Influence of chemical and architectural modifications on the enzymatic hydrolysis of poly(butylene succinate)

Abstract

Copolymers of poly(butylene succinate) (PBS) containing diethylene succinate sequences (PBSPDGS) with different molecular architectures were prepared via reactive blending in the presence of a Ti-based catalyst. In particular, a block copolymer with long sequences and a random one with very short sequences were synthesized, characterized and investigated in terms of enzymatic biodegradability. For comparison, the parent homopolymer PBS has been also prepared by the usual two-stage melt polycondensation. Preliminary biodegradation tests based on the highly sensitive film opacity assay indicated that lipase from Candida cylindracea was the most effective among four different commercially available lipases (e.g., those from Candida rugosa, Candida cylindracea, Aspergillus niveus and hog pancreas) and a serine protease (α-chymotrypsin from bovine pancreas), and that optimal test conditions were 50 enzyme U mL⁻¹, 30 °C and pH 7.0. Under such conditions, copolymers degraded to a much higher extent as compared to PBS. Moreover, the random copolymer degraded 100 times faster than the block one. ATRIR analysis and DSC measurements indicated that the enzyme attacked the amorphous phase first. Further, NMR analysis indicated that enzyme hydrolysis involved preferentially ester groups of DGS sequences, more hydrophilic than the others. These findings confirm previous evidence on the correlation between polymers biodegradation rate and their hydrophilic and amorphous degree. More importantly, they indicate (i) that dramatic increases in polyesters biodegradability can be obtained by introducing ether-oxygen atoms into the polymer chain and (ii) that biodegradability of oxygen etheroatom-containing copolyesters might be tuned within a wide range of rates through the modification of their molecular architecture.