Catalyst system, the catalyst system comprising (i) at least one metallocene complex of formula (I), (I) wherein Mt1 is Hf, X is a sigma-donor ligand, R1, R2, R3 are the same or different from each other and can be hydrogen or a saturated linear or branched C1-C10 alkyl, whereby the alkyl group can optionally contain up to 2 heteroatoms belonging to groups 14-16 of the periodic table, or R1 and R2 or R2 and R3 can form a ring having 4 to 6 C-atoms and 1 to 3 double bonds, R4 and R5 are the same or different from each other and can be saturated linear or branched C1-C10 alkyl, C5-C10 aryl, C6-C20 alkylaryl or C6-C20 arylalkyl groups, which can optionally contain up to 2 heteroatoms belonging to groups 14-16 of the periodic table, n can be 1 to 5, Ar is a C6-C20-aryl or -heteroarylgroup, which can be unsubstituted or substituted by 1 to 5 linear or branched C1-C10 alkyl group(s), and (ii) a boron containing cocatalyst.