Process for polymerizing olefins and polymerization catalyst therefor

A polymerization process which comprises polymerizing or copolymerizing olefins in the presence of an olefin polymerization catalyst formed from

• (A) a solid titanium component which contains magnesium, titanium, halogen and an electron donor as the essential components prepared by contacting of a magnesium compound with a titanium compound and an electron donor, said magnesium compound or a complex of said magnesium compound with said electron donor being reacted with the titanium compound in the liquid phase, or said magnesium compound having no reducibility and said titanium compound, both in liquid form, being reacted in the presence of said electron donor;
• (B) an organoaluminum compound;
• and
• (C) an organosilicon compound represented by the following formula
• SiR.sup.21 R.sup.22.sub.m (OR.sup.23).sub.3-m
• wherein R.sup.21 represents a cyclopentyl group, a cyclopentyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, an alkyl group having 2 to 4 carbon atoms substituted by a cyclopentyl group or a cyclopentyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, a cyclopentenyl group, a cyclopentenyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, a cyclopentadienyl group, a cyclopentadienyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, an indenyl group, an indenyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, an indanyl group, an indanyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, a tetrahydroindenyl group, a tetrahydroindenyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms, a fluorenyl group or a fluorenyl group substituted by 1 to 4 alkyl groups having 1 to 4 carbon atoms;
• R.sup.22 to R.sup.23 are identical or different and each represents a hydrocarbon group;
• and
• O.ltoreq.m<3.