VIII ##STR3## in which R9, R10, R11 and R12 are identical or different and are a hydrogen atom or a C1 -C8 -alkyl radical, at temperatures from -78??? to 150??? C. and under a pressure of from 0.01 to 64 bar, in the presence of a catalyst which is composed of an aluminoxane of the formula IX ##STR4## for the linear type and/or of the formula X ##STR5## for the cyclic type, where the radicals R13 i