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ADVANCES IN DENDRlTlC MACROMOLECULES

Volume 2

1995

This Page Intentionally Left Blank

ADVANCES IN DENDRITIC MACROMOLECULES Editor: G E O R G E R. N E W K O M E Department of Chemistry University of South Florida Tampa, Florida

VOLUME 2

1995

@) Greenwich, Connecticut

JAI PRESS INC. London, England

Copyright O 1995 by l A l PRESS INC. 55 Old Post Road, No. 2 Greenwich, Connecticut 06836 ]A/ PRESS L TD. The Courtyard 28 High Street Hampton Hill, Middlesex TW12 1PD England All rights reserved. No part of this publication may be reproduced, stored on a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, filming, recording, or otherwise, without prior permission in writing from the publisher.

ISBN: 1-55938-939-7 Manufactured in the United States of America

CONTENTS

LIST OF CONTRIBUTORS PREFACE George R. Newkome THE CONVERGENT-GROWTHAPPROACH TO DENDRlTlC MACROMOLECULES Craig 1. Hawker and Karen L. Wooley CASCADE MOLECULES: BUILDING BLOCKS, MULTIPLE FUNCTIONALIZATION, COMPLEXING UNITS, PHOTOSWITCHING Rolf Moors and Fritz Vijgtle IONIC DENDRIMERS AND RELATED MATERIALS Robert Engel SILICON-BASED STARS, DENDRIMERS, AND HYPERBRANCHED POLYMERS Lon 1.Mathias and Terrell W. Carothers HIGHLY BRANCHED AROMATIC POLYMERS: THEIR PREPARATION AND APPLICATIONS Young H. Kim DENDRlTlC BOLAAMPHIPHILES AND RELATED MOLECULES Gregory H. Escamilla lNDEX


LIST OF CONTRIBUTORS

Terrell W. Carothers

Department of Polymer Science University of Southern Mississippi Hattiesburg, Mississippi

Robert Engel

Department of Chemistry and Biochemistry Queens College of the City University of New York Flushing, New York

Gregory H. Escamilla

Department of Chemistry University of South Florida Tampa, Florida

Craig J. Hawker

IBM Research Division Almaden Research Center San Jose, California

Young H. Kim

DuPont Central Research and Development Experimental Station Wilmington, Delaware

Lon J. Mathias

Department of Polymer Science University of Southern Mississippi Hattiesburg, Mississippi

Rolf Moors

Institut fur Organische Chemie und Biochemie Universitat Bonn Bonn, Germany

Fritz Vogtle

Institut fur Organische Chemie und Biochemie Universitat Bonn Bonn, Germany

VII

LIST OF CONTRIBUTORS Karen L Wooley

Department of Chemistry Washington University St. Louis, Missouri

PREFACE

As recently illustrated by Professor Vogtle in a highlight (Angew. Chem., Int. Ed. Engl. 1994, 55, 2413) concerning dendritic chemistry, annual publications in this field are increasing at an incredible rate. As the combinations of building blocks and cores proliferate, the diversity of the resultant macromolecules will continue to expand our knowledge of new unnatural molecules with specific composition. This review series was organized to cover the synthetic, as well as chemical, aspects of this expanding field: the chemistry to and supramolecular chemistry of dendritic or cascade supermolecular compounds. Since one-step procedures to the related hyperbranched polymer are close cousins to the dendritic family, reviews have also been incorporated. In Chapter 1, Hawker and Wooley delineate the convergent growth approach to dendrimers, then relate their three-dimensional architectures to different block polymers. In Chapter 2, Moors and Vogtle describe Professor Vogtle's initial cascade molecules via the repetitive strategy, then expand his original concepts of its application by others, and lastly delineate the synthesis of a new series of tosylamide cascades. They also demonstrate the utility of his original Michael addition/reduction procedure by its application to differ cores. Chapter 3, composed by Professor Engel, describes ionic dendrimers which incorporated an internal transition metal center as well as his work based on ammonium and phosphonium centers. In Chapter 4, Mathias and Carothers review recent studies on silicon-based dendrimers and hyperbranched polymers. Chapter 5, by Kim, describes the preparation

IX

X

PREFACE

and utility of hyperbranched aromatic polymers. Lastly in Chapter 6, Escamilla reviews the historical as well as recent examples of ionic and nonionic bolaamphiphiles. I personally wish to thank these authors for their contributions to this volume and for their continuing contributions to this field. Future volumes in this series will highlight the work of others in the field of cascade/dendritic macromolecules. George R. Newkome Editor

THE CONVERGENT-GROWTH APPROACH TO DENDRITIC MACROMOLECULES

Craig J. Hawker and Karen L. Wooley

I. II. III. IV. V. VI. Vn. VIII.

ABSTRACT 2 INTRODUCTION . 2 DEVELOPMENT OF THE CONVERGENT-GROWTH APPROACH . . . 4 CHARACTERIZATION 10 CONTROL OF SURFACE FUNCTIONALITY 14 DENDRITIC BLOCK COPOLYMERS 21 HYBRID LINEAR-DENDRITIC BLOCK COPOLYMERS 29 PHYSICAL PROPERTIES OF DENDRITIC MACROMOLECULES . . . 3 3 FUTURE DIRECTIONS 36 ACKNOWLEDGMENTS 37 REFERENCES 37

Advances in Dendritic Macromolecules Volume 2, pages 1-39. Copyright © 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-9397

1

2

CRAIG J. HAWKER and KAREN L. WOOLEY

ABSTRACT The developing field of dendritic macromolecules has been characterized by two different, but complementary, synthetic strategies: the divergent-growth and convergent-growth approaches. The fundamental aspects of the convergent-growth approach are examined and described with the synthesis of dendritic polyether macromolecules based on 3,5-dihydroxybenzyl alcohol as the monomer unit. Using this series of dendritic polyether macromolecules, the techniques and methods for characterization of molecules, prepared by the convergent-growth approach, are demonstrated. Examples of the control of chain ends, internal building blocks, and the synthetic utility of the focal-point group are provided by the preparation of a number of unique macromolecular architectures. Finally, the physical properties of these three-dimensional macromolecules are compared with traditional linear polymers.

I. INTRODUCTION Analogous to the term dendrite, a noun describing an object resembling a tree in properties, growth, structure, or appearance, dendritic macromolecules are three-dimensional polymers with treelike, highly branched structures. It is now generally accepted that dendritic macromolecules are more accurately defined as systems possessing "perfectly" branched structures, characterized by large numbers of chain ends, all emanating from a central core, with at least one branch junction at each monomer unit. Dendritic macromolecules, which have also been termed cascade, arborol, starburst, or fractal structures,* are a fundamentally new class of polymers. They have received a considerable amount of interest recently with a large number of reviews and papers appearing in the literature detailing various aspects of their synthesis, characterization, and properties. Hyperbranched macromolecules^-^ are a related class of materials, also treelike and globular in nature, but less highly branched and significantly less regular than the dendritic systems and, while also interesting, are beyond the scope of this chapter. Initially, one may question the relevance of such exotic structures, and also the justification for devoting large amounts of time, resources and effort to their preparation and study. The answer to this question is related to one of the fundamental roles of science in today's society, namely, continued technological advancement, in part, driven by the production of new materials with enhanced and/or novel properties and by the more complete understanding of structure-property relationships for materials

The Convergent-Growth Approach

3

in use today."^ In view of these goals, the preparation and study of dendritic macromolecules and other two- and three-dimensional structures are becoming increasingly important.^ This has led to several instances where the microstructure of the polymers has affected the physical and chemical properties of the bulk material. Further characterization of physical properties and the development of a basic understanding of the relationship between microscopic structure and the overall microscopic and macroscopic properties of materials are currently of great potential and interest. These factors, coupled with the large degree of control possible over macromolecular architecture, demonstrate the significance of studying dendritic macromolecules. A number of excellent reviews on dendritic macromolecules has appeared. ^'^ This introduction, therefore, will cover only the basic aspects of dendritic macromolecules. For a more concise treatment, the reader is directed to the above reviews. Prior to 1990, the synthesis of dendritic macromolecules had been accomplished by a single methodology termed the divergent-growth or starburst approach. This synthetic strategy was pioneered by two independent groups of researchers headed by George Newkome^ and Donald Tomalia.^ Their seminal synthetic efforts, both published in 1985, but preceded by a number of theoretical treatments^^^ can be considered the first true attempts to prepare high molecular weight, regularly branched, dendritic macromolecules. A characteristic of the divergent-growth approach is that growth begins at a central polyfunctional core. Reaction of the core molecule with polyfunctional monomer units or building blocks then leads to the next generation compound with a concomitant increase in the number of chain-end functional groups. Deprotection or chemical transformation of these functional groups leads to the original reactive functionalities. This two-step process has been repeated to give larger dendritic macromolecules (Scheme 1), with growth continuing to generation 10. A fundamental aspect of the divergent-growth approach is the rapid increase in the number of chain-end functional groups. Associated with this is the increase in the number of reactions required to functionalize the chain ends fully. Incomplete reaction of these rapidly increasing terminal groups leads to failure sequences or imperfections in the next generation. These potential difficulties and the lack of control over the number and placement of functionalities at the chain ends led us to reevaluate the synthetic approach to dendritic macromolecules.

CRAIG J. HAWKER and KAREN L. WOOLEY

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11. DEVELOPMENT OF THE CONVERGENT-GROWTH APPROACH While the divergent-growth approach allowed an entry into the field of dendritic macromolecules and its use continues to produce novel and fascinating structures, the need existed to develop a complementary approach to synthesize dendritic macromolecules. To develop an alternate strategy, we decided to apply the disconnection method in organic

The Convergent-Growth

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Approach

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Figure 5. Preparation and structure of dendritic micelles, ((K02C)i5-[G4])2-[C], 20.

prepared by use of the convergent-growth approach described above. Hydrolysis of 19 then leads to a water soluble derivative 20 which has a hydrophobic aromatic polyether core surrounded by a hydrophilic carboxylate layer (Figure 5). The analogy with traditional micelles is readily apparent. There is, however, one unique and critical difference. The structure is static and does not vary with concentration. Therefore, there is no critical micelle concentration and no distortion at high concentration.


21

The dendritic micelle 20 was found to solvate hydrophobic molecules, such as pyrene, and a dramatic increase in the saturation concentration of hydrophobic molecules in water was observed. This increase was of a magnitude similar to that observed for traditional micelles derived from sodium dodecyl sulfate (SDS). However, unlike SDS micelles, 20 demonstrated solubihzing ability at concentrations as low as 5 x 10"^ M. This is consistent with its covalently bound static structure, and allowed the development of a novel, recyclable solubilization and extraction system. It was also found that a relationship existed between the solubilizing power of 20 and the electron density of the hydrophobic polycycUc aromatic molecule. This suggests that more sophisticated molecular recognition may be possible for the correctly designed system. The appUcation of such systems in areas such as drug delivery, catalysis, and artificial enzymes has great potential. Further property studies of dendritic micelle systems have been reported by Newkome.^^

V. DENDRITIC BLOCK COPOLYMERS If the structure of either 16 or 17 is examined, it becomes apparent that both can be considered as examples of block copolymers, since areas or blocks of the periphery are substituted with different chain ends. This raises the unique question as to the types of three-dimensional architectures that may be considered block copolymers. We have identified three different types of architectures that can be considered block copolymers for purely dendritic macromolecules and have termed them, dendritic surface-block (e.g., 16 and 17), dendritic segment-block, and dendritic layer-block copolymers. The three different architectures are represented schematically in Figure 6. As their name implies, dendritic segmentblock copolymers are characterized by different segments or fragments emanating radially from a central core, while dendritic layer-block copolymers have concentric layers of different chemistry around the central core. Since the preparation of the latter two block copolymers relies on different chemistries, the convergent growth approach was extended to the synthesis of dendritic aromatic polyesters based on 3,5-dihydroxybenzoic acid as the building block.^^ The combination of this chemistry in a controlled and systematic way with the polyether chemistry, discussed above, would then lead to either segment- or layer-block copolymers depending on the sequence of addition steps.

CRAIG I. HAWKER and KAREN L. WOOLEY

22 X X

Surface-block

Segment-block

Layer-block

Figure 6. Schematic representation of three different architectures that can be considered dendritic block copolymers.

The synthesis of the segment-block copolymers depends on the preparation of different dendritic fragments which are then coupled either to the monomer unit or to the core molecule in a stepwise fashion to give the desired product. For example the modified monomer unit, trichloroethyl 3,5-dihydroxybenzoate (21) is monoalkylated under standard conditions with the second-generation ether 4 to give the monophenolic 22. Switching to ester chemistry, esterification of 22 with the ester fragment 23 using DCC/DPTS as coupling agents afforded the third-generation dendrimer 24. As defined by the synthetic blueprint, 24 has both an ester block and an ether block attached to the same unique monomer unit at the focal point (Scheme 7). Coupling to the trifunctional core molecule 5 leads to a dendritic segment-block copolymer 25 which has a nominal molecular weight of 5370 amu. Since there is free rotation around all of the branching points in this molecule, there are a number of possible conformations which would lead to different degrees of mixing for the blocks. However, due to constraints arising from the branching sequence, a structural isomer is not allowed where all three polyester blocks are adjacent (Scheme 8).^^ While different chemistries are also employed in the synthesis of dendritic layer-block copolymers, the dendritic fragments used in the construction of these molecules are all the same and the block copolymer is formed through the addition of different monomer units in discrete steps of the synthesis to create layers. Many of the initial examples of

23


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dendritic macromolecules prepared by Newkome^^ and Tomalia^ can be considered dendritic layer-block copolymers since, by the choice of chemistry used in the growth sequence, altemating layers of functional groups are obtained. Dendritic layer-block copolymers have the potential for great use in the areas of catalysis and molecular recognition where active sites are incorporated into the internal structure of the macromolecule. It was decided, therefore, to explore the synthesis of such molecules

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The Con vergent-Growth Approach

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by the convergent-growth approach. There have been two reported examples of dendritic layer-block copolymers prepared by the convergent-growth approach. ^^'^^ One of the examples^^ incorporated both ether and ester chemistry into the structure, as described below. Starting with ether chemistry, the dendritic fragment 4 was prepared from benzyl bromide and the monomer unit, 3,5-dihydroxybenzyl alcohol 1. Changing the monomer unit to trichloroethyl 3,5-dihydroxybenzoate (21) allowed use of the same ether chemistry in constructing the third-generation compound 26, but permitted a change to ester formation in subsequent generation-growth steps (Scheme 9). Deprotection of 26, followed by generation growth and coupling to the triphenolic core 5 using DCC/DPTS chemistry, then gave 27. This dendritic layer-block copolymer 27 is characterized by two inner concentric layers of hydrolyzable ester-functional groups surrounded by three outer concentric layers of ether-functional groups (Scheme 10).^^ The concept of having layers of reactive functionalities or sites in the interior of dendritic macromolecules has been elegantly demonstrated recently by Newkome's group.^^ The synthesis of dendritic micelles, containing layers of triple bonds to which metal clusters can be attached.

1. Zn/HOAc 2. & DCC, DPTS

Scheme 10.

26

The Con vergent-Growth Approach

27

can be considered the first example of purposefully constructing dendrimers resembling more complex catalytic or enzymatic systems. Application of dendritic structures to this area of research has great potential, since the synthetic methodologies for constructing dendritic macromolecules are mature enough to allow control over the size and shape of the molecule, the number of active sites, the solubility of the overall molecule, and the nature of the interior. It was this last point that raised the important questions whetherthe interior of dendritic molecules exists as a unique and controllable microenvironment and how solvent affects groups located internally. To investigate these questions further, a solvatochromic probe was covalently attached to the focal point of a series of sizes of dendritic molecules, e.g., 28. The solvatochromic molecule was chosen to be a derivative of 4-(iV,A^-dimethylamino)-l-nitrobenzene and the chemistry used for coupling is outlined in Scheme Investigation by UV-Vis spectroscopy of a series of dendritic molecules from generation 0 to 6, containing the solvatochromic chromophore at their focal point, did indeed demonstrate that the probe was sensitive to changes in both the solvent and the size of the attached dendrimer. In solvents of medium to low polarity, the absorption maximum increased from generation 0 to generation 6. For example, in CCI4 the X^^ underwent a bathochromic shift from 366 nm for G = 0 to 383 nm for G = 6 (Figure 7). The relationship between X^^ and generation number was not linear and a marked discontinuity was observed between generation 3 and 4, correlating with a shape change from an extended to a more globular structure. Other evidence for such a conformational transition has been presented.^^ These results confirm that the influence of the building blocks of the dendrimer on the microenvironment of the solvatochromic probe increases as the size of the dendrimer increases, with an accompanying decrease in effects due to solvent. The interior of a dendritic macromolecule is, therefore, a unique microenvironment, the polarity of which can be manipulated by the size of the dendrimer, the nature of the intemal building blocks, and by the bulk solvent. The resemblance to the controlled microenvironments of some enzyme-active sites is tantalizing offering a number of unique opportunities and applications.

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dendrimers elaborated to only relatively few generations, the ruthenium(II) complex species associate at a greater distance from the dendrimer surface than they do with higher generation species.^^"^^ Finally, anionic surface dendrimers have been constructed as "balloons" attached along a Merrifield resin backbone."^^ Using an elaboration method based on the diethyl malonate and triethyl methanetricarboxylate procedures previously described,^^ dendrimers bearing double and triple branching hydrocarbon arms were prepared with carboxylate termini. These dendrimers are insoluble in all ordinary solvents and serve as high-capacity cation-exchange materials. V. IONIC MOLECULAR TRAINS AND CATENANES More than thirty years ago,^^ the first intentional preparation of a catenane provided significant imaginative impetus for further adventures in the realm of chemical topology. While catenanes themselves are not branched species, as is implied by the terms dendrimers or cascade molecules, they are materials which incorporate some of the most interesting of supramolecular effects.^^ Recently, using the tetracationic cyclophane 41 as a fundamental building block, several [2]- and [3]catenanes bearing charged rings have been synthesized.^^^"^ Examples of these materials are shown as 42 and 43. In addition to their synthesis and characterization, the ordering of the aromaticringsof the linked (but not covalently bound) rings, relative to each other in the catenane structures, has been investigated using a variety of techniques. It is anticipated that the investigation of the self-assembling characteristics of the components of these materials will

Ionic Dendrimers

95

42

41

8PF.

43

lead to a much clearer understanding of supramolecular self-assembling processes in general. A structurally intriguing material has also been reported recently in which networks of interlocking rings, incorporating manganese(n), copper(II), and pyridinium sites, are constructed to provide a molecularbased magnetJ^ Net electronic spin is provided by pendant nitrosyl functionaUties as well as the metal sites, resulting in a material which behaves as a magnet below 22.5 K. The capability for extending the interlocking rings to great distances in three dimensions and incorporating further electronic spin entities holds promise for constructing even better molecular magnetic systems using supramolecular concepts. Finally, note should be made of a series of catenanes employing rings which are essentially azacrowns.^^^^ While these catenanes are not by themselves charged, they can be constructed with coordinated metal ions, such as copper(I), to produce polycationic species, which exhibit intriguing physical characteristics.

96

ROBERT ENGEL

NOTES *STARBURST is a registered trademark of The Dow Chemical Company. ^Dabco is a registered trademark of Air Products and Chemicals, Inc.

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26. Denti, G.; Serroni, S.; Campagna, S.; Ricevuto, V.; Juris, A.; Ciano, M.; Balzani, V. Inorg, Chim. Acta 1992,198-200, 507. 27. Serroni, S.; Denti, G.; Campagna, S.; Ciano, M.; Balzani, V. J. Chem. Soc, Chem. Commun, 1991,944. 28. Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani, V. J. Am. Chem. Soc. 1992,114, 2944. 29. Serroni, S.; Denti, G.; Campagna, S.; Juris, A.; Ciano, M.; Balzani, V. Angew. Chem., Int. Ed. Engl. 1992,31,1493. 30. Dagani, R. Chem. Eng. News 1993, 71(5), 28. 31. Newkome, G. R.; Cardullo, R; Constable, E. C; Moorefield, C. N.; CargillThompson, A. M. W. J. Chem. Soc, Chem. Commun. 1993,925. 32. Newkome, G. R.; Lin, X. Macromolecules 1991,24, 1443. 33. Nagasaki, T; Ukon, M.; Arimori, S.; Shinkai, S. J. Chem. Soc, Chem. Commun. 1992, 608. 34. Constable, E. C; Cargill-Thompson, A. M. W. J. Chem. Soc, Chem. Commun. 1992, 617. 35. Constable, E. C; Cargill-Thompson, A. M. W. /. Chem. Soc, Dalton Trans. 1992, 3467. 36. Collin, J.-R; Lain^, R; Launay, J.-R; Sauvage, J.-R; Sour, A. J. Chem. Soc, Chem. Commun. 1993,434. 37. Bignozzi, C. A.; Roffia, S.; Chiorboli, C; Davila, J.; Indelli, M. T.; Scandola, P. Inorg. Chem. 1989,28,4350. 38. Bignozzi, C. A.; Argazzi, R.; Chiorboli, C; Roffia, S,; Scandola, F. Coord. Chem. Rev. 1991,111,261. 39. Tomalia, D. A.; Hall, M.; Hedstrand, D. M. J. Am. Chem. Soc 1987,109,1601. 40. Hall, H. K., Jr.; Polls, D. W. Polymer Bull. 1987,17,409. 41. Naylor, A. M.; Goddard, W. A., Ill; Kiefer, G. E.; Tomalia, D.A.J, Am. Chem. Soc 1989, 111, 2339. 42. Rengan, K.; Engel, R. / Chem. Soc, Chem. Commun. 1990, 1084. 43. Rengan, K.; Engel, R. J. Chem. Soc, Perkin Trans. 11991,987. 44. Engel, R.; Rengan, K.; Chan, C.-s. Phosphorus, Sulfur, and Silicon 1993, 77, 221. 45. Engel, R.; Rengan, K.; Chan, C.-s. Heteroatom Chem. 1993,4, 181. 46. Rengan, K.; Engel, R. J. Chem. Soc, Chem. Commun. 1992, 757. 47. Engel, R. Polymer News 1992,17, 301. 48. Torres, N.; Cherestes, A.; Engel, R. Unpublished results of this laboratory. 49. Kanazawa, A.; Ikeda; T; Endo, T. J. Polym. Sci. Part A - Polym. Chem. 1993, 31, 1441. 50. Kanazawa, A.; Ikeda, T; Endo, T J. Polym. Sci. Part A - Polym. Chem. 1993,31, 1467. 51. Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Saunders, M. J.; Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991,30,1178. 52. Hawker, C. J.; Wooley, K. L.; Pr6chet, J. M. J. J. Chem. Soc, Perkin Trans. 11993, 1287. 53. Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, R Polym. J. 1985,17,117.

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ROBERT ENGEL

54. Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, R Macwmokcules 1986, 79, 2466. 55. Tomalia, D. A.; Hall, M.; Hedstrand, D. M. J. Am. Chem. Soc, 1987,109,1601. 56. Tomalia, D. A.; Berry, V.; Hall, M.; Hedstrand, D. M. Macwmokcules 1987, 20, 1164. 57. Moreno-Bondi, M.; Orellana, G.; Turro, N. J.; Tomalia, D. A. Macwmokcules 1990,25,910. 58. Caminati, G.; Turro, N. J.; Tomalia, D. A. J, Am. Chem. Soc. 1990, 772, 8515. 59. Turro, N. J.; Barton, J. K.; Tomalia, D. A. Ace. Chem. Res. 1991,24, 332. 60. Gopidas, K. R.; Leheny, A. R.; Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1991, 775, 7335. 61. Newkome, G. R.; Yao, Z.-q.; Baker, G. R.; Gupta, V. K. J. Org. Chem. 1985, 50, 2003. 62. Wasserman, E. J. Am. Chem. Soc. 1960,82,4433. 63. Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1988,27, 91. 64. Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. R; Williams, D. J. Angew. Chem., Int. Ed. Engl 1988,27,1547. 65. Ashton, R R.; Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Stoddart, J. R; Williams, D. J. Angew. Chem., Int. Ed Engl. 1988,27,1550. 66. Ashton, R R.; Goodnow, T. T; Kaifer, A. E.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. R; Vicent, C ; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1989,28,1396. 67. Brown, C. L.; Philp, D.; Stoddart, J. R Synlett 1991,459. 68. Brown, C. L.; Philp, D.; Stoddart, J. R Synlett 1991,462. 69. Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. P.; Vicent, C ; Williams, D. J. J. Chem. Soc, Chem. Commun. 1991,630. 70. Ashton, R R.; Brown, C. L.; Chrystal, E. J. T; Goodnow, T T; Kaifer, A. E.; Parry, K. R; Philp, D.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. R; Williams, D. J. J. Chem. Soc, Chem. Commun. 1991, 634. 71. Anelli, R L.; Ashton, R R.; Spencer, N.; Slawin, A. M. Z.; Stoddart, J. P.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1991,30,1036. 72. Ashton, R R.; Brown, C. L.; Chrystal,E. J. T;Goodnow, T. T; Kaifer, A. E.; Parry, K. R; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. R; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1991,30,1039. 73. Ashton, R R.; Brown, C. L.; Chrystal, E. J. T; Parry, K. R; Pietraszkiewicz, M.; Spencer, N.; Stoddart, J. R Angew. Chem., Int. Ed. Engl. 1991,30, 1042. 74. Anelli, R L ; Ashton, R R.; Ballardini, R.; Balzani, V.; Delgado, M.; Gandolfi, M. T; Goodnow, T. T; Kaifer, A. E.; Philp, D.; Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. R; Vicent, C ; Williams, D. J. J. Am. Chem. Soc 1992,114,193. 75. Stumpf, H. O.; Ouahab, L.; Pei, Y; Grandjean, D.; Kahn, O. Scknce 1993, 261, 447. 76. Dietrich-Buchecker, C. O.; Khemiss, A.; Sauvage, J.-P. J. Chem. Soc, Chem. Commun. 1986,1376.

Ionic Dendrimers

99

77. Dietrich-Buchecker, C. O.; Guilhem, J.; Pascard, C; Sauvage, J.-P. Angew. Chem,, Int. Ed. Engl. 1990,29,1154, 78. Bitsch, F.; Dietrich-Buchecker, CO.; Khemiss, A.-K.; Sauvage, J.-P.; Van Dorsselaer, A. J. Am. Chem. Soc. 1991, 775,4023. 79. Dietrich-Buchecker, C; Sauvage, J.-R Bull. Soc. Chim. Fr. 1992, 729,113.


SILICON-BASED STARS, DENDRIMERS,AND HYPERBRANCHED POLYMERS

Lon J. Mathias and Terrell W. Carothers

I. II. III. IV. V.

INTRODUCTION STARS DENDRIMERS HYPERBRANCHED POLYMERS CONCLUSIONS REFERENCES

101 103 105 115 118 119

1. INTRODUCTION Recent intense research efforts have focused on the synthesis of multibranched polymers (i.e., cascade molecules) that can be characterized by their uniform branching, radial symmetry, dense packing, entanglementfree globular shapes, and large number of chain ends at their peripheries.

Advances in Dendritic Macromolecules Volume 2, pages 101-121. Copyright © 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-939-7

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102

LON J. MATHIAS and TERRELL W. CAROTHERS

Two distinct approaches to the synthesis of these multibranched polymers have evolved from the seminal works of Vogtle^ and Denkewalter.^ Tomalia's "dendrimers"^'* and Newkome's "arborols"^ were independently developed through a process termed "divergent" synthesis. This approach is characterized by polymer growth emanating from a central core via an iterative protection/deprotection reaction scheme. Polymer growth typically begins from a "core" molecule which undergoes exhaustive reaction with complementary monomers having two or more protected branch sites. Removal of the protecting groups and subsequent reaction of the liberated reactive sites leads to first-generation polymers. Repetition of this reaction process leads to polymers of desired molecular weight, molecular size and topology. The disadvantages of this synthetic approach include high synthetic cost (large excesses of reagents are typically employed), labor intensiveness, geometrically increasing number of "successful" reactions necessary for uniform polymer growth, and purification difficulties. The convergent synthetic approach, independently developed by Neenan and Miller^ and by Hawker and Frechet,^ begins at what will eventually become the outer surface of the dendrimer. Polymer "wedges" are synthesized via sequential reactions and contain single reactive functionalities at their loci. Wedges are then attached to a polyfunctional central core to complete dendrimer formation. The one major advantage of this methodology is its limited number of reactions compared to the increasingly large number of reactions necessary for divergent growth. A rapidly increasing number of cascade molecules has been synthesized and investigated. Some of the functional groups used in the formation of cascade structures include amines,^'^ amides,^'^ amidoamines,^ ethers,'^'^'^ hydrocarbons,^'^^ cations,^^ esters,^^ transition-metal complexes,^^ and silicons.^'^'^^ Silicon-based dendrimers distinguish themselves from other dendritic species in that they are usually fluids at room temperature, they possess very low Tg's, and they exclusively adopt globular geometries even at lower polymer generations. The multibranched silicon-based polymers offer novel alternatives to CN and CO-containing materials. Those reported to date range from stars containing multiple polymeric arms to highly symmetrical, three-dimensional dendrimers. This report organizes these silicon-containing polymers based on branching geometry and uniformity.

103

Stars, Dendrimers, and Polymers

II. STARS End-reactive polymers have been synthesized by "living" anionic polymerization using either multifunctional anionic initiators followed by functional group termination^^'^^ or utilization of blocked, functional anionic initiators followed by reaction with a multifunctional linker compound.^^ The latter method has the distinct advantage over the former in that gelation of multiple chain ends, so frequently encountered in the use of multifunctional anionic initiators, is alleviated through the use of monocarbanionic "arms" that are subsequently linked. The problem of gelation is particularly troublesome when performing polymerizations in nonpolar solvents, which in many instances are the solvents of choice for obtaining polymers with well-defined microstructures.^^ The use of blocked anionic initiators was prohibited in the synthesis of well-defined telechelic poly(dimethylsiloxanes) (PDMS) or polydienes with high c/5-1,4 contents because of initiator insolubility in nonpolar solvents. Dickstein developed thefirstblocked, amine-fiinctional initiator used for the anionic polymerization of well-defined poly(dimethylsiloxane) arms

• Y

1 '

HMPA Benzene/2S°C

MegSr

SiMeg

= INITIATOR

y

INITIATOR

v

rôf^s

"ÔLJ + CI

CI Si—CI CI

4-ARM STAR POLYMER

"LIVING" PDMS ARMS

Figure 1. Anionic synthesis of star poly(dimethylsiloxanes) via blocked amine-functional initiators


104

which were subsequently coupled to multihalogenated silane cores forming PDMS stars (Figure 1),^^ telechelic stars^"^ and rigid-rod star-block copolymers.^^ A blocked, functional initiator is made by reacting p-iNJ^bis(trimethylsilyl)amino)styrene with ^-^c-butyllithium in benzene, and is used for anionically ring-open polymerizing hexamethylcyclotrisiloxane (D3) in the presence of promoters such as hexamethylphosphoroamide (HMPA), tetrahydrofuran (THF), or dimethyl sulfoxide (DMSO). The resulting "living", monodisperse poly(dimethylsiloxane)

I X^^^^r

4

+

(CHi)2CISiH

Pt/Cart)on ^'^^""^

»

? ^^

CI

M= Si. Ge and Sn

m T m

0

Figure 9. Bochkarev's anionic synthesis of starburst perfluorinated poly(phenylenes)

of polymerization, allyltris(dimethylsiloxy)silane underwent competitive six-membered ring cyclization (giving the pseudo B2 segment 2,2dimethyl-6,6-bis(dimethylsiloxy)-1 -oxa-2,6-disilacyclohexane) and linear propagation to form a polymer (Figure 10).^^ Self-catenation of the analogous monomer vinyltris(dimethylsiloxy)silane gave a mixture of polymers having five-membered cyclics (from core 2,2-dimethyl-5,5bis(dimethylsiloxy)-l-oxa-2,5-disilacyclopentane) and vinyls at their loci. These polymers exhibited bimodal SEC traces with their higher molecular weightfractions(end-capped with allyl phenyl ether) corresponding to polystyrene standards of about 250,000. To disfavor unwanted cyclization entropically, this general reaction scheme was extended to include monomers 6-hex-l-enyltris(dimethylsiloxy)silane and 8-oct-l-enyltris(dimethylsiloxy)silane. The resulting polymers gave no indications of cyclization and resulted in molecular weights of about 12,(XX). However, SEC traces were multimodal in nature caused by monomer contamination with internal olefins (notoriously sluggish or completely unreactive towards hydrosilation reac-

Stars, Dendhmers, and Polymers

117

% xyî^^^^

O^'^'V/^^Sl-t-od HJ

\(. .SH-0—^H

^

!i±_v3ô^y

.Veô^y

^^^'ijs;y:^^'t;s^ Pseudo B«

O-Si-0

SiH

1

K"

-SHO-SHO-Sf

i

;i-o-î

cfo

\

v

-SHCX^

;iH

^^m

figure 10. Self-polymerization of the A-B3 monomer, allyltris(dimethylsiloxy)silane

tions). ^^Si NMR analysis typically detected four polymer T" regions (triple oxygen substitution on silicon, where n denotes the degree of branching) (Figure 11) which integrated to an average ratio of approximately 29% T^, 44% T^ 21% T^ and only about 6% T^ branching in

118

H


i

l

l

—Sh-

—SI—

—SI—

—Sh-

R—Sh-OSIH

R—SI-OSiH

R—SI-OSl^

R—il-OSi^

—SI— H

—SI— H

—ShH

—SI— I

T°

T'

i

I

i

I

T'

i I

T'

Figure 11. Schematic representation of the possible branching environments available from the self-polymerization of A-B3 monomers (Note: R represents an alkenylene group and the jagged line represents the hydrosilation addition of monomer)

these polymers. Although polymer size, branching content, and number of surface Si-H moieties vary among molecules of any given sample prepared via this "one-pot" polymerization scheme, polymer synthesis is rapid, surface modification is quite versatile, and polymer purification is easy. These hyperbranched poly(siloxysilanes) can be adequately described as being uniform in the nature of their surface functionality, mostly spherical in shape, and structurally intermediate between linear and perfectly hyperbranched A-B3 polymers.

V. CONCLUSIONS Star and dendritic polymers based on silicon have been synthesized possessing well-defined molecular weights, molecular weight distributions and uniform branching contents. The former were synthesized by relatively conventional star-polymerization chemistry while the latter were obtained with silicon-specific reactions that allowed controllable chain extension. The dendrimers exhibited properties consistent with reduced hydrodynamic volume and highly condensed structures, but with low glass transition temperatures (Tg). Less well-defined hyperbranched polymers containing silane and/or siloxane moieties have also been synthesized through "one-pot" polymerizations. While this allows rapid formation of relatively high molecular materials, the degrees of branching and structural regularities of these systems are much less than those of the dendrimers. These materials do display the micellelike


119

Structures desired and possess very low glass transition temperatures that lead to liquid like physical properties. Possible uses for these unusual molecular constructs rangefromdrug microcarriers to artificial erythrocytes and single-cell microreactors. Drug delivery would be facilitated by the liquid-like interiors of these systems, allowing the rate of release to be controlled by relative solubilities and diffusion. For artificial blood appUcations, incorporation of hydrophilic surface groups, especially oligooxyethylene-based materials, will allow good blood compatibility while the molecular interior can be modified by monomer functionalization before or after polymerization (e.g., fluorocarbon incorporation) to give an interior capable of dissolving and transporting blood gases. Use as microreactors could involve surface modification with hydrophilic and transport-active groups such as oligooxyethylenes or quaternary ammonium salts while maintaining a hydrophobic and oleophilic interior for dissolution of organic soluble starting materials and products. In all of these appUcations, modification of the interior and exterior portions of the molecules combines with enormously increased surface areas to make these attractive for investigation and commercial use. REFERENCES 1. a) Buhleier, E.; Wehner, W.; Vogtle, F. Synthesis 1978, 155; b) Vogtle, R; Weber, E. Angew. Chem. 1979, 97, 813; c) Vogtle, R; Weber, E. Angew. Chem. Int. Ed. Engl. 1979,18. 753. 2. Denkewalter, R. G.; Kolc, J.; Luskasavage, W J. U.S. Patent 4 289 872,1981. 3. Tomalia, D. A.; Naylor, A. M.; Goodard, W A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. 4. a) Tomalia, D. A.; Baker, H.; Dewald, H.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, R Polym. J. 1985,7 7,117; b) Tomalia, D. A.; Baker, H.; Dewald, H.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, R Macromolecules, 1986, 79, 2466; c) Padias, A B.; Hall, H. K., Jr.; Tomalia, D. A.; McConnell, J. R. Org. Chem. 1987, 52, 5305; d) Tomalia, D. A.; Hedstrand, D. M.; Wilson, L. R. Encyclopedia of Polymer Science and Engineering, 2nd ed.; John Wiley & Sons: New York, 1990; Index Volume, pp 46-92. 5. a) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K. J. Org. Chem. 1985, 50, 2004; b) Newkome, G. R.; Baker, G. R.; Saunders, M. J.; Russo, R S.; Gupta, V. K.; Yao, Z.; Miller, J. E.; Bouillion, K. J. Chem. Soc, Chem. Commun. 1986,752; c) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K.; Russo, R S.; Saunders, M. J. J. Am. Chem. Soc. 1986,108, 849.

120


6. a) Miller, T. M.; Neenan, T. X. Chem. Mater. 1990,2,349; b) Miller, T. M.; Neenan, T. X.; Zayas, R.; Blair, H. E. 7. Am. Chem. Soc. 1992,114,1018. 7. a) Hawker, C. J.; Fr^chet, J. M. J. J. Chem. Soc., Chem. Commun. 1990,1010; b) Hawker, C. J.; Fr^chet, J. M. J. J. Am. Chem. Soc. 1990, 772,7638; c) Hawker, C. J.; Fr^chet, J. M. J. Macromolecules 1990,25,4726. 8. Hall, H. K.; Polls, D. W. Polymer Bull. 1987, 77,409. 9. Kim, Y. H.; Webster, O. W. J. Am. Chem. Soc. 1990, 772,4592. 10. a) Wantanabe, H.; Yoshida, H.; Tokata, T. Macromolecules 1988, 27, 2175; b) Yoshida, H.; Wantanabe, H.; Tokata, T. Macromolecules 1991,24, 572. 11. a) Rengan, K.; Engel, R. J. Chem. Soc, Chem. Commun. 1990, 1084; b) Rengan, K.; Engel, R. J. Chem. Soc, Perkin Trans. 11991,987. 12. Hawker, C. J.; Lee, R. J.; Fr^chet, J. M. J. J. Am. Chem. Soc 1991, 775,4583. 13. a) Serroni, S.; Denti, G.; Campagna, S.; Juris, A.; Ciano, M.; Balzani, V. Angew. Chem. 1992,104,1540; b) Serroni, S.; Denti, G.; Campagna, S.; Juris, A.; Ciano, M.; Balzani, V. Angew. Chem. Int. Ed. Engl. 1992,57, 1493. 14. Uchida, H.; Kabe, Y; Yoshino, K.; Kawama, A.; Tsumuraya, T.; Masamune, S. J. Am. Chem. Soc 1990, 772, 7077. 15. a) Morikawa, A.; Kakimoto, M.; Imai, Y Macromolecules 1991, 24(12), 3469; b) Morikawa, A.; Kakimoto, M.; Imai, Y Macromolecules 1992,25, 3247. 16. a) Mathias, L. J.; Carothers, T. W J. Am. Chem. Soc 1991, 775,4043; b) Mathias, L. J.; Carothers, T. W; Bozen, R. M. Polym. Prepn, Am. Chem. Soc. Div. Polym. Chem. 1991,52(7;, 82. 17. a) Roovers, J.; Toporowski, P. M.; Zhou, L. L. Polym. Prepn, Am. Chem. Soc., Div. Polym. Chem. 1992,33(1), 182; b) Zhou, L. L.; Roovers, J. Macromolecules 1993, 26, 963. 18. Chang, R S.; Hughes,T. S.; Zhang, Y; Webster, G. R.; Poczynok, D.; Buese, M. A. J. Polym. ScL, Part A: Polym. Chem. 1993,57, 891. 19. Bochkarev, M. N.; Semchikov, Yu. D.; Silkin, V. B.; Sherstyanykh, V. I.; Maiorova, L. R; Razuvaev, G. A. Vysokomol. Soedin., Sen B 1989,57(9), 643. 20. Reed, S. F, Jr. J. Polym. Set. 1972,10,1087. 21. Schulz, D. N.; Sanada, J. C ; Willoughby, B. G. ACS Symp. Sen 1981,166,427. 22. a) Schulz, D. N.; Halasa, A. F J. Polym. Sci., Polym. Chem. Ed 1974, 72,153; b) Schulz, D. N.; Halasa, A. F J. Polym. Sci., Polym. Chem. Ed 1977, 75, 240. 23. Dickstein, W. H.; Lillya, C. R Macromolecules 1989,22,3882. 24. Dickstein, W. H.; Lillya, C. R Macromolecules 1989,22, 3886. 25. Bhattacharya, S. K.; Smith, C. A.; Dickstein, W. H. Macromolecules 1992,25,1373. 26. Kazama, H.; Tezula, Y; Imai, K. Macromolecules 1991,24,122. 27. Otocka, E. R; Hellman, M. Y; Blyler, L. L. J. Appl. Phys. 1969,40,4221. 28. a) Broze, G.; J6r6me, R.; Teyssi6, R Macromolecules 1981,14,224; b) Broze, G.; J6r6me, R.; Teyssi^, R Macromolecules 1982, 75,920. 29. Rebrov,E.A.;Muzafarov,A.M.;Papkov,VS.;Zhdanov,A.A.Dokl.Akad.Nauk. SSSR1989,309(2), 367.


121

30. Roovers, J.; Zhou, L.; Toporowski, P. M.; van der Zwan, M.; latrou, H.; Hadjichristidis, N. Macromolecules 1993,26,4324. 31. Karstedt, B. D. U.S. Patent 3 775 452,1973. 32. Mathias, L. J.; Carothers, T. W. Polym. Prepn, Am. Chem. Soc. Div, Polym. Chem. 1991,32(3h 633. 33. Carothers, T. W.; Mathias, L. J. Submitted for publication.


HIGHLY BRANCHED AROMATIC POLYMERS: THEIR PREPARATION AND APPLICATIONS

Young H. Kim

ABSTRACT I. INTRODUCTION II. POLYPHENYLENES A. Polymer Synthesis B. Characterization C. Chemical Modification of Polymer 3 D. Symmetrically Branched Polyphenylene E. Hydrophobic Binding Study of Polymer 3B F. Langmuir-Blodgett Films of Hyperbranched Polyphenylenes G. Blending with Other Polymers H. Star-Branched Polymerization III. POLYESTERS A. Single-Step Polymerization

Advances in Dendritic Macromolecules Volume 2, pages 123-156. Copyright © 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-939-7

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124 124 128 128 129 130 132 134 135 137 139 141 141

124

YOUNG H.KIM

B. Symmetrical Polyesters C. Applications IV. POLYPHENYLETHERS A. Synthesis B. Effect on Rheology V. POLYAMIDES A. Preparation B. Characterization C. Lyotropic Properties REFERENCES

143 144 145 145 150 151 151 154 155 155

ABSTRACT Preparation and characterization of highly branched aromatic polymers, polyphenylenes, polyesters, polyethers, and polyamides, were reviewed. These polymers were prepared from condensation of AB^-type monomers, which gave noncrosslinked, highly branched polymers. The polymer properties are vastly different compared to their linear analogs due to their resistance to chain entanglement and crystallization.

I. INTRODUCTION Branches in polymers play important roles in determining their properties, such as viscosity, density, and toughness. However, excessive branching is also known to cause deterioration of physical properties due to low probability of chain entanglement. Only recently, highly branched polymers have attracted increasing attention with the expectation that their unique structures will impart unusual properties creating novel applications. The highly branched polymer field has grownfromtwo fundamentally different disciplines. On one hand, well characterized small molecules, e.g., cascade compounds,* arborols,^ etc., were grown into higher molecular weight molecules by stepwise syntheses. As shown in Scheme 1 and 2, both divergent and convergent synthetic approaches were employed. These methods produce well-defined, large dendritic molecules. The convergent method also allows structural variation, as well as functional group variation. Possible uses for these materials are suggested in areas such as standards for particle size or molecular weight determination, lubricants, polymer-rheology modifiers, and molecular inclusion hosts, to mention but a few.

DIVERGENT SYNTHETIC SCHEME OF DENDRIMERS Protected

' b '

Core

G-1-P

G-1

G-2-P

w

-

I

& G-3-P

G-2

Scheme I .

CONVERGENT SYNTHETIC SCHEME OF DENDRIMERS Protected Monomer

v

Monomer

W-1 -P

4 ''

W-1

Scheme 2,

W-2-P

Highly Branched Aromatic Polymers

127

B B

Scheme 3.

Direct polymerization of the AB^-type monomers, where jc is 2 or greater, has also been attempted. This type of polymerization produces highly branched polymers having one A functional group and {x-l)n+l number of B functional, unreacted groups at the surface of the polymer, where n is the degree of the polymerization (Scheme 3).^ Through this methodology, highly branched copolymers can also be prepared with AB-type comonomers, resulting in the dilution of branching density and allowing further structural manipulation. Following Flory's seminal theoretical treatment of branched polymers, there have been numerous publications dealing with the theoretical as well as physical aspects of these highly branched molecules."^^^ Our research interest in thisfieldis based on the perception that these dendritic polymers could be useful as polymer-rheology control agents as well as spherical, multifunctional macromonomers. Hyperbranched polymers, which were not only thermally and chemically robust under the conditions used, but also could be economically obtained, were created to evaluate these concepts. The latter requirement led us to pursue the one-step polymerization of AB^-type monomers. We will review mostly the synthesis of aromatic polymers with "stable" chemical Unkages prepared by the single-step direct method, and we will briefly compare them with polymers made by more controlled multistep syntheses.

128

YOUNG H.KIM

11.

POLYPHENYLENES''^' A. Polymer Synthesis

Several chemo-selective aryl-aryl coupling reactions of aromatic halides and organometallics with transition-metal catalysts are known. ^^ For example, coupling reactions of arylboronic acids with aryl halides with Pd(0) catalyst,^^ arylmagnesium halides with aryl halides with Ni(II) catalysts,^^ and aryl trialkyl tin compounds with aryl halides with Pd(0) catalyst^^ are well known. The polyhalo-aromatic substances were converted into AB^-type monomers by selective metallation of one of the halo-fimctional groups in each monomer. Polymer 3 was best obtained in good yield under Suzuki's coupling conditions of refluxing a mixture of arylboronic acid and aryl halide in an organic solvent with aqueous carbonate and tetrakis(triphenylphosphine)palladium(O). The molecular weight of the resultant polymer depends on the organic solvent employed. Nitrobenzene, as the solvent, gave the highest molecular weights as summarized in Table 1. The polydispersities of the polymers prepared by this method are usually narrower than typical polymers prepared by condensation methods. Polymer 4 can also be prepared from the monoGrignard of 1,3,5-trichlorobenzene. The Grignard route is advantageous for large scale runs, but, in general, it gave polymers of greater polydispersity and a lower degree of branching. One possible cause of molecular weight limitations is the reactivity loss of the organometallic center due to increased steric hindrance from B(0H)2 Pd(0)

3 4

Scheme 4.

X»Bi X = CI


129

Table 1. The Effect of Coupling Reaction Conditions on the Molecular Weights of Polymers 3 and 4 Monomer

Metho(f

Polymerization

conditions

lA lA

A A

xylene with 1 N K2CO3 1 -methylnaphthalene

lA IB 2

A B B

nitrobenzene with 1 N Na2C03

M;

D'

3,820

1.50

6,560

2.02

32,000

1.13 1.81 18.2

with 1 N Na2C03

Notes:

anhydrous THF anhydrous THF, Mg turnings

3,910 6,470

Method A: Pd(0)-catalyzed boronic acid coupling reaction; Method B: Ni(II)-catalyzed coupling of phenyl Grignard compounds. Mj^ is the number average molecular weight. ^D is the polydispersity, M^/M^ where M^ is the weight average molecular weight.

increasing molecular weight. Another reasonable possibility is that the A functional group is consumed by intramolecular cyclization. Since there is only one A group per polymer molecule, such cyclization would rapidly abort polymer growth by a step-polymerization mechanism. B. Characterization

The molecular weights of these macromolecules are presented as gel permeation chromatographic (GPC) molecular weights against polystyrene standards, and are used only for qualitative purposes. It has usually been found that such GPC measurements of branched polymers underestimate the true molecular weights. In general, these polymers show enhanced solubility in various solvents. For example polymer 3 is readily soluble in o-dichlorobenzene (ODCB), tetrachloroethane, and tetrahydrofuran (THF), but not in CH2CI2. The high molecular weight polymer has diminished solubility when compared to its low molecular weight counterpart. Polymer 4 is the most soluble. These high solubilities can be explained by two synergistic properties of these polymers, namely, lack of crystalline packing and molecular entrapment of solvent within the cavities. The ^H NMR spectra of polymers 3 and 4 show broad peaks from 7.0-8.5 ppm. Infrared (IR) spectra of these polymers show two characteristic peaks at 847 and 740 cm~^ for 1,3,5-trisubstituted benzenes.

130

YOUNG H.KIM

Elemental analysis of polymer 3 showed that most of the bromine predicted from theory was present. By contrast, polymer 4 showed a chlorine content lower than expected, indicating possible reductive elimination of chlorine. Hyperbranched polymer 3 is stable in air up to 550 °C, but bums at 650 °C. Under nitrogen, the polymer loses about 50% of its weight at 550 °C with the evolution of HBr, leaving an amorphous form of carbon. The polymer has a glass transition temperature (Tg) at 238 °C, but no melting point. The molecular weight of the polymer, in the range of Af„ from 2000-35,000, did not influence the Tg (M^ is the number average molecular weight.) C. Chemical Modification of Polymer 3

The halogen groups in polymers can be readily converted to other functional moieties by various exchange reactions affording options of converting polymer 3 or 4 into "multifunctional initiators" or macromonomers. Metal-halogen exchange of the readily THF-soluble polymer 3 with alkyllithiums gave lithiate 3A, which is insoluble in THF, but stable for hours at -78 °C. In a small scale reaction, the lithiation yield was shown to be over 90%, based on successful trapping experiments with chlorotrimethylsilane. When the anion is quenched with methanol (MeOH) or acetonitrile, IR peaks appear at 700 and 779 cm~^ due to monosubstituted benzene, as well as peaks corresponding to trisubstituted benzene. The lithiate can also be treated with various electrophiles, such as CO2, yielding the water-soluble carboxylate (3B). Hydrolysis of this polymer provides a water-insoluble, but THF-soluble, polyacid (3J). The list of electrophiles used and the resulting functional groups are shown in Scheme 5. The degree of functionalization can be estimated from nuclear magnetic resonance (NMR) integration and elemental analysis. Usually 70-80% of the bromine is converted to the desired functionalities. The polymer also can be modified by coupling reactions. Capping the reaction of 3A with p-methoxyphenylmagnesium bromide with a Ni(II) catalyst provided 30. Pd(0)-catalyzed coupling with 2methyl-3-butyn-2-ol gave the acetylene derivative, 3Q. Borane reduction of carboxy-functional groups in 3J gave the corresponding hydroxylmethyl derivative 3K, which in turn was easily converted to the chloromethyl derivative (3L). A potential macroinitiator, 3R, is prepared by dehydration of polyol, 3F. Product 3R is a multifunctional analog of

3G {CH3)2SO,

I

CH3

3H CO,

H+ -^

|—COOLi

•

|—COOH

3B

BH3 •

3J

wy

3K Ncs/ppr^

Imidazolium salt -^

3S

3L

-^

BBr, •

|—CH2OCH3

I—CHzBr

3C

3M

HCON(CH3)2 -^

|—CHO 3T

{CH3)2CO

CH3

CH3COC,H3

p

H2SO4

3R (C5H5) 3D R = (CH3) 3FRMC6H5) CHgOCgH^MgBr

BBr.

30

3P

HCsC(CH3)20H

CH3

I

=

NaOH

4-OI I OH CH3

3Q (CH3)3SiCI |—Si(CH3)3 3i

Scheme 5. 131

|—CH2OH

3N

132

Table 2.

YOUNG H. KIM

The Effect of Polymer Functional Croup on the Thernnal and Solution Properties of Polymers 3 and 4 Solubility

Polymer

Functional group

T^CQ

THF

CH2CI2

221

+ -

-

121

+ +

+

+ +

+ -

3 3B

Br

3G 3H

H CH3

31

(CH3)3Si

3J 3K

C02H CH20H

3L

CH2C1

182

+ +

+ -

30

p-anisol

223

+

+

3P

/?-hydroxylphenyl

+

3R

a-vinyl phenyl

+

4

CI

C02Li 177 141

96

+ +

in ether

water

+

Notes: ^ h e glass transition temperature (T ) is the midpoint of the glass transition at a heating rate of 15 °C/min. ++ means very soluble, + means slightly soluble, and - means not soluble.

l,3-bis(l-phenylvinylbenzene), which is a difunctional initiator for anionic polymerizations in hydrocarbon solvent.^^ Cleavage of the methyl ether groups of 3 0 gave 3P, and deprotection via loss of acetone from 3Q gave the arylacetylene (3N). The properties of these polymers, such as solubility and Tg, are greatly affected by the terminal functional groups (Table 2). Several derivatives, such as those containing p-anisol, a-vinylbenzene, and hydroxylmethyl groups, are CH2Cl2-soluble. The hydroxymethyl derivative 3K requires a small amount of an H-donor solvent for dissolution. Polymers containing nonpolar terminal groups, such as H, CH3 and Si(CH3)3, show much lower Tg's than polymer 3, while polymers possessing polar group substituents, such as CH2OH or COOH, did not show any thermal events up to 400 °C. D. Symmetrically Branched Polyphenylene Miller et al. reported^^'^^'^^ that highly symmetrical, mostly single-element, highly branched polyphenylenes were prepared by the stepwise


3(4): 3(4F):

133

R = C6H5 R = CeF5 R^

R

3(10): R = CeH5 3(10F): R = CgF5

Scheme 6.

convergent synthesis approach shown in Scheme 6. The building block was prepared using 1,3,5-tribromobenzene, through 3,5-dibromophenylboronic acid (lA), by coupling with 3,5-dibromo-l-trimethyl-silylbenzene, in which the silane moiety serves as a halide-protecting group. Once the desired size of the. wedge building block was obtained, it was coupled to the trifunctional core material, 1,3,5-tribromobenzene. Polyphenylenes consisting of 4 phenyls [3(4)], 10 phenyls [3(10)], or 22 phenyls [3(22)] were prepared. A dendrimer with up to 46 rings [3(46)], which should have a 31A diameter size, was also prepared. The ^H NMR spectra of these substances show well-separated, assignable peaks, and the GPC peaks were very narrow. Branched phenylenes with perfluorinated terminal phenyls were also prepared. In contrast with polymer 3, a unique feature of these highly symmetrical phenylenes is their crystallinity. As summarized in Table 3,3(46) can be partially crystallized, and all lower homologues can be fully crystallized. When these dendrimers, with more than 10 phenylenes, were quenched rapidly, they show a Tg, which increases with molecular weight. The fluorinated homologues [3(4F) and [3(10F)] have more crystallinity and higher melting temperatures (T^), but possess a lower Tg's than the hydrocarbon analogs. One interesting aspect is the comparison of these highly symmetric dendrimers with 3G, which has a compositional similarity to the symmetrical phenylenes. Since the Tg of 3 is invariant with molecular weight, we believe that its molecular weight is in a plateau region where the Tg does not increase with the molecular weight. In spite of this, it seems

134

YOUNG H.KIM

Table 3. The Glass Transition and Melting Temperatures of Branched Polyphenylenes Compounds 3G 3(4) 3(10) 3(22) 3(46)

T^TQ 127 none 126 190 220

T^CQ none 174 271 339 512

Notes: ^ h e glass transition temperature (T ) is the midpoint of the glass transition at a heating rateofl5°C/min. ^The melting temperature (T ) is the onset of melt at a heating rate of 15 °C/min.

surprising that the Tg of 3G is lower than that of 3(46). This might be related to the symmetry and regularity of the aromatic arrays of the symmetrical phenylenes, which impose a more highly ordered packing structure. The nature of motions at and below Tg in this class of molecules, which affect ranges and the heat capacity changes at Tg, is not fully understood. Polymers based on 1,3,5-trisubstituted phenylacetylenic wedges and 1,4-disubstituted phenylene acetylenic linear segments with terminal solubilizing groups have also been reported.^'* E. Hydrophobic Binding Study of Polymer 3B

The structure and molecular modeling of poly mer 3B, which resembles a unimolecular micelle, reveal that the average opening of the cavities in the polymer is in theSA range. The complexation of polymer 3B with p-toluidine, a guest molecule, was studied by NMR in D20.^^ All ^H NMR peaks for the methyl singlet and AB quartets of the /7-toluidine protons shifted upfield and broadened as the polymer was added. Sodium acetate was used as an intemal standard with a chemical shift of 5 2.100. At the 3B to /7-toluidine ratio of 0.53, the chemical shift of the methyl group is 5 1.76 at 1.28 M Na2S04, and 5 2.03 at 0.13 M Na2S04. When the 3B to p-toluidine ratios were greater than 2.5 in a 1.9 M Na2S04 solution, the chemical shift reached a limiting value of 6 1.59. The equilibrium constant of the complex was determined in a 1 M NaCl solution by varying the ratio between the host and guest molecules. Since


135

the polymer can form multiple complexes with p-toluidine when the host-to-guest ratio is low, an accurate equilibrium constant is difficult to obtain. Assuming that only a 1:1 complex is formed, an equilibrium constant of about 510 M"^ is estimated. This polymer solution also demonstrated the ability to dissolve hydrophobic molecules, such as naphthalene and methyl red. The experiment indicated that an array of aromatic polymeric chains is capable of generating an environment resembling a micellar structure, a finding that led us to further studies of the polymer's surface properties. F. Langmuir-Blodgett Films of Hyperbranched Polyphenylenes

In spite of an irregular polymer architecture, some amphiphilic, hyperbranched polyphenylene derivatives were found to assemble, forming quite uniform Langmuir monolayers at the water/air interface. For example, 3J assembled into a monolayer at the water/air interface and provided a surface similar to the Langmuir film of fatty acids in the nucleating of mineral crystallization on the water/air interface. The number average molecular weight and dispersity of the hyperbranched polyphenylene were 32,000 and 1.03, respectively. The quaternary salt 3S was prepared by reaction of the chloromethyl derivative 3L with a slight excess of A^-methylimidazole. Elemental analysis of the product, which precipitated from ether, indicated that about 35% of the theoretical amount of benzylic chloride was converted to this imidazolium salt 3S, which is water-insoluble and only slightly soluble in MeOH. In contrast to ionic derivatives, the bromo derivative (3) did not form a Langmuir monolayer. The related hydroxymethyl derivative 3K formed an unstable Langmuir film, which collapsed below 20 mN/m, while, 3S, which possesses a classical amphiphilic structure in a two-dimensional presentation, formed stable monolayer films with collapse pressures as high as 60 mN/m without much hysteresis (Figure 1). The tangent of the Il-A curve has an intercept at approximately 9.6 A^ per repeating unit. If one considers that the degree of polymerization is 400 from the GPC data, the radius of the area for an average polymer on the air/water interface is about 70 A, fairly approximating the mean radius of gyration ^^^ of polystyrene of same molecular weight, which is about 50 A.^^ Since the polymers have an ellipsoid rather than spherical shape, we speculate that the flat side of the ellipsoids are at the water/air interface, thus, occupying a larger area than expected. The additional

YOUNG H. KIM

136

35 - ;

\

\

1

1

1 "

30 ^ 25 -

E ^

20-

E ^ 1 5 C 10 5 -

0 4

[1

8

12

16

20

Area/Repeating Unit (A^) Figure 1. Hysteresis isotherm of polymer 3S on a water/air interface. The compression and decompression rate is 50 Mm/min.

volume of about 35% due to the imidazole unit could also contribute to the discrepancy. On the other hand, the possibility of various conformations due to atropisomerism of phenyls and cavities around the phenyl rings could have an opposite effect. Deposition of a monolayer film of 3S at 20 mN/m surface pressure on a silicon wafer was achieved by z-type with transfer efficiency consistently in the range of about 0.5-0.6 (Figure 2). Such low transfer may imply that the hyperbranched polymer can be squeezed under compression, through either conformational change or intercalation. The overcompression could be reUeved by transfer onto a solid substrate where the molecule occupies an area larger than on the water/air interface. The thickness of the deposited monolayer measured by ellipsometry was about 32A for thefirstfew layers, but this number declined slowly as the number of depositions increased. The homogeneity of thefilm,judged from the standard deviation of thefilmthickness, also deteriorated. The


137

(0

c

CO

2

4

6

8

10

Number of Layers Figure 2. The transfer of a Langmuir-Blodgett film of polymer 3S to a silicon wafer.

large discrepancy between the radius of the molecule on the water/air interface and the monolayer film thickness might also be related to the unique structural feature discussed above. The structure of these amphiphiles is radically different from that of conventional Langmuir film-forming amphiphiles, where segregation of the hydrophobic and hydrophilic parts of molecules at the air/water interface is a prerequisite. This is the first example of a LangmuirBlodgettfilmthat is fabricated with a micellelike substance. It is intriguing that amphiphilicity is still required to form a stable monolayer film of hyperbranched polymers, in spite of their fundamental structural differences from conventional amphiphiles. G. Blending with Other Polymers

If polymeric molecules were to show an interaction with 3 in the way that small aromatic compounds form complexes with 3B, such interac-

138

YOUNG H.KIM

tions could provide the necessary free energy for polymer miscibility. In the glassy state, the complexation could function as a physically reversible cross-linking of the polymer chain. In the molten state, where such static interaction would not be sustained, a spherical hyperbranched polymer could affect the rheology of another polymer. Polystyrene and poly(vinylchloride) (PVC) were chosen to test this hypothesis. Polystyrene Blend

These polymers can be blended either in solutions or in the melt. A blend of up to 2% of polymer 3 with polystyrene appeared clear, but turbid blends resultedfromhigher percentages of polymer 3. The Tg's of blended polymers containing up to 30% of polymer 3 did not change from that of pure polystyrene. Transmission electron microscope (TEM) analysis of the 5% blend showed that there are bimodal distributions of polymer 3 domains, a small amount of approximately 10 nm domains, and a large portion of approximately 1 \i domains. In spite of this evidence of possibly poor mixing, some noticeable changes were found in the rheology and thermal stability of polystyrene. In comparing two polystyrene blends, one with 5% of polymer 3 and the other with 0.1% (control), the melt viscosity of the 5% blend was about 50% at 180 °C and 80% at 120 °C, respectively. The eifect was more drastic at higher temperatures and higher shear rates. The addition of polymer 3 also seemed to improve the thermal stability of polystyrene. When the molten blend polymer was kept at 180 °C for some time, the melt viscosity of the 0.1% blend increased, whereas that of the 5% blend remained constant. Polystyrene blended with 2% of polymer 3 was injection molded into 1/8-inch wide flex and tensile bars for mechanical measurement. It had no effect on the flexural modulus of polystyrene, but a significant improvement in the initial modulus with a concomitant sacrifice in the maximum strength was obtained. Weak cross-Unking of polystyrene by polymer 3 through aryl-aryl interaction might be responsible for the high initial modulus. PVC Blend

For the most part, this pair of polymers was mixed in solution. In contrast to polystyrene, polymer 3 had no noticeable influence on the


139

0^ K> "-^

o • (0 (0

, /^5

0-^ -H

t •

o

•

D O

•

i o

0'

Filled: with 5 % HBP Open: with 0 . 1 % H B P Circle: atlOO'^C Diamond: at 120°C Square: at140°C

H

9

in

8 1000 -^ 5, exhibit liquid crystalline phases and can be greatly stabilized in their bolaamphiphilic form (Figure 7). A key element for increased stability of these assemblies was

Dendritic Bolaamphiphiles and Related Molecules

167

HO HO HO HO HO HO HO HO HO HO HO HO

OH OU OH OH OH OH OH OH OH OH OH OH ^'•* Water

Figure 6. Morphology of a,(0-decosanediol at the air-water interface.

that the head groups should function as both a proton donor and acceptor. This functional duality permitted the incorporation of the head groups into the H-bridged network, resulting in an increasing degree of stability. Rico et al. reported^^ the synthesis of the bis-gluconamides and bislactobionamides (Figure 8) in higher yields than for the comparable

CnH2n+l

/ OH 20

HO-^ HO

ÔH \ / n

OH

21

Figure 7. General structures of amphiphilic and bolaamphiphilicpolyols.

168

G REGORY H. ESCAMILLA

R = H, Galactose n = 6-12

Figure 8. General structure of 6/s-gluconamides and 6/s-lactobionamides.

sugar bolaphiles (Scheme 3). Thompson and co-workers^"* also reported a series of chiral diether and tetraether phospholipids, which were explored as chemoselective thin films. B. Functional Vesicles

Fuhrhop et al.^^ demonstrated that incorporation of quinone moieties into either the head groups or within the hydrophobic spacer of a bolaphile gave redox-active membranes (Figure 9). Unsymmetrical bolaamphiphiles were used as models for photosynthetic electron acceptors (e.g. 29) and vesicle formation was verified by electron micrograph. The anticipated membrane organization would position the quinone moieties on the vesicle exterior. The location of the quinone moieties in this series was verified by their quantitative reduction by borohydride. Since borohydride does not diffuse through lipid membranes these "membrane quinones" must be part of the vesicle exterior. Anthraquinone-based bolaphile 30 (included in this nonionic section for convenience) was created in order to locate the quinone moiety within the center of a membrane; however, stable membrane formation was prevented due to steric considerations. It was possible to integrate this quinoid bolaphile into other host membranes, such as dihexyldecylphosphate. The absence of borohydride reduction supports the premise that the quinone functionality is located at the midpoint of the host membrane wall.^^ Structures 29, 30, and 31 were synthesized as


169

OH OHI

H2N,^NH2 22 21

H2N,>-yNH2 25

OH

HO

H ^N-Q^N OH H H

HO

26

Scheme 3. General synthetic route used for the construction of 6/s-gluconamides and 6/s-lactobionamides.

possible "pool quinones" with appropriate redox characteristics.^^ Electron transfer was also demonstrated by light-induced charge transfer between cationic porphyrins dissolved in water with dihexyldecylphosphate and dimethyldioctadecylammonium bromide vesicles doped with

GREGORY H. ESC AMI LL A

170

COOH

OOH

31

Figure 9. Tailored bolaphiles used to incorporate quinone moieties within micellar structures.

C. Biological Activity

Jayasuriya et al.^ proposed bolaforai amphiphiles synthetically tailored to provide an effective geometry for improved membrane disruption. Variation of either head group(s) or spacer(s) could provide a specific "tunability," allowing microorganisms specific disruption. Bolaphiles were envisioned^^ to disrupt the membrane via simple insertion of the hydrophobic spacer into the lipid membrane creating a mismatch between the preferred geometries of the bolaphile and lipid. When sufficient molecular invasion occurs, destabilization of the membrane increases (Figure 10). Alkane spacers ranged from decane to eicosane; however, the maximum activity was observed with pentadecane. Spacers with central double or triple bonds were made with the expectation that these functionalities would enhance the disruptive properties of these bolaamphiphiles. In the olefinic series, the geometry and

Dendritic Bolaampliiphiles and Related Molecules

171

Hydrophilic

Hydrophobic

Figure 10. Mechanism proposed for membrane destabilization via insertion of varying length hydrophobic spacers into a phospholipid layer and the resulting local defect.

position of the double bond were less important factors for membrane disruption efficiency than simply overall length. Bolaphiles possessing an internal ethyne group exhibited behavior similar to the olefins in that placement of unsaturation did not appear to be of significant importance. A general trend, however, was that maximum disruptive activity required an increased spacer length relative to the saturated series. Jayasuriya et al. described^ the use of a polymeric string (Figure 11) of bolaamphiphiles in membrane disruption studies. A comparison between the polymeric ("supramolecular surfactants") verses the monomeric bolaamphiphiles demonstrated an increase of 3 orders of magnitude for the former in membrane disruptive activity. The precise origin of this amplification is not yet understood; however, several theories have been proposed: (1) by their very nature, the polymeric bolaamphiphiles possess covalent linkages, which localize the "bolaamphiphile defects" by achieving a high local concentration; (2) domains of the supramolecular surfactant within the bilayer will be in equilibrium with nonaggregated membrane-bound polymers; and (3) repeat unit defects in the polymer are intrinsically more disruptive than the free monomer. A polyester structurally related to these "supramolecular surfactants", exhibited substantial protection for human CD4+ lympho-

172

GREGORY H. ESCAMILLA 32 O

O

HCKCH^CHÔ)^—e.(CHJn-C-0(CIi,CH20)gH O

O

[—(":.(CR,)n-l':-0(CH^CH20)^ ]z 33

o

^4

o

UOiCUfHp)^—C_(CHpx--C^C—(CH2)y--C--0(CH^CH^0)^H O

O

[—C--KCHpx—C=C—(CH2)y--C--0(CH,CH20)^]z 35 O

36

o

HCXCHXHÔ)^—C—(CH,)x—C==C—(CHpy---C:—C)(CH2CH20)gH O

O

1—I!:—(CHjx—C=C—(CHJy—I!:—0(CH,CH,0),Jz 37 Figure 11. Membrane-disruptive bolaphiles and their polymeric analogues,

cytes against HIV-1 during in vitro studies,^ thus offering a new route intp nonionic membrane-disrupting agents, in which activity and specificity could be tailored through molecular design. Selectivity of the bolaphiles to discriminate among lipid bilayers of varying cholesterol content has been considered by Nagawa and Regen.^^ Their results demonstrate that modest differences in membrane composition and packing can lead to large differences in structural lability, and that synthetic agents can be created to exploit these differences.^^ The concept of "tunability" has been supported by the fact that 33 (n = 14) readily lyses R mirabilis but not erythrocytes.

173

Dendritic Bolaamphiphiles and Related Molecules D.

Ion Transport

In an extension of bolaphilic architecture, Fyles et al.^^ have assembled an ion channel mimic (Figure 12), from what they have termed a "modular construction set" (Figure 13). Twenty one ion channels were Head

o=/^^\=o

Y

Z Wall

Core

o

O

Y

z

o < > 38 Figure 12. Schematic representation of the ion channel mimic.

Core Units coo'

cô^ coo" .^-"^ OOC^

O

r^^^

r^^^

o

oocs

o

o

coo-

ooc^^

O^

OOC^

O

O^

'COO'

OOC^

^v^

O

COO'

Ô '^Y^

^\

oocô

O

O^

"OOC

OOCK,

OOC^

o

O

O^

"COO"

Head Groups '^'^^^°^r^^

„/~-COOH

_s.

.OH

Wall Units o

O

0

Figwre 13. Components of the "modular construction set." 174

0

Dendritic Boiaamphiphiles and Related Molecules

175

HO--' -ÔH H0^7^0H

Core-4c(;2'

Scheme 4. Generalized synthesis of an ion channel mimic.

prepared and characterized through this modular approach. The authors envisaged a central "core" unit from which "wall" units radiate. A series of di-, tetra-, and hexacarboxylic acid derivatives of 18-crown-6 ether, serving as the cores, provided rigidity to direct the wall groups toward the two bilayer faces. The wall units, possessing the desirable oblong structural conformation, were composed of macrocyclic tetraesters derived from maleic anhydride; these macrocycles were selected via insight derived from literature data, model building, and molecular modeling studies (Scheme 4). Attachment of the walls to an appropriate core unit was followed by capping with one of three available head groups. The critical synthetic details conceming these ion channel mimics has been delineated.^^

176

GREGORY H. ESCAMILLA

Fyles et al.^^ described their results concerning the transport of alkali metal cations across vesicle bilayers mediated by these ion channels mimics. The relative activities of their most active transports were comparable to valinomycin but were 2- to 20-fold less active than gramicidin. Six examples in their modular series effectively acted as channels; four in this series acted as cation carriers. The range of observed transport activities, selectivities, and mechanism indicates that structural regulation of synthetic ion channel mimics is possible with this family of bolaphiles.^^ iii. IONIC BOLAAMPHIPHILES A. Molecular Assemblies

Roberts et al. "^ used bolaform phosphatidylcholine as a probe of water soluble phospholipase catalysis. These bolaphiles (Figure 14) contain two phosphatidylcholines, as the ionic head groups permitting the evaluation of the proposal that two phosphatidylcholines are required for phospholipase activity. Phospholipase activity was measured using micelles formed from these bolaphiles and phosphatidylcholine containing amphiphiles. Increased membrane stability of these bolaform

o

o o

o

43

Figure 14. Generalized structure of bolaform phosphatidylcholines.

Dendritic Bolaamphipliiles and Related Molecules

Ml

[2CH2N(CH,)2Ci CT(CH-)-^CfU:H.S-

^ v ÔCH^Ph

44

G=

-CH;

45

G=H

figure 15. Bolaphiles used for the generation of thermally stable membranes.

micelles lowered the rate of enzyme activity in comparison to micelles formed from the amphiphiles. The rationale for this observation was that reduction of the vertical diffusion of the individual bolaform phosphatidylcholine from the membrane was required for enzyme activity. A prime motivation for interest in synthetic bolaamphiphilic membranes was the enhanced membrane stability as found with archaebacterial membranes. Li and co-workers utilized the pioneering studies of Fuhrhop"^^ as well as the synthetic modifications described by Lo Nostro et al."^^ to generate bolaphiles 44 and 45 (Figure 15)."^^ As noted with archaebacterial membranes, these synthetic membranes tend to be rather stable and, in this particular case, stabilized toward temperature increases. This behavior was attributed to the location of these bolaphiles within the membrane, thus instilling a substantial resistance to their motion either along or out of the membrane."*^ Although the above examples dealt with molecular assembly of solvated bolaphiles, Ringsdorf and co-workers'^ have prepared self-assembled monolayers on negatively charged substrates. They noted literature precedence for the use of cationic bolaamphiphiles to reverse the surface charge of a mica substrate, thereby allowing subsequent adsorption of anionic polymers and construction of multilayers."^^ Organic

178


^' (C£)^'^^'^-'^^^^''-^ V_-/"

COO(CH:)6-NQ)

cr

46

Br"

©-'

(CH:)800C 47

l^r'

:00(CH2)II-NQ)

Br

^(£)N-(CH:

OOC €00(CH:)6HO' OH v r i ^ 49

Figure 16. Series of bolaphiles that adsorb onto mica surfaces.

monolayers, which are only a few nanometers in thickness, have possible applications in advanced optical and electronic sensors, biosensors, separation membranes, microlithography and pattern formation, and modification of surface properties."^ Thus, the influences of alkyl chain length upon self-assembled monolayers on mica were evaluated. Using phenylene based bolaphiles (Figure 16), the orientation of the bolaphile on the mica was shown to be dependent on spacer length: below a critical length, the bolaphile lays flat on the surface, while greater lengths adopt an end-on orientation. This behavior was explained by competition between hydrophobic and electrostatic effects. At short chain lengths, electrostatic effects dominate and the bolaphile lies flat on the surface. The ability to control self-assembly can be realized with the proper selection of the counterion. Reduction of the repulsion between the like charged head groups favors close packing, which is critical for the formation of layered structures.


179

Head Group

Figure 17. Generalized structure of a "gemini surfactant."

An ammonium ion head group, a rigid spacer, and attachment of long alkyl chains to the head groups led to the family of bolaphiles that Menger has labeled as "gemini surfactants" (Figure 17)7 The rigid spacer inhibits m/ramolecular chain-chain associations, whereas a flexible spacer would allow an u-shaped conformation in which the surfactant could behave as conventional twin-tailed amphiphile.^ Menger and co-workers synthesized these surfactants in order to gain additional structural information concerning membranes as well as their self-assembly. Computer modeling of assemblies made up of gemini surfactants show structures, such as thread-like micelles. Many dynamic properties of gemini surfactants, such as rapidly increasing viscosity with small increases in concentration, are very different from amphiphiles."*^ A theoretical explanation of experimental results, obtained for modified (non rigid spacer) gemini surfactants, was presented by Andelman and Diamant."*^ They noted that the aggregate morphology for dimeric surfactants is related mainly to the influence of the spacer on the specific area Z. The geometrical parameter, that determines aggregate shape, is the "packing parameter" denoted by the formula,"*^"*^ p = \)/Zl

(1)

where X) is the volume occupied by the hydrophobic moiety of the surfactant molecule, 1 is its length, and Z is the specific area per molecule."^^ It was found that the specific area of the surfactant at the air-water interface was dependent on interplay of three distinct factors: the geometrical effect caused by lengthening the spacer moiety, which increases Z; interaction among the surfactant monomers, which tends to decrease Z after the spacer reaches a critical length; and the conformational

180

GREGORY H. ESCAMILLA Hl+2mCm

CniH2m+l

Br 50

10<m< 18 4 < y < 16

Figure 18. Generalized structure of alkanediyl-a,a)-b/s(dimethylalkylammonium bromide)bolaphiles.

entropy of the spacer chain, which increases rapidly as spacer length increases and enhances the effect of the second factor.'*^ Neglected in the theoretical discussions were factors, such as van der Waals attraction and the excluded volume of the surfactant's hydrophobic tails. The model was developed for a specific case but should be valid for any bolaphile, even those without hydrophobic tails.'*^ This is still a very narrow model that needs further work to establish its generality. Skoulios et al.^^ described the structure of lyotropic mesophases formed from alkanediyl-a,co-bis(dimethylalkylammonium bromide) bolaphiles (Figure 18). As spacer lengths increase from 4 to 8 methylenes units, the concentration range of the lyotropic mesophases decreases, whereas for spacers of 10 and 12 methylenes, water-surfactant mixtures remain micellar throughout the whole range of composition. At lengths longer than 13 methylenes, lyotropic mesophases reappear. Bolaphiles of this type did not exhibit thermotropic liquid crystals when heated to high temperatures. A columnar mesophase with a cylindrical morphology was observed in specific concentration ranges. Dimensions of the column were in agreement with a structural model having the alkyl tails of the head groups oriented toward the center of the column. Exchange of these bolaphiles between bulk aqueous phase and micelles was found to be a single step process,^^ in which both head group alkyl chains exit simultaneously from, or incorporate into, the micelles. Alkanediyl-a,cobis(dimethyl-alkylammonium bromide) was also studied with respect to its behavior at the water-air interface.^^ Based on surface tension measurements, the location of the spacer varies with respect to spacer length: spacers of less than 10 methylenes lies along the air-solution interface; spacers of 10 to 12 methylenes conformationally bridge into the air side of the interface.


181

Br

Br"

51

CH^-^n ^

/

-

7Â, ° Br

52

Br53

Figure 19. Phenylene-based bolaphiles.

Ringsdorf et al.^^ described a series of three isomeric bolaphiles based on para-, meta-, and orr/io-phenylene derivatives with pyridinium head groups (Figure 19). Lyotropic mesophases were formed only by 51; however, 52 can form mesophases with the addition of 1-hexanol (1:1), as a co-surfactant. It was rationalized that the co-surfactant fits (Figure 20) between the hydrophobic alkyl chains of 52, thereby separating the charged head groups and thus stabilizing the macroassembly. A similar stabilization was seen in a,co-alkanediols and H2O as reported by Lahav

182

GREGORY H.ESCAMILLA

o#o*o#o4o Bolaphile

O

1-Hexanol

Figure 20. A representation of the "mixed" lyotropic nnesophase.

et al.^^ The introduction of rigid spacer units also stabilized the mesophase range of the bolamphiphile.^^ Bosch et al. reported the synthesis^"^ and later assembly properties'^ of bolaphiles (54, 55) prepared from dimeric acids. The objective was to demonstrate extension of the concept of supramolecular self-assembly to organic compounds having a substituted cyclohexane structure (Figure 21). Assemblies were observed with both bolaamphiphiles; however, 55 generates multilamellar arrangements whose structures depend on the counterion. Morphology of the aggregates was linked to the nature of the head groups and the nature of counterions. Nakatsuji et al.'^ have synthesized a dendritic series of bolaamphiphiles possessing three anionic head groups separated by spacers (Figure 22). Here, as with other examples, aggregates form at concen-

OSO^'Na

{, ^0S03 Na

X = I, CI, HCO3

Figure 21. General form of the cyclohexane based bolaphiles.

OR SO^'Na

"0 O O

Na "O^S"

-O

OR 56

Figure 22.

Generalized structure of a "three headed" bolaphile. 183

184

GREGORY H.ESCAMILLA

trations below critical micellar concentration (CMC).^'^^ Behavior of these bolaphiles in solution, in terms of CMC versus the number carbons in the lipophilic chains, runs counter to that observed for a homologous series of single-tailed surfactants; this behavior offers many new avenues of exploration to the synthetic chemist. B. Functional Vesicles

Bunton et al.^^ demonstrated catalytic behavior in the spontaneous hydrolysis of 2,4-dinitrophenyl phosphate promoted by alkane a,cobis(trimethylammonium) bolaphiles. The enhanced rate of hydrolysis followed the greater degree of organization within vesicles from surfactants possessing longer (Q-Cy) spacers. Notably, bolaphiles with 12 and 16 methylene spacers did not form micelles, but instead formed small clusters, and showed a lower rate enhancement versus micellizing surfactants. Fomasier et al.^^ utilized metallomicelles formed with either bolaphiles 58 or simple surfactants 57 as catalysts in the hydrolysis of

OH

57

Br

OH

58 Figure 23. Proximity of the charged head group to the binding site in 57 slows chelation of the metal relative to the bolaphile 58. M = Cu", Zn"; R = para-nitrophenyl picolinate.

Dendritic Bolaamphiphiies and Related Molecules

185

p-nitrophenyl picolinate. Micelles generatedfrompyridine bolaphile 57 (Figure 23) possessing Cu® or Zn^°^, enhanced the rate of hydrolysis compared with micelles generated from 6-{[(2-(n-hexadecyl)dimethylamino)ethyl]thio}methyl-2-(hydroxymethyl)pyridine bromide, which was not as effective a catalyst. Increased electrostatic repulsion between the terminal ammonium group and the terminal metal ion site possibly explain the difference in effectiveness of bolaform 58 versus classical micelle 57. Formation of a ternary complex consisting of substrate, metal, and bolaform amphiphile was suggested as the crux of the catalytic process.^^ These micelles exhibited substrate discrimination as indicated by the lack of catalytic enhancements when isomeric nicotinate or isonicotinate esters were utilized. Increased catalytic effectiveness of the micellar bolaamphiphiies was rationalized by: (1) hydrophobic factors that bring the substrate together with ligands and metal ions in a small volume, (2) the enhanced electrophilicity of divalent metal ions toward micellar-bound substrates, and (3) higher "local" pH at the micellar surface, relative to the bulk phase, which favors the acid dissociation of the hydroxy groups in the ternary complex.^^ Nolte and co-workers^^ recently reported a functional vesicle formed by assembly of hosts capable of binding guests, such as resorcinol and 4-(4'-nitrophenylazo)resorcinol (Figure 24). Two quaternary ammonium

Figure 24. Representation of bolaphile 59 used to prepare a functionalized vesicle

186


centers coupled with two long alkyl chains were incorporated in order to maintain the hydrophobic binding site at the exterior of the vesicle; the rigidity of the binding sites fixes the charged nitrogens in a specific orientation near the vesicle surface. In typical lipid fashion, the hydrophobic alkyl chains served to form the interior of the membrane. The combined interactions of solvent, binding sites, charged nitrogens, and hydrophobic alkyl chains govem the vesicular morphology, which was likened to a "golfball" due to its spherical shape and dimpled surface. Examination of a cast film of 59 by X-ray diffraction showed a clear periodicity of 53 A, which is approximately the length of two fully extended hexyldecylamine chains. Binding studies suggest that only half the total number of binding sites are available to react with guest. These data would indicate a bilayer membrane with a thickness of approximately 53 A. Nolte's "golfball" bears structural similarity to Menger's gemini surfactants,^ described earlier. Previous examples in this section have dealt with incorporation of moieties into the bolaphile for the expression of some desired chemical activity or property. Tirrell and co-workers^^ recently exploited the self-assembly properties of bolaphiles in another fashion. They used a bolaphile possessing a photopolymerizable spacer 47, and the preorganization of an assembled monolayer to generate polymerized layer, where the bolaphiles are covalently linked. Multiple layers of 47 were generated on a mica surface by alternating deposition of 47, followed by an anionic polyelectrolyte poly(2-acrylamido-2-methyl- 1-propanesulfonic acid) (PAMPSA), until the desired coating depth was reached. Photopolymerization was conducted at room temperature by irradiation for up to 30 minutes. Polymerization changes the apparent electrostatic properties of the layer interface and increases the lateral strength of the film. The nature of the first layer assembled on the substrate is the major factor controlling the stability of successive layers.^^ Applications of thin layers were noted previously in this review."^ C. Biologically Active Damha et al.^- have succeeded in making dendritically branched RNA-based bolaphiles in a convergent-growth approach (Figure 25). The resulting structure fits well within the description of a bolaamphiphile in that it is comprised of thymine and a modified adenosine spacer and terminal nucleotides as head groups. These branched RNAs


TTTTT^ ÂTTTTT TTTTT^ \ / \ / TTTTT ATTTTrrmT 111 1 ITl'lTl A ;:ATTrTr^ \ TTTTT^ 60

187

Tim TTTTTA ^rmr /^-^ ^"^^^ TTTTT

Figure 25. An RNA-based bolaphile.

could be useful in capturing and binding matching nucleotide sequences, thus, opening doors to potential antisense agents or ligands for affinity purification of the branch recognition factors, which can catalyze the maturation or splicing of precursor messenger R N A . D.

Transports

Diederich et al.^^ reported an example of a bolaphile designed as a transport agent specifically to carry nucleotide 5'-triphosphates across liquid organic membranes. Certain highly charged nucleotides designed to act as inhibitors of H I V reverse transcriptase cannot penetrate cell membranes, hence the motivation for the design and creation of these synthetic carriers.^^ The bolaphile 6 1 (Figure 26) forms 1:1 complexes with a variety of nucleotide 5'-triphosphates and does not leak from a CH2CI2 liquid membrane. Efficient transport was shown via u - t y p e cell experiments; however, with liposomes these synthetic carriers do not mediate specific transport of nucleotide 5'-triphosphates. At certain concentrations the transport acts as a detergent breaking the liposomal structure and leading to nonspecific leakage. From their experiments, the

r 0

4 Br

0

Hi-c/ 61 Figure 26.

A bolaamphiphilic transport for nucleotide 5'-triphosphates.

188


authors caution extrapolating from the carrier efficiencies in liquid organic membrane experiment to liposomes or cellular membrane transport.

IV. CONCLUSION Beyond these examples, basic questions remain concerning bolaamphiphile conformations within micelles.^^ Evidence indicates that a bent conformation is adopted,^ but micelles have been observed with very small hydrophobic cores and sometimes are seen on electron micrographs with diameters that correspond to the length of the bolaphile.^^'^^ Natural bolaamphiphiles found in thermophilic bacteria with their stereochemistry and stability in harsh environments illustrate further areas to explore concerning molecular self-assembly. More robust membranes or chiral membranes derived from chiral bolaphiles could provide avenues to molecular recognition based on stereochemistry not only at one site, but over an entire membrane.*^ Synthetic chemists can tailor membranes to provide a desired morphology or potential catalytic function. Bolaamphiphiles, like cascade macromolecules,^- have gained increasing attention recently, since they offer insight to micellar systems that can mimic enzymes, act as therapeutic agents, or as tools to investigate molecular recognition and assemblages. The works cited herein illustrate the goal Lehn set forth for supramolecular chemistry—do not focus just on structures but rather the expression of a desired chemical, biological, or physical property.--^-^" REFERENCES 1. Nagarajan, R. Chem. Eng. Commun. 1987.55, 251. 2. Fuoss. R. M.; Edelson, D. J. 7. Am. Chem. Soc. 1951, 73, 269. 3. Newkome, G. R.: Baker, G. R.; Aral, S.: Saunders, M. J.: Russo, R S.: Gupta, V. K.; Yao. Z.-Q.; Miller, J. E.: BouiUon, K. / Chem. Soc, Chem. Commun. 1986, 752. 4. Fuhrhop. J.-H.: Fritsch, D. Ace. Chem. Res. 1986, 79, 130. 5. Fuhrhop, J.-H.: Mathieu, J. Angew. Chem. 1984, 96, 124: Ange\v. Chem., Int. Ed. Engl. 1984, 23, 100. 6. Menger, F M : Littau, C. A. / Am. Chem. Soc. 1993, 775, 10083. 7. Menger, F M.: Littau, C. A. J. Am. Chem. Soc. 1991, 772. 1451. 8. Rosen, M. J. Chemtech 1993, 30.

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Jayasuriya, N.; Bosak, S.; Regen. S. L. / Am. Chem. Soc. 1990,112, 5851. Jayasuriya, N.: Bosak, S.: Regen, S. L J. Am. Chem. Soc. 1990,112, 5844. Amato, I. Science 1993. 260, 491. Hudson, R. H. E.; Damha, M. J. J. Am. Chem. Soc. 1993, 775, 2119. Fomasier, R.: Scrimin, R; Tecilla, R: Tonellato, U. J. Am. Chem. Soc. 1989, 777, 224. Cipiciani, A.: Fracassini, M C ; Germani, R.: Savelli. G.: Bunton, C. A. J. Chem. Soc, Perkin Trans. 1987, 547. Bunton. C. A.; Dorwin. E. L.: Savelli. G.: Si, V. C. Red Tra\: Chim. Pays-Bas. 1990, 709, 64. Kim. J.-M: Thompson, D. H. Langmuir 1992, 5, 637. Schenning, A. R H. J.: de Bruin. B.: Feiters. M. C : Nolte. R. J. M. AngeM\ Chem. 1994, 106, \lA\\AngeM: Chem., Int. Ed. Engl 1994. 33, 1662. Newkome, G. R.; Baker. G. R.: Arai, S.: Saunders, M. J.; Russo. R S.; Theriot, K. J.: Moorefield, C. N.: Miller, J. E.: Bouillon, K. J. Am. Chem. Soc. 1990, 772. 8458. Newkome, G. R.; Lin, X.: Yaxiong. C : Escamilla, G. H. J. Org. Chem. 1993. 58, 3123. Newkome, G. R.: Moorefield, C. N.: Baker, G. R.: Behera, R. K.: Escamilla, G. H.; Saunders, M. J. Ange^v. Chem. 1992.104, 901: Ange^v. Chem., Int. Ed Engl. 1992, i7.917. Fuhrhop. J.-H.; Spiroski, D.: Boettcher. C. J. Am. Chem. Soc. 1993. 775. 1600. Gros. L.; Ringsdorf. H.: Schupp. H. AngeM: Chem. 1981. 93. 311: A/iêw. Chem., Int. Ed. Engl. 19HI. 20. 305. Fendler. J.-H.: Tundo. R Ace. Chem. Res. 1984,17. 3. Lund, H.: Voigt, A. Organic Synthesis Collect.: Wiley: New York, 1943. Vol. IL p. 596. Lehn, J.-M. AngeM\ Chem. 1990, 702. 1347: Ange^v. Chem., Int. Ed Engl. 1990, 29. 1304. Munoz. S.: Mallen. J.: Nakano. A.: Chen, Z.: Gay, L: Echegoyen. L.: Gokel, G. W. J. Am. Chem. Soc 1993. 775. 1705. Fouquey. C : Lehn. J.-M.: Levelul. A.-M Adv. Mater 1990. 5. 254. Popovitz-Biro, R.: Majewski, J.: Margulis. L.: Cohen, S.: Leiserowitz. L.: Lahav. M. J. Phys. Chem. 1994. 98, 4970. Ueno. M.: Kawanabe. M.: Meguro. K. J. Colloid Interface Sci. 1975. 57. 32. Jeffers. R M.: Dean. J. J. Phys. Chem. 1965. 69. 2368. Popovitz-Biro. R.: Wang. J. L.: Majewski. J.: Shavit. E.: Leiserowtiz. L.: Lahav. NL J. Am. Chem. Soc. 1994,116, 1179. Henirich, F : Tschierske, C : Zaschke. H. Ange^v. Chem. Int. Ed. Engl. 1991. 30, 440. Garalli-Calvel. R.: Brisset, F: Rico. I.: Lattes. A. Syn. Comm. 1993, 23. 35. Thompson, D. H.: Svendsen. C. B.: Di Meglio. C : Anderson, V. C. J. Org. Chem. 1994,59,2945. Fuhrhop, J.-H.: Hungerbuhler. H.: Siggel, U. Langmuir 1990. 6, 1295. Siggel, U.: Hungerbuhler. H.: Fuhrhop J.-H. J. Chim. Phys. 1987. 84. 1055. Nagawa, Y: Regen. S. L. J. Am. Chem. Soc. 1991.113.1231.

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GREGORY H. ESCAMILLA Fyles, T M ; James, T. D.: Pryhilka. A.; Zojaji, M. / Org. Chem. 1993, 58. 7456. Fyles, T. M.: James, T. D.; Kaye, K. C. J. Am. Chem. Soc. 1993, 775, 12315. Lewis, K. A.; Soltys, C. E.; Yu, K.; Robens, M. F Biochemistry 1994, i i , 5000. Fuhrhop, J.-H.; David, H.-H.; Mathieu, J.; Liman, U.: Winter, H.-J.; Boekman, E. J. Am .Chem. Soc. 1986, 108. 1785. Lo Nostro, P.; Briganti, G.; Chen, S.-H. J. Colloid Interface Sci. 1991,142. 214. Moss, R. A.;li, G.; Li, J.-M. J. Am. Chem. Soc. 1994, 776, 805. Mao, G.: Tsao, Y.-H.; Tirrell, M.; Davis, H. T.; Hessel, V.; van Esch, J.; Ringsdorf, H. Langmuir 1994.10. 4\14. Mao, G.; Tsao, Y.-H.; Tirrell, M.: Davis, H. T; Hessel, V.; Ringsdorf, H. Langmuir 1993,9,3461. Karabomi, S.: Esselink, K.; Hilbers, P A. J.; Smit. B.; Kanhauser, J.; van Os, N. M.; Zana, R. Science 1994, 266. 254. Diamant, H.; Andelman, D. Langmuir 1994,10. 2910. Israelachvilli, J.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc, Faraday Trans. 2 1976, 72. 1525. Israelachvilli, J. Intermolecidar & Surface Forces, 2nd ed. Academic Press: London, 1991, Chapter 17. Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940. Frindi, M.; Michels, B.; Levy, H.; Zana, R. Langmuir 1994,10. 1140. Alami, E.; Beinert, G.: Marie, P; Zana, R. Langmuir 1993, 9, 1465. Festag, R.; Hessel, V.; Lehmann, P; Ringsdorf,H.; Wendorff, J. H.Reel. Trav. Chim. Pays-Bas. 1994, 77i, 222. Bosch, M. P; Parra, J. L.; Sanchez-Baeza, F Can. J. Chem. 1993, 71. 2097. Bosch,P;Parra,J.L.;delaMaza,A.AAigor. Chem. 1994,106.2151.Angew. Chem., Int. Ed. Engl. 1994, J i , 2078. Masuyama, A.; Yokota, M.; Zhu, Y-P; Kida, T.; Nakatsuji, Y J. Chem. Soc, Chem. Commun. 1994, 1435. Mao, G.; Tsao, Y; TirreU, M.; Davis, H. T Langmuir 1993, 9, 3461. Li, T.; Krasne, S. J.; Persson, B.; Kaback, H. R.; Diederich, F J. Org. Chem. 1993, 58. 380. Fuhrhop, J.-H.; Bach, R. In Advances in Supramolecular Chemistry; G. W. Gokel, Ed.; JAI Press: Greenwich, CT, 1992, Vol. 2, p. 25. Zana, R.; Yiv, S.; Kale, K. M. J. Colloid Interface Sci. 1980, 77.456. Yiv, S.; Zana, R. J. Colloid Interface Sci. 1980, 77. 449. Mekelburger, H.-B.; Jaworek, W.; Vbgtle, F Angew. Chem. 1992, 104. 1609; Angew. Chem., Int. Ed. Engl. 1992, 31. 1571. Newkome, G. R.; Moorefield, C. N. Advances in Dendritic Macromolecules: G. R. Newkome, Ed.; JAI Press: Greenwich, CT, 1994, Vol. 1, pp. 1-68.

INDEX 5-Acetoxylsophthalic acid, 142 Aliphatic polyesters, 141 n-alkane-l,2-diols, 166 a,a>-Alkanediols, 165-166 Alkyne-based bolaphiles, 162 Allyltris(dimethylsiloxy)silane, 116,117 3-Aminosophthalic acid, 151 Ammonium-centered dendrimers cationic, 75 ionic, 87-93 Amphiphiles, 158 bolaform, 159, 170 twin-tailed, 179 Amphiphilicity, 137 Amphotropic liquid crystals, 166 Anionic initiators, 103-105 Anionic polymerization, 103105,115,141 Anionic surface dentrimers, 94 Anion radical mechanism, 149 Annulene derivatives, repetitive synthesis of, 45

Anthraquinone-based bolaphile, 168-169 [91-10-[91-Arboral, 162 Arborols, 2, 42, 84, 102, 124 {see also Dendritic macromolecules) molecular assemblies, 160,161 repetitive synthesis of, 42-43,44 two-directional, 160, 161, 163 Archaebacterial membranes, 177 Aromatic amino acids, 151 Aromatic polyesters, 21 Aromatic polymers, highly branched (see Highly branched aromatic polymers) Aryl-aryl interaction, 138 Arylboronic acids, coupling reactions, 128 Atropisomerism, 136 Automorphogensis, 162 Axial chirality, 68 Azacrown ethers, 162 Azacrowns, 54-55, 60, 84, 95 Azobenzene, 42, 64

191

192

INDEX

"Balloon" dendrimers, 88, 90, crown ether-based, 162 92 defects, 171 Benzene derivatives, 53 defined, 158 1,3,5-Benzenetricarboxylic acid, ionic, 176-188 143 biogically active, 186-187 Bidentate ligands, 79 functional vesicles, 184-186 Bilayer membranes, ionic molecular assemblies, 176bolaamphiphiles, 186 184 Binding, hydrophobic, 134-135 transports, 187-188 Biological activity molecular self-assembly, 164, ionic bolaamphiphiles, 186165, 179, 182, 186,188 monomeric, 171 187 nonionic, 159-176 nonionic bolaamphiphiles, biological activity, 170-172 170-172 functional vesicles, 168-169 2,2'-Bipyridine, 76 ion transport, 173-176 2,2'-Bipyridyl, 78 molecular assemblies, 159Bipyridylpyrazine bridging 168 ligands, 76-84 polymeric, 171 2,2'-Bipyrimidyl, 80 Bolaamphiphilic membranes, 2,2'-Biquinolyl, 82 171-172, 177 Birefringence, 155 Bolaform amphiphiles, 158, 170 3,5-Bis(bromomethyl)nitro(see also benzene, 53 Bolaamphiphiles) Bis-gluconamides, 167-168 "tunability" of, 170,172 Bis-lactobionamides, 167-168 Bolaform electrolyes, 158-159 Bis-o-phenanthroline com(see also plexes, 79 Bolaamphiphiles) 1,3-Bis( 1 -phenylvinylbenzene), Bolaform phosphatidylcholine, 132 2,3-Bis(2-pyridyl)pyrazine, 77, 176 Bolaphiles 80 alkyne-based, 162 2,5-Bis(2-pyridyl)pyrazine, 79, anthraquinone-based, 16880 169 /7-(iV,A^-Bis(trimethylsily)amino)chiral, 188 styrene, 104 cychohexane-based, 183 Block polymers, dendritic, 21-27 lysine-based, 164 Bolaamphiphiles phenylene-based, 181 cationic, 177 racemic lysine, 164 conformations within RNA-based, 186-187 micelles, 188

Index

193

synthesis of, 101-102 systematic divergent construction of, 74-75 tosylamide cascades, 46-55 Cascade polymers, symmetrical, 143 Catalysis, dendritic layer-block copolymers for, 23, 27 Catenanes, 76 Cation-exchange materials, 94 Cationic bolaamphiphiles, 177 Cationic porphyrins, 169 CD4+ lymphocytes, 171-172 Charged dendrimers, structural types, 75-76 Charge-transfer transitions, 82 Capillary melt viscosities, of Chemoselective thin films, 168 PVC blends, 139 Chiral bolaphiles, 188 Carbon-carbon radical couChiral cascade molecules, 66-69 pling, polyphenylers, Chiral dendrimers, 67-69 149 Chiral dendritic imines, 67 Carbosilane dendritic macroChirality, in molecular monomolecules, 113, 114 layers, 165 Carboxylate anions, 93 Chlorotrimethylsilane, 130 Cascade compounds, 124 Cholesterol content, 172 Cascade macromolecules, 188 Cascade molecules, 2, 41-70 {see ^^C nuclear magnetic resonance (NMR) spectroscopy, also Dendrimers; den10,12,115 dritic macromolecules) Cobalt Salen, 63-64 chiral, 66-69 Colunmar mesophase, 180 crystallization properties, 69 molecular assemblies, 159-160 Complexed transition-metal ions, 76-86 multiple cobalt complexes, Convergent-growth synthesis, 163-64 37 photoswitching dendrimers, benzene derivatives as core 64-66 units for, 53 polyamine dendrimers, 56-60 cascade molecules, 69 research recommendations, characterization of, 10-14 69-70 dendrimers, 124-126 Salen units, 61-63

"three-headed," 183 unsymmetrical, 165 Bolaphilic polyols, 166 Bolytes, 159 {see also Bolaamphiphiles) Borane reduction, 130 Branching in highly branched aromatic polymers, 124, 132-134, 139-141 star, 139-141 symmetrical, 132-134 4-Bromo-2,6-dimethylphenol, 149 4-Bromomethylbenzonitrile, 14

194

dendritic block copolymers, 21-27 dendritic macromolecules, 19, 33-36 development of, 4-9 growth process, 5-6 hexacyclene as core units for, 54-55 hybrid linear-dendritic block copolymers, 29-33 limitations of, 7-9 stepwise, of monodisperse dendritic polyesters, 143 suface functionality, 14-21 symmetrically branched polyphenylenes, 132-133 tosylamide cascades, 53-55 value of, 102 Copolymers, 149 highly branched, 127 Co-surfactants, 181 Cotton effects, 67 Coupling reactions arylboronic acids, 128 carbon-carbon radical, 149 Pd(0)-catalyzed, 128, 130 polyphenylethers, 145-150 radical, 149 Suzuki's, 128 Williamson, 6, 7 Critical miscellar concentration (CMC), 184 Crosslinking, 144-145 Crown dendrimers, 60 Crowned arborols, 84 Crown ether-based bolaamphilphiles, 162 H-bonding, 163-164 Crown imines, 62 Crown units, dendrimers, 54, 59

INDEX

Cyano bridging, 86 Cychohexane-based bolaphiles, 183 Cyclam, 42, 59-60 Cyclamdendrimer, firstgeneration amine, 63 Cyclovoltanmiograms, 64 Cylindrical morphology, 180 Debenzylation, 143 Dendrimers, 102 (see also Cascade molecules; ionic dendrimers) charged, structural types, 7576 crown units, 54 divergent synthesis, 49-52 first-generation, 75 generation zero, 75 initiator core, 75 ionic, 73-95 notation system, 75 repetitive synthesis, 42-45 silicon-based, 102-103, 105115,118 structural concept of, 74-75 third-generation, 49 Dendrimer wedges, 143 Dendritic aromatic polyesters, 21 Dendritic block copolymers, 2127 hybrid linear-, 29-33 layer-, 21, 22-27 segment-21, 22 surface-, 21, 36 Dendritic bolaamphiphiles {see Bolaamphiphiles) Dendritic imines, of tris(2aminoethyl)amine (TREN), 62

Index

Dendritic macromolecules {see also Cascade molecules) characterization of products, 10-14 convergent-growth synthesis, 1-37, 124 defined, 2 dipolar, 18 divergent-growth approach, 2,3 functional groups at chain ends, 14 glass-transition temperature, 35 interiors as microenvironments, 27 intrinsic viscosity, 33-34 monolayer formation, 36 physical properties, 33-36 relevance of, 2-3 research recommendations, 36-37 solubility, 35 surface functionality, 14-21 systematic nomenclature, 6 Dendritic macromonomers, copolymerization of, 2933 Dendritic micelles, 19-21 synthesis of, 25, 27 Dendritic oligoamines, cascade synthesis of, 56 Dendritic polyether macromolecules, synthesis of, 6-9 Dendritic polymers, as polymerrheology control agents, 127 Dendritic poly(siloxanes), synthesis of, 105-107

195

Dendritic toolbox, 68, 69 3,5-Diacetoxybenzoic acid, 142 2,6-Diacylaminopyridine, 165 3,5-Diaminobenzoic acid, 151 1,4-Diazabicyclo[2.2.2]octane, 92 3,5-Di(benxyloxy)benzoic acid, 143 3,5-Dibromo-1 -trimethylsilylbenzene, 133 2,3-Dibromoanisole, 147 2,4-Dibromophenol, 146 3,5-Dibromophenyl-boronic acid, 133 Diels-Alder oligomerization, 43 Differential scanning calorimetry (DSC), 143 3,5-Dihydroxybenzaldehyde, 14 3,5-Dihydroxybenzil alcohol, 6 3.5-Dihydroxybenzoate, 25 3,5-Dihydroxybenzoic acid, 21, 141, 143 3,5-Dihydroxybenzyl, 19, 35 2,2-Di(hydroxymethyl)propionic acid, 141 Diisobutylaluminumhydride (DIBAH), 56, 58, 61 Dimeric surfactants, 179 4-{N, A^-Dimethylamino-1 nitrobenzene, 27 Dimethyl malonate, 160 2,4-Dinitrophenyl phosphate, 184 Dinuclear complexes, 79, 80 l,2-DiphenyH,2diaminoethane, 66 Dipolar dendritic macromolecules, 18 Dipole moments, 18-19

196

Directed self-assembly, of chiral components, 165 Dirutheium complexes, 79, 80 Diruthenium(II) complexes, 77 Disconnection method, 4-5 Dispersity of hyperbranched polyphenylenes, 135 of polypropiolactone, 140 Divergent-growth synthesis, 2 development of, 74-75 growth process, 3, 5 silicon-based dendrimers, 105 tosylamide cascades, 46-52 value of, 102 Divinyloligo(dimethylsiloxanes), 114 a,co-Docosanediol, 166, 167 Dodecanitrile, 60 "DumbbeUs," 161-162 Electron micrographs, 161 Elemental analysis, 10 Ellipsometry, 136 Energy transfer paths, in transition-metal ions, 82 Ester chemistry, 22, 25 Ether chemistry, 25 Extended hexyldecylamine, 186 Fast atomic bombardment mass spectrometry (FABMS), 63 First-generation dendrimers, 75 Flexible spacers, 159 vs. rigid spacers, 179 Focal point convergent-growth approach and, 29

INDEX

hybrid linear-dendritic block copolymers, 29, 31 nuclear magnetic resonance (NMR) spectroscopy of, 10, 11 Fractals, 2, 42 (see also Dendritic macromolecules) Fullerenes, 7 Functional groups, at chain ends, 14 Functional vesicles ionic bolaamphiphiles, 185 nonionic bolaamphiphiles, 168-169 Gelation, 161 Gel permeation chromatography (GPC), 12, 129 "Gemini surfactants,'' 159, 179, 186 Generation zero dendrimers, 75 Glass transition temperature, 118 branched aromatic polyesters, 144 dendritic macromolecules, 35 H-bonding, in nonionic bolaamphiphiles, 161164 H-bridged networks, 167 Hermophilic bacteria, bolaamphiphiles in, 188 Hexacyclen, 42 Hexacyclene as core unit for polyamine dendrimers, 59-60 as core units for convergent synthetic strategy, 54-55 dedrimers, generations of, 60

Index

Hexakis-provitamin Be, 70 Hexyldecylamine, extended, 186 Highly branched aromatic polymers, 123-155 branching in, 124, 132-134, 139-141 polyamides, 151-155 polyesters, 141-145 polyp henylenes, 128-141 polyp henylethers, 145-150 High-pressure liquid chromatography (HPLC), 12 HIV reverse transriptase, 187 ^H nuclear magnetic resonance (NMR) spectroscopy, 10,11,17 Homopolymers, 141 Hybrid linear-dendritic block copolymers, 29-33 Hydrolosis, ionic bolaamphiphiles, 184-185 Hydrophobic aromatic polyether core, 20 Hydrophobic binding, 134-135 Hydrophobic-hydrophilic interactions, 161 Hydrophobic spacers, 168, 170171 Hydrosilation, 106, 115, 116-117 "one-pot," 115, 118 5-Hydroxyisophthalic acid, 141, 143 Hydroxyl-terminated dendrimers, 91 Hyperbranched macromolecules, 2 Hyperbranched polyamides, 151-155

197

Hyperbranched polymers, 115118 A-B3 polymers, 118 rheology and, 150 silicon-based, 115-118 spherical, 138 stability of, 130 star-shaped, 140 synthesis of, 114-115, 127 Hyperbranched polyphenylene Langmuir-Blodgett films of, 135-137 rheology and, 150 Hyperbranched poly(siloxysilanes), 115-118 Infrared spectrum, 10, 17 Initator core, dendrimers, 75 Intercalation, 146 Intramolecular chain-chain associations, 179 Intramolecular cyclization, 129 Intrinsic viscosity, 33-34 Ion channel mimic, 173, 175 Ionic dendrimers, 73-95 ammonium-centered, 87-93 molecular trains and catenanes, 94-95 multinuclear, 76-84 multinuclear linear metal arrays, 84-86 phosphonium-centered, 87-93 structural types, 75-76 surface-charged, 93-94 Ionic dendritic bolaamphiphiles, 176-188 biological activity, 186-187 functional vesicles, 184-186 molecular assemblies, 176-184 transports, 187-188

198

Ionic molecular catenanes, 94-95 Ionic molecular trains, 94-95 Ion transport, nonionic bolaamphiphiles, 173-176 (£)/(Z)-Isomerization, 42, 64 Karstedt's catalyst, 114 Langmuir-Blodgett films, of hyperbranched polyphenylenes, 135-137 Langmuir monolayers, 135 Layer-block copolymers, 21, 2227 LEGO strategy, 43 Linear imines, 62 Lipid membranes, 168 "Living" polymerization, 140 anionic, 103-105 Low-angle light scattering, 12 Lyotropic properties mesophases, 180 polyamides, 155 polyamids, 155 Lysine-based bolaphiles, 164 Macromonomers, 130 Mark-Houwink constants, 107, 114 Masking, in convergent-growth approach, 7-9 Mass spectrometry for chiral cascade molecules, 67 for dendritic macromolecules, 10 Melt viscosity, of PVC blends, 139 Membrane-disruptive bolaamphiphiles, 171-172

INDEX

Membranes archaebacterial, 177 bilayer, ionic bolaamphiphiles, 186 bolaamphiphilic, 171-172, 177 Upid, 168 redox-active, 168 stability of, 177 Merrifield resin, 94 Metal-halogen exchange, 130 Metallation, selective, 128 Metallomicelles, 184 Methyl 4-bromomethylbenzoate, 19 Methyl 3,5-hydroxybenzoate, 143 A^-Methylimidazole, 135 Micellar bolaamphiphiles, 184-185 Micelles, 162 bolaamphiphile conformations within, 188 dendritic, 19-21, 25, 27 metallomicelles, 184 unimolecular, 93, 134 Michael addition, 42, 58, 60, 75, 93 Microorganisms, disruption of, 170 Modeling hydrophobic binding, 134-135 ionic dendrimers, 80 multinuclear dendrimers, 80 triethanolamine ammonium dendrimers, 92 "Modular construction set," 173-175 Molecular assemblies ionic bolaamphiphiles, 176184 nonionic bolaamphiphiles, 159-168

Index

Molecular-based magnets, 95 Molecular dumbbells, 161-162 Molecular recognition, 23, 165, 188 Molecular self-assembly ionic bolaamphiphiles, 179, 182, 186,188 nonionic bolaamphiphiles, 164, 165 Molecular weight hyperbranched polyphenylenes, 135 measurements, gel permeation chromatographic (GPC), 129 polyamides, 151, 154 polyphenylenes, limitations, 128-129 polyphenylethers, 145 star polymers, 140 Monomeric bolaamphiphiles, 171 Multifunctional initiators, 130 Multilamellar arrangement, bolaamphiphiles, 182 Multinuclear dendrimers based on bipyridylpyrazine bridging ligands, 76-84 linear metal arrays, 84-86 Multiple cobalt complexes, cascade molecules, 63-64 Multiple-metal complexes, 63 Multiple Salen systems, 63 Neumatic phase, 155 Ni(II) catalysts, 128 4-(4'-Nitrophenylazo)resorcinol, 185 /7-Nitrophenyl picolinate, 185

199

Nomenclature, dendritic macromolecules, 6 Nonionic cascade molecules, 84 Nonionic dendritic bolaamphiphiles, 158-176 biological activity, 170-172 functional vesicles, 168-169 ion transport, 173-176 molecular assemblies, 159-168 Notation system, for dendrimers, 75 Nuclear magnetic resonance (NMR) spectroscopy, 10-12,17,89,106,115, 117-118 Nucleotide 5'-triphosphates, 187 Octahedral coordination, 78 Oligoamines, cascade synthesis of, 56 Oligooxyethylene, 118 "One-pot" hydrosilation, 115, 118 Osmium(II), 82 Oxidative intiators, 149 Packing parameter, 179 Pd(0)-catalyzed coupling reactions, 128, 130 Pentaarylphosphorane, 89 Pentaethylenehexamine, 58-59 Peptides, repetitive synthesis, 43 Perfluorinated poly(phenylenes), 115 Phase transfer catalyst (PTC), for polyphenylether polymerization, 146 o-Phenanthroline, 78 2,2:6',2''-Phenanthroline, 78 Phenylacetylenic wedges, 134

200

Phenylene-based bolaphiles, 181 Phosphatidylocholines, 176 Phosphine, 89 Phospholipase catalysis, 176 Phosphonium, 75, 87-93 Phosphorane/ phosphonium ion dendrimers, 90 Photopolymerization, 186 Photostationary equilibrium (PSE), 64 Photoswitching dendrimers azobenzene, 42, 64 cascade molecules, 64-66 first-generation, 65 UV-VIS spectrum, 65 Photosynthetic electron acceptors, 168 ^^P NMR spectrum, 89 Poly(2-acrylamido-2-methyl-1 propanesulfonic acid) (PAMPSA), 186 Polyamides, 151-155 characterization, 154-155 lyotropic properties, 155 molecular weights, 154 preparation, 151-154 Polyamine dendrimers, 56-60 cyclam as core unit, 59-60 hexacyclene as core unit, 5960 pentaethylenehexamine as core unit, 58-59 tris(2-aminoethyl)aniine (TREN) as core unit, 56-57 Polyaza compounds, 42 Polybutadiene stars, 105, 114 Polydienes, 103 Poly(dimethylsiloxane) oligomers, 105

INDEX

Poly(dimethylsiloxanes) (PDMS), 103 Polydispersity, 13, 128 Polyesters, 141-145 applications, 144-145 single-step polymerization, 141-142 symmetrical, 142-144 Polyether macromolecules, synthesis of, 6-9 Polyhydroxy monocarboxylic acid, 141 Polymeric bolaamphiphiles, 171 Polymerization degree of, 135 direct, 127 ionic bolaamphiphiles, 186 "Uving," 140 one-step, 127, 141-142 polyamids, 154 polyesters, 141-142 polyphenylethers, 145 step-, 129 Polymers {see also Highly branched aromatic polymers) blending polyphenylenes with, 137-139 chemical modification of, 130-132 growth, 102 star-branched, 139-141 synthesis, polyphenylenes, 128-129 "wedges," 102 Polymetallic complexes, 84-86 Poly(methyl)methacrylate (PMMA), 141 Polyphenolic dendrimers, 7

Index

Polyphenylenes, 128-141, 144 blending with other polymers, 137-139 characterization, 129-130 chemical modification of, 130-131 hydrophobic binding, 134-135 hyperbranched, LangmuirBlodgett films of, 135137 molecular weight limitations, 128-129 polyme synthesis, 128-129 solubility, 129 star-branched polymerization, 139-141 symmetrically branched, 132134 Polyphenylethers, 145-150 Theological effects, 150 synthesis, 145-149 Polyphosphonium ion materials, 92 Polypropiolactone, 140 Poly(siloxanes), 105 starburst polymers, 107, 109, 111 synthesis of, 105-107 Poly(siloxysilanes), 115-118 Polystyrene blends, 138 Theological effects on, 150 PolystyTene-equivalent moleculaT weights, 12 Pool quinones, 169 PoTOUs VycoT glass, 76 PToton analysis, 115 PVC blends, 138-139 Pyridine bolaphile, 184-185 Pyridoxal derivative, 70

201

QuatemaTy phosphonium ions, 88 Quinone moieties, 168-169 Racemic lysine bolaphile, 164 Radical coupling Tadical-anion (SRNIO, 149 Tadical-Tadical, 149 Reactive end cappeTS, 105 Redox-active membTanes, 168 Repetitive synthesis of annulene derivatives, 45 defined, 42-43 of dendrimcTS, 42-45 of Newkome's aTvoTols, 4243,44 ResoTcinol, 185 Rheology, 150 Rigid spaccTS, 159 vs. flexible spaceTS, 179 RNA-based bolaphiles, 186-187 Ruthenium(II), 76, 82, 84, 85-86 Ruthenium/Thenium complexes, 86 Salen units, cascade molecules, 61-63 SEC analysis, 12, 105, 106, 115116 Segment-block copolymcTs, 21, 22 Selective metallation, 128 Self-aggregation, 165 Self-assembly, 165, 179, 188 directed, 165 ionic bolaamphiphiles, 186 monolayers, 177-178 supramolecular, 164, 182 Semiconductor metals, 86 Shear birefringence, 155

202

Short-arm star polymers, 140 Si-H addition, 115 Silicon-based polymers dendrimers, 102-103, 105-115, 118 hyperbranched, 115-118 stars, 103-105, 118 Single-step polymerization, polyesters, 141-142 ^^Si nuclear magnetic resonance (NMR) spectroscopy, 106,117-118 Size-exclusion chromatography (SEC), 12, 105, 106, 115-116 Sodium diethoxymethylsilanolate, 105 Sodium dodecyl sulfate (SDS), 21 Solubility dendritic macromolecules, 35 polyphenylenes, 129 Solvatochromic probe, 27 Solvatocrhomic dendrimers, 34 Spacers hydrophobic, 168, 170-171 length of, 180 rigid vs. flexible, 179 Spherical hyperbranched polymers, 138 Star-branched polymers, 139-141 Starburst molecules, 2 {see also Dendrimers) Starburst perfluorinated poly(phenylenes), 115 Starburst poly(siloxane) polymers, 107, 109, 111 Starburst synthesis, 3 {see also Divergent-growth synthesis)

INDEX

"Star" ionic dendrimers, 90 Star poly(butadienes), 114 Star polymers, 139-141 short-arm, 140 silicon-based, 103-105, 118 "Star" tetraarylphosphonium ion, 89 Step-polymerization, 115, 129 Stereogenic centers, in transition-metalcentered dendrimers, 79 Steric screening, 115 "String" dendrimers, 92 Supramolecular chemistry, 188 Supramolecular effects, 94-95 Supramolecular helix structure, 165 Supramolecular self-assembly, 164, 182 Supramolecular surfactants, 171 Surface-block copolymers, 21 Surface-charged dendrimers, 9394 Surface functionalization characterization of, 17 control of, 14-21 convergent-growth approach, 19 dipole moments, 18-19 Surface tension measurements, 180 Surfactants CO-, 181

dimeric, 179 "gemini," 159, 179, 186 supramolecular, 171 Suzuki's coupling, 128 Symmetrical polyesters, 143-144 Tartaric acid, 165 Telechelic starts, 104

Index

2,2':6',2"-Terpyridine, 85 Tetrabutylammonium fluoride, 146 Tetracationic cyclophane, 94 Thermal gravimetric analysis (TGA), 145-146 Thermal self-condensation, 142 3-D crystallites, 165 "Three-headed" bolaphiles, 183 Tosylamide cascades, 46-55 convergent synthesis strategy, 53-55 divergent synthesis strategy, 46-52 generations of, 49 starting materials for, 47 X-ray structure, 47-49 3-(Tosylamino)azobenzene, 64 Transesterification, 144 Transition-metal ions, 75 complexed, dendrimers containing, 76-86 multinuclear dendrimers, 7684 multinuclear linear metal arrays, 84-86 preparation, 76 Transport agents ionic bolaamphiphiles, 187188 nonionic bolaamphiphiles, 173-176 1,3,5-Tribromobenzene, 133 2,4,6-Tribromophenol, 145 Trichloroethy 3,5-dihydroxybenzoate, 22 Triethanolamine, 90 Triethyl methanetricarboxylate, 160

203

Tri(p-methoxymethyl)phenylphospine, 88 Tris(2-aminoethyl)amine (TREN) as core unit for polyamine dendrimers, 56-57 dendritic imine of, 62 first-generation dendrimers, 63 Tris(2,2'-bipyridyl) bridging ligand, 80 Tris(hydroxymethyl)aminomethane, 160 1,1,1 -Tris(4'-hydroxyphenyl)ethane, 6, 7 Tris(perfluorophenyl)germane, 115 Tris(perfluorophenyl)silane, 115 Tris(perfluorophenyl)stannane, 115 l,3,5-Tris(2,2':6,2"-terpyriden-4'yl)benzene, 85-86 "Tunability," of bolaform amphiphiles, 170, 172 Twin-tailed amphiphiles, 179 Two-directional arborols, 163 Unimolecular micelles, 93, 134 UV-Vis specroscopy, 27 Vesicles, 162 functional, 168-169, 185 Vesicular morphology, 186 Vinyltris(dimethylsiloxy)silane, 116 Viscosity intrinsic, dendritic macromolecules, 33-34 melt, of PVC blends, 139 Vycor glass, porous, 76

204

"Wedges," 102 Williamson coupling, 6, 7 Williamson synthesis, 69 Wittig reaction, 43

INDEX

X-ray diffraction ionic bolaamphiphiles, 186 polyamids, 155 X-ray structure, tosylamide cascades, 47-49

Advances in Dendritic Macromolecules Edited by George R. Newkome, Department of Chemistry, University of South Florida Volume 1,1994,198 pp.

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ISBN 1-55938-696-7 CONTENTS: Introductionto the Series: An Editor's Foreword, Albert Padwa, Emory University. Preface, George R. Newkome, University of South Florida. A Review of Dendritic Macromolecules, George R. Newkome and Charles N. Moorefield, University of South Florida. Stiff Dendritic Macromolecules Based on Phenylacetylenes, Zhifu Xu, Benjamin Kyan, and Jeffery S. Moore, The University of Michigan. Preparation and Properties of Monodisperse Aromatic Dendritic Macromolecules, Thomas X. Neenan, Timothy M. Miller, Elizabeth W. Kwock, and Harvey E. Bair, AT&T Bell Laboratories. HighSpin PolyarylmethylPolyradicals,Andrzej Rajca, University of Nebraska. A Systematic Nomenclaturefor Cascade (Dendritic) Polymers, Gregory R. Baker and James K. Young, University of South Florida. Index.

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Advances in Theoretically Interesting Molecules Edited by Randolph P. Thummel, Department of Chemistry, University of Houston Volume 1 , 1989,467 pp. ISBN 0-89232-869-X

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CONTENTS: lntroductionto the Series: An Editor's Foreword, Albert Padwa. Preface, Randolph P. Thummel. Isobenzofurans, Bruce Rickbom. Dihydropyrenes: Bridged [14] Annulenes Par Excellence. A Comparison with Other Bridged Annulenes, Richard H. Mitchell. [I.m.n.] Hericenes and Related Exocyclic Polyenes Pierre Vogel. The Chemistry of Pentacyclo [ 5 . 4 . 0 . 0 ~ ~ ~ . 0 ~ Undecane , ~ ~ . 0 ~ ~ (PCUD) ~] and Related Systems, Alan P. Marchand. Cyclic Cumulenes, Richard P. Johnson. Author Index. Subject Index. Volume 2, 1992, 223 pp. ISBN 0-89232-953-X

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