Enzymatic degradation of the most common aliphatic biopolyesters

The constant increase of the plastic production over the world has become a serious problem, since most conventional plastic materials come from fossil resources and are not biodegradable. This causes significant plastics accumulation in the environment, whose end-life must be managed. An effective and eco-friendly approach to solve such problem is the use of biodegradable materials. Biopolymers such as poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), poly(caprolactone) (PCL), poly(lactic acid) (PLA) and poly(propylene carbonate) (PPC) are among the most promising biodegradable commercial polyesters thanks to their high susceptibility to hydrolytic enzymes and to many microorganisms naturally occurring in the environment [1-2]. The current study is an investigation of the degradation ability of several hydrolytic enzymes belonging to different subclasses (i.e. lipase, esterase, proteinase, etc.) against some common aliphatic commercial polyesters. A deeper elucidation on the degradative ability and the mechanism of hydrolytic biodegradation can be useful for the management of biodegradable plastic wastes, the bioremediation of plastic-polluted environments, as well as the design of innovative biodegradable plastic materials. The end-of-life of biopolymers indeed, should also be addressed in view of a circular economy concept.

The enzyme screening was first carried out by investigating the capacity of fully degrading the target polymers in 24 h, then weight loss measurements of selected polyesters and target enzymes were performed. Solid residues after enzyme degradation were characterized by proton nuclear magnetic resonance (¹H NMR), gel permeation chromatography (GPC), infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and thermogravimetry (TGA). Liquid fractions were studied via GPC, ¹H NMR and high-performance liquid chromatography (HPLC). This, in order to understand molecular and chemical modifications induced at the surface and/or in the bulk of polymer materials over time. The results showed that the enzymatic degradation occurred homogenously from the surface through an erosion mechanism and involved both the amorphous and the crystalline regions. Cleaving action mode for each enzyme (endo- and/or exo-type) is also proposed.