Amino-termined poly(L-lactide)s as initiators for the polymerization of N-carboxyanhydrides: synthesis of poly(L-lactide)-block-poly(α-amino acid)s

Poly(L -lactide)-block-poly(L -amino acids) block copolymers were prepared via polymerization of α-amino acid N-carboxyanhydrides with amino-terminated poly(L -lactide)s as macroinitiators. Two types of macroinitiators were used, one with an aminopropoxy head group (number-average molecular weight M _n = 22000) and the other one with a phenylalanine end group (M _n = 18000). The first macroinitiator was obtained by polymerization of (L ,L )-lactide with an initiator prepared in situ from diethylzinc Et₂Zn and N-tert-butoxycarbonyl-1-amino-3-propanol, followed by deprotection of the amino group. The second macroinitiator was obtained by endcapping of poly(L -lactide) with N-tert-butoxycarbonylphenylalanine and deprotection of the amino group. ¹H- and ¹³C NMR spectroscopies confirm the block structure of the copolymers obtained. In differential scanning calorimetry curves only one melting transition characteristic of the poly(L -lactide) block is observed, on further heating decomposition occurs. By thermogravimetry two steps of decomposition are observed, the first one being assigned to the decomposition of the poly(L -lactide) block, and the second one to that of the poly(amino acid) block, by comparison with the thermal behaviour of the corresponding homopolymers.