Different mixed ligand lanthanocene complexes along with Al(i-Bu)(3) are capable of synthesizing partially syndiotactic high molecular weight polymethylmethacrylate. The Schiff base derivative of mixed ligand lanthanocene complexes (1-4) showed better efficiency (Mv = 227 x 10(3)-354 x 10(3)) compared to the methoxyethylindenyl derivative of mixed ligand lanthanocene complexes (5-9) ( Mv = 72 x 10 3, 172 x 10(3)). The effects of the catalyst, temperature, catalyst/MMA molar ratio, and catalyst/ Al(i-Bu)(3) molar ratio on the polymerization of MMA at 60 degrees C suggested that ligand size and radius of rare earth metals play an important role in determining the activity of the catalyst.