Kinetics of Ternary Copolycondensation for High-Performance PCTG Copolyester with Ester Interchange, Polycondensation, and Mass Transfer

Poly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate) (PCTG) is a high-performance engineering copolyester prepared using terephthalic acid (TPA), ethylene glycol (EG), and 1,4-cyclohexanedimethanol (CHDM). In this work, a complex reaction network of PCTG copolyesters involving homopolymerization and cross-polymerization was presented and the corresponding reaction mechanism was proposed. The reaction kinetics and mass transfer behavior were investigated using static thin film experiments, wherein the variation of the hydroxyl end-group concentrations of EG and CHDM with reaction time was separately determined. The results showed that the apparent reaction rate benefited from the high temperature, thin film thickness, and high vacuum conditions. Based on the pseudosteady-state assumption, the apparent reaction rate model coupled with mass transfer was evaluated and the model parameters including the reaction rate coefficient, equilibrium constant, and mass transfer coefficient were obtained, which were further used to simulate the polycondensation process. The concentration variation of EG and CHDM, and the rate variation of each reaction during polycondensation are discussed by combining the effects of homopolymerization, cross-polycondensation, and mass transfer. The concentration of EG was more sensitive to pressure than that of CHDM, which promoted the production of CHDM at high pressures. The proposed model is expected to provide guidance for the optimization of industrial production of PCTG/PETG and generalize to other copolyester systems.