Methyl Groups Boost Properties of PHA Plastics for Closed-Loop Recycling

A class of polyesters considered a promising alternative to common plastics, were it not for limitations like brittleness and thermal instability, have now been made more mechanically tough and thermally stable. Researchers replaced the reactive hydrogens in the monomer of these materials – polyhydroxyalkanoate (PHA) plastics – and found it enhanced PHA thermal and mechanical properties and enabled closed-loop chemical recyclability. The new approach could provide a route for increased use of sustainable PHA plastics. "The pathways described by Zhou et al. exemplify how polymers can be improved by revisiting some fundamental chemistry concepts rather than establishing complex chemistry models," writes Sophie Guillaume in a related Perspective. PHAs are a promising class of polyester that are biosynthesized by living microorganisms as well as through chemical synthesis using biorenewable feedstocks. As a result, they could offer a more sustainable alternative to widely used petroleum-derived plastics. Despite these attractive advantages, PHAs tend to be brittle and lack thermal stability and pathways to closed-loop chemical recyclability, which has limited their widespread commercial application. To address these three main challenges, Li Zhou and colleagues present a synthetic PHA platform that replaces reactive carbonyl-adjacent hydrogens in the monomer with methyl groups. Zhou et al. show that this relatively simple replacement results in a dimethylated PHA with enhanced mechanical toughness and melt processibility. What's more, the authors show that these PHAs could be chemically reverted to their starting monomers, enabling closed-loop recycling. "Together with the scalability of the polymerization and depolymerization steps, the findings of Zhou et al. constitute a notable step in the PHA domain to advance sustainability and circularity of polymers and could have profound implications for the next generation of industrial plastics," writes Guillaume.