Catalyst for the polymerization of olefins

Process for the preparation of a solid component of catalyst, active in the polymerization of propylene and other .alpha.-olefins in stereoregular polymers, said solid component containing silica, magnesium, halogen, titanium and a Lewis base, said process comprising:

• (i) treating a non-activated particulate silica support, by contacting said silica with a solution, in an inert hydrocarbon solvent, of a tin tetrahalide, operating with a weight ratio between the tin tetrahalide and the silica of between 1:1 and 20:1, at a temperature ranging from -30.degree. C. to 30.degree. C., for a time which is sufficient to at least partially block the hydroxylic groups of the silica, to obtain a blocked silica;
• impregnating said blocked silica with a solution, in an inert hydrocarbon solvent, of a magnesium dialkyl or magnesium alkyl halide, operating with a weight ratio between the magnesium compound and the blocked silica of between 0.1:1 and 10:1, at a temperature ranging from 20.degree. C. to the boiling point of the liquid phase, for a time which is sufficient to interact the magnesium compound with the tin compound and with any possible residual hydroxylic groups of the silica, to obtain a support;
• (iii) halogenating said support by contacting said support with a solution, in an inert hydrocarbon solvent, of a halogenating agent selected from the halides of tin, antimony or silicon, operating with a molar ratio between the halogenating agent and the magnesium compound introduced in step (ii) ranging from 0.1:1 to 100:1, at a temperature ranging from -20.degree. to 100.degree. C. and for a period of 0.5 to 5.0 hours, to obtain a halogenated support;
• (iv) titanating said halogenated support by contacting said halogenated support with an excess of a titanium tetrahalide, in the form of either a liquid or as a solution in an inert hydrocarbon solvent, operating at a temperature ranging from 20.degree. to 120.degree. C. and for a period of 0.5 to 5.0 hours, to obtain a titanated support;
• (v) contacting said titanated support with a Lewis base, in the from of either a liquid or as a solution in an inert hydrocarbon solvent, operating with a molar ratio between said Lewis base and the magnesium compound introduced in step (ii) ranging from 0.05:1 to 0.5:1, at a temperature ranging from 80.degree. to 120.degree. C. and for a period of 0.5 to 5.0 hours, to obtain a solid component of catalyst;
• and
• (vi) recovering said solid component of catalyst from the reaction products of step (v).