Upcycling of PET Waste by Incorporating a Malic Acid-Based Monomer for High Performance and Chemically Recyclable Copolyesters

Herein, a highly rigid monomer, 2-(2-hydroxyethyl)-1,3-dioxoisoindoline-5-carboxylic acid (HTI), was readily prepared from biosourced malic acid. A series of PET copolyesters were fabricated through an in situ chemical upcycling strategy, in which HTI units were inserted into the backbones of the PET waste via a depolymerization-repolycondensation process. The resulting copolyesters demonstrated outstanding thermal stability (T5% > 400 °C) and excellent optical transparency, with transmittance at 700 nm ranging from 86.8% to 90.6%. Incorporation of HTI units significantly improved the glass transition temperature and mechanical property of PET; notably, the copolyester containing 41 mol % HTI exhibited a tensile strength of 62.8 MPa and a Young’s modulus of 1.66 GPa. Furthermore, these PET copolyesters underwent complete methanolysis at 160 °C, enabling closed-loop recycling of the initial monomers in high yields. This work provides a new strategy for upcycling PET waste using biobased monomers, which incentivize both improved thermal-resistance, mechanical properties, and plastics reclamation.