the lower alkyl ester of terephthalic acid, and thereafter adding to the reaction system a phosphorus compound of the formula ##STR3## wherein R is a member selected from the group consisting of the alkyl, aryl and hydroxyalkyl groups, n is selected from the class consisting of 0 and 1, l is a whole number up to 2, including 0, m is a whole number from 1 to 3, with the proviso that the sum of l a