The invention relates to a novel process for the preparation of compounds of the formula I ##STR1## by polycondensation of a diol of the formula II ##STR2## with a dicarboxylic acid ester of the formula III ##STR3## In the above formulae, the substituents and symbols R.sub.1, R.sub.2, R.sub.3 and n are as defined in claim 1.The process comprises stirring the reaction medium which contains an excess of 2 to 50 mol. %, based on the diol of formula II, of the dicarboxylic acid ester of the formula III, in the presence of at least 0.03 mol. % of a suitable catalyst, until the bulk of the alkanol of the formula R.sub.3 OH formed during the reaction is distilled off, under a pressure of 0.7 to 1.3 bar, and thereafter stirring under a pressure of less than 5 mbar, in the temperature range from 100.degree. to 170.degree. C., and subsequently working up the resultant melt in conventional manner.