Heat unzips polymethacrylates

A catalyst-free method for pulling apart certain polymers could help speed the industrial adoption of controlled radical polymerization—a polymer-building reaction that’s popular with researchers but hasn’t yet found its way into many products. The new depolymerization returns polymethacrylates that are made in this way to their starting monomers with up to 92% conversion. These monomers could then be repolymerized to re-create the original polymer or combined with other monomers to create materials with different properties.

“This is a much better way than conventional recycling, where you usually start with a product and you go to a lower-end application,” says Athina Anastasaki, a polymer materials professor at the Swiss Federal Institute of Technology (ETH), Zurich, who led the depolymerization research.

Controlled radical polymerization lets chemists precisely tailor the molecular weights of the polymers they make. One type of this method, reversible addition−fragmentation chain transfer (RAFT) polymerization, does so by using capping groups, called RAFT agents. These groups sit at the end of the polymer chains and mediate the radical polymer growth.

But RAFT polymerization is more expensive than other approaches. This is in part because of the costly RAFT agents. Anastasaki reasoned that regenerating both the monomer and the RAFT agent would make the polymerization method more attractive to industry.

Chemists in Anastasaki’s lab unzip the polymethacrylate by making a dilute solution of the polymer in dioxane and heating it to 120 °C (J. Am. Chem. Soc. 2022, DOI: 10.1021/jacs.2c00963). They use a dithiobenzoate RAFT agent (example shown; dithiobenzoate in red).

Although others have depolymerized materials made from controlled radical polymerization, those examples used metal catalysts and higher temperatures, and they produced less monomer or degraded the RAFT agent.

William M. Gramlich, a polymer chemist at the University of Maine, says the work is interesting but is concerned about the dilute solutions used. Anastasaki is addressing that, as well as the use of dioxane, by using a flow system that would recycle the solvent.