Abstract
Only an estimated 9% of all plastic ever manufactured has been recycled, and the remainder persists in landfills, oceans, and, increasingly, in the human body as micro- and nanoplastics. Closing this loop demands chemistry that goes beyond shredding and remelting. This review surveys the mechanistically distinct strategies now converging on the plastic waste problem: classical end-of-life treatments and their limits; selective depolymerization, catalytic bond-breaking, and direct polymer functionalization; photochemical and electrochemical upcycling, including established semiconductor and molecular photocatalytic systems, together with a prospective metal-free, defect-engineered carbon nitride (O-VN-CN) platform developed by our group for ambient-condition processing of mixed plastic waste; enzymatic and chemo-biological recycling, exemplified by industrial PET biorecycling; and, on the front end, the molecular design of plastics whose recyclability is programmed into the monomer itself. Ten reversible and cleavable bond chemistries are organized into five design principles—reversible dynamic bonds, cleavable weak links, bidirectional controlled bonds, self-immolative cascades, and weak-C–C strategies—including a shuttle-controlled, bidirectional disulfide metathesis platform that enables clean, catalyst-free recovery of high-performance thermosets such as disulfide-doped polydicyclopentadiene. Together, these advances mark a decisive shift from managing plastic waste after disposal to engineering circularity by design, and this review closes by outlining the catalytic, biological, and materials-design frontiers most likely to determine whether that shift can be achieved at industrial scale.