Most plastics are recyclable, but are not actually recycled. One way to encourage recycling is to make plastics from polymers that can be chemically recycled, or broken down into monomers that can be recovered and made into new polymers. At the American Chemical Society Fall 2021 meeting, Cornell University chemist Geoffrey W. Coates presented a polymer that can be broken down into monomers and recovered from a mixture of plastics (Science, 2021, DOI: 10.1126/science.abh0626).
Speaking in a Division of Polymer Chemistry session Tuesday, Coates noted that the polyacetal polymer is stable up to 325 °C. However, once researchers combine this plastic with a strong acid catalyst and heat it above 73 °C, the polymer breaks down into its monomer constituents (shown). The monomers, including 1,3-dioxolane and derivatives, are liquids at this temperature. That makes it easy for the team to separate the monomers from a mixture of materials that don’t break down under these conditions. “We just went through our recycling bins at home,” says Brooks A. Abel, a polymer chemist now at the University of California, Berkeley, who helped develop the new plastic. They chopped up this mix of plastics, which still contained labels, glues, “and probably a little bit of Gatorade,” Abel said, and added it to a flask with their polymer. The team was able to distill out up to 98% of the monomer.
The new polymer could be used as a packing material, collected, broken down into monomers at a factory, and then reused. The polymer can only be chemically recycled under “Goldilocks” conditions, when heated in the presence of a strong acid, Coates said. “The odds are you would never have this polymer really hot, or with a strong acid, but definitely not with both,” he said. In addition, the monomer material 1,3-dioxolane is a commonly used solvent “that can be readily made from ethylene glycol and formaldehyde, two molecules that are very plentiful,” Coates said.
Not only is the polymer easy to recycle, he said, the group can also control its mechanical properties by carefully controlling the polymers’ lengths. The key was adding a pyridine proton trap to grab excess water from the reaction. Water can stop polymerization before the polymer chains get to the desired length. To link the monomers, they use an indium catalyst to rip off a bromide anion from the end of the chain, creating a carbocation. This reactive species then attacks another monomer, adding it to the end of the chain.
Craig Hawker, a materials scientist at the University of California, Santa Barbara, called the research groundbreaking. “This work capitalizes on an innovative approach: combining known building blocks and commercial initiators with a novel indium catalyst to produce high molecular weight materials,” he said. The depolymerization method is a major step forward, and permits a sustainable and circular life cycle, Hawker said.
Coates and his team are currently working on determining if the polymer is biodegradable, and optimizing the polymerization reaction with a zinc catalyst, as the metal is more abundant than indium.