Structural characterization and hydrolytic degradation of a Zn metal initiated copolymer of L-lactide and epsilon-caprolactone

A bioresorbable aliphatic polyester was synthesized by bulk copolymerization of a 1/1 M/M L,L-lactide/epsilon-caprolactone mixture using zinc metal as initiator. The actual composition of the copolymer was found to be 1.5/1 as deduced from H-1 NMR spectra obtained in DMSO-d6 solutions where higher resolution was obtained as compared with chlorinated solvents. Resonances due to L-lactyl units (L) exhibited triads stereosensitivity, epsilon-oxycaproyl units (C) being sensitive to dyads. Average lengths of both poly(lactic acid) and polycaprolactone sequences were evaluated and showed the presence of rather long PLA blocks. Furthermore, no C(L) over barC$ triad signal was found, suggesting the absence of transesterification rearrangements. 10 x 10 x 2 mm specimens made of the copolymer were allowed to age in isoosmolar pH = 7.4 phosphate buffer at 37 degrees C. Degradation was monitored by various analytical techniques such as SEC, X-ray diffractometry, DSC, and H-1 NMR. Data were compared with the behaviour of PCL and PLA homopolymers allowed to age under similar conditions. Crystallinity and composition changes are discussed in terms of preferential degradation in L- and C-containing amorphous domains, crystallized long PLA blocks being much more resistant.