Process for preparing polyolefins

A process for preparing a polyolefin, comprising polymerizing at least one olefin in the presence of a catalyst, which catalyst comprises a combination of the following components [I] and [II]:

• [I] a solid component obtained by intercontacting and reacting the following components (1) through (4):
• (1) a compound obtained by treating an oxide of an element of Groups II-IV in the Periodic Table with a compound represented by the general formula R.sup.1.sub.m Si(OR.sup.2).sub.n X.sub.4-m-n, used in an amount of 0.0001 to 2 mol per gram of the oxide of an element of Groups II-IV and heating them at a temperature in the range of 0.degree. C. to 500.degree. C. from 10 minutes to 24 hours, wherein R.sup.1 and R.sup.2 being each a hydrocarbon radical having 1 to 24 carbon atoms, a hydrocarbon radical having 1 to 18 carbon atoms containing oxygen, sulfur or nitrogen, or hydrogen, X being a halogen atom, m being 0.ltoreq.m<4 and n being 0.ltoreq.n.ltoreq.4, provided 0.ltoreq.m+n.ltoreq.4,
• (2) a reaction product obtained by the reaction of a magnesium halide and a compound represented by the general formula Me(OR.sup.3).sub.n X.sub.z-n, where the reaction ratio of the magnesium halide and the compound of the general formula Me(OR.sup.3).sub.n X.sub.z-n being in the range of 1:0.05 to 1:1 in terms of Mg:Me molar ratio, wherein Me, being Na, Mg, Ca, Zn, Cd, Al, B, Ga, Ge, Sn, P, Cr, Mn, Fe, Co, or Ni, with the limitation that silicon, titanium and vanadium being excluded, R.sup.3 being a hydrocarbon radical having 1 to 24 carbon atoms, X being a halogen atom, z being the valence of Me, and n being 0<n.ltoreq.z,
• (3) a compound represented by the general formula ##STR10## wherein R.sup.4, R.sup.5 and R.sup.6 being each a hydrocarbon radical having 1 to 24 carbon atoms, an alkoxy group, hydrogen, or a halogen atom, R.sup.7 being a hydrocarbon radical having 1 to 24 carbon atoms, and n being 1.ltoreq.n.ltoreq.30, and
• (4) a titanium compound and/or a vanadium compound;
• wherein conditions for the intercontacting and reacting of components (1) through (4) being:
• (i) intercontacting 0.01 to 5 grams of component (2) per gram of component (1) and at a temperature of 20.degree. to 400.degree. C.;
• (ii) reacting 0.01 to 5 grams of component (3) per gram of the contacted product of components (1) and (2);
• and (iii) reacting that reaction product with 1 to 20 wt. % of the titanium compound and/or vanadium compound of component (4);
• and
• an organometallic compound.