Synthesis and degradation of PLA–PCL–PLA triblock copolymer prepared by successive polymerization of ε-caprolactone and dl-lactide

Abstract

A PLA–PCL–PLA triblock copolymer was synthesized by ring-opening polymerization of successively added ε-caprolactone and dl-lactide in the presence of ethylene glycol, using zinc powder as catalyst. The resulting copolymer was compression molded to yield circular films from which 10×10×0.4 mm 3 square samples were cut. These samples were allowed to degrade in isoosmolar 0.13 M, pH 7.4 phosphate buffer at 37 °C. Various analytical techniques such as weighing, SEC, 1H NMR, IR, DSC, X-ray, ESEM and CZE were used to monitor property changes during degradation such as water absorption, weight loss, molecular weight distribution, composition, bulk and surface morphologies, thermal behavior, and release of water-soluble oligomers into the aqueous phase. Comparison is made with a PCL homopolymer under similar ageing conditions. The presence of PLA sequences did not alter the ability of PCL segments to crystallize during degradation, in agreement with the microphase separation between PLA and PCL blocks. Large amounts of lactic acid-rich soluble degradation by-products were released into the solution between 18 and 25 weeks. Copolymer films cracked down under drying beyond 25 weeks' degradation, while the surface remained smooth. The copolymer exhibited faster degradation as compared with PCL homopolymer, a feature which is of great interest for temporary therapeutic applications.