Unraveling the Unknown Effects of Impurities toward a Full Circular Economy of Polystyrene

Polystyrene (PS) is a widely used plastic but suffers from low recycling rates, contributing to significant environmental challenges. Catalytic depolymerization of PS produces styrenic liquid (SL), primarily composed of its monomeric form, i.e., styrene, offering a pathway for achieving a circular economy. However, the presence of impurities in SL disrupts polymerization, leading to the production of low-molar-mass PS. In this study, we systematically identified the critical molecular structures in SL responsible for poor polymerization performance, such as benzyl-containing molecules and α-substituted styrene, which act as chain transfer agents and/or retard polymerization through thermodynamic effects. Distillation process was employed to remove these impurities, yielding distilled SL with ∼92% styrene content. Polymerization of distilled SL produced high-molar-mass PS (up to 150 kDa), exhibiting comparable thermal and mechanical properties to virgin PS. Furthermore, the depolymerization of repolymerized PS demonstrated consistent behavior, demonstrating the feasibility of a fully circular PS recycling system. This work provides insights into impurity effects and establishes practical solutions for enabling sustainable and scalable PS recycling.