Hydrolytic degradation of PLA/PEO/PLA triblock copolymers prepared in the presence of Zn metal or CaH2

Various PLA/PEO/PLA triblock copolymers were prepared by ring-opening polymerization of L-lactide in the presence of poly(ethylene glycol), using CaH2 or zinc metal as co-initiator. The degradation behavior of these copolymers was investigated in a pH = 7.4 phosphate buffer at 37 degrees C. Various techniques such as weighing, size exclusion chromatography, differential scanning calorimetry, X-ray diffractometry,, and infra-red and H-1 nuclear magnetic resonance spectrometries were used to monitor the changes in water absorption, weight loss, molar mass distribution, thermal properties, degree of crystallinity and composition. The results showed that introduction of PEO sequences considerably increased the hydrophilicity of the copolymers as compared with PLA homopolymers. However, the degradability of PLA blocks was not enhanced because of the phase separation between PLA and PEO blocks. Bimodal molar mass distributions were detected at the later stages of degradation, which were assigned to the formation of crystalline degradation byproducts within the bulk material. (C) 1998 Elsevier Science Ltd. All rights reserved.