Anionic polymerization. Mechanism of methyl methacrylate polymerizations initiated by metal amides in liquid ammonia

Polymerizations of methyl methacrylate initiated by potassium and sodium amides in liquid ammonia solution proceed extremely rapidly even at low temperatures. The resulting polymers have relatively narrow molecular weight distributions M̄v/M̄n ∼ 1.5 and, unlike polymers of styrene prepared similarly, do not contain terminal amine groups. The molecular weight of poly(methyl methacrylate) prepared with potassium amide is independent of the monomer concentration and of the concentrations of potassium amide, potassium ion, and amide ion. The absence of terminal amine groups suggests that methoxide ion, produced by 1,2‐addition of amide ion to the monomer, is the actual initiating species. The substantial absence of a termination step is indicated by the narrow distribution of molecular weights. Several minor variations of this general reaction sequence may give the experimentally observed independence of molecular weight on reactant concentrations. One such modification is considered by way of illustration, although final selection of a detailed mechanism is not possible on the basis of the present results.