P-Propiolactone was reacted with triethylamine, pyridine, triphenylphosphine, potassium tert-butoxide, potassium (4-t-butylphenoxide) or potassium benzoate in CDCl3 or dimethylsulfoxide in an n.m.r. tube. The H-1 n.m.r. analyses revealed in all cases formation of large amounts of acrylate groups. beta-Propiolactone was polymerized at 20 degrees C with triethylamine, pyridine or triphenylphosphine at a monomer/initiator ratio (M/I) of 50:1, and the endgroups of the isolated poly(beta-propiolactone)s were characterized by H-1- and C-13-n.m.r. spectroscopy. Acrylate groups were found in all cases, even when the initiation with triphenylphosphine was conducted at -70 degrees C. Sodium methoxide and potassium t-butoxide even attack polypropiolactone, yielding acrylate endgroups by deprotonation and elimination. In the presence of methanol, transesterification with formation of methyl ester endgroups also takes place. Only small amounts of acrylate groups were found when tributyltin methoxide served as initiator at 60-100 degrees C. With aluminium isopropoxide acrylate endgroups were almost absent. Thus, the degree of deprotonation allows a differentiation between a true anionic polymerization and a nonionic insertion mechanism.