Preparation of ethene polymers using a ziegler catalyst system

In a process for the preparation of finely divided homopolymers of ethene or copolymers of ethene with minor amounts of a C.sub.3 -C.sub.8 -alpha-mono-alkene by polymerization of the monomer or monomers at from 30.degree. to 150.degree. C. under from 1 to 200 bar using a Ziegler catalyst system consisting of

• (1) a titanium-containing catalyst component obtained by
• (1.1) first of all producing a precipitate in an initially taken organic liquid by introducing and combining, with vigorous movement,
• (1.1.1) a 0.1-10 molar solution of titanium tetrachloride in a liquid saturated aliphatic and/or aromatic hydrocarbon of 5 to 10 carbon atoms,
• (1.1.2) a 0.1-10 molar solution of an organoaluminum compound of the empirical formula R.sub.n AlCl.sub.3-n, where n is from 1.5 to 3 and R is alkyl of 1 to 12 carbon atoms, in a liquid saturated aliphatic and/or aromatic hydrocarbon of 5 to 10 carbon atoms, and
• (1.1.3) a C.sub.2 -C.sub.8 -alpha-monoalkene,
• with the provisos that (i) a total of from 2 to 20 liters of the initially taken organic liquid, a total of from 0.5 to 5 g mol of the organoaluminum compound and a total of from 5 to 500 g mol of the alpha-monoalkene are used per g mol of the titanium tetrachloride employed in total, and (ii) per molar unit of the titanium tetrachloride employed in total, from 0.002 to 0.15 molar unit per minute is converted to a precipitate, and from 0.02 to 1 molar unit per minute of the alpha-monoalkene is introduced into the precipitation system, and
• (1.2) then producing a solid-phase product by combining, with thorough mixing, the precipitate obtained according to (1.1), which is present in a liquid saturated aliphatic and/or aromatic hydrocarbon of 5 to 10 carbon atoms, with a 0.1-10 molar solution of an organolithium compound of the empirical formula LiR, where R is alkyl of 1 to 12 carbon atoms, in a liquid saturated aliphatic and/or aromatic hydrocarbon of 5 to 10 carbon atoms,
• with the provisos that (i) the substances combined are left at from -10.degree. to 60.degree. C. for from 0.5 to 12 hours and (ii) from 0.1 to 20 g mol of the organolithium compound are used per g mol of titanium in the precipitate, and
• an aluminum-containing catalyst component which is a trialkylaluminum which contains 1 to 12 carbon atoms per alkyl group,
• with the proviso that the atomic ratio of titanium from the catalyst component (1) to aluminum from the catalyst component (2) is from 1:1 to 1:200,
• the improvement which comprises using as the titanium-containing catalyst component (1) one in which the precipitate of stage (1.1) has been produced at from -10.degree. to 10.degree. C. in an initially taken organic liquid which has a viscosity of from 5 to 15 mm.sup.2 /s at the precipitation temperature and consists of (a) a liquid saturated aliphatic hydrocarbon of 6 to 8 carbon atoms and (b) relatively small amounts, dissolved therein, of a homopolymer of isobutene.