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, DPs 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.