Even though many plastics are recyclable, most are not recycled because of the cost and labor needed. To help make plastic recycling easier, chemists want to find new polymers that require less work to reuse, but still have the same mechanical and physical properties that make current plastics useful.
Moving closer to this goal, Junpeng Wang and colleagues at the University of Akron developed a polymer that scientists can break down into monomers and then put back together to reuse (Nat. Chem., 2021, DOI: 10.1038/s41557-021-00748-5). These compounds are highly tunable, so chemists can change the properties of the polymer by swapping out the functional groups on the polymer backbone. Because of this variability, the researchers think that the polymers could find multiple uses, including as plastics or rubbers.
Wang and coworkers synthesized their polymers from cyclooctene monomers with a four-membered cyclobutane ring fused to the eight-member ring. The chemists can polymerize the cyclooctene compound below room temperature in 67% yield, and then get 90% of the monomer back at 50 °C, using the same ruthenium catalyst (shown). The reaction is controlled by the concentration, Wang says. “At high concentration, polymer forms and at low concentration, monomer forms,” he says. The polymer is stable up to 370 °C, and will only polymerize or depolymerize when the Ru catalyst is present, Wang says.
These reversible polymers have a carbon-carbon backbone, which means their strength, stretchability, and thermal stability are comparable with those of current plastics, such as polyethylene, Wang says. To make those carbon-carbon bonds from monomers with double bonds, the team used a reaction called ring-opening metathesis polymerization. If the monomers contained just the cyclooctene, the energy due to ring strain would be high enough that the polymer wouldn’t recyclize into monomers, making the reaction irreversible. Adding the highly-strained cyclobutane to the cyclooctene changes the energy of both monomer and polymer, and makes the polymerization reaction reversible, Wang says.
“Ring strain is really a relative term,” he says. Essentially, it’s the energy difference between the closed, cyclic form of the monomer and the open, polymerized form that dictates whether or not the reaction is reversible. Through computational studies, the group found that the cyclobutane locks the polymer and monomer into similar configurations, so that both forms have comparable energies. The cyclobutane isn’t “reducing the ring strain of cyclooctene itself, but actually raising the energy of the polymer,” Wang says. Because one form is no longer more stable than the other, the polymerization reaction becomes reversible.
Making recyclable polyolefins is a huge challenge, and Wang’s approach is very promising, says Colleen Scott, a polymer chemist at Mississippi State University. The prospect of fine tuning the properties of recyclable polymers to match those of current commercial polymers is especially exciting, she says.
Wang’s team discovered this reversible polymer during the COVID-19 pandemic, when chemistry labs across the world shut down. Unable to work on synthesis reactions in the lab, Wang and his team concentrated on computational studies on the ring strain in a series of cyclooctenes, he says. A student discovered that a previously published cyclobutane-fused cyclooctene compound had surprisingly low ring strain. “I couldn’t believe it,” Wang says. “I repeated it myself several times and every time it gave us very low ring strain.” The group realized that this polymer they already had should undergo depolymerization, he says. “We tested that and it worked very well.”