Carbon Dioxide Is Key to Safe Production of a Sensitive Polymer

If you watch an ice hockey match, you spend all three periods looking at methacrylate, a polymer type commonly used in paints and coatings, adhesives and glass substitutes. However, using this material for more sophisticated applications than blocking hockey pucks — for example, in drug-delivery mechanisms — requires an extremely controlled process called anionic polymerization that is difficult and even dangerous to carry out.

Now Cornell chemists have developed a precisely controlled, user-friendly, scalable anionic polymerization process for methacrylate mediated by carbon dioxide (CO2). This process, useful for advancing sophisticated applications of methacrylate, is already benefiting researchers like engineers working on metal-free batteries and has potential future applications in biomedical settings. Brett Fors, Frank and Robert Laughlin Professor of Physical Chemistry in the Department of Chemistry and Chemical Biology in the College of Arts and Sciences (A&S), said, "Making anionic polymerizations more accessible and safer will enable the scientific community to use these powerful methods to make next-generation materials."

Fors is the corresponding author of the paper "Controlled Anionic Polymerization Mediated by Carbon Dioxide," published in Nature Chemistry. The first author is doctoral student Paige Jacky, M.S. '23. Alexa Easley, Klarman Postdoctoral Researcher in Chemistry and Chemical Biology (A&S), is a contributing author. Easley said that anionic polymerization is a powerful way to make materials with well-defined molecular structures, where precise control enables scientists to understand how the material's structure and properties relate to each other. Easley said, "For special applications like drug delivery, if you're submitting a plan to the Food and Drug Administration that contains an entire polymer, you need to have a well-known structure that Plexiglas does not have. Glass substitute is great for the job it does, like protecting us at sporting events, but it cannot be used for these biomedical applications that require more structural control." Current anionic polymerization methods are difficult and generally dangerous to apply; they require toxic metals, explosive or spontaneously flammable chemicals in air, or other compounds that contaminate the final material. Some of these methods require very low temperatures and are sensitive to moisture and air. By contrast, the Cornell team's method, called CO2-mediated reversible deactivation anionic polymerization (CMAP), is simple and requires just a single container. It uses a carbon dioxide atmosphere plus an initiator and works through easy-to-achieve heating rather than deep cooling. It produces well-defined materials. The simple synthesis of the initiator and the "one-pot" nature of the process make anionic polymerization an accessible tool that can be used outside specialist chemistry laboratories.

Jacky said the keys to the method are carbon dioxide and heat. Inspired by recent research on reversible initiators (chemicals used to both start and stop a reaction), she turned to carbon dioxide: in this context, relatively abundant, inexpensive and non-toxic. Typically, carbon dioxide is considered a terminating agent in anionic polymerizations, but she discovered that "if you apply heat, the process can be reversed." Carbon dioxide rapidly starts and stops the reaction. It also protects and stabilizes enolate, a key component that is highly reactive. The researchers tested the method using an apparatus that Easley developed during her Klarman Fellow research to track molecules for carbon dioxide capture.

More work needs to be done to scale the CMAP method for industry, but the researchers believe this work will inspire future similar reversible deactivation strategies for other anionically polymerizable vinyl monomers. They also hope the method will make anionic polymerization of methacrylate materials more accessible to a broader scientific community. It is already being used by some researchers, including Easley. While a graduate engineering student at Texas A&M University, Easley tried to make a polymer for a metal-free battery but the only available method — using pyrophoric initiators — did not work. "I tried many ways to do this and never succeeded," she said. But now, using CMAP, "we succeeded."