A process for the preparation of isotactic propylene polymer compositions involving the polymerisation of propylene and optionally one or more comonomers selected from ethylene and α-olefins containing 4 to 12 carbon atoms in one or more reaction steps in the present of a catalyst system comprising 45 to 99 wt.-% of a metallocene catalyst and 1 to 55 wt.-% of a Ziegler Natta catalyst having a non-phthalate internal donor and having been prepolymerised with a monomer (I) of the general formula CH2=CH-CHR1R2 (I) wherein R1 and R2 are either individual alkyl groups with one or more carbon atoms or form an optionally substituted saturated, unsaturated or aromatic ring or a fused ring system containing 4 to 20 carbon atoms.