Polymerization of L ‐ and DL ‐phenylalanine N ‐carboxyanhydride by poly( N ‐methyl‐ L ‐alanine)

Polymerizations of L ‐ and DL ‐phenylalanine N ‐carboxyanhydride in nitrobenzene by poly ( N ‐methyl‐ L ‐alanine) of varying degrees of polymerization ( n = 1–30) were investigated. Poly( N ‐methyl‐ L ‐alanine) was prepared by the polymerization of N ‐methyl‐ L ‐alanine NCA with N ‐methyl‐ L ‐alanine diethylamide and the degree of polymerization was controlled by the molar ratio [NCA]/[Catalyst] + 1. This polymer was shown to be an asymmetrically selective catalyst which polymerized L ‐phenylalanine NCA at a faster rate than DL ‐phenylalanine NCA. With increasing degree of polymerization the stability of the secondary structure of poly( N ‐methyl‐ L ‐alanine) increased. This was confirmed by circular dichroism spectra. However, the degree of asymmetric selection did not increase as the stability of the secondary structure of poly( N ‐methyl‐ L ‐alanine) increased. These findings indicate that the interaction of a growing polypeptide in an ordered structure with NCA molecules prior to the reaction does not lead to an asymmetric selection, and that the mechanism of the asymmetric selection by poly( N ‐methyl‐ L ‐alanine) should be different from those proposed so far.