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.
Propagation
center|640px|thumb|Organolithium-initiated polymerization of styrene
Propagation in anionic addition polymerization results in the complete consumption of monomer. This stage is often fast, even at low temperatures.
Sequential monomer addition is the dominant method, also this simple approach suffers some limitations.
Moreover, this strategy, enables synthesis of linear block copolymer structures that are not accessible via sequential monomer addition. For common A-b-B structures, sequential block copolymerization gives access to well defined
block copolymers only if the crossover reaction rate constant is significantly higher than the rate constant of the homopolymerization
of the second monomer, i.e., k<sub>AA</sub> >> k<sub>BB</sub>.
End-group functionalization/termination
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, -NH<sub>2</sub>, -OH, -SH, -CHO,-COCH<sub>3</sub>, -COOH, and epoxides.
thumb|400px|center|Addition of hydroxide group through an epoxide.
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.
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Chain transfer can occur when an agent can act as a Brønsted acid. In this case, the pKa value of the agent is similar to the conjugate acid of the propagating carbanionic chain end. Spontaneous termination occurs because the concentration of carbanion centers decay over time and eventually results in hydride elimination.