Hydrolytic and thermal degradation of random copolyesters of epsilon-caprolactone and 2-oxepane-1,5-dione

The hydrolytic and thermal stability of random copolyesters of epsilon-caprolactone (epsilon-CL) and ca. 30 mol-% 2-oxepane-1,5-dione (OPD) have been investigated. Compared with poly(epsilon-caprolactone) (PCL) of a comparable molecular weight, the hydrolytic degradation of the copolyester is faster in a phosphate buffer (pH = 7.4) at 37°C as confirmed by the time dependence of water absorption, weight loss, melting temperature, and molecular weight. This difference is a result of the higher hydrophilicity imparted to the copolyester by the ketone of the OPD units. The thermal degradation has been studied by thermogravimetric analysis (TGA), (HNMR)-H-1 spectroscopy, and size exclusion chromatography (SEC). The activation energy of the thermal degradation under nitrogen has been found to be lower for the copolyesters than for PCL, which indicates that the OPD co-units have a deleterious effect on the thermal stability of PCL. The thermal degradation primarily occurs by pyrolysis of the ester functions.