ß-D,L,-Butyrolactone was polymerized at 50°C in deuterated solvents of different polarity, and both the conversion and the formation of crotonate groups were directly determined from the reaction mixture by means of1H-NMR spectroscopy. Two classes of initiators, namely nonionic ones, such as tertiary amines or triphenylphosphine, and ionic ones, such as potassium salts of alcohols, phenols, and carboxylic acids, were used. In almost all polymerizations, formation of crotonate end groups was detected. Two deprotonation mechanisms are discussed. This side reaction is the main reason why high molecular weight poly(ß-C,L-butyro-lactone) could not be obtained even when the monomer/initiator ratio was varied by a factor of 40. In contrast, the nonionic initiator tributyltin methoxide yielded poly(ß-D,L-butyrolactone) free of crotonate groups. When potassium salts ofN-protectedL-amino acids were used as initiators, again crotonate end groups were formed even at +5°C, and the resulting poly(ß-D,L-butyrolactone) was almost atactic.