Polycondensation equilibrium and the kinetics of the catalyzed transesterfication in the formation of polyethylene terephthalate

The polycondensation equilibrium constant for bis(β‐hydroxyethyl) terephthalate was found to be 0.50. By means of this value for determining the relative reactivity of two kinds of hydroxyl groups, the kinetic data were found to fit a third‐order reaction for the transesterification: one order each with respect to hydroxyl group, methoxy group, and catalyst. An order with respect to hydroxyl group, which had previously been in doubt, was proved in a series of experiments in which the concentration of ethylene glycol was varied. Third‐order rate constants were determined for the reactions catalyzed by lead, zinc, and calcium salts, and the activation energies were found to be 12.5, 15.0, and 14.5 kcal./mole, respectively. The results are consistent with the author's theory of aromatic transition states and are difficult to explain on the basis of ionic theories of reaction mechanism.