanthogenate or unstable xanthogenates like glycerine xanthogenate. At the other end of the pH scale, i.e. at and above pH 14, a steep increase in xanthogenate decomposition rate takes place too, with dithiocarbonate being the predominant primary decomposition product, and a considerably larger rate difference between various xanthogenates being observed than in acid decomposition. Intermediate formation of an orthoxanthogenate by addition of a hydroxy ion to the CS double bond has been proposed to explain the course of this reaction. Remarkable differences in stability between ethyl or 1,4-butandiol xanthogenate on the one hand, and glycol and glycerine xanthogenate on the other, have been observed in the pH range 7-13 (Philipp and Fichte, 1960): the so-called stable xanthogenates mentioned first are slowly decomposed at a nearly constant rate over a wide range of pH with evolution of free CS2 as the predominant product of decomposition due to reaction with water molecules probably forming primarily a hydration complex with the xanthogenate anions. With glycol or glycerine xanthogenate, however, a rather large amount of carbonyl sulfide, bound at least partially as alkyl monothiocarbonate, was already observed at a pH of about 9, besides formation of a large amount of sulfide; a stepwise desulfuration of the xanthogenate was proposed as a possible mechanism for this route of dexanthation. The decisive point of difference between the two groups of xanthogenates is not the number but the mutual position of the hydroxy groups. Obviously, the above-mentioned stepwise desulfuration is favored by a vicinal hydroxy group, at least in the case of aliphatic alcohols. The course of xanthation and dexanthation of mono- and polysaccharides takes an intermediate position between the so-called 'stable' and the 'unstable' xanthogenates, but resembles more that of the stable ones despite the existence of vicinal hydroxy groups in the saccharide molecule. The location of these hydroxy groups within an anhydropyranose ring obviously exerts a stabilizing action on the xanthogenates formed. According to Philipp (1957b) primary as well as secondary hydroxy groups of monosaccharides can be xanthogenated, the maximal level of xanthogenate formation being of course lower with xylose

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than with glucose. Studies on emulsion xanthation of various polysaccharides, i.e. a short-chain cellulose (ß-cellulose), a beech xylan, an ivory nut mannane and an alginate at 4 % polymer concentration in 5 N NaOH at 28 0C with an excess of CS2 (see Fig. 4.4.19), revealed a rather similar course of reaction for ßcellulose and xylan, except for the very plausible fact that the maximal DS of 0.95 with xylan amounted to two sorts only of that of ß-cellulose with a DS of 1.45. With the mannan on the other hand, a significantly faster formation and decomposition of the xanthogenate with a maximal DS of 1.6 can be concluded from the data shown in Fig. 4.4.23 (see later). Obviously, the cis position of the hydroxy groups at C-2 and C-3, in contrast with the trans position in cellulose, leads to a faster decomposition, similar to that observed with glycol xanthogenate, quite in agreement with the observed shift of the ratio of trithiocarbonate to sulfide formed as by-products in favor of sulfide. On xanthation of alginate under the conditions used here, a definitely lower maximal DS (ca. 0.7) than with xylan was found, possibly due to a shielding action of the anionic group already present in the C-6 position. From the viewpoint of the industrial viscose process, a moderate amount of xylan units in the dissolving pulp obviously does not disturb the xanthation reaction, but mannose units consume more than the adequate amount of CS2 and transform it rather quickly to undesired by-products.

Figure 4.4.19. Course of xanthation of various polysaccharides: (a) low DP cellulose; (b) beech xylan; (c) ivory nut mannan (Philipp, 1957b), γ-value = 100 - DS. Characteristics of cellulose xanthogenate formation and decomposition Generally, xanthation of cellulose complies with the principles of this reaction outlined above: xanthation and CS2 hydrolysis proceed independently in a reaction-controlled process. The formation of by-products, especially trithiocarbonate, increasing with the temperature of reaction due to the difference in activation energies (EA =13 kcal/mol for xanthation, Ξ 21 kcal/mol for CS2 hydroly-

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151

sis). All three hydroxy groups of the AGU can participate in the reaction. After the pioneering work of Hess et al. (1951) on the heterogeneous course of cellulose xanthation, and of Matthes (1952) on transxanthation via free CS2> decisive progress in understanding the mechanism and the kinetics of cellulose xanthation and dexanthation was achieved in the late 1950s and 1960s. Especially to be mentioned are the comprehensive studies of the groups of Samuelson (e.g. Samuelson, 1948; Dunbrant and Samuelson, 1965) on xanthogenate group stability and its spectrophotometric assessment, of the group of Treiber (e.g. Treiber et al., 1955 and 1956; Treiber and Fex, 1956) on the colloid chemistry of cellulose xanthation and xanthogenate solution, of Hovenkamp (e.g. Hovenkamp, 1963 and 1965) on the role of sodium dithiocarbonite in the xanthation process, and of Dautzenberg (e.g. Dautzenberg et al., 1972) on the formation of low molecular sulfidic products during xanthation and dexanthation. Two important characteristics, have to be considered in connection with cellulose xanthogenate formation and decomposition, i.e. (i) the influence of polymer supramolecular structure on maximal DS obtainable, and on substituent distribution; (ii) the existence of a quasi-equilibrium of dexanthation and rexanthation via free CS2 as the active agent in aqueous alkaline solutions of this 'moderately unstable' xanthogenate. With regard to supramolecular order or 'state of dispersity' of the polymer, two borderline cases can be realized: (i) a xanthation of low DP cellulose homogeneously dissolved in aqueous alkali with liquid CS2; (ii) a xanthation of rather well-ordered fibrous sodium cellulose with gaseous or liquid CS2 (so-called fiber xanthation). A so-called 'emulsion xanthation', i.e. the reaction of a cellulose suspension in aqueous NaOH with liquid CS2, leading to gradual dissolution of the polymer during reaction, takes an intermediate position between these borderline cases. The industrial xanthation process usually corresponds quite closely to a fiber xanthation. In homogeneous xanthation, a strongly preferential C-6 substitution takes place. With sufficiently high CS2/NaOH input all three hydroxy groups of the AGU can be xanthogenated up to a DS of nearly 3 in a homogeneous or emulsion xanthation (Geiger and Weiss, 1953). With increasing substitution of hydroxy groups by xanthogenate groups, the rate constant of homogeneous xanthation decreases, while that of dexanthation remains nearly constant. An enhanced hydroxy concentration in homogeneous xanthation leads to an increased xanthation rate constant especially for the C-6 position, obviously due to a further breakdown of intra- and/or intermolecular cellulosic hydrogen bonds. Fiber xanthation of an alkali cellulose, on the other hand, is characterized by a limited maximal DS of about 0.9-1.0 even with a large excess of €82 and by a preferential substitution at the C-2 position. CS2 physically dissolved in the

152

4 Λ Ε st erification of Cellulose

adhering lye is the active agent also in the xanthation of fibrous alkali cellulose, and the reaction rate increases with increasing €82 pressure according to Grotjahn (1953). Figure 4.4.20 presents the course of DS with time of reaction for different temperatures in the range of practical interest, employing a large excess of CS2From a quantitative evaluation of the kinetic data can be concluded that the reaction proceeds according to the scheme Na-CeII

k-1

Cell-xanthogenate

k-2

Cell Il

with cellulose xanthogenate as a moderately stable intermediate and the ratio of the rate constants k\lk^ being about 10 (Philipp, 1956). The experimentally observed maximal DSx values of about 0.9-1.0 in fiber xanthation in connection with this ratio of rate constants, indicate that obviously the so-called true alkali uptake of 1 mol of NaOH/mol of AGU of the alkali cellulose employed is the limiting factor for the DSx value, which comes rather close to a value of DS = 1 due to slow decomposition of the xanthogenate during its process of formation. A quantitative calculation presented in Philipp (1956) confirms this assumption. Furthermore, the maximal DSx remains constant within the total range of sodium cellulose I formation, i.e. at steeping lye concentrations between 14 and 22 % (see Fig. 4.4.21), and the same holds true for the rate constant of xanthogenate formation and decomposition.

2

3

4

Time[h]

Figure 4.4.20. Course of γ-values on alkali cellulose fiber xanthation at different temperatures (O 20 0C, Δ 28 0C, D 35 0C) (Philipp, 1957c).

At a steeping lye concentration above 22 %, the maximal DSx decreases due to a strongly diminished xanthation rate caused by lack of free water as solvent for the CS2 (Bartunek, 1953). At a steeping lye concentration below 14 %, the maximal DSx, as well as the true alkali uptake, decrease sharply indicating an

4.4.2 Cellulose esters with reagents derived from carbonic acid (H2 CO^)

153

alkali-cellulose formation in the less well-ordered regions only (see chapter 4.2). Simultaneously, the rate constant of xanthation increases significantly and the energy of activation decreases: at a concentration of steeping lye of 6 % NaOH, the maximal DS amounts to 0.24, the energy of activation to only about 7 kcal/mol, and the rate constant of xanthation is twice the value observed after steeping with 18 % NaOH. The low-ordered regions of alkali cellulose are obviously much more rapidly xanthogenated in possibly a diffusion-controlled process than the crystalline regions. Xanthation of the crystalline regions of sodium cellulose I can be classified as a so-called lattice layer reaction, with the 1-0-1 lattice distance gradually increasing during the reaction, but with some delay at the beginning in comparison with the course of DSx (Hess et al., 1951), indicating again a faster xanthation of the less well-ordered regions. ^NoOH-uptake

1Q

0.5

O

5

10 15 20 NaOH [wt.%]

25

Figure 4.4.21. Maximal DS of fiber xanthogenate and 'true' NaOH uptake in mol NaOH/mol AGU(see chapter 4.2) of alkali cellulose in dependence on steeping lye concentration.

A comparison of alkali-cellulose samples prepared by steeping with 18 % NaOH of different pulps resulted in significant differences in the rate constant of fiber xanthation of up to 25 %, with alkali cellulose from !inters exhibiting the lowest value (Philipp, 1956). Table 4.4.20. Xanthogenate group distribution in fiber xanthogenate and viscose.

Sample

DS at C-2/C-3

Fiber xanthogenate (DS 0.61) Viscose, non-ripened (DS 0.58) Viscose, moderately ripened (DS 0.49) Viscose, extensively ripened (DS 0.28)

0.38 0.34 0.16 O

DS at C-6 0.17 0.24 0.32 0.32

Technique: Preparation of DA-xanthogenate, tosylation, iodination. Total DS via N-content of DS-xanthogenate. Distribution via analysis of iodinated sample.

154


The preferential substitution at the C-2 position in xanthation with a limited amount of CS2 (see Table 4.4.20) has been correlated in early work with the higher acidity of this hydroxy group, but is obviously mainly caused by a low availability of the C-6 hydroxy group in the ordered structure of the alkali cellulose during fiber xanthation, while on homogeneous xanthation with freely available hydroxy groups in all three positions the C-6 position is obviously favored. Cellulose fiber xanthogenates at a DSx level of about 0.5 can be easily and completely dissolved in 1-2 molar aqueous NaOH to give a viscous polymer solution containing, besides the cellulose xanthogenate, trithiocarbonate, carbonate and sulfide, as well as small amounts of di- and monothiocarbonate, perthiocarbonate and thiosulfate as by-products. Also, some monothiocarbonate substituents at a DS level below 0.04, have been detected in the cellulose xanthogenate moiety (Bernhardt, 1926). This cellulose xanthogenate solution undergoes rather complex chemical and colloidal changes on standing ('ripening'), which are of high relevance to viscose preparation and spinning. The overall DSx decreases continuously during this ripening process. A fast decrease is observed in the number of xanthogenate groups at C-2, while the level of partial DSx at C-6 remains rather constant over a long period or was even found to be temporarily enhanced (Fig. 4.4.22). 60

CO Q

20

O

5

10 15 Time[h]

20

25

Figure 4.4.22. Course of partial DSx during viscoseripening(· C-6, · C-2, A C-3) DS [%] = % of total DS (König et al., 1993). Furthermore, a rather constant level of free CS2 of about 1 % of the total amount bound to cellulose could be detected in these cellulose xanthogenate solutions and was found to appear again even after precipitation throughout washing and redissolution of the cellulose xanthogenate (Philipp and Dautzenberg, 1967). From a quantitative evaluation of these facts and other observations, most researchers including the authors group assume a quasi-equilibrium of dexanthation and rexanthation in these aqueous alkaline cellulose xanthogenate solutions resulting in a redistribution of xanthogenate groups by transxanthation,

4.4.2 Cellulose esters with reagents derived from carbonic acid (H2CO 3)

155

although other opinions have been published (König et al., 1993). This quasiequilibrium of de- and rexanthation can be formulated according to CeII-OH + OH'+ CS2 K

Cell-O-CSS' + H2O

[CeIl-O-CSS-J[H2Q] ~ [CeIl-OH][CS2][OH"]

with the quasi-equilibrium constant K being about 105 for the comparatively stable C-6 xanthogenate, and about 103 for the more labile C-2/C-3 xanthogenate in 1-2 N NaOH at room temperature. From model experiments can be concluded that the rate constant of dexanthation at the C-2/C-3 position is about 16 times higher than the corresponding one at C-6, while the rate constant of rexanthation at C-6 is about 4 times higher than that at C-2 in 1-2 N aqueous NaOH at 20 0C. From this quasi-equilibrium, some free CS2 is continuously drained by irreversible reactions with OH or SH ions. While liberation of CS2 by reaction of xanthogenate with water molecules is the dominating route of dexanthation up to an NaOH concentration of about 2 N, xanthogenate decomposition to dithiocarbonate prevails at higher alkali concentrations, and no transxanthation can occur e.g. with 10 N NaOH as the medium (Table 4.4.21). Table 4.4.21. Decomposition of cellulose xanthogenate in aqueous NaOH.

MoI of NaOH/1 0.1 1.0 2.0 6.5 10.0 a

% CS2a 99 98 70-90 14-18 O

%CS 2 02-a

1

2

10-30 82-86 100

Percentage relative to xanthogenate.

In consequence of the transxanthation process (Matthes, 1952) outlined here briefly, xanthogenate substituents are not only transferred from the C-2 to the C6 position but also are more evenly distributed along and between the cellulose chains, resulting in a drop in solution viscosity. During ripening, the concentration of low molecular by-products, especially of Na2CS3, steadily increases at the expense of hydroxy ions, and the tendency of the system to coagulate on addition of electrolytes (NaCl, M^Cl) is enhanced. After extensive overall dexanthation, the viscosity increases rather steeply due to loss of hydrophilic anionic groups until syneresis of the system takes place (Götze, 1967).

156


The course of cellulose xanthogenate decomposition in solution can be retarded or accelerated by adding compounds interfering with the transxanthation via free CS2*. the irreversible drain of free CS2 can be enhanced by addition of H2Ü2 or retarded by addition of Na2SU3 via the level of disulfide formed in the system, which reacts very rapidly with CS2 to give perthiocarbonate. Addition of small amounts of polyhydric alcohols with vicinal hydroxy groups, like glycol or glycerine, results in fast, irreversible C$2 consumption and the formation of rather large amounts of COS, leading to an increased overall decomposition rate of the xanthogenate. Besides this, a fast drop in viscosity is observed probably due to the participation of COS in the exchange of substituents, resulting in a more uniform substituent distribution. A similar effect, i.e. a significantly reduced viscosity of the system, has been observed according to Philipp (1957a) on addition of a few percent COS to the liquid CS2 employed in xanthation. Like all other xanthogenates, cellulose xanthogenate is rapidly decomposed in a strongly acidic medium, e.g. in 1 N Η^βΟφ via free cellulose xanthogenic acid, to give CS2 and cellulose II, the physical structure of which largely depends on the chemical and physical state of the xanthogenate solution and on the conditions of acid treatment, and which can be influenced, via the ratio of coagulation rate to decomposition rate, by the presence of zinc ions and special additives (see the next section). Cellulose xanthogenate is rather easily decomposed already at a temperature of 90-100 0C with evolution of CS2- Cellulose xanthogenate is precipitated from its aqueous alkaline solution by numerous transition metal cations, especially those forming widely insoluble sulfides like Zn2+, Hg2+ or Ag+. Some of these xanthogenates show a spontaneous decomposition to the corresponding metal sulfide as observed for example with silver or mercury salts of cellulose xanthogenate. Cellulose xanthogenate is a rather reactive cellulose ester well suited for subsequent steps of derivatization. Some typical routes are indicated in Fig. 4.4.23. Reaction of cellulose xanthogenate with nonsubstituted or substituted alkyl halides leads to full esters of cellulose xanthogenic acid and permits the attachment of various functional groups onto the cellulose chain. These full esters are much more stable than the cellulose xanthogenate itself, and they are soluble in various organic liquids. Of special analytical interest in determining the pattern of substitution is the stable and organosoluble ester formed with Λ^,Λ^-diethyl chloroacetamide (Matthes, 1952). Some reactions proceeding with the elimination of one sulfur atom or of CS2 open up a route to special cellulose derivatives like aryl-substituted cellulose thiocarbamates or cyanoethylcellulose. Furthermore, the xanthogenate group can be employed for covalent crosslinking between the cellulose chains or for preparing radical sites on the cellulose chains for subsequent grafting (see chapter 4.1). Of special analytical interest for a convenient titration of xanthogenate groups is the oxidation of two SH functions to a disulfide bridge by iodine.

4.4.2 Cellulose esters with reagents derived from carbonic acid f//2C(9jj R = Alkyl, CH2-COOH, 2 — CON(C2H5)2, \

O

CI-R l

* Cell-O-C; H2C-CH-CN, H2O - CS2, - NaOH

,S

CeII-O-C' ^SeNa®

157

* CeH-O-CH2-CH2-CN

1

H9N-R - /VaSH

CeII-O-C' S

NHR

1

H2CN2, H 2 O - CS2, - NaOH, - N2

I2

CeII-O-CH3

CeII-O-C

V-sx Figure 4.4.23. Consecutive reactions of cellulose xanthogenate.

Survey of the commercial viscose process After its invention by Cross, Bevan and Beadle in 1893 (Cross et al., 1893) the viscose process of manufacturing cellulose rayon filament and staple fiber has been practised for many decades as the only one and later as the dominating one in the commercial production of chemical fibers. Important aspects of the process are its versatility and adaptability to end-use requirements and one century of process engineering experience. As severe shortcomings, from the ecological hazards connected with the handling and disposal of CS2 and ^S, to the low speed of spinning in comparison with melt-spun synthetic fibers have to be mentioned. These disadvantages, however, could be at least partially compensated by recent developments, which will be adequately emphasized in the following context. The scheme in Fig. 4.4.24 gives an overview of the numerous steps of chemical and physical treatment of cellulose during the viscose process. Hard wood as well as soft wood sulfite or prehydrolysis sulfate pulp with an α -cellulose content of between 91 and 96 %, an ash content of < 0.1 %, a very low content of calcium and heavy metal ions and a high uniformity at all three structural levels is used today as the starting materials. The conversion to alkali cellulose (sodium cellulose I) is usually performed by continuous slurry steeping with aqueous NaOH of about 18 % concentration and subsequent continuous pressing to a cellulose content of 32-35 % and an alkali content of between 15 and 16%. After shredding and oxidative depolymerization ('preripening', see chapter 2.3) to the appropriate level of DP, xanthation of the alkali cellulose takes place in a dry or wet churn process (or frequently in a hybrid process starting with dry alkali cellulose followed by subsequent addition of aqueous

158


NaOH) with a total amount of 28-30 % CS2, at a temperature of about 30 0C for several hours. The cellulose xanthogenate, with a DS of about 0.5, is dissolved in dilute aqueous NaOH, usually under high-intensity mechanical agitation, to give a viscous solution containing about 8 % cellulose and about 6-7 % total NaOH. Besides Na cellulose xanthogenate and free NaOH, this viscose solution contains trithiocarbonate and carbonate at the 1 % level, sulfide and perthiocarbonate at the 0.1 % level, and small amounts of thiosulfate, dlthiocarbonate and monothiocarbonate. The subsequent viscose ripening for 1-3 days at constant temperature, at the level of or below room temperature, serves the purpose of adjusting the degree of substitution and the viscosity of the solution to the level

Steeping with 18% NaOH CeII-OH + NaOH—> CeII-O0Na0 + H2O Alkali cellulose shredding

Preripening of alkali cellulose

Oxy 'dative depot ymerization

Xanthogenation with CS2 (sulfidation)

CeII-O0Na0+ CS2 CeII-O-C-S 0 Na 0 Il S

Dissolution of xanthogenate in dilute aqueous NaOH

Viscose ripening

CeII-O-C-S 0 Na 0 + H2O 5 CeII-OH + Na0OH0 + CS2

Filtration of the spinning solution

Spinning into acid bath (H2SO4, Na2SO4)

CeII-O-C-S 0 Na 0 + H0 S Il Ä m CeII-O-C-S 0 H 0 CeII-OH + CS2

Filament aftertreatment Figure 4.4.24. Scheme of the viscose process.

4.4.2 Cellulose esters with reagents derived from carbonic acid (H2CO^)

159

desired for the spinning process, and to redistribute the xanthogenate substituents for achieving a higher uniformity of substitution along and between the chains. This step is often combined with several procedures of filtration in order to eliminate persistent fiber fragments and to reduce the gel particle content of the system to a sufficiently low level. Much research and development work has been put into reducing the CS2 input from about 35 % 20 years ago, to less than 30 % today, and about 25 % being the goal of further development. This reduction in CS2 input necessary for ecological reasons has to be performed without impeding the quality of the spinning solution by a higher content of fiber fragments and gel particles due to a lower DS of the xanthogenate. To solve this problem, a better yield of the CS2 input for xanthation (less by-product formation) and a higher uniformity of the xanthogenate had to be striven for. Two routes have been successfully pursued for this purpose in recent years. The first one consists of the supply of a highly reactive pulp obtained by special pulping procedures, by loading the pulp with surfactants, facilitating xanthation and by providing a pulp with a higher chainlength uniformity, for example by radiation depolymerization. As recently shown by Fischer et al., (1996) the high-DP part of a dissolving pulp usually carries less than the adequate amount of xanthogenate groups and thus can lead to difficulties in viscose filtration and spinning due to the presence of low substituted fiber fragments and gels. The second route is characterized by a still better mutual adaptation of the various steps of viscose preparation. The principle of the conventional viscose-spinning process consists of pressing the viscose solution through corrosion-resistant spinnerets with about 100 holes in the case of rayon filaments spinning, several 1000 holes in the case of rayon staple spinning, with a hole diameter of between 50 and 100 μηι, into an acid bath of aqueous t^SC^ and Na2SC>4 at a temperature of about 40 0C, and conveying the thread of cellulose II successively formed via godets onto a bobbin. Structure formation of the thread is governed by the rate ratio of cellulose xanthogenate coagulation and decomposition on the one hand, and the mechanical forces exerted on the forming thread at various stages of the spinning process on the other. But also the state of ripening of the spinning solution has a strong bearing on the fiber structure formed due to its close interconnection with both of the factors mentioned. Via the process parameters of spinning and aftertreatment, the mechanical properties of the threads can be varied within wide limits. For further details the reader is referred to Götze (1967). But only one point shall be mentioned briefly: by the presence of zinc ions in the spinning bath, often in combination with special additives based on amines and/or polyethylene oxides, so-called skin-core-filaments with a different fibrillar architecture in the outer skin and the inner core can be produced resulting in textile properties that are outstanding for special applications. The effects obtained are based on a hindered diffusion of the HÔ ions into the fiber structure due to the clogging of

160


micropores by sulfidic zinc compounds (Klare and Grobe, 1964). Recent developments are aiming to reduce this zinc content for ecological reasons without compromising filament properties, and to produce a significant increase of the spinning velocity above its present level of about 170 m/min. This poses, of course, new physicochemical problems with regard to xanthogenate decomposition and filament structure formation, as well as engineering problems concerning for example a computerized and automated starting of the spinning process, or the hydrodynamics of conveying the forming thread through the spinning bath. From the present state of development, the forecast seems justified that despite the existence of alternative processes and despite ecological problems not yet fully solved, the viscose process will keep its place in the foreseeable future due to its versatility and due to the fact that viscose rayon filament and staple fiber are still indispensable in many areas of the textile industry. Properties of cellulose xanthogenate Cellulose fiber xanthogenate at the conventional DS level of about 0.5 is a yellowish fibrous mass, easily soluble in water or dilute aqueous alkali, exhibiting the properties of a poly electrolyte in these solutions. It can be precipitated from these aqueous systems by lower aliphatic alcohols or other water-miscible organic liquids, by salting out with low molecular electrolytes or by adding cations of heavy metals. Cellulose xanthogenate is unstable in aqueous media over the whole range of pH and is rapidly decomposed by acids with the evolution of CS2. Its thermal stability is rather low, decomposition starting already below 100 0C with the liberation of CS2. Applications of cellulose xanthogenate Concerning annual production capacity, cellulose xanthogenate is the number one among cellulose derivatives, but it is used as an intermediate only and not as a final product of chemical cellulose processing. Its quite predominant application is a transient solubilization of cellulose for converting the short wood-pulp fibers into endless filaments or staple fibers of cellulose II. But also films of cellulose II, especially for food packaging purposes, are still manufactured in many countries from aqueous alkaline cellulose xanthogenate solutions (viscose). Besides this, cellulose xanthogenate solutions can be employed to convert cellulose into specially shaped products, by putting the viscose into the appropriate form before decomposition. An example of commercial relevance is the production of cellulose sponges (macroporous sponges) by thermal decomposition of viscose in chest-like forms after previous addition of crystalline Na2SC^. Finally, the preparation of macroporous cellulose beads from viscose shall be mentioned as a recent development in the area of carrier and separation materi-

4 A.2 Cellulose esters with reagents derived from carbonic acid (7/2^(9 3)

161

als. In this process, drops of viscose are coagulated and decomposed to cellulose II beads in an organic liquid of suitable density and boiling point, which is inmiscible with water, at a temperature of about 90 0C (Dautzenberg et al., 1985a and b). Chlorobenzene was found to be especially suitable for this process. After the decomposition, the low molecular by-products are washed out with water. By variation of composition and state of ripening of the viscose, as well as of the conditions of decomposition, the pore structure of the beads obtained can be varied within wide limits and adapted to special end-use requirements.

4.4.2.3 Cellulose carbamate General comments on formation and decomposition of cellulose carbamate and its possible applications Cellulose carbamates with a low DS of about 0.3 have received considerable attention in recent years as alkali-soluble intermediates in an alternative process of artificial cellulose-fiber spinning, the so-called carbamate process (Segal and Eggerton, 1961; Ekman, 1984; Lang et al., 1986). Cellulose carbamates are formed in a high-temperature reaction between cellulose and urea via isocyanic acid as active intermediate. From their aqueous alkaline solution these carbamates can be spun in an acid bath to filaments, subsequently decarbaminated by alkali to threads of cellulose II. The chemistry of this process looks very simple, but in reality is probably still more complicated than that of the viscose process, as numerous condensation equilibria of C-N bond formation and cleavage have to be considered. Furthermore, only a small DS range, between 0.2 and 0.3, is available for preparing alkali-soluble cellulose carbamates, because with increasing DS a growing tendency of crosslink formation counteracts the solubilizing action of the hydrophilic substituents. Due to these facts and still unsolved problems of decarbamation, the process is now practised on a pilot scale only, despite looking very promising at first. Chemistry of cellulose carbamate formation and decomposition On heating cellulose with urea above its melting point of 133 0C, carbamate ester groups can be introduced into the cellulose chain by reaction of hydroxy groups with isocyanic acid formed as an active intermediate on decomposition of urea (Fig. 4.4.25, A). This reaction is catalyzed by metal salts, especially zinc sulfate. Suitable external conditions have to been chosen in order to eliminate the ammonia formed as by-product of urea decomposition and to minimize isomerization of isocyanic acid to cyanic acid, as the latter favors crosslinking by condensation reactions. But also the isocyanic acid can give rise to condensation structures, for example by biuret formation (Fig. 4.4.25, B). Generally, crosslinking between polymer chains impeding solubility is enhanced by in-

162


creasing the temperature and the time of reaction, but, on the other hand, a sufficiently large number of hydrophilic substituents must be introduced to cleave the interchain hydrogen bonds in the subsequent process of dissolution. The extent of crosslinking can be estimated by comparing the DS obtained by mass increase of the purified product and the DS obtained via its nitrogen content, with the latter usually having the lower value. According to recent 13C NMR studies (Nehls et al., 1994), reaction of hydroxy groups by carbamate ester groups takes place exclusively at the 2 position. Due to the low DS level set by the crosslinking tendency, a intimate contact between cellulose and urea and an equal temperature throughout the whole mass are necessary to ensure a sufficiently uniform substituent distribution along and between the polymer chains. For cleaving crosslinks formed via CN bonds, also a treatment of the reaction mass with liquid ammonia has been considered besides for the main purpose of extracting excess urea.

H2N A)

\

C=O

14O0C

— HN = C = O

^

H |sj

+ NH3

lsocyanic acid

CeII-OH ~~ + HN = C = "~ O

CeII-O-C-NH2 - Il O CeII-O-C-NH2 _îm^ CeII-OH - NH3, Na2CO3

H2N

C =O / + CeII-OH Cross/inked ΗΝχ %e//u/ose

— B) HN=C = O —

H2N

/ H2N

+

C 7

C= O

H

=0

2N Biuret

Figure 4.4.25. Scheme of the carbamate process. Cellulose carbamate with a DS of between 0.2 and 0.3 can be dissolved in aqueous NaOH of optimal solvent power, i.e. a concentration between 10 and 11%, eventually containing additionally some zincate or berylate (see chapter 4.3). In this alkaline medium the carbamate groups are irreversibly decomposed

4.4.2 Cellulose esters with reagents derived from carbonic acid (T^COjj

163

to carbonate and ammonia at a rate depending on NaOH concentration and temperature, and unsubstituted cellulose is formed which can eventually coagulate to a low ordered cellulose II after sufficient decarbamation. A redistribution of substituents cannot take place in this system, as the decomposition is irreversible, in contrast with transxanthation via free CS2 in the viscose process. Also, in contrast with cellulose xanthogenate, cellulose carbamate is rather stable in an acid medium and can be coagulated as a cellulose carbamate thread by spinning in an acid bath. Brief description of the cellulose carbamate process of fiber spinning As the starting material, a highly reactive dissolving pulp at a DP level of about 300 is employed, the latter being obtained either by the pulping process itself or by irradiation depolymerization, or by an intermediate alkalization and oxidative depolymerization of the alkali cellulose. For an intimate contact between cellulose and reagent, the pulp is swollen in an aqueous solution of urea of about 40 % concentration at a solid-to-liquid ratio of about 1 : 3 for several hours at ambient temperature, then pressed off, milled and dried. The mass containing a large excess of urea and eventually some zinc sulfate as catalyst is than reacted for 1 to 2 h at 140-150 0C, either in a rotating kiln in the presence of air, or in a stirred reactor with an inert medium (hydrocarbon) caring for the uniform transmission of heat and impeding isomerization of isocyanic to cyanic acid. The yellowish-to-brown colored reaction product, with a DS between 0.25 and 0.30, is extracted with water or with liquid ammonia to recover excess urea and eliminate colored by-products. A hydrolysis step at high pressure and temperature may be included for partial cleavage of crosslinks before the product is dissolved in aqueous sodium hydroxide of 10-11 % concentration and kept for some time at a temperature of about 5 0C for gradually reducing the DS without an early coagulation of the system. The viscosity of the solution decreases initially during this process but increases again with further lowering of the DS. After excessive filtration, the solution with a polymer content of 6-7 % is spun in an acid bath with about the same speed as in the viscose process to obtain a cellulose carbamate thread. In order to produce artificial fibers of sufficiently high wet tenacity, the carbamate groups have to be eliminated by a subsequent alkaline treatment as far as possible, followed by an acid treatment step for deswelling. The problem of eliminating the last residual carbamate groups from the cellulose chains within the fiber structure limits at present the textile quality of threads spun by the carbamate process, but probably the quality level of a conventional rayon staple fiber can be obtained. In comparison with the viscose process, the carbamate process has the advantage of a better ecological compatibility, and much of the conventional equipment of a viscose plant can be used, but on the other hand it still lacks the versa-

164


tility and the ultimate quality level of rayon filament and staple spun from viscose. The present technology of the carbamate process resembles some kind of 'via rope walk', as in several points it has to find an acceptable balance between counteracting effects. Purified cellulose carbamate with a DS of about 0.3 is a white mass, containing some covalent crosslinks, and is soluble in aqueous alkali under slow decomposition to cellulose II, carbonate and ammonia, whereas it is insoluble and rather stable in dilute aqueous acid. The only application of cellulose carbamate known so far is its use as an intermediate for solubilizing cellulose in the carbamate process for artificial fiber spinning, practised now on a pilot scale.

4.4.3 Esters of cellulose with organic acids 4.4.3.1 General remarks The formation of cellulose esters of organic acids proceeds along the routes of esterification of alcoholic hydroxy groups, well known from low molecular organic chemistry: usually the anhydride or the chloride of the acid in question is employed as the agent, while the reactivity of the acid itself suffices only in some cases to obtain an appreciable degree of esterification, even at large excess. An acyl cation RCO+ can be generally assumed as the active intermediate, the formation of which is favored either by an acid catalysis in the case of the free acid as the reagent, according to RCOOH + H+ -> RCO+ + H2O or by the adjuvant action of a tertiary base like TEA or pyridine, with the acid derivatives serving as the agent, according to RCOCl + NR'3 -^ RCO-NR3+ CIAs a very effective adjuvant base, 4-dimethylaminopyridine was recommended for various esterifications of cellulosic hydroxy groups in homogeneous systems (Philipp et al., 1983). Esterification of alcoholic hydroxy groups generally takes place as an equilibrium reaction, with the ester bonds formed being susceptible to hydrolytic cleavage in an aqueous acid medium, and at sufficiently high alkalinity an irreversible saponification of the ester groups can occur. While practically any aliphatic or aromatic acid chloride can be reacted with cellulosic hydroxy groups at 100 0C in the presence of pyridine as a free base, application of pyridinium hydrochloride is frequently more favorable in the case of an acid anhydride as the esterifying agent, as it promotes formation of the acid chloride as an active intermediate. Especially in synthesizing long-chain aliphatic or aromatic esters of cellulose, the use of a chlorinated acid anhydride like chloroacetanhydride, in combination with the free acid to be esterified, can

164


tility and the ultimate quality level of rayon filament and staple spun from viscose. The present technology of the carbamate process resembles some kind of 'via rope walk', as in several points it has to find an acceptable balance between counteracting effects. Purified cellulose carbamate with a DS of about 0.3 is a white mass, containing some covalent crosslinks, and is soluble in aqueous alkali under slow decomposition to cellulose II, carbonate and ammonia, whereas it is insoluble and rather stable in dilute aqueous acid. The only application of cellulose carbamate known so far is its use as an intermediate for solubilizing cellulose in the carbamate process for artificial fiber spinning, practised now on a pilot scale.

4.4.3 Esters of cellulose with organic acids 4.4.3.1 General remarks The formation of cellulose esters of organic acids proceeds along the routes of esterification of alcoholic hydroxy groups, well known from low molecular organic chemistry: usually the anhydride or the chloride of the acid in question is employed as the agent, while the reactivity of the acid itself suffices only in some cases to obtain an appreciable degree of esterification, even at large excess. An acyl cation RCO+ can be generally assumed as the active intermediate, the formation of which is favored either by an acid catalysis in the case of the free acid as the reagent, according to RCOOH + H+ -> RCO+ + H2O or by the adjuvant action of a tertiary base like TEA or pyridine, with the acid derivatives serving as the agent, according to RCOCl + NR'3 -^ RCO-NR3+ CIAs a very effective adjuvant base, 4-dimethylaminopyridine was recommended for various esterifications of cellulosic hydroxy groups in homogeneous systems (Philipp et al., 1983). Esterification of alcoholic hydroxy groups generally takes place as an equilibrium reaction, with the ester bonds formed being susceptible to hydrolytic cleavage in an aqueous acid medium, and at sufficiently high alkalinity an irreversible saponification of the ester groups can occur. While practically any aliphatic or aromatic acid chloride can be reacted with cellulosic hydroxy groups at 100 0C in the presence of pyridine as a free base, application of pyridinium hydrochloride is frequently more favorable in the case of an acid anhydride as the esterifying agent, as it promotes formation of the acid chloride as an active intermediate. Especially in synthesizing long-chain aliphatic or aromatic esters of cellulose, the use of a chlorinated acid anhydride like chloroacetanhydride, in combination with the free acid to be esterified, can Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

4.4.3 Esters of cellulose "with organic acids

165

be of technical advantage: the chlorinated acid anhydride acts as an 'impelling agent' according to (ClAc)2O + 2 RCOOH -» R(CO)2O + 2 ClCH2COOH and the preparation and excess application of an expensive acid anhydride can thus be avoided. With acid chlorides of higher aliphatic or of aromatic acids, esterification of cellulose can be performed also according to the SchottenBaumann reaction with alkali cellulose as the starting material, but a large excess of reagent is required here of course. Besides the direct action of free carbonic acids, their anhydrides or their acid chlorides on cellulosic hydroxy groups, transesterification reactions frequently provide a suitable route to cellulose esters. These reactions proceed either by interaction of hydroxy groups of the polymer with a labile ester or a salt of the acid in question, or by reaction of acid anhydrides or chlorides with labile ester or ether groups attached to the polymer and serving as a leaving group in esterification. As a technique of the future, enzymatic esterification of cellulose may be envisaged, although it has not been realized up to now with the polymer itself, but only with pyranosidic and furanosidic low molecular saccharides: a pancreatin-catalyzed transesterification reaction between saccharidic hydroxy groups and vinyl acetate in a THF/TEA mixture was reported (Lay et al., 1996) resulting in a site-specific acetylation of the C-6 position of the saccharide. Besides the well-known principles of esterification, some characteristics have to be kept in mind in connection with cellulose ester formation. Also, esters carrying two different substituents ('mixed esters') or ether-esters of cellulose can be prepared, the former playing a role also on the commercial scale. The degree of substitution obtained and the distribution of the substituents along and between the macromolecules is largely governed by the accessibility of the hydroxy groups, just as in other reactions of the polymer. In a thoroughly heterogeneous system with a low degree of cellulose swelling, the reaction can be limited to the surface of the fiber material resulting in a very low average DS. In systems exhibiting a high degree of swelling or turning from the heterogeneous to the homogeneous state in the course of reaction, a DSof 3, or of nearly 3, can be reached under suitable conditions frequently, and the same holds true, of course, after previous dissolution of the polymer in a nonderivatizing solvent like DMA/LiCl. The preference of substitution within the AGU at low DS depends largely on the reaction system considered, but often a substitution reaction at C-6 is found to be favored. Although in principle any aliphatic or aromatic acid residue can be attached to the cellulose backbone by esterification, only a limited number of these derivatives has been thoroughly studied and only rather few, especially the acetate and some mixed esters with acetate groups, have gained commercial relevance. So

166


far, esterification of cellulose has mainly served the purpose of modifying the material properties of the polymer, and rather high DS values were required with the question of arriving at a DS of exactly 3 often being of scientific and practical interest. In connection with the recent trend of cellulose chemistry to tailored derivatives as building blocks for defined complex supramolecular structures, the preparation of regioselectively substituted organic esters receives increasing attention and will be adequately considered in this subchapter. Subsequently, a brief systematic survey shall be presented on the characteristics of formation and decomposition of the various classes of organic cellulose esters, on the role of supramolecular structure in these processes as well as on properties and areas of application of the products. This survey will be structured according to the classes of organic acids in question, i.e. unsubstituted and substituted aliphatic carbonic acids, aromatic carbonic acids and sulfonic acids, and it will also include the urethanes of cellulose, especially the carbanilate, as a class of derivatives closely related to the esters.

4.4.3.2

Cellulose formate

The formylation of cellulose belongs to the rather few esterification reactions of this polymer with organic acids proceeding to high DS values with the free acid itself. As illustrated by Fig. 4.4.26, DS values of about 2.5 are obtained with 98100 % formic acid at room temperature after a reaction time of about 2 weeks, with the formic acid simultaneously acting as a 'derivatizing solvent', leading finally to a macroscopically homogeneous medium (Takahashi et al., 1986; Philipp et al., 1990).

fe2

Λ Q

1

O

Λ

8

Reaction time [d]

12

Figure 4.4.26. Course of cellulose formylation in HCOOH (Philipp et al., 1990).

Addition of F^SC^, HCl or ZnC^ was found by these authors to increase decisively the rate of esterification (see Fig. 4.4.26). A lowering of the formic acid concentration and/or an increase in water content result in a decrease in formyl group content, as to be concluded from the preparation of DMS O- soluble cellulose formates of a DS of about 0.6 in a reaction system consisting of formic acid, phosphoric acid, water, and cellulose, and becoming homogeneous after about

4.43 Esters of cellulose with organic acids

167

24 h (Schnabelrauch et al., 1992). In several studies, a preferential reaction at the 6 position has been reported, followed by one at C-2. Esterification of cellulose with concentrated formic acid combined with dissolution of the polymer is generally accompanied by a severe hydrolytic chain cleavage by scission of the glycosidic bonds. A special route to cellulose formate, interesting from the viewpoint of synthetic organic chemistry, has been described by Vigo et al. (1972), by reacting cellulose in DMF with thionyl chloride, and employing a formimminium compound of cellulose as an intermediate, according to the scheme in Fig. 4.4.27. H I SOCI2 + 0 = C-N(CH3)2

CeII-CI Chlordesoxycellulose - SO2

Il ι θ +CeII-OH ι +CeII-OH ° CI-S-O-C=N(CH3)2Cle ^CeII-O-S = O *· CeII-CI+ CeII-O-S —OH - HCI, - DMF -HCI \ + H2O

O Il Cell—O —S-OH Cellulose sulfite

__ CeII-CI - DMF

^ - SO2

H I θ CI-C=N(CH3)2Cle

+ CeII-OH - HC/

H l Φ * Cell-0-C=N(CH3)2Cle

- (CH3J2NH2CI I + H2O O Il CeII-O-C-H

Cellulose formate

Figure 4.4.27. Scheme of reaction route to cellulose formate via formimminium compounds (Vigo et al., 1972). As compared with cellulose acetate and the higher fatty acid esters, cellulose formate has to be classified as an unstable derivative: already the moisture content of the air leads to a slow liberation of formic acid, and according to Fujimoto et al. (1986) a cellulose formate with a DS of 2-2.5 is completely decomposed by 10 h boiling with water. From studies on sulfation of cellulose formates with a DS of 2-2.5 in DMF it could be concluded that sulfation takes place not only at the free hydroxy groups but probably also by a transesterification of formyl groups, in contrast with the behavior of cellulose acetate (Philipp

168


et al., 1990). Also, a comparatively low thermal stability had been assessed by DTA measurements in the same study. The course of cellulose formate formation is strongly affected by the supramolecular order of the polymer: the rate of esterification/dissolution in concentrated formic acid was found to increase in the order cotton !inters < wood pulp < viscose rayon, and could be considerably enhanced by a suitable preactivation of the cellulose sample. The course of a strictly heterogeneous formylation with 90 % aqueous formic acid was observed to depend strongly on cellulose physical structure on the one hand, and on the reaction temperature on the other, and has been employed by several authors to obtain a so-called lateral order spectrum of the sample in question (Marchessault and Howsmon, 1957; Philipp and Baudisch, 1965). According to these studies, structural regions of the sample of successively lower accessibility are made available for formylation by a stepwise increase of reaction temperature in the range between -5 0C and +40 0C, resulting, under otherwise constant conditions, in characteristic lateral order patterns for different samples. It has to be emphasized that these patterns can by no means be considered as absolute lateral order spectra but only as a kind of 'finger print' for classifying various cellulose materials, especially different types of viscose filaments. From the viewpoint of material properties, cellulose formates are characterized by their high susceptibility to hydrolytic ester cleavage, as already mentioned, and by a good solubility over a wide range of DS in various polar solvents like DMSO, DMF, concentrated formic and acetic acid, and dichloropropionic acid (Philipp et al., 1990). According to Schnabelrauch et al. (1992) solubility in DMSO was already observed at a DS of 0.6. Up to now, cellulose formates have not been produced on an industrial scale or applied commercially, and also the attempts to employ this unstable derivative as an intermediate in artificial cellulose fiber spinning so far have not met with practical success.

4.4.3.3

Cellulose acetate

General comments on reactions and products Cellulose acetate was described as the first organic ester of cellulose already by Schutzenberger (1865 and 1869), who reacted cotton cellulose with acetanhydride in a sealed tube at 180 0C and arrived at an ethanol-soluble product. Fourteen years later, Franchimont (1879) recognized the catalytic efficiency of H2SÜ4 and also of HC1U4 in this process. Both these observations provided the basis for commercial cellulose acetate manufacture starting already at the beginning of this century and performed up to now with acetanhydride as the esterifying agent and sulfuric acid (or in special cases also perchloric acid) as the catalyst. The raw materials are cotton !inters or refined wood pulp. Acetylsulfuric acid, formed by reaction between the agent and a catalyst, acts as an important

4.4.3 Esters of cellulose with organic acids

169

intermediate in this process providing the necessary level of acetyl cations for esterification. A considerable decrease in chain length due to hydrolytic cleavage of glycosidic bonds is an inevitable consequence of the strongly acidic system employed, resulting in a DP level of about 300 for the cellulose acetate, with a starting material of DP of 800-1600. Acetylation of cellulose is industrially performed either retaining the gross morphology of the fibers ('fiber acetylation'), or with a transition from an initially heterogeneous to a homogeneous reaction system ('solution acetylation'). In both cases a fully substituted cellulose triacetate (CTA) is obtained, as the reaction product in the fibrous state or dissolved in the reaction system, respectively. This derivatization to a DS of 3 is necessary in order to secure complete organosolubility of the product, as a lower average DS leads to an inhomogeneous distribution of acetyl groups along and between the polymer chains. While a fiber CTA is directly used for film casting or filament spinning after dissolution in e.g. CH^C^, the CTA obtained by solution acetylation is usually converted without isolation to a product with a DS of about 2.5 by partial deacetylation in a homogeneous acid system containing some water. In this way a homogeneous distribution of acetyl groups is obtained, and the so-called 'secondary cellulose acetate' or 'cellulose 2,5-acetate' is easily and completely soluble in the convenient solvent acetone, and can be converted to filaments or films by a so-called dry spinning process. Regarding material properties, cellulose acetate resembles more a synthetic plastic than a cellulosic, showing some similarities to cellulose trinitrate, but without the inflammability hazards of the latter. CTA and secondary acetate exhibit good mechanical properties and good stability under atmospheric conditions, including rot and water resistance. It can be processed, however, only via the solution state or in the presence of a large amount of plasticizer. Melting is accompanied by decomposition due to the high melting point of 225-250 0C for cellulose 2,5-acetate and above 300 0C for cellulose triacetate. Most of the approx. 0.9 million tonnes produced annually is employed for the production of filaments, fibers, films, membranes and cigarette filters. Besides its industrial relevance, acetylation of cellulose plays an important scientific role as a model reaction in elaborating new routes of synthesis for regioselectively substituted cellulose esters, and new analytical techniques for their comprehensive characterization. Chemistry of cellulose acetylation and deacetylation, including effects of cellulose accessibility on the course of reaction Cellulose, i.e. !inters or wood pulp, can be acetylated either by direct esterification of hydroxy groups or by a transesterification, employing a labile primary substituent, e.g. a nitrite group (Mansson and Westfeld, 1980), as the leaving

170


group. The reaction can be performed in a strictly heterogeneous way, retaining the gross morphology of the original fiber, with transition from a heterogeneous to a homogeneous state in a system capable of dissolving the CTA form, or in a strictly homogeneous way after previous dissolution of the unsubstituted polymer in a derivatizing or nonderivatizing solvent system. The first two routes are of industrial relevance in manufacturing CTA as a large-scale product, while the homogeneous route has been amply studied in recent years to prepare welldefined, partially acetylated products. In contrast with formic acid, acetic acid is not capable to esterify cellulose to a significant extent, and the more reactive acetanhydride is quite predominantly employed, mostly as a liquid, in special cases, also in the vapor phase. Ketene (CH2=C=O) can in principle also be used, if the course of reaction permits the intermediate formation of acetanhydride. Acetyl chloride represents a still more reactive esterifying agent, which is frequently employed in scientific studies, especially in homogeneous acetylation, in combination with a tertiary amine as an adjuvant base. As an interesting variation of the general procedure, acetylation of cellulose in DMF/pyridine with an alkali or alkaline earth salt of acetic acid in the presence of p-toluenesulfonyl chloride, has been reported (Shimizu and Hayashi, 1988). Introduction of acetyl groups by transesterification has also been achieved with ethylene diacetate, with the cellulose dissolved in the system /?-formaldehyde/DMSO at elevated temperature (Johnson, 1969; Johnson and Nicholson, 1976). Just like any esterification, acetylation of cellulose is an equilibrium reaction, which can be shifted to the ester side by applying an adequate excess of reagent and by minimizing the water content in the system, and which can be decisively accelerated by the presence of a suitable catalyst, promoting formation of the acetyl cation CH^CO+ as the reactive intermediate. Minimization of the water content is performed here by the interaction between water and acetanhydride (or acetyl chloride). Formation of acetyl cations is promoted in the case of acetanhydride as the reagent by adding t^SC^ (5-10 % of the weight of the cellulose) or HC1O4 (1-2 % of the cellulose weight) as an acid catalyst, forming acetylsulfuric acid or acetylperchloric acid, respectively. Methanesulfonic acid can be used, too, but is less effective, just as afe some Lewis acids like ZnC^. With acetyl chloride as the agent, tertiary amines like TEA or pyridine are well suited as the adjuvant base, forming an acylium complex according to:

R-C-Cl + N-R2 6

R,

/n D R-C-N-R '2

Il

O

\

R3

cr


111

Still more effective, especially in a homogeneous system with a nonpolar reaction medium, is the stronger, basic 4-dimethylaminopyridine (Philipp et al, 1983). The acid-catalyzed acetylation with acetic anhydride results in a dramatic drop in DP due to hydrolytic chain cleavage, for example from a DP of 15002000 of the bleached and scoured cotton !inters employed, down to DP values of 350-500 for a CTA prepared by the dissolution process. The preparation of high-DP cellulose acetates with DP values > 1200 was reported by Kulakova et al. (1971) in a system consisting of Ac2O/acetyl chloride and acetic acid. Turning now more closely to acetylation of cellulose in the acetic anhydride/acetic acid system, i.e. a heterogeneous system at least at the beginning of esterification, the decisive effect of cellulose accessibility, determined by the supramolecular and morphological structure of the polymer, must be emphasized first: the course of esterification is not only determined by the chemical reaction itself, but also depends largely on sorption, swelling and diffusion phenomena, which affect reaction rate and product quality. Similar to formylation, highly accessible regions are esterified first and/or under milder conditions, but in contrast with formic acid, swelling in acetanhydride is rather small and the active intermediate, i.e. acetylsulfuric acid or acetylperchloric acid, possesses a larger molar volume than formic acid, thus making penetration of the fiber moiety more difficult. Generally, the disordered regions of the fiber structure are considered to be more rapidly acetylated than the crystalline regions. Regarding the chemical interaction between cellulose, acetanhydride, acetic acid and catalyst (£[2804 or HClO4) the following statements may be condensed from the large number of experimental studies published already in the first half of this century (Malm and Hiatt, 1954). (i) A prerequisite of any thorough and uniform acetylation is the adequate activation of the cellulose fibers, predominantly performed by using acetic acid, (ii) The catalyst is strongly chemisorbed onto the fibers, as to be concluded from the large heat of sorption, which increases in the order of catalytic activity, i.e. ZnCl2 < H2SO4 < HClO4. (iii) Sulfuric acid as a catalyst not only promotes the formation of acetyl cations as the reactive species in esterification, but also leads to introduction of sulfate half-ester groups at a level of some percent of the acetyl groups, which must be removed in a subsequent step of stabilization (see chapter 4.1). In contrast with H2SO4, perchloric acid as a catalyst does not lead to an analogous esterification, but is known to cause a more severe chain-length degradation than H2SO4. (iv) Acetylation as well as sulfation obviously occur at a higher rate at the C-6 position compared with C-2/C-3. (v) The state of interaction between acetanhydride and catalyst influences the overall course of reaction, as shown by a comparison between a fresh and an aged acetylation mixture.

172


During acetylation of cellulose with Ac2O/HAc/catalyst, either a two-phase system can be maintained during the whole course of reaction, resulting in a socalled fiber CTA, or the two-phase system can be gradually transformed to a homogeneous one, yielding a so-called solution CTA. In fiber acetylation the preactivated cellulose is reacted with a large excess of Ac2U in the presence of HAc, usually with HC1Ü4 as the catalyst at slightly elevated temperature for 1 to several hours. Formation of the triacetate is indicated by a change in the birefringence in the fibers from a positive to a negative value. Also, Ac2U vapor can be employed for fiber acetylation, with the crystal modification of the fiber CTA depending on reaction temperature (CTA I below 80 0C, CTA II above 80 0C). This vapor process can be modified by adding some propionic or butyric acid anhydride to the vapor phase, obtaining the appropriate mixed ester, i.e. cellulose acetopropionate or acetobutyrate. Besides the preparation of the triester, fiber acetylation can be employed to give a morphologically limited partial acetylation, e.g. of only the surface of paper sheets, by suitable adaptation of reaction conditions. Also, an acetylation of whole wood fibers from Southern Pine with AC2Ü has been reported by Rowell (1982) and Shiraishi and Yoshioka (1986), resulting in a 20 % add-on, with a preferential introduction of acetyl groups into the lignin component compared with the holocellulose. The gradual transition from a heterogeneous to a homogeneous system in solution acetylation is achieved by a large excess of glacial acetic acid acting as a solvent for CTA, applying a moderate excess of Ac2U and !!2804 as the catalyst at a reaction temperature of about 50 0C and a reaction time of several hours. Complete dissolution does not occur until a DS of almost 3 is reached. Swelling and dissolution of the polymer is facilitated by the presence of methylene chloride, substituting some of the glacial acetic acid with this good solvent for CTA. Both these procedures again require an adequate preactivation of the polymer by acetic acid treatment with or without part of the catalyst. Both procedures are practised industrially to obtain a CTA solution that can be converted to an acetone-soluble product by homogeneous deacetylation to a DS of about 2.5 without intermediate isolation of the CTA. Some further details of the technical process will be presented in the subsequent section. Fiber CTA and solution CTA differ with regard to solubility in various media, colloid chemical behavior and rate of deacetylation, even at the same average DP and DS. This difference is obviously caused by a stronger interchain cohesion in the case of fiber CTA, resulting in larger supramolecular aggregates, even in dilute solutions of fiber acetates, compared with solution acetates (Bischoff, 1963). The commercial relevance of the chemical process of acetylation has promoted a mathematical modeling of the course of reaction on a predominantly phenomenological level, providing useful interpolation data on the change of DS with time of reaction in dependence on various reaction parameters.


173

Homogeneous acetylation of cellulose and cellulose derivatives in various systems has been amply studied for the last 30 years in connection with the application of new organic solvent systems, new routes of synthesis for regioselectively substituted cellulose derivatives, and new approaches for their comprehensive analytical characterization. From the results of this work it can be concluded that the reactivity of cellulosic hydroxy groups in homogeneous acetylation can vary widely in dependence on the system considered, and that a preferential or even a regioselective acetylation of one or two of the sites within the AGU can be achieved by an appropriate procedure of synthesis: homogeneous acetylation of free hydroxy groups in a partially acetylated sample of DS\c = 12 indicated a strong influence of the esterifying agent, as with acetanhydride a preferential substitution at C-6, with acetyl chloride a preferential substitution at C-2, and with the C-3 position showing the lowest reactivity in both cases (Nehls et al., 1994). Among the numerous nonderivatizing solvent systems for cellulose, so far only a solution in DMA/LiCl and a melt solution in jV-ethylpyridinium chloride have been successfully employed for acetylation of this polymer (McCormick and Chen, 1982; Miyamoto et al., 1984 and 1985; Kamide et al., 1987; Husemann and Siefert, 1969 and 1970), indicating a preferential substitution at the C6 position, With many other systems a smooth acetylation with the conventional reagents Ac2U and acetyl chloride is inhibited by a violent interaction between one of the solvent components and the agent. These detrimental side reactions can be widely avoided by employing derivatizing solvent systems. So, for example, the cellulose trinitrite formed on dissolving the polymer in the N2O4/DMF system could be transesterified with Ac2U to a cellulose acetate of DS = 2 with the C-2 position reacting the fastest (Mansson and Westfelt, 1980). In the paraformaldehyde/DMSO solvent system, obviously all the hydroxy end groups of the methylol side chains are preferentially acetylated with Ac2O/pyridine. A high DS of the acetyl groups could also be obtained in this system by transesterification of ethylene diacetate in the presence of Na acetate at 90 0C (Seymor and Johnson, 1978). In the system chloral/DMF/pyridine, cellulose was found to dissolve with complete substitution of the hydroxy groups by the appropriate half-acetal groups, which could be acetylated to a DS of 2.5 by Ac2U or acetyl chloride (Clermont and Manery, 1974). The free hydroxy groups within the AGU of rather stable partially substituted cellulose derivatives can be acetylated to an extent depending on the system considered. A complete substitution of all residual free hydroxy groups has been reported for tosyl cellulose (DS 0.9-2.3) by reaction with 3 mol of Ac2U per mol of hydroxy groups in the presence of sodium acetate (10 % Ac2U) in pyridine at 60 0C for 6 h (Heinze et al., 1996a), and for TMS-cellulose (DS = 2) with an excess of Ac2U (Stein and Klemm, 1988). Acetylation of the free hydroxy groups in a benzyl ether of cellulose with DS = 2, in benzene with Ac2U in the

174


presence of TEA has been studied by Philipp et al. (1983); an addition of DMAP to the system was found to increase the DS of acetyl groups from 0.1 to 0.35, while a further increase of Ac2O input did not lead to any significant effect. Starting from 6-O-trity!cellulose with a DS of 0.98, acetylation with Ac2O/pyridine resulted in a regioselectively substituted cellulose acetate with partial DS values of 0.15 at C-2, 0.10 at C-3 and 0.0 at C-6 after detritylation with gaseous HCl in CF^C^ (Yasuda and Yoneda, 1995). Some results of our own on the preparation of regioselectively (in the C-2 and C-3 positions) substituted cellulose acetates via 6-0-silylcellulose lead to a DS value of 1.1 starting from TMS-cellulose with DS = 1.9. The complete desilylation without deacetylation takes place with 1 N HCl in THF within 15 min. Ac2Ü proved to be superior to acetyl chloride in avoiding an early loss of primary substituent groups, which could be selectively removed after acetylation by HCl in an aprotic medium like THF (Stein and Klemm, 1988). As already emphasized, acetylation of cellulose is an equilibrium reaction, deacetylation occurring with an excess of water in the presence of an acid catalyst providing a sufficiently high accessibility of the acetyl groups. A homogeneous partial deacetylation of CTA in aqueous acetic acid, with ^804 as the catalyst, is practised on an industrial scale in order to reduce the DS^C to about 2.5. Energies of activation of 16.6 kcal/mol or 18.3 kcal/mol (Eicher, 1986) have been reported for this process in the case of solution CTA, while a much higher value of 25.2 kcal/mol was observed by Bischoff (1963) for fiber triacetates, which was assumed to be caused by a dissociation of supramolecular clusters with increasing temperature enhancing the availability of the acetyl groups for hydrolysis. In an aqueous acid system, deacetylation at the C-6 position obviously proceeds faster than at C-2/C-3. A preferential deacetylation at these secondary C atoms, on the other hand, can be performed in amine-containing systems of special composition: Miyamoto reported a preferential deacetylation at C-2 and C-3 in the presence of hydrazine (Miyamoto et al., 1985). The data summarized in Table 4.4.22 illustrate that preferentially C-6substituted cellulose acetates can be obtained from CTA by the action of a ternary mixture of DMSO/water and an aliphatic amine like e.g. dimethylamine, or hexamethylenediamine, which rather selectively deacetylates the two secondary positions. Hydrazine, on the other hand, proved to be less effective under the conditions employed. During a homogeneous aminolysis of CTA by ethylene diamine after dissolution in dimethylacetamide in a water free system, Deus et al. (1991) observed a very uniform deacetylation at all three positions of the AGU in comparison to other routes of deacetylation. The joint relevance of the two prerequisites for deacetylation, i.e. the presence of an aqueous medium and the accessibility of the acetyl groups to hydrolysis, are illustrated by the behavior of powdered cellulose acetate (DS > 2.5) against water and acetone: the DS of an aqueous suspension of the powder remains unchanged for a long period of


175

time due to the hydrophobicity of the particles. The acetate powder dissolved in dry acetone also exhibits no detectable change in DS over a long period. Suspension of the particles in a water/acetone mixture, accompanied by considerable swelling, however, results in a significant decrease in the acetyl content within a few hours at room temperature. This is obviously promoted by a small number of acid groups present in the polymer (Ludwig and Philipp, 1990). The irreversible alkaline saponification of acetyl groups in solid samples depends on their accessibility, i.e. the state of swelling too, as demonstrated in Fig. 4.4.28 for the decrease in acetyl DS on treatment with 0.1 N NaOH in water/acetone mixtures of increasing acetone content. In our laboratory-scale studies, an efficient hetero-saponification of acetyl groups was achieved by treatment of the swollen sample with 1 N KOH in EtOH. Table 4.4.22. Homogeneous deacetylation of CTA in amine-containing media at 80 0C (Wagenknecht, 1996; Deus and Fribolin, 1991).

Amine Example mol/mol ofAGU HMDA 2.3

DMA

4.5

t (h)

DSAca

DSAc (NMR)

Pattern of substitution C-2 C-6 C-3

2.5 4.5 9 14 24

2.60 2.41 1.87 1.33 0.75

2.65 2.4 1.95 1.5 0.75

0.8 0.65 0.45 0.2 0.05

0.85 0.75 0.55 0.45 0.1

1.0 1.0 0.9 0.85 0.6

5 11 15 20 24

2.55 2.06 1.84 1.59 1.45

2.55 2.0 1.8 1.6 1.2

0.75 0.5 0.35 0.3 0.2

0.8 0.5 0.5 0.4 0.3

1.0 1.0 0.95 0.9 0.7

a

Functional group analysis. HMDA NH2-(CH2)6-NH2. DMA HN(CH3)2.

In the absence of water, the acetyl groups of cellulose acetates dissolved in an aprotic liquid are rather stable even in the presence of acid anhydrides or acid chlorides at room or slightly elevated temperature. Within these limits, the acetyl group serves as an effective protecting group in a subsequent homogeneous esterification of residual hydroxy groups, as demonstrated e.g. in our studies on sulfation of cellulose 2,5-acetate with SO3 or ClSO3H, resulting in a complete sulfation of residual hydroxy groups without loss of acetyl groups. A transesterification with elimination of the acetyl group obviously takes place only

176


with high boiling acid chlorides at high temperatures, as indicated by Frautschi et al. (1983) for the reaction of cellulose acetate with palmitoyl chloride at 120 0C under nitrogen, resulting in a 50 % conversion. On heating of cellulose acetate under DTA conditions, deacetylation was observed besides dehydration and glycosidic bond cleavage already at an early stage of thermal decomposition (Jain et al., 1986 and 1987b).

20 4.0 60 Acetone [mol%] Figure 4.4.28. Effect of acetone content and temperature (· 30 0C, · 40 0C, A 50 0C) on the course of saponification of cellulose acetate (DS = 2.9) in 0.1 N NaOH (3 h, liquid-to-solid ratio 200 : 1) (Lukanoff et al., 1969).

Finalizing this section on the chemistry of acetylation and deacetylation of cellulose, the important role of modern techniques of instrumental analysis for a comprehensive characterization of the samples involved must be accentuated. Examples mentioned here explicitly are the 1H and 13C NMR spectroscopic studies by the group of Kamide, including a complete signal assignment (Kamide and Saito, 1994), and the combined solid state NMR and Raman spectroscopic investigations (VanderHart et al, 1996) on the polymorphs of CTA with new aspects of correlating the molecular and supramolecular structure of cellulose acetates. Survey of the industrial process of cellulose acetylation All the industrial processes practised today are aimed at the manufacture of a fully substituted cellulose triacetate (DS > 2.9) as the primary product. This is either isolated and processed as it is, or converted to a so-called secondary acetate with a DS of between 1.8 and 2.5 (predominantly near 2.5), by a partial


111

deacetylation under homogeneous conditions. A heterogeneous procedure yielding a uniform secondary acetate is not yet available. Most of the 832,000 tons of cellulose acetate produced worldwide in 1988, mainly in the USA (50 %), in Western Europe (16 %) and Japan (13 %), is manufactured by the process of solution acetylation and subsequently converted to secondary acetate, while only a minor amount is obtained by fiber acetylation. For both these processes, high-ΖλΡ, scoured and bleached cotton !inters are predominantly employed as the raw material, but also an adequately refined softwood sulfite pulp or even a special great prehydrolysis sulfate pulp can be used. Formation of cellulose II on alkali refining of the pulp should be avoided as it may impede a smooth course of acetylation. A low ash content, a very low content of alkaline-earth and heavy-metal cations, a low content of organosoluble extractives, as well as a low content of pentosans and mannans, are further requirements to be met by an acetate great wood pulp. Studies on acetylation of a nonrefined pulp, with 87 % α-cellulose content (Matsumura and Saka, 1992), indicated the formation of a considerable amount of glucomannan acetate, insoluble in glacial acetic acid, employed as the solvent for CTA. The raw material is dried to a residual water content of 4-7 % and then preactivated by treatment with glacial acetic acid (30-100 % of the cellulose weight), eventually in the presence of some F^SC^. In solution acetylation with either glacial acetic acid alone or in combination with methylene chloride is employed as a solvent for the CTA formed, arriving finally at a polymer concentration of between 10 and 20 % in the CTA solution. Acetylation is generally performed with an excess of acetic anhydride (1040 % above the amount needed for CTA formation) in the presence of l-^SC^ as the catalyst. In the 'acetic acid process' the exothermic reaction is performed in a kneader equipped with effective cooling and mixing facilities, adding the acetic anhydride stepwise and employing 2-5 % H^SC^ (calculated on cellulose weight) as the catalyst. The mixture, passing gradually from a fiber suspension to a viscous solution, is kept for several hours at a temperature of 50 0C, reaction temperature and reaction time determining the decrease in DP. The CTA formed is subsequently converted to secondary acetate without isolation of the CTA by adding water or dilute aqueous acetic acid to the system at an excess of 5-10 % Ü2O above that needed to decompose excess acetanhydride. This excess of water is sufficient for an effective decomposition of most of the sulfate half-ester groups in the cellulose chain and to decrease the DS of acetyl groups to the level required. But it is still low enough to keep the cellulose acetate in solution. After some hours of treatment at 40-80 0C the reaction mass is buffered with magnesium acetate. Then the cellulose acetate is precipitated with water under stirring, subsequently cooked under pressure with aqueous 1 % mineral acid for further stabilization, washed and then dried under vacuum to a moisture content of 13 %. The yield amounts to about 95 % of the theoretical one.

178


In the methylene chloride process, a mixture of about 2 parts of CI^C^ and 1 part of glacial acetic acid is employed as the medium for swelling and dissolving the activated polymer. The level of the t^SC^ concentration can be kept here at about 1 %, calculated on cellulose, due to a faster and higher swelling of the reaction mass. The acetylation itself is performed under similar conditions as in the acetic acid process, with the low-boiling CH2C12 serving as an internal thermostat to keep the reaction temperature at a level of 50 0C in the mixing vessel equipped with stirring facilities. Desulfation and partial deacetylation are in principle performed as already mentioned, with the exception that the CI^C^ is distilled off and recovered before precipitation of the cellulose acetate. An advantage of the methylene chloride process is the much lower amount of dilute aqueous acetic acid to be disposed of as a waste product of the process. In fiber acetylation, the activated cellulose is reacted with an excess of acetanhydride in the presence of a large amount of a nonsolvent for CTA (CC^, benzene, toluene) and about 1 % HC1O4 (calculated on cellulose) in a rotating sieve-drum, mounted in a stainless steel vessel at a temperature of up to 50 0C for one to several hours. After the reaction the CTA is separated from the liquid phase, buffered, washed and freed from residual nonsolvent by steaming, and dried to a low residual moisture content of about 1 %. A stabilization step is usually not necessary here as no perchloric acid ester groups are bound to the cellulose. Continuous processes for cellulose acetylation have been described too, but obviously are scarcely practised due to the inferior uniformity and general quality of the products obtained. Properties of cellulose acetate Cellulose triacetate is a semicrystalline polymer, crystallizing in the two allomorphs of CTA I and CTA II. Ample research efforts have been made to elucidate the detailed structure of these modifications and their conditions of formation (Buchanan et al., 1987). By VanderHart et al. (1996) the structural difference is traced back to a different backbone conformation, and a different chain polarity, i.e. parallel in CTA I and antiparallel in CTA II is considered as probable. In dependence on polymer concentration, DP, temperature and solvent, CTA can form various liquid crystalline phases. For details the reader is referred to the comprehensive work of Zugenmaier (1994) and Guo and Gray (1994). Commercial cellulose acetates, i.e. CTA and cellulose 2,5-acetate, are highmelting, high-strength and tough polymer materials, exhibiting a high UV stability and film transparency, combined with low inflammability. CTA melts at 306 0C, and cellulose 2,5-acetate at 225-250 0C with decomposition. The presence of butyrate groups besides the acetate groups decreases the melting point of


179

cellulose acetates considerably, and the solubility and the compatibility with other polymers are enhanced. Concerning the scientifically interesting and technically important point of cellulose acetate solubility, the hydrophobicity and the high resistance to hydrocarbons are to be mentioned as characteristic material properties of high-DS cellulose acetates. Table 4.4.23 presents an overview of solvents for the various cellulose acetates in dependence on DS. Table 4.4.23. Solubility of cellulose acetate with different patterns of substitution in various liquids.

Liquid

Water DMF Acetone (< 0.01 % H2O) Acetone (1 % H2O) Pyridine Pyridine/H2O ( 1 : 1 v/v) Ethyl lactate

DSAC range of solubility for partially deacetylated cellulose acetate in C-2/-3/-6 position3 in C-2/-3 position13 0.8-1.0 1.8-2.7 insoluble 2.3-2.6 0.8-2.7 0.6-2.0 1.6-2.7

insoluble 1.3-2.8 insoluble 2.5-2.6 1.2-2.8 1.2-1.6 2.6-2.8

Deacetylation of cellulose triacetate: a with CH3COOHTH2SO4 (Deus and Fribolin, 1991). b with amine/DMSO/H2O (Philipp et al., 1995).

This classification of course only holds true on the prerequisite of a sufficiently uniform acetyl-group distribution along and between the polymer chains. Otherwise no complete solubility at all can be expected. Besides this decisive factor, the substituent distribution within the AGU plays an important role too: from the results published in Miyamoto et al. (1985), Deus and Fribolin (1991) and Philipp et al. (1995) it can be concluded that the hydrophile/hydrophobe ratio, i.e. the ratio of hydroxy to acetyl groups at the C-6 position, predominantly determines the solubility of the sample in various solvents. So, for example, solubility in water has been observed for low-DS cellulose acetates in the DS region of around 0.8 only for samples carrying a large amount of hydroxy groups at C-6 in the case of statistical acetyl group distribution, whereas after regioselective deacetylation of a CTA at the C-2 and C-3 positions the reaction products remained insoluble in water in the same DS region. Worth mentioning in this connection is our observation that commercial cellulose 2,5-acetate, with its free hydroxy groups rather equally distributed on the three positions at C-2, C-3 and C-6, proved to be insoluble in an absolutely dry acetone, with a water

180


content below 0.02 %, whereas it readily dissolved in standard-grade acetone with a water content of about 1 %. Another point of interest to be traced back, however, to a different course of reaction, is the difference in solubility between solution CTA and fiber CTA described in Bischoff (1963): while the solubility of a solution CTA in acetone at -40 0C reached the 20 % level, a fiber CTA exhibited a solubility of less than 5 %, obviously due to a stronger chain aggregation persisting from the supramolecular structure of the native cellulose. The rheology of cellulose acetate solutions at various levels of polymer concentration, DS and solvent has been widely studied, including that of liquid crystalline systems and of thermoreversible gels formed in e.g. water/dioxane as the solvent (Altena et al., 1986). From the numerous investigations published by many groups, promoted by the industrial relevance of the dissolved state for cellulose acetate processing, only two rather arbitrarily chosen examples shall be presented here in order to illustrate the broad spectrum of topics studied: Klenkova and Khlebosolova (1977) compared the rheological behavior of CTA with that of cellulose tripropanoate and cellulose tributyrate, concluding from their results a high chain flexibility of CTA and a predominance of the DP above the DS in determining the rheological properties of CTA in solution. Burchard and Schulz (1989) studied the intermolecular interaction between cellulose acetate macromolecules by employing globular proteins as the probe and concluded from their results the presence of reversible as well as irreversible supramolecular aggregates, assuming the existence of some kind of fringed micelle for CTA in solution. Obviously, the state of solution of cellulose acetates still presents numerous open problems to polymer and colloid science due to the large number of variables involved, but also offers further approaches to give defined supramolecular structures by employing cellulose acetates with a tailored substituent distribution. Application of cellulose acetate Cellulose acetate is commercially available either as the triacetate (DS 2.9-3.0; acetyl content ca. 45 %) or as cellulose 2,5-acetate (DS 2.4-2.5; acetyl content ca. 40 %), and as a specialty also in the DS range 1.85-2.0 (acetyl content ca. 35 %). Besides products carrying the acetyl group as the only ester group, mixed esters with a varying amount of propanoic or butanoic ester groups are manufactured in order to improve melt processibility. The classical areas of cellulose acetate application are the manufacture of filaments for textile use and of films via a solution of the secondary acetate (DS ~ 2.5) in acetone. The filaments are formed in a so-called 'dry spinning process' by evaporation of the low-boiling solvent during thread formation between spinneret and godet. A spinning dope containing between 20 and 30 % polymer (preferentially 25 %) is pressed through a spinneret with 20-100 holes


181

in a 4-6 m long spinning column and exposed to a stream of hot air at 80100 0C resulting in formation of the solid filament by solvent evaporation. The filaments are stretched in a still plastic state to enhance their mechanical properties. A spinning speed between 300 and 800 m/min is generally employed. In spinning cellulose triacetate filaments from a solution in methylene chloride/methanol, the stretched filaments showing a core shell structure due to partial crystallization are heat-set at 180-220 0C for some minutes or seconds in order to reduce water retention and water absorption and to improve the wash and wear properties of the finished goods. Cellulose triacetate (fiber triacetate) finds its predominant application in the production of high-quality cine film as it exhibits an excellent dimensional stability combined with very low flammability, in contrast with films from cellulose nitrate. Besides films, textiles from CTA filaments are on the market, produced by dry spinning of a CTA solution in e.g. a methylene chloride/methanol mixture. With regard to textile properties, cellulose acetate filaments take an intermediate position between rayon and synthetics, resembling much more the latter. Due to this competition with synthetics, no growth in production and market share can be expected in the future, but textiles from cellulose acetate will keep their place despite their rather high production cost due to some special assets regarding e.g. handle and dyeability. About 130,000 tonnes per year of cellulose acetate filaments are still produced, especially for linings and women's apparel wear. As a third, also a classical area of cellulose acetate application, its use as a plastic material, must be mentioned. Especially mixed esters containing butyrate, besides the acetate groups (cellulose acetobutyrate) can be melt processed, especially by injection molding to produce consumers goods with attractive mechanical properties and attractive appearance; but also in this field cellulose acetate stands in hard competition with synthetic plastics. Thermoplastic processing of cellulose acetate to high-quality consumer goods is realized today along the two routes of: (i) thermoplastic shaping of cellulose acetate proper in combination with about 30 % softener (mostly phthalates); (ii) melt processing of cellulose acetobutyrates of varying ester-group ratio. A growing market for cellulose acetate can be seen, however, in two more recent areas of application, i.e. as a material for cigarette filters and for separation membranes. As a material for cigarette filters, cellulose acetate obviously meets in an unique manner the requirements of filtering efficiency and taste quality. During the recent decades, cellulose acetate (DS 2.5-3) has found a new, interesting and prosperous area of application in the manufacture of separation membranes for ultrafiltration, reverse osmosis and hemodialysis. These membranes are prepared from solutions of the polymer in a suitable liquid or a mixed solvent, combining solvent evaporation, polymer precipitation by a nonsolvent

182

4 A Esterification of Cellulose

and eventually subsequent annealing of the solid product. By these numerous degrees of freedom, the pore size of the membrane can be varied within rather wide limits and adapted to the special end-use intended. In any case the pore structure is asymmetric, exhibiting a pore-size gradient across the membrane with a fine porous 'separation active' layer at one side (pore size in the nm range) and a coarse, porous supporting layer (pore size in the μιη range) at the other. In reverse osmosis predominantly employed for desalting of sea water, these cellulose acetate membranes have the advantage of good stability against chlorine chemicals in the necessary disinfection cycles, but show a lower flux rate as compared with the competing synthetic products from aromatic polyamides. Also, in hemodialysis in the so-called 'artificial kidney', cellulose acetate membranes are still widely employed due to their good blood compatibility.

4.4.3.4

Cellulose esters of higher aliphatic acids

In principle, cellulose esters of higher aliphatic acids are synthesized along the same routes as described for cellulose acetate, i.e. employing the acid anhydride with a suitable catalyst or the acid chloride in the presence of a tertiary base as the predominant acylation systems. It must be taken into account that the higher acid chlorides and acid anhydrides are less reactive than e.g. acetyl chloride and acetic anhydride, and that these higher anhydrides and chlorides are rather special and therefore expensive chemicals. The 'impeller technique' employing the appropriate carbonic acid in combination with chloroacetic anhydride is of special interest in connection with the higher cellulose esters, and effort has been made to find catalysts of very high efficiency. The propionylation of cellulose of course resembles most closely acetylation, and can be performed as a solution propionylation with the anhydride and an acid catalyst. Also, a cellulose suspension in dioxane/pyridine can be employed for propionylation to high DS, in this case with propionic acid chloride as the agent. Farvardin and Howard (1985) studied the heterogeneous propionylation of cellulose in systems consisting of propionic acid, propionic anhydride and an appropriate metal chloride as the catalyst in an aprotic solvent, comparing various metal chlorides and solvents with regard to their effect on reaction rate. The kinetics was described by two consecutive first-order reactions with the second one proceeding faster than the first one. For a homogeneous acylation of cellulose to esters, with an aliphatic chain length of between three and eight carbon atoms, a 2 % polymer solution in DMA/LiCl with 9 % LiCl, and a mixture of the appropriate acid with its anhydride in the presence of dimethylcyclohexylcarbodiimide or pyrrolidinopyridine as the catalyst was employed (Samaranayake and Glasser, 1993). A very low excess of reagent was reported to be necessary for reaching high DS values, with the sites at C-6 and C-2 being more reactive than that at C-3. Regioselectively substituted propionylcelluloses with


183

the ester groups in the C-2/C-3 positions have been prepared from 6-0trimethylsilyl and 6-0-tritylcelluloses by reacting this compounds with an excess of propionic anhydride in the presence of pyridine and subsequent desilylation or detritylation with HCl (Iwata et al., 1992). The 2,3-propionates proved to be more stable in the acid medium than the 2,3-acetates and could be isolated without loss of ester groups. The propionylation of partially substituted methyland ethylcelluloses to give stable ether-esters has been reported by Guo and Gray (1994), with free hydroxy groups in the C-6 position being preferentially esterified during the homogeneous reaction. Mixed esters containing aliphatic residues from C-3 to C-5, besides acetyl groups, can be prepared in the conventional way with the appropriate acid anhydrides in the presence of Ρ^Οφ For the preparation of higher aliphatic cellulose esters care must be taken in drying the solvent employed. A preactivation of the polymer with an aliphatic amine was found to be advantageous in synthesizing higher esters from the butyrate up to the stearate, with the acid chloride or the anhydride as the agent. The hydrophobicity of the product increased with DS and with the molar volume of the substituent. On esterification of a hydrolyzed cellulose ('microcrystalline cellulose') with pelargonic acid chloride, up to a DS of 3 has been reported by Battista et al. (1978). In a medium of DMF and pyridine, a mixture of ptoluenesulfonyl chloride and the Na-salt of the appropriate aliphatic or aromatic acid was found to be effective in preparing higher cellulose esters (Shimizu et al., 1993a). A homogeneous transesterification was reported in Shimizu et al. (1993b) for a cellulose trinitrite by lauroyl chloride in a N2Ü4/DMF solution of holocellulose (delignified wood consisting of cellulose and hemicelluloses). A special route to higher aliphatic esters of cellulose has been proposed (Kwatra et al., 1992): in this 'vacuum acid chloride technique' the cellulose is reacted directly with the appropriate acid chloride without the presence of a solvent at a sufficiently high temperature. The HCl formed is eliminated from the system continuously by vacuum. A palmitoyl ester of cellulose was obtained with a yield of 90 %. The reaction was found to be chemically and not diffusion controlled; adequate kinetic models were reported. A full signal assignment of the 1H and 13 CNMR spectra of cellulose triacetate, tripropionate and tributyrate has been published (Buchanan et al., 1987) with the conclusion that only small changes in chemical shift (usually within 1 ppm) take place depending on the size of the ester group. An NMR spectroscopic study of two regioselectively substituted, mixed cellulose triesters, i.e. 6-0-acetyl-2,3-0-propionylcellulose and 6-0-propionyl-2,3-0-acetylcellulose has been published by Iwata et al. (1996). Some physical properties of higher aliphatic cellulose esters in the solid state are presented in Table 4.4.24 in comparison with cellulose acetate. Obviously the intermolecular interaction between the polymer chains decreases with in-

184


creasing length of the ester side chain, as indicated for example by the change in melting point and in the elastic modulus of the crystalline regions (Nishino et al., 1995) of these semicrystalline solids. According to Buchanan et al. (1989) no principle change in polymer backbone conformation is induced by increasing the length of the ester side chains. Already the cellulose butyrate melts without decomposition at 192 0C and thus can be melt processed. The higher esters of cellulose, as investigated in the range from C-3 to C-18 of side chain length, are increasingly hydrophobic, but soluble in many organic liquids of medium to low polarity. Methylene chloride is a good solvent for many of these cellulose derivatives. An especially broad spectrum of solvents is known for esters of medium chain length, e.g. the valerate and the caproate. Many of these higher aliphatic esters of cellulose form liquid crystalline systems with suitable solvents, and especially the higher members of the homologous series, e.g. the octadecanoate, were found to be suitable for the preparation of Langmuir-Blodgett monolayers and multilayers (Kawaguchi et al., 1985). Rheological studies (Klenkova and Khlebosolova, 1977) on semiconcentrated solutions of cellulose triacetate, tributyrate and acetobutyrate, in dependence on DP, substituent group and DS, indicated a predominant effect of DP, with cellulose triacetate showing the highest value of T]Q under comparable conditions. Correlations between the rheological properties of these solutions and the physical properties of threads and films prepared therefrom were concluded from this study. Rheological investigations of dilute solutions of cellulose tripropionate (Casay et al., 1995) lead to the assumption of worm-like chains in these systems, in between the limiting models of a random coil and a rigid rod. 100 Acetate

80 60 40 20

O

0.5

1.0

1.5 DS

2.0

2.5

3.0

Figure 4.4.29. Complement (C 5a) activation (y-axis in %) by cellulose basedmembranes with various ester substituents (Vienken et al., 1995). Complement activation as a criterion of membrane hemocompatibility is given in relation to a nonmodified cellulose standard (= 100%).

Melting point (0C) 225-250 306 234 183 122 94 88 91 106 105

Char point (0C) ca. 230 315 > 315 > 315 > 315 > 315 290 > 315 315 315

1.30 1.28 1.23 1.17 1.13 1.10 1.07 1.00 0.99 0.99

Density (g/ml)

% Moisture regain (75 % r.h.) 6-6.5c 3.8 1.5 0.7 0.3 0.2 0.2 0.1 0.1 0.1

Tensile Solubilities in b strength3 Methylene Acetone (kg/mm2) chloride + + 7.3 + 4.9 + + 3.1 + + 1.9 + + 1.4 + + 1.1 + +

0.6 + 0.6 + 0.5 + a Measurements on films; b soluble (+), insoluble (-); c 65 % r.h. (Malm and Hiatt, 1954).

(2,5-Acetate) Acetate Propionate Butyrate Valerate Caproate Heptanoate Laurate Myristate Palmitate

Triester

Table 4.4.24. Some properties of higher rc-aliphatic triesters of cellulose in comparison with cellulose acetate

+ + +

+ + +

Ethyl acetate — —

+ +

+ + +

— —

Toluene

186


Higher aliphatic esters of cellulose find application as specialty plastics, predominantly as mixed esters, especially as acetobutyrates of cellulose, which can be melt processed. Furthermore, cellulose acetobutyrates are used as components in melt coatings for paper. Cellulose propionate has been proposed for the preparation of microspheres for the encapsulation of antibiotics. Higher members of the series find current interest in the preparation of Langmuir-Blodgett layers. Introduction of palmitoyl groups into a cellulose acetate was found to increase albumin binding and hemocompatibility of films formed therefrom. Systematic studies of the effect of aliphatic ester group and DS on the hemocompatibility of cellulose-based hemodialysis membranes (Vienken et al., 1995) indicated a DS optimum for each system investigated, which was shifted to lower DS values with increasing side chain length, with an optimal compatibility being obtained with a low-substituted cellulose stearate (Fig. 4.4.29).

4.4.3.5

Esters of cellulose with substituted monocarboxylic aliphatic acids

Most of the work published in this area has been performed with chlorinated or fluorinated acetic acids or their anhydrides or chlorides, respectively. Halogenation at the methyl group generally increases the reactivity in esterification. Application of chloroacetic anhydride as a catalyst to esterification of cellulose with other less reactive agents has already been mentioned. Bludova et al. (1984) compared the heterogeneous course of reaction of cellulose with formic acid, acetic acid, trichloroacetic acid and trifluoroacetic acid, and reported for CFßCOOH a thorough reaction of amorphous as well as of crystalline regions, whereas with CC^COOH only the amorphous regions were found to be acylated and dissolved. A preferential substitution at the C-6 position was observed in both cases. According to Pikler et al. (1980) a monochloroacetate of cellulose can be prepared by reaction of alkali cellulose with an excess of chloroacetyl chloride in DMF. The chlorine content of the product was reported to increase strongly with the reaction temperature between 70 and 100 0C under otherwise fixed reaction conditions, and an energy of activation of 80 kJ/mol was calculated from this dependency. In contrast with propionic acid itself, 1,2dichloropropionic acid can be directly reacted with cellulose in the presence of HC1O4 as a catalyst (Jain et al., 1980). 2,2-Dichloropropionic acid esters of partially substituted carboxymethylcellulose were obtained by reacting the unmodified cellulosic hydroxy groups with the appropriate acid chloride in pyridine at 20 0C for 4 h, employing a fine suspension of CMC in the reaction system (Schnabelrauch et al., 1990). A more convenient and effective procedure for subsequent modifications of CMC has been described (Vogt et al., 1996). CMC was treated in a dipolar-aprotic solvent like DMA or DMSO with ptoluenesulfonic acid, yielding a highly reactive gel-suspension of the polymer.

4.4,3 Esters of cellulose wiih organic acids

187

This mixture allows the direct esterification of unmodified hydroxy groups of CMC, as exemplified by acylation with carbonic acid chlorides or anhydrides and with isocyanates as well as by sulfation, phosphatation and silylation. Of some relevance to cellulose derivatization as well as to cellulose dissolution is the interaction between the polymer and CF3COOH of 98-100 % concentration. This acid dissolves cellulose already at room temperature without considerable degradation, and regenerated cellulose without any ester groups can be recovered from these solutions by precipitation in an aqueous medium. There was some discussion on whether or not the cellulose is esterified on dissolution in CFßCOOH, which could be settled by a 13C NMR spectroscopic study (Nehls et al., 1995): On dissolving cellulose in concentrated trifluoroacetic acid, most of the derivatization does not occur before a clear solution is obtained. As shown by the 13C NMR spectra in Fig. 4.4.30, at first only the C-6 position is affected, followed later on by the C-2 and to a smaller extent also the C-3 position, arriving after about 28 days at a total DS of 1.6. C-2.3,5

100

90

80 ό [ppm]

70

60

Figure 4.4.30. 13C NMR spectra of cellulose after different times of reaction in trifluoroacetic acid (Nehls et al., 1995): (a) 10 h; (b) 2 days; (c) 28 days; index ' means esterified position, index " means influenced by C'. Several routes of synthesis to cellulose trifluoroacetates were recently developed and compared by Liebert et al. (1994), i.e. (i) esterification with a mixture of CF3COOH and trifluoroacetic acid anhydride; (ii) esterification with CF3COOH and partially hydrolyzed POC^, arriving at a DS of TFA groups of up to 1.6 for the reaction product soluble in DMF, DMSO, or pyridine;

188


(iii) reaction of cellulose with phenyltrifluoroacetate resulting in insoluble products with a DS of TFA groups of 0.3 only; (iv) reaction of TMS-cellulose of DSsi = 2.8 with CF3COOH and partially hydrolyzed POCl3 in Ct^C^, with the TMS groups obviously acting as the leaving groups and arriving at a DS of trifluoroacetate groups of up to 2.4, with complete elimination of the silyl substituents, the products being soluble in DMF, THF and acetone. Cellulose trifluoroacetates of high purity in the DS range 1.5-2.1 could be prepared along route (i) and a subsequent 'thermal purification' at 150 °C/80 Pa for elimination of excess reagent, solvents and by-products. The esterification was accompanied by a moderate chain degradation from e.g. DP 1400 to DP 800. The trifluoroacetates exhibited good solubility in DMF, DMSO, THF and pyridine and thermal stability up to 250 0C. The 13C NMR data revealed again a preferential substitution at the C-6 position. Contact with water at room temperature led to a quick and complete decomposition to regenerated cellulose. In the authors opinion, cellulose trifluoroacetates can be considered as versatile intermediates for subsequent steps of derivatization. Methacrylate esters of cellulose with a DS of up to 2.0 have been prepared with methacryloyl chloride as the agent in the presence of pyridine in DMF as the medium (Svistunova et al., 1964). The free hydroxy group could be subsequently acetylated, and an analogous route of synthesis was described for mixed oleate/acetate esters of cellulose (Iodannidis et al., 1966). Another route to mixed cellulose esters containing acetyl and methacryloyl groups was described in Pohjola and Aarmikoivu (1976) and Pohjola et al. (1976), starting from a melt solution of cellulose in A^-ethylpyridinium chloride, which was reacted with 0-10 mol of acetanhydride and 3-7.5 mol of methacryloyl chloride per mol of AGU in the presence of pyridine, arriving at products with a total DS between 0.5 and 2.5 and a DS of vinyl groups between 0.1 and 0.9. As shown quite recently by Zhang and McCormick (1997), the DMA/LiCl system is well suited for a homogeneous esterification of dissolved cellulose with various unsaturated carbonic acids or their anhydrides, e.g. crotonic, methacrylic, vinylacetic or cinnamic acid. A^TV'-Dicyclohexylcarbodiimide was employed as a condensation agent and 4-dimethylaminopyridine (or 4-pyrrolidinopyridine) as an acylation catalyst. Reaction products obtained with crotonic or methacrylic acid (or their anhydrides) exhibited poor solubility, due to side reactions favoring the high reaction temperature required here. The reaction with vinylacetic or cinnamic acid, however, proceeded facile to products readily soluble in DMSO. A direct route to acetoacetates of cellulose was recently published in Edgar et al. (1995) by reacting a cellulose solution in DMA/LiCl with bis-/butylacetoacetate or acetoacetic acid chloride arriving at esters with a DS of up to 3. Solubility in various media was determined by the level of DS, low-DS


189

products being dissolved in tÔ. Also, the preparation of levolinic acid esters of cellulose with DS values up to 1 has been reported (Vladimirova et al., 1965). So far, cellulose esters with substituted monocarboxylic aliphatic acids have not been manufactured on a commercial scale and have found, with the exception of the trifluoroacetates, only limited scientific interest in the organic chemistry of cellulose.

4.4.3.6

Esters of cellulose with di- and tricarboxylic aliphatic acids and their derivatives

Publications in this area dealing mostly with compounds carrying oxalic, malonic, maleic or succinic acid residues. A comprehensive review has been published (Allen and Cuculo, 1973). The routes of synthesis are analogous to those presented for monocarboxylic acid esters. They start from cellulose or a partially substituted cellulose ester or ether in a heterogeneous or a homogeneous system. They employ the acid anhydride or acid chloride as the esterifying agent, with the peculiarity that these agents can react bifunctionally with the result of crosslinked and therefore insoluble products. Crosslink formation may be reduced by masking one of the acid functions with a less reactive group like an ester or amide moiety. Cellulose oxalates with up to 2 acid equivalents bound per mol of AGU and probably considerable crosslinking were obtained in a heterogeneous reaction of spruce sulfite pulp with oxalyl chloride in glacial acetic acid or DMF. The presence of 4-dimethylaminopyridine enhanced the add-on considerably in the case of native pulp, but reduced it significantly in the case of mercerized pulp, probably due to the changed pore structure of the sample impeding penetration of the voluminous acid chloride-DMAP complex (Philipp et al., 1983). Organosoluble cellulose oxalates could be obtained by reacting the polymer with an oxalic halfester acid chloride ROOC-COCl in the presence of pyridine in nitrobenzene (Frank and Caro, 1930). The synthesis of a cellulose trimethoxalate has been described by Rebek and Jurkowisch (1977), who reacted cotton cellulose with methoxalic acid anhydride in the presence of pyridine at 60 0C, arriving after 4 h at a DS of 2.9. A rapidly proceeding succinylation of cellulose with succinic anhydride in methanesulfonic acid at 25 0C has been described by Hirabayashi (1984), leading to only a small amount of crosslinking, which however rendered the products incompletely soluble. Esterification of cellulose with e.g. succinamic, maleamic (and phthalamic) acid by a pad bake technique in the presence of ammonium sulfamate to DS values between 0.5 and 1, has been described (Cuculo, 1971; Allen and Cuculo, 1976):

190

4 A Esterification of Cellulose

O O Il Il CeII-OH + H 2 N-C-(CH 2 ) 2 —C-OH 75O 0 C

Aqueous medium

Cell-O-C — (CH 2 ) 2 -C-OH + NH3 O

O

The products proved to be soluble in 5-12 % aqueous NaOH with the amide group being saponified, and a carboxylated crosslinkable cellulosic compound being formed. As curiosities, the preparation of a cellulose furoate by esterification with furoic acid anhydride and pyridine in a dipolar aprotic solvent, and of a cellulose citrate with a rather large amount of free carboxyl groups shall be mentioned (Shaposhnikova et al., 1965; Touey and Kiefer, 1956). Esterification of free hydroxy groups in partially substituted cellulose acetates with a DS^c between 2 and 3 has been accomplished with various dicarboxylic acid chlorides or anhydrides in the presence of a tertiary amine like pyridine and a metal acetate as catalyst, leading to soluble as well as insoluble products depending on reaction conditions (Malm and Fordyce, 1940). A promising route to new cellulose derivatives consists in the attachment of unsaturated ester groups with C-C double or triple bonds onto the polymer skeleton, as shown recently (Klemm and Vogt, 1995) by the esterification of free hydroxy groups in carboxymethylcellulose with maleic acid anhydride in a dipolar aprotic solvent, or by introduction of C-C triple bonds via esterification with acetylene dicarboxylic acid methyl ester after dissolving the cellulose in DMA/LiCl.

4.4.3.7

Cellulose esters with aromatic acids

In contrast with the broad variety of aliphatic esters experimentally studied, the spectrum of aromatic esters of cellulose investigated so far in some detail is rather small and quite predominantly limited to the synthesis and characterization of cellulose benzoates (including ring-substituted products) and phthalates. Besides the esters of aromatic carboxylic acids, that of /?-toluenesulfonic acid, known as tosylcellulose, will be considered here in some detail as an interesting intermediate in cellulose derivatization chemistry. With regard to the high boiling point of the acid anhydrides and acid chlorides in question, which are employed also as the esterifying agents, aromatic ester synthesis sometimes can be performed at rather high temperature with an excess of agent serving as the reaction medium.


191

According to Braun and Bahlig (1994) a cellulose tribenzoate with a DS between 2.8 and 2.9 is obtained in a one-step reaction with benzoyl chloride in the presence of pyridine. By Mannschreck and Wernicke (1990) nitrobenzene is recommended as a medium for preparing a tribenzoate of cellulose with benzoyl chloride in the presence of pyridine at 130-140 0C, while a monobenzoate could be conveniently prepared by reacting alkali cellulose with an appropriate amount of benzoyl chloride. Also, higher substituted products were obtained in a Schotten-Baumann-type reaction with NaOH and benzoyl chloride, but pyridine as a base proved to be more effective for this purpose. An unconventional route of synthesis has been described by Isogai et al. (1988), who obtained a cellulose benzoate of DS 2.5 and a DP of about 800 by ozonization of a cellulose tribenzyl ether of DP 1200. Cellulose tribenzoate is a hard and brittle solid with a glass transition temperature of 155 0C and a melting temperature of 274 0C, and soluble in DMF, CHCl3 and CH2Cl2 (Braun and Bahlig, 1994). For a smooth film formation, at least 20 % softener is required. The tribenzoate was found to be thermally stable up to 250 0C. Differential scanning calorimetry and TG data between 20 and 450 0C were published by Jain et al. (1986), indicating debenzoylation and radical formation at high temperature. According to Mannschreck (1990) cellulose tribenzoate is a versatile sorbent for separating various enantiomers. Derivatization to benzene-ring-substituted cellulose benzoates of high DS has been accomplished with the appropriate free acids containing -NO2, -Cl, or -OCH3 in the presence of pyridine and p-toluenesulfonyl chloride. The position of the substituent within the benzene ring proved to be of minor importance for the course of reaction (Shimizu et al., 1993a, b). A remarkable catalytic effect of 4-dimethylaminopyridine was observed in the esterification of the free hydroxy groups of a cellulose benzyl ether of DS = 2 with 1 mol of 4-nitrobenzoyl chloride in the presence of 1 mol of TEA per mol of AGU at room temperature in benzene as the reaction medium. By addition of 0.2 mol of DMAP/mol of AGU to the system, the DS of benzoate ester groups increased from less than 0.01 to 0.3-0.4 in this homogeneous reaction (Philipp et al., 1983). As illustrated by the data in Table 4.4.25, see also Fig. 4.4.31, the TMS group acts as a leaving group at or above 100 0C, with an excess of benzoyl chloride serving as the reaction medium, and the benzoate ester groups are prelimanary introduced at the C-6 position with elimination of the volatile trimethylchlorosilane. At low temperature in the presence of a tertiary amine, on the other hand, the TMS group is an effective protecting group, and free hydroxy groups in the C-2/C-3 position are benzoylated (Stein and Klemm, 1988; Klemm et al., 1990). Cinnamates of cellulose with a DS of up to 3 have been prepared in a homogeneous reaction of the polymer dissolved in DMA/LiCl with cinnamoyl chloride in the presence of pyridine at 30-60 0C, with a preferential substitution being observed at the C-6 position (Ishizu et al., 1991). This homogeneous reac-

192


tion was compared in Ishizu et al. (1991) with the heterogeneous one of a cellulose suspension in cinnamoyl chloride and pyridine, and with a Schotten-Baumann-type reaction with aqueous NaOH. Table 4.4.25. Conditions and results of the benzoylation of TMS-cellulose with acid chlorides (Stein and Klemm, 1988; Klemm et al., 1990). TMScellulose (DS)

Acid chloride R-COCl (mol/molof AGU)

,Z. 4-Ό

ο

Amine

—

\— MO

/

Reaction temperature (0C) AoU

Cellulose ester (DS)

1.57

(2.5)

2.30

(5.0) 2 62

·

16

~&

R=^jV-CH 2 -CH 2 -Br

(3.5)

°

2.53

i-55

R=-/ = VNO

TEAb

25

0.56

1-99

R = -(^)-NO2

TEAC

25

0.43

1.99

R=^Q-CH 2 -CH 2 -Br

TEAC

^

0.39

a

Standard reaction conditions: without solvent, 30 min, N2. Solvent DMF, 4 h. c Solvent benzene, catalyst DMAP.

b

/

OH

Ô

foil L/elL

Γϊ +ι n

^

Γ* U

OSi(CH 3 ) 3

80 -16O00C

Cl

OH

/

/^nII ' L/eiL

> ~*

33

O—C — R 11

°

/° (Et3N)" 'Cl O

O

Il

C

Il

R

,°- X

OSi(CH3)3

HC,/«,, 25 C

° >2°mi"

S'*-" OH

Figure 4.4.31. Conversion of TMS-cellulose to cellulose esters by different routes.


193

Ample research work has been invested in the phthaloylation of cellulose and some of its derivatives like partially substituted ethylcellulose and especially partially substituted cellulose acetates (Fig. 4.4.32).

CeII-OH

CeII-O HO (OH)

(H3C- C=0)>

(H 3 C-C = O) Figure 4.4.32. Scheme of phthaloylation of cellulose and of cellulose acetate.

Cellulose phthalate half-esters show a pH-dependent solubility in aqueous media that is useful for the manufacture of process auxiliaries. Phthalic anhydride is generally employed as the agent in the presence of a basic catalyst. By Levesque et al. (1987) the esterification of cellulose and chitosan with the anhydrides of phthalic acid, nitrophthalic acid and trimellitic acid in the presence of TEA and 4-dimethylaminopyridine in DMSO is described, soluble products being obtained at a molar ratio of anhydride per AGU of > 1. Numerous publications are dealing with the synthesis and application of the commercially relevant cellulose acetophthalates. These products are generally obtained by reacting cellulose acetate in the DS range between 1.7 and 2.5 with phthalic acid anhydride in the presence of a basic catalyst like pyridine or TEA, in a dipolar aprotic or rather nonpolar medium (DMSO, DMF, dioxane, acetone, benzene). Also, tetrahydro- or hexahydrophthalic acid anhydride have been reported as esterifying agents, and besides the catalysts mentioned above also picoline, lutidine, 4-dimethylaminopyridine and 1,4-diazadicyclo-2,2,2-octane have been employed. Furthermore, the phthaloylation of cellulose acetate in a melt of the anhydride has been performed. Malm et al. (1957) phthaloylated a cellulose acetate of DS 1.8 in glacial acetic acid in the presence of sodium acetate. The authors emphasized the significant effect of the water content in the reaction system on reagent yield. Wagenknecht et al. (1987) investigated the phthaloylation of several cellulose acetates with phthalic acid anhydride in dioxane at 100 0C, and in acetone at 56 0C, varying the DS of the starting material, the catalyst, the reagent input ratio and the time of reaction. With acetates of a

194


DS of 2 and 2.5 (strictly homogeneous course of reaction) a complete substitution of all the free hydroxy groups was accomplished, whereas at a lower DS of acetyl groups and an at least partially heterogeneous course of reaction, the DS of phthaloyl groups remained below the amount of free hydroxy groups, resulting in a total DS of less than 3. DMAP and l,4-diazadicyclo-2,2,2-octane proved to be superior to pyridine and TEA as catalysts. The decisive effect of the water content in the reaction system on the DS of phthaloyl groups obtained under given conditions is confirmed by the data in Table 4.4.26. Table 4.4.26. Effect of water content on the phthaloylation of cellulose acetate (DS = 2) in acetone (Wagenknecht et al., 1987) (2.4 mol of phthalic acid anhydride and 1 mol of TEA/AGU; 56 0C, 4 h).

% H2Ü in acetone 1.8 0.3 0.03 0.03

State of drying of CA oven-dry oven-dry air-dry oven-dry

DS 0.61 0.71 0.80 0.98

Cellulose acetate phthalates are produced commercially as a specialty product, and are mainly employed in coatings for e.g. tablets, and as a process auxiliary in the photographic industry.

4.4.3.8

Esters of cellulose with organic acids carrying sulfonic or phosphonic acid groups

Besides the cellulose esters with carboxylic acids considered so far, esters of the polymer can be formed also with sulfonic or phosphonic acid groups or the appropriate acid chlorides. Phosphonic acid esters like the methylphosphonate of cellulose have been discussed already in section 4.4.1 of this chapter, and also the esters with aliphatic sulfonic acids like methylsulfonic acids, leading to socalled 'mesy!cellulose', shall only be mentioned here, as they are without scientific or commercial relevance. The esters of cellulose with /7-toluenesulfonic acid, the so-called tosylcelluloses, on the other hand, deserve some more attention as they form versatile intermediates in the organic chemistry of cellulose derivatization. Cellulose tosylates can be prepared in a heterogeneous as well as a homogeneous system of reaction. In both cases a preferential substitution at the C-6 position is observed, without a pronounced regioselectivity being detected at low and medium DS values (Takahashi et al., 1986).

4.4. 3 Esters of cellulose with organic acids

195

A heterogeneous procedure has been employed in former studies, reacting a cellulose suspension in pyridine at room temperature up to 80 0C with a large excess of p-toluenesulfonyl chloride (Hess and Ljubitch, 1932; Honeyman, 1947) according to CeII-OH + H 3 C - ^ S O 2 C I

-

CeIhO-SO

Besides requiring a long reaction time of even days, and a high reagent-tocellulose ratio of up to 40 : 1, this 'heterogeneous procedure' has the disadvantage of excessive side reactions, i.e. chlorination and eventually also formation of aminodesoxy groups. The chlorination to chlorodesoxycellulose additionally is favored by a high reaction temperature. Furthermore, the products obtained usually exhibit a poor solubility. These shortcomings can be avoided by performing the esterification in a thoroughly homogeneous system after previous dissolution of the polymer in a nonderivatizing solvent system, e.g. DMA/LiCl. A suitable homogeneous procedure in this solvent system has been recently described by Rahn et al. (1996). In this study 0.6-9 mol of tosyl chloride/mol of AGU were employed, starting from cellulose with a DP between 280 and 5100, arriving (in the presence of TEA as the base) at tosylcelluloses of DS values between 0.4 and 2.3. Reaction time was 24 h at 8 0C. The higher reaction rate of the C-6 position as compared with those at C-2 and C-3 has been confirmed in this study. The reaction products were soluble in DMSO irrespective of the DS obtained, while the solubility in other solvents like DMF, acetone, THF, or chloroform was found to depend on DS. The range of solubility of these homogeneously prepared cellulose tosylates is significantly broader than that of conventionally synthesized ones, and the former show good film-forming properties e.g. for the preparation of membranes, and can be processed by means of a thermally induced phase separation process. According to this, tosylcelluloses of good and uniform solubility can only be prepared by this homogeneous process, which takes place with only minimal side reactions (DS^\ = 0.01-0.02) (Rahn et al., 1996; Heinze, 1997). In a broad variety of subsequent reaction steps, the tosylate function can be employed as a protective group as well as a leaving group. Employing the tosylate group as a protecting group, aliphatic and aromatic, and also unsaturated mixed esters of cellulose could be obtained by esterification of free hydroxy groups. The appropriate anhydride instead of the acid chlorides is recommended for this purpose in order to avoid chlorination of the polymer chains. A complete acylation of all the free hydroxy groups of cellulose tosylates was observed with acetic anhydride or propionic anhydride without a decrease in the DS of tosylate groups, and even introduction of stearyl groups was accomplished of up to 84 % of the total amount of free hydroxy groups. By acylation of free hydroxy groups,

196


soluble cellulose derivatives with a controlled hydrophile/hydrophobe ratio can be synthesized. Besides this, amphiphilic esters (phthalates, trimellitates and sulfates) of cellulose tosylates with unusual solubility properties can be obtained. Sulfation of the tosylates results in water-soluble cellulose sulfate halfesters with reactive tosylate groups, which are well suited for the design and experimental realization of new supramolecular cellulosic structures (Heinze and Rahn, 1996a). A protective action of tosylate groups has been observed, too, in the reaction with isocyanates. The function of the tosylate group as a leaving group has long been known in the synthesis of desoxycelluloses, e.g. by reaction with NaI to a 6-Oiododesoxycellulose according to Cell-O-SO2-^)-CH3 + NaI

- CeII-I + Na-O-SO 2

Acetylacetone has been proposed as reaction medium (Rahn et al., 1996), which permits rather short times of reaction due to its high boiling point. The availability of cellulosic compounds with reduced functionality and controlled reactivity along this route has been emphasized recently (Heinze et al., 1996b; see also chapter 4.4.1.6). Cellulose tosylates generally show a satisfactory chemical and thermal stability. A comprehensive thermoanalytic characterization of homogeneously prepared tosylcelluloses from 20 0C up to 500 0C has been published by Heinze et al. (1996a), indicating a lower temperature of decomposition as compared with cellulose itself due to detosylation and simultaneous backbone degradation. An integration of tosylate groups into a partially substituted cellulose acetate resulted in a lowering of the decomposition temperature and an increased char yield (Takahashi et al., 1986; Heinze et al., 1996b).

4.4.3.9 Phenyl carbamates of cellulose Cellulose can be smoothly reacted with phenyl isocyanate according to CeIl-OH+

—

~

in a dipolar aprotic medium in the presence of pyridine, yielding a trisubstituted product ('cellulose tricarbanilate') with a sufficiently high excess of reagent after about 10 h reaction time at 70-100 0C. The reaction system changes gradually from a heterogeneous one to a homogeneous one, and this carbanilation is accompanied by only a negligible chain degradation. It thus represents a suitable route to convert cellulose 'polymer-analogues' to a soluble derivative for subse-

4.4.4 Concluding remarks on cellulose esterification

197

quent characterization in solution. After the reaction the excess isocyanate can be decomposed by addition of dry methanol. After precipitation with a water/methanol mixture the reaction product is recovered as a white solid, which is soluble in various dipolar aprotic solvents like DMF, DMSO, THF or acetone (Burchard and Husemann, 1961; Schroeder and Haigh, 1979; Rantanen et al., 1986). A laboratory procedure published by Burchard and Husemann (Burchard and Husemann, 1961) for preparing cellulose tricarbanilate is given in the Appendix. A strictly homogeneous route to cellulose tricarbanilate is available by dissolving the sample in DMA/LiCl and reacting it with an adequate amount of phenyl isocyanate in the presence of some pyridine as catalyst (Terbojevich et al., 1995). Besides the well-established position of cellulose tricarbanilates in general solution characterization of cellulosics (Burchard and Schulz, 1989), and especially in the determination of the molar mass distribution of cellulose samples by GPC (Saake et al., 1991), some ring-substituted phenyl carbamates of cellulose have recently gained interest as sorbents for the Chromatographie separation of enantiomers. A photocontrolled chiral recognition was reported by Yashima et al. (1995) with 4-phenyl-azo-phenyl carbamates of cellulose and amylose in connection with a photoresponsive cisltrans isomerization, the trans isomer showing a higher selectivity. The optical resolving ability of two regioselectively carbanilated cellulose and amylose samples has been compared by Kaida and Okamoto (1993), one of the samples carrying a 3,5-dimethylphenyl carbamate residue in the C-2/C-3 position and a 3,5-dichlorophenyl carbamate residue in the C-6 position, while the other sample had attached the chloro-substituted residue in the 2,3-position and the methyl-substituted one in the C-6 position. A comprehensive macromolecular characterization of samples of bis-3,5dimethylphenyl carbamates of cellulose with molar masses between 2 χ ΙΟ4 and 4 χ 106 in dilute solution in l-methyl-2-pyrrolidone has recently been presented by Tsuboi et al. (1995). The authors emphasized the remarkable optical anisotropy of this polymer and concluded from their light scattering, sedimentation and viscosity data a worm-like chain behavior in solution. Concentrated solutions of regioselectively functionalized cellulose phenylcarbamates ('cellulose carbanilates') can form lyotropic liquid crystalline mesophases. Their optical properties depend on the pattern of substitution within the AGU as well as on the specific substitution within the phenylring by CHs-, F- or Cl- (Derleth and Zugenmaier, 1997)

4.4.4 Concluding remarks on cellulose esterification The esterification of cellulose plays a central role in chemical conversion of this polymer. From the scientific point of view it represents a very broad spectrum of chemical compounds and material properties, and it is the most important point

4.4.4 Concluding remarks on cellulose esterification

197

quent characterization in solution. After the reaction the excess isocyanate can be decomposed by addition of dry methanol. After precipitation with a water/methanol mixture the reaction product is recovered as a white solid, which is soluble in various dipolar aprotic solvents like DMF, DMSO, THF or acetone (Burchard and Husemann, 1961; Schroeder and Haigh, 1979; Rantanen et al., 1986). A laboratory procedure published by Burchard and Husemann (Burchard and Husemann, 1961) for preparing cellulose tricarbanilate is given in the Appendix. A strictly homogeneous route to cellulose tricarbanilate is available by dissolving the sample in DMA/LiCl and reacting it with an adequate amount of phenyl isocyanate in the presence of some pyridine as catalyst (Terbojevich et al., 1995). Besides the well-established position of cellulose tricarbanilates in general solution characterization of cellulosics (Burchard and Schulz, 1989), and especially in the determination of the molar mass distribution of cellulose samples by GPC (Saake et al., 1991), some ring-substituted phenyl carbamates of cellulose have recently gained interest as sorbents for the Chromatographie separation of enantiomers. A photocontrolled chiral recognition was reported by Yashima et al. (1995) with 4-phenyl-azo-phenyl carbamates of cellulose and amylose in connection with a photoresponsive cisltrans isomerization, the trans isomer showing a higher selectivity. The optical resolving ability of two regioselectively carbanilated cellulose and amylose samples has been compared by Kaida and Okamoto (1993), one of the samples carrying a 3,5-dimethylphenyl carbamate residue in the C-2/C-3 position and a 3,5-dichlorophenyl carbamate residue in the C-6 position, while the other sample had attached the chloro-substituted residue in the 2,3-position and the methyl-substituted one in the C-6 position. A comprehensive macromolecular characterization of samples of bis-3,5dimethylphenyl carbamates of cellulose with molar masses between 2 χ ΙΟ4 and 4 χ 106 in dilute solution in l-methyl-2-pyrrolidone has recently been presented by Tsuboi et al. (1995). The authors emphasized the remarkable optical anisotropy of this polymer and concluded from their light scattering, sedimentation and viscosity data a worm-like chain behavior in solution. Concentrated solutions of regioselectively functionalized cellulose phenylcarbamates ('cellulose carbanilates') can form lyotropic liquid crystalline mesophases. Their optical properties depend on the pattern of substitution within the AGU as well as on the specific substitution within the phenylring by CHs-, F- or Cl- (Derleth and Zugenmaier, 1997)

4.4.4 Concluding remarks on cellulose esterification The esterification of cellulose plays a central role in chemical conversion of this polymer. From the scientific point of view it represents a very broad spectrum of chemical compounds and material properties, and it is the most important point Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

198


of intersection between general organic chemistry and the special chemistry of cellulose derivatization, promoting the introduction of modern reaction theory into cellulose chemistry. Furthermore, esters like the nitrate or the carbanilate are indispensable for the macromolecular characterization of cellulose in solution and for assessing molar mass distribution. From the commercial point of view, esterification of this polymer is by far the most widely employed route. Cellulose xanthogenate in about 90 % of the total production of cellulose derivatives. The area of organic cellulose esters has been investigated rather thoroughly over many decades, and discoveries of really new types of compounds have been rather scarce in recent years. The inorganic esters, on the other hand, have been studied more, emphasizing definitely the nitrate and to some extent the sulfate, and leaving ample space for further exploration. At present, future developments in cellulose esterification are envisaged by the authors as the synthesis of compounds with well-defined and pre-set patterns of substitution along the polymer chain, as well as within the single AGU, including double and triple substitution with different groups, in order to provide macromolecular entities for the design of well-defined supramolecular cellulosebased architectures. For achieving this goal in an adequate extension, homogeneous and heterogeneous reactions, as well as combinations of both, will have to be pursued, implying a deeper insight into the relations between chemical reactivity and physical structure of cellulose, besides the application of the full repertoire of theoretical principles and experimental techniques of modern organic chemistry in the field of cellulose esterification.

References Albright, L.F., in Encyclopedia of Chemical Technology, New York: John Wiley & Sons, 1981, 3rd. Edn., Vol. 15, pp. 841-853. Allen, T.C., Cuculo, J.A., /. Polym. ScL, Macromol Rev. 1973, 7, 189-262. Allen, T.C., Cuculo, J.A.,Appl. Polym. Symp. 1976, 28, 811-829. Altena, F.W., Schroder, J.S., Van de Hüls, R., Smolders, C.A., /. Polym. ScL, Part B, Polym. Phys. 1986, 24, 1725-1734. Anger, H., Berth, G., Wagenknecht, W., Linow, K.J., Acta Polym. 1987, 38, 201-202. Arthur, J.C., Bains, M.S., Patent US 3790562, 1974; Chem. Abstr. 1974, 8I9 39285. Arthur, J.C., Bains, M.S., Patent US 3891621, 1975; Chem. Abstr. 1975, 83, 114817.

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4.5 Etherification of Cellulose 4.5.1 General remarks on etherification Cellulose etherification is a very important branch of commercial cellulose derivatization that started considerably later than the conversion of the polymer to esters. Preparation of a cellulose ether was reported for the first time in 1905 by Suida, who reacted the polymer with dimethyl sulfate to give a methylcellulose. The first patent claiming the preparation of soluble nonionic alkyl ethers of cellulose was issued in 1912 to Lilienfeld, and by 1920 the synthesis of some other important classes of cellulose ethers like carboxymethylcellulose, benzyl-

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4.5 Etherification of Cellulose 4.5.1 General remarks on etherification Cellulose etherification is a very important branch of commercial cellulose derivatization that started considerably later than the conversion of the polymer to esters. Preparation of a cellulose ether was reported for the first time in 1905 by Suida, who reacted the polymer with dimethyl sulfate to give a methylcellulose. The first patent claiming the preparation of soluble nonionic alkyl ethers of cellulose was issued in 1912 to Lilienfeld, and by 1920 the synthesis of some other important classes of cellulose ethers like carboxymethylcellulose, benzylComprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

208

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cellulose or hydroxyethylcellulose had been described. Industrial production started in the two decades between 1920 and 1940, beginning with carboxymethylcellulose (CMC) in the early 1920s in Germany. The worldwide industrial manufacture of cellulose ethers has presently arrived at a level of about half a million tons annually, with CMC dominating by far, followed by methy!cellulose and hydroxyethylcellulose (Table 4.5.1). Table 4.5.1. Production capacity (t/a) of economically important cellulose ethers (Brandt, 1986).

Ether

Production capacity

Carboxymethylcellulose Methylcellulose Hydroxyethylcellulose

300,000 t/a 70,000 t/a 54,000 t/a

Among the various routes to synthesis of cellulose ethers, which will be described in detail in the following sections, only two are of commercial relevance, i.e. (i) the reaction of hydroxy groups with an alkyl chloride in the presence of strong alkali-metal hydroxides, according to the Williamson ether synthesis, consuming 1 mol of alkali/mol of alkyl chloride reacted; (ii) the ring-opening reaction of an alkylene oxide with the hydroxy groups, which is catalyzed by alkali-metal hydroxides without significant alkali consumption, and which often results in longer side chains due to further add-on of alkylene oxide onto the newly formed hydroxy groups. Industrial etherification of cellulose is exclusively performed in a heterogeneous system, starting from alkali cellulose. Due to side reactions with the water present in the aqueous system in large excess (calculated on a molar basis) and competing with the cellulosic hydroxy groups for the etherifying agent, reagent yield remains considerably below the 100 % margin, and a further processing to remove by-products from the crude cellulose ether is usually required for highpurity products. The etherification of cellulose in the dissolved state can be realized too and is of scientific interest today in connection with the control of the functionalization patterns of the polymers and with the synthesis of new types of cellulose ethers. The abundant variability of cellulose ether structures described up to now and the remarkable broad spectrum of cellulose ethers commercially available can be traced back to two characteristics besides the well-known possibility of varying the DS and the distribution of the substituents: firstly, the chemical constitution of the alkyl halide and to some extent also of the alkylene oxide can be changed, making anionic and cationic cellulose ethers available besides the neutral ones.

4.5.1 General remarks on etherification

209

Moreover, not only carbon-based cellulose ethers can be prepared, but also various silyl ethers have been synthesized by reaction of the polymer especially with trialkylchlorosilanes (see section 4.5.5). Secondly (and this point is still more relevant in connection with commercial cellulose ethers), the two routes of ether synthesis outlined above can be combined by adding simultaneously or consecutively an alkyl chloride and an alkylene oxide to the aqueous alkaline reaction system, arriving at so-called mixed ethers of cellulose with two or even three different ether functions. Furthermore, numerous routes of a subsequent functionalization of cellulose ethers considerably increases the number of structures and products available. Research and development activities in recent decades have been centered on the full exploitation of this 'mixed ether principle' for tailoring properties to the broad variety of end-use requirements. Besides this, the minimization of chain degradation during the process in order to obtain a high solution viscosity of the product and the enhancement of reagent yield with the option to decrease the input of chemicals for ecological reasons, played a major role. Cellulose ethers on a commercial scale are generally used as end-products, but serve as interesting intermediates too. In laboratory-scale research they are used either for further chemical modification of the ether group primarily introduced, or for subsequent reaction of remaining hydroxy groups present in a partially substituted cellulose ether. The most important properties of cellulose ethers are their solubility combined with chemical stability and non-toxicity. Water solubility and/or organosolubility can be controlled within wide limits via the constitution and the combination of ether groups at the cellulose chain, as well as via the DS, and to some extent via the pattern of substitution. Accordingly, cellulose ethers are generally applied, in the dissolved or highly swollen state, to many areas of industry and domestic life, with the spectrum of applications ranging from auxiliaries in large-scale emulsion or suspension polymerization, through to additives for paints and wall paper adhesives, to viscosity enhancers in cosmetics and foodstuffs. For the sake of clearness and conciseness, the following chapter is structured according to the constitution of the ether group: the first and most voluminous section deals with aliphatic cellulose ethers, comprising alkyl ethers, substituted alkyl ethers, hydroxyalkyl ethers and mixed aliphatic ethers of cellulose. The following section on aryl and aralkyl ethers of cellulose is centered on triphenylmethylcellulose and related substances as interesting intermediates in today's cellulose chemistry. As a special feature of this book, the third section describes in a rather detailed manner the preparation, properties and subsequent reaction routes of silyl ethers of cellulose, emphasizing adequately the authors' work in this area. Each of the sections begins with a comprehensive discussion of the chemical aspects, followed by a brief consideration of the role of cellulose

210


supramolecular structure by etherification, turning then to the properties and the main areas of application of the various classes of cellulose ethers. A brief description of the industrial process is included for some ethers.

4.5.2 Aliphatic ethers of cellulose Aliphatic ethers of cellulose have been extensively investigated since the beginning of this century, and comprise also large-scale industrial products of this class of cellulose derivatives like methylcellulose, carboxymethylcellulose (CMC), and hydroxyethylcellulose (HEC). Aliphatic cellulose ethers can be classified in various ways, i.e. (i) from an applicational point of view into nonionic (methylcellulose, HEC) and ionic (CMC) ones; (ii) on the basis of the main routes of synthesis, into those obtained by the Williamson synthesis with consuming one mol of base per mol of ether groups introduced and those obtained by ring-opening reactions of epoxides as reagents with a catalytic amount of alkali; (iii) from the viewpoint of a systematic description according to the type of functional groups attached to the backbone. In this context the last-mentioned route will be followed, structuring the section according to alkyl ethers, carboxyalkyl ethers, hydroxyalkyl ethers and ethers with special functional groups. 4.5.2.1

Alkyl ethers of cellulose

Chemistry of cellulose alkylation By far the most important representative of this class of cellulose ethers carrying an unsubstituted alkyl group is methylcellulose, which is available over the whole DS range 0-3 along various routes of synthesis. The commercial products with a DS between 1.5 and 2.0 are obtained by a Williamson reaction of alkali cellulose with gaseous or liquid CH3Cl. The lye employed for cellulose alkalization contains at least 40 % NaOH (in contrast with about 18 % in the viscose process). The methylation of cellulose, which is usually classified as an SN2 reaction, is the result of the nucleophilic attack of the cellulosic alkoxido group on the acceptor C atom of the methyl chloride. CeII-OH + Na+ OH' —- CeII-OI Na+ + H2O CeII-OI Na+ + C+H3-CI

- CeII-Q-CH3 + Na+ Cl'

The etherification of cellulose in the presence of alkali hydroxide is, however, accompanied by the hydrolysis of methyl chloride, with the water present in the

210


supramolecular structure by etherification, turning then to the properties and the main areas of application of the various classes of cellulose ethers. A brief description of the industrial process is included for some ethers.

4.5.2 Aliphatic ethers of cellulose Aliphatic ethers of cellulose have been extensively investigated since the beginning of this century, and comprise also large-scale industrial products of this class of cellulose derivatives like methylcellulose, carboxymethylcellulose (CMC), and hydroxyethylcellulose (HEC). Aliphatic cellulose ethers can be classified in various ways, i.e. (i) from an applicational point of view into nonionic (methylcellulose, HEC) and ionic (CMC) ones; (ii) on the basis of the main routes of synthesis, into those obtained by the Williamson synthesis with consuming one mol of base per mol of ether groups introduced and those obtained by ring-opening reactions of epoxides as reagents with a catalytic amount of alkali; (iii) from the viewpoint of a systematic description according to the type of functional groups attached to the backbone. In this context the last-mentioned route will be followed, structuring the section according to alkyl ethers, carboxyalkyl ethers, hydroxyalkyl ethers and ethers with special functional groups. 4.5.2.1

Alkyl ethers of cellulose

Chemistry of cellulose alkylation By far the most important representative of this class of cellulose ethers carrying an unsubstituted alkyl group is methylcellulose, which is available over the whole DS range 0-3 along various routes of synthesis. The commercial products with a DS between 1.5 and 2.0 are obtained by a Williamson reaction of alkali cellulose with gaseous or liquid CH3Cl. The lye employed for cellulose alkalization contains at least 40 % NaOH (in contrast with about 18 % in the viscose process). The methylation of cellulose, which is usually classified as an SN2 reaction, is the result of the nucleophilic attack of the cellulosic alkoxido group on the acceptor C atom of the methyl chloride. CeII-OH + Na+ OH' —- CeII-OI Na+ + H2O CeII-OI Na+ + C+H3-CI

- CeII-Q-CH3 + Na+ Cl'

The etherification of cellulose in the presence of alkali hydroxide is, however, accompanied by the hydrolysis of methyl chloride, with the water present in the Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

4.5.2 Aliphatic ethers of cellulose

211

system at large molar excess leading to methanol, which can react further with methyl chloride to form dimethyl ether. This by-product formation accounts for 20-30 % of the CH3Cl consumption, resulting in a reagent yield for etherification of maximally 80 %. For etherification, as well as for the by-product formation, 1 mol of NaOH is consumed per mol of CH3Cl converted, and besides the organic by-products, a large amount of NaCl is inevitably produced in this process. Methylation of cellulose by the Williamson reaction is generally performed at elevated temperature with cellulose in the solid state (see section 4RoIe of cellulose supramolecular structure in alkylation'). As demonstrated by the results of laboratory-scale methylation (Philipp et al., 1979) in Fig. 4.5.1, the course of reaction is characterized by a fast initial state, followed by a slow leveling off of the DS just below 2 even at a large excess of methyl chloride. 2.0 1.5

1.0

0.5

100

200 300 Reaction time [min]

400

100

200 Reaction time

300 [min]

400

Figure 4.5.1. Course of total NaOH consumption (left) and degree of substitution of methyl groups (right) with time of methylation of alkali cellulose with an excess of CH3Cl at different temperatures: · 70 0C, O 80 0C, · 90 0C (Philipp et al., 1979).

While an increase in reaction temperature from 70 to 80 0C considerably enhances the initial reaction rate, a further increase to 90 0C has a minor effect only. This is consistent with the assumption that the overall rate is determined by the chemical reaction only at lower temperature, while above 80 0C the reagent transport rate across the phase boundary and within the alkali cellulose moiety is the dominating factor. At a reaction temperature above 80 0C, as employed in the technical process, methyl chloride is known to be a more efficient etherification agent than the corresponding bromide or iodide due to a higher molar volume. This agrees well with a diffusion-controlled reaction. Regarding the substituent distribution within the AGU of partially methylated products, obtained by the Williamson reaction, a slight preference for the C-2 position compared with C-6 is generally reported, while the C-3 position contains ether groups to a definitely lower extent (Dönges, 1990). According to

212


Rosell (1988) a partially methylated sample exhibited a methylation of 70.0 % at O-2, 61.5 % at O-6 and only 35.4 % at O-3 position. The substitution pattern along the chain obviously depends on the procedure of synthesis employed and may deviate from a statistical distribution (Arisz et al., 1996). Table 4.5.2. Systems employed in laboratory methylation of cellulose. Methylation system

References

DMSO/NaOH/CH3I

Ciucanu and Kerek, 1984; Needs and Selvendran, 1993 Hakamori, 1964; D'AmbraetaL, 1988 Klemm and Stein, 1995 Mischnick, 1991; Kulshin et al., 1991 Prehm, 1980; Mischnick, 1991

DMSO/LiH/CH3I DMF, THF/NaH/CH3I CH2Cl2/2,6-di-i-buty lpyridine/ (CH3)3O+[BF4] (CH3)3PO4/2,6-di-i-butylpyridine/ CF3SO3CH3 (methyltriflate)

The course of cellulose methylation can be widely modified by varying the methylation agent, the base which is required in any case, the reaction medium, and the state of dispersion of the system. Instead of methyl chloride, also methyl iodide, dimethyl sulfate, diazomethane (prepared in situ from nitrosomethylurea) or special agents like trimethyloxonium tetrafluoroborate, or methyltriflate (see Table 4.5.2) can be employed. Methyl iodide proved to be very suitable for the methylation of cellulosic hydroxy groups in a homogeneous medium of the polymer (e.g. after dissolution in DMA/LiCl or in tetraalkylammonium bromide), the reaction medium and the alkyl halide. As a base, NaH or LiH, metallic Na dispersed in an ammonia cellulose NH3 system, or di-i-butylpyridine have been proposed (see Table 4.5.2). Methylation of cellulose has also been performed in aqueous solutions of tetraalkylammonium hydroxides, with the polymer in the highly swollen or dissolved state, arriving here at water-soluble products already at a DS of about 0.6, due to a more equal ether-group distribution along the polymer chain (Bock, 1937). Besides water, also dipolar aprotic liquids like DMSO, DMF or THF served as reaction media in cellulose methylation. Some of these systems, suitable for laboratory-scale etherification, are listed in Table 4.5.2. According to the authors' experience, systems of CH3I and NaH in THF or of CH3I and finally powdered NaOH in DMSO proved to be very suitable for the permethylation of free hydroxy groups of partially substituted tritylcelluloses (Camacho Gomez et al., 1996) or trialkylsilylcelluloses (Erler et al., 1992a),


213

without loss of the substituent already present. CD3I can be employed as well, if it is advantageous for the subsequent instrumental analysis of the product. Methylation can also be performed with the polymer dissolved in an aprotic system like DMSO/paraformaldehyde (Nickelson and Johnson, 1977) or DMA/LiCl. In the latter system, copolymers with a nonstatistical distribution of the different repeating units were prepared using finely powdered NaOH as the base and CH3I as the agent (Liebert and Heinze, 1997). The etherification could take place only at the points of contact between the NaOH particles and the polymer. The alkyl halide/NaOH/DMSO system has been successfully employed in recent years for preparing regioselectively or completely substituted methylcellulose with a homogeneous solution of the polymer: cellulose acetates could be converted to highly substituted methylcelluloses by a simultaneous deacetylation and etherification (Kondo and Gray, 1990). A regioselectively substituted 2,3dimethylcellulose was prepared by alkylation of 6-O-tritylcellulose and subsequent detritylation with HCl (Kondo and Gray, 1991). A small amount of water in the system proved to be essential for obtaining full methylation of the O-2 and O-3 positions. The detritylated product could be further alkylated to various 6O-derivatives of 2,3-0-methylcellulose (Kondo, 1993). Turning now to ethylcellulose and higher alkyl ethers it must be stated first that the Williamson ether synthesis under heterogeneous starting conditions becomes more and more inefficient with increasing molar volume of the alkyl halide. In the appropriate range of reaction temperature the process is diffusioncontrolled and by-product formation prevails with increasing alkyl chain length. Ethylation can still be performed by analogy to methylation by reacting alkali cellulose with ethyl chloride, arriving at a substitution pattern with about equal partial DS at C-2 and C-6, and again a low degree of etherification at C-3 (Dönges, 1990). An activation energy of 10.3 kcal/mol at a reaction temperature below 30 0C, and of 4.4 kcal/mol at higher temperature were reported (Chakrabarti et al., 1986), indicating again the dominant role of diffusion in the latter case. An efficient propylation required either a previous partial methylation for 'widening' the polymer structure, or employing a tetraalkylammonium hydroxide of high swelling power in aqueous solution as the base and reaction medium (Schenck, 1936; Timell, 1950). The synthesis of alkyl ethers of cellulose with longer side chains usually requires nonaqueous systems, more severe basic reaction conditions, and rather long reaction times often at elevated temperature. The preparation of long-chain alkyl ethers of cellulose by reaction of cellulose acetate with the appropriate alkyl bromide in the presence of NaOH in DMSO as the reaction medium is described by Basque et al. (1996). Table 4.5.3 presents a survey of some longchain alkyl ethers and their preparation, starting from a suspension of dry cellu-

214


lose in isopropanol or in DMSO and reacting it with the appropriate alkyl bromide in the presence of NaOH or NaH (Blasutto, 1995). Table 4.5.3. Reaction conditions for long-chain cellulose ether preparation in DMSO, taken from Blasutto et al. (1995).

Reagent 1 -Bromooctadecane 1 -Bromohexadecane 1 -Bromotetradecane 1 -Bromooctadecane 1 -Bromododecane 1 -Bromotetradecane 1 -Bromohexadecane 1-Bromooctane 1-Bromooctane 1 -Bromooctadecane 1 -Bromotetradecane

Base NaOH NaOH NaOH NaH NaH NaH NaH NaH NaH NaH NaH

Reaction time (h) 92 48 133 72 15 25a 19 3 25 26 26

a

The suspension of NaH in DMSO was heated to 40 0C to accelerate the formation of the anion (CH3-SO-CH2)". The cellulose was added after cooling at room temperature.

Due to the chemical stability of the ether group, and a DS-dependent solubility in various media, partially substituted methyl- and ethylcelluloses are well suited to serving as the starting material for a subsequent functionalization of residual hydroxy groups under homogeneous conditions of reaction. Examples are the preparation of various organic ester ethers of cellulose from an ethylcellulose of DS = 2 with various acylanhydrides and acyl chlorides in benzene in the presence of 4-dimethylaminopyridine as the catalyst (Philipp et al., 1983), and the preparation of mixed methyl/allyl ethers of cellulose from a methylcellulose of DS = 1.6 by reacting it with allyl chloride or methallyl chloride in the presence of NaOH in DMSO (Kondo et al., 1987). The same authors also succeeded in the synthesis of a triallylcellulose by deacetylation/etherification of a cellulose acetate of DS = 1.8 with allyl chloride and NaOH in DMSO. The laboratory procedure for the methylation of cellulose is presented in the Appendix. Role of cellulose supramolecular structure in alkylation Methylation of alkali cellulose with CH3Cl represents a typical 'heterogeneous derivatization reaction', with the accessibility of the cellulose chains to the reagent determining the course of conversion in this diffusion-controlled process. A


215

still larger influence of accessibility on the course of reaction was observed with more voluminous alkylating agents such as dimethyl sulfate or ethyl chloride. The high steeping lye concentration required in alkali-cellulose formation for an effective etherification, not only supplies the necessary alkalinity for the chemical reaction, but also enhances the availability of the cellulose molecules to the reagent by a further decrease in overall supramolecular order (Fink et al., 1995). 2.0

1.5

co 1.0 Q

0.5

20

40 60 80 NaOH consumption

100

Figure 4.5.2. Relation between total NaOH consumption and degree of substitution of methyl groups on methylation of alkali cellulose (31.8 % cellulose, 30 % NaOH) at different temperatures: · 70 0C, O 80 0C, · 90 0C (Philipp et al., 1979).

The strongly heterogeneous character of alkali-cellulose methylation had been emphasized already by Hess and co-workers (Hess et al., 1933), who observed the WAXS reflexes of trimethylcellulose already at a low overall DS, and assumed a high DS in near-surface areas of the fiber and a negligibly low DS in their center at an early stage of reaction. In the presence of a sufficient amount of methyl chloride and NaOH, the still existing hydrogen bond system between the cellulose chains is further disturbed on methylation, and the hydrophilic character of the still free hydroxy groups is irreversibly liberated by partial etherification (Dönges, 1990). The overall reaction, however, can come to a quasi-standstill before complete consumption of the NaOH due to diffusion hindrance, as demonstrated in Fig. 4.5.2, by the limiting DS of about 1.7 reached after an alkali consumption of about 80 % (Philipp et al., 1979). On the other hand, rather small differences in the course of alkali-cellulose methylation were found between a spruce sulfite pulp and bleached cotton !inters. Also, preactivation of the cellulose was found to be of minor influence only. This is understandable in so far as the process of alkali-cellulose formation, especially at high steeping lye concentrations, results in a leveling of structural differences between the different cellulose materials. This holds true especially also for the industrial process of methylation, as there, alkalization is

216


usually preceded by a dry grinding, which by itself already decreases differences in e.g. X-ray crystallinity (Fink and Walenta, 1994). Completely substituted trimethylcellulose exhibits a well-defined WAXS fiber diagram, with a period of 10.3 A in the fiber-axis direction, and it can be brought to crystallization from solution or from its melt (Hess et al., 1928). The state of supramolecular order and the side chain conformation of liquid crystalline systems of ethy!cellulose in CHCl3 have been studied by NMR (Yim et al., 1992). Survey of the technical process of cellulose methylation (Dönges, 1990; Brandt, 1986) After dry grinding or chopping, normal-grade wood dissolving pulp is transformed into alkali cellulose by treatment with 35-70 % aqueous NaOH (34 mol/mol of AGU), and after an eventual preripening for viscosity reduction (see preripening process) the alkali cellulose is methylated with an excess of CH3Cl employed either in the gaseous or in the liquid state. In the 'gaseous process' the alkali cellulose is warmed with part of the CH3Cl to about 50 0C in a corrosion-resistant pressure vessel by an effective stirring device. A reaction temperature of between 60 and 100 0C is maintained for some hours. Reagent, evaporating together with the by-products, is removed, condensed and recycled into the reactor, together with fresh reagent, in order to keep a constant concentration of methyl chloride in the reaction system. The 'liquid methyl chloride process' can be performed as a continuous process requiring a reaction time of less than 1 h. In this process the alkali cellulose is slurried in excess reagent, and this slurry then is pumped through a partially heated reaction tube. By-products and excess reagent are evaporated. The 'liquid process' can also be operated in the presence of an inert organic liquid, e.g. dimethyl ether, dimethylglycol or toluene, in order to reduce the reaction pressure in the case of higher boiling liquids and/or to reduce by-product formation. Both processes can also be employed for the production of mixed ethers, with the second reagent added before or after methylation, and the course of the reaction temperature being program-controlled. In many of the technical procedures the alkali is completely consumed, otherwise a neutralization step is necessary before washing the product with hot water of 80-90 0C, i.e. well above the gelation temperature, for removal of sodium chloride and other by-products. In this way, the NaCl content is decreased to about 1 % for normal grade and about 0.1 % for high-quality methylcellulose. Eventually, the product crosslinks to a low degree with glyoxal for a retarded dissolution in water by slow hydrolysis of the crosslinks. Drying of the product is performed in conventional equipment.


217

Ethylcellulose is manufactured analogously to methylcellulose, with ethyl chloride as the reagent, but at a higher temperature, usually above 110 0C. A reaction time of 8-16 h is required, and about half of the reagent input is consumed for side reactions, i.e. of ethanol and diethyl ether. Reagent yield is reported to increase with the steeping lye concentration (55-76 % NaOH), and a stepwise addition of the lye in the process was found to be advantageous. Further product processing is performed as described for methylcellulose. Properties of alkylcelluloses, especially methylcellulose Alkylcelluloses are white-to-yellowish nontoxic solids, exhibiting a graded solubility in various media, in dependence on substituent and DS. Hydrophobicity increases with the length of the alkyl chains and with the DS. Commercial methylcelluloses in the DS range 1.5-2.0 are to be classified as amphiphilic, while commercial ethy!celluloses with a DS above 2 are definitely hydrophobic. Methylcellulose is chemically very stable and the viscosity of an aqueous solution is independent of pH in the range from pH 2 to 12. Ethylcellulose can form peroxides in the presence of oxygen and light, and eventually needs stabilization by an antioxidant. Methylcellulose in the DS range up to 2 proved to be biodegradable in the presence of water: According to Seneker and Glass (1996) sequences of at least six AGU with an unsubstituted C-2 position are required for an enzymatic attack, demonstrating once more the relevance of regioselectivity of substitution to interaction with biological systems. Ethylcellulose and the higher alkyl ethers are hardly degraded by cellulolytic enzymes even at much lower DS. For a fully substituted methylcellulose, a melting range between 227 and 240 0C under decomposition has been reported (Hess et al., 1935). Commercial ethylcelluloses with a DS above 2 are thermoplastic and can be extruded to give films at a softening temperature of 130 0C and a flow temperature of 140-160 0C. The most relevant applicational properties of methyl- and ethylcelluloses are the solubility and solution properties. As can be seen from Table 4.5.4, methylcelluloses of increasing DS, i.e. of increasing hydrophobicity, exhibit solubility in liquids of decreasing polarity, and the same holds true for ethylcellulose, taking into consideration the more hydrophobic nature of the substituent. These statements, however, are valid only for products manufactured by the conventional etherification of alkali cellulose in a heterogeneous system, and therefore exhibiting a nonuniformity of substituent distribution along the polymer chains. A more even distribution, as realized by methylation of cellulose dissolved in a tetraalkylammonium hydroxide, results in complete water solubility already at a DS of about 0.6. The effect of regioselectivity on physical product properties, as e.g. solubility and crystallinity, was recently studied by Kondo (1997). He compared 6-O-methy!cellulose with non-regioselectively methylated samples and

218


correlated differences in product properties to differences in their hydrogen bond systems. The excellent solubility and poor crystallinity of 6-0-methylcellulose were traced back to a lack of interchain hydrogen bonds, while intramolecular hydrogen bonds were assumed to persist even after dissolution of the sample. The optical transparency of aqueous methylcellulose solutions can be enhanced by using a mixed ether with a small amount of hydroxyalkyl groups. Table 4.5.4. Solubility of methyl- and ethylcelluloses in dependence Cellulose ether

Solvent

DS range of solubility

Methylcellulose Methylcellulose Methylcellulose Methylcellulose Methylcellulose Ethylcellulose Ethylcellulose

Aqueous NaOH Water Ethanol Acetone Toluene Water Organic liquids

0.25-1.0 1.4-2.0 >2.1 >2.4 >2.7 0.7-1.7 > 1.5; preferably >2

A phenomenon of high scientific and practical relevance is the gelation of aqueous solutions of methylcellulose with a DS in the range between 1.7 and 2.3 at elevated temperatures. Commercial products of DS 1.8 form gels at 54-56 0C. This gelation is reversible along a hysteresis loop of gelation and redissolution, and it plays an important role in methylcellulose purification and processing (see scheme in Fig. 4.5.3).

100

-Cloud point "- Cteer point

λ

Flocculation

σ Q_

Dissolving

C-2 > C-3 has been observed. A MS value of about 4 is considered necessary here to obtain water solubility. Starting from a conventional alkali cellulose prepared by steeping with 18 % aqueous NaOH and subsequent pressing, two procedures of hydroxypropylation were compared by Asandei et al. (1995): (i) a slurry procedure with an organic diluent, employing rc-hexane, i-butanol and alkali cellulose at a ratio of 1.54 : 0.5 : 1 and a propylene oxide/AGU ratio of 7-11 at 60 0C for 2-6 h under pressure;


239

U 12 10 >s

I8

I6

CL

4 2 O

0.5 1.0 mol NoOH mol AGU

1.5

Figure 4.5.14. Effect of alkali concentration on the reactivity of the positions X, 6, 2, and 3 (top to bottom) in the hydroxyethylation of cellulose (Dönges, 1990).

(ii) a procedure without diluent, employing gaseous propylene oxide at a ratio of 2-6/AGU at 40-50 0C and low pressure for 2-6 h. With the first procedure MS values up to 4.2, and with the second procedure up to 4.0, were obtained, despite the much lower reagent input in the latter case. The hydroxy groups in hydroxyalkylcellulose can be employed for subsequent esterification or etherification. Acetylation has been used for analytical purposes in assessing the pattern of functionalization. Etherification of HEC by methylation, ethylation or hydroxypropylation is performed on a technical scale in the manufacture of mixed cellulose ethers. The introduction of tertiary amino groups and quaternary ammonium groups into HEC is described by Katsura et al. (1992). The efficiency of phase-transfer catalysis in the hydrophobic modification of HEC by introduction of dodecylphenylglycetyl ether groups has been discussed by Emett (1996). By Lee and Kwei (1996) the reaction of HPC with hexyl-, octyl-, dodecyland octadecyl isocyanate is described, together with the supramolecular and morphological properties of the products. The simultaneous reaction of alkali cellulose with CS2 and ethylene oxide has been investigated (Lukanoff and Philipp, 1967): a presumed spacing effect of hydroxy ethyl groups was obviously overcompensated by an enhanced side product formation via the ethylene glycol formed in the system and by a reduction of CS2 activity in the presence of ethylene oxide, resulting in a significant decrease in the degree of substitution of xanthogenate groups and indicating a detrimental effect for the viscose process instead of the expected beneficial one. Covalent crosslinking of hydroxyalkylcellulose can be achieved by conventional bifunctional agents like glyoxal, re-

240


suiting in simultaneous crosslinking and chain scission. Besides these subsequent covalent reactions presented here in some detail, the intermolecular interaction between hydroxyalkylcelluloses and surfactants, e.g. sodium dodecyl sulfate, has been studied (e.g. Zugenmaier and Aust, 1990). Role of the supramolecular structure of cellulose on hydroxyalkylation As a heterogeneous process, the hydroxyalkylation of cellulose is affected by the supramolecular structure of the polymer too. This influence is diminished by strong swelling in the alkaline system, by the spacing action of the hydroxyalkyl side chains, and by the high reactivity of the C-6 position promoting a more uniform substituent distribution along the polymer chains, and it is additionally covered by the strong effect of reagent distribution within the reaction system (Dönges, 1990). Yokota (1986) compared cellulose hydroxypropylation in a slurry process with organic diluent at a low alkali concentration of 0.4 mol/mol of AGU on the one hand, and with more concentrated aqueous alkali at a low liquor ratio on the other. A very heterogeneous progress of reaction from fiber to fiber in the first case was observed, and a more uniform higher state of order in the second. The introduction of the substituents resulted in an increased 1-0-1 lattice spacing, which appeared to be more uniform across and along the fibrils in the case of the more concentrated aqueous alkali. Survey of the technical process of cellulose hydroxyalkylation In commercial hydroxyalkylation the shredded or milled material is reacted with ethylene oxide and NaOH, usually in a slurry process with /-propanol, i-butanol or acetone as the diluent, employing 0.5-1.5 mol of NaOH/mol of AGU. This is added either before the diluent or by pouring directly into the suspension. With isopropanol as the diluent usually 10-12 mol of H2O/mol of AGU are present in the reaction system. The reaction proceeds at 30-80 0C for 1-4 h, with the MS controlled by the amount of reagent applied. The reagent yield for the main reaction is reported to be about 70 % but decreases down to 50 % at high MS. In order to decrease the ethylene oxide consumption for the side reactions and to enhance product uniformity, a two-stage process can be practised, arriving at a low-substituted water-insoluble product in the first stage, and then performing the second stage with only a catalytic amount of alkali and the main part of the ethylene oxide, making use of the hydroxyethylene groups introduced in the first stage as a spacer. After neutralization with e.g. HCl, the low molecular byproducts are washed out by water/alcohol mixtures. Hydroxypropylcellulose is manufactured in a slurry process similar to HEC, but requires a higher reaction temperature of up to or above 100 0C, and a longer reaction time due to its lower reaction rate. It is manufactured under pressure with liquid propylene oxide or hexane as the reaction medium. Purification can


241

be accomplished by washing with hot water, as HPC exhibits gelling in hot water like methylcellulose. In the manufacture of HEC-based mixed ethers, usually the hydroxyethylation is accomplished first, followed by etherification with the second reagent. Properties of hydroxyalkylcelluloses HEC and HPC are white, odorless, physiologically inert powders, the solubility of which depends largely on the kind of substituent and the pattern of substitution. The biodegradability of HEC decreases with increasing DS, while the length of the side chains is of minor relevance to enzymatic attack. HPC is much more hydrophobic than HEC and can be extruded without a softener at 160 0C, whereas HEC is not thermoplastic and is decomposed in aqueous solution already above 100 0C. HEC exhibits solubility in cold, as well as in hot water at an MS above 1.0, and becomes soluble at higher MS also in mixtures of water with some polar organic liquids like lower alcohols. HPC requires an MS of about 4 for solubility in cold water and shows gelling at about 40 0C and precipitation at about 45 0C. The apparent viscosity of aqueous solutions of HEC and HPC depends on the DP, the polymer concentration and the shear rate of the solution. The [7]]-M relationship has been reported (Dönges, 1990) to be for HEC: 77 =1.1 x 10-2DP0-87 η = l χ ίο-3 Mw°·7 An impression of the concentration and shear rate dependency of the apparent viscosity of nonionic cellulose ethers in general is presented for 2 % aqueous solutions in Fig. 4.5.15. The figure indicates the broad spread of the viscosity range for commercial ether types with the appropriate viscosity level usually being included in the designation of the type in question. Besides the strong decrease of apparent viscosity with increasing shear rate most of these products exhibit a pronounced viscoelastic behavior in aqueous solution. As a peculiar feature, the tendency of HEC, and especially of HPC, to form liquid crystalline aqueous systems has to be mentioned. These systems and their optical behavior have been comprehensively studied in recent years (e.g. Giasson et al., 1991). Crosslinked films of HPC from aqueous solutions by 7irradiation and drying were prepared, and the TEM micrographs of these films revealed a persistence of chiral nematic structures from the solution to the solid film. Promising routes to preserve the liquid crystalline order of hydroxyalkylcellulose solution in the solid state consist of either preparing the liquid crystalline system in a polymerizable liquid, e.g. acrylamide, with subsequent formation of a solid matrix by radiation polymerization, or using a mixed ether with a substituent susceptible to polymerization crosslinking (Hohn and Tieke, 1997).

242


100.000

10.000

4.000

10.000

1.000 4.000

40.000

?1.000

10.000

^ 400 >> 'S 100

1 .000 'g

8

400

~

U (Λ

>

100

40 10 4

40 10

10

10*

Rotational speed [s"1]

2 4 6 Concentration [wt %]

Figure 4.5.15. General scheme of the concentration (left) and shear dependency (right) of the apparent viscosity of nonionic cellulose ethers (Dönges, 1990).

Areas of application of hydroxyalkylcelluloses The first place is kept by hydroxyethylcellulose with an annual production of 54,000 t worldwide (Dönges, 1990). Some other hydroxyalkylated products of commercial relevance are listed in Table 4.5.12, together with the appropriate DS values. Table 4.5.12. Commercial nonionic mixed ethers of cellulose (Dönges, 1990)

Sample Hydroxybutylmethylcellulose

Formula -OCH3 -Q-CH2-CHOH-CH2-CH3 Ethylhydroxyethylcellulose -Q-C2H5 -OCH2-CH2OH Hydroxyethylhydroxypropylcellulose -OCH2CH2OH -OCH2CHOH-CH3

DS 2 0.05 0.7-1.2 0.8-2.7 0.8-1.2 0.65-0.9

HEC is predominantly used in aqueous systems as a thickener or binder, or as a protective colloid and suspension stabilizer. It exhibits an excellent salt compatibility and can be converted to transparent films from aqueous solution. Numerous types covering a wide range of apparent viscosity are commercially available.


243

The main areas of application of HEC are: • • • • •

Dispersion (Latex) paints · Pigment carrier Ceramic binder · Textile size Adhesives · Emulsion polymerization Oil exploitation Paper sheet formation (wet strength additive together with glyoxal as crosslinker)

Besides this widespread use of HEC as a product, hydroxyethylation to low DS has been considered for hydrophilizing cellulose and for loosening its physical structure by the spacer action of the hydroxyethyl side chains. An interesting development of an ecocompatible artificial fiber from low DS (DS ca. 0.2) alkali-soluble HEC (Diacik et al., 1977), finally failed, as obviously complete alkali solubility to a gel-free spinning dope and sufficiently high wet strength of the fibers obtained proved to be incompatible. Areas of application of the less hydrophilic, thermoplastic and organosoluble hydroxypropylcellulose are known in the food industry and pharmaceuticals due to the high biocompatibility of this product, which is also of potential interest as a speciality product in the electronics industry. This spacer effect of hydroxypropylation to give a low DS has been successfully employed to convert macroporous bead cellulose to a more uniform, rather continuous, network structure by combining hydroxypropylation with subsequent crosslinking with epichlorohydrin(Loth, 1991). Ether bond crosslinking of cellulose with epichlorohydrin l-Chloro-2,3-epoxypropane (epichlorohydrin) combines the reactivity of an alkyl halide with that of an alkylene epoxide towards cellulosic hydroxy groups. Due to this bifunctionality, it acts as an efficient crosslinking agent in an aqueous alkaline medium according to the reaction scheme depicted in Fig. 4.5.16. Besides the catalytic amount of NaOH required for epoxy ring cleavage, the stoichiometric amount of 1 mol/mol of epichlorohydrin is necessary here for the epoxide formation. A considerable part of the epichlorohydrin is consumed in the formation of low molecular by-products, especially of glycerol (see Fig. 4.5.17), and the part of the reagent reacting with cellulose is used for bifunctional crosslinking, as well as for the monofunctional formation of 1,2dihydroxypropylcellulose. Thus, the crosslinking efficiency of epichlorohydrin lags far behind the total consumption, and the number and distribution of crosslinks formed depends largely on the detailed procedure of epichlorohydrin application. Three different procedures were compared with regard to the resulting structural changes of a

244


cellulose powder (Dautzenberg et al., 198Od). The decrease of Guam solubility with increasing degree of crosslinking (defined as the average number of hydroxy groups/AGU involved in ether crosslinks) depended significantly on the procedure employed (see Fig. 4.5.18). CeII-OH + CH2-CH2-CH2-CI -CeII-CH2-CH-CH2 \ / O

OH'

ι ι OH Cl

CeII-O-CH2-CH-CH2 O

CeII-O-CH2-CH-CH2 + CeII-OH -CeII-O-CH2-CH-CH2-O-CeII OH

V

CeII-O-CH2-CH-CH2+ H2O —-CeII-CH2-CH-CH2 \ I O

' ' OH OH

CH2-CH-CH2-CI + 2H2O-QTq-CH2OH-CHOH-CH2OH 'HCI

\ /

Figure 4.5.16. Scheme of cellulose crosslinking by epichlorohydrin

100

α. 2 W c ο υ c 40 •ο

ο 20 υ •α.

LU

O

2

4 Time [h]

6

Figure 4.5.17. Course of epichlorohydrin consumption for cellulose etherification (·) and for total consumption (O) (Dautzenberg et al., 198Od).


245

100

.75

50

25

0.2 0.6 1.0 Degree of crosslinking

1.2

Figure 4.5.18. Decrease of Guam solubility of a cellulose powder on crosslinking with epichlorohydrin by different procedures of alkalization: (O) high liquid ratio, acetone as diluent, two liquid phases; (·) high liquid ratio, aqueous NaOH (25 % w/w), no diluent; (·) low liquid ratio, aqueous NaOH (25 % w/w), spray procedure (Dautzenberg et al., 198Od).

Alkali solubility and WRV, on the other hand, did not decrease uniformly with progressive crosslinking, but passed a maximum with all the procedures employed due to the counteracting effects of 'structure widening' by introduction of covalent spacer at a high state of swelling and 'structure tightening' by formation of covalent crosslinks (see Fig. 4.5.19 and 4.5.20). 300

200

100 0.2

0.6

1.0

Degree of crosslinking

Figure 4.5.19. WRV of a cellulose powder versus degree of crosslinking with epichlorohydrin (Dautzenberg et al., 198Od).

On the supramolecular level the reaction with epichlorohydrin resulted in an increase in the 1-0-1 lattice distance after neutralization and in a fairly severe destruction of the fibrillar architecture of the particles. The interplay between the hydrophilic spacing and the structure tightening crosslinking by etherification

246


with epichlorohydrin has also been emphasized in a study on modification of bead cellulose (Loth and Philipp, 1989). 60

O CO

20

0.1

0.3

0.5

0.7

Degree of crosslinking

Figure 4.5.20. Solubility of cellulose powder in 5 % NaOH versus degree of crosslinking with epichlorohydrin (Dautzenberg et al., 198Od).

Etherification by epoxidation has also been performed with cellulose dissolved in DMA/LiCl. Diamantoglou reported DS values of about 0.5 with powdered NaOH or with LiOH as the base and propylene oxide or epichlorohydrin as the reagent, whereas carboxymethylation with monochloroacetic acid arrived at a DS < 0.1 only, in the same system (Diamatoglou and Kühne, 1988). These findings demonstrate again the difference in reaction mechanism between a carboxymethylation requiring a stoichiometric amount, and an epoxidation needing only a catalytic amount. On the other hand, using an excess of NaOH and ClCH2COONa, CMC of high DS of up to 2.3 can be synthesized (Heinze et al., 1994b). Etherification of cellulose by epoxidation has also been employed to introduce functionalized side chains via ether linkages into the macromolecule. This was studied especially as a route to synthesize cellulose derivatives with cationic nitrogen functions attached (see section 4.5.3.2).

Formation and reactions of hydroxymethyl(4methyloP)cellulose and related derivatives Formally, hydroxymethylcellulose can be considered as the first member in a series of hydroxyalkyl ethers of cellulose, but regarding the mode of formation and the instability of methylolcellulose with the simplified formula CeIl-OCH2OH, it represents a half-acetal of the polymer. Methylolcellulose was identified and characterized in connection with the discovery of the solvent system DMSO/paraformaldehyde or DMSO/formaldehyde (Johnson et al., 1976; Baker et al., 1981) about 20 years ago. At elevated temperature usually of about


247

140 0C, the DMSO/paraformaldehyde system dissolves even high molecular cellulose quickly and completely without significant chain degradation. It has therefore been studied as a possible route to artificial fiber spinning, as a solvent for cellulose characterization in solution (Gruber and Gruber, 1978), and as a system for subsequent cellulose derivatization in solution. The present state of knowledge of cellulose methylolation can be summarized as follows: due to several chemical equilibria existing in systems of cellulose/formaldehyde/polar aprotic liquid, and interacting with each other (see Fig. 4.5.21), methylolcellulose is not a well-defined cellulose derivative. Its composition depends largely on parameters like component ratio, mode of component addition, rate of heating, and final reaction temperature and reaction time. (CH20)n^^ n CH2O CeII-OH + CH2O =^ Cell-O-CH2OH CeII-OH +(CH2O)x =^ Cell-O-(CH2O)x-H 2 CeII-OH + CH2O ^=* Cell-O-CH2-O-CeII + H2O Figure 4.5.21. Scheme of reaction involved in the methylolation of cellulose.

Methylolation can take place with a large excess of formaldehyde above 80 0C, or, more comfortably, with paraformaldehyde in DMSO at 135-140 0C. A methylolated cellulose obtained in various polar liquids at elevated temperature with gaseous CH2O, as well as with paraformaldehyde, was reported (Baker et al., 1981). The authors characterized the product by 1 HNMR spectroscopy after complete acetylation and isolation of the stable acetates. A high MS of between 15 and 25 was required for cellulose dissolution in the various solvents employed, which, however, could be subsequently lowered to a MS of between 0.5 and 3.0 without precipitation of the polymer (see Table 4.5.13). Table 4.5.13. Initial and final MS of methylolcellulose prepared in different solvents (Baker et al., 1981).

Solvent

Initial MS

Final MS

DMSO DMF NMP DMA Pyridine

18.8 23.6 21.9 20.9 15.1

0.5 2.0 1.5 1.5 3.0

NMP, W-methylpyrrolidone.

248


Obviously, an initially nonuniform distribution of long methyl side chains changes gradually to a more uniform one of shorter side chains by cleavage of CH2O entities. The C-6 position was shown to be the preferred site of reaction, followed by the C-2 position (Nehls et al., 1994). Even a DS of 3 can be realized according to Kinstle and Irving (1983). A molecular substitution of 1.5-2.4 was found to be necessary for dissolving cellulose at 85 0C in DMSO with an excess of formaldehyde, but the level of formaldehyde concentration subsequently could be lowered considerably before precipitation occurred (Baker et al., 1981). The MS level required obviously depends also on the polar liquid employed. Besides DMSO, DMA/LiCl and DMF/LiCl represent good solvents for methylolcellulose. The methylol groups are easily split-off by water or methanol. Already a small amount of water is reported to increase significantly the gel content of a methylolcellulose solution in DMSO/paraformaldehyde (Gruber and Gruber, 1978). Methylolcellulose in the dissolved state with its strongly solvated but chemically unstable hydroxymethyl groups, has been employed in several studies for subsequent steps of derivatization. By Kinstle and Irving (1983) acetylation with acetyl chloride, ionic grafting of acrylonitrile with sodium hydride, and the synthesis of a methylolcellulose octadecylcarbamate by reaction with octadecyl isocyanate in the presence of stannic octoate at 50 0C in DMF/LiCl as the solvent are reported. Sulfuric acid half-ester formation can be accomplished with the SO3/DMF complex, but predominantly takes place at the methylol hydroxy end groups, and most of the sulfur introduced is split-off with the side groups in an aqueous medium. Periodate oxidation of methylolcellulose in an aqueous medium was reported to proceed to a high degree of 2,3-dialdehyde cellulose formation with gradual decomposition of the methylol groups (Morooka et al., 1989). Methylolcellulose in DMSO/paraformaldehyde exhibits a very high intrinsic viscosity, exceeding that of cellulose in FeTNa (Gruber and Gruber, 1978). The relation [η] (cm3/g) = 3.38 χ K)-2 DPW

°·84 has been reported (Baker et al., 1981). The existence of chain aggregates, even in very dilute solutions, cannot be excluded (Gruber and Gruber, 1978). At a polymer concentration above 18 %, solutions of methylolcellulose in DMSO exhibit interesting liquid crystalline properties (Gilbert and Fornes, 1989), with the optical data confirming the heterogeneity of the chemical structure along the polymer chains. A methylolcellulose-based route to artificial fibers has been developed with the cellulose/DMSO/paraformaldehyde system on a semitechnical scale, but did not reach the level of industrial production, and obviously cannot compete today with the development of the amine oxide spun fibers.

4.5.3 Various functionalized alkyl ethers of cellulose

249

Methylolcelluloses functionalized at the acetal group have been synthesized with various aldehydes or aldehyde derivatives: trichloroacetaldehyde (chloral) is known to dissolve cellulose in the presence of a dipolar aprotic liquid with formation of a substituted methylol derivative.

Q CeII-OH + CCI3-C*

π

CeIhO-CH-OH ι CCI3

The formation of various cellulose hemiacetals by reaction of the polymer dissolved in DMA/LiCl with the dimethylacetals of benzaldehyde, cyclohexanone and acetaldehyde has been studied (Ikeda et al., 1990), e.g. CeII-OH + CH

CeII-O-CH-OMe +MeOH

) On increasing the temperature to 70 0C and removal of the methanol formed in vacuo, a DS of acetal groups of up to 2 could be obtained. A reversible bimolecular reaction between cellulosic hydroxy groups and the dimethylacetal is assumed. Finally, the formaldehyde viscose spinning process shall be mentioned briefly with the 5-methylol derivative of cellulose xanthogenate: CeII-O-C-S-CH2OH S affecting filament formation and filament structure. This compound, as well as some transient -Q-CH2-O- ether crosslinks are formed on adding formaldehyde at the 0.1-1 % level to the viscose spinning dope or to the spin bath. In this way, cellulose xanthogenate decomposition is retarded, and the stretchability of the nascent filament is enhanced, resulting finally in a high tear strength and a changed morphology of the threads (Bartsch et al., 1974).

4.5.3 Various functionalized alkyl ethers of cellulose Besides the hydroxyalkylcelluloses and CMC, several other functionalized alkyl ethers of cellulose, e.g. cyanoethylcellulose: aminoethylcellulose: sulfoethylcellulose: phosphoromethylcellulose:

CeIl-O-CH2-CH2-C=N CeIl-O-CH2-CH2-NH2 CeIl-O-CH2-CH2-SO3H CeIl-O-CH2-PO3H2


249

Methylolcelluloses functionalized at the acetal group have been synthesized with various aldehydes or aldehyde derivatives: trichloroacetaldehyde (chloral) is known to dissolve cellulose in the presence of a dipolar aprotic liquid with formation of a substituted methylol derivative.

Q CeII-OH + CCI3-C*

π

CeIhO-CH-OH ι CCI3

The formation of various cellulose hemiacetals by reaction of the polymer dissolved in DMA/LiCl with the dimethylacetals of benzaldehyde, cyclohexanone and acetaldehyde has been studied (Ikeda et al., 1990), e.g. CeII-OH + CH

CeII-O-CH-OMe +MeOH

) On increasing the temperature to 70 0C and removal of the methanol formed in vacuo, a DS of acetal groups of up to 2 could be obtained. A reversible bimolecular reaction between cellulosic hydroxy groups and the dimethylacetal is assumed. Finally, the formaldehyde viscose spinning process shall be mentioned briefly with the 5-methylol derivative of cellulose xanthogenate: CeN-O-C-S-CH2OH S affecting filament formation and filament structure. This compound, as well as some transient -Q-CH2-O- ether crosslinks are formed on adding formaldehyde at the 0.1-1 % level to the viscose spinning dope or to the spin bath. In this way, cellulose xanthogenate decomposition is retarded, and the stretchability of the nascent filament is enhanced, resulting finally in a high tear strength and a changed morphology of the threads (Bartsch et al., 1974).

4.5.3 Various functionalized alkyl ethers of cellulose Besides the hydroxyalkylcelluloses and CMC, several other functionalized alkyl ethers of cellulose, e.g. cyanoethylcellulose: aminoethylcellulose: sulfoethylcellulose: phosphoromethylcellulose:

CeIl-O-CH2-CH2-C=N CeIl-O-CH2-CH2-NH2 CeIl-O-CH2-CH2-SO3H CeIl-O-CH2-PO3H2

Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

250


and related compounds have been studied. Some of them have found a limited practical application. The synthesis of these functionalized ethers takes place along the routes already discussed, i.e. by reaction with an alkyl halide, or by a Michael addition of a compound with an activated C=C bond onto a cellulosic hydroxy group. Ordered according to the functional group in the end-product, this section presents an overview on the mode of preparation, the consecutive reactions, the properties, and the potential areas of application of these cellulose ethers.

4.5.3.1

Cyanoethylcellulose and related cellulose ethers

Among the functionalized alkyl ethers of cellulose to be considered in this section, cyanoethylcellulose and some related derivatives have been most widely studied due to their scientific relevance for cellulose ether formation and cellulose ether consecutive reactions and due to their practical importance in the furnishing of cellulosic textiles. Cyanoethylation is the classical example of cellulose etherification by Michael addition of an activated C=C bond to a partially anionized cellulosic hydroxy group in an aqueous alkaline medium. A simple scheme of this reaction is presented in Fig. 4.5.22. The mesomeric structure of the anionic ether primarily formed should in principle permit a subsequent anionic grafting of acrylonitrile onto the polymer backbone, which, however, is inhibited by the very fast addition of a proton to form the neutral cyanoethyl ether. —

OH®

^

H®

~

CeII-OH ^=^ CeII-OI +H 2 O CeII-OI + CH 2 -CH-C = N ^^ CeII-O-CH 2 -CH-C = N + H®

CeII-O-CH 2 -CH 2 -C = N Figure 4.5.22. Scheme of cyanoethylcellulose formation.

Besides the C=N group, several other substituents are able to activate the C=C bond to an extent sufficient for the addition reaction. Typical examples of reagents are: • • • •

Acrylonitrile Methacrylonitrile oc-Chloroacrylonitrile Allyl cyanide

· · · ·

Acrylamide oc-Methyleneglutaronitrile irarcs-Crotonitrile Vinyl sulfonate, acrylic acid esters

4.5.3

Various functionalized alky I ethers of cellulose

251

A decreasing order of reactivity has been reported according to acrylonitrile > amethylene glutaronitrile = croton nitrile = allyl cyanide > methacrylonitrile, corresponding to a decreasing polarizability of the C=C bond (Lukanoff et al., 1967). α-Chloroacrylonitrile was also found to be less reactive than acrylonitrile itself (Lukanoff et al., 1969). Numerous systematic investigations have been performed on laboratory-scale cyanoethylation of cellulose in order to assess the relevance of the various reaction parameters and to elucidate the interaction between cyanoethylcellulose formation and its various routes of decomposition. The pioneering work of Bikales shall be mentioned explicitly. He reported (Bikales, 1974) the preparation of a fibrous cyanoethylcellulose of DS = 2.75 (12.6 % N) from regenerated cellulose with acrylonitrile and aqueous NaOH at 50 0C. The results of numerous subsequent studies can be summarized as follows: cyanoethylation of cellulose proceeds as a equilibrium reaction usually with the polymer remaining in the solid state and can be performed either by simultaneous (one-step process) or by subsequent (two-step process) addition of the components aqueous NaOH and acrylonitrile, usually at a temperature between 30 and 50 0C, within some hours. At low alkali concentration and a temperature not higher than 30 0C a hydrolytic cleavage of the C=N bond can be widely avoided, which is favored at higher OH~ concentration and higher temperature. As a low molecular by-product, 2,2 'dicyanodiethylether is formed by cyanoethylation of water to an extent largely depending on the reaction conditions. In the low DS range, a preferential substitution at the C-6 position has been reported (Nehls et al., 1994). Cyanoethylation of cellulose in the fibrous state at a low alkali concentration of e.g. 2-4 % was found to depend considerably on cellulose physical structure with regard to rate and final DS of etherification (Lukanoff et al., 1979). The reaction rate increased significantly after a preactivation of the cellulose sample by mercerization with 18 % NaOH or by pretreatment with liquid NH3 (see Fig. 4.5.23), and could also be enhanced by addition of DMSO to the system for increasing the solubility of acrylonitrile (Schleicher et al., 1974). The efficiency of the activating pretreatment increased with its effect on supramolecular order and with decreasing NaOH concentration in the system. A rate difference of about one order of magnitude has been observed between a strictly homogeneous and a strictly heterogeneous course of reaction, while the order of reactivity of various vinylic compounds remained the same in both cases. An energy of activation of 15.5 kcal/mol for the homogeneous and of 11.7 kcal/mol for the heterogeneous course of reaction was reported (Lukanoff et al., 1979).

252


3,0 2,0 CO Q

1,0

10

30

50

70 90 Time [min]

110

Figure 4.5.23. Course of cyanoethylation of beech sulfite pulp in 2 % aqueous NaOH: (·) pulp without preactivation, (O) pulp activated with liquid NH3 (Lukanoff et al, 1979). The heterogeneous course of cyanoethylation of a preactivated cellulose sample can turn to a homogeneous one after a brief initial reaction phase due to formation of an alkali-soluble cyanoethyl ether above a DSN of 0.3-0.4, which can then be further etherified under homogeneous conditions, until above a DS of about 1.2 precipitation occurs in the aqueous system and further cyanoethylation takes place at the solid polymer (Koura et al., 1977). A completely homogeneous course of reaction up to a DS^ of 0.8 could be realized after dissolving the cellulose in 7V-methylmorpholine TV-oxide with the small amount of Nmethylmorpholine present in the system already sufficing for the basic catalysis of cyanoethylation (Philipp et al., 1986). It is worth mentioning that alkalisoluble products in the DS region of about 0.4 and water-soluble cyanoethylcellulose in the DS range between 0.7 and 1.0 required for their preparation a suitable preactivation of the starting material for securing a sufficiently uniform substituent distribution along the polymer chains. Cyanoethylcellulose obtained in a thoroughly homogeneous procedure exhibited organosolubility already at a DS above 0.8, while a DS above 2.0 was necessary along a heterogeneous route of synthesis. It can be concluded from these results that the solubility of cyanoethylcellulose in various liquids is primarily governed by the DS, as to be expected, but also that the substituent distribution plays an important role. A combined cyanoethylation and xanthation by simultaneous action of acrylonitrile and CS2 onto a conventional alkali cellulose resulted in an enhanced xanthation rate and a higher final xanthogenate DS, possibly due to a combined spacing and CS2 solubilizing action of cyanoethyl side groups, as demonstrated by a DSx of > 0.9 and a D5N of 0.3, with a reagent input of 4 mol of CS2 and 1 mol of acrylonitrile/mol of AGU (Lukanoff et al., 1969). Acrylamide can be added to cellulose in a similar manner as acrylonitrile with formation of the carbamoylethyl ether of cellulose:


ο

253

ο

'' CeII-OH + CH2 = CH-C^

NH2

OH" " -^- CeII-O-CH2-CH2-C^

NH2

The reactivity, however, of acrylamide in a Michael addition is significantly lower than that of acrylonitrile, and the amide group is more easily saponified to a carboxyl group than the nitrile group. The functional groups of cyanoethyl and carbamoylethylcellulose are susceptible to various consecutive reactions. Most important is the decomposition in an aqueous medium of higher alkalinity at elevated temperature to give carboxyethylcellulose as the stable end-product, with free acrylonitrile and carbamoylethylcellulose acting as intermediates (see scheme in Fig. 4.5.24). Main reaction (NaOH) CeII-OH + CH2 = CH-C=N ·

CeII-O-CH2-CH2-C=N

Side reaction (NaOH)

/Û -OU-^ = M L/n 2 -On W — IN

-^MU

u ^

*°

»- OU -/"Û- Γ* ΟΠ2-ΌΠ O^

+ NaOH

INlM2

'

CH2=CH-COONa + NH3

xx° (NaOH) *° CeII-OH + CH2 = CH-C. — CeII-O-CH2-CH2-C. NH2 NH2 CeII-O-CH2-CH2-Ct. CH2 = CH-C = N +H 2 O

+ NaOH

NH2 (NaQH)

CeII-O-CH2-CH2-COONa + NH3

HO-CH2-CH2-C = N

(NaOH)} 2 CH2 = CH-C = N +H 2 O ^ — (NCCH2CH2J2O Figure 4.5.24. Main and side reactions in the formation of cyanoethylcellulose. The rate constant of cyanoethylcellulose cleavage was observed to increase linearly with the sodium hydroxide concentration (see Fig. 4.5.25). As another route from cyanoethylcellulose to carbamoylethylcellulose, oxidation with H2O2 in an aqueous alkaline system has been reported.

254


E x103 C JO

ω §

Ä oz

0,5 1,0 1,875 2,5 NaOH concentration [mol/l]

Figure 4.5.25. Rate constant of cyanoethylcellulose cleavage versus NaOH concentration (Lukanoff et al., 1977). According to Englebretsen and Harding (1992) the nitrile group of cyanoethylcellulose can be reduced to an aminopropyl substituent with diborane. An amidoxime has been prepared from carbamoylethylcellulose with hydroxylamine in a neutral aqueous system at 70 0C (Kubota and Shigehisa, 1995). The technical process of cyanoethylation usually aims to produce not a commercial product 'cyanoethylcellulose', but a furnishing of cellulosic textile goods or of paper. In the so-called 'two-step process' the cellulose is firstly impregnated with aqueous NaOH of e.g. 2 % concentration for l h at 50 0C, and then reacted with an excess of acrylonitrile at the same temperature. The reaction is stopped by addition of acid and the fibrillar material, usually containing 3-4 % nitrogen corresponding to a DS of about 0.5, is washed free of byproducts and dried. In the One-step process' often employed for obtaining higher degrees of cyanoethylation, the cellulose is soaked with NaOH and acrylonitrile at low temperature, which is then raised to a level of about 50 0C for the etherification. The One-step process' was found suitable to obtain DS values above 2 resulting in organosoluble products. Modifications of these two principle process routes are a continuous procedure for cotton cloth with an impregnation with aqueous NaOH as the first step and an etherification with acrylonitrile in the gaseous state in a reaction chamber as the second one, and also a so-called high solids process with small amounts of acrylonitrile and aqueous NaOH being reacted with the cellulose. Regarding material properties, cyanoethylation results in an improved rot resistance already at low DS, in an enhanced thermoresistance, and more favorable dielectric properties compared with cellulose. Cyanoethyl ethers of cellulose exhibit a graded solubility in aqueous NaOH, water and polar organic liquids, including acrylonitrile, depending on the DS and on the procedure of etherification. Cyanoethylcellulose of high DS becomes thermoplastic at a temperature of about 160 0C and is very hydrophobic. The latter property can still be enhanced by introduction of additional fluorinated substituents. The main area of application of cyanoethylation is the rot proof furnishing of cellulosic textiles, especially of cotton cloth. Besides this, cyanoethylated prod-


255

ucts are used for speciality papers. Films cast from cyanoethy!cellulose solutions have been recommended as separation membranes due to their enhanced rotting and chemical stability compared with e.g. cellulose acetate (Chen et al, 1991). The blood clotting efficiency of water-soluble carboxyethylcarbamoylethylcellulose has been studied (Kamide et al., 1987).

4.5.3.2 Functionalized cellulose ethers with basic N-functions The chemically most simple route to cellulose ethers with an amino group is the reaction with ethylene imine in an aqueous alkaline medium arriving at aminoethylcellulose according to CeII-OH + CH2-CH2 ^^ CeII-O-CH2-CH2-NH2 \ / NH quite by analogy to hydroxyethylation with ethylene oxide. This route was practised in the first half of this century for an amination of viscose rayon to low DS in order to improve dyeability, but is now abundant due to the toxicological hazards involved. Subsequently, the aminoethylation of cellulose found limited attention in the preparation of weak anion exchangers to be used in various Chromatographie techniques, e.g. affinity chromatography, or in connection with enzyme immobilization. The aminoethyl ether group was introduced by reaction with either ethylene imine or 2-aminoethyl sulfate. A procedure for the first route is described by Podgornyi and Gur'ev (1981) employing cellulose suspension in toluene and reacting it with ethylene imine in the presence of benzyl chloride in an autoclave at 70 0C for 1Oh. Aminoethylation with 2-aminoethyl sulfate in the presence of aqueous NaOH at a temperature between 70 and 120 0C was claimed for obtaining ion-exchange materials from cellulose powders (Bischoff and Dautzenberg, 1977). The reaction proceeds according to CeIl-OH + NaO3SO-CH2-CH2-NH2 + NaOH -» CeIl-O-CH2-CH2-NH2 + Na2SO4 The primary amino group of aminoethy!cellulose can serve as a reactive site in subsequent transformations for the reaction with N-acetylhomocysteinethiolactone in order to obtain tailored Chromatographie sorbents (Podgornyi and Gur'ev, 1981). More recent developments were centered not so much on compounds with primary amino groups, but predominantly on cationic alkyl ethers with tertiary amino functions or quaternary ammonium groups. A large number of synthesis routes and a broad variety of products has been described. Amino functionalization of cellulose became a very attractive area of organic cellulose chemistry,

256


although only a very limited number of products is produced commercially as a speciality in rather small amounts. The most important route to cationic cellulose ethers is still the coupling of an N-functionalized compound onto the polymer via displacement of a labile halogen atom (Fig. 4.5.26). But also a coupling of cationic groups onto the polymer via an epoxidation is widely used (see Fig. 4.5.27). Representatives of the first mentioned route are diethylamino-ß-chloroethane employed in the preparation of diethylaminoethylcellulose and 3-chloro-2hydroxypropyltrimethylammonium chloride for introducing propyltrimethylammonium chloride side chains into the polymer. C2H5 CeII-OH + NaOH + CI-CH2-CH2-Nx r μ C2H5 CeII-O-CH2-CH2-Nx

^2M5

+ NaCI + H2O C2H5

CeII-OH + CI-CH2-CH-CH2-N(CHg)3 Cl'

Na

°H

OH CeII-O-CH2-CH-CH2-N(CHg)3+ Cl' + NaCI + H2O OH

CeII-O-CH2-C^1+ H2N-(CH2Jn-NH2 L/l

o

n>2

Cell-O-CH2-C-NH-(CH2)n- NH2+ HCI CeII-CI + H2N-CH2-CH2-NH2 NaOH CeII-NH-CH2-CH2-NH2 + NaCI + H2O CeII-OH + Br-(CH2Jn-Br + NaOH Cell-O-(CH2)n-Br + NaBr + H2O CeII-O-(CH2X1-Br + H2N-(CH2Xn-NH2 Cell-O-(CH2)n-NH-(CH2)m-NH2 + HBr Figure 4.5.26. Introduction of amino groups into cellulose via halogen functions (simplified scheme).


257

The other routes shown above are primarily of scientific interest. They permit however, the introduction of N-functionalized side chains with one or two amino functions and a controlled spacer length for potential application in Chromatographie techniques. CeII-OH + CH 2 -CH-CH 2 + NR3 — CeII-O-CH 2 -CH-CH 2 -NR 3 + Cl' OH

Cl

CeII-OH + CH 2 -CH

cat.+ oligomer —- CeII-O-CH2-CH

\/

cat.+ oligomer

OH

CeII-O-CH 2 -CH-CH 2 + H2N-(CH2Jn-NH2 Cell-O-CH2-CH-CH2-NH-(CH2)n-NH2 OH

Figure 4.5.27. Routes of formation of cationic cellulose ether by linkage via epoxy groups (Gruber et al., 1996). N-functionalization via an 4epoxy coupling' is often performed with glycidyltrimethylammonium chloride employing NaOH as a catalyst. A one-step procedure for modifying cellulose by substitution with quaternary ammonium functions was recently published by Gruber et al. (1996), who reacted the polymer with epichlorohydrin and a tertiary amine in the presence of a sterically hindered amine as the catalyst (Fig. 4.5.2.7). Also, cationic oligomers with epoxy coupling groups were employed by this author for pulp cationization (Fig. 4.5.28).

Figure 4.5.28. Cationic epoxide oligomer for cationic cellulose ether formation (Gruber et al., 1996). The introduction of tertiary and quaternary N-functions can also be realized by the Michael addition of cationic acryl and methacryl esters or the corresponding substituted amides:

258


CH2=CH

/Me

O=C-O-CH 2 -CH 2 -N' x

CH2=C-CH3

Me

O=C-NH-CH JH22-CH 22-CH 2 -N x Me

CH 2 —C

CHg

O=C-NH-CH 2 -CH 2 -CH 2 -NMe 3 + Cl" Employing a low-DS Na-cellulose sulfate in aqueous alkaline solution (0.02 M NaOH) after a reaction time of 3 days at 35-60 0C, maximal D5N values of 0.36 for the tertiary N-function and 0.27 for the quaternary ones were obtained. Most probably substitution occurred preferentially at the C-2/C-3 position. It must be emphasized that the reaction proceeded smoothly in an aqueous system only, while in aprotic liquids like DMSO or DMF the D5N remained below 0.1 (Wagenknecht, 1996). Finally, two possible routes to aminoalkylation starting from cyanoethylcellulose shall be mentioned (see Fig. 4.5.29), which are of scientific interest but demonstrate the feasibility of two well-known pathways of low molecular organic chemistry at the cellulose macromolecule.

Cell-O-CH 2 -CH 2 -ct MU

Nn2

H2O

CeII-O-CH 2 -CH 2 -C^N v THF

" ^ NaOB^

CeII-O-CH2-CH2-NH2

BH3-SMeX

CeII-O-CH 2 -CH 2 -CH 2 -NH 2 Figure 4.5.29. Reaction routes from cyanoethylcellulose to aminoalky!cellulose.

The products synthesized by the various routes of N-functionalization have also been subjected to subsequent reaction steps, e.g. a quaternization of tertiary amino groups by alkyl halide or dimethyl sulfate, a crosslinking by epichlorohydrin or an additional substitution with anionic groups by carboxymethylation. Besides cellulose itself, partially substituted cellulose esters and ethers, especially HEC, hemicelluloses (predominantly xylans), and other polysaccharides like amylose, have been N-functionalized, preferentially with diethylamino-ß-


259

chloroethane or 3-chloro-2-hydroxypropyltrimethylammonium chloride. According to Ebringerovä and Hromädkovä (1996), 2-hydroxypropyltrimethylammonium groups were introduced into a beech wood hemicellulose dissolved in NaOH and a DS of up to 1.0 has been arrived with the 2 position being the preferred site at low DS. Katsura et al. (1992) prepared cationic ethers with tertiary and quaternary N-functions from HEC, amylose and amylopectin dissolved in aqueous NaOH, arriving at DS^ values up to 0.5 and concluding a preferential substitution at C-2 from NMR results. A fairly regioselective cationization was performed with regioselectively substituted Na-cellulose sulfates dissolved in aqueous NaOH by reaction with 3-chloro-2-hydroxypropyltrimethy!ammonium chloride (Wagenknecht, 1996). The results obtained with a preferentially C-2/C3 substituted and a preferentially C-6-substituted cellulose sulfate in comparison with samples with a rather statistical distribution of the ester groups within the AGU are summarized in Table 4.5.14. Table 4.5.14. Etherification of cellulose sulfuric acid half-ester with Cl-CH2-CHOH-CH2-NMe3Cl in excess.

Cellulose DS$ 0.25 0.72 0.22 0.25 0.45 0.70

sulfate Preferential site of reaction C-6 C-6 C-2/C-3 C-2/C-3/C-6 C-2/C-3/C-6 C-2/C-3/C-6

DSN 0.76 0.31 0.39 0.80 0.40 0.26

For preparing these cellulosic ampholytes, 4-8 mol of NaOH and 3-6 mol of etherifying agent were employed in a reaction time of 4 h at 60 0C. Thus the reaction proceeds much faster than the Michael addition of cationic acryl esters onto the same cellulose sulfates. Also, from this study a preferential etherification at the C-2/C-3 position can be assumed. Application-oriented research and development in the N-functionalization of cellulose has so far been pursued by three routes: (i) preparation of anion-exchanging sorbents for Chromatographie purposes of low DS (usually 0.1-0.2), starting from alkali cellulose or a slurry with e.g. isopropanol added, retaining the solid structure of the polymer throughout the process and arriving at a water-insoluble product which eventually is additionally crosslinked for reduced swelling; (ii) cationization of cotton or wood pulp for changing the surface properties in e.g. sorption processes, and maintaining, of course, the solid state of the polymer;

260


(iii) synthesis of water-soluble cationic cellulose ethers as process auxiliaries in e.g. the paper industry or in water processing, usually proceeding in a homogeneous system for securing a uniform substituent distribution along the chains, and either arriving at a rather high DS or employing already a water-soluble starting material like HEC. A comprehensive contribution to (ii) was recently published by Gruber et al. (1996) who compared three routes of pulp cationization, i.e. the abovementioned one-step quaternization with epichlorohydrin and a tertiary amine, the attachment of cationic oligomers with epoxy end-groups via ether linkages, and the competing route of cellulose radical grafting with e.g. a combination of acrylamide and diallyldimethylammonium chloride, initiated with Ce4+. They compared these routes with regard to filler retention effect, beatability and sheet strength in paper making. Today's commercial manufacture of water-soluble cationized cellulose ethers as speciality products usually starts from a water-soluble HEC, which is chemically modified either with glycidyltrimethylammonium chloride in an aqueous alkaline medium, or by radical grafting, employing preferentially diallyldimethylammonium chloride as a cationic monomer. These products can be considered as special types of hydroxyalkylcelluloses, which due to their cationic charges can form polyelectrolyte complexes with anionic polymers or surfactants. So for example a 200-fold increase in solution viscosity of an aqueous solution of sodium dodecyl sulfate at the critical micelle concentration, by addition of a 1 % aqueous solution of a cationic cellulose ether, was reported (Gruber and Kreeger, 1996). Hair cosmetics is considered today as a particular field of application of these cationic cellulose products.

4.5.3.3 Sulfoalkyl ethers of cellulose Sulfoalkyl ethers of cellulose are prepared with cellulose in the presence of alkali or with alkali cellulose at elevated temperature with (i) chloroalkane sulfonate according to CeII-OH + CI-CH2-CH2-SO3 Na

NaOH

CeII-O-CH 2 -CH 2 -SO 3 Na + H2O + NaCI (ii) alkylation with, e.g. propane sultone, according to CeII-OH + CH2-CH2^CH2 X

o-so2

NaOH

CeII-O-CH 2 -CH 2 -CH 2 -SO 3 Na +H 2 O

4.5.3 Various functionalized alky I ethers of cellulose

261

(iii) ethylene sulfonate (vinyl sulfonate) by Michael addition according to

CeIhOH + CH2 = CH-SO3 H

NaOH

CeIhO-CH 2 -CH 2 -SO 3 Na +H 2 O Ebringerovä and Pastyr (1980) compared these three routes with delignified wood as the starting material and arrived at sulfur contents of 3.3-5.3 %, corresponding to a range of DS§ from 0.1 to 0.3 and an order of reactivity of the agents of propane sulfone < chloroalkyl sulfonate < vinyl sulfonate. An increase in the NaOH concentration and/or the temperature of reaction (> 65 0C) resulted in a higher DSS. The procedure of alkalization was found to be essential for the course of sulfoalkylation. Sulfomethylcellulose has been prepared with Cl-CH2SO3Na in the presence of aqueous NaOH of higher concentration at 60-90 0C, and was proposed as a cation-exchanger. Sulfoethylcellulose is synthesized by reaction of cellulose with Cl-CH2-CH2SO3Na in the presence of strong alkali or by Michael addition of CH2=CHSO3Na in an aqueous alkali medium. Also, a thionic acid HSO3-O-CH2CH2SO3H has been proposed as an etherifying agent. The Na salt of Sulfoethylcellulose is soluble in water above a DS of 0.3, and less sensitive to precipitation by low molecular electrolytes than carboxymethylcellulose. Thermal degradation of Sulfoethylcellulose and some related compounds has been studied up to 60O 0 C (Oppermann, 1995; Sazanov et al., 1981). Etherification of cellulose with divinyl sulfone CH2=CH-SO2-CH=CH2 in an aqueous alkali medium results in an efficient crosslinking via CH2-CH2-SO2-CH2-CH2- ether linkages (Anbergen and Oppermann, 1990). For the preparation of sulfopropylcellulose, usually the route of propane sultone ring cleavage and addition in an aqueous alkaline medium is employed. Sulfoalkylcelluloses find limited application as cation-exchanger materials in Chromatographie techniques.

4.5.3.4 Miscellaneous functionalized alkyl ethers of cellulose Phosphonomethylcellulose CeIl-O-CH2PO3H2 can be prepared by reaction of alkali cellulose with Cl-CH2PO3H2 at 100-120 0C with a DS of up to 0.5. The anionic ether becomes water-soluble above a DS of 0.15, but the solution is reported to be sensitive to low molecular electrolytes, e.g. H+ and OH" ions of higher concentration. Above pH = 10 the disodium salt is formed (Brandt, 1986). Also, HEC has been transformed to a mixed ether containing phosphoromethyl groups. The formation of halogen-containing cellulose ethers of the structure CeIl-OH-(CH2)^-X, where n = 3, 4; X = Cl, Br, was reported by Klavins and

262


Prikulis (1982) by using the dihalogenoalkane in dioxane or DMF at 80-100 0C. In a subsequent step, the ether was allowed to react with 1,6-diaminohexane in order to obtain Chromatographie materials for affinity chromatography.

4.5.4

Aralkyl ethers and aryl ethers of cellulose

4.5.4.1

Arylmethyl ethers

As the most important ether of this type benzylcellulose was first reported by Leuchs in 1917. Further ary!methyl ethers contain different types of alkyl residues (e.g. Koenig and Roberts, 1974), halogen substituents (e.g. Frazier and Glasser, 1995), and functional groups like methoxy, nitro, and amino groups mainly in para-position of the benzyl units or in some cases aryl groups other than phenyl (Harkness and Gray, 1991; Isogai et al., 1985). The typical synthesis pathway consists in the reaction of cellulose with the corresponding arylmethyl halogenides in presence of a base (e.g. Isogai et al., 1984, 1985): CeII-OH+ HaI-CH 2 -Ar

base

CeII-O-CHp-Ar

Ar =

CH(CH3)2

OCHo

CH = CHn

NH,

The reaction proceeds under heterogeneous conditions (Braun and Meuret, 1989; Obayashi et al., 1992; Zhdanov et al., 1993) using sodium hydroxide in water or, on the other hand, in an homogeneous medium, e.g. with sodium hydroxide in DMA/LiCl (McCormick and Shen, 1981; McCormick, 1978; Isogai et al., 1984; Kim, 1987). In some cases phase transfer catalyse techniques have been used (Daly et al., 1979; 1982; 1984).

262


Prikulis (1982) by using the dihalogenoalkane in dioxane or DMF at 80-100 0C. In a subsequent step, the ether was allowed to react with 1,6-diaminohexane in order to obtain Chromatographie materials for affinity chromatography.

4.5.4

Aralkyl ethers and aryl ethers of cellulose

4.5.4.1

Arylmethyl ethers

As the most important ether of this type benzylcellulose was first reported by Leuchs in 1917. Further ary!methyl ethers contain different types of alkyl residues (e.g. Koenig and Roberts, 1974), halogen substituents (e.g. Frazier and Glasser, 1995), and functional groups like methoxy, nitro, and amino groups mainly in para-position of the benzyl units or in some cases aryl groups other than phenyl (Harkness and Gray, 1991; Isogai et al., 1985). The typical synthesis pathway consists in the reaction of cellulose with the corresponding arylmethyl halogenides in presence of a base (e.g. Isogai et al., 1984, 1985): CeII-OH+ HaI-CH 2 -Ar

base

CeII-O-CHp-Ar

Ar =

CH(CH3)2

OCHo

CH = CHn

NH,

The reaction proceeds under heterogeneous conditions (Braun and Meuret, 1989; Obayashi et al., 1992; Zhdanov et al., 1993) using sodium hydroxide in water or, on the other hand, in an homogeneous medium, e.g. with sodium hydroxide in DMA/LiCl (McCormick and Shen, 1981; McCormick, 1978; Isogai et al., 1984; Kim, 1987). In some cases phase transfer catalyse techniques have been used (Daly et al., 1979; 1982; 1984). Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

4.5.4 Aralkyl ethers and aryl ethers of cellulose

263

Moreover, benzylcellulose has been prepared by etherification of cellulose acetate (DS > 2) under simultaneous deacetylation (Shibata et al., 1983; Nakamura, 1984) and primarily introduced substituents at the aryl units were subsequently modified, e.g. nitro groups reduced to amino groups. In the field of arylmethyl ethers benzylcellulose was of commercial importance in the 1940th, particularly in Europe. It is a thermoplastic cellulose derivative with a melting range of 90 - 155 0C, insoluble in water, stable against water and strong bases and acids, and is soluble in organic solvents such as esters, hydrocarbons, and chlorinated carbons. Synthesis and properties of benzylcellulose have been described in detail in previous reviews (Braun and Meuret, 1989; Engelskirchen, 1987). The typical heterogeneous synthesis leads to DS values of about 2.0, the homogeneous reactions to products with DS values up to 3.0. Similar results have been obtained in case of benzylcelluloses containing further functional groups in the aryl moieties. In the last years benzylcelluloses with a regioselective distribution of the ether groups have been prepared using protective group methods (Isogai et al., 1984; Kondo, 1993; 1994). Based on such types of cellulose ethers with a well-defined molecular structure, investigation on hydrogen bond systems were carried out by FTIR- and solid-state CP/MAS 13C NMR spectroscopy. Together with completely etherified benzylcellulose and p-methylbenzylcellulose the 2,3-di-Oether has been used in research on thermotropic mesophase properties (Kageyama et al., 1985). To determined the degree of substitution in benzylcellulose and other cellulose derivatives with low DS (e.g. < 0.1) spectroscopic methods (UV/vis, Raman, near-IR) using multivariant data analysis have been described (Sollinger and Diamantoglou, 1996). Surface imaging of benzylcellulose containing in a cellulosic dialysis membrane was carried out with confocal Raman microscopy.

4.5.4.2

Triphenylmethyl (6trityP) and related ethers

Tripheny!methyl ('trityl') cellulose is the most important ether of a class of organosoluble cellulose derivatives that contain aromatic groups in the substituents. Trity!cellulose, as well as benzhydryl(diphenylmethyl)cellulose, benzyl(phenylmethyl)cellulose and phenylcellulose, generally are formed by the reaction of phenylmethyl halides with cellulose in the presence of an organic base. In order to gain considerable degrees of substitution, a sufficient activation of the starting cellulose material is absolutely necessary. Only benzylcellulose was commercially available in the mid-1930s in large quantities and was applied as a basic substance to lacquers. At present these cellulose ethers are mainly of scientific interest. Since the first preparation of Otritylcellulose by Helfrich and Koester (1924), triphenylmethylation (tritylation) has become an intensively studied and useful method for a preferred substitution

264


of primary hydroxy groups in cellulose chemistry. The products have been used as intermediates in the synthesis of a number of selectively substituted cellulose derivatives as shown in detail below. It was found already in the 1940s (Hearon et al., 1943) that heterogeneous tritylation in pyridine, using decrystallized cellulose regenerated from viscose, from cuprammonium solutions, or especially from cellulose acetate by treatment with 15 % aqueous ammonia, yields colorless products with DS values from 0.81 to 1.21. To evaluate the extent to which the trityl group is specific for the O-6 position, a chemical analysis was carried out. Following tritylation, the remaining free hydroxy groups are covered by carbanilation with phenyl isocyanate. Subsequent detritylation and /?-toluenesulfonylation of the free hydroxy groups formed leads to a completely substituted tosylcellulose carbanilate, which was treated with sodium iodide for displacing the tosyl groups. Provided that the nucleophilic displacement takes place only at the tosyl esters of primary hydroxy groups (see also chapter 4.4.3.8), the content of iodine indicates the amount of O-6 tritylation in the starting product. Starting from a sample of DS of trityl groups of 1.03, shows that at least 90 % of the primary hydroxy groups were tritylated. It is noteworthy in this context that the selectivity of the reaction can be influenced by the reaction time and the molar ratio of anhydroglucose units/trityl reagent, as illustrated by Honeyman (1947). The rate of reaction is initially 58 times faster at the primary than at the secondary hydroxy groups. However, with increasing conversion of the primary hydroxy group this ratio decreases and become equal at small amounts of remaining primary hydroxy groups. A summary of the early results was given by Green (1963) (see Table 4.5.15). Table 4.5.15. Selectivity of tritylation of regenerated cellulose.a Reaction conditions b

Mol/mol 1.5

Degree of substitution at: c

Time 0-6 0-2/0-3 1 0.16 0.00 4 0.48 0.03 8 0.76 0.07 96 0.70 0.26 9.0 1 0.02 0.51 4 0.99 0.10 24 0.58 0.99 48 0.90 0.99 a Adopted from Green (1963, with permission), from cellulose acetate. b MoI of trityl chloride per mol of AGU. c Reaction temperature 1000C.


265

Based on the mentioned investigations, Gray and Harkness (199Oa and 199Ob) synthesized 6- O-trity!cellulose heterogeneously in pyridine, starting from regenerated, commercial cellulose acetate, which was deacetylated for 19 days with a 15 % aqueous solution of ammonium hydroxide. It was verified by the absence of a carbonyl stretching band in the infrared spectrum that no acetyl group remains on the polymer, as assumed by other authors (Hall and Home, 1973; Hagiwara et al., 1981). In the Appendix an example of the preparation of 6-0trity!cellulose with DS 0,97 is given (Gray and Harkness, 199Ob). As described before, typical tritylation procedures for regioselective 6-O protection of the AGU of cellulose use activated or regenerated cellulose as the starting material and heterogeneous starting conditions (Helfrich and Koester, 1924; Green, 1963; Yalpani, 1985). In this case, the tritylated celluloses become soluble during the reaction. In order to compare the reactivities of unsubstituted and increasingly methoxy-functionalized trityl chlorides with each other, and to exclude some of the problems of a heterogeneous start of the reaction (solubility of the polymers and accessibility of the hydroxy groups, for instance), Camacho Gomez et al. (1996) used a homogeneous derivatization procedure. For this reason, the well-investigated DMA/LiCl cellulose solvent system (Dawsey and McCormick, 1990) was selected (Fig. 4.5.30).

DMA / LiCI, pyridine 250C or 7O0C

R1 H

H

H

H

H

H

OCH3

OCH3

OCH,

OCH,

OCHo

R4 = H or C(C6H4R1) (C6H4R2) (C6H4R3)

OCH3

Figure 4.5.30. Tritylation of cellulose with trityl chloride and methoxy-substituted trityl chlorides.

As shown for the corresponding 4-methoxy-substituted diphenylmethyl chlorides (Erler et al., 1992b), the insertion of electron-donating substituents into the aryl moieties of triphenylmethyl chloride increases the rate of the reaction drastically. So, the reaction of unsubstituted trityl chloride with cellulose at 25 0C

266


and at comparable reaction times leads to products with very low DS values, being insoluble in organic solvents. The insertion of one methoxy substituent into the aryl moiety of the triphenylmethyl chloride, results in soluble cellulose derivatives (DS 0.7) after 24 h at 25 0C. The di- and trisubstituted chlorides give soluble derivatives with DS values of about 1 after 24 h at 25 0C. In this way, the tritylation of cellulose at room temperature is possible for the first time. Table 4.5.16 summarizes results of the homogeneous tritylation at 70 0C and of acidic detritylation at room temperature. Figure 4.5.31 shows the 13C NMR spectra of cellulose ethers with DS values of about 1, synthesized at 70 0C. The peak assignments were carried out based on the assignments of Takahashi et al. (1986) for trityl- and methylcelluloses, as well as on the peak assignments of the corresponding cellulose diphenyl ethers. As no significant splitting of the peaks for the C-6 and C-I carbon atoms could be observed, it may be concluded that the tritylation in all cases proceeds with high 6-O-regioselectivity. Figure 4.5.32 shows the complete spectrum of the monomethoxytrityl ether. Table 4.5.16. Tritylation of cellulose with methoxy-substituted trityl chlorides (3 mol of reagent/mol of AGU, DMA/LiCl, 70 0C) and detritylation (37 % HCl aq. in THF, 1 : 25 v/v at 25 0C) after subsequent permethylation (Camacho Gomez et al., 1996).

Substituent

Trityl

4-Monomethoxytrityl

4,4 '-Dimethoxy trityl

4,4',4"Trimethoxytrityl a b

Tritylation Time (h) 4 24 48 4 24 48 4 24 48 4 24 48

DS*

0.41 0.92 1.05 0.96 0.92 0.89 0.97 1.05 0.90 0.96 0.92 0.93

Relative rate DSb 1 0.43 0.83 1.12 2 1.03 — 1.03 - 2 XlO5 1.17

Relative detritylation rate 1

18

100

—

6 XlO6

—

590

— -

DS calculated from elemental analysis. DS determined by gravimetry.

The described results demonstrated that soluble 6-0-triphenylmethy!cellulose with DS values of about 1 were obtained at 25 0C using methoxy-substituted


267

triphenylmethyl chlorides in DMA/LiCl/pyridine. The tritylation of these derivatives is higher than 96 % at the 6-O position. Even after relatively long reaction times, using an excess of the reactant and/or at a higher temperature, the substitution at the 2-O and 3-O positions was less than 11 %, and comparable with that of unsubstituted triphenylmethyl ethers of cellulose. The 4-methoxy, 4,4'-dimethoxy- and 4,4',4"-trimethoxytriphenylmethyl groups can be removed faster and under milder conditions than the unsubstituted trityl group. The introduction of just one 4-methoxy substituent into the trityl group caused a 10 times faster tritylation, as well as a 20 times faster detritylation. The 4-methoxysubstituted trityl groups are, therefore, useful tools for subsequent reactions in basic media and for the synthesis of regioselectively 2-O- and 3-O-substituted derivatives of cellulose. Typical examples are given in the Appendix for regioselective carboxymethylation of cellulose. C-2,3,5

100

80 6 [ppm]

60

Figure 4.5.31. 13 CNMR spectra (DS ~ 1) of: (a) trityl-, (b) 4-monomethoxytrityl-, (c) 4,4'-dimethoxytrityl-, and (d) 4,4',4"-trimethoxytrity!cellulose (Camacho Gomez et al., 1996).

268

4.5 Etherification of Cellulose C-9,10,11

C-12

C-10'

lC-91 C-11'

C-7

C-2,3,5

C-1

I

160

.

I

UO

.

I

120

C-6

LJL^~> .

I

100 6 [ppm]

,

I

80

,

I

60

Figure 4.5.32. 13C NMR spectrum of 4-(monomethoxtrityl)cellulose (Camacho Gomez et al., 1996).

A triphenylcarbinol (TPC)-moiety-containing cellulose derivative (TPC cellulose) was prepared by a two-step reaction (Arai and Kawabata, 1995). First, microcrystalline cellulose was dissolved in SO2/diethylamine/DMSO and homogeneously reacted with /?-bromobenzyl bromide to obtain tn-O-(pbromobenzyl)cellulose. Secondly, the tri-(9-(p-bromobenzyl)cellulose was reacted with buty!lithium and then with Michlers ketone. The DS of the obtained ethanol-soluble TPC cellulose was up to 0.56. The leuco form of the TPC moiety in this cellulose showed a small extent of ionic dissociation in ethanol under irradiation with UV light of λ > 290 nm, accompanied by a large degree of decomposition of the structure. With additions of acid and then of alkali, the TPC structure was reversibly isomerized from the leuco form to the colored form and then from the colored form to the leuco form. However, repeated cycles of the additions of acid and alkali resulted in considerable fatigue with the number of cycles (Arai and Kawabata, 1995). The preparation, as well as certain fine structural and thermal properties of partially benzhydrylated cotton cellulose (DS 0.31-1.22), have been described (Stanonis and King, 1960; Stanonis and Conrad, 1966; Stanonis et al., 1967; Cannizzaro et al., 1973). Their method employs mercerization and solvent exchange to 2,3-lutidine/DMF before treatment with benzhydryl bromide at temperatures of 120 0C for various intervals of time. Whereas the tritylation of cellulose with triphenylmethyl chloride in DMA/LiCl solution at 70 0C and with pyridine as a base proceeds over 48 h to soluble products with DS values of up to 1 (see above), the reactivity of diphenylmethyl chloride is insufficient under these conditions (see Fig. 4.5.33 and Table 4.5.17).


269

OCH3

10

30

Time [h]

Figure 4.5.33. Conversion plots of some diphenylmethyl chlorides and triphenylmethyl chlorides for cellulose etherification in DMA/LiCl at 70 0C using pyridine as a base. Broken lines denote tangents to curves at the origin (Erler et al., 1992b).

Provided that diphenylmethyl etherification of cellulose proceeds via a carbenium ion at the reaction center, the insertion of electron-donating substituents will increase the rate of the reaction. The results obtained with mono- and dimethoxy-substituted diphenylmethyl chlorides, shown in Fig. 4.5.32 and Table 4.5.20 (see later), give clear evidence for the influence of the methoxy substituents. The correlation of the reaction rate at the beginning of the conversion (tangent to the conversion plot, -log A(DS)/AO versus the sum of the substituent constants according to the Hammett equation ( Σ σ ΐ ) results in a straight line. In addition, p-chloro-substituted benzhydryl chloride does not react because of the positive σΐ-value of the chloro substituent. Furthermore, use of the most reactive dimethoxy compound enables the synthesis of diphenylmethyl ethers of cellulose in the DMA/LiCl solvent system at 70 0C within an acceptable time. As shown with this reagent, THF- and 1,4dioxane-soluble products were obtained after 8 h at 70 0C (DS = 0.83).

270


Table 4.5.17. Comparison of the reactivity of some chlorides in the homogeneous etherification of cellulose (Erler et al., 1992b). 1

2

3

MA

CeIl-OH + CI-C(R RR) v '

R1

R2

T=V

R3

/=V-

CH3OnQCH30V=V

QCH30V=V.

Q1V=V

/°\_

/""V

/"V.

a

c

a

DS

1

2

3

A(DS

:)

Σσ+

CeII-Q-C(R RR) v '

A(DS) At

h

Ιοα

c

P

H

0.02 3.00 χ 10"4

- 3.52

H

0.33 2.25x10-2

-1.65

-0.78

H

1.01 0.66

-0.18

-1.56

H

T=V.

O

0.11

SEA by using the silicon content determined by elemental analysis. An excess of TDMSCl leads to thexyldimethyldisiloxane during the work-up procedure, which forms a host-guest compound with the cellulose derivative, and a complete removal of thexyldimethyldisiloxane is impossible. Table 4.5.20. Silylation of cellulose in DMA/LiCl solution (reaction time 24 h).

Reaction conditions TDMSCl Temperature Yield Reprecipitation EAa (mol/mol (0C) ( %) from AGU) DMF 25 82.0 1.03 1.2 2.5 25 THF 82.5 1.53 3.5 THF 25 81.0 1.94 3.5 50 76.2 THF 2.11 6.0 THF 2.11 50 96.0

DS

HPLCb

1.07 1.51 1.64 1.77 1.90

a

Based on the Si content determined by elemental analysis; ^ based on the permethylation analysis and calculation according to DS = 2 MFdi + MFmono (MF = mole fraction).

4.5.5.3 Properties and structure characterization The trimethylsily!celluloses are soluble in common organic solvents. Under conditions of spin coating and Langmuir-Blodgett techniques (see below) they form films and ultrathin layers. Using a common spinneret, fibers are formed (Greber and Paschinger, 198Ic). In relation to DS^ they show solubility in strong polar solvents like DMSO and in nonpolar solvents like n-hexane (Fig. 4.5.37). The thexyldimethylsilyl (TDMS) celluloses were soluble in DMF (DS < 1), THF (DS > 1), and chloroform (DS > 1.5). For characterization of regioselectivity the knowledge of the distribution of functional groups within the AGU and along the polymer chains is essential. Therefore, suitable methods of structure analysis by 1H and 13C NMR spectros-

4.5.5 Silyl ethers of cellulose

281

copy, as well as by HPLC, after complete methylation and complete chain degradation have been developed (Erler et al., 1992a). n-Hexan\ CH2Cl2-

B- Acetone, ethylacetate

O

0.5

15

1.0

2.0

2.5

DS

Figure 4.5.37. Solubility of TMS celluloses in relation to D% (Klemm et al., 199Oa). In the case of TMS cellulose, e.g., with DSsi 1.55 (Fig. 4.5.38), a complete 6-O silylation and an additional O-2 and O-3 silylation have been observed by 13 C NMR spectroscopy. C-2.2',3,5

C-1"

C-1

100

80

60 40 6 [ppm]

20

O

Figure 4.5.38. 13 CNMR spectrum of trimethylsilylcellulose, DS = 1.55, in DMF-J7 (Klemmetal., 199Oa). In the case of silylcelluloses with bulky alkyl groups, an O-6 silylation and free hydroxy groups in positions 2 and 3 could be demonstrated. As a typical example Fig. 4.5.39 shows the 13C NMR spectrum of teri.-butyldimethylsilylcellulose. An important result of NMR spectroscopy in structural analysis of functionalized celluloses consists of two-dimensional 1H/1!! NMR techniques after subsequent derivatization of the original polymers. This method is suitable even for products with high molecular weight. In the case of the previously described TDMS celluloses, the polymers were treated with sodium hydride and methyl

282


iodide in THF solution, which leads to the completely methylated products (see Fig. 4.5.36). In this case it is additionally necessary to substitute all silyl groups by acetate residues to get a better resolution of the spectra so that the 1 Hy1H COSY (homonuclear chemical shift correlation spectroscopy) technique is useful for peak identification. From this point of view, the obtained methylated polymers were treated with tetrabutylammonium fluoride to remove the TDMS groups completely. The obtained methylcelluloses could be completely acetylated with acetic anhydride in pyridine.

C-6

100

90 80 ό [ppm]

70

1

60

Figure 4.5.39. 13 CNMR spectrum of teri.-butyldimethylsilylcellulose, DS = 0.96, in DMF-J7 (Klemm et al., 199Oa).

In the case of HPLC analysis, the methylated silylcelluloses lead, with trifluoroacetic acid and water, to the corresponding methylglucoses (mixture of anomers) by desilylation and chain degradation. The determination of the OSHPLC was carried out after separating the methylglucoses by reversed-phase HPLC on a LiChrospher-aminephase column, which separates them into groups of unmodified, mono- and dlfunctionalized AGU. The mole fractions (MF) were calculated after integrating the corresponding peaks, £>SHpLc according to 2 MFdi + MFmono. For instance, the peaks of 2,3-di-O-methylglucose indicate silylation at position 6, that of 3-O-methylglucose indicating 2,6-di-O-silylated AGU. Figure 4.5.40 shows the structural characterization of two typical TDMS celluloses after the described subsequent modification. The NMR results demonstrate that the heterogeneously synthesized polymer 6-O-thexydimethylsilylcellulose (DS = 0.69, e.g.) leads to 6-O-acetyl-2,3-di-O-methylcellulose after stepwise derivatization. Only one peak is detectable of all the AGU protons. The double peak of the H-6 proton is caused by the diastereotopic effect of the neighboring asymmetric carbon atom. The homogeneously prepared polymer 2,6-di-O-TDMS cellulose (DS, e.g. 1.02) leads to a nonuniform 6-O-acetyl-2,3-di-O-methyl-co-[2,6-di-%). Reference Koschella, A., Klemm, D., Macromol Symp. 1997, 720, 115-125.

Appendix (Volume 2)

371

296-Di-O-thexyldimethylsilylcellulose Λ,

O

OH

' HO

N

Ο,. + TDMS-CI

OH

H

O^

OTDMS

TDMS-CI =

thexyldimethylchlorosilane

Cellulose (15.0 g, 92.6 mmol, AVICEL PH-IOl, dried for 1 day over potassium hydroxide under vacuum at 105 0C) was suspended in 300 ml of N,Ndimethylacetamide (DMA) and stirred for 2 h at 120 0C. After cooling down to 100 0C, 22.5 g of LiCl (dried for 1 day over potassium hydroxide under vacuum at 150 0C) was added. The mixture was stirred at 25 0C until a clear solution was obtained. Imidazole (30.29 g, 445 mmol, 4.8 mol/mol of AGU) was dissolved in DMA and added to the cellulose solution. Thexyldimethylchlorosilane (66.2 g, 371 mmol, 4 mol/mol of AGU) was added dropwise. The mixture was stirred for 24 h at 25 0C. After some hours precipitation of the silylated cellulose occurs. The mixture was poured into 3 1 of pH 7 buffer solution. The separated polymer was carefully washed with water and dried under vacuum, first at 25 0C then at 100 0C. Further purification was carried out by reprecipitation from THF solution in pH 7 buffer. Yield: 42 g FTIR(KBr)^SOSCm-1 v(OH); 2958, 2871cm-1 V(CH2, CH3); 1466cm-1 6(CH2, CH3); 1252cm-1 8(Si-C); 1119-1037 cm-1 v(C-O-C); 833 cm-1 v (Si-C) The polymer is soluble in THF, hexane and chloroform.

372

Appendix (Volume 2)

6-O-Thexyldimethylsilyl-293-di-O-methylcellulose

NaH

O^ + CH 3 I "

THF, Id1 250C "H3CO 3d, 5O0C

6-O-Thexyldimethylsilylcellulose (50.Og, 0.172mol, DSSi 0.9) was suspended in 1.5 1 of THF. After addition of 41.3 g (1.72 mol, lOmol/mol of modified AGU) of sodium hydride, 107 ml (1.72 mol, 10 mol/mol of modified AGU) of methyl iodide was added slowly during 1.5 h. Nearly l h after the start of the methyl iodide addition, an exothermic reaction occurs and it was necessary to cool the flask with ice. The mixture was stirred overnight at 25 0C and for 3 days at 50 0C. After cooling down at room temperature the inorganic salts were separated by centrifugation. The clear solution was concentrated using a rotary evaporator and precipitated into 6 1 of pH 7 buffer solution. The polymer was separated, washed and dried carefully. Yield: 45. 17 g FTIR(KBr): 2957cm-1 V(CH2, CH3); 1466cm-1 5(CH2, CH3); 1252cm-1 5(Si-C); 1126-1041 cm-1 v(C-O-C); 831 cm-1 v(Si-C) Reference A. Koschella, Klemm, D., Macromol Symp. 1997, 720, 115-125.

Appendix (Volume 2)

373

Trimethylsilylcellulose methoxyacetate, synthesis via cellulose methoxyacetate in DMA OCOCH2OCH3 .0

(CH3)3Si — NH"-Si(CH3)3

TMS-CI (DMA)

15min, rt / 5h, 8O0C

OCOCH 2 OCH 3 O OR

R ' = COCH2OCH3 , Si(CH3J3 according to DS

Dried cellulose methoxyacetate (Ig, DS - 1.1, determined by saponification analysis) was dissolved in 50 ml of DMA under stirring and an inert atmosphere. Then 1.96 ml (0.019 mol) of hexamethyldisilazane were added at room temperature within 15 min, as well as TMS-Cl, at catalytic concentration. The reaction mixture was stirred for 5 h at 80 0C. The excess of hexamethyldisilazane was removed with a water-jet vacuum pump at higher temperature. The reaction product was precipitated in buffer solution of pH 7, dispersed, filtered off and washed with distilled water many times until the filtrate was free of chloride ions. The product was dried at 50 0C under vacuum. Yield: 1.5 g (94 %) of pure product with D5Si =1.9 and DS ester groups =1.1. FTIR (KBr): 1766 (v OOester), 1252 (δ Si-C), 842 (ν Si-C) cm'1 The DS of ester groups was based on analysis by alkaline saponification. The DS §i was determined by gravimetric analysis (SiO2 content). The sample was soluble in THF, dichloromethane, chloroform and toluene. References Siegmund, G., Diploma thesis, Friedrich-Schiller-University of Jena, 1993. Stein, A., Thesis, Friedrich-Schiller-University of Jena, 1991, p. 82.

374

Appendix (Volume 2)

6-Carboxycellulose, homogeneous synthesis with phosphoric acid

1. NaNO2/ H3PO4

a 2

.

.O

2.NaBH4 (1) 5.0 g of cellulose were dissolved (using a special flask, content 1.5 1, height 45 cm) in 200 ml of 85 % phosphoric acid. After 2 h at room temperature, 5.0 g of powdered sodium nitrite were added under vigorous stirring during 15 min. Within 5 h without stirring, a stable foam was formed. It was destroyed by vigorous stirring and another 5.0 g of sodium nitrite were added. The addition was repeated after 3 h. After a total reaction time of 10 h, 50 ml of 85 % formic acid were added in order to destroy the excess sodium nitrite. All escaping gases were absorbed with ethanol. If the reaction time is 8 h, three portions (5.0 g of sodium nitrite were added) after 4 and 6.4 h. The polymer was precipitated with 800 ml of ice-cold acetone and transferred into a beaker. Precipitation was completed by addition of 2 1 of ice-cold ether (caution, very exothermic reaction). After filtration the material was washed with distilled water until the liquid becomes neutral and then first with 0.5 1 of 50 % ethanol and with 0.5 1 of absolute ethanol. The product was dried under vacuum at 5O0C. Fourier transform (FT)IR ( KBr ) ACOOH 174° cm"1 (2) 4.0 g of carboxycellulose was added during 3 h to a 10 % NaBH4 aq. solution (50 ml) under stirring at room temperature. After 16 h, without stirring, this solution was neutralized with acetic acid. The precipitated product was separated by centrifugation (15 min at 2000 g). The sodium salt was precipitated in 600 ml of acetone and dried under vacuum at 50 0C. FTIR (KBr) vcoo_ 1580-1630 cm

1 A

Appendix (Volume 2)

Cellulose starting material

DP

Cellulose powder

160

Viscose staple fiber

300

Spruce sulfite pulp

600

Cotton !inters

1400

Reaction time (h) 3 5 8 10 3 5 8 10 3 5 8 10 3 4 5 8 10

375

Content of carboxy groups (%) 7.7 57.9 62.0 63.0 60.1 60.4 65.0 67.9 62.0 68.9 73.5 75.0 35.7 52.3 73.4 78.0 81.0

References Heinze, Th., Klemm, D., Loth, F., Nehls, L, Angew. Makromol. Chem. 1990, 178, 95-107. Heinze, Th., Klemm, D., Schnabelrauch, M., Nehls, L, in Cellulosics: Chemical, Biochemical and Material Aspects, Kennedy, J.F., Phillips, G.O., Williams, P.A. (Eds.), New York: Ellis Horwood, 1993, pp. 349-355.

Subject index accessibility 171, 214 acetal structures 310 acetate borate ester 141 acetophthalates 193 acetosulfation 123f acetylation 170ff, 176ff - acid catalyst 170 - cellulose trinitrite 173 -DMA/LiCl 173 - W-ethylpyridinium chloride 173 - fiber acetylation 178 - industrial process 176 - mathematical modeling 172 - methylene chloride process 178 - partial acetylation 172 - preactivation 177 - preferential substitution 173 -rate 171 - raw material 177 - reactivity of cellulosic hydroxy groups 173 -reagent 170 - solution acetylation 172 - technical process 172 - two-phase system 172 - vapor process 172 6-0-acetyl-2,3-di-O-methyl-co-[2,6-diO-acetyl-3-O-methyl]cellulose 282f - 1H NMR spectrum 283 -HPLC 283 acrylamide 252 activation 43,61, 171,317,322 - activating agent 61 - with liquid NH3 43 acylation 182, 286ff - heterogeneous 182 -homogeneous 182 6-aldehydecellulose 304 aldonic acid end-groups 303 aliphatic ethers 210, 213f, 221, 234 - carboxymethy!cellulose 221 - hydroxyalkyl ethers 234

- long-chain alkyl ethers 213 - methy!cellulose 210 -preparation 214 - subsequent functionalization 214 alkali cellulosates 32 - preparation 32 - properties 32 alkali cellulose 49ff, 216, 339 - applications 50 - preparation 339 - properties 50 alkali uptake 146 ß-alkoxy elimination 303 alkyl ethers 207 alkylation 21Of, 214 - product formation 211 - role of cellulose supramolecular structure 214 -S N 2 reaction 210 alky !cellulose 217ff -applications 219 -commercial 219 -ASrange 217 -Relation 218 - hydrophobicity 217 - liquid crystalline systems 220 - microcapsules 220 -properties 217 -solubility 217 - solution properties 217 - ultrathin films 220 -viscosity 217 amidoxime 254 amine oxide process 321 amino groups 256 - introduction into cellulose 256 aminodesoxycellulose 144 aminoethylation 255 aminoethylcellulose 249 ammonia cellulose 58 amphiphilic esters 196 amylose 258 anion-exchanging sorbents 259

Comprehensive Cellulose Chemistry; Volume 2: Functionalization of Cellulose D. Klemm, B. Philipp, Ύ. Heinze, U. Heinze, W. Wagenknecht Copyright © 1998 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-29489-9

378

Subject index

anticlotting activity 132 aromatic esters of cellulose 190ff - acetate 193 -benzoates 190 - cinnamates 191 - homogeneous reaction 191 -phthalates 190,193 artificial fibers 163 asymmetric membranes 293

bacterial cellulose 317, 344 - acetylation 344 base uptake 55 bead cellulose 243 benzhydryl(diphenylmethyl)cellulose 263 benzoylation 192 benzylcellulose 263 benzyl(phenylmethyl)cellulose 263 bleaching 317 block copolymers 27f - biodegradability 28 - cellulose and lignin 28 - cellulose triacetate 28 - principle 27 - synthesis 27 - synthetic prepolymer 28 bromodesoxy cellulose 144 f-butyldimethylsilylcellulose 281 i-butyldimethylsilylcellulosecinnamate 291

C-6 oxidation 305 Cadoxen 90, 95, 231, 79ff Cannizzaro reaction 310 carbamate method 321 carbamate process 161, 163 - fiber spinning 163 carbamoylethy!cellulose 253 carbanilation 196f, 264 - regioselective 197 carbonyl sulfide 145 - esterification with 145 carboxy groups 305 6-carboxycellulose 304, 374

- synthesis 374 carboxycellulose 306f - 1 3 CNMR 306 - ionotropic gels 307 -p^s 307 - sulfation 307 carboxyethylcarbamoylethylcellulose 255 carboxyethylcellulose 230, 253 carboxyl groups 303 - determination 303 carboxymethylation 22Iff, 229f, 267 - continuous process 230 -DMA/LiCl 225 - dry process 229 - kinetic study 222 - laboratory procedure 222 - phase-separation process 224 - regioselective 224 - slurry process 222,229 - technical process 229 carboxymethy!cellulose 186, 22Iff, 227ff, 353, 355, 357 - acid chloride 227 -activation 228 - anionic polyelectrolyte 234 - application 233 - block-like structures 224 - 1 3 CNMR 228 - crosslinking 227,228 - degree of substitution 223 -DP 231 -ASrange 233 - gel particles 231 - heterogeneous synthesis 353 - highly substituted 222 - intermediate derivatization 232 - lactone formation 227 - molar mass 231 - pattern of substitution 223 - preparation 222 - properties 230 -quality 233 - reactivity ratio 223 - subsequent derivatization 227 - substituent distribution 229

Subject index -sulfation 228 - synthesis in DMA/Lid 355 - synthesis via cellulose trifluoroacetate 357 - water solubility 230 2,3-0-carboxymethyl6- O- triphenylme thy !cellulose 36 If - detritylation 362 - synthesis via 6-O-trity!cellulose 361 cation exchangers 140 cationic cellulose 257 cationization 257, 259 - regioselective 259 celluloid 111 cellulose 331 - dissolution in DMA/LiCl 331 cellulose 2,5-acetate 169 cellulose acetate 121,138,176, 178ff, 274 -application 180 - deacetylation 121 -films 180 - liquid crystalline phases 178 -NMR 176 - phosphory lation 138 - plastic material 181 -properties 178 - Raman spectroscopy 176 -rheology 180 - separation membranes 181 -solubility 179 - sulfation 121 - supramolecular aggregates 180 - textile properties 181 cellulose acetate phosphate 136 - polytetraphosphoric acid 136 cellulose acetate sulfate 121, 123 - competitive esterification 123 - heterogeneous deacetylation 121 - regioselectivity 123 cellulose acetobutyrates 186 cellulose borate 140, 142 - application 142 - properties 142 - synthesis 140 - thermal stability 142 cellulose carbamate 16If

- crosslinking 161 - decomposition 161 -formation 161 - substituent distribution 162 cellulose citrate 190 cellulose dichloroacetate 345 - synthesis 345 cellulose dithiocarbonic acid 147 cellulose ester 182ff, 186, 192 - application 186 - liquid crystalline systems 184 -NMR 183 - palmitoyl ester 183 - physical properties 183 cellulose ethers 208, 256, 318 - cationic 256 - production capacity 208 -regioselective 318 cellulose formate 166ff, 225, 342 -preparation 166 -properties 168 - synthesis 342 - thermal stability 168 - transesterification 167 cellulose furoate 190 cellulose gels 84 cellulose grafting 322 cellulose hemiacetals 249 cellulose I 41ff, 61 cellulose II 41ff, 61 cellulose III 58 cellulose methoxyacetates 347 - synthesis 347 cellulose monothiocarbonate 146 - maximal DS 146 cellulose nitrate lOlff, 108ff, 144 - acid hydrolysis 108 - application 111 - decomposition 102, 108 - degradation 108 - film-forming properties 111 -formation 102,105 - industrial production 101, 109 - nitrating acid 104 - nitrating agent 102 - nitrogen content 101, 103

379

380

Subject index

- properties 111 - softeners 111 - solubility in organic liquids 111 - stabilization 110 - viscosity adjustment 110 cellulose nitrite 112ff - addition compound 113 -application 114 -isolation 113 - nitrosyl compounds 114 - NMR studies 113 -properties 114 - stability of the nitrite groups 113 -synthesis 112 cellulose oligophosphate 134 cellulose oxalates 189 cellulose phenylcarbamate 341 - synthesis 341 cellulose phosphate 133ff, 137, 139f, 338 - application 140 -NMR 139 - pattern of substitution 137 - preparation 338 -properties 139 - reaction routes 134 - Schotten-Baumann reaction 139 - solubility 139 - solution viscosities 140 cellulose phosphite 136 cellulose phosphonate 136 cellulose phosphonite 136 cellulose purity 317 cellulose solvents 88 cellulose sulfate 116f, 12If9 125, 128f, 131f,332, 334ff - acidic character 128 -application 131 - biological activity 131 - C-2/C-3-substituted 337 - C-6-substituted 336 - chain degradation 125 - defined patterns of substitution 128 - distribution of sulfuric acid half-ester 122 -DS

111

- DS from cellulose acetate 121 - film-forming properties 131 - gelforming properties 132 - [η]-Mw relationship 129 - preparation 336 -properties 128 -purification 121 -SO3-DMF 332 -solubility 117 - solution viscosities 129 - substitution patterns 125 -synthesis 116 - synthesis via cellulose trifluoroacetate 334 - synthesis via trimethylsily!cellulose 335 - thermal stability 129 - thermoreversible 129 cellulose tosylate 350 - homogeneous synthesis 350 cellulose triacetate 169, 176, 343f - preparation 343 - selective deacetylation 344 cellulose tribenzoate 191 -benzene-ring-substituted 191 cellulose tricarbanilate 196f, 340 - homogeneous route 197 - laboratory procedure 197 - molar mass distribution 197 cellulose trifluoroacetate 188, 225, 346 - synthesis 346 cellulose trimethoxalate 189 cellulose triniträte 332 - preparation 332 cellulose trinitrite 138 - phosphory lation 138 cellulose xanthogenate 339 - preparation 339 cellulose xanthogenic acid 156 cellulose-4-nitrobenzoate 348f - synthesis via cellulose trifluoroacetate 348f cellulose-metal complexes 320 cellulosic ampholytes 259 cellulosics 292 - cinnamoyl-group-containing 292 chain degradation 93

Subject index - oxidative 93 chain stiffening 82,92 chemical synthesis 3f, 6 - cationic polymerization 4 - polycondensation 3 - protecting groups 4 - ring opening polymerization 3 - stereoregular chain structure 4 - transglycosidation 6 chlorination 135, 143 - homogeneous route 143 chlorodesoxycellulose 143 churn process 157 CMC see carboxymethylcellulose 13 CNMR 266,280 13 C NMR spectroscopy 272 colloid chemistry 151 comb-like cellulose derivatives 290 commercial processes 321 - artificial fibers, films 321 commercial viscose process 157, 159 - alkali cellulose 157 - ecological hazards 157 - skin-core-filaments 159 -spinning 159 - structure formation 159 controlled activation 278 copolymerization of ß-D-glucose 3 - Acetobacter xylinum 3 - 7V-acetylglucosamine 3 copper complexes 72ff, 76f COS see carbonyl sulfide crosslinking 6ff, 14ff, 243f, 261, 323, 363 -applications 16 - covalent 6 - crosslink density 6 - diisocyanates 7 - divinyl sulfone 261 - epichlorohydrin 244, 363 - ester bond 8 - ether bonds 6 - formaldehyde 6 - ionic 7 - macroradicals 7 - material properties 15

381

- mechanical properties 16 - morphological structure 14 - oxidative coupling 7 - polycarboxylic acids 8 - principles 6 - self-crosslinking 6 - solubility in Cuam 15 - supramolecular structure 14 - water retention 15 crosslinking by Michael addition 12, 16 - di vinyl sulfone 12 -hydrogels 12,16 crosslinking with alkyl halides and epoxides 12f -epichlorohydrin 13 crosslinking with formaldehyde 8ff, 16 - acetal bridges 8 - dry process 9 - formaldehyde liberation 10 - high-performance crosslinker 11 -kinetics 9, 11 - mechanism 9, 11 - methylol derivatives !Off - methylolated 16 - tetrafunctional crosslinker 11 - urea compounds 16 - urea derivatives 10 - wet process 9 crystalline order 54 crystallinity 40, 45 - crystallite size 40 - lattice dimensions 40 Cuam 74ff, 93ff Cuen 77,84 cyanoethylation 250ff - heterogeneous 252 - homogeneous 252 - Michael addition 250 - reaction rate 251 - reactivity 251 cyanoethylcellulose 226, 249ff, 258, 364 - application 254 - consecutive reactions 253 - decomposition 251 ff - formation 250 - material properties 254

382

Subject index

- oxidation 253 - preparation 364 cyanopropyldimethylsilylcelluloses 274

deacetylation 174f - alkaline saponification 175 -homogeneous 175 decrystallization 54, 58 desilylation 287 desoxy cellulose 142ff, 196 - iminodiacetic acid group 144 - preparation 143 - subsequent functionalization 144 - thermal decomposition 145 detritylation 264, 266 2,3-dialcohol cellulose 31Of - 1 3 CNMR 311 dialdehyde cellulose 304 2,3-dialdehyde cellulose 309 - preparation 309 2,3-dicarboxycellulose 304, 31 If - 1 3 CNMR 311 - complexing properties 312 dicarboxylic acid methyl ester 190 dicarboxymethylcellulose 226 diethylaminoethy!cellulose 256 2,3-diketocellulose 304 4,4' -dimethoxytripheny!methyl groups 267 4,4' -dimethoxytrity!cellulose 267 4-dimethylaminopyridine 164, 171, 285 2,3-dimethy!cellulose 213 2,3-Di-O-methylcellulose 352 diphenylmethyl ethers 269 diphenylmethylsilylcelluloses 274 direct esterification 169f - homogeneous 170 dissolution 34, 52, 72, 90 - dimethyldibenzylammonium hydroxide 52 - triethylbenzy!ammonium hydroxide 52 dissolving pulp 321 2,6-Di-O-thexyldimethylsilylcellulose 371 DMA/LiCl 87,93f - structures 87

donor-acceptor complex 72 DP see degree of polymerization

emulsion xanthation 150 enzymatic esterification 165 epoxidation 246 equilibrium reaction 164 esterification 99f, 186ff - 1,2-dichloropropionic acid 186 - inorganic acids 100 - TMS-cellulose 188 - trifluoroacetic acid 187 esters of cellulose 99ff, 112, 115, 133, 140ff, 161, 164, 166, 168ff, 182, 186, 189ff - aromatic acids 190 - cellulose acetate 168 - cellulose borates 140 - cellulose carbamate 161 - cellulose formate 166 - cellulose nitrate 101 - cellulose nitrite 112 - cellulose phosphate 133 - cellulose sulfates 115 - desoxycellulose 142 - di- and tricarboxylic aliphatic acids 189 - dithiocarbonate esters 147 - higher aliphatic acids 182 - inorganic acids 100 - mesylcellulose 194 - monothiocarbonic acid 145 - organic acids 164 - pheny 1 carbamates 196 - phosphonic acid esters 194 - production capacity 99 - substituted monocarboxylic aliphatic acids 186 - tosy !cellulose 194 etherification 207, 246, 270 - reactivity 270 ethers 285 - 2,3-substituted 285 ethers of cellulose 210 - aliphatic ethers 210

Subject index ethylation 213 - activation energy 213 ethylcellulose 213,216 - liquid crystalline systems 216

FeTNa 82ff,93f, 331 - cellulose interaction 84 - characterization of cellulosic materials 84 - complex binding 84 - degradation 83 - intrinsic viscosity 83 - medium for etherification 84 - molecularly dispersed system 84 -preparation 331 - replacement of Na+ by K+ 84 fiber acetylation 172 fiber xanthation 151 ff - lattice layer reaction 153 - lye concentration 152 - maximal DS 151 - rate constant 153 filament spinning 93 film formation 93 flame retardation 139 flash photolysis 273 flocculation 218 fluorodesoxy cellulose 143 formylation 166ff - hydrolytic cleavage 167 - preferential reaction 167 -rate of 168 functionalized alkyl ethers 249, 255 - with quaternary ammonium groups 255 - with tertiary amino functions 255 g graft copolymers 17f, 24, 26, 141 - acrylonitrile fibers 26 -analysis 18 - antimicrobial finish 26 - applications 24 - cellulose fibers 24 - filtering processes 26

383

-homopolymer 17 - ion-exchange 26 - properties 24 - routes 18 - side chains 18 - super-absorbing materials 26 grafting 17, 19ff - cationic acrylics 26 - conditions 25 - effect of swelling 23 - mechanochemical treatment 20 - monomers 19 - morphological structure 17, 22 - perfluorinated compounds 26 - radiation grafting 21 f - radical polymerization 17 - radical site 19 - redox reaction 19 - supramolecular structure 17, 22 - surface grafting 22

Hammett equation 269 hemodialysis 140 1 HNMR 280 1 W 1 HCOSY 282 hollow fibers 94 HPLC 226,281 HPLC see high performance liquid chromatography hydrogen bond system 73 hydrogen bonds 318 hydrophile/hydrophobe ratio 179 hydroxyalkylation 234f, 237, 240 - by epoxides 235 - heterogeneous process 240 - heterogeneous reaction 237 - hydroxyalkyl chains 235 - reaction rate 237 - reagent yield 235,237 - spacing action 240 - technical process 240 - two-stage process 240 hydroxyalkylcellulose 237, 239, 24If - application 242 - liquid crystalline systems 241

384

Subject index

- pattern of substitution 237 -properties 241 - reaction with isocyanate 239 - reactivity ratio 237 - subsequent esterification 239 - subsequent etherification 239 -viscosity 241 hydroxyethylation 235 hydroxyethylcellulose 234, 236, 242f - application 242 - ecocompatible artificial fiber 243 - length of the side chains 236 -MS 236 hydroxymethylcellulose 246 - formation 246 hydroxypropylation 237f - relative rate 237 - slurry procedure 238 hydroxypropylcellulose 234, 236 -MS 236 i impeller technique 182 impelling agent 165 in vitro synthesis 3 - cellulase 3 - functionalized cellulose 3 - micellar aggregation 3 induced phase separation 225 interaction with aliphatic amines 62ff - accessibility 64 - addition complexes 63 - addition compounds 62 - degree of order 64 - diamines 63 - ethanolamine 63 - ethylene diamine uptake 65 - higher amines 63 - increase in accessibility 62 - methylamine and DMSO 62 - polyamines 63 - primary amines 62 - steric hindrance 63 - water sorption 66 interaction with alkali hydroxides 33, 35ff, 43, 47

- alkali uptake 35 - chain conformation 38 - chemical processes 35 - comparison of NaOH and KOH 37 - conformational changes 39 - diffusion-controlled reaction 39 - effect of temperature 39 - fibrillar morphology 43 - general comments 33 - hydration states 36 - 2 3 NaNMR 36,38 - NaOH ion dipoles 38 - reactive structural fractions 47 - sorption isotherm 35 - water uptake 36 interaction with ammonia 57ff - addition compounds 57 - degree of order 58 - dry process 60 - dyeability 60 - fibrillar architecture 58 - handling 60 - lattice transitions 57 - NH3/DMF mixture 59 - recrystallization 58 - structure 57 - swelling power 57 - textile processes 60 - textile processing 57 - water regain 58 - wet process 60 interaction with guanidinium hydroxide 54ff - accessibility 56 - activation technique 57 - adduct formation 55 - base uptake 55 - fibrillar structure 57 - regenerated samples 56 - solutions of GuOH 55 - swelling 55 - water sorption 55 - X-ray patterns 55 interaction with hydrazine 61 interaction with inorganic salts 86f - spinning of threads 87

Subject index - suitable cations 86 - thiocyanate 86 interaction with tetraalkylammonium hydroxides 5 Iff - applications 54 - hydrate complex 53 - model of dissolution 53 - structure 54 - uptake of base 52 intermolecular interaction 240 intracrystalline swelling 61 iododesoxycellulose 142 IR spectroscopy 288 isomerization 292

ketocellulose 308 - selectively oxidized 308 2-ketocellulose 304 3-ketocellulose 304 1 Langmuir-Blodgett layers 184,323 Langmuir-Blodgett technique 293 lateral order spectrum 168 LB see Langmuir-Blodgett level-off degree of polymerization 45, 58, 65, 87 liquid crystalline systems 323 LODP see level-off degree of polymerization

m mercerization 49 - cold mercerization 49 - hot mercerization 49 mesophase 323 mesy !cellulose 142 metal complexes 7Iff, 76ff, 85f, 90, 92ff - acid-base interaction 80 - application 93 - bisdiolato complex 85 - bisdiolato crosslinks 77 - cellulose zincate interaction 86 - characterization of cellulose 95 - colored 92

385

- coordination equilibria 81 - covalent functionalization 94 - cuprate anions 85 - determination of foreign substances 95 - formation 72f - heteroleptic complex 82 - heteroleptic copper complex 77 - homoleptic cationic complex 78 - hydroxamic acid functions 72 - hydroxy bonds 81 - interchain crosslinking 82 - ligand exchange 77 - ligand-exchange processes 72 -main routes 71 - precipitation 92 - properties 92 -reformation 81 - solution viscosity 92 - spectrophotometric investigation 96 - state of solution 92 - supramolecular aspects 90 - toxicity 93 -type 90 -with 1,3-diaminopropane 76 - with ethylene diamine 76 - zincate 90 methacrylate ester 188 methoxyl group content 220 4-methoxytripheny!methyl groups 267 methylation 21 Iff, 216f -agent 212 - diffusion-controlled reaction 211 - gaseous process 216 -laboratory 212 - liquid methyl chloride process 216 - reaction temperature 211 - reagent yield 217 - regioselective 213 - technical process 216 methylcellulose 207, 21Of - degree of substitution 211 - substituent distribution 211 Λ^-methylmorpholine TV-oxide 321 methylolcellulose see hydroxymethylcellulose - artificial fibers 248

386

Subject index

- distribution of side chains 248 - liquid crystalline properties 248 -MS 247 - subsequent derivatization 248 - viscosity 248 Michael addition 257 microfibril structure 44 mixed cellulose esters 288 mixed esters 183 mixed ethers 227, 233 molar-mass distribution 316 mole fractions 226 molecular modeling 323 molecular weight distribution 95 4-(methoxytrityl)cellulose 268 MS see molecular substitution

Na-cellulose 40ff, 48 - kinetics 43 -modifications 41 - permodoid reaction 41 - phase diagram 42 Na-cellulose see sodium cellulose nano-structure s 322 Nioxam 79 Nioxen 79f nitrating system 103 nitration 103, 105ff, 110 -action of N2O4 103 - batch process 110 - changes in supramolecular structure 108 - continous process 110 - course of reaction 107 - equilibrium constant 106 -mechanism 103 - nitronium cation 105 - nitronium salts 103 - reaction temperature 107 - stabilization process 106 - substitution pattern 107 - sulfate groups 106 Ni-tren 8Of nonionic mixed ethers 242 Normann compound 74, 85, 90

ft-octyldimethylsilylcellulose 293 oligophosphate crosslinks 138 organic ester ethers 214 oxidation 302ff, 308ff - content 305 - formation of carbonyl groups 303 - formation of carboxy, aldehyde and keto groups 302 - heterogeneous 309 - homogeneous 309 -partial 302 - primary hydroxy groups 304 - ruthenium tetroxide 308 - secondary hydroxy groups 308 - selective 304 -with Mn(III) 308 - with nitrogen dioxide 305 - with periodate 304,309 - with phosphoric acid 305 - with sodium chlorite 311 - with sodium nitrite 305 oxidized cellulose 304 oxycelluloses 302,312 - poly electrolyte properties 312 -viscosity 312

paper making 260 Pd-en 81 permethylation 212 pervaporation membranes 132 phenylcellulose 263 phenyldimethylsilylcelluloses 274 phosphating agents 137 phosphonomethy!cellulose 261 phosphoromethylcellulose 249 phosphorylating agents 133 phosphorylation 133ff - cellulose acetate 136 - crosslinking 133 - H3PO4 and urea 134 - hydroxyethy !cellulose 136 - with ternary systems 134 photoconducting 273 photosensitive side chains 290

Subject index photosensitivity 292 phthaloylation 193 physical structure 40 polyelectrolytes 131, 160, 221, 232, 308 - anionic 131 polymer degradation 309 polymer skeleton 2ff - biosynthesis 2ff - chemical synthesis 2ff - enzymatic synthesis 2ff polymer-analogous reactions 319 polymerization 319 - enzyme-catalyzed 319 polyolato complex 75 pore and void structure 45, 60 preactivation 183, 251 preferential substitution at the C-6 194 process auxiliaries 260 propionylcellulose 182 - regioselectively substituted 182 propylation 213 protecting group 120, 175, 285 pulping 317

quaternization 258

radical grafting 260 raw material 316 reductive amination 310 regioselective functionalization 319 - enzymatically catalyzed 319 regioselectivity 271 ripening 154

salt stability 233 Schotten-Baumann reaction 146, 165 Schotten-Baumann-type reaction 192 selective membranes 323 separation membranes 255 silyl ethers 274 silylamine 278 silylation 59, 274f, 278f -control of DS 278

387

-DMA/LiCl 275 - heterogeneous 278 - homogeneous 279 - regioselective 275 - with chlorotrimethylsilane 274 - with hexamethyldisilazane 275 - with thexyldimethylchlorosilane 279 silylation reagents 274 6-O-sily !cellulose 174 silylcellulose 127 - sulfation 127 silylcellulose sulfation 127f -DS 127 - insertion reaction 128 - mechanism 127 silylethers 282, 284f - functionalization patterns 284 - structural characterization 282 - subsequent reactions 285 sodium cellulose 4Of -formation 41 sodium glycolate 221 solvent 246 - DMSO/paraformaldehyde 246 stereoregular reactions 319 structure formation 318 submodifications 317 substituent migration 319 sulfating agents 120 -reactivity 120 sulfation 11 off, 124, 126, 332 - chain degradation 117 - chlorosulfonic acid 119 - heterogeneous 116 - labile ether groups 124 -of CMC 124 - protecting group 119 - reaction rate 120 - reagent distribution 120 - regioselective 118 -SO3/DMF 118 - solvents 118 - sulfating reagents 116 - via nitrite groups 124 - via trialkylsilyl ether groups 126 sulfoalkyl ethers 260

388

Subject index

sulfoethylcellulose 249, 261 sulfomethy!cellulose 261 sulfopropy!cellulose 261 supramolecular architectures 322f supramolecular structures 290, 318 swelling 34ff, 52, 72, 90, 165, 171 - alkali uptake 35 -aqueous KOH 39 - chain conformation 38 - 13 C NMR spectrum 291 - ethanolic NaOH 39 - hydration shell 34, 36 - increase in fiber diameter 34 - NaOH ion dipoles 38 - steeping temperature 34 - swelling agents 52 - water uptake 36 t thermoreversible substitution 319 thexyldimethylchlorosilane 278 6-O-Thexyldimethylsilylcellulose 370 6-O-Thexyldimethylsilyl2,3-di-O-methylcellulose 372 thiocyanate route 321 TMS cellulose 137, 173, 192, 225, 274, 279ff, 286, 293f, 365, 367 - acylation 286 - DS range of the solubility 279 -LB films 293f - phosphorylation of 137 - properties 280 -solubility 281 -structure 280 - synthesis in DMA/LiCl 367 - synthesis in pyridine/THF 365 tosylate group 195 - leaving group 195 - protecting group 195 tosylcellulose 142, 173, 190, 195f - acylation 195 - heterogeneous procedure 195 - homogeneous procedure 195 - thermoanalytic characterization 196 transesterification 125, 141, 146, 165, 169, 183

transxanthation 154 trially!cellulose 214 tri-0-(/?-bromobenzyl)cellulose 268 4,4' ,4'' -trimethoxytriphenylmethyl groups 267 4,4' ,4 " -trimethoxy tritylcellulose 267 trimethy!cellulose 215 -WAXS 215 trimethylsilylcellulose methoxyacetate 373 - synthesis 373 triorganosilylcelluloses 276 - subsequent derivatives 276 triphenylcarbinol 268 triphenylmethylcellulose 263ff 2,4,5-tris(hydroxymethyl)1,3-dioxopentamethylene 310 tritylation 264ff, 269 -DMA/LiCl 269 - heterogeneous 264 - homogeneous 265 - regioselectivity 266 - selectivity 264 - with methoxy-substituted trityl chlorides 265 - with trityl chloride 265 6-O-tritylcellulose 265, 359 - preparation 265 - synthesis 359 tritylcellulose 263ff, 267 - subsequent reactions 267 two-dimensional 1W1H NMR 281

ultrathin films 293

viscose 51, 154 -ripening 154 viscose process 49f, 147, 321 - preripening 50 - slurry steeping process 50 - standard alkali cellulose 50 viscose ripening 158 viscosity 231

Subject index w water retention value 46 water uptake 52 WAXS 40ff, 55, 58,61, 108 WAXS see wide-angle X-ray scattering wide-angle X-ray scattering 4Off Williamson ether synthesis 208 Williamson etherification 221 WRV 61,65,245 WRV see water retention value

xanthation 149ff - heterogeneous 151 -kinetics 151 -mechanism 151 - mono- and polysaccharides 149 - various polysaccharides 150 xanthogenate 147, 150, 156f, 160 - applications 160

- consecutive reactions 157 - decomposition 147 - formation 147 - maximal DS 150 - model experiments 147 - pattern of substitution 156 -properties 160 - subsequent derivatization 156 xanthogenate decomposition 149 - rate 149 xanthogenate formation 147f - energy of activation 148 - industrial 147 - rate constant 148 xanthogenate group distribution 153 xanthogenation 339 xylans 258

Zincoxen 90

389

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