High-stiffness propylene polymers and a process for the preparation thereof

A process for producing a high-stiffness propylene polymer nucleated with a polymeric nucleating agent containing vinyl compound units, comprising the steps of

• modifying a catalyst system, the procatalyst component of which contains magnesium, titanium, halogen and an electron donor,
• by polymerizing a vinyl compound of the formula
• wherein R1 and R2 together form a 5 or 6 membered saturated or unsaturated or aromatic ring, at a weight ratio of the vinyl compound to the catalyst amounting to 0.1 to below 3, in the presence of said catalyst, a cocatalyst and a strongly coordinating external donor,
• by carrying out the modification in a medium which does not essentially dissolve the polymerized vinyl compound and
• by continuing the polymerization of the vinyl compound until the concentration of unreacted vinyl compounds is less than about 0.5 wt-%,
• to produce a modified catalyst composition, said modification being carried out essentially before any prepolymerization step of the catalyst with an olefinic monomer, and
• polymerizing propylene optionally together with comonomers in the presence of said modified catalyst composition,
• wherein the strongly coordinating external donor is at least one compound of formula III or formula IV as follows:
• R4R5C(COMe)2 (III)
• wherein R4 and R5 are the same or different and are a linear, branched or cyclic aliphatic or an aromatic hydrocarbon group;
• and
• R1nR2mSi(R3O)4−n−m (IV)
• wherein R1 and R2 can be the same or different and they stand for a linear, branched or cyclic aliphatic, or aromatic hydrocarbon group;
• R3 is methyl or ethyl;
• n is an integer 0 to 3;
• m is an integer 0 to 3;
• and
• n+m is 1 to 3, with the proviso that the coordinating effect of the donor is stronger than that of a donor of formula (IV) wherein R1 is methyl, R3 is methyl and R2 is a cyclohexyl group.