Abstract
A bioresorbable aliphatic polyester was synthesized in bulk from L,L-lactide and e-caprolactone in 1/1 mol ratio using zinc metal as initiator. The actual composition of the copolymer was found to be 1.5/1 as shown by 1H NMR spectra obtained in DMSO-d6 solutions. In this solvent, 1H NMR resolution was improved, as compared to chlorinated solvents. For L-lactyl units, splittings due to triads were found, beside the known diad effect related to e-oxycaproyl units. The improved 1H NMR resolution permitted the calculation of average lengths of both the poly(lactic acid) and polycaprolactone sequences. Moreover, no CLC triad signal was found, a finding which was confirmed by 13C NMR in CDCl3. The hydrolytic degradation of the copolymer was investigated under standardized conditions, i.e. 10 x 10 x 2 parallelepiped specimens allowed to age in isoosmolar pH = 7.4 phosphate buffer at 37°C, the control being a PCL homopolymer. Degradation characteristics were monitored by various analytical techniques such as SEC, X-ray diffractometry, DSC and 1H NMR. Heterogeneous degradation was observed for the slightly crystalline copolymer, but not for PCL, probably because of the high crystallinity and very slow degradation rate. Dramatic morphological changes were detected in both cases with increase in crystallinity. For PCL, crystallinity increased due to the crystallization of initially amorphous domains. In contrast, for the copolymer, the crystallinity increase mainly resulted from the vanishing of amorphous regions. Compositional changes were also observed for the copolymer with increase in lactyl content, which could be attributed to the PLA-type crystalline structure.