Chain copolymerizations, living

K. Satoh, M. Matsuda, K. Nagai, M. Kamigaito, AAB-sequence living radical chain copolymerization of naturally occurring limonene with maleimide an end-to-end sequence-regulated copolymer, J. Am. Chem. Soc. 132 (2010) 10003-10005. 

This article progresses to discuss the newly emerging field of controlled/living radical copolymerization, and the novel materials that can be made using these highly versatile synthetic methods. Finally we cover in brief ionic chain copolymerization and cross-linking copolymerization. 

The real advantage of GRP lies in the ability to synthesize block copolymers from free radical monomers without the very stringent polymerization conditions necessary for ionic polymerization. Since the chains remain living, one monomer can be completely polymerized and then another monomer added to form a second block on the same (living) chains. This process can be repeated to form multiblock polymers. This aspect of GRP will be addressed in the following as part of the modeling of copolymerization reactions. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

An issue in living radical copolymerization is that the conditions for dormant chain activation can vary substantially according to the particular propagating radical. The problem may be mitigated by two factors. 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

The role of reactive centers is performed here by free radicals or ions whose reaction with double bonds in monomer molecules leads to the growth of a polymer chain. The time of its formation may be either essentially less than that of monomer consumption or comparable with it. The first case takes place in the processes of free-radical polymerization whereas the second one is peculiar to the processes of living anionic polymerization. The distinction between these two cases is the most greatly pronounced under copolymerization of two and more monomers when the change in their concentrations over the course of the synthesis induces chemical inhomogeneity of the products formed not only for size but for composition as well. 

In our own research, the functional termination of the living siloxanolate with a chlorosilane functional methacrylate leading to siloxane macromonomers with number average molecular weights from 1000 to 20,000 g/mole has been emphasized. Methacrylic and styrenic monomers were then copolymerized with these macromonomers to produce graft copolymers where the styrenic or acrylic monomers comprise the backbone, and the siloxane chains are pendant as grafts as depicted in Scheme 1. Copolymers were prepared with siloxane contents from 5 to 50 weight percent. 

An alternative route for the preparation of styrenic macromonomers is the reaction of living chains with 4-(chlorodimethylsilyl)styrene (CDMSS) [192]. The key parameter for the successful synthesis of the macromonomers is the faster reaction of the living anionic chain with the chlorosilane group rather than with the double bond of the CDMSS. Anionic in situ copolymerization of the above macromonomers (without isolation) with conventional monomers leads, under appropriate conditions, to well-defined comb-like chains with a variety of structures. 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

Block copolymerization was carried out in the bulk polymerization of St using 18 as the polymeric iniferter. The block copolymer was isolated with 63-72 % yield by solvent extraction. In contrast with the polymerization of MMA with 6, the St polymerization with 18 as the polymeric iniferter does not proceed via the livingradical polymerization mechanism,because the co-chain end of the block copolymer 19 in Eq. (22) has the penta-substituted ethane structure, of which the C-C bond will dissociate less frequently than the C-C bond of hexa-substituted ethanes, e.g., the co-chain end of 18. This result agrees with the fact that the polymerization of St with 6 does not proceed through a living radical polymerization mechanism. Therefore, 18 is suitably used for the block copolymerization of 1,1-diubstituted ethylenes such as methacrylonitrile and alkyl methacrylates [83]. 

The synthesis of a gradient copolymer is sometimes desirable in a living polymerization to produce a copolymer with a variation in composition along the polymer chain. This is achieved either by carrying out the copolymerization without addition of monomer to maintain a constant feed composition or addition of monomer to deliberately change the feed composition from its original state. 

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