The highly stable carbon–carbon backbone of polystyrene (PS) leads to severe environmental accumulation. To overcome the polymer deterioration and low economic value associated with traditional mechanical recycling and pyrolysis, the molecular-level conversion of waste PS into high-value chemicals and advanced materials is recognized as a critical pathway. This review categorizes the chemical upcycling of PS into two main pathways: “Breaking the Backbone” (destructive cleavage) and “re-using the skeleton” (non-destructive editing). While systematically summarizing multi-energy-driven (e.g., thermal, photochemical, and electrical) deconstruction strategies that cleave the inert C(sp3)–C(sp3) main chain, this review also examines site-selective C(sp2)–H post-polymerization modification. By introducing functional groups onto the intact polymer skeleton, waste PS is converted into value-added materials. Preliminary life-cycle assessments (LCA) indicate that preserving the skeleton mitigates the energy penalty of complete backbone cleavage, offering a pragmatic and low-carbon route for plastics recycling.
