A catalyst comprising an aluminoxane and a transitional metal component comprising at least one zirconocene of the formula I and at least one zirconocene of the formula Ia or alternatively, at least two zirconocenes of the formula I ##STR12## 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 -ary