Ball Mill Grinding Mechanochemistry for Sustainable Chemical Transformations of Post-Consumer Polymer Waste

Abstract

Chemical upcycling/repurposing can transform plastic waste into value added products, helping mitigate the increasing amount of plastic waste in landfills and improving upon inefficient conventional mechanical recycling. Therefore, the development of more sustainable and broadly applicable chemistries for polymer transformations (i.e., post-polymerization modification (PPM) and depolymerization-repolymerization) is critical for the future of plastic waste management. Solid-state mechanochemistry via ball mill grinding (BMG) represents an attractive, more sustainable method for these polymer transformations, requiring less solvent, enabling shorter reaction times, and unlocking mechano-exclusive products. Detailed herein are reports of direct C-H functionalizations of post-consumer polystyrene (PS) and depolymerization-repolymerization of mixed polyester (PE) waste, both enabled by mechanical force, for access to new materials that incentivize chemical recycling. An initial proof-of-concept study revealed that small amounts of solvent, used for liquid-assisted grinding (LAG), significantly decreased the glass transition temperature of PS via plasticization, preventing mechanochemical degradation while enabling trifluoromethylation, in a first-of-its-kind PPM reaction facilitated via BMG. Mechanochemical amination was subsequently investigated as a platform for the formation of creep resistant, reprocessable, and degradable networks consisting of polyimine-thermoplastic reactive blends from post-consumer PS. While LAG can be utilized to prevent degradation, its absence was exploited for the depolymerization of complex polymer substrates. Mechanochemical aminolysis methodology was developed to enable the fast, catalyst-free, and air tolerant deconstruction of aliphatic PEs such as poly(lactic acid) (PLA), polyhydroxyalkanoates (PHAs), and mixed PLA/PHA laminated films into diol monomers. The monomers were further functionalized with α-lipoic acid and crosslinked to form reprocessable and biodegradable covalent adaptable disulfide networks (CANs) with potential applications in additive manufacturing. Overall, several novel mechanochemical PPM and depolymerization reactions were developed and applied to post-consumer polymer waste, and the mechanical and thermomechanical properties of the resultant materials reported. The methods developed throughout these thesis projects demonstrate the operational simplicity and sustainability of polymer mechanochemical transformations conducted under BMG conditions for the future of plastic waste remediation.