Reinventing recycling: Chemists develop depolymerization method for ultrahigh molecular weight plastic

14 Aug 2023 --- A team of US researchers has developed an alternative method of depolymerizing and recycling plastics that reportedly uses less energy without compromising plastic quality.

We speak to Dr. Brent Sumerlin, lead researcher and professor at the University of Florida’s Department of Chemistry, to learn the science behind the development. Their experimental strategy induces depolymerization of polymers meaning they revert completely to the much smaller monomer molecules from which they were originally made. 

The resulting monomer can then be used to prepare new polymers with similar or better properties than the plastics from which they were derived.

“Our report shows that by designing the polymer from the outset with eventual depolymerization in mind, we can dramatically reduce the amount of energy and time needed to revert the polymer to monomer,” he explains.

“We essentially designed the polymer chains to have sensitive units at their ends that can be triggered to break at much lower temperatures than polymers without these end groups. Once the ends of the polymers have broken off, the polymer chains rapidly unzip to release the monomers.”

The scientists focused on finding a solution for the drawbacks of industry-standard reprocessing of plastics which often produces lower-quality recycled plastic as the polymers are broken down into smaller fragments.  

“Unfortunately, this process is rather aggressive and leads to the polymer chains breaking down and losing some of the properties that we find most useful. This is where there’s an opportunity for fundamental science to help,” says Sumerlin.

Dr. Brent Sumerlin.New recycling strategies
The depolymerization study employed thermolytically labile end-groups via reversible-deactivation radical polymerization for the bulk depolymerization of polymethyl methacrylate (PMMA) – more widely known as plexiglass – at “significantly lower temperatures” than previously reported methods.

“This depolymerization methodology enables catalyst- and solvent-free reversion to monomer on a multigram scale at temperatures up to 250 degrees Celsius lower than current industrial methods. These reactions are performed in an efficient and high-yielding manner, establishing a viable route to depolymerize PMMA on large scales,” write the chemists.

Furthermore, Sumerlin stresses that while most know the importance of plastic recycling, the process is fraught with challenges and inefficiencies. “First, there’s the issue of consumer compliance – how do we ensure more plastic is submitted for recycling instead of finding its way into the environment or landfills?” 

“Once the plastic reaches the recycling facility, a whole other set of challenges presents itself, with the most obvious being that plastics of various types need to be sorted because it’s difficult, for various reasons, to produce anything useful from mixed recycled plastics.”

He argues that polymer chemists in the last century succeeded in creating materials that were “too robust, lasted too long and cost too little.” But today, there is an appreciation among chemists that they’re the ones who are best positioned to solve these problems.

“While plastics are amazing materials, we all realize the importance of considering their ultimate fate after usage to address concerns about their persistence in the environment,” Sumerlin states.

The experimental work carried out by a team of researchers in Sumerlin’s group and led by graduate students James Young and Rhys Hughes is published in the Chem journal. 

“Our work is a response to the call to action proposed by the UN Sustainable Development Goals,” concluded Sumerlin. “New recycling strategies have become imperative to reduce the negative impact of plastic on the environment.”

By Radhika Sikaria