Quaternized copolymer

A quaternized copolymer obtainable by: (A) copolymerization of straight-chain, branched or cyclic, ethylenically unsaturated C2 to C100 hydrocarbons, which may bear oxygen- or nitrogen-functional substituents which cannot be reacted with amines to give amides or imides or with alcohols to give esters, with ethylenically unsaturated C3- to C12-mono- or dicarboxylic acids or acid derivatives, which can be reacted with amines to give amides or imides or with alcohols to give esters, to give a copolymer with a number-average molecular weight of 500 to 20 000; (B) partial or full amidation or imidation or esterification of the carboxylic acid functions by reacting them with oligoamines having 2 to 6 nitrogen atoms or alcoholamines, each of which comprises primary or secondary nitrogen atom or a hydroxyl group and a quaternizable tertiary nitrogen atom; (C) partial or full quaternization of the tertiary nitrogen atom with a quaternizing agent. The quaternized copolymer thus prepared is suitable as a fuel additive in direct-injection diesel engines.