Polylactones. 55. A-B-A triblock copolymers of various polypeptides. Syntheses involving 4-aminobenzoyl-terminated poly(epsilon-caprolactone) as B block.

A telechelic poly(epsilon-caprolactone) having a degree of polymerization (DP) around 25 and two 4-aminobenzoyl chain ends was used as a macroinitiator for the ring-opening polymerization of various alpha-amino acid N-carboxyanhydrides (NCAs). Glycine-NCA, L-alanine-NCA, L-phenylalanine-NCA, and gamma-benzyl-L-glutamate-NCA served as monomers and the NCA/macroinitiator ratio was varied between 20:1, 40:1, and 100:1. In the case of L-Phe-NCA, a ratio of 200:1 was also used. It was demonstrated by means of model studies, that the NCAs may react almost quantitatively with the 4-aminobenzoyl end groups despite their relatively low nucleophilicity. The isolated triblock copolymers were characterized by viscosity measurements and by (1)H NMR spectroscopy with regard to their composition (which in most cases paralleled the feed ratios). However, in the case of gamma-Bzl-Glu-NCA mixtures of di- and triblock copolymer were obtained. The secondary structures of the solid copolymers were examined by IR spectroscopy and (13)C NMR CP/MAS spectroscopy. It was found that the alpha-helix/beta-sheet ratio of the poly(L-Ala) and poly(L-Phe) blocks increases with their average length, according to the NCA/macroinitiator ratio.