Polyolefinic compositions

A polymerization process for the preparation of

• a polymer composition comprising (percent by weight):
• a) 57%-74%, of a crystalline propylene polymer having an amount of isotactic pentads (mmmm), measured by 13C-MNR on the fraction insoluble in xylene at 25° C., higher than 97.5 molar %;
• b) 8 to 15%, of an elastomeric copolymer of ethylene and propylene, the copolymer having an amount of recurring units deriving from ethylene ranging from 30% to 50%, and being partially soluble in xylene at ambient temperature;
• the polymer fraction soluble in xylene at ambient temperature having an intrinsic viscosity value ranging from 2.5 to 3.5 dl/g;
• and
• c) 18-28%, of ethylene homopolymer having an intrinsic viscosity value ranging from 1.5 to 4 dl/g;
• said composition having a value of melt flow rate ranging from 35 to 60 g/10 min, and the amount of hexane extractables lower than 4.0 wt %,
• comprising the step of preparing the polymer composition in at least three sequential polymerisation stages wherein crystalline polymer (a), elastomeric polymer (b) and polyethylene (c) are prepared in separate subsequent stages, operating in each stage, except the first step, in the presence of the polymeric material formed and the catalyst used in the immediately preceding polymerisation stage.