Waste Polystyrene Upcycling via the Birch Reduction with Ball-Mill Grinding

The development of commodity polymer upcycling methods, which can turn plastic waste into new functional polymers, is an important approach to reducing the burden of plastic on the environment. Here we report a Birch reduction that is compatible with polystyrene (PS), PS derivatives, and several types of waste PS using a 1 min ball-mill grinding approach. In most cases, full conversion of the PS repeat units was achieved, yielding primarily the dearomatized skipped diene repeat unit, without chain scission, and with minimal cross-linking. With our optimized conditions, a PS standard was converted to the reduced product with a repeat unit composition of ca. 79% skipped diene, 8% conjugated diene, 8% monoene (from over-reduction), and 7% multi-substituted arene (presumably from reductive arene coupling) units. For several PS derivatives, high reduction performance was maintained, and reductive defunctionalization was observed for halogenated- and sulfonated-PS derivatives without the need to alter the protocol. This is relevant as it couples upcycling and removal of chlorinated pollutants in one permutation. Importantly, full conversion of waste PS samples could be achieved on a gram scale without increasing the 1 min reaction time. Additionally, we demonstrate that additives in the waste material do not slow or hamper the reaction in reaching full conversion. This study represents an important step toward developing a sustainable upcycling method for PS waste plastics.