The free-radical polymerizations of methyl methacrylate (MMA), ethyl methacrylate, isopropyl methacrylate, and 2-methoxyethyl methacrylate were carried out in the presence of various Lewis acids. The MMA polymerization in the presence of scandium trifluoromethanesulfonate [Sc(OTf)(3)] in toluene or CHCl3 produced a polymer with a higher isotacticity and heterotacticity than that produced in the absence of Sc(OTf)(3). Similar effects were observed during the polymerization of the other monomers. ScCl3, Yb(OTf)(3), Er(OTf)(3), HfCl4, HfBr4, and In(OTf)(3) also increased the isotacticity and heterotacticity of the polymers. The effects of the Lewis acids were greater in a solvent with a lower polarity and were negligible in tetrahydrofuran and N,N-dimethylformamide. Sc(OTf)(3) was also found to accelerate the polymerization of MMA. On the basis of an NMR analysis of a mixture of Sc(OTf)(3), MMA, and poly(methyl methacrylate), the monomer-Sc(OTf)(3) interaction seems to be involved in the stereochemical mechanism of the polymerization.