Process for preparing polypropylene compositions having high impact strength at low temperatures

Claim 1

Process for preparing polymeric compositions containing at least 50% by weight of isotactic polypropylene comprising the following steps: (a) polymerizing propylene to isotactic polypropylene by polymerizing propylene in a hydrocarbon solvent at a temperature of from 50° C to 80° C and at a pressure between 3 and 10 kg/cm 2 gauge, in the presence of hydrogen and of a stereospecific catalyst comprising the product obtained by mixing a titanium trihalide with a dialkyl aluminum monohalide, to obtain a suspension of substantially isotactic polypropylene in the hydrocarbon solvent; (b) feeding ethylene continuously into the polymerization suspension while feeding propylene discontinuously into the suspension to obtain, during the propylene feeding, a molar ratio of ethylene to propylene of from 60/40 to 85/15 and, in the feed as a whole, a molar ratio of at least 80/20 between the total amount of ethylene fed and the total amount of propylene fed, both monomers being polymerized at a temperature of from 60° C to 80° C and at a pressure lower than 10 kg/cm 2 gauge, until the resulting copolymer makes up from 8% to 18% by weight of the total composition.