Process for production of saturated, high-molecular weight polyesters

Claim 1

In a two-stage process for the production of saturated, high-molecular weight polyesters whose acid component consists at least 70 molar percent of terephthalic acid and whose diol component consists at least 70 molar percent of 1,4-butanediol wherein the first stage is an interesterification reaction conducted at 150°-250° C. in the presence of an interesterification catalyst and wherein the second stage is a polycondensation reaction conducted at 200°-300° C. at reduced pressure in the presence of a polycondensation catalyst, the improvement which comprises conducting the polycondensation reaction in the presence of 0.005 - 2% by weight, based on the terephthalic acid, of a compound of the general formula ##STR2## having a molecular weight of 1,000 - 1,000,000 wherein a and b are each positive integers from 1 to 100; n is a positive integer from 10 to 10,000; R 1 and R 2 are each hydrogen, alkyl of 1-12 carbon atoms or phenyl; and R 3 and R 4 are each hydrogen, alkyl or hydroxy-alkyl of 1-4 carbon atoms, said compound either being added before the interesterification reaction, or during the interesterification reaction up to the beginning of the polycondensation reaction or, when the compound is a free acid, at any time up to the end of the polycondensation reaction, or said compound being formed during the interesterification or polycondensation reaction.