Abstract
In the polymerizations of alanine, γ‐ethyl glutamate, and leucine N‐carboxyanhydrides (NCA's) initiated by tertiary amines and some secondary amines such as N‐methyl‐L‐alanine dialkylamide, a stereoselectivity was observed: the polymerization rates of L‐ and D‐NCA's were identical to each other and larger than that of DL‐NCA. However, this selectivity was not observed in the polymerizations of valine and isoleucine NCA's initiated by N‐methyl‐L‐alanine dialkylamide. The stereoselective polymerizations of valine and isoleucine NCA's were induced only with tetriary amines such as tri‐n‐butylamine. N‐Methyl‐L‐alanine di‐alkylamide has been shown to initiate the polymerization of usual α‐amino acid NCA according to the activated‐NCA mechanism, but it initiated the polymerizations of valine and isoleucine NCA's according to the primary amine‐type mechanism. This is because in the latter NCA's the N–H group is masked by the adjacent C^β^‐branched alkyl substituent against the approach of the secondary amine. Poly(DL‐alanine)s produced in the stereoselective polymerization had higher viscosities and were more stereoblock‐like than those produced without the stereoselectivity. These experimental results indicate that the stereoselective polymerization is possible only when the polymerization proceeds through the activated‐NCA mechanism.