The anionic polymerization of methyl methacrylate in toluene in the presence of tetraalkylammonium-trialkylaluminum 1:2-complexes, NR4+[Al2R＇X-6](-), has living and controlled character up to -20 degrees C. Quantitative monomer conversions are usually reached within a few minutes. The resulting polymers have narrow molecular weight distributions (1.05 < (M) over bar w/(M) over bar(n), < 1.20) and are highly syndiotactic (rr approximate to 0.75). Kinetic studies show that the mechanism of the polymerization is rather complex, implying an equilibrium between three active species of different reactivities.