Preparation of homo- and copolymers of propene by means of a Ziegler-Natta catalyst system

A process for the preparation of a homopolymer of propene or a copolymer of propene with minor amounts of other C.sub.2 -C.sub.12 -.alpha.-monoolefins by polymerization of the monomer or monomers at from 20.degree. to 160.degree. C. and from 1 to 100 bar by means of a Ziegler-Natta catalyst system consisting essentially of

• (1) a titanium component which is based on a finely divided, shape-imparting silica gel and contains titanium, magnesium, chlorine and a benzenecarboxylic acid derivative,
• (2) an aluminum component of the formula
• AlR.sub.3
• where R is alkyl of not more than 8 carbon atoms, and
• (3) a silane component of the formula
• R.sub.n.sup.1 Si(OR.sup.2).sub.4-n
• where R.sup.1 is, methylphenyl or ethylphenyl, R.sup.2 is methyl or ethyl and n is 1 or 2.
• with the provisos that the atomic ratio of titanium from the titanium component (1) to aluminum from the aluminum component (2) is from 1:10 to 1:800 and a molar ratio of the aluminum component (2) to the silane component (3) is from 1:0.01 to 1:0.8, wherein the titanium component (1) used is one which is obtained by a method in which first
• (1.1) in a first state, (I), a carrier is prepared from (Ia) a finely divided silica gel which has a particle diameter of from 1 to 1,000 .mu.m, a pore volume of from 0.3 to 5 cm.sup.3 /g and a surface area of from 100 to 1,000 m.sup.2 /g, is of the formula SiO.sub.2.aAl.sub.2 O.sub.3, where a is from 0 to 2 and possesses a moisture content such that it loses from 1 to 20% by weight, based on the initial total weight of the silica gel, of water at 1000.degree. C. in the course of 0.5 hour, (Ib) an organomagnesium compound of the formula MgR.sup.3 R.sup.4, where R.sup.3 and R.sup.4 are each C.sub.2 -C.sub.10 -alkyl and (Ic) a gaseous chlorinating agent of the formula ClZ, where Z is Cl or H, in a manner such that first
• (1.1.1) in a first substage, the finely divided silica gel (Ia) and the organomagnesium compound (Ib) are combined in a liquid inert hydrocarbon with constant thorough mixing at from 10.degree. to 120.degree. C., from 1 to 10 molar parts of the organomagnesium compound (Ib) being used by 10 molar parts of silicon of the silica gel (Ia), and the combined substances are then kept at from 20.degree. to 140.degree. C. for from 0.5 to 5 hours, then
• (1.1.2) in a second substage, the gaseous chlorinating agent (Ic) is passed into the mixture obtained from the first substage with constant thorough mixing, from 2 to 40 molar parts of the chlorinating agent (Ic) being used per molar part of the organomagnesium compound (Ib), thereafter
• (1.2) in a second stage, a solid-phase intermediate is prepared from (I), the carrier obtained in the first stage (II), a C.sub.2 -C.sub.8 -alkanol, (III) titanium tetrachloride and (IV) a phthalic acid derivative of the formula ##STR2## where X and Y together are oxygen or X and Y are each chlorine or C.sub.1 -C.sub.10 -alkoxy, in a manner such that first
• (1.2.1) in a first substage, the carrier (I) and the alkanol (II) are combined in a liquid inert hydrocarbon with constant thorough mixing, from 1 to 5 molar parts of the alkanol (II) being used per molar part of magnesium of the carrier (I), and the combined substances are kept at from 20.degree. to 140.degree. C. for from 0.5 to 5 hours, then
• (1.2.2) in a second substage, the titanium tetrachloride (III) is introduced into the reaction mixture resulting from the first substage with constant thorough mixing, from 2 to 20 molar parts of the titanium tetrachloride (III) being used per molar part of magnesium of the carrier (I), with the proviso that the phthalic acid derivative (IV) is introduced at least in the course of one of the substages (1.2.1) or (1.2.2), from 0.01 to 1 molar part of the phthalic acid derivative (IV) being used per molar part of magnesium of the carrier (I), then
• (1.3) in a third stage, the solid-phase intermediate obtained from the second stage is subjected, at from 100.degree. to 150.degree. C. for a period of from 0.2 to 5 hours, to a one-stage or multistage or continuous extraction of titanium tetrachloride or with a mixture of titanium tetrachloride and an alkylbenzene of up to 12 carbon atoms, whose content of titanium tetrachloride is at least 5% by weight, a total of from 10 to 1,000 parts by weight of the extraction agent being used per 10 parts by weight of the solid-phase intermediate obtained from the second stage, and finally
• (1.4) in a fourth stage, the solid-phase produce formed in the third stage is washed with a liquid inert hydrocarbon until the hydrocarbon contains less than 2% by weight of titanium tetrachloride, and the titanium component (I) is thus obtained.