Methyls make PHAs more practical for plastics

Although the polyesters known as polyhydroxyalkanoates, or PHAs, have been around for almost a hundred years, they haven’t caught on as major components of plastics. Thanks to the ester bonds in their backbones, PHAs are biodegradable. But they’re brittle and thermally unstable, qualities that make them inferior to commonly used polyolefins, which are not biodegradable.

Two research groups independently discovered they could make PHAs that have properties on par with polyolefins by modifying the monomers with methyl groups. These new PHAs could be used to make plastic bags, bottles, straws, and utensils that can potentially biodegrade or be recycled into their starting monomer or other valuable chemicals.

Chemists led by Colorado State University’s Eugene Y.-X. Chen recognized that PHAs are prone to decomposition via a cis-elimination reaction that involves the hydrogens on the carbon next to the carbonyl in the PHA backbone. They reasoned that by replacing these hydrogens with methyl groups, they could prevent the cis-elimination reaction. “It’s very simple chemistry,” Chen says.

The researchers were delighted to find that the PHA they made had properties that were similar to polymers such as polypropylene and high-density polyethylene. What’s more, the chemists could take the polymer and turn it back into the starting β-lactone monomer—something that’s hasn’t been possible to do with other PHAs. With that β-lactone monomer, Chen says, “we can repolymerize again and close the chemical loop,” (Science 2023, DOI: 10.1126/science.adg4520).

Separately, chemists led by Cornell University’s Geoffrey W. Coates made a series of PHAs starting from carbon monoxide and 2-butene feedstocks. The researchers created four β-lactone diastereomers and polymerized them in various combinations. They found the PHAs they made with a mix of diastereomers had the best properties. “In the polymer field, sometimes highly crystalline, perfect polymers are really brittle,” Coates says. By using a mix diastereomeric monomers, they could make the polymer more malleable (Nat. Chem. 2023, DOI: 10.1038/s41557-023-01187-0).

Coates’s team can break their PHAs down into tiglic acid, which can be used make flavors, fragrances, or pharmaceutical intermediates. Or tiglic acid can be turned into 2-butene, which can then be transformed into the β-lactone monomers.

Yan Xia, a chemist at Stanford University who designs polymers, says that the work from Chen’s and Coates’s groups “provides inspiration to design chemically recyclable polymers that are scalable under industrially friendly conditions and competitive or even superior in properties compared with commercial single-use plastics.”

Coates sees plenty of opportunities to explore the chemistry of PHAs. He says it’s similar to what chemists have been doing with polyolefins since the 1950s—with one key difference. “The big advantage of these is they have ester groups in the backbone. They could potentially biodegrade. They’re not hydrocarbon backbones that are going to be hydrocarbon backbones for centuries.”