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 H-1 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 beta-sheet lamellaes prevented efficient cyclizations and stabilized imidazolide endgroups. (c) 2005 Wiley Periodicals, Inc.