These materials incorporate thermolytically labile groups into widely used glass-substitute polymers, allowing solvent and catalyst-free degradation of the polymer into a high-purity composition of its monomers. These materials can therefore be recycled into reusable monomers at the end of their life cycle. Poly(methyl methacrylate) (PMMA) is a glass substitute used to manufacture automobiles and aircraft. Industrial sources manufacture 4 million tons of PMMA annually but recycle only 10 percent of that amount . Conventional routes to recycle PMMA require high temperatures and result in impure, difficult-to-reuse products. An efficient process for recycling PMMA into a pure composition of its methyl methacrylate monomer at an industrial scale would therefore increase the sustainability of the polymer industry.
Researchers at the University of Florida have synthesized PMMA with thermolytically labile pendent or chain-end groups that can be used to induce catalyst-free degradation into methyl methacrylate monomers. This simple recycling process takes place at lower temperatures than conventional PMMA degradation requires.
Synthesis of PMMA with thermolytically labile phthalimide groups that drive degradation into highly pure methyl methacrylate monomer
Upon formation of the PMMA, a long chain of carbon-carbon single bonds forms, two for every methyl methacrylate monomer. Recycling PMMA into a fresh polymer requires degrading this carbon backbone in a controlled manner to generate products with a high purity of methyl methacrylate monomers, so that these monomers can be reused to form a fresh polymer. One path to destabilizing the PMMA backbone is to liberate some attached functional groups, creating radicals.
For these polymers, UF scientists have created materials that enable on-demand creation of radicals in the PMMA backbone, and therefore incorporate recycling into their design. The first material is a copolymer including a small fraction of monomers containing phthalimide groups that decorate the PMMA backbone, known as pendent groups. The phthalimide pendent groups are thermolytically labile, generating radicals at high temperatures and inducing backbone fragmentation and degradation.
Alternatively, the material can generate radicals via its chain-end groups. The material is then a homopolymer constructed only from identical methyl methacrylate monomers, but a thermolytically labile phthalimide group is still present at the α-end of the chain. As before, these chain-end groups generate radicals at high temperatures and cause the polymer to degrade into a high-purity composition of reusable monomers.
