The polymn. of Me methacrylate (MMA) in the presence of the neutral chloro zirconocene enolates, Cp2ZrCl[OC(OMe)=CMe2] (1), Me4C2(Cp)2 ZrCl[OC(OMe)=CMe2] (2), and Me2C(Cp)2ZrCl[OC (OMe)=CMe2] (3), was investigated. None of these compds. is catalytically active on its own. They could be activated by adding Ph3CB(C6F5)4 (4); however, only if the initial concn. of enolate is higher than that of the perfluorinated Ph borate. Polymers were produced with a no. av. molar mass of up to 100 000 g/mol and a dispersion index of 1.1-1.3 with mixts. comprising 1 and 4 or 2 and 4. The d.p. depends only on the quantity of excess enolate and not on the abs. concn. of the initiator components. In contrast, a mixt. of 3 and 4 was not very active. Kinetic modeling of the systems as well as NMR spectroscopic investigations indicate that a bimetallic mechanism can describe chain growth. The activity of these catalytic systems depends on the structure of the ligand. The two zirconocenes with bridged cyclopentadienyl ligands, (2) and (3), convert MMA considerably slower. In addn. to the Me zirconocene enolates with unbridged cyclopentadienyl ligands, the corresponding chloro zirconocene enolates are also suitable for the polymn. of MMA. The latter compds. offer the advantage that prepns. for the prodn. of the initiators are comparatively simple.