New Polymer Materials (Advances in Polymer Science,Vol.94)

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94

Advances in Polymer Science

New Polymer Materials With contributions by B. Boutevin, Y. Ikada, M. Irie, Y.Tabata, T Takekoshi

With 59 Figures and 27 Tables

Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo Hong Kong

ISBN-3-540-51547-X Springer-Verlag Berlin Heidelberg New York ISBN-0-387-51547-X Springer-Verlag New York Berlin Heidelberg Library of Congress Catalog Card Number 61-642 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions of the German Copyright Law of September 9, 1965, in its version of June 24, 1985, and a copyright free must always be paid. Violations fall under the prosecution act of the German Copyright Law. © Springer-Verlag Berlin Heidelberg 1990 Printed in Germany The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Bookbinding: Liideritz & Bauer, Berlin 2152/3020-543210 Printed on acid-free paper

Editors

Prof. Akihiro Abe, Tokyo Institute of Technology, Faculty of Engineering, Department of Polymer Chemistry, O-okayama, Meguro-ku, Tokyo 152, Japan Prof. Henri Benoit, CNRS, Centre de Recherches sur les Macromolecules, 6, rue Boussingault, 67083 Strasbourg Cedex, France Prof. Hans-Joachim Cantow, Institut ftir Makromolekulare Chemie der Universit~it, Stefan-Meier-Str. 31, 7800 Freiburg i. Br., FRG Prof. Paolo Corradini, Universith di Napoli, Dipartimento di Chimica, Via Mezzocannone 4, 80134 Napoli, Italy Prof. Karel Du~ek, Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague 616, (~SSR Prof. Sam Edwards, University of Cambridge, Department of Physics, Cavendish Laboratory, Madingley Road, Cambridge CB30HE, England Prof. Hiroshi Fujita, 35 Shimotakedono-cho, Shichiku, Kita-ku, Kyoto 603, Japan Prof. Gisela Henrici-Oliv6, 1332 Neal Road, Cantonment, Florida 32533, U.S.A. Prof. Dr. Hartwig H6cker, Deutsches Wollforschungs-Institut e. V. an der Technischen Hochschule Aachen, Veltmanplatz 8, 5100 Aachen, FRG Prof. Hans-Henning Kausch, Laboratoire de Polym6res, Ecole Polytechnique F6d6rale de Lausanne, 32, ch. de Bellerive, 1007 Lausanne, Switzerland Prof. Joseph P. Kennedy, Institute of Polymer Science. The University of Akron, Akron, Ohio 44325, U.S.A. Prof. Anthony Ledwith, Pilkington Brothers plc, R&D Laboratories, Lathom Ormskirk, Lancashire L40 5UF, U.K. Prof. Seizo Okamura, No. 24, Minamigoshi-Machi Okazaki, Sakyo-Ku, Kyoto 606, Japan Prof. Salvador 'Oliv6, 1332 Neal Road, Cantonment, Florida 32533, U.S.A. Prof. Charles G. Overberger, Department of Chemistry. The University of Michigan, Ann Arbor, Michigan 48 109, U.S.A. Prof. Helmut Ringsdorf, Institut ffir Organische Chemic, Johannes-GutenbergUniversit~it,J.-J.-Becher Weg 18-20, 6500 Mainz, FRG Prof. Takeo Saegusa, Department of Synthetic Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Kyoto, Japan Prof. J. C. Salamone, University of Lowell, Department of Chemistry, College of Pure and Applied Science, One University Avenue, Lowell, MA 01854, U.S.A. Prof. John L. Schrag, University of Wisconsin, Department of Chemistry, 1101 University Avenue. Madison, Wisconsin 53706. U.S.A. Prof. John K. Stille, Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, U.S.A. Prof. Dr. G. Wegner, Max-Planck-Institut fiir Polymerforschung, Ackermannweg 10, Postfach 3148, 6500 Mainz, FRG

Prof. Dr. Giinther Heublein, 11.12. 1933--17. 7. 1989

Giinther Heublein Giinther Heublein, Professor of Organic Chemistry, FriedrichSchiller Universit/it Jena, died of cancer on July 17, 1989. Prof. Heublein studied Chemistry at the University of Jena where he also received his P h . D . in 1961 and Dr. sc. nat. in 1967. He worked under the guidance of Prof. Drefahl and started his scientific career with contributions to the stereochemistry of diastereomers, with investigations of the effect of EDA complexes in organic reactions, and the detection of reactive intermediates. In 1970, he entered the field of Polymer Chemistry and devoted his efforts to cationic polymerisation. This decission and the following period are characteristic of his attitude towards science: he always was eager to find the topics of current importance, ready to learn over and over again, and enthusiastic to perform thorough work in the selected field. Extensive investigations were made on the influence of donors and acceptors on the course of cationic polymerisation of vinyl monomers and the acceptor effect on molecular weight and copolymer composition. Research on the selective polymerisation of olefins from technical C4-fractions led to more than 20 patents which are applied in several countries. Prof. Heublein held the chair of Organic Chemistry from 1968. He was author or co-author of more than 300 scientific papers, of 4 books, and held more than 70 patents. He worked actively on many scientific commissions and was member of the International Steering Committee of the International Symposium on Cationic Polymerisation, the 7th of which took place with great success in Jena in 1985. From 1981 he was on the editorial board of the J. Macromol. Sci.-Chem.'and, f r o m 1983, one of the editors of Advances in Polymer Science. Those who knew him are grateful for the standards he set and keep alive the memory and name of a distinguished scientist and a good man. Gottfried G16ckner

Table of Contents

Polyimides T. Takekoshi

. . . . . . . . . . . . . . . . . . . . .

Photoresponsive Polymers M. Irie

. . . . . . . . . . . . . . . . . . . . . . .

27

Telechelic Oligomers by Radical Reactions B. B o u t e v i n

. . . . . . . . . . . . . . . . . . . . .

69

Phagocytosis of Polymer Microspheres by Macrophages Y. Tabata, Y. Ikada . . . . . . . . . . . . . . . . . .

107

Author Index Volumes 1-94

. . . . . . . . . . . . . .

143

. . . . . . . . . . . . . . . . . . . .

157

Subject Index .

Polyimides T o h r u Takekoshi General Electric, Corporate Research and Development, Schenectady, N Y 12301, USA

A survey of scientific and patent literature in the last 5-10 years is presented. The science and technology on polyimides have been rapidly diversified as minor but important cummulative technical breakthroughs have been made. Special emphasis is given to the area of processable polyimides and polyimide matrix resins for structural composites.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

1 New Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Diamines . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Dianhydride . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 6

2 Polyimides with Improved Proeessability . . . . . . . . . . . . . . . .

9

3 Crystalline Polyimides . . . . . . . . . . . . . . . . . . . . . . . .

11

4 Polyimides for Structural Composites . . . . . . . . . . . . 4.1 Acetylene Terminated Polyimides . . . . . . . . . . . . 4.2 PMR-15 . . . . . . . . . . . . . . . . . . . . . . . 4.3 Bismaleimides . . . . . . . . . . . . . . . . . . . . 4.4 Biphenylene Terminated Oligoimides . . . . . . . . . . 4.5 Polyimides with Benzocyclobutene Groups . . . . . . . .

. . . . . .

13 14 16 16 17

5 Polyimide Forms . . . . . . . . . . . . . . . . . . . . . . . . . .

19

6 Polyimides for Electronic Applications . . . . . . . . . . . . . . . . .

20

7 Polyimides with Other Specific Properties

. . . . . . . . . . . . . . .

21

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

Introduction

8 Conclusion

9 Acknowledgement 10 References

. . . . . .

. . . . . .

. . . . . .

. . . . . .

13

. . . . . . . . . . . . . . . . . . . . . . . . .

22

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

Advances in Polymer Science 94 Springer-Verlag Berlin Heidelberg 1990

2

T. Takekoshi

Introduction Since the first commercialization of Kapton polyimide nearly three decades ago, an impressive number of new polyimides of unique properties have been introduced for various industrial and consumer product applications. Commercial and semi-commercial production of numerous new monomers have made it possible to explore seemingly infinite varieties of structurally different polyimides and copolyimides. As a result we seem to have a more finely tuned understanding of structure-property relationship necessary for a specific application. Research and development activities in new polyimides seem to be intensifying rather than reaching its maturity. In this article, recent advances in polyimides are reviewed in the area of scientific activity as well as commercial developments.

New Monomers One of the most important developments in the last decade was the increasing need for polyimides with various specific properties such as improved processability. New monomers were developed to meet such needs. New monomers as well as monomers which have become available recently because of new improved processes, are discussed below.

1.1 Diamines Bell et al. [I] synthesized an exhaustive number of position isomers of diaminodiphenylmethane (MDA), diaminodiphenyl ether (DDE) and diaminobenzophenone (DBP) and demonstrated that the meta linked diamines gave polyimides with markedly lower glass transition temperatures (Tg) than those ofpara-linked structures. Interestingly, ortho-linked structures were ineffective in decreasing Tg probably because of possible steric restriction imposed on the rotational freedom of main chains. Melt fusible LARC-TPI developed by NASA is based on this discovery and is composed of 3,3'-DBP and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). Diamines containing one or more stable linkages between aromatic rings such as - - O - - , - - S - - , - - 8 0 2 - , - - C O -N, --C(CH3)z--, and - - C ( C F 3 ) z - - , have been used to synthesize many tractable polyimides. Some of the standard procedures to prepare these diamines are described below. (a) Reaction of activated bishaloaromatic compounds and aminophenolates yields bis(aminophenoxy)arylene for example [2, 3].

2

H

Y

+

H2N~ - ~ - /

Z = - S O 2 - - , --CO--,

~

etc.

~

X = F, C1

"~-~-/'NHz

Polyimides

3

(b) Friedel-Crafts reaction of nitrobenzoyl chloride with nucleophilic aromatic ether gives bis(nitrobenzoyl)arylene ethers which can be readily reduced to the corresponding diamines [4].

Y = --O--,

--O--Ar--O--,

etc.

(c) Nucleophilic displacement reaction of 4-halonitrobenzene with various bisphenolates affords bis(4-nitrophenyl)arylene ethers which can be hydrogenated to the diamines [5].

20=~!

+ eo-Ar-O®

> Oz~O-Ar-O~NOz Hz > H 2 N ~ O _ A r _ O ~ H

z

Bis(3-aminophenyl)arytene ethers are useful monomers to prepare polyimides of lower Tg's. For example, 1,3-bis(3-aminophenoxy)benzene has been used as building blocks for various thermally processable PI systems. The compound was originaly synthesized by Fink [6] according to the following Ullmann synthesis under rather harsh conditions.

The improved synthetic method was reported [7] in which readily available m-dinitrobenzene was subjected to nitro-displacement reaction with various bisphenols. The resulting bis(3-nitrophenyl)arylene ethers were hydrogenated to afford bis(3aminophenyl)arylene ethers.

2

+ ®O-A r-O®

DMF Hz

r

Ortho or para activated nitro aromatic compounds have been known to undergo nitro-displacement reaction to form various ortho and para substituted aromatic ethers [8]. In the above case, the powerful electron withdrawing effect of one nitro group activated the other nitro group although they are meta to each other. Other meta substituted bis(aminophenyl)arylene ethers reported are 1,3-bis(3-amino-

4

T. Takekoshi

phenoxy)-5-chlorobenzene [9], 2,6-bis(3-aminophenoxy)pyridine [10], and 2,2-bis(aminophenoxy)biphenyls [1 t]. Also reported are diamines containing other heterocyclic rings such as 2,5-bis(4aminophenyl)pyrazine [12], 4,4'-diaminobipyridyl [13], N,N'-bis(4-aminophenyl)piperazine [14], and 2,5-bis(4-aminophenyl)-3,4-diphenylthiophene [15]. Perfluoroalkylene bridged diamines are described with increasing frequency, included in this group are 2,2-bis(3 or 4-aminophenyl)hexafluoropropane [16, 17] and 1,I-bis(4-aminophenyl)-l-phenyl-2,2,2-trifluoroethane [18, 19]. Polyimides derived from cycloalkylene bridged diamines such as 5-amino-3-(4aminophenyl)-l,l,3-trimethyldihydroindene [20] and bis-(3-aminophenoxy)spirobiindane [21] are reported to exhibit good thermal stability and processability. Diamines with bulky substituents such as phenyl have been used to increase the solubility of resulting polyimides [19, 22]. Other diamines listed in Table t, will be discussed in the following sections. TaMe 1. List of aromatic diamines

Diamine

References

B]

H'N~~z~H'

[2, 3]

[4] [53]

[6, 7]

[8]

H z ~

H2

H 2 ~

H2

[91

[lO]

[12]

Polyimides Table 1. continued Diamine

References

H 2 ~ H 2

[13] 114]

H 2 ~ H z

[15] [16, 17]

CF3"~,.-~-/

[18, 19]

F~

[9o1

H z ~ H 2

I2o]

Hz@~ H2

[21]

Hz~Hz

[22]

H z N ~

.

,

/'-'~

Hz

[49]

[5l]

[44] H

[58]

Hz

H3~

H., H2

H ~ CCH3 H3

Hz Hzf,,l.(( I)_I~IH z H~(~ CH~

[1 lO]

6

T. Takekoshi

1.2 Dianhydrides Generally, synthesis of dianhydrides is somewhat more complex than that of diamines and until recent time pyromellitic dianhydride (PMDA) and benzophenone-3,3',4,4'tetracarboxylic dianhydride (BTDA) had been the only commercially produced aromatic dianhydrides. Some of the significant commercial products developed recently, are Upilex by UBE Ind. and Ultem by General Electric. The former is based on biphenyl-3,Y,4,4'-tetracarboxylic dianhydride (BPDA) and the latter on 2,2bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride or bisphenol A dianhydride (BPA-DA). BPDA is produced by oxidative coupling of inexpensive phthalic acid esters in the presence of palladium catalyst [23, 24].

~

OzR

O= Pd

OzR

Another process reported is based on dechloro coupling of 4-chlorophthalic acid salt in the presence of a palladium catalyst and sodium formate [25]. Synthesis of various ether containing dianhydrides, i.e. bis(ether anhydride)s was pioneered by Koton and Florinski [26] as shown by the following reaction scheme. 2 B

r•H3

H~C~O-Ar-O~ + HO-Ar-OH

H3 H

KOH/Cu >

H~ KMnO4

:>HO2C~-Ar HO=C-~-~

~ ' ~

O2H

~_../~C OzH

However, less than quantitative yields of reactions and difficulty in purification of the product hampered the practical use of this scheme. The same compounds can be prepared by "nitro displacement" of 3 or 4-nitrophthalimide with difunctional phenol salts. In this reaction, the nitro group is displaced and leaves as a nitrite ion [27]. The products bis(etherimide)s are hydrolyzed to the tetracarboxylic acids which are in turn converted to the dianhydrides [28].

>

It

-)-]-O-Ar-O-t-(-- / I

HO,CJ j

H

Bis(ether anhydride)s

Polymides

7

Similarly, nitro displacement of 4-nitrophthalimide with bissulfinates afforded bis(sulfoneimide)s from which bis(sulfoneanhydride)s were produced [29].

2 RN 1(~ ) I * eOzS-Ar-SOzE>

~o,

SOi-Ar--S

OcAr-S Nucleophilic displacement of hexafluorobenzene with 4-hydroxy-N-methylphthalimide followed by hydrolysis and cyclization gave tetrafluorophenylenedioxybis(phthalic anhydride) [30].

C H 3 ~ C H 3

O

)-

Field et al [31] reported synthesis of fused aromatic system dibenzoanthracenetetracarboxylic dianhydride by interesting photo-oxidative cyclization.

hu -Hz

Synthesis of trifunctional heteroaromatic anhydride was reported by Kanakarajan et al [32, 33]. Other dianhydrides listed ~n Table 2, will be discussed in the following sections.

T. Takekoshi

8

Table 2. List of dianhydrides Dianhydride

Reference

[23, 251

O•O-Ar-O• ~~'Oz-Ar-S~

[26, 28]

[291

[3ol

[31]

0

[32, 331

[44]

[45, 46]

Polyimides Table 2. continued Dianhydride

Reference

[471

[112, 114]

2 Polyimides with Improved Processability High performance characteristics of polyimides are represented by high mechanical strength and modulus, extraordinary non-flammability, excellent electrical properties and solvent resistance, as well as outstanding thermoxidative properties. However, extremely high glass transition temperatures of earlier polyimides associated with rigid fused heteroaromatic ring systems hampered development of various applications other than films and coatings in which the elegant two step polyamic acid process [34] was used. Researchers in the USSR [35] described earlier systematic observations that introduction of flexible linkages into polyimide main chains significantly lowered glass transition temperatures without greatly sacrificing thermal stability. However, a convenient and practical process to'synthesize such structures was not available until recent times. Nitro-displacement process developed at General Electric afforded a family of polyetherimides represented by Ultem 1000 resin whose structure is shown below.

-•N

H3

The polymer has a glass transition temperature of 217 °C and TGA weight loss of only 1% at 460 °C in air at a heating rate of 10 °C/min. Excellent melt stability over a wide process window is attributed to the completely imidized structure and stable chain end capping. Such control of the structure is attainable only by improved polymerization processes which are governed by procassability of the polymer itself. Polyetherimides (PEI) were synthesized from bis(ether anhydride)s described in the preceding chapter with various diamines. Because of excellent high temperature solubility and melt stability, PEI are excellent systems to demonstrate various alternative polycondensation processes applicable to the polyimide formation. PEI have been prepared by one-step high temperature solution polymerization in phenolic solvents [36] or

10

T. Takekoshi

in non-polar solvents [37] as well as solventless melt polymerization [38]. The polymerization has been also performed by amine-imide exchange [39] and ether-phenol exchange [40] reactions in melt condition. PEI can be also prepared by the nitrodisplacement reaction between bisphenol salts and bis(nitroimide)s in dipolar solvents [27, 41] as shown by the following equation.

~--~N-Ar--I~'~

+ eO-ArL'Oe

°

Upilex Type R by UBE Ind. is produced from BPDA and ODA. It is based on a unique combination of the new monomer synthesis described in the preceding section and one step high temperature solution polymerization in a phenolic solvent [42]. High quality films and fibers can be produced from the solution because a water forming reaction is not involved [43]. The polymers produced by such a process have a completely imidized structure and provide for superior properties than polymers prepared by solid state imidization ofpolyamic acids. For example long term oxidative and hydrolytic stabilities and retention of electrical properties are substantially better. One of the effective methods to improve the solubility is to introduce bulky nonpolar substituents onto polymer chains. Harris [44] prepared dianhydrides highly substituted with phenyl groups as shown below.

Ar+2

r

The polyimides derived from the dianhydrides had high Tg's of 238 to 466 °C and they were soluble in chlorinated hydrocarbon solvents. Because of their solubility, copolyimides containing the following acetylene substituted diamine monomers were readily prepared by homogeneous high temperature solution polycondensation in m-cresol.

~

C

H

Polyimides

11

Alston and Grants [19] described the structure-property relationship of polyimides derived from phenyl substituted monomers including the following dianhydride.

Woo [45] synthesized soluble polyimides with high Tg (225-285 °C) from 1,4phenylenebis[2-(2-alkylsuccinic anhydride)].

Teshirogi [46] also reported polyimides derived from the similar bis(succinic anhidride) and bis(glutaric anhydride) shown below.

Sillion et al [47] prepared polyimides containing benzhydrol groups by using the reduction product of benzophenonetetracarboxylic acid dimethyl ester shown below.

CH302C-~ ~~O2CH3 HOzC, ~.--~-J OH ~COzH The polyimides were soluble in dipolar solvents such as NMP. Interestingly, the polyimides were readily transformed to non-hydroxylated polymers via a redox disproportionation as illustrated below.

+ H20 Improvement of processability is also a key technology involved in melt processable linear crystalline polyimides, matrix resins for structural composites, and other new applications which will be discussed in the following sections.

3 Crystalline Polyimides Melt processable, crystalline, high temperature polymers such as polyetheretherketone (PEEK) have been developed for critical application areas where processability, chemical resistance and toughness are required. PEEK has been applied as thermo-

12

T. Takekoshi

plastic matrix resin in graphite fiber composites to impart high fracture toughness. Crystalline polyimides are expected to fit into similar areas of application with an additional advantage of having various process options. However, the majority of polyimides with high Tg are noncrystalline polymers. Polypyromellitimides such as Kapton generally show little crystallinity. The lack of significant crystatlinity may be primarily attributed to the immobility of rigid main chains at the process temperatures. The insertion of increased number of ether groups in the main chains improves such chain mobility as represented by decreased Tg. Earlier investigations indicated that some of these ether-containing polyimides showed crystallinity as shown below [35].

At:

Ar :

Recently St. Clairs [48] reported that polyimides prepared from a bis(ether anhydride) and p-phenylenediamine and benzidine were crystalline as shown below

Ar

=

Harris et al. [49] prepared a highly crystalline polyimides from oxybisphthalic anhydride and 1,2-bis(4-aminophenoxy)ethane as shown below.

i

--

1,2

Polyimides

13

However, melting temperatures of the above polyimides are well over 400 °C and expected to be difficult to process. More recently it has been shown that some of the polyetherimides derived from bis(ether anhydride)s and aromatic diamines containing more than one ether group show crystallinity and that their melting temperatures are in a range of 300 to 400 °C [36, 50]. Because of high symmetry and low Tg, polyphenylene sulfide has a great tendency to crystallize. Polyimides derived from aromatic diamines containing cummulative phenylene sulfide groups as shown below, were also found to crystallize in a moderate temperature range of 300 to 400 °C [51].

LARK-TPI developed by NASA is a melt procesable polyimide derived from BTDA and 3,3'-diaminobenzophenone and useful as a high temperature adhesive [1].

Characterization of this polyketoimide indicated that the properties of the sample varied according to annealing conditions because of partial crystallization [52]. Hergenrother et al [53] prepared diamines containing both ether and keto groups. Several polyimides derived from these diamines had melting temperatures of 350 to 440 °C.

4 Polyimides for Structural Composites 4.1 Acetylene Terminated Polyimides Thermid MC-600, a low melting oligoimide with acetylene end groups was first developed at the Hughes Aircraft Co. [54, 55] and is now produced by the National Starch and Chemical Corp. The structure is based on all m e t a linked diamine 1,3bis(3-aminophenoxy)benzene for better processability.

On heating, it cures by addition reaction with little off-gassing to give low void cured resins which have 315 °C service temperature. The reaction mechanism of the cross linking is complex. A part of the acetylene groups trimerize to form a benzene ring

14

T. Takekoshi

but the major part dimerize to form eneyne structure which is believe to cycloaromatize on further heating [56, 57].

Preformed eneyne could be built into linear polyimides using 1,4-bis(3-aminophenyl)1-buten-3-yne.

The polyimides crosslinked on heating at around 270 °C [58]. Thermid IP-600 series are the isoimide form of MC-600 whose structure is shown below.

Polyisoimides have better solubility and improved melt flow characteristics [59, 60]. The isoimide groups rearrange to imides on heating above 300 °C. Better processability is also achieved by substituting hexaffuoroisopropylidene-bis(phthalic anhydride) (6FDA) for BTDA, Thermid FA-700 [61-63]. Siloxane containing oligoimides with acetylene end groups were reported to exhibit good processability and good adhesive properties [64].

4.2 PMR-15 PMR-15 is a state of the art thermoset potyimide mainly used as a matrix resin for graphite fiber composites in the aerospace industry. It was originally developed at NASA [65] and represents Polymerization from Monomeric Reactants with a molecular weight of 1500. Composite prepregs are fabricated with a mixture containing dimethyl ester of benzophenonetetracarboxylic acid (BTDE), monomethyl ester of norbornenedicarboxylic acid (NE) and MDA in methanol. The chemistry of PMR-15 is illustrated in Fig. t. Usage of readily removable solvent and a wide process window make this resin the material of choice in aerospace application. However, the actual chemistry is complex and its quality control is difficult. Johnston et al. [66] showed that the intermediate amic acid was formed via anhydride which was formed by reverse cyclization of acid ester groups rather than direct ester aminolysis.

Polyimides

15

r O'H 1

L'-~-~O,CHd

+

L

n+1

HO2C~

~.,,~-'~CO:H

_t

A I-CH30H

A I "H:O I

A

/ i

+

n = ~'.o87

Crossllnked polymer Fig. I. PMR-15 polyimide resin system

One of the difficult problems is the analysis of the PMR-15 monomeric mixture for reliable quality control [67]. During the production of BTDE, excessive heating can produce some triester (BTTE). Lauber [68] demonstrated that the presence of BTTE led to final composites of inferior properties. More recently Roberts et al. [69, 70] developed a precise and sensitive HPLC method to analyze the PMR-15 momomer mixture. The results greatly helped to elucidate the complex chemical transformation of PMR-15 resin. Further increase of use-temperature up to 371 °C was achieved by use of thermally more stable diamines in place of MDA [71]. A new thermoset polyimide introduced by Ciba-Geigy is related to PMR-15. The oligomer is end capped with allylnorbornenedicarboxyl anhydride.

CI.~CI+C~''~

It is reported to have epoxy-like good processability and high reactivity [72-74].

16

T. Takekoshi

4.3 Bismaleimides Bismaleimide of MDA (Kerimid 601) was introduced by Rhone-Poulenc around 1970 [75]. The resin cures on heating via Michael addition of amino groups to maleimide unsaturations and polyaddition of the maleimide group itself. An attractive feature of bismaleimide resin (BMI) is its relatively low cure temperature. However, brittleness and micro crack formation in cured products are some of the problems attributed to very high cross link density. Another problem is a relatively high melting temperature of the uncured resin resulting in a narrow process window. Elimination of the solvent, thus hot melt application is used for some of the newer BMI resins. The brittleness can be improved by addition of rubbers (Compimid 453, Boots Technochemie) or with other components [76-79]. Effect of rubber toughened BMI resin was described by Shaw [80]. Bell et al [81] prepared bismaleimides from less toxic isomers of MDA. Attempts to make more processable BMI such as shown below, are found in much of the patent literature [82-84].

Process characteristics of various BMI resins are reviewed by Briscotl and Walton [85]. Difluoromaleimides showed improved thermal stability and ease of curing [86].

4.4 Biphenylene Terminated Oligoimides Stille et al. [87] introduced biphenylene as reactive end groups capable of being cured at elevated temperatures via an addition mechanism. When heated above 350 °C, biphenylene undergoes a thermal ring opening to form primarily its cyclic dimer, tetrabenzocyclooctatetraene.

Polyimides

17

In the presence of aromatic polymers, it reacts to form biphenylated aromatics as major products. Biphenylene end-capped polyimide oligomers were prepared using PMR technique and its curing poperties were investigated [88].

Ro,

The graphite composite fabricated from this resin and cured at 390 °C indicated that the molecular weight increased but crosslinking efficiency was low. The result was improved by use of a trifunctional amine to achieve increased crosstinking density. Biphenylene end groups were found to react at 350-400 °C with internal 1,2-diarylacetylene group to form phenanthrene crosslink [89].

The processability of the system was improved by using hexafluoropropylidene connecting groups such as 6FDA and 2,2-bis-[4-(4-aminophenoxy)phenyl]hexafluoropropane shown below [90].

Low molecular weight bisbiphenylene compounds were used as a plasticizer for various polymers including polyimides. The plasticized polymers were cured on heating [91].

4.5 Polyimides with BenzocyclobubeneGroups Benzocyclobutene (BCB) polymerizes at much lower temperatures of about 250 °C than the closely related biphenylene. The reaction undergoes electrocyclic ring opening to form o-xylylene which polymerizes to produce cyclic dimer and poly-o-xylylene [92].

18

T. Takekoshi

Tan and Arnold [92] synthesized bis(benzocyclobuteneimide)s of the following structures. ×=

The compounds cured thermally to form crosslinked polymers with 343 °C stability in air. The curing characteristics of oligoimides end-capped with aminobenzocyclobutene were also described [93]. o-Xylylene undergoes the Diels-Alder reaction with various dienophiles as shown below.

Monomers containing both BCB and acetylene groups were shown to undergo polyaddition via Diels-Alder cyclization [94].

Similarly, mixtures of imide oligomers containing BCB groups and those containing maleimide groups were shown to polymerize at moderate temperatures of 20t)-250 °C to give thermally stable polymers [95].

Potyimides

19

Mixtures of bisbenzocyclobutene and bismaleimide such as shown below were also polymerized to form stable polymers.

d~F3

v

The system provided for a wide process window and various crosslink densities according to the variation in the above monomer ratios [96]. Properties of mixtures containing benzocyclobutene end-capped oligoimides and bismaleimides were reported by Denny et al. [78]. Cyclopolymerization of bis(benzocyclobutene)s with other difunctional dienophiles such as aromatic biscyanates and bisacetylenes were also described [97]. A similar system, a p-xylylene dimer namely p-cyclophene was also explored for reactive end groups. Boldwin et al. [98] prepared oligoimides shown below.

The oligomer cured at significantly higher temperatures of 270--330 °C to give thermally stable resin.

5 Polyimide Foams The current polymer foam market is dominated by gas imbibed polystyrene, polyolefins and polyurethanes used in consumer products, packagings and construction markets. However, there is a growing market for high performance foam products. Insulation materials for aerospace, transportation and some construction industries are required to have higher temperature performance and non-flammability. In this respect polyimides are naturally the material of choice. Thermoplastic polyetherimide Ultem can be formulated with high temperature foaming agents such as 5-phenyldihydro-l,3,4-oxadiazinone to produce structural form products [99] by injection molding. PEI could be readily imbibed with various volatile organic compounds such as chlorinated hydrocarbons [100] and acetone [101].

20

T. Takekoshi

Polyimides have been prepared by the reaction of dianhydrides with diisocyanates. Carbon dioxide is the condensation byproduct in this reaction and serves as foaming gas [102-104].

O

Ar

+ OCN_Ar~NCO

~

Ar

Ar

+ CO2

With use of appropriate catalysts, mixtures of dianhydrides and diisocyanates were directly converted to rigid foam products without use of foaming agents [105, 106]. In situ formation of imide forms like the polyurethane process, was developed by Riccitietlo et al. [107]. The system contains a unique furfuryl alcohol-acid mixture which generates heat to initiate the isocyanate-anhydride reaction. An alternate process to produce polyimide foam is based •n PMR technique. A mixture containing diester of BTDA (BTDE) and diamines is heated in a closed mold. The imidization takes place with formation of methanol and water vapor which serve as foaming gas [108, 109].

6 Polyimides for Electronic Applications Polyimides including those already described above are used as insulators, passivation coatings, alpha particle barriers, planarization coatings and even as part of IC elements as interlayer dielectrics. Photo-imagable polyimides could be applied in microelectronic systems in the same manner as photoresists are used. Several systems developed are based on polyamic acids solution in which some of the carboxyt groups are derivatized to photo-sensitive groups. Some of the shortcomings are complex process steps, low photo sensitivity and most importantly the unstable shelf life of such polyamic acid solutions. In this review, the topics will be limited to some important basic technical progress reported recently. A unique photo-imagable polyimide, not intermediates but completely imidized form, was described by Pfeifer and Rhode [110]. The polyimides were prepared from BTDA and diamines whose ortho positions to the amino groups were extensively substituted with alkyl groups.

Tg = 384°C

Tg = 439°C

Importantly, a high degree of methyl substitution increased both solubility and Tg of the polyimides. Non-planner conformations and restricted rotational motion of the main chain are the probable cause of such effects. The polyimide solutions can be readily applied and have exellent film forming properties. Because of high Tg they are very stable under harsh processing conditions. In addition, the films showed high photo sensitivity and good contrast.

Polyimides

21

Another significant development is the recognition that certain polyimides exhibit very low values of the coefficient of linear thermal expansion (CLTE). In constructing electronic devices, matching CLTE of polyimides to those of substrate materials such as metals and ceramics is obviously important to avoid formation of serious internal stresses. Numata et al. [111] prepared polyimides from PMDA and BPDA with aromatic diamines which were composed of single or cummulative 1,4-phenylene and 2,5pyridinediyl structures. The polymers showed generally very low CLTE of below 2 x 10 -5 °C -1. One of these polyimides had a CLTE value of 4 x 10 -7 °C -1. Polyimides derived from 3,3",4,4"-tetracarboxy-p-terphenyl dianhydride were also shown to have very low CLTE [112, 113]. Upilex S Type which is primarily composed of BPDA and p-phenylenediamine was reported to have a very good dimensional stability and very low CLTE [114].

7 Polyimides with Other Specific Properties Earlier commercial polyimides derived from PMDA and BTDA are usually colored in deep yellow or yellowish orange. When two anhydride groups of diananhydrides are separated into two phthalic anhydride groups and linked together with sp3 carbon atom such as --C(CF3)2-- group or hetero atoms such as --O--, --S--, and --SO2--, they tend to produce polyimides of significantly lighter color. Quantitative comparison on degree of coloration is generally difficult because many factors cause coloration of polyimides other than the structure of polymer repeating units. The majority of dianhydrides are relatively difficult to purify to a colorless state. Most of the aromatic diamines are very sensitive to air oxidation and the purification by distillation is limited to those with low molecular weight. Because polyimide preparation usually involves a high temperature process exceeding 200 °C, it is difficult to prevent some oxidation of the amino group. In the case of BTDA, formation of an imino group may take place, particularly in the presence of an excess of diamine, by the reaction between the keto group and the amine.

Aromatic imines are usually intensely yellow. Other possible byproducts which may be formed in the polyimide system are the previously discussed isoimide and imideimine formed by the following reaction.

22

T. Takekoshi

Both isoimides and imideimines have a deep yellow color. Despite those difficulties, St. Clare et al. [115, 116] determined the qualitative relationship between the color of polyimides and the structures of diamines and dianhydrides from which they are derived. According to the above authors, bulky electron withdrawing groups such a s - - S O 2 - - , and --C(CF3)2 - and also m e t a linked aromatic ether structures in diamine moieties provided for light color and an increase in solubility. The effects were attributed to the reduced charge transfer interaction. However, simple disruption of conjugated unsaturation extending the imide groups may be considered as the major reason. According to our observation, the majority of polyetherimides which do not contain electron withdrawing groups were nearly colorless [36]. Polyimides with a low level of color are important in some critical applications such as lightweight flexible solar radiation reflectors, protective coating for solar cells, liquid crystal orientation films in LC display devices, LC light shutters, etc. Hasuda et al. [117, 118] described the construction of multi-layer thermal control structures for communication satellites. The outer layer was made of silvered Ultem 1000 polyetherimide film. The material was chosen because of good thermal radiation characteristics and a low solar radiation absorbance as well as excellent stability against solar radiation. Bilow et al. [ 17] described polyimides prepared from hexafluoroisopropylidene-4,4'-bis(phthalic anhydride) and 2,2'-bis(3-aminophenyl)propane as a protective coating for solar cells positioned in space. The coating has good transparency and exellent stability against heat, UV, and low energy particles. Recently, numerous patent disclosures have appeared in the area of colorless polyimides for LC alignment control films [119].

8 Conclusion The research and development efforts on polyimides have been intensified in recent years. Exploration of various new monomers and new synthetic methods as well as new polymer processes have made it possible to develop new applications in important high technology markets. Backed by high added value of new materials in sophisticated industrial societies, research and development activities in the field of polyimides seem destined to continue prospering.

9 Acknowledgement The author would like to express his sincere gratitude to Professor Charles G. Overberger of the University of Michigan for his support and encouragement.

10 References 1. Bell VL, Stump BL, Gager H (1975) J. Polym. Sci., Polym. Chem. Ed. 14:2275 2. Kawakami JH, Brode GL, KwiatokowskiGT, BedwinAW (1974)J. Polym. Sci, Polym.Chem. Ed. 12:565

Polyimides 3. 4. 5. 6. 7.

23

Hergenrother PM, Watelyn NT, Havens SJ (1987) J. Polym. Sci., Part A, Polym. Chem. 25 : 1093 Schoenberg JE, Anderson SP (1983) US Pat. 4,405,770 to National Starch & Chem. Williams AL, Kinney RE, Bridger RF (1967) J. Org. Chem., 32:2501 Fink W (1968) Herv. Chim. Acta, 51 : 954 Jap. Pat. Disclosures 62-05035, 62-068817, 62-270636, Intl. Pat. 8701378 (1987) to Mitsui Toatsu 8. Takekoshi T (1987) Polymer J. 19:191 9. Ger. Pat. 3 429 903, Jap. Pat. Discl. 62-116 547 (1987) to Mit~ui Toatsu. 10. Jap. Pat. Discl. (1987) 62-116563 to Mitsui Toatsu. 11. Belg. Pat. (1977) 855653 to Ciba-Geigy. 12. Jap. Pat. Discl. (1987) 62-270623 to Daicel Chem. 13. Jap. Pat. Discl. (1986) 61-181828 to Hitachi. 14. Lee TB, Feld WA (1988) Am. Chem. Soc., Potym. Prepr. 29:2t4 15. Imai Y, Maldar, NV Kakimoto M (1984) J. Polym. Sci., Chem. Ed. 22:2189 16. Lau KSY, Landis AL, Kelleghan WJ, Beard CD (1982) J. Polym. Sci., Polym. Chem. Ed. 20: 2381 17. US Pat. 4,592,925 (1986) and 4,645,824 (1987) to Hughes Aircraft Co~ 18. Kray WD, Rosser RW (1977) J. Org. Chem. 42:1t86 19. Alston WB, Gratz RF (1987) In: Weber, WD, Gupta MR (eds) Recent advances in polyimide science and technology", Soc. Plast. Eng., p 1. 20. Cobuzzi CA, Chaudhari MA (1985) Natl. SAMPE Tech. Conf., 17:318 21. Jap. Pat. Discl. 62-050375 (1987) to Mitsui Toatsu 22. Gannett TP, Gibbs HH (1986) US Pat. 4,576,857 to Du Pont 23. Itatani H, Yoshimoto H (1973) J. Org. Chem., 38:76 24. Kajima M et al. (1973) J. Catalysis 29:92 25. Shoji F, Kataoka F (1986) Jap. Pat Discl. 61-167642 26. Koton MM, Florinski FS (1968) Zhur. Org. Khim. 4:774 27. Takekoshi T, Wirth JG, Heath DR, Kochanowski JE, Manello JS, Webber MJ (1980) J. Polym. Sci., Polym. Chem. Ed. 18:3069 28. Takekoshi T, Kochanowski JE, Manello JS (1985) J. Polym. Sci., Polym. Chem. Ed. 23:1759 29. Europ. Pat. 245729 (1987) to Bayer 30. USSR Pat. (1985), 1 100868 31. Fields EK, Wisenburg ML, Behrend SJ (1987) US Pat. 4,638,072 to Standard Oil (Indiana) 32. Kanakarayan K, Czarnik AW (1986) J. Org. Chem. 51:5241 33. Kanakarayan K, Czarnik AW (1988) Am. Chem. Soc., Polym. Prepr. 29:246 34. Sroog CE, Endrey AL, Abramo SV, Bert CE, Edwards WM, Olivier KL (1965) J. Polym. Sci. A3: 1373 35. Adrova NA, Bessenov MI, Laius LA, Rudakov AP (1970) Polyimides, Technomic, Stamford, CT (Progress in Materials Science Series, vol 7) 36. Takehoshi T, Kochanowski JE, Manello JS, Webber MJ (1986) J. Polym. Sci., Polym. Syrup. 74:93 37. Takekoshi T, Kochanowski JE (1976) US Pat. 3,991,004 to General Electric 38. Takekoshi T, Kochanowski JE (1974) US Pat. 3,803,085 to General Electric 39. Takekoshi T, Kochanowski JE (1974) US Pat. 3,850,885 to General Electric 40. Takekoshi T (1977) US Pat. 4,024,110 to General Electric 41. White DM, Takekoshi T, Williams FJ, Relies HM, Donahue PE, Klopfer HJ, Louks GR, Manello JS, Matthews RO, Schluenz RW (1981) J. Polym. Sci., Polym. Chem. Ed. 19:1635 42. Sasaki Y, Inoue H, Itatani H, Kashima M (1981) US Pat. 4,290,936 to UBE Ind 43. Yamane H (1985) Proceedings of Second International Conference on Polyimides, Soc. Plast. Eng., Ellenville, NY, Oct. 1985, p 86 44. Harris FW (1982)Proceedings from First Technical Conference on Polyimides, Soc. Plast. Eng., Ellenville, NY, Nov. 1982, p 1 45. Woo EP (1986) J. Polym. Sci., Part A, Polym. Chem. 24:2823 46. Teshirogi T (1987) J. Polym. Sci., Part A, Polym. Chem. 25:31 47. Quenneson M, Garapon J, Bartholin M, Sillion B, Verdet L, (1985) Proceedings of Second International Conference on Polyimides, Soc. Plast. Eng., Ellenville, NY, Oct. 1985, p 74 48. St. Clair TL, St. Clair AK (1977) J. Polym. Sci., Polym. Chem. Ed. 15:1529

24

T. Takekoshi

49. Harris FW, Karnavas AJ, Das S, Cucuras CN, Hergenrother PM (1986) Am. Chem. Soc., Polym. Mater. Sci. Eng., Prepr. 54:89 50. Takekoshi T, Anderson PP (1986) US Pat. 4,599,396 to General Electric 51. Takekoshi T, Anderson PP (1987) US Pat. 4,716,216 to General Electric 52. Burks HD, St. Clair, TL, Hou T (t986) SAMPE Quarterly 18:1 53. Hergenrother PM, Wakelyn NT, Havens SJ, (1987) J. Polym. Sci., Part A, Polym. Chem. 25: 1093 54. Bilow N, Landis AL, Miller LJ (1974) US Pat. 3,845,018 to Hughes Aircraft 55. Landis AL, Bitow N, Boschan RH, Lawrence RE, Aponyi TJ (1974) Am. Chem. Soc., Polym. Prepr 15 : 537 56. Hergenrother PM, Sykes GF, Young PR (1973) Am. Chem. Soc., Div. Pet. Chem., Prepr. 24: 243 57. Sefcik MD, Stejskal EO, McKay RA, Shaefer J (1979) Macromolecules 12:423 58. Reinhardt BA, Arnold FE (1981) J. Appl. Polym. Sci. 26:2679 59. Landis AL, Naselow AB (1982) Natl. SAMPE Tech. Conf. Ser. 14:236 60. Murray T, Tessier N (1986) 31st Intl. SAMPE Symp., Las Vegas, NM, Apr. 1986, p 693 61. Harris FW, Pamidimukkolas A, Gupta R, Das S, Wu T, Mock G (1984) J. Macromol. Sci., Chem., A21:1117 62. Capo DJ, Schoenberg J (1986) 18th Natl. SAMPE Tcch. Conf., Oct. 1986, Seatle, WA, p 710 63. Unroe MR, Reinhardt BA, Arnold FE (1985) Am. Chem. Soc., Polym. Prepr. 26:136 64. St. Clair TL, Maudgal S (1986) US Pat. 4,624,888 to NASA 65. Serafini T, Delvig P, Lightsey G (1972) J. Appl. Polym. Sci. 16:905 66. Johnston JC, Meador MAB, Alston WB (1987) J. Polym. Sci., Part A, Polym. Chem. Ed. 25: 2175 67. Lauber RW, Alston WB, Vannucci RD (1978) NASA Tech. Report 78~53 68. Lauber RW (1976) NASA TMX-73444 69. Roberts GD, Vannucci RD (1986) SAMPE J. 22:24 70. Roberts GD, Lauber RW (1987) J. Appl. Polym. Sci. 33:2893 71. Vannucci RD (1987) 32nd Intl. SAMPE Symp., Apr. 1987, p 602 72. Plastics World, Jan. 1988, p 64 73. Renner A, Eldin SH (1987) US Pat; 4,666,997 to Ciba-Geigy 74. Renner A (1987) US Pat. 4,667,003 to Ciba-Geigy 75. Mallet MAJ, Damory FP, (1974) Am. Chem. Soc., Div. Org. Coat. Plast. Chem., Prepr. 34:173 76. King J J, Chaudhari MA, Zahir SA (1984) Proceedings of 29th Natl. SAMPE Conf., 1984, p 392 77. Chaudhari MA, Galvin TJ, King JJ (1985) Proceedings of 30th Natl. SAMPE Conf., t985, p 735 78. Denny LA, Goldfab IJ, Farr MP (1987) Am. Chem. Soc., Div. Polym. Mater. Sci. Eng., Prepr. 56:656 79. Carduner KR, Chattha MS (1987) Am. Chem. Soc., Div. Polym. Mater. Sci. Eng., Prepr. 56: 660 80. Shaw SJ (1987) In: Weber WD, Gupta MR (eds) Recent advances in polyimide science and technology, Society of Plastics Engineers, Poughkeepsie, NY, p 290 81. Bell VL, Young PR (1986) J. Polym. Sci., Part A, Polym. Chem. 24:2647 82. Eur. Pat. 253891 (t986) to Mitsui Toatsu 83. Intl. Pat. 8700835 (1987) to AMOCO 84. Jap. Pat. Discl. 61-287938 (1986) to Mitsui Toatsu 85. Briscoll SB, Walton TC (1987) SAMPE J. 23:9 86. Green HE, Jones RJ, O'Rell MK (1979) US Pat. 4,173,700 87. Stille JK, Droske JP (1984) J. Macromol. Sci., Chem. A21 : 913 88. Takeichi T, Stille JK (1986) Macromol, 19:2093 89. Takeichi T, Stille JK (1986) Macromol. 19:2103 90. Takeichi T, Stille JK (1986) Macromol. 19:2108 91. Brand RA (1981) US Pat. 4,269,953 to General Dynamics 92. Tan L, Arnold FE (1985) Am. Chem. Soc., Polym. Prepr. 26:176 93. Denny LR, Soloski EJ (1988) Am. Chem. Soc., Polym. Prepr. 29:176 94. Tan L, Arnold FE (1985) Am. Chem. Sci., Polym. Prepr. 26:178 95. Tan L, Soloski EJ, Arnold FE, (1986) Am. Chem. Soc., Polym. Prepr. 27:240 96. Tan L, Soloski EJ, Arnold FE (1986) Am. Chem. Soc., Polym. Prepr. 27:453

Polyimides

25

97. Tan L, Soloski EJ, Arnold FE (1987) Am. Chem. Soc., Div. Polym. Mater. Sci. Eng., Prepr. 56: 650 98. Baldwin LJ, Meador MB, Meador MA (1988) Am. Chem. Soc., Polym. Prepr 29 : 236 99. Smearing RW, Floryan DE (1985) US Pat. 4,543,365 to General Electric 100. Krutchen CM, Wu P (1985) US Pat. 4,535,100 to Mobil Oil 101. Hoki T, Matsuki Y (1986) Eur. Pat. 186308 to Asahi Chem 102. Meyers RA (1969) J. Polym. Sci., Part A-l, 7:2757 103. Carleton PS, Farrissey WJ, Rose JS (1972) J. Appl. Polym. Sci. 16:2983 104. Alvino WM, Edelman LE (1975) J. Appl. Polym. Sci. 19:2961 105. Farrissey WJ, Rose JS, Carleton PS (1970) J. Appl. Polym. Sci. 14:1093 106. Riccitiello SR, Sawako PM (1979) US Pat. 4,177,333 to NASA 107. Hamermesh CL, Hogenson PA, Tun CY, Sawako PM, RiccitieUo R (1979) 1lth Natl. SAMPE Tech. Conf., 1979, p 574 108. Gagliai J (1980) US Pat. 4,241,193 to International Harvester 109. Lee R, Okey DW, Ferro GA (1985) US 4,535,099 to IMI-Tech Corp 110. Pfeifer J, Rohde O (1987) In: Weber WD, Gupta MR (eds) Recent advances in polyimide science and technology, Society of Plastics Engineers, Poughkepsie, NY, p 336 111. Numata S, Oohara S, Fujisaki K, Imaizumi J, Kinjo N (1986) J. Appl. Polym. Sci. 31 : 101 112. Eur. Pat. 247731 and Jap. Pat. Discl. 62-258338 (1987) to Hitachi Chemical 113. Jap. Pat. 61-18I 829 (1986) to Hitachi 114. High performance polymer films, Nikkei New Materials, Sept. 1986, p 21 115. St. Clair AK, St. Clair TL, Slemp WS, Ezzell KS (1985) Proceedings of Second International Conference on Polyimides, Ellenville, NY, Oct. 1985, p 333 116. St. Clair AK, St. Clair TL, (1986)Am. Chem. Soc. Div. Polym. Mater. Sci. Eng. 55:396 117. Ichino T, Sasaki S, Hasuda Y (1986) Denshi Tsushin Gakkai Ronbunshi, J 69-C: 199 118. Ichino T, Sasaki S, Hasuda Y (1987) US Pat. 4,666,760 to NTT 119. For example, Eur. Pat. (1986) 240249, Jap. Pat. Discl. (1987) 62-057421 to Nitto Electric, Jap. Pat. Discl. (1987) 62-231935-231937 to Canon, Eur. Pat. (1987) 234882 to Mitsui Toatsu

Editor: Ch. G. Overberger Received July 11, 1988

Photoresponsive Polymers Masahiro Irie Institute of Advanced Material Study, K y u s h u University, K a s u g a - K o e n 6-1, Kasuga, F u k u o k a 816, Japan

Several proposals for the construction of artificial photoresponsive polymer systems are reviewed. On photoirradiation, photoresponsive polymers change reversibly their physical and/or chemical properties, such as conformation, shape, surface wettability, membrane potential, membrane permeability, pH, solubility, sol-gel transition temperature, and phase separation temperature of polymer blends. The dynamics of the conformation changes detected with a time-resolved light scattering measuring system combined with a short laser pulse source is included in the discussion. Photoresponsive polymers are interesting practically as well as academically.

1 Introduction

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

28

2 Photoeontrol of Polymer Conformation - - A Guiding Principle of Molecular Design of Photoresponsive Polymers . . . . . . . . . . . . . 2.1 Molecular Design . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Dynamics of C o n f o r m a t i o n Changes . . . . . . . . . . . . . . . .

29 29 38

3 Photoeontrol of Physical and Chemical Properties of Polymers . . . . . . 3.1 Shape - - Macro-Size Effect . . . . . . . . . . . . . . . . . . . . 3.2 Surface Wettability . . . . . . . . . . . . . . . . . . . . . . 3.3 M e m b r a n e Potential . . . . . . . . . . . . . . . . . . . . . . 3.4 M e m b r a n e Permeability . . . . . . . . . . . . . . . . . . . . 3.5 p H . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

42 42 51 53 55 57

4 Photostimulated Phase Transition . . . . 4.1 A General View . . . . . . . . . . 4.2 Phase Separation o f Polymer Solutions 4.3 Sol-Gel Transition . . . . . . . . . 4.4 Miscibility of Polymer Blends . . . .

. . . . .

58 58 59 62 64

5 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . .

65

6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

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. . . . .

. . . . .

Advances in Polymer Science 94 © Springer-Verlag Berlin Heidelberg 1990

28

M. Irie

1 Introduction Biological systems have developed various kinds of photoactive organs to adapt themselves to the environmental electromagnetic radiation, sunlight. Plants, for example, have evolved photosynthesis systems to utilize the light as an energy source. At the same time, they have also developed systems which measure and respond to the light intensity and its duration, thereby seeking out optimum conditions for their life processes. These systems use light as information [1]. As in biological systems, light can be utilized in polymer chemistry not only as an energy source for polymer synthesis but also as an information source or a trigger for the reversible control of the physical and chemical properties of polymers. Biological photoresponsive processes consist of many complex biological reactions ranging from the capture of photons to photoresponsive behavior, such as phototropism or phototaxis [2]. Although we are yet unable to reconstruct the biological photoresponsive system as it is, it is worthwhile mimicking its processes and applying them to the molecular design of synthetic photoresponsive polymers, so that the physical and chemical properties can be changed reversibly by photoirradiation. The feature common to the biological photoresponsive systems is that they contain photochromic molecules embedded in biopolymer matrices to absorb light and use the photoisomerization for controlling the conformation and assembly of the biopolymers. The photoinduced structural changes of the biopotymers perturb subsequent biochemical reactions. As in biological systems, photoresponsive polymers consist of a photoreceptor, which contains photochromic molecules, and a functional part (see Fig. 1). At first, an optical signal is captured by the photochromic molecules and then the isomerization of the chromophores in the photoreceptor converts it to a chemical signal. The latter signal is transferred to the functional part via "a chemical circuit" and controls the polymer properties. The number of photochromic chromophores used in the biological systems is limited, while in the synthetic photoresponsive systems, the number of chromo-

I IPh°t°r°c°pt°rl

I

Fig. 1. The structure of a photoresponsivepolymer Table 1. Physical and chemicalproperties of photoresponsive polymerscontrolled by photoirradiation Solution

Solid

Viscosity pH Solubility Metal Ion Capture Capability

Membrane Potential Membrane Permeability Surface Wettability Shape Sol-Gel Transition Miscibility of PolymerBlends Tg

Photoresponsive Polymers

29

phores that can be used is virtually limitlesL Depending on the purpose, we can choose suitable chromophores from a great number of synthetic photochromic compounds. In addition, many aspects of polymer properties can be used for the functional part. Table 1 summarizes the properties so far reported to be photocontrolled reversibly. In the first part of this article, the fundamental idea for molecular design of photoresponsive polymers is described. The second part deals with several examples of photostimulated property changes of the photoresponsive polymers. In the third part, photostimulated phase transitions of photoresponsive polymer systems are discussed.

2 Photocontrol of Polymer Conformation -A Guiding Principle for Molecular Design of Photoresponsive Polymers 2.1 Molecular Design It is well known that many photosensitive molecules can be transformed under photoirradiation to other isomers, which return to the initial state either thermally or photochemically [3]. This reaction is schematically expressed as ]Iv

A~ ¸ ' B hvP,A

and referred to as photoisomerization. The chromophores capable of this reaction are called photochromic molecules. The photoreceptor of Figure 1 contains such molecules. Table 2 shows some typical photoisomerizations, which include (a) trans-cis isomerization, (b) zwitter ion formation (c) radical formation, (d) ionic dissociation, and (e) ring-formation and ring-cleavage. These isomerizations are always accompanied by certain changes in the physical and chemical properties of the chromophores, such as dipole moment and geometrical structure. These changes may induce the changes in the properties of the polymer in which the chromophores are incorporated. The conformation of polymers governs their various physico-chemical properties in solution. To begin with, we outline the guiding principle for designing polymers which change the conformation reversibly by photoirradiation, since they are relevant as a general model of photoresponsive polymers. Figure 2 illustrates several proposals to use photochromic chromophores as a tool for conformation changes. The first mechanism (1) was proposed for the first time by Lovrien [4] in 1967. If a polymer is in equilibrium interaction with some photoisomerizable low molecular weight chromophores, it may undergo a conformation change when irradiated with light, because the interaction between the polymer and the chromophores chantes. The example described by Lovrien is the mixture of poly(methacrylic acid) and chrysophenine G(1, CHP) in water. CHP changes

3O

M. lrie

Table 2. Photoisomerizations Type of reaction

Example

(a) Trans-cis isomerization N~N

%c c% (b) Zwitter ion formation

~ NR O 2 Ph

(c) Radical formation

-O

Ph

Ph

2 .~,'~>---Ph Ph Ph Ph Ph

Ph

H3 ? N H3

(d) Ionic dissociation

H3C~I ~*~--~N/CH3 H3C

CN

CH3

H:~

NC CN H3C~CH3

(e) Ring-formation and ring-cleavage

NC

H30--~ S~C~(~,.~S~...~--CH3

trans

form.

S,O3Na %

/.z/CH-~N,,~. ~ o.

SO3Na {I)

CN

H3C"-~ SC~H3S.,.~"'CH3

the hydrophobic property when the configuration changes from all

C2HsO~~--N~ ~

CHs

CN-

%

oct.,

to

c-t-c

31


~

3

I

hv:A

2o

~ h~,'.,%

2b

~

hv;A

5

h ls,'A

fi ~

htp',A hv'-2"A

~

Fig. 2. Schematic illustration of photostimulated conformation changes of polymer chains

Upon ultraviolet irradiation, trans CHP isomerized to the cis form (around 10~), and the aqueous solution viscosity decreased as much as 80 9/0. The conformation change was interpreted as follows. The anionic linear and planar all-trans CHP would attach itself to the hydrophobic poly(methacrylic acid) backbone, leading to an extended polymer conformation. In the cis form, azoMyes are much more hydrophilic. Consequently, the cis form was envisaged as binding less strongly so that the polymer chain would be less extended. In 1974, the above system was reexamined by Van der Veen and Prins [5]. They, however, did not observe the change in viscosity as much as Lovrien did. The reduced viscosity for 43 ~ cis CHP was only 5 ~ lower than the value for CHP completely converted to the trans form. It was concluded that the viscosity decrease reported by Lovrien had been caused by some impurity contained in the dye and/or the polymer. 1.9

-~

1.7 1.6

0.B5

Fig. 3. Photoinduced reversible conformation changes of HEMAVPy copolymer -- CHP complex in water at 30 °C [6]. (C)): reduced viscosity of 0.334 dl/g copolymer (HEMA content, 0.62); ( 0 ) : absorbance at 404 nm

0.75 t%

0.65 0.55 0

I I 1 20 rain 60

1 t I I 0 20 rain 60

I

I

0

8

I

I

16h24 Time

!

I

0

8

I

I

16h24

32

M. Irie

Being so attractive, efforts to confirm the photoregulation mechanism postulated by Lovrien were continued. In 1977, Negishi et al. [6] found a pronounced photostimulated viscosity change of an aqueous solution of 2-hydroxyethyl methacrylate (HEMA) -- N-vinylpyrrolidone (VPy) copolymer a and CHP or acid yellow 38 (2). The reduced viscosity reversibly decreased as much as 12%, as shown in Figure 3. The interaction between the polymer and CHP was sucessfully controlled by photoirradiation. The result suggested that the mechanism should strongly depend on the pair of polymer and dyes [7].

NaO3S

2 The second mechanism (2) utilizes the change induced in the intramolecular interaction between pendant groups by photoirradiation. The system reported for the first time is poly(methacrylic acid) with pendant azobenzene groups [4]. In an aqueous solution, the viscosity was found to increase by ultraviolet irradiation (Fig. 2(2 a). The trans to cis photoisomerization was considered to decrease the hydrophobic interaction between the azobenzene chromophores, allowing the polymer coil to expand. Mat6jka et al. [8] extended the study to styrene -- maleic anhydride copolymer with pendant azobenzene groups (3) and measured the photoresponsive behavior in less polar solvents.

( CH-CH--CH2--CHI i O| )x ( CHc=oI -- c=0CHI'"CH2~ ~-~_x o//C"o/C%o

I

!

OH NH

+ d

This copolymer exhibited a reversible photodecrease of the viscosity in 1,4-dioxane solution. A decrease of 24-30 % was found in the reduced viscosity after the solution was irradiated with ultraviolet light. In tetrahydrofuran, the viscosity decrease was 1-8 %. The contraction of the dimensions of the copolymer coil was explained as follows. Cis form azobenzene has a dipole moment of 3.t D, while the dipole moment of the trans form is less than 0.5 D. Therefore, the trans to cis isomerization induces strong dipoles in the pendant groups. These dipoles tend to orient in parallel and attract each other in less polar solvents, making compact coil conformations preferrable, as shown in Fig. 2(2)-b. In the dark, the viscosity of the copolymer


33

solution returned to the original value, though the process occurred much more slowly. The rate was 1/2.5 to 1/7 of the rate o f c i s to trans isomerization of the pendant azobenzene chromophores, which was measured by optical absorption. The discrepancy requires further examination of the postulated mechanism for the conformation change. Even when the azobenzene chromophores are incorporated into the polymer backbone, the dipole moment increase of azobenzene residues by photoirradiation can also induce a change in polymer chain conformation. The solution viscosity of poly(dimethylsiloxane) containing azobenzene residues in the main chain decreased upon ultraviolet irradiation, and the effect was attributed to the trans to cis photoisomerization [9]. The photodecrease of the viscosity depended on the polarity of the solvent. It was 24 ~ in non-polar heptane, but negligible in polar dichloroethane. In the mechanism (2), the driving force for conformation changes depends on solvent polarity. In aqueous solutions, a hydrophobic property change is effective to induce the conformation change, case a of Fig. 2, while in less polar solvents the dipole moment change becomes useful, case b. Polymer chain stacking owing to the aggregation of pendant merocyanine forms postulated for photoirradiated poly(spiropyran methacrylate) and poly(spiropyran acrylate) in benzene and toluene [10] may be classified into the mechanism (2). The third mechanism (3) is the simplest one. When trans-cis photoisomerizable chromophores are incorporated into the polymer backbone, the photoinduced configuration change of the chromophores is expected to induce a conformation change of the polymer chain. Azobenzene (4) is the most widely used as the trans-cis photoisomerizable photoreceptor molecule. It undergoes isomerization from the trans to the cis form under ultraviolet irradiation, while the cis form can return to the trans form either thermally or photochemically [11]. R

9

N

I

z,

N

1

R (z) 151 During the isomerization, azobenzene undergoes a large structural change. The distance between 4 and 4' carbons decreases from 9.0 to 5,5 A [12]. Table 3 summarizes the reported polymers having photoisomerizable unsaturated linkages in their backbones, mostly containing azobenzene groups, Polyamides with azobenzene groups in the backbone are among the earliest in which trans-cis isomerizabte chromophores were used to regulate the polymer conformation [13, 14]. The intrinsic viscosity [11]of polyamide (6) in polar N,N-dimethylacetamide was found to decrease from 1.22 to 0.5 dl/g upon ultraviolet irradiation

34

M. lrie

Table 3 Photoresponsive polymers with photoisomerizable unsaturated linkages in the polymer backbone Structure

Reference COOH

COOH 7

-"~N H~

N= N ~ N

HCO'-'~CH2"~mCO-)'gn

[14]

C•H3 k

CH3

C.H3

CH3

Ph

11

Ph

[19]

(410 > ~ > 350 nm) and to return to the initial value in 30 h in the dark at 20 °C (Fig. 4). The decrease is not due to the intramolecular dipole-dipole interaction, because the polarity of the solvent used was very high and the photo-effect was not observed for polymers with long flexible methylene chains. The slow recovery of the viscosity in the dark was accelerated by visible light irradiation (~. > 470 nm). When alternately irradiated with ultraviolet and visible light, the viscosity reversibly changed as much as 60 ~ (Fig. 5). Before photoirradiation, the polyamide has a rod-like conformation. The isomerization from the trans to the cis form kinks the polymer chain, resulting in a


35

1.5;

t~ 1.0

"~0 5

ii

............

0

0.2 0.4 Concentration,g/dr

i

Fig. 4; Viscosities of polyamide (6) in N,N-dimethylacetamide at 20 °C ( t ) in the dark before photoirradiation and (O) under irradiation with ultraviolet light (410 n m > 2 > 350 nm) [14]

0,6

1.0 8

~7 o.s a

1.2 "U

1.0

o" 0.8 0.6 b 0

i

50

i

t00 Time

i

150

i

200 min

Fig. 5. Changes in (Q) content of the trans azobenzene residues in polyamide (6) backbone and (©) viscosity of the polyamide in N,N-dimethylacetamide on alternate irradiation with ultraviolet (410 nm > 2 > 350 nm) and visible (2 > 470 nm) light at 20 °C [14]. Polymer concentration was 0.9 g/dl compact conformation and a decrease in the viscosity. The conformation changes from the compact form to the initially extended one by either heat or visible light irradiation, causing the viscosity to increase.

-~N~N~,N~N\N~N.

~----~ /'N'~--N\N----#N~N

36

M. Irie

Similar experiments were carried out for polyamides (8)and (9) by Blair et al. [15]. They, however, observed no decrease in intrinsic viscosity under ultraviolet irradiation, in contrast to the above case. But they found a small decrease in reduced viscosity at a high polymer concentration. The absence of the photodecrease in the viscosity is probably due to the inclusion of flexible piperazine segments in the polymer chain. Neckers et al. [16, 17, 18] demonstrated that polyureas with backbone azobenzene groups (t0) also underwent a photoviscosity effect when ultraviolet-irradiated. Stille et al. [19] reported that the intrinsic viscosity of potyquinoline (1/) with backbone stilbene groups in di-m-cresyl phosphate/m-cresol decreased as much as 24% under ultraviolet light. The decrease was ascribed to the trans to cis isomerization of the stilbene groups. Because of its simplicity the mechanism (3) has been widely applied to other polycondensation or potyaddition polymers. The fourth mechanism (4) invokes the electrostatic reptilsion between photogenerated charges as the driving force for conformation changes. Triphenylmethane leucoderivatives (12) have been used as photoreceptor molecules. The chromophore dissociates into an ion-pair under ultraviolet irradiation, generating an intensely green colored triphenylmethyl cation. The cation thermally recombines the counter anion as follows.

cH3,N N.3 R

CH3/

X

\CH3

hv

T X

12

13

The triphenylmethane leucohydroxide residues were incorporated into the pendant groups b y copolymerizing the vinyl derivative (12, R = C H = C H 2, X = O H ) with N,N-dimethylacrylamide [20]. Upon ultraviolet irradiation (X > 270 nm), the solution became deep green, and at the same time its reduced viscosity increased from 0.55 to 1.6 dl/g, as depicted in Fig. 6. After the light was shut, the viscosity returned to the initial value with a half-life of 3.1 min. The close correlation between the viscosity change and the absorption intensity at 620 nm implies that the electrostatic repulsion was responsible for the expansion of the polymer conformation. Formation of strong dipoles along the polymer chain and the intramolecular interaction of the dipoles with the polymer chain would also change the chain conformation, as expected by the fifth mechanism (5), This approach to induce con-

H3C CH3 H3C CH3

f~~j

/NO2

hv', & NO 2

-O

R

14

15

Photoresponsive Polymers / 0.4 ~ 4 ~ ~

37

Dark

Dark

/

E

0.3

E

o0.2 ~3

~0,1 A

0 ~1

1.5 "7 1.0

I

I

!

I

fLL

0.5 "b

i

.I

0

10

I

..........

I

20 30 Time

i

I

i

40min

Fig. 6. Changes of (A) absorption at 620 nm and (B) viscosity of poly(N,N-dimethylacrylamide) having pendant triphenylmethane leucohydroxide groups (9. l tool %) in methanol at 30 °C on exposure to ultraviolet light (2 > 270 rim) [20]. Polymer concentration was 0.06 g/dl

50

formation changes has used spirobenzopyran or azobenzene groups as photoreceptor molecules. Spirobenzopyran (14) is well known for undergoing under ultraviolet light irradiation a ring opening reaction leading to the formation of merocyanine having a strong dipole. This reaction can be reversed either thermally or photochemically. We can use the dipole moment change as a driving force for conformation changes of the polymer chain by incorporating spirobenzopyrans into the pendant groups. A typical example is poly(methyl methacrylate) with pendant spirobenzopyran groups [21]. Figure 7 shows its viscosity (the content of the pendant groups is

0.4

Cr~

-~ o.3 f

y

J

0.2 i

|

0.5 1.0 Concentration, g / d l

Fig. 7, Viscosities of poly(methyl methacrylate) having spirobenzopyran groups (13 mol%) at 30 °C: (A) in benzene (0) in the dark (O) under irradiation (2 > 310 nm); (B) in dichloroethane [21]

38

M. lrie

13 m o l e ~ ) in benzene in the dark as well as during the photoirradiation 0~ > 310 nm). The data for dichloroethane are also included to illustrate the viscosity behavior in polar solvents. In benzene, the intrinsic viscosity during irradiation is 17~ lower than the viscosity in the dark. The viscosity change in polar dichloroethane is only 1~o. The viscosity change decreases almost in parallel with increasing microscopic polarity of the solvents. These solvent effects suggest that the polymer chain shrinks mainly due to specific solvation of the photogenerated merocyanines by the poly(methyl methacrylate) ester groups. This intramolecular solvation competes with the solvation by solvents. The intramolecular attraction between the pendant merocyanine and the polymer chain overcomes the merocyanine -- solvents interaction, giving the polymer a more coiled conformation. The intramolecular dipole -- dipole interaction between pendant merocyanine groups in a polymer as suggested for poly(spiropyran methacrylate) and poly(spiropyran acrylate) [t0] is less likely to decrease viscosity for the following reasons: (1) the benzene viscosity of polystyrene having pendant spirobenzopyran groups showed no response to photoirradiation, and (2) the photodecrease of the benzene viscosity of the poly(methyl methacrylate) reached a maximum at a spirobenzopyran content as low as 17 mole %. Poly(methacrylic acid) with pendant spirobenzopyran groups also showed photostimulated conformation changes in methanol [22]. Visible light irradiation of the solution increased the viscosity, while ultraviolet light irradiation caused the solution viscosity to decrease. Alternate irradiation of visible and ultraviolet light brought about reversible viscosity changes with an amplitude as large as 40~, and it was possible to repeat the cyclic changes in viscosity many times. Photostimulated conformation changes observed for polypeptides with pendant azobenzene residues may also be classified as the mechanism (5) [23, 24]. We have described five mechanisms proposed so far for the induction of polymer conformation changes, along with some typical photoresponsive polymers. The photochromic reactions shown in Table 1 have been successfully utilized to induce the conformation changes. An optical signal is captured by the photochromic molecules and converts to chemical signals (geometrical structure and/or dipole moment changes) owing to the isomerization of the chromophores. The chemical signals are transferred to the polymer chain and eventually a change in polymer conformation comes about. This process is observed in common for photoresponsive polymers, and its underlying idea can be applied for controlling various properties of polymers.

2.2 Dynamics of Conformation Changes It is of particular interest to know how fast long polymer chains change their conformation in response to a short laser pulse. The dynamics of this process can be studied in photoresponsive polymers which undergo photostimulated conformation changes [25]. The isomerization of photochromic chromophores can be induced in less than 10 -8 s with a short laser pulse. The conformation change subsequent to it can be followed with a time resolved light scattering system combined with the short laser pulse source (Fig. 8).

PhotoresponsivePolymers

39

Ar ion laser t.__..~._~.,, ~ Sampleceil _ Shutter

~[ass fiber-

Photodetector

Monochrometer~

Anotyzing system

}

1

YAGlaser Fig. 8. Time-resolvedlight scattering measuring system combined with a short laser pulse

The Debye equation-Eq. (1)

K~ -

-

1 16za(s2) = -- + - sin 2(0/2) + 2A2

Ro M~

3Z~w

(1)

relates the light scattering intensity R 0 to the weight average molecular weight M w, the mean square radius of gyration (s2), and the second virial coefficient A2 of a polymer in dilute solution. Here, K = (2n2n2/NgL~) (dn/dc) z, c is the polymer mass concentration, no is the refractive index of the solvent, dn/dc is the specific refractive index increment, ko is the wavelength of the incident light and N A is the Avogadro constant. Expansion of the polymer coil leads to an increase in (s2), which decreases R 0. Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in N,N-dimethytacetamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser experiment, the polymer was brought to a compact conformation by eontimaous ultraviolet irradiation, and then the unfolding process was traced by the laser flash photolysis method.

/

40

M. Irie

Figure 9 shows the oscillograms demonstrating the decrease and increase of optical absorption at 310 and 410 nm for polyamide (6) and the model compound (16) during and after the laser flash. oCONH~

N

COOH

16

Although a relatively slow relaxation process can be seen, we may say that the cis to trans isomerizations of both the polyamide and the model compound are complete in 100 ns. Figure 10 shows a typical oscilloscope trace illustrating the change in light scattering intensity during and after the laser flash. The decrease in light scattering intensity reflects the conformation change involving a decrease in (s2). The initial rapid increase in scattering intensity is due to a concurrent decrease in the optical absorption at 514nm, as depicted by oscillogram (b). For comparison, Fig. t0c shows an oscillogram illustrating the change in light scattering intensity observed on the model compound (•6). This trace only reflects changes in optical absorption and specific refractive index increment. These results indicate that the conformation change of the total polymer chain occurs in about 1 ms. Solvent quality changed the unfolding rat e of the polyamide coil. In good solvents, such as N,N-dimethylacetamide and N,N-dimethylformamide, the folding proceeded rather rapidly, whereas the unfolding was slow in poor solvents containing a miscible non-solvent. When the solvent quality was made poorer, the unfolding rate was retarded. Although solvent quality influences the relaxation time, we may say that the total polymer chain unfolds within the time interval from 0.5 to 1.1 ms.

Polyamide(6) .L

"h,: 31Onto

2my

a

5 0 ns -'1 I-

Model( 16 ) ,,,.,,

~,: 310 nrn

..L

2my T

Uo: 99mv

c

-I

50ns t-

Uo'- 6 8 m y

~, : 410 nm

.~ b

50ns q I-

_L 5 mv T

;~: 410 nrn f

.

.

.

.

50ns

.-.

Uo:109mv d

-t

t--

m

_L. 5mY

T Uo: 84 mv

Fig. 9a--d. Cis to trans isomerization of polyamide (6) and model compound (16) at 22 °C in N,Ndimethylacetamide (1.1 x 10 -2 g/dl) measured by optical absorption [25]. The oscillograms illustrate the decrease and increase of optical absorption at 310 nm (cis form) and 410 nm (trans form) during and after irradiation with 20 ns flash of 530 nm light

Photoresponsive Polymers a Polyamide(6)

41 L.S.

..L

2mY 500#s

~

b

Uo:6OmV

Polyamide(6)

0.A.

..L

5 mV == ~L__

T I

L

500#s ~ c

j

-

-

Uo:42 mV

Model(/6)

L.S.

.L

500#$

-4 l--

2 mV T

Uo: 46rnV

Fig. 10a-c. Chain unfolding and cis to trans isomerization of polyamide (6) in N,N-dimethylacetamidesolution (0.31 g/ dl) [25]. The oscillograms illustrate changes of fight scattering intensity (a, e) at 514nm and optical absorption at 514 nm (b) during and after 20 ns flash of 530 nm light. Traces a and b: polyamide (6); trace c: model compound (16)

The large difference in the response times for optical absorption and light scattering suggests a two step mechanism for the photostimulated unfolding process. During the isomerization of the backbone azobenzene residues, the total chain conformation maintains in the initial compact conformation. After the isomerization is completed, the conformation relaxes to a more stable extended conformation in 1 ms.

Compact Conformation 10 -7 s $ c i s - t r a n s isomerization Compact Conformation 10 -3 s + unfolding Extended Conformation

This scheme implies that the compact conformation having t r a n s azobenzene residues is a constrained form capable of storing a certain amount of strain energy. The strain energy causes coil expansion, and it is released during the unfolding process. The photostimulated conformation change of polystyrene with pendant azobenzene groups in cyclohexane has also been studied by the time-resolved light scattering technique [26].

42


3 Photocontrol of Physical and Chemical Properties of Polymers 3.1 Shape -- Macro-Size Effect It is possible to amplify the photostimulated conformation changes of polymer chains in solution to a macroscopic change in the size of gets or solids. The use of structural changes of photoisomerizable chromophores for the size change of polymer solids was proposed for the first time by Merian [27]. He studied a nylon filament fabric, 6 cm wide and 30 cm long, dyed with 15 mg/g azo-dye. After exposure to a xenon lamp at a distance of 30 cm, he found the dyed fabric shrank 0.33 m m (Fig. 11). Since this finding, many materials exhibiting photostimulated deformations have been reported. Polymer films mixed with low-molecular-weight photochromic compounds, such as nylon film-p-carotene [28], nylon film-~-cyanostilbene [28], and polystyrene - spirobenzopyran [29] were shown to undergo photostimulated reversible size changes.

c oz

co

NH ,< ~ NH

o~,.

Fig. I1. Contraction of fabric dyed with trans-cis photoisomerizable dye [27]

RI--OC 1 OH

0.4 [3

|

0

1

2

3

4

5 Time

6

7

8h9

Fig. 12. Photostimulated contraction of nylon film (A) in the presence of ~-cyanostilbene and (B) in the absence of the chromophore [28]


43

Figure 12 illustrates Blair's data on the mixed system nylon film-~-cyanostilbene

(17)[281. CN

c>i -o /

,x

17 When irradiated, this film contracts by 0.8 ~ , while in the dark it recovers to the initial length in about 4 h. Curve B is for a nylon film without ~-cyanostilbene under the same conditions and it displays no photo-shrinkage. A covalently bound photochromic chr0mophore is expected to give a more direct effect on the deformation of polymers. The systems studied were polyimide with backbone azobenzene groups [30], polyamide with backbone stilbene groups [31], polyquinoline with backbone stilbene groups [19], polytetrahydrofuran with backbone viologen groups [32], poly(ethyl acrylate) with spirobenzopyran [33, 34] or azobenzene groups [35] as cross-linking agents, and poly(n-butyl acrylate) with pendant azobenzene groups [36].

2.0

._J

1.0

0

t

Statl UV

t 1°

30

20

Slop

Time [min)

Fig. 13. Photomechanical behavior of poly(ethyl acrylate) cross-linked with spirobenzopyran groups [33]. Influence of temperature and stress time in min

O A III • V

Temperature

mm

Load, g

15 29.6 45 15 15

44.5 44 42 22 32.5

21.9 21.9 21,9 35.7 59.4

44

M. lrie

Among these systems, polyimide with azobenzene chromophores in the backbone was first studied by Agolini and Gay in 1970 [30]. They reported 0 . 5 ~ contraction of the polymer film at 200 °C taking place under ultraviolet irradiation. No correlation, however, was reported between the degree of isomerization and film contraction. At the high temperature they studied, the cis isomer content in the photostationary state would be very low, because a rapid thermal reverse reaction from the cis to the trans form should occur. Thus, the isomerization is not the sole origin of the contraction of the system. The most pronounced photo-contraction effect was observed for poly(ethyl acrylate) cross-linked with spirobenzopyran groups (18) [33]. This is shown in Fig. 13.

i

CH-CO2Et I 0 CH2

H2C--O--ICI --~--CH3 CH3

J~-..TX( 0 ~ (

[( )T X

~

CH2

+ I

I

H2C t

~

))'--NOzl

~H-CO2Et I

'

CH2 I

,N~

X 1( )1

lmNH( ))---~ CN3--C--C--O--CH 2

H~ CH-CO2Et i (is) Initially, it was assumed that the contraction is due to an entropy increase of the polymer chain associated with a higher flexibility of the open-ring merocyanine form than the parent ring-closed spiropyran. However, careful reexamination of the action spectrum of the contraction, i.e. the dependence of film shrinkage on the wavelength of irradiation, led to the unexpected finding that the contractio n spectrum closely fitted with the absorption spectrum of open-form merocyanine and that ultraviolet light causing the isomerization from the spirobenzopyran to the merocyanine form was very inefficient for film contraction. Thus, the film shrinkage appeared to be induced not by the photochemical isomerization of the spirobenzopyran to the merocyanine form, but by the increase in local temperature arising from a non-radiative transition of the photoexcited merocyanine.

PhotoresponsivePolymers -~-

0.:3 - - L j

%

45 Z.2

-J-

~'l

_L

A

0.2

x

O.II

°°~~°

;~I{trons-~ds) =365 n m

)~2(cis_~.tco~sl=436 nm 0

~J

i 8, 12, 14, 2 '0 2 4 -28-#

4

I-I

o

i

600

Time {rain)

Fig. 14. Schematicrepresentation of the photomechanicaleffect induced in poly(ethylacrylate) network with azoaromatic cross-links upon irradiation [35] One example, which exceptionally exhibited a real photochemical contraction effect in the film state, is poly(ethyl acrylate) cross-linked with 4,4'-dimethacryloylaminoazobenzene studied by Eisenbach [35]. Figure 14 shows the data. The film contracts upon irradiation with ultraviolet light, which causes the trans to cis isomerization of the azobenzene chromophores, while it expands by irradiation with visible light, which induces the cis to trans isomerization. Both contraction and expansion are induced by photoirradiation. This finding indicates that the structural change of the cross-linking azobenzene chromophores in the polymer network is responsible for the contraction/expansion behavior. However, the observed contraction was very small, being only about 0.15-0.25 ~. In order to minimize the local heating effect, Prins et al. [37] proposed experimenting with solvent-swollen gels, in which rapid thermal conduction is supposed to suppress this unfavorable effect. The gel system used by Prins et al. was a mixture consisting of low molecular weight chrysophenin G(1) and a water-swollen gel of poty(2-hydroxyethyl methacrylate) (PHEMA) cross-linked with ethylene glycol dimethacrylate (1.1 w t ~ ) . Upon irradiation, the dye changed the configuration from all trans to the e-t-c form. The difference in the hydrophobic property of the two forms brough about a change in the intermolecular interaction between the dye and PHEMA. The increase in the hydrophilicity of the dye by the isomerization from all trans to the c-t-c form contracted the polymer gel, because this action liberated the hydrophilic dyes from the polymer chain to the surrounding solution.

I19

46

M. lric

A similar photo-effect on the swelling of cross-linked poly(methacrylic acid) (PMA) was observed in the presence of 4-phenylazophenyl trimethylammonium ions (19) [38]. An increased cis content of the chromophore by photoirradiation at a pH in the range of conformation unfolding of PMA led to an increased ionization and thus enhanced swelling. The conversion efficiency of light energy to mechanical work was calculated to be 3 x 10 -7. Even when a solvent-swollen gel is used, there remains the question as to the relative contribution of the local heating and the real photochemical reaction to the observed photo-shrinking. Mat6jka et al. [36, 39] carefully examined the contribution of the former to the photostimulated shrinkage of a maleic anhydride-styrene copolymer with covalently bound pendant azobenzene groups (MAH-STY-AAB) swollen in diethylphthalate. They measured the temperature of the gel by inserting a thermocouple into the gel along with the photo-generated force. The copolymer was irradiated at an elongation of 1.25 %. The irradiation caused a reversible increase in the force by 1% (Fig. 15). As seen in the figure, the force change rate is much faster than the isomerization reaction, and the response correlates well with the change in the temperature of the gel. The latter fact suggests that the decisive role played in the contraction process be the local heating due to light absorption, apparently not to the photoisomerization of the photochromic chromophores. When intensely photo-irradiated, the sample was heated by the absorption of radiation even under a careful thermostatic control. The rise in temperature inside the gel was determined to be 1.2 K. With an interference filter (X = 370 nm) it was possible to minimize the heating effect. The slow increase of the force as seen in Fig. 15(b) may be ascribable to the photochemical trans-cis isomerization, though the effect is less than 1~o.

---m.-

£1

0.474

299

0.47 0 t

I

1

)

1

298

I

LightDark Light Dark Light Dark .....

t

I

I

I

I

b 299

0.4?3

._-

298

0.470 0.026

t ,

" ~ ' ~ 0.02."

C

,,

Dark

Light I

1

0

25

I

I

50 t

75

I

100 min

Fig. 15a-c. Effects of radiation on retractive force f (in Newtons) at constant length of a sample of poly(MAH-STY-AAB) swollen in diethylphthalate and on temperature Ti inside the sample [36]. () for, f; (------) for T i. a . l = 1.25 without interference filter, b. t = 1.25 with interference filter, c. l = 1.05 without interference filter


47

The large heat effect observed even in the contraction of the solvent-swollen gel strongly suggests that many previous studies reporting to have observed photostimulated contractions have to be reexamined to check and evaluate the real photochemical effect. The following criteria may be useful to judge whether the effect is due to photochemistry or photoheating. (1) When the recovery rate of deformation in the dark after light is switched off is faster than the rate of thermal isomerization of the chromophores in the film, the contraction is due to photoheating. The recovery rate should always be slower than or almost equal to the rate ofisomerization even when the recovers is induced by photoirradiation, if the process is associated with the photochemical reaction of the chromophores. (2) At elongations l smaller than the inversion elongation llnv, the temperature increase causes the force to decrease, while at I larger than l~nv,an increase in the modulus with temperature causes the contraction force to increase. Therefore, photoheating should give rise to reverse effects depending on l. On the other hand, the photochemical effect is independent of I. If the photostimulated contraction depends on l, the contraction is due to photoheating. Photostimulated expansions observed for small elongation in polyamide film containing stilbene chromophores in the polymer backbone may be explained by the thermal effect [31]. The solution viscosity of this polymer decreased upon photostimulated trans-cis isomerization, while the film showed a decrease in force. In this system, the photoheating effect dominates the photochemical one. According to the above criteria, photostimulated deformations observed in a hydrogel by Ishihara et al. [40] may be ascribed to the photochemical effect. They prepared a gel of PHEMA with pendant azobenzene groups (20). CH3 I --~ CH2-- C - - ~

CH3 i CH^-- C ---~

C=O

C=O

o

o

I

I

1

I

CH, o.

o

c_O_\©

20

It was swollen in water, and then irradiated with ultraviolet light. Figure 16 shows the observed photoresponsive behavior. The gel swelling decreases by as much as 7 ~ in 1 h. Upon visible irradiation, "the gel again swells slowly in 10 h. The swelling rate is slower than the photoisomerization rate, and both the shrinking and swelling processes are induced by photoirradiation. These results support the view that the effect is photochemical. Although many systems showing photostimulated deformations have been reported, the deformations were limited to less than 10 ~. Small deformations make it difficult to judge whether the effect is due to photochemistry or photoheating. If the deformation is larger than 20 ~o, we may safely say that it is due to the photo-

4~

M. lrie

UV

14~

% 12

UV

I -o10

Vis

E

i Vis I

I

I,/~'

/

8

6

,,z

0

J

1

2

t

3 Time

s

4

I

h 5

Fig. 16. Photostimulated changes in the swelling degree of P H E M A having pendant azobenzene residues in water at 25 °C [40] UV: 2 = 350 ___ 50 nm; Vis: 2 > 470 nm

/A--. 10

chemical effect. One photo-deformable material of interest is poty-4-(N,N-dimethylamino)-N-r-D-glutamanilide, which displayed a dilaton amounting to 3 5 ~ in N,Ndimethylformamide when exposed to light in the presence of CBr4 [41]. This large deformation was due to the ionization of N,N-dimethylanilide groups. Although the system was irreversible and no attempt was made to make it reversible, such a pronounced effect is informative for the design of reversible photo-deformable polymers. We may infer from the studies on the conformation change in solution that the electrostatic repulsion between photogenerated charges is more effective for conformation changes than the trans-cis isomerization of unsaturated linkages. On due consideration, we decided to take advantage of electrostatic forces to obtain gels exhibiting large reversible deformations [42, 43]. Polyacrylamide gels containing a small amount of triphenylmethane leucohydroxide or leucocyanide groups (21) were prepared by free radical copolymerization of di(N,N-dimethylaniline)-4-vinylphenylmethane leucohydroxide (12, X = 0 H , R =

C=O I NH

I CH2 t

NH I

C=O

--~

CH2?H~ C=O I NH2

CH~

H3C)N,-~.-, H3C

1

~

/CH3 N. CH3 21

X = OH, CN


49

C H - - C H 2 ) or leucocyanide (12, X = C N , R = C H = C H 2 ) in dimethylsulfoxide in the presence of N,N-methylene-bisacrylamide. The gels were swollen to equilibrium by allowing them to stand in water overnight. Then the changes in their weight and dimensions induced by ultraviolet light were measured. A disk-shaped gel (10 mm in diameter and 2 m m in thickness) having 3.7 m o l % triphenylmethane leucohydroxide residues showed photostimulated reversible dilation in water. Figure 17 shows that upon ultraviolet irradiation (X > 270 nm), the gel swells and the weight increases by as much as 3 times its original weight in 1 h. The dilated gel contracts in the dark to its initial weight in 20 h. The cycles of dilation and contraction of the get were repeated several times. The gel having leucohydroxide residues swelled even in the dark when the aqueous solution became acidic, owing to the chemical ionization of the residues. In order to make the gel insensitive to pH changes, the hydroxide residues were replaced by cyanide groups. The weight of the leucocyanide gel remained constant in the range of p H 4-9. Figure 18 shows the photoresponsive behavior of the gel having 1.9 mole % leucocyanide residues in water. Upon ultraviolet irradiation, the gel weight increases as much as 18 times. In the dark, the gel contracts again slowly to the initial weight. Figure 19A and B show the rate of coloration at 660 nm and the gel expansion rate under continuous light irradiation. The triphenylmethyl cation is well known to have a very strong absorption at 622 nm. Upon ultraviolet irradiation, the color

t-.,--~,---+-Dark-~- ~

=I Dark-~-~

3

Fig. 17. Photostimulated dilation and contraction of polyacrylamide gel having pendant triphenylmethane leucohydroxide groups (3.7 mol %) with light of wavelength longer than 270 nm at 25 °C [43]. Wo is the weight before photoirradiation '

20

2

'

';z2() Time

~-~,--+--~

' 2'2 '

''iOh'

Dark - -

15

10

o

2'oh Time

Fig. 18. Photostimulated dilation and contraction of polyacrylamide gel containing 1.9 mole % pendant triphenylmethane leucocyanide groups with light of wavelength longer than 270 nm at 25 °C [43].Wo is the weight before photoirradiation

50

M. h'ic

1.0

t

*

~ ~ o ~

i

,

i

b

°---'~°~°-

~100 o

--o50 1

0 0

~

0

;

8'0

1;o 2;0

Fig. 19a, b. Photostimulated (a) color change and (b) dimension change of polyacrylamide gel containing 1.9mole% triphenylmethane leucocyanide groups in water [43]. 10is the initial diameter of the disk-shaped gel before photoirradiation

Time, rain

of the gel changes quickly from pale green to deep green in less than 3 rain and then remains almost constant. In the dark, the color returns to the initial pale green in several hours. The size of the gel, on the other hand, increases slowly and reaches the saturated value in about 2 h. The photostimulated dilation is 2.2 times. The slow response o f the size change in comparion with the color change indicates that the gel dilation is due not to the thermal effect but to the photochemical ionization. The formation o f charges, fixed cations and free anions, generates an osmotic pressure difference between the gel and the outer solution, and this osmotic effect is considered to be responsible for gel expansion. It is worthwhile to note that the gel expansion is suppressed by the addition of salts, such as NaC1 and KBr. N o photostimulated dilation was observed in the presence of 10 -2 M NaC1 or KBr for the gels having leucohydroxide or teucocyanide groups. The potyacrylamide gel described abobe is the first example showing a reversible deformation o f more than 100%. The effect is purely photochemical and reversible. However, this gel has a serious disadvatage in that the response time is slow. To improve this point the effect of electric field on the gel deformation was examined [44]. A rod-shaped polyacrylamide gel containing triphenylmethane leucocyanide groups (25 m m in length and 2 m m in section diameter) was prepared in a capillary tube. The gel rod was inserted between two parallel platinum electrodes in a small

Fig. 20. Photostimulated vibrational motion of a rod shaped polyacrylamide gel having 3.1 mol~ triphenylmethane leucocyanide groups under an alternating electric field (+0.8 v/cm, 0.5 Hz) in water in the presence of 4 x I0-4 M NaC1


51

water pool (Teflon, 36 x 19 x 15 mm) and fixed at its one end on the pool wall. It vibrated in response to the alternating electric field (+8.5 V/cm, 0.5 Hz) under ultraviolet irradiation, as shown in Fig. 20.

3.2 Surface Wettability

Surface wettability is an important property of polymer solids and plays an important role in printing, dyeing and adhesion. This property depends on the surface free energy, which is expressed in terms of a sum of the dispersion energy and polar energy terms of the surface tension. These terms vary with the molecules which are attached to the polymer surface. If we introduce photoisomerizable chromophores, which change the polarity reversibly by photoirradiation, the surface wettability is expected to become photo-controlled. Figure 21 shows a wettability change measured by the contact angle of a water droplet on the surface ofa HEMA - - methacryloyl-2-hydroxyethyl-phenylazobenzene copolymer [45, 46]. The absorbance change at 325 nm, which gives information about the content of trans form azobenzene chromophores, is also included in the figure. The contact angle (cos 0) increases from 0.22 to 0.41 under ultraviolet irradiation, while it decreases to 0.22 with visible irradiation. At the same time, the absorbance decreases with ultraviolet irradiation, while it increases with visible irradiation. The close correlation between the contact angle and the absorbance at 325 nm indicates that the wettability change of the polymer surface is attributable to the structural change of the azobenzene chromophores. A wettability change was also observed when triphenylmethane leucohydroxide was introduced [47]. The contact angle with water of the surface of polystyrene having triphenylmethane leucohydroxide chromophores increased from 0.2 to 0.8. This increase in wettability is due to an enhanced hydrophilicity of the film surface brought about by ionic dissociation of the chromophores. 2-Hydroxytriphenylmethanol (22) is also an useful photochromic chromophore

I

0.4

'

1.1

E

c

t.o

"6 OJ

8 0.3

Fig. 21. Photoinduced change in (O) wettability and (O) absorb-

0.9

(3

o

0.2

I

0

I

'o.7

!

5 10 15min UV ir rodiation time I

I

I

I

0 5 10 rain 15 Vis irrodietTon time

ante of PHEMA film having pendant azobenzene groups [47].

UV:2 = 350 5:50 nm;Vis: 2 > 470 rim. Azobenzenecontent was 0.387

52

M. lric

to induce the surface wettability change [48]. The photochemical reaction of the chromophore is as follows:

÷

OH

22

H~o

0

23

Upon ultraviolet irradiation, the chromophore liberates water to a carbonyl group, which reverts to the hydroxyl group by reaction with water. According to the above reaction scheme, the surface polarity of poly(n-butyl methacrylate) with pendant 2-hydroxytriphenylmethanol is expected to decrease upon photoirradiation, if the chromophores are located in the surface region. Before photoirradiation, the contact angle (cos 0) of a water droplet at the polymer surface was about 0.1. After ultraviolet irradiation for 2 min, cos 0 decreased below --0.15, which indicates that the surface is highly hydrophobic. In the dark, the surface again became less hydrophobic. The decrease in cos 0 by photoirradiation was also observed for poly(n-butyl methacrylate) mixed with 2-hydroxytriphenylmethanol, though the increase of 0 was very small ( ~ 4 °) even at a content of 45 mol %. In addition, cos 0 after photoirradiation did not depend on the content, but stayed constant within experimental errors up to the content of 45 mol ~o. The marked difference in wettability behavior between the polymer having the chromophores in the pendant groups and the polymer mixed with the chromophores suggests the difference in the location of the chromophores in the polymers. A change in the hydrophilic nature of the polymer surface on irradiation of poly(p-phenylazoacrylanilide) (PAAn) or its copolymer with HEMA may be used to control the adsorption -- desorption behavior of proteins or organic substances onto the polymer [49]. Adsorption of lysozyme onto the copolymer ofp-phenylazoacrylanilide and HEMA was found to decrease from 4.6 eg to 1.8 cg per gram of adsorbent on ultraviolet irradiation, which induces .the isomerization from the trans to the cis form. The decrease in adsorption ability upon ultraviolet irradiation is explained by a reduction of the hydrophobic interaction between the protein and the polymer, which results from the appearance of hydrophilic cis-form azobenzene on the surface. Photostimulated changes in the adsorbance of a polymer may be utilized for adsorption chromatography [50]. Figure 22 is concerned with the adsorption of Cephalosporin C on a photoresponsive PAAn column, and the desorption by irradiation with ultraviolet light. When an aqueous solution of Cephatosporin C was passed through a column in the dark, the absorbance of the effluent did not differ from that of the solution. When it was irradiated with ultraviolet light, the absorbance of the effluent increased. This shows that Cephalosporin C was desorbed from the adsorbent as a result of hydrophobicity change. These findings suggest that isolation and purification of biomaterials be in principle possible by use of this adsorbent in systems in which water is the only solvent.


53

1.40

E

UV

~ 1.30 Fig. 22. Chromatography of Cephalosporin C on cross-linked polystyrene coated with PAAn at 25 °C [50]. Flow rate: 1 ml/min; fraction volume: 3 ml. Concentration of Cephalosporin C was 2 x 10-4 mol/L (0) in the dark; (O) during ultraviolet irradiation

u

1.20 " O O Q 0 m

< 1.10 I

.......

5

I

I

10 15 Fraction n u m b e r

.....

20

3.3 Membrane Potential Membrane characteristics, such as membrane potential and ion permeability, can be controlled by photoirradiation when photoisomerizable chromophores are incorporated into the membrane. The membrane can fabricate an organic-photosensor, which changes the potential in an on-off fashion when light irradiation is used as an input signal. Such an application of organic membranes is of considerable interest in connection with the development of molecular based electronic devices. The research was initiated by Kato et al. [51] in 1976, who used an acetyl cellulose film containing photochromic spirobenzopyran and phosphatidyl chloride. Figure 23 shows a schematic diagram of the apparatus used for the measurement of the membrane potential. The concentration ratio, T = C1/C2, of the electrolytes in compartments I and II is a parameter to vary the potential in the dark. In the dark before photoirradiation, the membrane exhibited a steady state potential difference Ato of --28 mv. The membrane potential shifted to --10 mv when the membrane was irradiated with ultraviolet light, and it reverted to the initial value upon visible irradiation. The change in the membrane potential was thus reversible. The potential change, AtOp - - AtOd, decreased with increasing salt concentration at constant 7- At high concentration, the membrane potential is determined mainly

I

E

L l IT

~M

I

Fig. 23. Experimental set-up for measuring light induced membrane potential change [51]. A, amplifier; B, recorder; E, saturated calomel electrode; L, light source; M, membrane; P, rubber packing; S, magnetic stirrer bar

54

M. trie

by the diffusion potential. On the other hand, the Donnan potential is important at low salt concentration. The light-induced potential change resulted from a change in charge density of the membrane, Donnan effect. Therefore, a significant potential change is induced at tow salt concentration. A similar photostimulated potential change was observed for a poly(methacrylic acid) membrane having pendant spirobenzopyran groups [52]. The membrane potential decreased by visible light irradiation in the low NaC1 concentration region ( < 5 x 10 -2 mol/1), while it increased in the higher NaC1 concentration region. The salt concentration dependence showed that the photoinduced potential change at higher concentration was caused by a change in the ion transport number resulting from the conformation change of the polymer chain, while the potential change at lower concentration was induced by the charge density change of the membrane due to the photoisomerization of spirobenzopyran groups, just as in the above mixed system. The photoinduced membrane potential change was also detected even in the absence of the ion concentration gradient between the two cell compartments. Anzai et al. [53, 54] prepared a poly(vinyl chloride) membrane containing spirobenzopyran with a long alkyl chain. The response of the membrane potential upon photoirradiation is shown in Fig. 24. When the membrane between two compartments I and II containing NaC1 solutions of the same concentration (1 mol/l) was irradiated with visible light from the side of compartment I, no membrane potential was detected. The potential increased as much as t00 mv upon ultraviolet irradiation. Visible irradiation brought the potential back to zero. The reversible on (positive potential)/off (zero potential) cycle was attained by use of ultraviolet/visible light switching as an input signal. Another interesting property of this membrane is that the polarity of the photoresponse in the membrane potential was reversed by irradiation from the side of compartment II, as shown in Fig. 24(B). The photoresponsive behavior is almost the same as that observed when irradiated from the side of compartment I, except for the polarity. This finding suggests that the surface potential at the membrane/solution interface facing the light source differ from that at the opposite interface, owing to the difference in charge density between the two membrane b

b b I(A)I

>

1

E 100 C

rl _7

-6 C

C

8 E

..Q

C

(8) I

o

-I00

C

a

a

3 rain

a

l

d

1

d

Fig. 24. Time responses of membrane potential across poly(vinyl chloride) membrane containing spirobenzopyran with a long alkyl chain upon UV and visible light irradiation [54]. NaC1 concentration in solution cl and ca:l M. a) UV irradiation from the side of cl ; b) visible irradiation from the side of c~; c) UV irradiation from the side of c2; d) visible irradiation from the side of c2


55

surfaces. Photoirradiation gives a transient asymmetric membrane. As expected, prolonged ultraviolet irradiation reduced the potential. Membranes containing azobenzene-modified crown ether and crown ether linked spirobenzopyran also showed changes in the photostimulated membrane potential [55]. 2,3-Dipheuylindenone oxide was also effective in changing the membrane potential of poly(vinyl chloride) by photoirradiation [56].

3.4 Membrane Permeability Membrane permeability is khown to depend on the swelling degree of hydrophilic membranes through which water soluble solutes permeate [57]. This fact leads to the idea that the permeability can be photocontrolled by use of photodeformable polymer gels. Figure 25 shows time-permeation curves of proteins with various molecular weights for a PHEMA gel membrane with pendant azobenzene groups [58]. In the dark, the amounts of permeation of insulin, lysozyme and chymotripsin increase linearly with time. Albumin shows no permeation. The difference is probably due to the size of the proteins. Ultraviolet irradiation diminishes the permeation rate. In particular, the permeation of chymotripsin is completely suppressed. In Sect. 3.1, it was shown that the swelling degree of a polymer gel in water was decreased by ultraviolet irradiation and recovered to the original level by visible light irradiation. The photodecrease in the permeation rate of proteins is due to the decrease in the swelling degree of the polymer membrane. We tried to photocontrol water permeation through a porous poly(vinyl alcohol) membrane coated with the polyacrylamide gel containing triphenylmethane leucocyanide groups [59]. The photoresponsive behavior of the gel has already been described in Sect. 3.1. Figure 26 shows that the rate of water permeation through

I

x

30 E 20

Fig. 25. Permeation profiles of proteins through the PHEMA membrane having pendant azobenzene residues at 30 °C [58]. The arrows represent the UV irradiation to the membrane; (©) insulin, (O) lysozyme, (A) chymotripsin, (&) albumin

o

0

I00

200

Time, min

300

56

M. Irie

-~10 e" E :3

0

=,5 Ligh E r,

I

!

5

10 Time

?

I/~1

15

20 0

II~

I,

f

5 0 Time

5

1

I

10 15 Time

f

,

20

Fig. 26. Photo-effects on water permeation through a porous poly(vinyl alcohol) gel with triphenylmethane leucocyanide groups (content, 1.9 tool %). ( ~ ) an unmodified membrane; ( 0 ) a coated membrane in the dark; (O) a coated membrane under UV irradiation

an unmodified porous membrane is very fast, while the rate is markedly decreased when the membrane is coated with photoresponsive polyacrylamide. Upon irradiation with ultraviolet light the permeation rate increases by as much as 60 times, but in the dark it again decreases. Photostimulated size changes of the adsorbed polymer gel control the permeation of water. The addition of NaC1 to water quenched the photo-effect. This is because the salt suppresses the photostimulated swelling of the polymer gel. The transport property of poly(L-glutamic acid) membrane (24) was also photocontrolled by incorporating triphenylmethane leucoderivative, pararosaniline, groups [60]. The pararosaniline groups in the membrane dissociate into ion-pairs upon ultraviolet irradiation, yielding hydroxide ions. --NH-CH-CO--NH-CH-CO~NH-CH-CO--NH-CH-CO--

CO

CO

t

I

NH

NH

I

I

©

x_©+©_c'_ _x

©

COOH

--NH-CH-CO--NH-CH-CO--NH-CH-CO~NH-CH-CO--

CO

I

CO0-

H

CO

I 1

NH

hv

x_©_:_©_x!x_©_c_©_x,

OH I

.........

(2~:)

(25)

OH-- . . . . .

"J

X=NH;


57 Time

0

50

I

I

min

100 I

2.5 •r.d) 2.3

~ 2.1 x

1.9

n

t.7

0 a

5 Time

10rain Time 50

0

0.80

!

!

0.79 3=

0.78

0.77 b

!

I

0

5

I

,i

rain

100 !

Fig. 27a. Changes in the permeability coefficient of styrene glycol, P,, across a membranecomposedofpoly (L-glutamic acid) containing 15.5 mol % pararosaniline groups on UV irradiation and dark adaption at pH 8.6 and at 25 °C; b. changesin the degree of swelling, H, of the membrane on UV irradiation and dark adaption at pH 8.6 and at 15 °C [60]

lOmin

Time

The hydroxide ions accelerated acid dissociation of the neighboring L-glmamic acid moieties, which, in turn, changed the conformation of the polymer from the randam coil to a helix. Figure 27 shows the rate of styrene glycol permeation through a poly(L-glutamic acid) membrane with 15.5 mol% pararosaniline groups at pH 8.6 together with the swelling behavior. The rate increases by ultraviolet light irradiation. In the dark, the rate again decreases to the initial value. The permeation rate correlates closely With the swelling degree of the gel. The swelling degree is also consistent with the conformation response to light. These findings indicate that the photoinduced increase in the permeability of styrene glycol arises from the increase in the degree of swelling resulting from the conformation change of poly(L-glutamic acid). Photocontrol of the transport property was also studied for poly(L-glutamic acid) membrane with azobenzene-4-sulfonic acid residues [61].

3.5 pH Photocontrol of pH has been tried by use of poly(methacrylic acid) having spirobenzopyren pendant groups [62]. Figure 28 shows the response of pH of an aqueous solution of this polymer to visible light irradiation (~. > 470 nm). The pH increases

58

M. lrie .......

H

Dark t - - - - - 7,

='~=

Dark

---

7.4

3Z

7.3

I

I

I

0

10 rain

20

/]

i

/

t

I

1

2

3

4

Fig. 28. Response of pH of an aqueous solution of poly(methacrylicacid) having spirobenzopyran units (16 mol %) under visible irradiation (2 > 470 nm) and in the dark at 25 °C [62]. Concentration of the polymer was 0.08 g/dl

h Time

on exposure to the visible light, and returns to the initial value in the dark. The mechanism of the pH change is explained as follows. Hydrophobicity of the merocyanine is considered to be weaker than that of the spirobenzopyran because of its zwitter ionic structure. The increase of hydrophobicity due to the isomerization of the merocyanine to the spirobenzopyran form by visible light irradiation causes the polymer chain to contract. The conformation change of the polymer chain decreases the dissociation of the carboxylic acid residues in the pendant groups, giving rise to an increase in pH. The conformation change of polyamide with pendant azobenzene groups (6) in aqueous solution also induced a change in pH [14]. Ultraviolet irradiation caused the pH of an aqueous solution of the polyamide to decrease and visible irradiation returned the pH to the initial value.

4 Photostimulated Phase Transition 4.1 A General View Photoresponsive polymers so far described change their properties in proportion to the number of photons that they absorb. Thus, when they contain more photochromic chromophores, which undergo an isomerization by absorbing a definite number of photons depending on the quantum yield, their properties change more. To make a. sensitive photoresponsive polymer, i.e. one which responds more efficiently to a fewer photons, we have to introduce an amplification mechanism into the system. A convenient way to achieve this end is to utilize the phase transition of polymers. At a temperature close to the phase transition temperature, the system is in an unstable state, and hence a small perturbation may bring about a large effect on it. When such a system is perturbed by a photochromic reaction, the absorption of a few photons will induce a large property change. Figure 29 shows a schematic illustration of the photostimulated phase transition from the state X to the state Y. When the photochromic chromophores in the polymer

Photoresponsive Polymers Stote X

59

Potymer-B I

k2

% % %

Polymer-A I

State Y I

Ta

Fig. 29. Schematic illustration of photostimulated phase transition from the state X to the state Y

Tb Temperature

chain are A isomers, the polymer changes the state at a temperature of T,. We assume that the phase transition temperature will rise to Tb when A isomers convert to B isomers. Then, if the isomerizafion from the A to the B isomer can be induced by photoirradiation with light o f wavelength X, at temperature T(T~ < T < T ¢ , the state will change isothermally at T from Y to X by photoirradiation. In this system, a few photons can induce a marked change in such properties as phase separation in polymer solutions, gel-sol transition, and miscibility of polymer blends.

4.2 Phase Separation of Polymer Solutions Poly(N-isopropylacrylamide) in water has a lower critical temperature at 31 °C. Temperatures at which this system undergoes phase separation are expected to vary when photoisomerizable chromophores are introduced into the polymer. Such an

100

8O

60

Fig. 30. Transmittance changes at 750 nm of a 1% aqueous solution of poly(N-isopropylacrylamide) with pendant azobenzene groups (2.7 mol%) when heated at a rate of 2 °C/min. (O) before photoirradiation; (0) under photostationary state with ultraviolet irradiation (410nm > ,l > 350 nm)

u

a 40

E c-

~ 20

I--

15

20 25 Temperature (*C)

30

60

M. lrie

attempt was made by incorporating azobenzene chromophores into the pendant groups [63]. Figure 30 shows the transmittance change at 750 nm which occurred when a 1~o aqueous solution of poly(N-isopropylacrylamide) with 2 . 7 m o l e ~ pendant azobenzene groups was heated. In the dark before photoirradiation, the solution begins to be turbid at 18.5 °C and the transmittance decreases to one-half the initial value at 19.4 °C. Upon ultraviolet irradiation (410 n m > k > 350 nm), the phase separation temperature rises to 26.0 °C. Between 19.4 and 26.0 °C, ultraviolet irradiation solubilizes the polymer and the solution becomes transparent, while visible irradiation decreases the solubility of the polymer and leads to phase separation. The maximum difference in phase separation temperature was observed at a very small azobenzene content of 2.7 mol ~o. Below and above this content, the phase separation was not affected by photoirradiation. This fact indicates that the phase transition temperature depends on a subtle balance between the polymer's ability of hydrogen bond formation with water and the intermolecular hydropholic force. The isomerization of a small number of azobenzene chromophores (2-3 mol ~ ) effects the balance, resulting in an efficient phase separation. A large effect, a large turbidity change in this case, was induced by a small number of photons in the temperature range from 19.4 to 26.0 °C. Below 19.4 and above 26.0 °C, the photostimulated phase separation was not observed. These findings are consistent with the schematic illustration in Fig. 29. Similar phase separation was observed in theta solvents containing polymers with pendant photochromic chromophores. In a theta solvent, the interaction between the polymer and the solvent is in balance with intra- and inter-polymer interactions. The isomerization of the pendant chromophores alters this balance. The system studied was a cyclohexane solution of polystyrene with pendant azobenzene groups [64]. Cyclohexane becomes a theta solvent for polystyrene at 35 °C. Moderate molecular weight polystyrene (Mw = 5 x t04) with pendant azobenzene groups is soluble in

~100

~o

0 c-

ee°°°°°*

• ° e ° e ° l

•

e

• °Ooooeoolol

°eooooooooe

"~ ~0 E

8 20

~

0

I

100

I

l

~

200

t

300

~

l

400

500

Time(s)

Fig. 31. Changes in transmittance at 650 nm o f a cyclohexane solution containing polystyrene having pendant azobenzene groups (content, 6.1 mol ~ ) on alternate irradiation with ultraviolet (410 nm > 2 > 350 nm) and visible (2 > 470 nm) light at 30 °C [641


61

this solvent at 30 °C. The solution became turbid upon ultraviolet irradiation (410 nm > ~ > 350 nm)..Prolonged irradiation caused the polymer to precipitate. The solution became transparent when irradiated with visible light (X > 470 nm). The photoresponsive behavior is shown in Fig. 31. The phase separation is ascribable to the isomerization of the pendant azobenzene groups. Introduction of non-polar trans form azobenzene chromophore into the pendant groups little affects the polymersolvent interaction, while the photogenerated cis form tends to decrease the polymersolvent interaction. Thus, upon ultraviolet irradiation, the polymer-solvent interaction decreases considerably until the polymer precipitates. The precipitation behavior of the polymer is interpreted by a photostimulated change of the critical miscibility temperature T¢. For polystyrene dissolved in cyclohexane, the polymer precipitates at temperature below T¢. According Fox and Flory [65], T~ depends on the molecular weight M as T¢

=

ToO -- b/M °'5)

(2)

where T o is the value of To for M = oe and b an empirical constant. We assume that T o and b change when the trans azobenzene groups convert to the cis form. Figure 32 shows a schematic illustration of the molecular weight dependence of Tc by plotting T¢ against M -°'s. First, we consider a mono-dispersed polystyrene of molecular weight M containing pendant azobenzene groups. When these groups are in the trans form, the polymer solution phase separates at t t, which corresponds to T~ of Fig. 29. The isomerization of the chromophores from the trans to the cis form causes the phase separation temperature to rise to t~, which corresponds to TB of Fig. 29. This means that phase separation of the solution is induced between t t and t¢ by ultraviolet irradiation, which causes the trans-cis isomerization. Next, we consider the phase separation of a poly-dispersed polystyrene with pendant azobenzne groups at constant temperature TM. With all azo groups in the trans configuration, any fractions of M < MT are soluble in cyclohexane at TM, but after photoirradiation, the fractions of M r < M < M.r become insoluble and

{¢

--

TM tt

....

Tm ....

C)

i----"~a~5

t

I. . . . .

t. . . . .

I I I

I I I

MT

M

"%.~T e (T)

I I i M-1/2

Mc

"~"

Fig. 32. Molecular weight (M) dependenceof critical miscible temperature, T,. Te(C) and Te(T) indicate the values of T¢ for polystyrene with cis and trans azobenzene groups at M = oo, respectively. See text for M, tt, te, TM,and Tm

62

M. Irie

J

e

/d IC

1b /

t

|

i

]5

i

t

°

k

i

i

20 25 Etution counts 1

106

I

I

105 10~ Molecu[er weight

30 I

Fig. 33. Irradiation time dependence of molecular weight distribution of insoluble polystyrene having azobenzene groups (content, 4.8 mol %) at 30 °C [64]. Irradiation time: (a) 10, (b) 20, (c) 40, (d) 60, and (e) 90 s. Concentration of the polymer was 0.08 g/dl

103

precipitate from the solution. Actually, irradiation here has the same effect as lowering the solution temperature from T M to Tin. The latter may be applied to fractionate the poly-dispersed polymers by photoirradiation. Figure 33 shows the photoirradiation time dependence of the molecular weight distributions of precipitated polymers. The fractions precipitated in the initial 10 s have the largest molecular weight ( M , = 7.1 x 104) and the smallest Mw/Mn (t.21). As time goes, the smaller molecular weight fractions succesively precipitate. The molecular weight (M,) of the fraction precipitate after 30 s is estimated to be 3.8 x 104. I f the phot~induced solubility change is large enough, the system can be used as a photoresist. Polystyrene with pendant spirobenzopyran groups was found to be useful as a negative type photoresist [66]. The photoresponsive behavior described above is quite efficient. Only 5 mol% azobenzene pendant groups were enough to cause the phase separation. In the system containing spirobenzopyran groups, the efficiency was much higher. The isomerization of 2molto spirobenzopyran chromophores in the pendant groups raised T~ considerably and led to phase separation.

4.3 Sol-Gel Transition The formation of a three dimensional infinite network in a polymer by a chemical or physical process leads to a gel. Polymer gels are classified into two types, irreversible and reversible. The latter are formed by cross-linking due to physical interaction between certain points on different polymer chains, and the sol-gel phase transition is induced by a change in temperature. At temperatures below Tgo~,


63

0 -I v

-2!

Fig. 34. Gel-sol transition of polystyrene (Mn = 1.5 × 104) with (O) 62% eis azobenzene and (@) 100% traps azobenzene groups in CS2 measured by the bail drop method [68]. Arrows indicate the gel melting temperature. The content of the azobenzene groups in the polymer was 10.5 tool% and the polymer concentration in CS2 was 200 g/1

-4

-o'o

50 ' Temp. (°C)

- 4'0

-30

the gelation temperature, the polymer solution stops flowing, while above Tg¢~ the gel melts to flow. Photoirradiation induces a reversible sol-gel phase transition in a polymer in which photoisomerizable chromophores are incorporated. The system so far reported is a CS2 solution of polystyrene with pendant azobenzene groups [67, 68]. Tgel can be conveniently determined by a ball drop method. A steel ball is placed on the top of the gel, and the temperature at which the depth-temperature curve begins to deviate from the horizontal line is taken as Tge~. Two CS2 solutions, one containing polystyrene with partially photoisomerized azobenzene groups (cis content, 6 2 ~ ) and the other containing polystyrene with 100% trans azobenzene groups, were placed in separate tubes, cooled to --78 °C, and allowed to stand for 5 h. Then a steel ball was placed on the top of each gel, and the temperature was raised at a slow rate of 0.5 °C/min. Figure 34 shows how the ball-sinking behavior of the two gels depends on temperature. The ball for the polystyrene gel having trans azobenzene groups begins to sink at --56 °C, whereas the ball for the gel having partially cis azobenzene groups still remains on the top at this temperature. For the latter the ball begins to fall at --47 °C. This Tge~ reverted to --56 °C, when the gel was irradiated for 20 min with visible light at --78 °C, prior to raising the temperature. This behavior can be attributed to the action of visible light which converts the pendant groups from the cis to the trans form. The above finding indicates that the gel-sol transition can be induced isothermally between --56 and --47 °C by changing the irradiation wavelength. Ultraviolet irradiation (400 nm > ~. > 310 nm) converts the sol to the gel state, whereas visible irradiation (L > 450 nm) induces the transition from the gel to the sol state. In fact, a reversible gel-sol transition was observed at --52 °C.

sol ~

~'2

400 n m >

gel kl > 310 nm, ~-z > 450 nm

64

M, Irie

The formation of a photo-reversible gel is explained as follows. The azobenzene groups in the surface region of the polymer coil help the coil overlap. When they change to a more polar cis form, the interactions are strengthened. The change in the interpolymer interactions is responsible for the change in the gel melting temperature.

4.4 Miscibility of Polymer Blends Miscibility of polymer blends is an attractive subject from both scientific and industrial view points. Recently, several attempts have been reported to make immiscible pairs of polymers miscible at room temperature by incorporating a third component. Pearce et al. [69] observed that the immiscible pair of polystyrene and poly(n-butyl methacrylate) became miscible when only 1.8 mot % hydroxy groups were introduced into polystyrene. It was also shown that the cloud point temperature of poly(methyl vinyl ether), PMVE, blended with deuterated polystyrene is 40 °C higher than the temperature of the blend with undeuterated polystyrene [70]. These demonstrate that small changes in polymer properties alter miscibility markedly. This is expected because the miscibility, or the phase separation of polymer mixtures, depends on a subtle balance of inter- and intra-molecular interactions. With the incorporation of photoisomerizable chromophores into one of the paired polymers, we should be able to control the miscibility of the polymer blends by photoirradiation. This prediction was confirmed for PMVE blended with polystyrene having pendant stilbene groups (26) [71].

,,

26

hv

•

27

Stilbene isomerizes from the trans to the cis form when irradiated with ultraviolet light (400 n m > ~. > 300 rim). The photogenerated cis form returns to the trans form by irradiation with light of 254 nm. Thermal isomerization from the cis to the trans form is negligible, even at 200 °C. Figure 35 shows the temperature dependence of the inverse intensity of light transmitted through PMVE blended with polystyrene having either all trans or partially cis pendant stilbene groups (cis content, 25 mol%). Both blends are transparent at room temperature. When heated from room temperature at a rate of 5 °C/min, the blend containing cis stilbene groups turns optically opalescent at 78 °C, while the blend containing trans stilbene groups still remains transparent at that temperature. The latter turns opalescent at 101 °C. The cloud point temperature of the blend containing 0.25 cis fraction is 23 °C lower than that of the blend containing all trans stilbene groups. Although the molecular basis of the miscibility change is not yet fully understood, the interaction between phenyl rings and C O C H 3 groups is considered to play an important role. Probably, the introduction of bulky stilbene pendant groups perturbs the efficient packing of PMVE with polystyrene, causing the the phase separation


65

50

!

_~ 30

J/

"E:

o/

/

0

j/

10 t

I

I

I

I

70

80

90

I00

II0

120

T/°C

Fig. 35. Temperature dependence of the inverse of the transmittance fight, 1/Tr, after passage through blend of PMVE with polystyrene having (O) all trans stilbene groups and ( ~ ) partially cis stilbene groups (cis content, 0.25) [71]. Content of the stilbene groups in the copolymer was 15.2 m o l ~ temperature to lower. Conversion o f the stilbene groups from the trans to the cis form further decreases the cloud point temperature. D u r i n g the isomerization, the molecule change the geometry from p l a n a r to "propeller-shaped". Thus, the molecular volume increases by the isomerization. The expansion o f the molecular Volume will decrease the intermolecular interaction forces between the phenyl rings a n d C O C H 3 groups, thus bringing a b o u t a decrease in the cloud p o i n t temperature.

5 Acknowledgement The a u t h o r wishes to thank Professor K. H a y a s h i o f O s a k a University for continuous encouragement. H e is deeply indebted to Professor W. Schnabel o f the H a h n Meitner Institute for his stimulating contributions to the laser experiments o f Sect. 2.2. H e is also grateful to Emeritus Professor H. F u j i t a o f O s a k a University for critically inspecting the manuscript a n d giving invaluable comments. He also thanks all o f his co-workers for their devoted contributions.

6 References 1. 2. 3. 4. 5. 6.

Montagnoli G, Erlanger BF (1983) Molecular models of photoresponsiveness, Plenum Etzold H (1965) Planta 64:254 Brown GH ~,971) Photochromism, Wiley-Interscience Lovrien R (1967) Proc. Nat. Acad. Sei. 57:236 Van der Veen G, Prins W (1974) Photochem. Photobiol. 19:191 Negishi N, Takahashi M, lwazawa A, Matsuyama K, Shinohara I: Nippon Kagaku Kaishi 1977: 1035 7. Negishi N, Ishihara K, Shinohara I (1982) J. Polym. Sci., Chem. Ed. 20:1907 8. Mat~jka L, Dn~ek K (1981) Makromol. Chem. 182:3223 9. Irie M, Suzuki T (1987) Makromol. Chem. Rapid Commun. 8:607 10. Goldburt E, Shvartsman F, Fishman S, Kronganz V (1984) Macromolecules 17:1225 11. Zimmerman G, Chow L, Paik U (1958) J. Am. Chem. Soc. 80:3528 12. Hampson GC, Robertson JM: J. Chem. Soc. 1941:409 13. Irie M, Hayashi K (1979) J. Macromol. Sci., Chem. A13:511

66 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.

M. hie trie M, Hirano K, Hashimoto S, Hayashi K (t981) Macromolecules 14:262 Blair HS, Pogue HI, Riordan JE (t980) Polymer 21:1195 Kumar GS, DePra P, Neckers DC (1984) Macromolecules 17:1912 Kumar GS, DePra P, Zhang K, Neckers DC (1984) Macromolecules 17:2463 Kumar GS, Savariar C, Sattran M, Neckers DC (1985) Macromolecules 18:1525 Zimmerman EK, Stitle JK (1985) Macromolecules 14:1246 Irie M, Hosoda M (1985) Makromol. Chem. Rapid Commun. 6:533 Irie M, Menju A, Hayashi K (1981) Macromolecules 12:1176 Menju A, Hayashi K, Irie M (1981) Macromolecules 14:755 Ueno A, Osa T (1980) Yuki Gosei Kagaku 38:267 a. Ciardelli F, Cartini C, Salaro R, Altomare A, Pieroni O, Houben JL, Fiss A (1984) Pure Appl. Chem. 56:329 b. Fissi A, Pieroni O, Ciardelli F (1987) Biopolymer 26:1993 Irie M, Schnabel W (1983) Maeromolecules 14:1246 Irie M, Schnabel W (1985) Macromolecules 18:394 Merian E (1966) Text. Res. J. 36:612 Blair HS, Law TK (1980) Polymer 21 : 1475 Blair HS, Pogue HI (1982) Polymer 23:779 Agolini F, Gay FP (1970) Macromolecules 3:349 Osada Y, Katsumura K, Inoue K (1981) Makromol. Chem. Rapid Commun. 2:47 Kohjiya S, Hashimoto T, Yamashita S, Irie M: Chem. Lett. 1985:1479 Smets G, Evans G (1973) Pure Appl. Chem. Macromol. Chem. 8:357 Smets G, Breaken J, Irie M (1978) Pure Appl. Chem. 50:845 Eisenbach CD (1980) Polymer 21 : 1175 Mat~jka L, Ilavsky M, Du~ek K, Wiehterle O (1981) Polymer 22:1511 Van der Veen G, Prins W (1971) Nature Phys. Sci. 230:70 Chuang JC, de Sorgo M, Prins W (t973) J. Mechanoehem. Cell. Motility 2:105 Mat~jka L, Du~ek K, Ilavsk~, M (1979) Potym. Bull. 1 : 659 Ishihara K, Hamada N, Kato S, Shinohara I (1984) J. Polym, Sci. Chem. Ed., 22:121 Aviram A (1978) Macromolecules 11:1275 Irie M, Kungwatchakun D (1984) Makromol. Chem. Rapid Commun. 5:829 Irie M, Kungwatchakun D (1986) Macromolecules t9:2476 Irie M (1986) Macromolecules t9:2890 Ishihara K, Okazaki A, Negishi N, Shinohara I, Okano T, Kataoka K, Sakurai Y (1982) J. Appl. Polym. Sci. 27:239 a. Ishihara K, Kato S, Shinohara I (1983) J. Appl. Polym. Sei. 28:1321 b. Negishi N, Tsunemitsu K, Shinohara I (1981) Polym. J. 13:411 Ishihara K, Hamada N, Kato S, Shinohara I (1983) J. Polym. Sci., Chem. Ed., 21: 155I Irie M, Iga R (1987) Makromol. Chem. Rapid Commun. 8:569 Negishi N, Ishihara K, Shinohara I, Okano I, Kataoka K, Sakurai Y, Akaike T: Chem. Lett. 1981 : 681 Negishi N, Ishihara K, Shinohara t, Okano T, Kataoka K, Sakurai Y (198t) Makromol. Chem. Rapid Commun. 2:95 Kato S, Aizawa M, Suzuki S (1976) J. Membrane. Sci. 1 : 289 Irie M, Menju A, Hayashi K: Nippon Kagaku Kaishi 1984:227 Anzai J, Sasaki H, Ueno A, Osa T: Chem. Lett. 1985:1443 Anzai J, Sasaki H, Ueno A, Osa T (1986) Makromot. Chem. Rapid Commun. 7:133 Anzai J, Sasaki H, Ueno A, Osa T: J. Chem. Soc. Chem. Commun. I983:1045 Sasaki H, Ueno A, Anzai J, Osa T (1986) Bull. Chem. Soc. Jpn. 59:1953 a. Yasuda H, Lamaze CE, Peterlin A (1971) J. Polym. Sci. A-2, 9:1117 b. Zentner GM, Cardinal JR, Kim SW (1978) J. Pharm. Sci. 67:1352 Ishihara K, Shinohara I (1984) Potym. Sci., Polym. Lett. Ed. 22:515 Irie M, Kungwatchakun D: to be published. Sato M, Kinoshita T, Takizawa A, Tsujita Y (I988) Polym. J. 20:729 Sato M, Kinoshita T, Takizawa A, Tsujita Y (1988) Polym. J. 20:761 Irie M, Hayashi K, Menju A ('1981) Polym. Photochemistry 1:233 Kungwatchakun D, Irie M (1988) Makromol. Chem. Rapid Commun. 9:243

Photoresponsive Polymers 64. 65. 66. 67. 68. 69. 70. 71.

Irie M, Tanaka H (1983) Macromolecules 16:210 Fox TG, Flory PJ (1951) J. Am. Chem. Soc. 73: 1909, 1915 Irie M, Iwayanagi T, Taniguchi Y (1985) Macromolecules 18:2418 Irie M, lga R (1985) Makromol. Chem. Rapid Commun. 6:403 Irie M, Iga R (1986) Macromolecules 19:2480 Pearce EM, Kwei TK, Min BY (1984) J. Makromol. Sci. Chem. 21 : 1481 Halary JL, Ubrich JM, Monnerie L, Yang H, Stein RS (1985) Polymer Commun. 21 : 73 Irie M, Iga R (1986) Makromol. Chem. Rapid Commun. 7:751

Edited by H. Fujita Received June 8, 1989

67

Telechelic Oligomers by Radical Reactions B. B o u t e v i n E c o l e N a t i o n a l e S u p e r i e u r e de C h i m i e de M o n t p e l l i e r L a b o r a t o i r e de C h i m i e A p p l i q u e e U R A D 11930 C N R S / F r a n c e , 8 R u e E c o l e N o r m a l e , M o n t p e l l i e r / F r a n c e

The synthesis of telechelic oligomers by a radical reaction can be carried out either by functional initiators or by telomerization. This review describes the different methods of synthesis used to obtain these oligomers. First, we mention the preparation and use of the functional diazoic initiators, functional peresters, hydrogen peroxide, hybrid initiators (which contain two kinds of labile groups) and macromolecular polyinitiators. Then, new initiators disubstituted (tetraphenyl ethanes or thiurams functional or not) which are real precursors of living oligomers are described. In each case, the kinetical data (decomposition rate constants and efficiency) are provided. In the second part, surveys concerning telechelic telomers are mentioned. The different telogens used and their known transfer-constants are supplied: aliphatic and aromatic disulfides, disubstituted benzoyl disulfides, functional xanthogens and polyhalogenated compounds. Also novel monoaddition reactions of functional monomers onto both end groups of dithiols and ditrichtoromethyl compounds are summarized.

1 Introduction

. . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Synthesis of Tdechelic Oligomers by Radical Initiation

71

. . . . . . . . . .

71

2.1 F u n c t i o n a l I n i t i a t o r . . . . . . . . . . . . . . . . . . . . . . . 2.2 T e l o m e r i z a t i o n . . . . . . . . . . . . . . . . . . . . . . . . .

71 72

3 0 l i g o m e r i z a t i o n s by Functional Initiators . . . . . . . . . . . . . . . . 3.1 T e r m i n a t i o n R e a c t i o n o f V a r i o u s M o n o m e r s . . . . . . . . . . . . 3.2 Efficiency o f t h e I n i t i a t o r . . . . . . . . . . . . . . . . . . . . . 3.3 D e a d - e n d P o l y m e r i z a t i o n . . . . . . . . . . . . . . . . . . . . 3.4 D i a z o i c C o m p o u n d s . . . . . . . . . . . . . . . . . . . . . . . 3.5 O x y g e n a t e d C o m p o u n d s . . . . . . . . . . . . . . . . . . . . . 3.6 H y d r o g e n P e r o x i d e . . . . . . . . . . . . . . . . . . . . . . . 3.7 T e t r a a l k y l t h i u r a m Disulfides . . . . . . . . . . . . . . . . . . . 3.8 D i s u b s t i t u t e d T e t r a p h e n y l E t h a n e s . . . . . . . . . . . . . . . . .

73 73 74 75 76 79 83 86 89

4 Telomerization . . . . . . . . . . . . . . . . . . . . . . . . . . .

91 91 93 94 96

4.1 4.2 4.3 4.4

Disulfides . . . . . . . . . . . . A c i d Disulfides . . . . . . . . . . Xanthogens . . . . . . . . . . . . The Uses of Halogenated Telogens .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . . .

Advances in Polymer Science 94 © Springer-Verlag Berlin Heidelberg 1990

70

B. Boutevin 4.5 The Use of Non-conjugated Dienes . . . . . . . . . . . . . . . . 4.6 The Uses of C o m p o u n d s which Exhibit CC13 end groups . . . . . . . 4.7 Synthesis of Tetechelic Products by R h o d i u m Catalysis . . . . . . . .

98 99 100

5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102

6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102

TelechelicOligomersby Radical Reactions

71

1 Introduction The synthesis of telechelic oligomers has been brought up in numerous genera1 publications. There are four main ways of synthesis: -- polymerization by condensation or polyaddition; -oxidative cleavages of polymers; -ionic polymerizations; -- radical polymerizations. This paper deals with this last method. Yet, the publications are quite general and deal with these four topics without giving a complete overall review. Furthermore, they are spaced in the time from the seventies up to now. Lastly, they are either general or very specific (for instance they mention topics such as dienes, functionality or molecular distribution). With reference to the general reviews of particular note are the surveys carried out by Brossas et al. [1] (about 100 references), then Athey [2, 3] (roughly 50 references), Barentsevich [4] (66 references) and finally those published in 1987 by Heitz [5, 6] and Brosse et al. [7] (76 and 260 references, respectively). For specific papers on oligomers prepared from dienes, special mention is made to French's [8] and Schnecko's [9] works (126 and 68 references respectively). Then, we can mention a special study by Entelis [10, 11] based on the functionality and the molecular distribution of telechetic polymers. Before going into the methods for radical reactions it most be said that polycondensation or polyaddition have led to more industrial preparation. In this connection epoxy resins, the polyurethanes obtained from prepolymers and, more recently, more specialized polymers such as the PEBAC (ATOCHEM), amid-ether or polyimids (KHERIMIDE from RHONE POULENC must be mentioned). Moreover, it is interesting to note that the ionic methods (cationic or anionic ones) have not produced industrial products (except dihydroxy poly (dimethyl siloxanes), poly (tetrahydrofuranes)) but they have facilitated theoretical studies both on the analytical aspects and the materials we can obtain. In this paper we are going to the syntheses of telechelic oligomers by the radical method. Such a topic has already been studied very often. In this field of research, there are two main parts for the obtaining of such oligomers: -the use of functional initiators, -the telomerization.

2 Synthesis of Telechelic Oligomers by Radical Initiation 2.1

Functional

Initiator

Generally, this initiator is symbolized by G - - A - - A - - G where G represents the functional group and A the part of the molecule which is cleaved by heat or radiation: G-A-A-G

0o~hv 2 GA'

G A ' + M --~ GAM' " ~ G - A - ( M ) ~ + t

(1) (2)

72

B. Boutevin

The above reactions represent the initiation and propagation steps. The two last steps determine whether the oligomer will be telechelic or not. Termination :

combina~

GA(M)2. AG

(3 a)

2 GA(M).

(3 b)

GA(M)~~ td~oPnra~.P-o%r~

The combination step itself gives the telechelic compound; however, there is a lack of knowledge about both the last steps. Heitz [4] gave interesting data on this topic. Transfer:

GA-(M)~ + XY --. GA-(M). Y + X"

(4)

Using a functional Y lads to the telechelic compound but generally one of both end groups of XY is not functional and in this case, it does not produce the difunctionality of the oligomer.

2.2 Telomerization The initiator A 2 is not necessarily functional, but in this case, the telogen is difunctional or may be potentially functional: G - - T - - T - - G A 2 ~ 2 A"

(5)

A" + G T T G --. ATG + GT"

(6)

G T ' + M ~ GTM" 2_~ GT(M)~+I

(7t

As previously, these reactions above correspond to the initiation and propagation steps. As for the termination and transfer steps, the latter one is mainly favored. Actually, the G T T G compound has been chosen since it reacts quite easily with radicals in the middle: GT(M). + G T - T G ~ GT(M)nTG + GT

(8)

If the termination occurs by recombination, we obtain the difunctionality; on the other hand, every disproportionation diminishes this last characteristic. Furthermore, we might expect the difunctional telomers to exhibit molecular weights much lower than those of the telechelic oligomers obtained with the difunctional initiators without the transfer step. Thus, in both cases, the disproportionation adversely effects the obtaining of the difunctional compound.

Telechelic Oligomers by Radical Reactions

73

30ligomerizations by Functional Initiators We can distinguish three main categories of functional initiators: - - diazoic compounds, - - hydrogen peroxide, - - oxygenated substances. Each of them is quite wide and several regroupings can be expected. Hydrogen peroxide is rather close to the third variety but its reactivity and its solubility in water show that such reactant represents, itself, a unique category for its industrial applications. Before studying each series of initiator, we should mention again that difunctionalization is connected with the way the radicals can recombine. Thus, we will study first the behavior of different monomers to the reaction of termination.

3.1 Termination Reaction of Various Monomers Bamford and Tipper [12] gave the results gathered in Table 1 for the termination step. Table 1. Percent of recombination (p) of various monomers at 25 °C Monomers

P

Styrene p CI styrene p OCH3 styrene Methyl/ethyl acrylate Methyl/ethyl methacrylate Acrylonitrile Methacrylonitrile

100 100 81 100 33 100 35

As regards styrene, several authors [13] found 80% instead of 100%. Heitz [5] noticed that the reactivity of the intermediate radical takes an important part. Actually, if it is primary, it tends to be very reactive and attacks either the polymer (especially at high conversion rate) and this leads to trifunctional compounds, or reacts with the solvent (monofunctional species), or reacts with the initiator (telechelic compound). Heitz and Guth [14] showed that the functionality of the product resulting from the addition of ethylene onto AIBN in tertiobutanol is 1.7. The reactivity of the primary radicals allows one to predict t.8 [15] but we must consider that the transfer is an important step. When methanol is used as solvent, the amount of product obtained by transfer is 10 % and with benzene it reaches 20 %. With reference to vinyl acetate, few data have been given but Brosse et al. [7] described the synthesis of oligomers the molecular weights of which are Mn = 500 to 4,000. They mentioned that the initiation with H202 leads to a polymer with a Mn

74

B. Boutevin

which depends upon the vinyl acetate concentration, and the functionality is 2 anyway [16]. For butene, Brosse et al. [17] gave the following series: 1-butene > 2-butene > isobutene The molecular weights Mn are below 1,000 and their functionality varies between 2 and 4. This lack of reactivity explains why these monomers are mostly used in copolymerization. As for fluorinated monomers, Rice et al. [18] noticed that the termination of H 2 C = C F 2 and F z C = C F - - C F 3 occurs by recombination. Besides this parameter which depends on the recombination or the disproportionation of macromolecular radicals, we must consider, in the case of telechelic oligomers, the reactions with primary radicals obtained from the direct decomposition of the initiator:

GA(M)~, + AG"

~GA(M). AG [GA(M). H + AG(minus H)

(9a) (9b)

Besides, it is natural that in those reactions where we are seeking to obtain low molecular weights, we must use a higher initiator concentration with reference to the M A Y O equation and consequently the reactional mixture is rich in AG' radicals. Thus, we must consider those reactions (of termination of macroradical with primary radical (Eq. (9)) which have not necessarily the same reactivity ratio, as the termination of two macroradicals. And this makes it often possible to obtain telechelic oligomers for monomers which give a non-negligible amount of disproportionation reactions in traditional polymerization. The price of the difunctional initiator is also a crucial parameter. Actually this is the most expensive compound and it is essential that it is entirely used. Thus, in classical polymerization, we are seeking to use an important initiator amount in order to obtain a high conversion rate in the monomer. On the other hand, when we prepare a telechelic oligomer, the initiator must be consumed completely and this leads to two control parameters: -- the efficiency of the initiator on one hand, and - - on the other hand, the kinetical conditions to use up completely the functional initiators. 3.2 Efficiency o f the Initiator

The different reactions involved are Kd

R 2 ---~ 2R" K¢

(10)

R" + M - - ~ RM'

(11)

R" + R" Ki, R2(inactivated product)

(12)


75

The efficiency can be written according to/the following formula: f=

K~(R') (M) K~(R') (M) + K~(R') z

=

(M) (M) + Kffk~(R')

(13)

Such an equation shows easily that the initiators are more active at high monomer concentrations. Moad et al. [19] studied the change of the effeciency with the monomer concentration and from the obtained curve, they found: K e

K--T"(R') = 10 The crucial igoint is the determination of f. Heitz [5] suggested a new relation : f =

A(M) e/2(DPn)~.m A(I2)

(t4)

where A(M) and A(I2) are the monomers and initiators consumptions and e represents the functionality of the oligomers. Determining e is also difficult and this is true for all the types of polymerizations. Heitz [4] suggested the formula: DPn e = 2 - 2 DPno

(15)

where D P and DP----orepresent the real and without transfer values of DP~.

3.3 Dead end Polymerization Tobolski [20, 21] showed that log (M)° - K(I2)lo/2 ( 1 - e(-Kdt)/2) (M) with

(16)

2 Kpf 1/2 K - K1/21z- t/2

d X~Te

By assuming that the initiator is consumed faster than the monomer, (Izo0 = 0 and (M)o~ is determined from the equation: log

(M)o (M)~o

-- K(12)~/2

(i7)

Knowing K permits the calculation of (M)~o. With reference to the volume contractions in the polymerization of high monomer concentration, Tobotski gave more elaborated equations [21] which are still widely used.

76

B. Boutevin

The term involving the energy, EpEgU2ETeU2 directs the polymer decomposition rate, whereas E a directs that of the initiator. In most cases, 20 < E a < 40 Kcal/mol; 25 < Eve N 5 Kcal/mol and 6,5 < Ep < _ 8 Kcal/mol and it is easy to show that an increase of the temperature favors more the initiator-decomposition than the monomer-disappearance. Practically, we control the temperature in order to obtain a faster initiator decomposition than that of the monomer and the time is calculated to be greater than 10 initiator half-lives. 3.4 Diazoic Compounds

The synthesis is that from Strecker; R

\

R

C=O + H2N-NH 2 ~

CHf

N

C=N-N=C

CN R

C-NH-NH-C CHf

~ C H 3 Hnr

R

\CH 3

CHf

HCN R C N

/

CN

CN

R-C-N=N-C-R [ [ CH 3 CHs

The most commun diazoic compound is AIBN ( R = C H a ) but CH3COCH2CH2R (R---CO2H , CH2OH ) can be used as a starting material and then th~ corresponding products are [5]: ~H3

~H3

CN - C - N = N - C - C N CH 2

1 I ~H2

CH2

J I ~H2

R

R

From AIBN, we can prepare another series of diazoic initiators: O AIBN + ROH

CH31

|CHs

O

.~

R = CH3, (CH2) n - O H Therefore, we have two series of diazoic compounds the nomenclature of which has been remarkably described by Brosse et al. [7]: - - The xx'-azobis (x cyano-alkanol) and, - - The di (x-hydroxyalkyl)-2,2'-azobisisobutyrate.

77


A wide range of products are potentially available and their applications are numerous. Table 2 gives kinetic oata (K a, activation energy etc...). It is interesting to note recent or innovating work in this area. First, Ghatge et al. [23] obtained a diazo initiator which is a precursor ofisocyanate according to the following reactions:

~N HO2C-(CH2)2-~-N=N~CO2H

+ 2N(Et)3

CH 3

CN + Et3NHOzC-(CH2) 2- ~ -N=N~

CO 2H

/

CH3 o O O CN , H II I EtOC-Cl_loocE t - O - C - O - C - ( C H z ) z - ( ~ - N = N ~ O E t CH3 O

CN

O

-2 to 0°C, N 3 - C - ( C H 2 ) / - CI- N = N ~ C N CH 3

3

This is noteworthy since the azide leads to an isocyanate during the thermal polymerization. O Jl

CN i

CN J

N3C-(CH2)2-~-N=N

~N 3 + nM ~

OC=N-(CH2)2-~-(M)n

CH 3

CH3

In the same way, Heitz's works [25, 26] are also interesting. ~H3

~H3

CH3

CH 3

N C - C - ( S t y ) . - C . - C N H2/co H2N_CH2_t_(Sty) _~._CH2_NH 2 CH 3

~H 3

CH 3

l CH3

1 240 ° C/30' h, HO2C-

~H 3 2

CH 3

+

-(StY)n- - C O 2 H CH 3

CH 3

78

B. Boutevin

~H3 O C = N - C H 2 - C - (Sty).- C - C H z - N=CO ~Hs

2 COC12;

CH 3

a) C1-O-OEt/NET3

b, N~N3

CH 3

CH3 1

~H3

CH 3

CH 3

... O C = N - C - ( S t y ) . - C - N = C O

Table 2, Kinetical parameters of several diazoic functional initiators Solvent

I05 • Kd (T °C)

Ref,

E KJ/

mol

~ H3 @H

CH3-C-N=N-

[81

16.0 (80)

CN

~ H3 HO-(CH3)3-C. - N = N -

0.78

[31

130

0.72

[3]

4.6 (70) 9.0 (80)

130 --

0.68 --

[3] [81

Et

8.3 (80)

120

MeOH

4.6 (95)

127

acetone

4.3 (80)

acetone

4.5 (70)

acetone H20

crN

~

H3 Br-(CH2)a-~-N=NCN CH 3 HO2C-(CHz)z- + - N = N CN

~ H3 R-O-C-C-N=N-

If

I

[3]

O CH 3 CH 3 O

0.70

[3]

CH 3

~ H3

Toluene

HO-CH2CH2-O~-~-N=NO CH 3

20 (80)

1.6 (60)

130

2.0 (80) 2.0 (80)

125 130

[25] Dioxane DMF

TelechelicOligomersby RadicalReactions

79

These surveys permit the obtaining of telechelic diisocyanates and diamines, unknown so far. The side-reactions must also be considered with diazoic derivatives:

~H3 NC-C I f CH

~H3 C-CN I CH 3

coupling

CH 3. AIBN ---* NC-~" CH 3~ " ~

~ H3

/CN

N C - ~ - H + H2C=C CH3 ~CH3

disproportionation

~ H3 CH-C-N=C=C I I CH 3 CH 3

H3 ~ H3 NC-C C I I CH 3 CN

~ H3 C-CN I CH 3

trimer

The coupling was already observed in 1950 by Bicker et al. [28]. On the other hand, the transfer constant to the polymer is always low since the cyanoisopropyl radical has a rather weak reactivity.

3.5 Oxygenated Compounds As Heitz mentioned, the problem is more complex with the peracids: O

tl

O

II

R-CO-O-C-R. O R - C / / O O%C-R -* 2 R - C / / M_~ polymer \O--O / [ \O _CO2 " 2 R. ~

polymer

80

B. Boutevin

when R is aliphatic, the R" radical is mainly produced and when R is an acyl group, there is a mixture of O - C O " and O'. Therefore, two series of polymeric products can be obtained. O O This also occurs for the dicarbonates tl t1 which lose a CO 2 RO-C-O-O-C-OR molecule and the RO" radicals initiate the polymerization (except for R = t-But and perhaps for R = cyclohexyl [29]). Furthermore, these authors showed that the reactivity of the RO' radical was much weaker than that of the RO--CO' group; this was observed from the 14C tracing of the carbonyl group. However, few functional oxygenated compounds were mentioned except by some teams. In 1962, the Thiokol Company [30] developed the synthesis of dicarboxylic polybutadienes from functional peresters prepared, according to Clover and Houghton, [31] as indicated in the following scheme:

/R. O=C\o/C=O 10 to 20°C

2h

+ H202

, HO2C- R-CO-O2-

OC- R-CO2H

Thus, 70 ~o of the initiators lose a C O 2 molecule when added on to the monomer. We can note the particularly interesting kinetical study carried out by continuously adding both the initiator and the monomer in order to obtain an expected average molecular weight (all was calculated by computer). The patent is for the synthesis and processing, and this needs to be elaborated. Rice and Sandberg [18] suggested another interesting survey, using fluorinated compounds:

CIOC--(CF2)3--COC1+ R O H

~ RO2C--(CF2)3--COCt

NaOH/n20 ,, R O z C - ( C F z ) 3 - C O - O O - C O - ( C F z ) 3 - C O 2 R H202

VDF/nFP, RO2C_(CFz)3_(CH2CF2)n_(CF 2_~ F)p_(CF2)3_CO2 R CF3 functional copotymer Mn = 3,000 Barentsevich et al. [4] suggested a series of initiators listed in Table 3. It is possible to introduce CO2H o r CC13 as end groups that can be changed. We present the results obtained with polyfunctional oxygenated or nitrogenous initiators. Ivanchev [36] proposed a rationalization of particular functional initiators. More recently, Simionescu et al. [37] have written a complete review the synthesis of initiators di- and polyfunctional.


81

First, we list the traditional initiators whose reactivity decreases in tim series: O

II

O

O

II

II

ROCO-OCOR

O

O

II

II

> RCO-OCR > RCO-OR > RO-OR > RO-OH > (R3E--O)2-where E = Si, B, Sn

Table 3. Kinetical characteristics of the oxygenated initiators Product

Solvent

l0s Kd (T °C)

EaKJ/ moL

f

Ref.

Acetone Toluene

4.8 (70) 0.8 (70)

105

093 --

[33]

CCI3-O-C~ O O-

Toluene

0.5 (70)

--

0.90

[34]

O HO2C_(CH2)2_C// \ O-

BuOH DMF Acetone

0.6 (90) 1.6 (70) 4.3 (70)

104 ---

0.20 ---

I4]

--

--

[35]

//O C1CH2-~-C\o-

[32]

Otl CH3_O_C_ O_

H20

Then, the polyfunctional initiators are classified into three parts: • X-R-OO-R-Y • H2C=CH

\

RO-O-R' • -(R"-O-O-R").where X and Y represent the functional groups, R a divalent group, R' an alkyl group a n d R " an alkylene one. This first class has previously been discussed. In the second type of monomer, the m o n o m e r must exhibit a lateral group labile thermically. F o r instance, Ivanchev proposed that /CHs H2C=C \CO2-CH2-CH2-OOBu.

82

B. Boutevin Furthermore, azoic compounds [37] can be used such as:

H2C=CH-O-N=N-C-CH

3

These monomers are precursors of graft polymers and curing agents. The third part is composed of bi-initiators which contain a number of limited labile groups on perfectly defined organic molecules. For instance, is azo-peroxy, peroxy-peroxy and azo-carbonate the two functions are cleaved at different temperatures and this enables one to add labile end-groups in the polymers. They then can be further used to prepare block copolymers. Table 4 lists several examples o f such products with their kinetical characteristics. O O With II I1 lvanchev et al. [39] prepared adpolydispersed (R-OO-C-(CHz)z-CO ~ polymer with a functionality of 1.50-1.85. They obtained 50 ~o of bifunctional, 38 ~o of monofunctional and 12 ~o of a mixture composed of tri- and tetrafunctional products. The functionality and the yield depend upon the experimental procedure, i.e. the higher the temperature, the lower the functionality.

Table 4. Kineticat characteristics of potyfunctional initiators Product

Solvent

c//O ROO-C-CH2-

• Et

'\O-

10s Kd 6.7 (80)

EKJ/mol (T °C)

• Et

14.0 (80)

126.5

3.5 (t lo)

158.8

R-O0-C-O-C\ o~H3 //O R-N=N-C. -(CH2)3-C\ I OCH3

~ H3 CN

0.4

[401

0.4

[411

t.9 (63) p O CI

--

[421

-

[42l

--

[42]

1.9 (73) a 1.9 (45) p OCl

O

~H3 R - O - O - ,C,-(CHz)2-C, - N = N 11 I O CN a = azo; p = peroxy

Ref.

--

3.7(110) 156.3

~O

f

1.9 (73) a

12.5 (80) a 0 CI 3.6 (80) p


83

Lastly, this third group also contains macromolecular poly-initiators which have been developed for a long time by A. E. Woodwards and G. Smets [43]. The principle of the reaction is the polymerization of a monomer (M,) with the phtaloyle peroxide - - ( O O O C - - O - - C O - - O - - C O - - O - - C O ) --. The obtained homopolymer still exhibits perester groups which the authors used for initiating polymerization of a second monomer (M2) and for obtaining multiblock copolymers. The different M1/M 2 monomers blends used were Styrene/MMA, Sty/VaC and Sty/N-vinylpyrolidone. Ivanchev used the polyperesters: --(OC--(CH2h--CO--OO--CO--(CH2)6 - C 0 - 0 - 0 ) I8

In the same way, Matsushima et al. [44] prepared block copolymers from styrene, vinylacetate, vinylchloride and acrylic monomers with the following initiators: -- (CO-- (CH 2)4- CO2 - - (CH2 CH2 Oh -- C O - - (CH 2)4-- CO--OO)s.3

Finally, Heitz [45] recently synthesized polyesters which had azo groups: .CI/OH

AIBN + H O - R - O H

0to-5°C

~H3

~H3

, HO-R-O-(CO-~-N=N-~-CO2RO).CH 3

H

CH 3

3.6 Hydrogen Peroxide This peroxide is certainly the most used initiator for different reasons, its price, its solubility in water and a reaction temperature lower than 50 °C. Brosse et al. [7] have proposed the most complete review, by far. From the Fenton's reagent [46] H202/FeSO,, HaO 2 + Fe2+ ~ F e 3+ + H O -

+HO',

in an aqueous middle, a radical is generated and is able to initiate a polymerization. Furthermore, Fe a ÷ is also able to decompose H202: H202 + Fe 3+ ~ F e 2+ + H ÷ + H O O " H O O ' ~ 0 2 "q- H' These redox systems have been prepared either in aqueous medium or in an organic system (HCIOJH202 or BF3/etherate/H202). We can add an alcohol to the system and the mechanism is as follows: H202 + Fe 2 + ~ Fe 3 + + O H - + HO" HO" + H - - R - - O H ~ H20 + HO--R" HOR" + mM ~ HOR(M)n ~ HO--(M)2n--OH

84

B. Boutevin

The yield depends upon the nature of alcohol. tBuOH > nBuOH > iPrOH > EtOH However, t-120 2 is not used only with a redox system but directly as a peroxide H 2 0 2 ~ 2 HO'. Because of its particular solubility and that of the nonomers, a co-solvant has also been used (very often an alcohol). Generally this solvent induces secondary effects according to the solubility of the polymer in the alcohol:

Solubility not soluble

{

Product

Example

rich in H202 : oligomer (O)

diene

rich in M : polymer (P) 1 type of product

soluble

/ Mn = 500 (O) /

VAC

Mn 1,500-11,000(P) (Mn 500-4,000)

The different parameters which influence the reaction, the oligomer/polymer ~ratio essentially and the molecular weight Mn are the temperature, the nature of the alcohol, and the monomer and hydrogen peroxide concentrations. Moreover, the lower the monomer concentration, the lower is the molecular weight Mn. The most interesting case concerns the poly(butadiene) hydroxytelechelic (PBHT) and deals with the repartition of the hydroxy groups in the chain and the nature of the chain (1-2, 1 4 c/s and trans). As for the nature of the hydroxy groups, the PHAM's N M R study on the polymerization of butadiene in 2-butanol [44] is of special interest. Table 5.

HO-H2C \ ( HO-H2CN__ --\ or - C -

HO-HzC\=/

Mn=500

Mn=2,800

56 30 10

35 46 19

IL

CH-CHzOH unidentified

4

functionality (~/o) Conversion (~o) Barentsevich [4] confirmed and explained the 1-2 nature of the first base unit, even in the case of initiation by ( C I - - C H 2 - - O - - C O - - O ) 2 and this came from the allylic radical of butadiene. R - C H 2 - ( C H == CH .... CH2).


85

If R = OH or ~CH2C1, the conjugation is decreasing and we obtain more 1-2 bonds. However, this effect disappears when the chain length increases. With reference to the functionality, it is generally greater than 2 for the polydienes. Heitz [5], who carefully studied the radical's reactivity, showed that the HO' radical is very reactive and it is able to initiate side reactions. This may explain the functionality in the range of 2.2 to 2.3 and also the presence of mono-, di- and trifunctional compounds although the difunctional ones are more numerous at a low conversion rate. Schnecko [9] confirmed these results and gave an interesting graph from Falkova's survey [48]. This shows that the HO" radical has been added onto the butadiene by making first a 1,2 base unit but the propagation is rather a 1,4 type. Thus, for low Mn, the 1,2 bond amount is greater than for high Mn. Furthermore, the HO" radical reacts with the growing-chain and leads to the trifunctional products. Moreover transfer reactions to solvents, initiators and other compounds lead to monofunctional compounds. However, functionality, greater then two, is obtained, so we observe that trifunctional compounds favoured over monofunctional ones. 100 % 80

.'= 60 tO

"6 g ~0

I.L

20

0

I

I

i

I

20

&O

60

80

% ~00

Conversion

Finally, Brosse [7] showed that the functionality depends upon Mn. It remains constant between 1,000 and 4,000 and then it increases up to 5 or 7 when Mn varies between 4,000 and 10,000 [49]. From a kinetical point of view, it is interesting to notice the transfer constants to the initiator and to the alcohols [50] at 120 °C for the different monomers: butadiene (B), isoprene (I) and vinvlacetate (VAc) listed in Table 6. The transfer constant decreases in the following series of solvent [7]: M e O H > E t O H > nPrOH > iPrOH The dienes might be copolymerized with numerous monomers such as styrene in most cases and also p-bromostyrene [51], some acrylates [52] and acrylonitrile for instance.

86

B. Boutevin

6. Various transfer constants for the polymerization of dienes and vinylaeetate initiated by thermally decomposed hydrogen peroxide

Table

Monomer

C~o n

C~¢ntanol

C.2oz

B

0.3

--

I VAc

5 x 10 -4 10-3

1.6 --

0.190 0.215 0.300

×

10 -6

Two other types of compounds must be added to this kind of functional initiators although the scientific community does not regard them as initiators. These are: S

S

II

tl

R2N-C-S-S-C-NR

z and

R(I)2C--C(I)2R

3.7 Tetraalkylthiuram Disulfides The tetraalkylthiuram disulfides were mentioned for the first time by Tobolsky et al. [53]. We can find them classified in the Polymer Handbook [54] as "disulfides (bis dialkylthiocarbanoyle)". In 1955 Tobolski [53] noticed that the thiocarbamate disulfides could be as reactive as benzoyl peroxide. However, when the initiator concentration increased, they e~hibited a retarding behavior and such as effect was more important for M M A than for styrene. In the same way, in 1955 Otsu et al. [55] showed that the compounds: R

R

\N-C-(S)

.,/ r~

-C-N /

II

II

S

S

\D,

"~

where n = 1,2 and R = O ; R ' - C H 3 ; 4 ; C2H5; R = R ' = e t h y l e n e oxide are excellent thermal initiators and photochemical sensibilizers of styrene and MMA, but they are inefficient for acrylonitrile, vinylidene chloride and vinylacetate. It can be seen that for R = R ' = M e the inhibiting or retarding effect is more important. However, Japanese researchers, because of their interest is such initiators investigated a lot in 1960 [56] and 1982 [57]. Otsu [56] prepared triblock copolymers VAc--S--VAc as follows: Et\ Et

~ S /2

//Et\ q~

Et/

"~

~

;

,]2

After precipitation of polystyrene in methanol (called TPSt) the further reaction was carried out photochemically at 30 °C in benzene. TPSt + VAc - ~ (VAc)p-(S).-(VAc)p-


87

The triblock copolymer was then hydrolyzed. The oligostyrene exhibited between 170 and 430 base units for an initiator-monomer ratio ranging from 5 x 10 - 3 (yield = 37%) to 2 x t0 -2 (yield = 47%). The average VAc number in the end-blocks was 650. Moreover, the authors showed that 75 % of PVAc was in the triblock copolymer and 25 % in the PVAc homopolymeL The authors attributed such reactivity to the intermediates radicals: Et2N-~-S's + "(Sty). [ --~ T.PSt

or

Et2N-~" + 'S(Sty). S The --(Sty). and --(Sty) --S" radicals are more reactive than others because (Et2N~S)2 S does not lead to any product with VAc either by thermal or photochemical initiation. In 1982, Konishi [57] performed the same synthesis by using H 2 C = C H - - C O - NH--C(CH3)2CHzSO3H (called ASO3H ) as monomer for the block copolymerization with T--PSt which had a DPn 150. The photochemical block copolymerization (with Hg) in methylene chloride with triethylamine led to the results listed in Table 7. Table 7. Reactants and products amounts for the blockcopolymerization of T.PSt with ASO3H T.PSt

ASO3H

%

%

50.0 60.0 66.5

50.0 40.0 33.5

%

homo PS

inter-

Block 1

Block 2

34 (30)~ 41 (35)~ 35 (38)"

0 3 (48)a 9 (60)a

mediate 94 90 96

19.3 9.5 3.7

47.0 46.5 52.3

a designatesthe ASO3H% in the copolymer Such products were precursors for biological semipermeable membranes. Table 8 lists several kinetical constants of these xanthogcns. Recently, a new team investigated the use of functional thiuram [62, 63]; Clouet et al. studied the polymerization of MMA with: C2H5 \N_C_S_S_C_N -C2H: ~ ~ A

C2H 5

/ \C2H4-OH

[64]

88

B, Boutevin

and they pointed outthe efficiency of such compounds as both transfer ager/ts and initiators (Table 9).

Table 8. Transfer constants of xanthogen for MMA or styrene Monomer

T (°C)

Cr

Ref.

MMA Me\

/ Me N-C-SS-C-N Me/ ~ ~ \Me

70

O.Ol15

[53]

20

5.500 (hv)

[58]

60

0.080(hv)

[58]

/Me O-C-S-C-N

S MeN /Me N-C-S-S-C-N

Me/

~

~

Et\ /N-C-SS-C-N Et ~ ]IS

\Me

60

1.110

[591

70

0.0114

80

0.568

80 95 95 115 115 130 130

0,620 0,780 0.860 0.939 1.035 0.984 1.150

[53] [60] [61] [611 [60] [60] [611 [60] [61]

60

0.724

[59]

60

1.750

[59]

/Et \Et

/Et N-C-SS-C-N Et / ~ ~ \0

TabLe 9, Kinetical parameters of A T (°C)

CT,

106xKi~

60 70 85 95

0.230 0.265 0.350 0.455

0.53 4.41 26.00 48.50

a K~ = 2 • f. K d (constant Of decomposition of common initiators)

TelechelicOligomersby Radical Reactions

89

Furthermore, the authors noticed that the DPn increased with the monomerconversion-rate since the initiator decomposed very quickly. The products exhibited very good thermal properties; actually at 300 °C the oligomers were still stable because of the absence of double bonds obtained in traditional radical polymerization of MMA. The authors also prepared polymers which exhibited phosphonated end-groups from thiuram [65]: S

I

S

H

(CzH 50)2-p. - N - C H 2 C H z - N - I~-S- S - C - N - C H 2 C H 2 - N

II i

I

O CH 3

CH3

I

~'-(OC2Hs)2

[

CH 3

CH30

The initiators were synthesized in two steps: H - N - C H z C H z N]- H I + CI-~-(OEt)2 c~c!a, H - N - C z H a - N I[ CH 3

CH 3

O

CH 3

~-(OEt)2

CH 3 0

(B) CHCI 3

(B) + CS2 + 12 ~

thiuram

The polymerizations of MMA and styrene were also studied and the authors obtained polymers whose molecular weights were 2,000 to 40,000 by varying the initiator/monomer molar ratio. Finally, Clouet et al. [66] prepared "polymeric iniferters", precursors of multiblock copolymers. The compounds:

R

-×

R

"×

were condensed with CS2 and led to poly(phosphonamides) linked by thiuram bridges, thermically labile, used for initiating the polymerization of MMA and styrene [67].

3.8 Disubstituted Tetraphenyl Ethanes In t955, Pierson et al. [68] studied the influence of

I

I

CN CN

90

B. Boutevin

on the polymerization of styrene. After 17 h at 50 °C, the conversion rate was 4.5 ~o and the transfer constant was determined (C T = 2.8). But the low yield value showed that these compounds exhibited the same behavior as both transfer agent and inhibitor of polymerization. Mayo et al. [69] noticed that (I)3C--C~3 played the same role. According to these authors these compounds did not play any role in the emulsion polymerization of butadiene and this explained why such research had been given up for 20 years. C. Braun and Bledzki [70--72] contributed greatly to the development of these investigations. M M A and styrene were mainly studied and three initiators were used:

T?

~-C-C-~

J L

R R

where R represents CN or O ~ or OSi(Me)3 groups. The products obtained exhibited the following structure:

?

T

R

R

where4 < n < 300

Their syntheses were carried out in two steps in the temperature range 80 to 100 °C: first, the oligomers were prepared and then the molecular weight increased with an increase of the conversion rate or by decreasing the initiator concentration. With M M A [73, 74] the authors showed that the telechelic oligomers from MMA were able to be cleaved between the two last carbon atoms and this was carried out from the model molecules:

?

~H3

(b-C-CH2-C-R CN

CO2Me

(1)l ~H3 and

(I)-C--C- R

t

with R = H, CHa, C2H5

I

CN COEMe

Such a product facilitated the preparation of diblockcopolymer [71 ] in a temperature range of 60 to 100 °C and these compounds can also be used as curing agents of saturated polyesters [61]. With styrene, the oligomers cannot be cleaved whereas with the a-methylstyrene [75] such a cleavage is possible from 50 °C. These products along with the M M A or the styrene lead to diblockcopolymers and we can regard these oligomers as living polymers: in this case the monomer used in the first step has to be the m-substituted one. So far, no classical functional oligomer (diacids, diols etc...) has been prepared by such a reaction.


91

4 Telomerization F o u r main methods o f telomerization can be suggested as leading to difunctionat compounds but in each case it is essential to use an initiator which is either a generator o f free radicals or a redox catalyst. The first two methods deal with the telomerization o f a non-functional m o n o m e r with a functional telogen (first method) or a telogen which can be further modified in order to exhibit the functional group (second method). The two last methods consist o f performing the monoaddition on b o t h sides o f the chain extremity o f a non-conjugated diene (third method) with a functional telogen or on a telogen with a functional monomers (four method). We are going to study each o f these methods.

4.1 Disulfides They are used the most. The first investigations were carried out by Costanza and Pierson [68, 76] on styrene with aromatic disulfides especially. These authors showed that the aliphatic disulfides are not very reactive even though the aliphatic R - - S H thiols are very good transfer agents. F o r instance, the transfer constant of B u - - S - - S - Bu is 1.54x 10 -3 at 55 °C [77] and 6.80× 10 -3 at 99 °C [78]. For the activated disulfides (Table I0): (R-O-~-CH2-S~-2, O whatever the nonomer, the corresponding values are rather low but the results obtained from the literature do not allow us to form a conclusion, they are only regarded as interesting transfer agents to use. The results from the aromatic disulfides on both styrene (Table 11) and M M A (Table 12), on which Japanese teams performed a lot o f investigations, are move interesting. We can try to interpret their results although the authors thenselves did not do that. The transfer constant seems to be slightly influenced by the nature o f the substituent in the para position, whatever its inductive effect.

Table 10. Comparison between the transfer constants of non-aromatic disulfides with various monomers Disulfide

R

Monomer T (°C)

Cx

Ref.

RO2C--(CH2)lo-- SS--(CH2)lo--CO2R R O 2 C CH 2 SS CH 2 CO2R

Et Et Et Et Et Et Et Me

Styrene VAc VAc MMA Styrene Styrene Styrene Styrene

0.005 0.0132 1.5 0.00065 0.0005 0.015 0.2 0.1

[76] [79] [80] [79] [76] [79] [78] [80]

50 60 60 60 50 60 99 99

92

B. Boutevin

On the other hand, if the substituents is in the ortho position, the more the group donates electrons, the higher the transfer constant. The Thiokol Corporation [83] used another disulfide which exhibits acid end groups :

HO2C-CH2- - ~ S -

S--~CH2-CO 2H

Finally, Okano et al. [84] have recently prepared an aromatic disulfide with isocyanate end group according to the reaction: O 2 N - @ - C I + Na2S ~ H 2 N - O - S - N a + H202) H 2 N _ O _ S _ S _ O _ N H l)

z

HCl ) O = C = N - O - S - S - O - N = C = O

2) COCI2

This disulfide has been used in photopolymerization of styrene in bulk or in the presence of T H F and the degrees of polymerization are in the range o f 20 to 130 for telogen/monomer ratio o f 0.1 to 0.01 and for m o n o m e r conversion rate o f 20-30 %. Table 11. Transfer constants to the styrene of monosubstituted aromatic disulfides [68, 76]

/CH 3 (0.22) (0.11} CH3--~)-S

(O.09)

.CH2-C02H (0.005) ~SCH2Br {1) /CH2CI (1.3)

/CH20H (0.58) HOH2C----S-O~

(0.24) H2N~ k / - ~ S (0.21) RNH/lk--"/

/NH2(3) -

(0.18) C H 3 0 - - ~ S -

(0.2z,) RO-C-HN II 0 (0.11) Et 02~ CO2Et(0.005) /~CO2H (0.01) (0.17) H02CJ-x~S (Q~)--S- (0.06)

--CH2--S--(0.02)

93


Table 12. Transfer constants to the MMA of substituted phenyl disulfides ( x 104) at 50 and 60 °C [81]

Disulfides

T (°C)

(~S)z (p CH 3-~S)2 (p Br--~S)~ (p Ct--OS)2

~p o c u ~ - - . s h

50

60

38 31 46 72

85 44 98 117

44

(p NO 2-~S)2 NO2--OS)2 (EtOzC--CHz--S)2 (~_CHz--S)2

52

127 176 ---

198 508 6.5 a 62.7 b

a Ref. [79]; b Ref. [82]

In 1986, Akemi et al. [85] carried out the same synthesis and obtained triblock cotelomers as previous researchers - - the monomers used where firstly HEA/S/HEA and secondly FA/S/FA (where FA is fluoroacrylates). In the second case, medical applications (antitrombogenic compounds) were developed. Several surveys on disulfides with fluorinated monomers [86, 87] should also be mentioned, Thus, Yacubovich et al. [87] added tetrafluoroethylene OSS~ according to the following scheme:

~-S--S--~ + C2F4 +

O-S-C2F4-S-O

A2

I ~-S-C2F~-S~ 60"/, t~-S--(C2F4)2- S~ 17*I. .~

(I)--S--C2F4H

,..,,,,.~CFv.. L' ~ S j C F 2

5*•, 5 */.

~Ae20 0 - S O 2 - C 2 F 4 - S O 2 - 0

Sharp et al. [86] carried out the photochemical reaction of non-functional disulfides on different molecules: C H a - - S - - S - - C H 3 + H2C --- CF 2 -~ C H 3 - - S - - C H 2 - - C F 2 - - S C H 3 C 6 F s - - S - - S - - C 6 F 5 + H2C = CH 2 ~ C6Fs--S--C2H4--SC6F 5

4.2 Acid Disulfides The acid disulfides exhibit the structure:

O O Few studies on acid disulfides have been performed, whatever the functionality of these telogens. Tables 13 and 14 sum up their basic transfer constants to M M A and Styrene.

B. Boutevin

94 TaMe 13. Transfer constants to the MMA of substituted benzoyl disulfides at 60 °C (according to Tsuda and Otsu) [88] Disulfides

Cr ( × 104)

(qb--CO--S)z (p CH3--q~--CO--S)z (p B --dP--CO--S)2 (p CH30--~--CO--S)2 (p CN--~--CO--S)2 (p NO2--dP--CO--S)2

10.0 11.0 16.7 14.6 290.0 694.0

Table 14. Transfer constants to the styrene of substituted benzoyl disulfides Disulfides

T (°C)

CT ( × 104)

Ref.

((I) CO--S--)2

50 60 60 99 60 60 60 60 60 60

50 107 36 1,100 96 46 745 t96 3,190 6,650

[68] [89] [88] [78] [88] [88] [88] [88] [88] [88]

60

3,400

[68]

60

61,000

[59]

(CH30--~--CO--S)2 (CH3--~--CO--S)2 (Br--~--CO--S)2 (CI--~--CO--S)2 (CN--q)--CO--S)2 (NO2--q~--CO--S)2

( [~co-s-)-(O/--AN--C - S)2 k__/ S

4.3 Xanthogens Their transfer constants are very high (1 < CT < 20). These telogens have been studied since 1955 and there are several publications on this subject. First several authors planned their use as initiators for thermal o r photochemical polymerization for different monomers. Thu s, Otsu [55] c o m p a r e d the activity o f these xanthogens, sulfides, disulfides and thiuram with several monomers - - M M A , S., A . N . , VAc, VDC. The results allowed him to classify products as telogens and not as functional initiators. In the same way, these surveys showed that they are photoinitiators without having retarding or inhibiting effects as are observed for several disulfides.


95

Furthermore, Pierson et al. [76] determined the transfer constants of diene disulfides and among them the Dixie:

o CH3

S

O

/

CH3

O

At 50 °C, in presence of styrene, the transfer constant is C T = 5.3. It is a very high value and such a reactant is regarded as a good transfer agent. In the same period of time, some non-functional xanthogens were studied and the corresponding transfer constants were determined (Table 15). Basically, the monomers were the styrene or the butadiene/styrene mixture. The objectives were to synthesize new elastomers and to study the curing or the vulcanization by different agents. Fokina et al. [90] studied the modification of xanthogens according to the reaction: RO-C-S-S-C-OR

II

S

LI

+ nM --* R O - C - S - ( M ) . - S - C - O R

Ir

S

S

II

S

NH3 HS_(M)n_SH CICH2CH2OH , . , HOCH2CH2S-(M).-SCH2CH2OH with M is St, Isoprene, Divinyl benzene DVB, or St/DVB and St/Butyl ( ~ S between 5 and 10%) 103 < M n

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