Process for the preparation of ultrahigh molecular weight polyethylene having a high bulk density

A process for the preparation of pulverulent polyethylenes having viscometrically measured molecular weights of at least 10.sup.6 g/mol and bulk densities of 430 to 460 g/liter by polymerization of ethylene at polymerization temperatures of 30.degree. to 130.degree. C. and pressures of 0.05 to 4 MPa in the presence of a mixed catalyst and a molecular weight regulator,

• said mixed catalyst comprising a titanium component and an activator, said titanium component being produced by a reaction having a first stage and a second stage,
• in said first stage, a Ti(IV) compound of the formula Ti(OR.sup.1).sub.4-n X.sub.n wherein n is an integer from 1 to 4, the R.sup.1 's are independently hydrocarbon radicals and X is halogen, is reacted with a first organoaluminum compound which is selected from the group consisting of organoaluminum compounds having the formula AlR.sup.2 3, wherein the R.sup.2 's are independently alkyl radicals having 1 to 12 carbon atoms, and polymeric organoaluminum compounds which are obtained from the reaction of lithium aluminum hydride, trialkylaluminum, or dialkylaluminum hydrides, the alkyl radicals of which each have 1 to 16 carbon atoms, with diolefins having 4 to 20 carbon atoms, at -40.degree. C. to 140.degree. C. in a first molar ratio of titanium to aluminum of 1:0.1 to 1:0.6 to form a titanium (III) compound as a first reaction product and
• in a second stage, said first reaction product is aftertreated with isoprenylaluminum as second organoaluminum compound at -10.degree. to 150.degree. C. in a second molar ratio of titanium to aluminum of 1:0.4 to 1:5, thereby forming said titanium component, and forming said titanium component into said mixed catalyst by adding said activator, which is a third aluminum compound selected from the group consisting of triisobutylaluminum, isoprenylaluminum, and mixtures thereof in a third a molar ratio of titanium in said Ti (IV) compound to aluminum in said third aluminum compound of 1:1 to 1:15.