The effects of Lewis acids, that is, rare earth metal trifluoromethanesulfonates, on the free-radical polymerization of N-methylmethaerylamide (MMAM), N-isopropylmethacrylamide (IPMAM), N-tert-butylmethacrylamide (tBMAM), N-phenylmethacrylamide (PMAM), and methacrylamide were examined under various conditions. A catalytic amount of Yb(OSO2CF3)(3)-significantly affected the stereochemistry during the radical polymerization. Polymerization solvents strongly influenced the effect of the Lewis acids. Methanol was the best solvent for increasing the isotactic specificity during the polymerization of MMAM and IPMAM, whereas tetrahydrofuran was more effective for the tBMAM and PMAM polymerizations. (C) 2003 Wiley Periodicals, Inc.