Kinetic Phenomena in Mechanochemical Depolymerization of Poly(styrene)

Abstract

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 solidstate 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.

Depolymerization of Polymethacrylates with Ball-Mill Grinding

“…7% depolymerization conversion for PS, and proposed a mechanism in which the depolymerization was initiated by PS-peroxy radical species (which were detected by ESR) . Sievers and co-workers further supported the importance of oxygen in PS depolymerization and proposed that the more thermodynamically stable peroxy radical chain ends may prevent recombination (or other side reactions) and may also provide a more thermodynamically favored pathway for depolymerization . Notably, in our previous studies with PMS, the milling atmosphere (air vs Ar) did not significantly influence the depolymerization conversion .…”

“…Notably, in our previous studies with PMS, the milling atmosphere (air vs Ar) did not significantly influence the depolymerization conversion . Sievers and co-workers proposed that the low T c of PMS (61 °C) allows the depolymerization process to occur spontaneously due to favorable thermodynamics (and therefore oxygen has negligible influence) . Even though PMMA has a lower T c than PS (220 °C vs 310 °C, pure monomer values), which likely explains the higher depolymerization conversions for PMMA, it is still much higher than the grinding temperature (43 °C).…”

“…Previous ESR studies have already established that PMMA reacts with oxygen to form peroxy radicals . Peroxy radical chain ends have been proposed to play an important role in the depolymerization of polystyrene (PS). , Balema et al reported up to ca. 7% depolymerization conversion for PS, and proposed a mechanism in which the depolymerization was initiated by PS-peroxy radical species (which were detected by ESR) .…”

Modeling Mechanochemical Depolymerization of PET in Ball-Mill Reactors Using DEM Simulations

“…One promising alternative is the depolymerization of polymers in the solid state via mechanically-induced reactions. − Mechanochemical reactions are typically performed in ball mills, in which contacts and collisions between grinding surfaces (balls and reactor wall) supply the energy required to chemically transform the (usually particulate) solid reactants caught between these surfaces. ,− Mechanochemistry has been successfully demonstrated on a laboratory scale for the production of lignocellulosic biomass, , cellulose, − ammonia − and lignin. − Particularly for poly(ethylene terephthalate) (PET), Štrukil and Tricker et al recently demonstrated its complete depolymerization to monomers inside ball mills. Moreover, mechanochemical routes have recently been explored for the depolymerization of various polymers such as polystyrene (PS), , polyethylene (PE), and poly(α-methylstyrene) (PMS) and in the dechlorination of polyvinyl chloride (PVC). , In addition to the ability to efficiently process solid reactants, ball milling is a highly scalable industrial process being utilized in a wide variety of grinding applications, from minerals and cement, to chemicals and pharmaceuticals. − Despite these advantages, mechanochemical reactions are often seen and modeled as “black-boxes”, which hinders the fundamental understanding of mechanically induced reactions . In attempts to model mechanochemical reactions, semiempirical models have been proposed across various branches of mechanochemistry. ,,,, However, these models are often limited by extrapolation issues, which restrict their utility in exploring conditions such as reactor geometry or grinding media material that would lead to optimal performance.…”