Dimethylzirconocene precatalysts are shown to initiate the polymerization of methyl methacrylate (MMA) when activated by a Lewis acidic methyl abstractor in the absence of any ZnEt2 additive. The order of addition of the various components is crucial to forming a well-defined active species. Conversion of the precatalysts Cp2ZrMe2 (1a), rac- [C2H4(eta(5)-C9H6)(2)]ZrMe2 (2a), {Me2C(eta(5)-C5H4)(eta(5)-C13H8)}-ZrME2 (3a), {Me2C(eta(5)-C5H4)(eta(5)-C9H4)(eta(5)-C9H6)}ZrMe2 (4a), {Me2C(eta(5)-C5H4)(eta(5)-2-MeC9H5)}ZrMe2 (5a), and (Cp)-(Cp*)ZrMc(2) (6a) to their cationic methylzirconocene derivatives (2b-6b) followed by addition of methyl methacrylate results in controlled polymerization to poly(methyl methacrylate) with the exception of 3b which is inactive.