New life for old polyethylene

Sandwich bags, milk jugs, and other plastic items made from polyethylene could find new life as upgraded materials. A reaction that converts the popular polymer’s C–H bonds to C–N bonds and works on post-consumer materials now opens an avenue to new polymer properties.

“With this reaction, you can now take all of the different polyethylenes—low density, high density, high molecular weight, low molecular weight—and then place a variety of groups on the polymer by making the C–N bond,” says John F. Hartwig, a chemistry professor at the University of California, Berkeley, who led the project.

It’s possible to dial in a polymer’s properties by changing the substituents on the nitrogen that’s added to the polymer, says Nicodemo Ciccia, a graduate student in Hartwig’s lab and the paper’s first author. “The goal was to develop one single method that would allow you to get a whole library of materials,” he says.

The reaction is a variation on work Hartwig’s lab reported in 2014, where the chemists used a copper-catalyzed reaction to transform C–H bonds in small alkanes into C–N bonds. But applying that chemistry to polyethylene was not straightforward. The scientists had to rethink the ligand they used. They discovered that a ligand with long, alkyl side chains worked best ( Science 2023, DOI: 10.1126/science.adg6093).

While the researchers worked out most of the chemistry by dissolving the polyethylene in a solvent, they also used the reaction on melted low-density polyethylene without any solvent (shown).

“One of the motivations was to take the waste plastics and convert them into something of higher value,” Hartwig says. But the chemists didn’t know if their reaction would tolerate the impurities, colors, stabilizers, and plasticizers in post-consumer plastics. Their experiments indicate that it does.

Frank Leibfarth, an expert in polymer chemistry at the University of North Carolina at Chapel Hill, who was not involved with the work, says the reaction is a creative way to attach amides to polyethylene. “This method will provide polymer scientists a new tool to leverage the most abundant plastic waste feedstocks for new materials discovery efforts, which could contribute an important piece of the puzzle toward realizing a more circular plastic economy,” he says in an email.