tion-metal component, at least one zirconocene of the formula I and at least one zirconocene of the formula Ia or alternatively at least 2 zirconocenes of the formula I ##STR4## in which R1 and R2 are identical or different and are a hydrogen atom, a C1 -C10 -alkyl group, a C1 -C10 -alkoxy group, a C6 -C10 -aryl group, a C6 -C10 -aryloxy group, a C2 -C10 alkenyl group, a C7 -C40 -arylalkyl group,