Depolymerization of Polymethacrylates with Ball-Mill Grinding

Abstract

In this report, we demonstrate that polymethacrylates can undergo appreciable depolymerization, with mild conditions and without reactive end groups, using ball-mill grinding (BMG). For instance, the important commercial polymer, poly­(methyl methacrylate) (PMMA) underwent up to 41% depolymerization to monomer within 8 min of grinding. We explored the factors that influence the depolymerization conversion and found temperature (heating to only 43 °C), addition of a small amount of liquid (μL of tert-butyl alcohol), and ball number/size to be the most important parameters. We also applied this method to other polymethacrylates and found the pendent substituent to influence the extent of depolymerization. Addition of a liquid was found to reverse the effects of the substituent that were observed with bulk grinding. For instance, among the polymethacrylates studied, PMMA had the lowest depolymerization conversion in bulk and one of the highest depolymerization conversions in a slurry. Notably, the molecular weight of the residual polymer is remarkably low, ca. 1 kDa, demonstrating the excellent degradation performance of BMG. Overall, this work provides important mechanistic insight into depolymerization reactions under ball-milling conditions and represents an important step in expanding BMG as a tool for the chemical recycling of polymethacrylates.