Anionic polymerization living systems

Block copolymers have commercial potential for obtaining products that can incorporate the desirable properties of two or more homopolymers. This potential has led to an intense effort to find reaction systems that proceed as living polymerizations. Some anionic chain polymerizations proceed as living polymerizations under conditions where no viable chainbreaking reactions occur, and this has resulted in useful block copolymers (Sec. 5-4). 

Polymers with even narrower mass distributions, e.g. with PDI values close to 1, arise in living polymerization systems, in which no chain termination processes can occur at all, such that all chains remain bound to the metal centre from which they have started to grow at the same time. Living polymerizations, which offer useful opportunities, e.g. with regard to the production of block copolymers by exchange of one monomer for another, occur in anionic polymerizations of styrenes or butadienes such as are induced by simple lithium alkyls. For a-olefin polymerization catalysts of the type discussed above, living polymerizations are rare. These more elaborate catalysts can thus release a newly formed polymer chain within a time interval of typically less than one... 

In a series of papers43 4S), the kinetics of anionic polymerization of ethylene oxide in conjunction with different catalysts were studied. These studies expand our understanding of the mechanism of living polymerization systems and provide new information on the processes of active center association. Herein, primarily, lies the specific nature of the heteroatomic systems, as compared with the vinyl monomers studied earlier 9 ... 

The first unequivocal proof of feasibility of homogeneous polymerization free of termination and chain-transfer was reported in 1956, and the concept of living polymers and its ramifications were fully developed in those publications 2). Although the initial work dealt with anionic polymerization of styrene and the dienes, the underlying ideas were soon applied by other workers to a great variety of polymeric systems, as shown by the brief review of some of those developed recently. 

Sequential addition of different monomer charges to a living anionic polymerization system is useful for producing well-defined block copolymers. Thermoplastic elastomers of the triblock type are the most important commercial application. For example, a styrene-isoprene-styrene triblock copolymer is synthesized by the sequence... 

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