A vanadium-containing catalyst system particularly suited to the polymerization of olefin polymers. The catalyst system includes a supported, first catalyst component prepared by contacting silica with hexaalkyldisilazane and thereupon with (1) a compound or complex which includes at least one carbon to magnesium covalent bond and (2) a compound which includes at least one carbon to Group 13 metal covalent bond. The sequence of contact of the silica with compound or complex (1) and compound (2) is optional. However, unless the compound or complex (1) and the compound (2) contact the silica simultaneously, the product of this contact is next contacted with whichever of compound (1) or (2) does not initially contact the silica. The product of the step of contacting with compounds (1) and (2) is washed and then contacted with a vanadium compound which includes at least one halogen atom. Finally, the product of the vanadium compound contacting step is contacted with an alcohol. A second component of the catalyst system is an organoaluminum compound cocatalyst. The catalyst system also includes a halocarbon compound promoter as a third catalyst component.