Studies on the formation of poly(ethylene terephthalate): 3. Catalytic activity of metal compounds in transesterification of dimethyl terephthalate with ethylene glycol

The rate constant of transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG) in the presence of various metal compounds as catalysts at 197°C, calculated from the quantity of formed methanol, was used to evaluate each metal compound in its catalytic activity. First, in the case of highly basic metal salts the rate constants were found to be extremely large in the initial stage of the reaction. These values depended on the basicity of the metal salts, but decreased rapidly with the progress of the reaction, and reached values which were not appreciably dependent on the basicity. Secondly, in the case of metal salts of lower basicity such a phenomenon was not observed, and single rate constants were recognized throughout the reaction period. These rate constants were logarithmically correlated by a straight line (mountain-shaped) in a plot vs. stability constants (logβ1) of dibenzoyl methane (DBM) complexes of the corresponding metal species. Also in the case of highly basic metal salts, the rate constants in the latter stage followed the same straight line. Consequently, the stability constant of DBM complex of each metal species was found very useful as a forecast of catalytic activity of the metal compound. The compound of metal species with values of logβ1 ranging from 9 to 11 was most active as the catalyst.