In-situ reactor blend of Ziegler-Natta catalysed, nucleated polypropylene and a metallocene catalysed polypropylene

A process for the preparation of isotactic propylene polymer compositions comprising the steps of:

• (a) prepolymerising a Ziegler-Natta type catalyst comprising a magnesium halide support, a titanium component and an internal donor (ID), wherein the internal donor is other than a phthalic ester and the Ziegler-Natta type catalyst is free from phthalic esters, in the presence of an aluminium alkyl co-catalyst and an external donor (ED), 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 to obtain a catalyst composition (II) comprising 25 to 95 wt. % of an isotactic polymer based on said monomer (I);
• (b) mixing said catalyst composition (II) with a supported metallocene catalyst (III) suitable for the production of isotactic polypropylene in a weight ratio (II):(III) of 1:99 to 55:45 in an inert medium to obtain a catalyst blend (IV), wherein the metallocene catalyst (III) comprises a metallocene complex, a co-catalyst system comprising a boron-containing co-catalyst and/or aluminoxane co-catalyst, and a silica support;
• (c) using said catalyst blend (IV) for polymerising 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 to obtain an isotactic propylene homo- or copolymer (V);
• and
• (d) melt-mixing said isotactic propylene homo- or copolymer (V) with additives including at least one of antioxidants or acid scavengers and optionally a nucleating agent, followed by pelletisation.