From End Group to Tunable Comonomer: Sustainable Upcycling of Waste PET Into High‐Value Added Copolyesters via a Controlled One‐Pot Transesterification Process

Physical and chemical recycling of polyethylene terephthalate (PET) often yields low‐grade materials or is costly, limiting sustainable reuse. This study employed a one‐pot transesterification reaction using value‐added CHDM. The mechanism of the CHDM‐mediated depolymerization–repolymerization was systematically investigated, proposing a two‐step reaction mechanism based on hydroxyl–ester transesterification (HET) and ester–ester transesterification (EET). The role of CHDM from end groups to copolymer units was clarified, providing theoretical guidance for the upcycling of waste PET. This process achieves direct one‐pot upcycling without intermediate separation. By adjusting the feed ratio of CHDM to PET, recycled PECT copolyester (r‐PECT) can be obtained. The physical properties of this material are comparable to virgin PETG. Films processed from waste PET exhibit excellent optical transmittance and high cell viability, supporting their potential applications in packaging and biomedicine. Furthermore, the strategy demonstrates broad applicability, successfully applied to various polyester wastes. Technoeconomic analysis predicts a 30.8% material cost reduction versus virgin PETG. Life cycle assessment shows that the global warming potential of 1 kg of heat‐shrinkable film produced through this route is 1.10 kg CO ₂ ‐eq kg ⁻¹ , 64.7% lower than for virgin PETG film. Thus, this work provides a reliable and feasible upcycling pathway for industrial applications.