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

Commodity polymer upcycling methods, which turn plastic waste into new functional polymers, represent 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 method. In most cases, full conversion was achieved, yielding primarily the dearomatized skipped diene repeat unit (up to ca. 80%), with minimal cross-linking and without chain scission. For PS derivatives, high reduction performance was maintained, and reductive defunctionalization was observed for halogenated- and sulfonated-PS derivatives. Importantly, full conversion of waste PS samples was achieved on a gram scale without increasing the reaction time. The resulting reduced polymer could also be cross-linked via thiol-ene reactions, giving network materials with distinct properties from PS. This study represents an important step toward developing a sustainable upcycling method for PS waste plastics.