Mechanistic studies of isospecific polymerization of methacrylates by a cationic, chiral ansazirconocene ester enolate complex, rac-(EBI)Zr+(THF)[OC((OPr)-Pr-i)=CMe2][MeB(C6F5)(3)](-) [1; EBI = C2H4-(Ind)(2)], are reported. Complex 1 effects isospecific and living (co)polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA), producing highly isotactic homopolymers and block copolymers with narrow molecular weight distributions and high initiator efficiencies. Investigations of statistical copolymerizations of MMA and BMA have yielded monomer reactivity ratios of r(MMA) = 0.62 and r(BMA) = 0.72 (the Kelen-Tudos method), indicating that the copolymer formed instantaneously has a somewhat alternating character. Studies of MMA polymerization kinetics have shown that propagation is first order in both concentrations of the monomer and the active species. The single MMA addition product, rac-(EBI)ZrMe[OC(OMe)=C(Me)CH2C(Me-2) C((OPr)-Pr-i)=O] (3), has been isolated from the reaction between the neutral rac-(EBI)ZrMe[OC((OPr)-Pr-i)=C(Me)(2)] (2) and MMA; however, subsequent MMA additions do not proceed, unless the single addition product is further treated with B(C6F5)3 to generate the eight-membered-ring cyclic ester enolate species, rac-(EBI)Zr+[OC(OMe)=C(Me)(CHC)-C-2(Me-2)C((OPr)-Pr-i)=O] [MeB(C6F5)(3)](-) (4), which corresponds to the structure of the intermediate for the single MMA addition product of 1. All results presented here are consistent with the conclusion that isospecific polymerization of methacrylates by 1-which models the proposed isospecific propagating species for the methacrylate polymerization by chiral ansa-zirconocene complexes-is enantiomorphic site controlled and proceeds in a monometallic, intramolecular Michael addition mechanism.