To develop a novel polycondensation method for the preparation of poly (amino acid)s, we screened a transition metal or a rare-earth triflate as a Lewis acid for the polycondensation of activated amino acid esters in N,N-dimethylformarhide solutions at room temperature. The polymerizations of 4-nitrophenyl L-leucinate (1a) and 4-nitrophenyl L-valinate (1b) scarcely proceeded without any Lewis acid at room temperature. In the presence of 5 mol % metal triflates, especially scandium(III) trifluoromethanesulfonate, the polymerizations of both monomers were promoted effectively. The products, which were collected by the reaction mixture being poured into water, were recognized as poly(L-valine)s by Fourier transform infrared spectroscopy, gel permeation chromatography analysis, and H-1 NMR spectroscopy. These results showed that a metal triflate as a Lewis acid could coordinate to a carbonyl oxygen of activated L-valinate and L-leucinate even in a highly polar solvent, such as NN-dimethylformamide; therefore, the polymerizations of activated L-valinate and L-leucinate were promoted. Because steric hindrance derived from the isobutyl group in 1b was less than that of the isopropyl unit in la, the effect of the metals was not as sensitive for the polymerization of 1b. (c) 2006 Wiley Periodicals, Inc.