Recyclable Polyhydroxyalkanoates via a Regioselective Ring-Opening Polymerization of α,β-Disubstituted β-Lactone Monomers

Abstract

The economical and societal issues associated with the depletion of fossil feedstocks and severe accumulation of nondegradable plastics in nature have pushed the increasing efforts toward the development of biodegradable polymers with unique recyclability. The bacterial polyhydroxyalkanoates (PHAs) represent a unique class of biodegradable polymers that have found a variety of applications in packaging, agricultural, biomedical, and pharmaceutical fields. Due to the high production costs and limited volumes for the bacterial fermentation process, the chemical synthesis of PHAs via ring-opening polymerization of lactone monomers has attracted intensive attention. However, most β-lactones feature a substituent group at the β-position, and they cannot be recycled back into the monomers or other intermediates in quantitative yields. Rare examples were reported regarding the synthesis of PHAs based on β-lactones with α,β-disubstituted groups or a ring fusion at both α- and β-positions. Here, we report a type of β-lactone fused with a five-membered ring fusion at α- and β-positions (cis-6-oxabicyclo[3.2.0]heptan-7-one, β-CPL), from the ring-expansive carbonylation of cyclopentene oxide (CPO). With the efficient catalysts, the ring-opening polymerization of β-CPL under ambient conditions produces a novel type of polyhydroxyalkanoates with controlled linear and/or cyclic topologies; the produced PHAs feature a high melting temperature (∼185 °C) and high molecular weight (Mn = 1.01 × 105 g mol–1, Đ = 1.36) via a regioselective fashion through an O-acyl cleavage. In particular, the quantitative formation of α,β-unsaturated carboxylic acid during pyrolysis represents a rare example that the recyclability can be accomplished via a transformation of α,β-unsaturated carboxylic acid to β-lactone monomers.