Hydrogen-Bond-Assisted Activation Strategy for Accelerated Sustainable Copolyester Synthesis Using rBHET Recovered from PET Glycolysis

Abstract

This study examines how structurally distinct C8 diols regulate the melt polycondensation of dimethyl 1,4-cyclohexanedione-2,5-dicarboxylate (DMSS) with recycled bis(2-hydroxyethyl) terephthalate (rBHET) recovered from post-consumer poly(ethylene terephthalate) bottles. The effects of 1,4-cyclohexanedimethanol (CHDM) were compared with those of aromatic 1,4-benzenedimethanol (BDM) and linear 1,8-octanediol (ODD). Fourier-transform infrared, 1H nuclear magnetic resonance (NMR), X-ray diffraction (XRD), melt-torque, thermal, molecular-weight, and density functional theory (DFT) analyses indicated that CHDM-containing systems undergo faster melt-viscosity development at lower final polycondensation temperatures than BDM- and ODD-containing analogues. The binary DMSS-CHDM and ternary DMSS-rBHET-CHDM systems reached a rapid viscosity build-up at 135 and 150 °C with weight-average molecular weight (Mw) values of 5.95 × 105 and 9.45 × 105 g mol–1, respectively, whereas the CHDM-free systems required higher final temperatures (220 °C) under comparable conditions. Spectroscopic changes in the carbonyl/unsaturated vibration region and the enol-associated 1H NMR resonance at δ 3.18–3.19 ppm, together with DFT calculations, are consistent with hydrogen-bond-assisted stabilization of an enol-associated DMSS environment. XRD and scanning electron microscopy further showed that CHDM-containing copolyesters developed more ordered chain packing, with crystallinities of 80% for DMSS-rBHET-CHDM and 71% for DMSS-CHDM, compared with 73 and 46% for the BDM- and ODD-containing systems, respectively. These results suggest that diol topology and hydrogen bonding capability can influence DMSS reactivity, chain growth behavior, and solid-state organization, providing a route to resource-efficient PET-derived copolyesters.