ing, as the transition-metal component, at least 2 metallocenes of the formula I ##STR4## which are stereorigid and whose moiety formed by Zr and the substituents R1 -R4 has Cs symmetry or slightly distorted Cs symmetry, and in which R1 and R2 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 -alkyl group, a C1 -C10 -alkoxy group, a C6 -C10 -aryl group, a C6 -C10 -arylo