A catalyst containing the combination which comprises:(a) an aluminoxane, a compound of the formula R.sub.x NH.sub.4-x, BR'.sub.4, R.sub.x PH.sub.4-x, BR'.sub.4, R.sub.3 CBR'.sub.4 or BR'.sub.3, or a mixture thereof wherein X is an integer from 1 to 4 and the radicals R are identical or different and are a C.sub.1 -C.sub.10 -alkyl or a C.sub.6 -C.sub.18 -aryl or 2 radicals R together with the atom joining them form a ring, and the radicals R' are identical or different and are a C.sub.6 -C.sub.18 -aryl which optionally are substituted by alkyl, haloalkyl or fluorine, and(b) a metallocene of the formula VI ##STR1## in which M.sup.1 is titanium, zirconium, hafnium, vanadium, niobium or tantalum,R.sup.1 is (C.sub.3 -C.sub.20)alkyl, (C.sub.6 -C.sub.14)aryl, (C.sub.4 -C.sub.10)alkoxy, (C.sub.2 -C.sub.10)alkenyl, (C.sub.7 -C.sub.20)arylalkyl, (C.sub.7 -C.sub.20)alkylaryl, (C.sub.6 -C.sub.10)aryloxy, (C.sub.1 -C.sub.10)fluoroalkyl, (C.sub.6 -C.sub.10)halogenoaryl, (C.sub.2 -C.sub.10)alkynyl; a radical --SiR.sup.6.sub.3, in which R.sup.6 is (C.sub.1 -C.sub.10)alkyl; a halogen atom; or an aromatic radical having 5 or 6 ring members in the ring, at lease one of which is a heteroatom,R.sup.2, R.sup.3 and R.sup.4 are identical or different and are hydrogen; (C.sub.1 -C.sub.20)alkyl, (C.sub.6 -C.sub.14)aryl, (C.sub.1 -C.sub.10)alkoxy, (C.sub.2 -C.sub.10)alkenyl, (C.sub.7 -C.sub.20)arylalkyl, (C.sub.7 -C.sub.20)alkylaryl, (C.sub.6 -C.sub.10)aryloxy, (C.sub.1 -C.sub.10)fluoroalkyl, (C.sub.6 -C.sub.10)halogenoaryl, (C.sub.2 -C.sub.10)alkynyl; a radical --SiR.sup.6.sub.3, in which R.sup.6 is (C.sub.1 -C.sub.10)alkyl, a halogen atom, or a heteroaromatic radical which has 5 or 6 ring members and can contain one or more heteroatoms, or adjacent radicals R.sup.1 -R.sup.4 ; with the atoms joining them, form one or more rings,R.sup.5 is hydrogen,R.sup.7 is a radical ##STR2## in which M.sup.2 is carbon, silicon, germanium or tin.