Synthesis and Characteristics of a Biobased High-Tg Terpolyester of Isosorbide, Ethylene Glycol, and 1,4-Cyclohexane Dimethanol: Effect of Ethylene Glycol as a Chain Linker on Polymerization

A solution for overcoming the low reactivity of terephthalic acid and isosorbide (ISB) is proposed that uses 1,4-cyclohexane dimethanol and ethylene glycol. Using the different reactivities, volatilities, and degree of steric hindrances among the three diols, a highly heat-resistive biobased terpolyester (PEICT; glass transition temperature = 93–143 °C) was synthesized with a high degree of polymerization (weight-average molecular weight 65 400; number-average molecular weight 25 400). After esterification, most of the oligomer end groups were found to consist of ISB, which decreases the overall reactivity of transesterification due to its characteristics. However, this end group changed gradually into ethylene units, which accelerated the transesterification and chain growth in the polycondensation process via chain scission at the carbonyl carbon adjacent to the ethylene unit. To substantiate this mechanism, the Fukui function was used to calculate the reactivity difference between monomers. The sequence distribution was analyzed using 13C-nuclear magnetic resonance to elucidate the function of each diol unit in transesterification. Finally, a polycondensation process for the PEICT terpolyester is proposed.