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

DETAILED DESCRIPTION We 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.
degree.
C to 80.
degree.
C and at a pressure between 3 and 10 kg/cm.
sup.
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.
degree.
C to 80.
degree.
C and at a pressure lower than 10 kg/cm.
sup.
2 gauge, until the resulting copolymer makes up from 8% to 18% by weight of the total composition.




Description:
BACKGROUND OF THE INVENTION Isotactic polypropylene, as defined in Natta et al U.
S.
Pat.
No.
3,112,300, is a polypropylene which consists essentially of isotactic macromolecules, i.
e.
, macromolecules having substantially the isotactic structure and being insoluble in (non-extractable with) boiling n-heptane.
While said polypropylene is adapted to use in many commercially important applications, its impact strength at temperatures of 0.
degree.
C.
or less is rather low, particularly for socalled commercial grade polypropylene.
Different ways of improving the impact strength of the polypropylene at low temperatures without unacceptable adverse effect on its other properties, including its flexural rigidity and thermal resistance have been proposed.
The technique which is most widely used for achieving that objective consists in polymerizing propylene in contact with a Ziegler/Natta stereospecific catalyst until most of the propylene is polymerized and then, during the final stage of the propylene polymerization feeding a different olefin, in particular ethylene, to the polymerization zone and continuing the polymerization until the amount of the added olefin, e