A copolymer obtainable by free-radical copolymerization of (i) monomer units of the structure M1 in which the variables are each hydrogen, alkyl groups, carboxyl groups or carboxyl derivative moieties, and (ii) monomer units of the structure M2 in which R5 is the radical of a carboxylic ester of the formula -A-CO—O—R10 where A is an alkylene group and R10 is a hydrocarbyl radical, and in which R6, R7 and R8 are each hydrogen or alkyl radicals, and subsequent polymer-analogous reaction of the product formed with a long-chain hydrocarbyl alcohol. This copolymer is suitable for improving the cold flow properties of middle distillate fuels, especially those which consist of or comprise biofuel oils.