Ring‐opening homopolymerization and copolymerization of lactones. Part 2. enzymatic degradability of poly(β‐hydroxybutyrate) stereoisomers and copolymers of β‐butyrolactone with ɛ‐caprolactone and δ‐valerolactone

Abstract

In this study, we have prepared poly([R,S ]-β-hydroxybutyrate) (P([R,S ]-β-HB) or PHB) from [R ,S]-β-butyrolactone ([ R,S]-β-BL), using different aluminoxane catalyst systems (triethylaluminium/water, triisobutylaluminium/water, trioctylaluminium/water and tetraisobutyldialuminoxane/water). By varying the ratio of catalyst to water and using a method of fractionation of polymers, PHB with different isotactic diad fractions (i) (from 0.41 to 0.72) and crystallinities were obtained. Copolymers poly(butyrolactone-co-caprolactone) (P(BL-co-CL)) and poly(butyrolactone-co-valerolactone) (P(BL-co-VL)) have also been synthesized from the ring-opening copolymerization of [ R,S]-β-BL with either ε-caprolactone (CL) or δ-valerolactone (VL) using tetraisobutyldialuminoxane (TIBAO) catalyst. The enzymatic degradability of these polymers was studied in aerobic and anaerobic media. The objective of this work was to determine the influence of the tacticity and crystallinity of the polymers on their degree of biodegradation and on their initial degradation rate. It was shown that the degradation rate measured for bacterial PHB 100% [R] was the highest and the degree of aerobic biodegradation reached after 36 days was around 94%. A 40–50% biodegradation was obtained for synthetic PHB, highly isotactic and predominantly syndiotactic. The non-crystalline and atactic PHB synthesized from TIBAO catalyst had a very high degree of biodegradation of around 88%. This result may suggest that not only are the [R ]-BL units hydrolysed but also the [S ]-BL units. The influence of the crystallinity on the initial degradation rate was observed for the copolymers P(BL-co-CL) and P(BL-co -VL) of various feed ratios. All these copolymers synthesized from TIBAO catalyst, exhibit a high degree of biodegradation of around 85% except for copolymers containing a very high portion of unsubstituted units, CL or VL. The anaerobic biodegradation of PHB and copolymers P(BL-co -CL) is much lower than the aerobic biodegradation, as are the initial rates, even for bacterial P([R ]-HB). © 1999 Society of Chemical Industry