Enzymatic degradation of novel ethero-oxygen atom-containing copolyesters based on Poly(butylene succinate)

In recent years, biodegradable polymers have gained much attention as green materials and biomaterials. Aliphatic polyesters, due to their favorable features of biodegradability and biocompatibility, are one of the most important classes of synthetic biodegradable polymers: polycaprolactone, poly(hydroxybutyrate), poly(L-lactide), poly(butylene succinate) (PBS), etc. are nowadays available commercially [1]. Among all, PBS is used in a wide range of applications, such as packaging film, bags, flushable hygienic products and garden mulch [2], but shows a quire slow rate of degradation [1]. As is well known enzymatic biodegradation of a polymer is controlled by several factors: the most important one is the nature of the polymer itself, i.e. its chemical structure; hydrophilicity and consequently water uptake increases degradation rate; moreover, the degree of crystallinity is a crucial factor, since enzymes preferably attack the amorphous domains of a polymer. Conditions like temperature, pH and concentration of enzyme also play a important role [1]. In our study, the effect of chemical modification as well as of molecular architecture on PBS biodegradation rate has been investigated. In addition, different enzymes and different operating conditions have been explored. Materials and methods. Copolymers of PBS containing diethylene succinate sequences with different molecular architecture have been prepared in our laboratories via reactive blending in the presence of Ti-based catalyst (PBSPDGS). In particular, a block copolymer with long sequences and the random one have been considered. For comparison the parent homopolymer PBS has been also synthesized by the usual two-stage melt polycondensation. Four different commercially available lipases were tested from Candida rugosa, Candida cylindracea, Aspergillus niveus and hog pancreas and a serine protease (alpha-chymotrypsin from bovine pancreas) (50U/ml in 0.1M phosphate buffer pH 7.4, 30°C, 100 rpm). To evaluate the biodegradability of the polymers under investigation, two kind of tests were carried out: i) turbidimetric film assay, which has high sensitivity to very small extents of degradation and ii) weight loss measurements. ATRIR analysis and DSC measurements have been performed to correlate degradation rate with crystallinity degree. Lastly, NMR analysis was performed to follow changes in composition of the copolymers under investigation. Results and Discussion. Preliminary results based on turbidimetric film assay, indicate that the most active enzyme is the lipase from Candida cylindracea, and that the best operative conditions are: [E]=50 U/ml (at higher [E] biodegradation is too fast for correct monitoring), T= 30°C (close to ambient temperature; high degradation rate) and pH =7.0 (highest degradation rate). Copolymers degrade to a much higher extent than PBS. Moreover, random copolymer degrades faster than the block one, probably due to its lower degree of crystallinity. ATRIR and DSC analyses confirm that the amorphous phase is the region attacked first by enzyme. Lastly, in the case of PBSPDGSblock an evident decrease of DGS content is observed by means of NMR analysis with the proceeding of degradation, indicating that enzyme hydrolysis involves preferentially ester groups of DGS sequences, probably because of their higher hydrophilicity. Conclusions. Relevant results have been obtained from the present research. They can be summarized as follow: 1. Copolymerization is an efficacious way to increase PBS biodegradability. 2. Biodegradation rate can be modulated changing the molecular architecture, which affects the crystallinity degree. References [1] Bikiaris et al., (2005). Polymer Degradation and Stability 91: 31. [2] Soccio et al., (2008). European Polymer Journal 44: 1722.