Process for preparing polyolefins

A process for preparing polyolefins by polymerizing or copolymerizing olefins in the presence of a catalyst comprising a solid component and an organometallic compound component, characterized in that said solid component is obtained by copulverizing

• 1. a magnesium dihalide
• 2. a compound represented by the general formula Al(OR).sub.m X.sub.3-m wherein R is alkyl and/or aralkyl having 1 to 20 carbon atoms and may be the same or different, X is halogen and 0<m.ltoreq.3,
• 3. a tetravalent titanium compound, and
• 4. a trivalent titanium compound being present in a molar ratio with tetravalent titanium compound (3) of from 50:1 to 1:50 and in that said organometallic compound component is a mixture of
• 5. a trialkylaluminum and
• 6. a compound represented by the general formula AlR'.sub.n X.sub.3-n wherein R' is alkyl having 1 to 10 carbon atoms and may be the same or different, X is halogen and 0<n.ltoreq.3, said compound being present in a molar ratio with trialkylaluminum(5) of from 2:1 to 1000:1.