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Kinetic Phenomena in Mechanochemical Depolymerization of Poly(styrene)
Summary
Researchers studied the kinetics of breaking down polystyrene plastic into its monomer styrene using mechanochemical ball milling at room temperature. They found that styrene is produced at a constant rate during milling, and that continuous removal of the monomer is essential to prevent it from repolymerizing. Iron surfaces and molecular oxygen were shown to promote the depolymerization process, offering insights for improving mechanical recycling of plastic waste.
Synthetic polyolefinic plastics comprise one of the largest shares of global plastic waste, which is being targeted for chemical recycling by depolymerization to monomers and small molecules. One promising method of chemical recycling is solid-state depolymerization under ambient conditions in a ball-mill reactor. In this paper, we elucidate kinetic phenomena in the mechanochemical depolymerization of poly(styrene). Styrene is produced in this process at a constant rate and selectivity alongside minor products, including oxygenates like benzaldehyde, via mechanisms analogous to those involved in thermal and oxidative pyrolysis. Continuous monomer removal during reactor operation is critical for avoiding repolymerization, and promoting effects are exhibited by iron surfaces and molecular oxygen. Kinetic independence between depolymerization and molecular weight reduction was observed, despite both processes originating from the same driving force of mechanochemical collisions. Phenomena across multiple length scales are shown to be responsible for differences in reactivity due to differences in grinding parameters and reactant composition.