Catalyst for the polymerization of olefins

Procedure for the preparation of a solid component of catalyst, active in the polymerization of propylene and other .alpha.-olefins into stereoregular polymers, composed of a silica support and a catalytically active part including magnesium, halogen, titanium and a Lewis base, which includes:

• (i) treating a non-activated silica support by contact of said silica with a solution, in an inert hydrocarbon solvent, of a magnesium dialkyl or halide of magnesium alkyl, operating with a weight ratio between the magnesium compound and the silica of 0.1/1 to 10/1, at a temperature ranging from 20.degree. C. to the boiling point of the liquid phase, for a period which is sufficient to completely, or almost completely deposit the magnesium compound onto the silica;
• (ii) halogenating the support treated in (i) by contact of said activated support with a solution, in an inert hydrocarbon solvent, of a halogenating agent selected from silicon, tin or antimony halides, operating with a molar ratio between the halogenating agent and the magnesium compound deposited in step (i) of 0.1/1 to 100/1, at a temperature ranging from -20.degree. C. to 100.degree. C. and for a period of 0.5 to 5.0 hours;
• (iii) titanating the support halogenated in (ii) by contact of said halogenated support with an excess of a titanium tetrahalide either liquid or in solution in an inert hydrocarbon solvent, operating at a temperature ranging from 80.degree. to 120.degree. C. and for a period of 0.5 to 5.0 hours.
• (iv) forming the solid component of catalyst by contact of the support titanated in (iii) with a Lewis base either liquid or in solution in an inert hydrocarbon solvent, operating with a ratio between said Lewis base and the magnesium compound absorbed in step (i) of 0.5/1 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;
• and
• (v) recovering the solid component of catalyst from the reaction products of step (iv).