Anionic Polymerization of Methyl Methacrylate in the Presence of Piperidine

Anionic polymerization of methyl methacrylate (MMA) initiated by lithium tert-butoxide (t-BuOLi) was investigated in different mixtures of benzene and piperidine. The latter compound activates the associated alkoxide, as evidenced by the observed increase in the rate of polymerization, proportional to piperidine concentration and also by the lowering of the overall kinetic order of the polymerization reaction with respect to initial monomer concentration. However, at higher piperidine concentration the rate of the polymer growth significantly decreases after a short period of time; a probable reason for this retardation or termination effect is the decay of active growth centers by a termination reaction with the methacrylate carbonyl group. The molecular weight of the polymer is significantly lowered by even a small addition of piperidine as a result of increased initiator efficiency that leads to a higher absolute concentration of active centers and approaches the theoretical limit given by the stoichiometric ratio of monomer and initiator concentrations. The microstructure of the product is affected by the presence of the polar solvent to a considerably lesser degree than in the case of classical alkylmetal initiators. The isotacticity slowly decreases with piperidine concentration over the whole investigated range. The high stability of the complex active center of growth formed by the alkoxide initiator and also the gradual change in the character of the ionic pair at the end of the growing polymer chain are responsible for the relatively small changes of the microstructure.