Synthesis and polymerization of N ‐carboxyanhydrides of N ‐benzyl‐β‐amino acids

The NCAs of the N ‐benzyl derivatives of β‐alanine, β‐ DL ‐aminobutyric acid, and β‐ DL ‐aminoisobutyric acid (nonplanar six‐membered rings) were prepared by reacting the corresponding N ‐chloroformyl derivatives, obtained on reaction of the N ‐benzyl amino acids with phosgene, with triethylamine. Contrary to the others, the NCA of N ‐benzyl‐β‐alanine polymerized readily on heating in vacuo or in solution, using n ‐hexylamine or methanolic sodium methoxide as initiators. With n ‐hexylamine the molecular weights of the polymers obtained in benzene, dioxan, and dimethylsulfoxide, were in accordance with DP = [NCA]/[Initiator], as was found with conventional five‐membered ring NCAs of α‐amino acids. With sodium methoxide initiation, DP s of the polymers obtained were smaller than the (NCA)/[Initiator] ratio, contrary to what was found previously with α‐amino acid NCAs. The possibility that stereochemical factors are responsible for the differences in polymerization activity of various. N ‐alkyl β as well as α amino acid NCAs is discussed.