Process for the preparation of impact resistant propylene polymer compositions

A process for the preparation of propylene polymer compositions carried out in the presence of a catalyst system comprising the product obtained by contacting:

• (a) a solid catalyst component having an average particle size ranging from 15 to 80 μm comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two internal electron donor compounds, wherein one of the electron donor compounds is present in an amount from 50 to 90% by mol with respect to the total amount of donors and selected from succinates and another electron donor compound is selected from 1,3 diethers,
• (b) an aluminum hydrocarbyl compound wherein the hydrocarbyl is selected from C3-C10 branched aliphatic or aromatic radicals and optionally
• (c) an external electron donor compound, the process comprising the following steps:
• (i) contacting the catalyst components (a), (b) and optionally (c) for a period of time ranging from 0.1 to 120 minutes, at a temperature ranging from 0 to 90° C.;
• (ii) polymerizing propylene in the optional presence of at least one of ethylene and C4-C10 alpha olefins, thereby producing a propylene (co)polymer being at least 85% by weight insoluble in xylene at 25° C.;
• and
• (iii) in a successive step carried out in gas-phase, in the presence of the product coming from (ii), polymerizing mixtures of ethylene with α-olefins CH2═CHR wherein R is a hydrocarbon radical having 1-10 carbon atoms, thereby producing an ethylene copolymer
• wherein the propylene polymer composition has a ratio between the intrinsic viscosities of the propylene (co)polymer and that of the ethylene copolymer in tetrahydronaphthalene at 135° C. of the portion soluble in xylene and of the portion insoluble in xylene at room temperature from 0.3 to 5 and
• wherein the propylene polymer composition has an impact strength at determined according to ISO 180/1A at −20° C. from 5 to 10 KJ/m2.