The technique of G. Challa was employed for following the reaction of oligomeric poly(ethylene terephthalate) in the presence of antimony trioxide. This technique is based on determining the changes in glycol and bishydroxyethyl terephthalate concentrations with time at a given temperature. The linear trimer of ethylene terephthalate, prepared from terephthaloyl chloride and bishydroxyethyl terephthalate, was employed as starting material. Rate constants were not corrected for catalyst concentrations. The reaction was studied over the temperature range 221–251°C. The equilibrium constant k p / k ′ for the polycondensation reaction magnified image was found to be 0.36. Observed reaction rates were low but faster than in the uncatalyzed reaction studied by Challa. At 231°C. the values of the rate constants found in the presence of 0.025% by weight antimony trioxide were k p = 0.151.mole −1 hr. −1 and k ′ = 0.33 l./mole‐hr., and the redistribution rate constant was k R = 0.11 l./mole‐hr. From data at four temperatures the estimated activation energies were E p = 29 kcal. and E R = 24 kcal. The polycondensation rates were low compared with rates calculated from literature data. A mechanism to explain the difference requires that bishydroxyethyl terephthalate endgroups compete successfully with oligomer endgroups for antimony trioxide catalyst and that the bishydroxyethyl terephthalate catalyst product is unreactive in polymerization.
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Stevenson et al. (1968) studied this question.