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Unformatted text preview: Annu. Rev. Phys. Chem. 1990. 41:525-57 Copyright 1990 by Annual Reviews Inc. All rights reserved BLOCK COPOLYMER THERMODYNAMICS: Theory and Experiment Frank S. Bates Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Glenn H. Fredrickson ~ AT&T Bell Laboratories, Murray Hill, New Jersey 07974 KEY WORDS: order and disorder. INTRODUCTION Block copolymers are macromolecules composed of sequences, or blocks, of chemically distinct repeat units. The development of this field originated with the discovery of termination-free anionic polymerization, which made possible the sequential addition of monomers to various carbanion-ter- minated ("living") linear polymer chains. Polymerization of just two dis- tinct monomer types (e.g. styrene and isoprene) leads to a class of materials referred to as AB block copolymers. Within this class, a variety of molec- ular architectures is possible. For example, the simplest combination, obtained by the two-step anionic polymerization of A and B monomers, is an (A-B) dioblock copolymer. A three-step reaction provides for the preparation of(ABA) or (BAB) triblock copolymer. Alternatively, "living" diblock copolymers can be reacted with an n-functional coupling agent to produce (AB)n star-block copolymers, where n = 2 constitutes a triblock copolymer. Several representative (A-B)n block copolymer architectures ~ Present address: Department of Chemical and Nuclear Engineering, University of Cali- fornia, Santa Barbara, California 93106. 525 0066-426X/90/11014)525502.00 www.annualreviews.org/aronline Annual Reviews Annu. Rev. Phys. Chem. 1990.41:525-557. Downloaded from arjournals.annualreviews.org by UNIVERSITY OF FLORIDA on 01/14/08. For personal use only. 526 BATES & FREDRICKSON are illustrated in Figure 1. As a consequence of the "living" nature of these reactions, the resulting block and overall molecular weight distributions are nearly ideal, i.e. Mw/Mr~ < 1.1, where Mw and MN represent the weight and number-average molecular weights, respectively. Since the original studies of anionic block copolymerization in the 1950s (1, 2) a variety of new polymerization mcthods (e.g. condensation, Ziegler- Natta, etc.) have contributed to an expanding number of block copolymer classes (e.g. ABC) and novel architectures (e.g. graft-block). Although some of the developments have resulted in important new materials (e.g. polyurethanes), anionic polymerization remains the only viable method for (A-B)n Block Copolymer Architectures k.. J,~ triblock Figure 1 Schematic illustration of several (A-B), type block copolymer architectures. Solid and dashed lines represent A and B block chains. The n = 1 and n = 2 architectures are commonly referred to as diblock and triblock copolymers, while n __ 3 are denoted starblock copolymers....
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