Anionic polymerization

Ionic polymerization in which the active centers are anions.

Note 1: The anions may be free, paired, or aggregated.

Modified from the earlier definition.

Anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of vinyl monomers with strong electronegative groups. This polymerization is carried out through a carbanion active species. Like all chain-growth polymerizations, it takes place in three steps: chain initiation, chain propagation, and chain termination. Living polymerizations, which lack a formal termination pathway, occur in many anionic addition polymerizations. The advantage of living anionic addition polymerizations is that they allow for the control of structure and composition.

Anionic polymerizations are used in the production of polydiene synthetic rubbers, solution styrene/butadiene rubbers (SBR), and styrenic thermoplastic elastomers.

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 Szwarc. The early work of Michael Szwarc and co – workers in 1956 was one of the breakthrough events in the field of polymer science. When Szwarc learned that the electron transfer between radical anion of naphthalene and styrene in an aprotic solvent such as tetrahydrofuran gave a messy product, he started investigating the reaction in more detail. He proved that the electron transfer results in the formation of a dianion which rapidly added styrene to form a "two – ended living polymer." Being a physical chemist, Szwarc set forth in understanding the mechanism of such living polymerization in greater detail. His work 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 had great ramifications in future research in polymer synthesis, because Szwarc had found a way to make polymers with greater control over molecular weight, molecular weight distribution and the architecture of the polymer.

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