Kinetic insights into mechanochemical polypropylene depolymerization during ball milling

Mechanochemistry and specifically ball milling drives the depolymerization of commodity polymers with potential application for plastic recycling. The depolymerization is initiated by homolytic cleavage of the polymer chains and monomers cleave off from these activated chain ends. The efficiency of the process is determined by the cleavage rate but also by the number of monomers produced per chain end. This is influenced by side reactions such as recombination and disproportionation. The rates of these reactions are difficult to assess due to the transient nature of the radical intermediates. Here, we used radical scavenging combined with infrared spectroscopy to determine radical formation rates arising from backbone cleavage of polypropylene and perform microkinetic modelling of subsequent reactions. Monomer formation rates were determined with a ball mill setup that allows for online analysis of gaseous products. Microkinetic modeling revealed that reaction rate constants determined for thermal degradation underestimate the monomer formation rate during ball milling. This provides convincing evidence for the previously speculated destruction of a radical cage by the ball mill action. The termination reactions are suppressed by separating radicals in space through rigorous movement of the grinding spheres, increasing the efficiency of depolymerization.