Process for preparing polyolefins

A process for preparing a polyolefin by polymerizing or copolymerizing an olefin or olefins in the presence of a catalyst comprising a solid catalyst component and an organometallic compound, the solid catalyst compound being prepared by reacting the following components [I], [II] and [III]:

• I a reaction product obtained by the following components (1), (2) and (3) in the presence of a compound represented by the general formula R2OH wherein R2 is a hydrocarbon radical having 1 to 20 carbon atoms or an organic radical containing an element selected from the group consisting of oxygen, nitrogen, sulfur, and chlorine:
• (1) a silicon oxide and/or aluminum oxide;
• (2) a reaction product obtained by reacting a magnesium halide and a compound represented by the general formula Me(OR)nXz-n wherein Me represents an element of Groups Ia to IVa in the Periodic Table, z represents the valence of the element Me, n is 0<n≦z, X is a halogen atom, and R is a hydrocarbon radical having 1 to 20 carbon atoms;
• and
• (3) a titanium compound represented by the general formula Ti(OR1)mX4-m wherein R1 is a hydrocarbon radical having 1 to 20 carbon atoms, X is a halogen atom, and m is 0≧m≧4;
• II an organoaluminum compound represented by general formula Al(OR3)pRq4X3-(p+q) wherein R3 and R4, which may be the same or different, are each a hydrocarbon radical having 1 to 24 carbon atoms, X is a halogen atom or hydrogen atom, and p and q are 0≧p<3, and 0≧q ≧3, provided 0<p+q≧3;
• and
• III a silicon compound represented by the general formula R5aR6bR7cSi(OR8)dX4-(a+b+c+d) wherein R5, R6, R7 and R8, which may be the same or different, are each a hydrocarbon radical having 1 to 20 carbon atoms, X is hydrogen or a halogen atom and a, b, c and d are 0≧a<4, 0≧b≧4, 0≧c<4 and 0<d≧4.