A maleimide/.alpha.-olefin copolymer composed of structural units of the formula I ##STR7## where R.sup.1 is a tetramethylpiperidinyl radical of the formula II ##STR8## R.sup.3 is hydrogen, C.sub.1 -C.sub.6 -alkyl, formyl, C.sub.2 -C.sub.6 -alkanoyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.5 - or C.sub.6 -cycloalkoxy, cyanomethyl, 2-hyrdoxy-ethyl, benzyl or a radical of the formula --CR.sup.4 =CH--CO--OR.sup.5, R.sup.4 is hydrogen, C.sub.1 -C.sub.6 -alkyl or a radical of the formula --CO--OR.sup.5, and R.sup.5 is C.sub.1 -C.sub.18 -alkyl, C.sub.5 -C.sub.8 -cycloalkyl, C.sub.7 -C.sub.18 -aralkyl, phenyl or tolyl, and
- up to 8 mol %, based on II, of R.sup.1 may furthermore be hydrogen, C.sub.1 -C.sub.22 -alkyl or C.sub.5 -C.sub.8 -cycloalkyl, and
- R.sup.2 is a mixture of C.sub.18 -C.sub.28 -alkyl groups, said mixture comprising at least two C.sub.18 -C.sub.28 -alkyl groups which may differ by no more than two carbon atoms, and each of said two C.sub.18 -C.sub.28 -alkyl groups being present in an amount of at least 30% of said mixture, and
- said copolymer having a number average molecular weight of from 1,000 to 50,000.