Kilogram-Scale Production of Sustainable PCF Copolyesters Based on Novel Cyclic Diol THFDM Derived from 5-Hydroxymethylfurfural: Trade-Off between the THFDM Structure and Various Properties of Copolyesters

By a novel kilogram-scale production method, copolyesters of poly(1,4-cyclohexyldimethylene-co-hydroxymethyl tetrahydrofuran 2,5-furandicarboxylate) (PCTF) with the entire composition range were synthesized using a 5 L steel reactor. The composition and microstructure of copolyesters were investigated by NMR spectra, which confirmed the random distribution of segments. The 2,5-hydroxymethyl tetrahydrofuran (THFDM) content in copolyesters was lower than that of the initial feeding ratio, which was attributed to the lower reaction reactivity (giving 1.35 × 10–2 and 1.06 × 10–2 min–1 for the FDCA reaction with CHDM and THFDM, respectively) and the lower boiling point of THFDM compared with that of 1,4-cyclohexanedimethanol (CHDM). It is worth mentioning that the cyclic structure and ether groups in THFDM attained a balance between their opposing influence on the Tg of the copolyesters, resulting in only a slight change in the Tg (ranging from 89.3 to 76.2 °C). With the incorporation of THFDM, the crystallization ability of copolyesters decreased and the d-spacing increased, as confirmed by wide-angle X-ray diffraction (WAXD) and isothermal crystallization kinetics analysis. The inclusion–exclusion model proposed by the Wendling–Suter theory provided an explanation for the crystallization behavior. For the mechanical properties, the mechanical behavior could be adjusted to realize the brittle-to-ductile transition, which was significantly influenced by the crystallization degree. The investigation of the secondary relaxations caused by the transition of conformation of CHDM and THFDM rings proved the switching of the mechanical behavior. Notably, large-scale production of THFDM has been realized, making it possible to synthesize this promising material for practical usage.