The NCAs of the following five amino acids were polymerized in bulk at 120 degrees C without addition of a catalyst or initiator: sarcosine (Sar), L-alanine (L-Ala), D,L-phenylalanine (D,L-Phe), D,L-leucine (D,L-Leu) and D,L-valine (D,L-Val). The virgin reaction products were characterized by viscosity measurements C-13 NMR spectroscopy and MALDI-TOF mass spectrometry. In addition to numerous low molar mass byproducts cyclic polypeptides were formed as the main reaction products in the mass range above 800 Da. Two types of cyclic oligo- and polypeptides were detected in all cases with exception of sarcosine NCA, which only yielded one class of cyclic polypeptides. The efficient formation of cyclic oligo- and polypeptides explains why high molar mass polymers cannot be obtained by thermal polymerizations of alpha-amino acid NCAs. Various polymerization mechanisms were discussed. alpha 2008 Wiley Periodicals, Inc.