Polyglycine and polyalanine obtained by polymerization of their N-carbonic anhydrides often appeared to contain more acid groups than amino groups. This is ascribed, at least partly, to the formation of hydantoin groups. A small amount of 2,5-diketopiperazine was isolated from polyglycine. A discussion of the results and their bearing upon the calculation of molecular weights is given.
Sluyterman, L. A. A. E., & Labruyere, B. (1954). Side reactions in the polymerization of a-amino acid N-carbonic anhydrides : titration data of polyglycine and polyalanine. Recueil des Travaux Chimiques des Pays-Bas, 73(4), 347-354. https://doi.org/10.1002/recl.19540730407