Sustainable Terephthalic Polyesters with Medium/Long Methylene Sequence: Structure–Property Relationships and Closed-Loop Chemical Recycling

Traditional short-chain terephthalic polyesters such as poly(ethylene terephthalate) (PET) are among the most produced plastics in today’s polymer industry, while their large-scale chemical recycling has been a long-standing challenge. The long-chain polyesters have shown good recyclability under mild conditions, whereas the structure–property relationships and chemical recycling performance of long-chain terephthalic polyesters are still unclear. Herein, we synthesized a series of medium/long-chain terephthalic polyesters with different methylene sequence lengths (CH2 number in diol monomer nCH2 = 6, 8, 10, 12) and investigated their thermal properties, crystallization behavior, mechanical properties, and chemical recycling performance. All terephthalic polyesters show good crystallizability; their melting temperature decreases, and melting enthalpy increases gradually as nCH2 increases from 2 to 12. All polyesters adopt a triclinic crystal lattice while slightly different crystalline lamellar structures. The medium/long-chain terephthalic polyesters exhibit improved ductility and good melt processability, enabling them to be used as three-dimensional (3D) printing materials. In addition, the medium- and long-chain terephthalic polyesters are chemically recyclable and can depolymerize under mild conditions (e.g., 140 °C in methanol). The monomers obtained by depolymerization can be reused to synthesize again the according polyesters. This study provides new inspiration for developing sustainable high-performance polyesters with great industrial potential.