You often hear plastic pollution referred to as a problem. But the reality is that it's a lot of problems. Depending on the properties we need, we form plastics from different polymers, each of which is held together by a special type of chemical bond. So the method we use to break down one type of polymer may not be compatible with the chemistry of another.
This problem is why, even though we have been able to find enzymes that break down common plastics such as polyesters And PETThis is only a partial solution to the problem of plastic waste. However, researchers are not sitting back and enjoying the triumph of partial solutions, and they now have very sophisticated protein design tools that can help them.
This is the story of a brand new enzyme that researchers have developed to break down polyurethane, a polymer commonly used to make foams, among other things. The new enzyme is compatible with an industrial recycling process that breaks down the polymer into its basic building blocks, which can be used to form fresh polyurethane.
Destruction of polyurethane
Basics of chemical bonds that bind polyurethanes. The rest of the polymer is indicated here by the letters X.
A new paper describing the development of this enzyme reveals the scale of the problem: in 2024, we produced 22 million metric tons of polyurethane. The urethane bond that defines them involves nitrogen bonded to carbon, which in turn is bonded to two oxygen atoms, one of which is bonded to the rest of the polymer. The remainder of the polymer linked by these bonds can be quite complex and often contains ring structures related to benzene.
Digesting polyurethanes is a challenging task. Individual polymer chains are often highly cross-linked, and bulky structures can make it difficult for enzymes to access bonds they can digest. A chemical called diethylene glycol can break down some of these molecules, but only at elevated temperatures. And this leaves behind a complex mess of chemicals that cannot be used in any useful reactions. Instead, it is usually incinerated as hazardous waste.
