A radical way to upcycle polymers

By selectively pulling hydrogen atoms from branched polyolefins, chemists have come up with a way to chemically modify common polymers. The chemistry could expand the life span of these polymers beyond single-use plastics by upcycling them into higher-value materials, polymers that are easier to recycle, or both. The University of North Carolina at Chapel Hill’s Frank Leibfarth presented the method Wednesday in a symposium organized by the Polymeric Materials: Science and Engineering Division at ACS Spring 2021, a meeting of the American Chemical Society.

The branched polyolefins polypropylene, low-density polyethylene, and linear low-density polyethylene (LLDPE) account for more than 40% of the world’s plastic production, according to PlasticsEurope, but very little of that material gets recycled. Leibfarth’s group focuses on turning branched polyolefins into materials that are either more economically valuable or more sustainable. “In the ideal case, they would actually be both,” Leibfarth said.

Jill Alty, who recently completed her PhD in Leibfarth’s lab, collaborated with UNC’s Erik Alexanian to identify a reagent that could functionalize C–H bonds in branched polymers via a radical mechanism. Chemists have tried this method in the past, but earlier reagents tended to remove tertiary hydrogens, which resulted in the polymer backbone’s breaking at that site. That breakage degraded the material’s quality by reducing the polymer’s molecular weight. A couple of years ago, the UNC chemists found they could preferentially pluck off primary and secondary hydrogens from branched polyolefins using a bulky radical-generating reagent. But part of this reagent wound up incorporated in the polymer, limiting its use.

The chemists recently expanded the types of substituents that can be incorporated in the polymer by designing a new radical-generating reagent that creates radicals on the polymer chain. Then, a separate radical-trapping reagent adds substituents to the radical sites. The team used the reaction to turn LLDPE into an ionomer—a type of high-value, charge-containing material. Popular ionomers include Dow’s copolymer Surlyn, a material that’s used on the outside of golf balls. Surlyn must be made with ethylene at pressures above 100 MPa. Leibfarth says the new radical method can create ionomers from LLDPE in an extruder—a standard piece of polymer-making equipment that’s basically an oven with a screw in it.

“The ability to functionalize commercial polyolefins in a facile manner and convert them into higher value products is the essence of polymer upcycling,” Stuart Rowan, a polymer expert at the University of Chicago who was familiar with the radical reaction, said in an email. He called Leibfarth’s work “an elegant method to functionalize polypropylene and convert it into ionomers. It will be interesting to see how far his team can push this technology and develop a new, valuable route to upcycling commodity plastics.”