摘要：

A catalyst for the polymerization of olefins comprising (A) a solid catalyst component essentially containing magnesium, titanium, an electron donor compound, and a halogen which is prepared by contacting a magnesium compound, a titanium halide compound, and an electron donor compound, (B) an organoaluminum compound, and (C) an organosilicon compound and a process for polymerizing an olefin using the same are disclosed, the organosilicon compound (C) being represented by formula (I) : ##STR1## wherein R.sup.1 and R.sup.2, which may be the same or different, each represents an alkyl group having from 1 to 3 carbon atoms; R.sup.3 and R.sup.4, which may be the same or different, each represents an alkyl group having from 1 to 3 carbon atoms, or a halogen atom; and m and n each represents 0 or an integer of 1 or 2. A polyolefin having high stereoregularity and broad molecular weight distribution can be obtained in high yield.

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