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Form of addition polymerization initiated with anions

In polymer chemistry, anionic addition polymerization is a form of chain growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but traditionally vinyl monomers are used. Often anionic polymerization involves living polymerizations, which allows control of structure and composition.]] As early as 1936, Karl Ziegler proposed that anionic polymerization of styrene and butadiene by consecutive addition of monomer to an alkyl lithium initiator occurred without chain transfer or termination. Twenty years later, living polymerization was demonstrated by Michael Szwarc and coworkers. In one of the breakthrough events in the field of polymer science, Szwarc elucidated that electron transfer occurred from radical anion sodium naphthalene to styrene. The results in the formation of an organosodium species, which rapidly added styrene to form a "two – ended living polymer." An important aspect of his work, Szwarc employed the aprotic solvent tetrahydrofuran. Being a physical chemist, Szwarc elucidated the kinetics and the thermodynamics of the process in considerable detail. At the same time, he explored the structure property relationship of the various ion pairs and radical ions involved. This work provided the foundations for the synthesis of polymers with improved control over molecular weight, molecular weight distribution, and the architecture. The use of alkali metals to initiate polymerization of 1,3 dienes led to the discovery by Stavely and co workers at Firestone Tire and Rubber company of cis 1,4 polyisoprene. This sparked the development of commercial anionic polymerization processes that utilize alkyllithium initiators.

One of the remarkable features of living anionic polymerization is the absence of a formal termination step. In the absence of impurities, the carbanion would remain active, awaiting the addition of new monomer. Termination can occur through unintentional quenching by impurities, often present in trace amounts. Typical impurities include oxygen, carbon dioxide, or water. Termination intentionally allows the introduction of tailored end groups. Living anionic polymerization allow the incorporation of functional end groups, usually added to quench polymerization. End groups that have been used in the functionalization of α haloalkanes include hydroxide, NH2, OH, SH, CHO, COCH3, COOH, and epoxides. An alternative approach for functionalizing end groups is to begin polymerization with a functional anionic initiator. In this case, the functional groups are protected since the ends of the anionic polymer chain is a strong base. This method leads to polymers with controlled molecular weights and narrow molecular weight distributions.

Cowie, J.; Arrighi,V. Polymers: Chemistry and Physics of Modern Materials; CRC Press: Boca Raton, FL, 2008.