It was first found that (diisopropylamido)bis(methylcyclopentadienyl)lanthanides (MeC5H4)(2)LnN(i-Pr)(2)(THF) (Ln = Yb (1), Er (2), Y (3)) exhibit extremely high catalytic activity in the polymerization of methyl methacrylate. The reactions can be carried out over a quite broad range of polymerization temperatures from -78 to 40 degrees C. The catalytic activity of the complexes increases with an increase of ionic radii of the metal elements, i.e. Y > Er > Yb. The results of GPC (gel permeation chromatography) indicate that the number-average molecular weights (M-n) of polymers obtained exceed 100 x 10(3) and the molecular weight distribution (M-w/M-n) becomes broad with the increase of temperature. Furthermore highly syndiotactic PMMA (87.7%) can be obtained by lowering the reaction temperature to -78 degrees C.