Abstract: Catalyst components for the polymerization of olefins, obtained by reacting a tetravalent titanium halide or halogen alcoholate and an electron-donor compound with a solid comprising a porous metallic oxide containing hydroxyl groups on the surface, on which is supported a magnesium dihalide or a magnesium compound which does not contain Mg-C bonds and can be transformed into a dihalide, characterized in that the quantity of Mg supported on the oxide prior to the reaction with titanium compound, and present in the catalyst component after the reaction with the Ti compound, is from 5% to 12% weight with respect to the weight of the catalyst component.

Abstract: Disclosed are catalyst components for the polymerization of olefins comprising the product of the reaction of a tetravalent titanium halide or alkoxy titanium halide and an electron-donor compound with a solid obtained by the reaction of a metal oxide containing surface hydroxyls, preferably together with chemically uncombined water, with an organometallic magnesium compound used in a quantity which does not cause reduction of the titanium in the subsequent reaction with the tetravalent titanium compound.

Abstract: Components of catalysts for polymerizing olefins are obtained by grinding together an anhydrous magnesium halide, an ester of an aromatic carboxylic acid and a silicon compound having alkenyl groups and alkoxy groups bound to the silicon atom, of the type of vinyl-triethoxy-silane, and by successively reacting the ground product with a halogenated titanium compound, liquid under the reaction conditions, such as TiCl.sub.4.The present invention provides not only the aforesaid catalyst components for the polymerization of olefins, but also catalysts obtained therefrom and the use of the same, particularly in the polymerization of the alpha-olefins CH.sub.2 .dbd.CHR wherein R is an alkyl or aryl radical with from 1 to 8 carbon atoms.In the case of the polymerization of alpha-olefins, the catalytic components, when mixed with organometallic aluminum compounds, yield final catalysts which result in an increase in the polymer yield without lowering the isotacticity index of the polymer.