Recent advances in polyhydroxyalkanoates degradation and chemical recycling

Plastics are nowadays essential to our daily life for a wide range of applications. In order to face the demand of polymer markets, given the depletion of fossil feedstocks from which they are still most commonly produced, and with the aim to develop more ecofriendly plastic materials, the need for renewable and/or recyclable polymers is huge. Polyhydroxyalkanoates (PHAs) are a class of polyesters that could meet the challenges of such a circular economy, as they currently stand as promising bio-based, degradable and recyclable alternatives to traditional non-degradable commodity polymers that are polyolefins. PHAs typically feature different side-chain substituents on the repeating units, which beside the stereochemistry along the polymer backbone and the intrinsic characteristics of the macromolecules, are key parameters that dictate and enable tuning of their thermal, mechanical, and recyclability performances. PHAs are thus a large family of versatile polymers that are currently of topical interest in light of their end-of-life options. This review discusses the chemical recycling of natural, biosynthetic and synthetic PHAs, mainly focusing on the most common examples, namely poly(3-hydroxybutyrate) (PHB), and its related copolymers. The most relevant non-biotechnological approaches, including pyrolysis-type processes, and solvolysis with especially hydrolysis and alcoholysis, whether they are catalyzed or not, are then addressed. The latest advances on the degradation, depolymerization and upcycling of PHAs, show promising outcomes for a close-carbon cycle economy with a favorable environmental impact, as exemplified from the most recent literature.