Process for preparing polyolefin

A process for preparing a polyolefin which comprises polymerizing an olefin by using a catalyst system combining an auxiliary catalytic component of an organoaluminum compound and a solid catalytic component obtained by reacting a mixture of a vanadium trichloride ether complex and a titanium trichloride ether complex with an aluminum halide and/or a boron halide in a solvent in which the resultant ether complex of said aluminum halide and/or said boron halide is soluble and said solid catalytic component is insoluble, and separating the resultant precipitate containing said solid catalytic component;

• wherein the olefin is ethylene or a mixture of ethylene and an .alpha.-olefin comonomer having from 3 to 12 carbon atoms, in a ratio such that the resulting copolymer contains less than 10 wt. % of said comonomer units;
• and wherein the molar ratio of the vanadium trichloride ether complex to the titanium trichloride ether complex is in the range of 0.001 to 3,000.