Poly(α-alkyl γ-glutamate)s of microbial origin: I. Ester derivatization of poly(γ-glutamic acid) and thermal degradation

A series of poly(α-alkyl γ-glutamate)s (PAAG-n with n=1–10) were prepared from poly(γ-glutamic acid) (PGGA) of microbial origin by a two-steps synthesis procedure. The poly(α-ethyl γ-glutamate) was firstly obtained by esterification of PGGA and then transesterified with the selected alkanol to obtain the corresponding PAAG-n. The modification reactions were found to occur without alteration of the original enantiomeric composition but with a significant reduction in molecular weight. The thermal degradation of PAAG-n was examined by thermogravimetric analysis combined with IR and NMR spectroscopy. At low temperatures (<300°C) decomposition was found to take place by depolymerization with releasing of the corresponding alkyl pyroglutamate. Decarboxylation of PAAG-n to an unsaturated polyamide seemed to be the main process occurring at high temperatures (400–450°C).