Copolymers containing A) from 45 to 54 mol % of divalent structural units of the formula 1 ##STR1## in which a and b=0 or 1 and a+b=1, B) from 35 to 53 mol % of divalent structural units of the formula 2 --H.sub.2 C--CHR.sup.2 -- (2) and C) from 1 to 25 mol % of divalent structural units derived from polyolefins, where the polyolefins can be derived from monoolefins having 3 to 5 carbon atoms, wherein a) R.sup.1 is an alkyl or alkenyl radical having 10 to 40 carbon atoms or an alkoxyalkyl radical having 1 to 100 alkoxy units and 1 to 30 carbon atoms in the alkyl radical, and b) R.sup.2 is an alkyl radical having 10 to 50 carbon atoms, c) the alkylvinylidene content of the polyolefins on which the structural units C) are based is at least 50 mol %, and d) the number of carbon atoms in the polyolefin molecules on which the structural units C) are based is from 35 to 350.