Thermosets are durable plastics that can hold their shape indefinitely—but that makes them notoriously difficult to recycle. A new, degradable thermoplastic made by researchers at the Massachusetts Institute of Technology breaks this mold, offering good performance and degrading on command.
Led by organic chemist Jeremiah Johnson, the team started with polydicyclopentadiene (pDCPD), a tough, lightweight thermoset used for automobile and aviation components. They introduced cleavable bonds in the polymer strands using a monomer that contains a silyl ether group. This resulted in materials that fully degrade when exposed to fluorine (Nature 2020, DOI: 10.1038/s41586-020-2495-2). The new polymer is “virtually the same in all of its major properties” as pure pDCPD, Johnson says, and the degraded fragments can be reincorporated into new polymerization reactions to make a partially recycled plastic.
After their initial success, Johnson’s team tried a second approach. pDCPD owes its strength not only to its long polymer strands, but also the dense junctions that cross-link them together. So the researchers wondered if placing cleavable bonds in these junctions would have the same effect. But installing silyl ether bonds in the cross-links couldn’t produce a degradable plastic, no matter how many cleavage sites they incorporated. This suggests that having cleavable bonds is not enough—it also matters where researchers place them.
When he saw the MIT group’s results, says University of Illinois at Urbana–Champaign polymer chemist Jeffrey Moore, “my first reaction was, why didn’t we do this years ago?” Moore, who was not involved in the study, says this simple design shows “you don’t have to compromise” between structural performance and degradability. He is also intrigued by the way Johnson’s team “elegantly” characterized the degraded fragments.
By breaking the polymer network into soluble chunks of a controlled size, the researchers were able to directly interrogate the molecular interactions that hold these polymer networks together at higher resolution than ever before, Johnson explains.
Johnson hopes this strategy of carefully placing cleavable bonds will help make degradable versions of other thermoset materials, such as epoxies and vulcanized rubber. Moore agrees that the proof of concept in pDCPD will likely spark interest in other thermoset systems.
Moore says that scientists often focus on engineering new material properties with little thought to what happens after that material has done its job. This new thermoset shows how a plastic’s deconstruction can be conceived alongside its synthesis, he says.