Enzymatic Transformation of Bacterial Polyhydroxyalkanoates into Repolymerizable Oligomers Directed towards Chemical Recycling

Summary: The enzymatic transformation into an oligomer was carried out with the objective of developing the chemical recycling of bacterial polyesters. Poly( R ‐3‐hydroxyalkanoate)s (PHAs), such as poly[( R ‐3‐hydroxybutyrate)‐ co ‐12%( R ‐3‐hydroxyhexanoate)] and poly[( R ‐3‐hydroxybutyrate)‐ co ‐12%( R ‐3‐hydroxyvalerate)], were degraded by granulated Candida antarctica lipase B immobilized on hydrophilic silica (lipase GCA) in a diluted organic solvent at 70 °C. The degradation products were cyclic oligomers having a molecular weight of a few hundreds. The obtained cyclic oligomer was readily repolymerized by the same lipase (lipase GCA) to produce the corresponding polyester in a concentrated solution. The cyclic oligomer was copolymerized with ε ‐caprolactone using lipase to produce the corresponding terpolymers having an $\overline M _{\rm w}$ of 21 000. This is the first example of the enzymatic chemical recycling of bacterial PHAs using lipase. Poly( R ‐3‐hydroxybutyrate) [P(3HB)] was also degraded into the linear‐type R ‐3HB monomer to trimer by P(3HB)‐depolymerase (PHBDP) in phosphate buffer at 37 °C. The degradation using PHBDP required a longer reaction time compared with the lipase‐catalyzed degradation in organic solvent. The monomer composition of the oligomer depended on the origin of the PHBDP. The R ‐3HB monomer was predominately produced by PHBDP from Pseudomonas stutzeri , while the R ‐3HB dimer was produced by PHBDP from Alcaligenes faecalis T1. Repolymerization of these oligomers by lipase in concentrated organic solvent produced a relatively low‐molecular‐weight P(3HB) (e.g., $\overline M _{\rm w}$  = 2 000).Degradation of P(3HB) by lipase in organic solvent into repolymerizable cyclic oligomer and degradation of P(3HB) by PHBDP in buffer into hydroxy acid type R ‐3HB dimer.