The present invention relates to processing aids for the processing of synthetic polymer compositions based on carboxylic esters, where the processing aids include, partial esters which contain at least 18 carbon atoms, have a number-average molecular weight of from 300 to 10,000, and can be prepared by the reaction of saturated or unsaturated aliphatic and/or cycloaliphatic polycarboxylic acids or anhydrides of these with saturated or unsaturated aliphatic, cycloaliphatic and/or aromatic monoepoxides and/or monoalcohols optionally containing ether groups, where at least 10% and not more than 90% of the carboxyl groups of the polycarboxylic acids have been reacted and the polycarboxylic acids and anhydrides of these contain from 8 to 100 carbon atoms. The invention further relates to the use of these processing aids in synthetic polymer compositions, and also to synthetic polymer compositions, which comprise these processing aids.