Synthesis and biodegradability of novel copolyesters containg γ-butyrolactone units

Abstract

Copolymers were synthesized by ring-opening polymerization of γ-butyrolactone (BL) with cyclicesters. Comonomers used were l-lactide (LLA), glycolide (GL), β-propiolactone (PL), δ-valerolactone (VL) and ε-caprolactone (CL). Tetraphenyl tin was used as an initiator. The copolymerization was carried out in bulk at 140°C for 4 days and the polymers were characterized by 1H n.m.r., 13C n.m.r., g.p.c. and d.s.c. The BL contents of the copolymers varied in the range 0 and 26 mol%. The number average molecular weights were from 1.3 × 10 3 to 1.5 × 10 5. When a small amount of the BL unit was introduced into the polymer chain, increased flexibility and excellent biodegradability were imparted to the polymer. However, excess of BL resulted in low molecular weight polymers with substantially low yields. D.s.c. measurements showed that the copolymers had one single endothermic peak at lower temperatures than those of the T ms of each homopolymer. The BL rich copolymers were shown to be amorphous. The statistical nature of the synthesized copolymers was confirmed with n.m.r. analysis. The copolyesters were hydrolyzed in distilled water at 70°C and their hydrolyzability was found to be affected by the chemical structure and polymer composition. The hydrolyzability of glycolide or lactide copolymers was high in comparison with other copolyesters. The BL-rich copolyesters were easily hydrolyzed. The copolymers were also hydrolyzed with lipases from Rhizopus arrhizus, R. delemar and Candida cylindracea in phosphate buffer solution (pH 7.0) at 37°C. Copolymers without substituents, such as the poly(BL- co-ε-caprolactone)s, were easily enzymatically hydrolyzed. It is noteworthy that the non-enzymatic hydrolysis was not affected by the presence of substituents.