Process for the preparation of heterophasic propylene polymer compositions

A process for the preparation of a propylene polymer composition comprising:

• (A) 50-90% by weight of a propylene (co)polymer, wherein the propylene (co)polymer contains a fraction that is insoluble in xylene at 25° C., and
• (B) 10-50% by weight of an ethylene copolymer, wherein the ethylene copolymer contains a fraction that is soluble in xylene at 25° C.;
• wherein the propylene polymer composition has a total ethylene content from 5-10 wt. %, based upon the total weight of the propylene polymer composition, and a flexural modulus greater than 1200 MPa;
• said process comprising:
• (i) a first step of polymerizing propylene in the optional presence of ethylene and/or C4-C10 alpha olefins, to produce the propylene (co)polymer, wherein the propylene (co)copolymer contains a fraction that is insoluble in xylene at 25° C.;
• and
• (ii) a successive gas-phase copolymerizing step, carried out in the presence of the propylene (co)polymer, a mixture of ethylene and one or more α-olefins CH2═CHR in which R is a hydrocarbon radical having 1-10 carbon atoms, to produce the ethylene copolymer;
• the process being carried out in the presence of a catalyst system comprising the product obtained by contacting:
• (a) a solid catalyst component comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two internal electron donor compounds,
• wherein the at least two internal electron donor compounds comprise a succinate as a first internal electron donor present in an amount from 35-90% by mol with respect to the total amount of internal electron donor compounds present in the solid catalyst component, and a 1,3-diether comprises the second internal electron donor, wherein the total concentration of the internal electron donors is lower than 14.0% by weight with respect to the total weight of the solid catalyst component;
• (b) triethylaluminum;
• and
• (c) optionally, an external electron donor compound at a molar ratio of 5-500 with respect to triethylaluminum.