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Abstract

Propylene polymers which comprise from 25 to 97% by weight of a propylene polymer (I) containing from 0 to 15% by weight of copolymerized C.sub.2 -C.sub.10 -alk-1-enes and also comprise from 3 to 75% by weight of a further propylene polymer (II) containing from 15 to 80% by weight of copolymerized C.sub.2 -C.sub.10 -alk-1-enes, are obtainable by two-stage polymerization of propylene and C.sub.2 -C.sub.10 -alk-1-enes in the presence of a Ziegler-Natta catalyst system which comprises a titanium-containing solid component a) comprising a compound of magnesium, a halogen, silica gel as support and a carboxylic ester as electron donor compound, and also as cocatalysts an aluminum compound b) and a further electron donor compound c), where, in a first polymerization stage, propylene is polymerized in the presence or absence of further C.sub.2 -C.sub.10 -alk-1-enes at from 50.degree. to 100.degree. C., pressures in the range from 15 to 40 bar and a mean residence time of from 0.5 to 5 hours and subsequently, in a second polymerization stage, a mixture of propylene and further C.sub.2 -C.sub.10 -alk-1-enes is polymerized at from 30.degree. to 100.degree. C., pressures in the range from 15 to 40 bar and mean residence times of from 0.5 to 5 hours onto the propylene polymer obtained from the first polymerization stage, and the silica gel used as support for the titanium-containing solid component has a mean particle diameter of from 5 to 200 .mu.m, a mean particle diameter of the primary particles of from 1 to 10 .mu.m and voids or channels having a mean diameter of from 1 to 10 .mu.m, which voids or channels have a macroscopic volume as a proportion of the total particle in the range from 5 to 20%.

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