Imidazole‐initiated polymerizations of α‐amino acid N‐carboxyanhydrides – simultaneous chain‐growth and step‐growth polymerization

The N ‐carboxyanhydrides (NCAs) of sarcosine (Sar), D , L ‐leucine ( D , L ‐Leu), D , L ‐phenylalanine ( D , L ‐Phe), and L ‐alanine ( L ‐Ala) were polymerized in dioxane. Imidazole served as initiator and the NCA/initiator ratio was varied from 1/1 to 40/1. The isolated polypeptides were characterized by 1 H NMR spectroscopy, by MALDI‐TOF mass spectrometry, by viscosity measurements, and by SEC measurements in the case of poly(sarcosine). Cyclic oligopeptides were found in all reaction products and in the case of polySar, poly( D , L ‐Leu), and poly( D , L ‐Phe) the cycles were the main products. In the case of poly( L ‐Ala), rapid precipitation of β‐sheet lamellaes prevented efficient cyclizations and stabilized imidazolide endgroups.