Living anionic polymerization

Living anionic polymerization is a living polymerization technique involving an anionic propagating species.

Living anionic polymerization was demonstrated by Szwarc and co workers in 1956. Their initial work was based on the polymerization of styrene and dienes. One of the remarkable features of living anionic polymerization is that the mechanism involves no formal termination step. In the absence of impurities, the carbanion would still be active and capable of adding another monomer. The chains will remain active indefinitely unless there is inadvertent or deliberate termination or chain transfer. This gave rise to two important consequences:

The following experimental criteria have been proposed as a tool for identifying a system as living polymerization system.

However, in practice, even in the absence of terminating agents, the concentration of the living anions will reduce with time due to a decay mechanism termed as spontaneous termination.

Polymerization reactions excluding a termination and transfer step are particularly useful for the synthesis of functionalized polymers with well defined architectures. The consumption of the monomer results in stable, anionic polymer chain ends, allowing reactions with a variety of electrophilic functional groups post polymerization.

However the efficacy of these reactions depends on a number of variables such as chain-end structure, solvent, temperature, and concentration. Alternatively, by controlling the functionality of the initiator, one can prepare polymers having different geometries such as symmetric and asymmetric stars, comb shaped, etc.

Synthesis of block copolymers by sequential monomer addition is one of the most important applications of living polymerization as it offers the best control over structure. The nucleophilicity of the resulting carbanion will govern the order of monomer addition, as the monomer forming the less nucleophilic propagating species may inhibit the addition of the more nucleophilic monomer onto the chain. An extension of the above concept is the formation of triblock copolymers where each step of such a sequence aims to prepare a block segment with predictable, known molecular weight and narrow molecular weight distribution without chain termination or transfer.

Sequential monomer addition is the dominant method in the literature, mainly due to its simplicity. Nevertheless, also this concept has serious limitations. For example, for a given set of three monomers (A, B, and C), sequential living anionic polymerization can afford all possible linear BC structures (i.e., ABC, ACB, and BAC)exclusively in the highly unlikely scenario that the following requirements are fulfilled: (a) similar monomer reactivity, (b) high crossover efficiency, and (c) absence of side reactions.

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