Polymerization of methyl methacrylate (MMA) was conducted with the catalyst systems of rac-Et(Ind)(2)Zr(CH3)(2)/Ph(3)CB(C6F5)(4), rac-Et(IndH(4))(2)Zr(CH3)(2)/Ph(3)CB(C6F5)(4), and rac-Me(2)Si(Ind)(2)Zr(CH3)(2)/Ph(3)CB(C6F5)(4). These catalysts only were found to be inactive. However, the addition of suitable Lewis acids like alkylzinc and alkylaluminum compounds gave highly isotactic PMMA. The isotactic pentad [mmmm] in PMMA as well as the propagation rate constant (k(p)) depended significantly upon the zirconocene compounds. Both the microstructure ([mmmm]) and propagation rate constant (k(p)) increased in the following order : Me(2)Si(Ind)(2) < Et(IndH(4))(2) < Et(Ind)(2). The microstructure of PMMA was also affected slightly by alkylzinc compounds : (CH2)(2)CH=CH2 < CH2CH3 > (CH2)(3)CH3. When the polymerization was initiated by the rac-Et(Ind)(2)Zr(CH3)(2)/Ph(3)CB(C6F5)(4)/Zn(C2H4CH=CH2)(2) catalyst system at 0 degrees C in toluene, the molecular weight of PMMA increased in proportion to the conversion of MMA. The increase of temperature from 0 to 80 degrees C caused a decrease in isotactic pentad content by approximately 10%. The isotactic polymerization of MMA was revealed to proceed by an enantiomorphic site-controlled mechanism.