Catalysts for polymerizing alpha-olefins and process for polymerizing alpha-olefins in contact with said catalysts

Catalysts for the polymerization of alpha-olefins containing at least 3 carbon atoms and mixtures thereof with ethylene, comprising the product obtained by contacting the following starting components:

• (a) a hydrocarbyl Al compound not containing halogen atoms directly bound to Al;
• (b) a hydrocarbyl electron-donor capable of forming complexes or substitution reaction products with the Al compound (a) in such amount that 15% to 100% of the Al compound is combined with the electron-donor compound;
• and
• (c) a solid component comprising, at least on the surface, the reaction product of a Mg dihalide with a halogenated teatrvalent Ti compound selected from the group consisting of Ti tetrahalides and Ti halogen alcoholates and with a hydrocarbyl electron-donor capable of forming complexes with Mg dihalides, the molar ratio electron-donor/Ti in said product being higher than 0.2 and the molar ratio halogen atoms/Ti being comprised between 10 and 90, component (c) being further characterized in that at least 80% by weight of the tetravalent Ti compounds contained therein is insoluble in boiling n-heptane and in that at least 50% by weight of the Ti compounds insoluble in n-heptane is insoluble in TiCl.sub.4 at 80.degree. C., and also in that the surface area of the product insoluble in TiCl.sub.4 at 80.degree. C., and the surface area of component (c) as such, is higher than 40 m.sup.2 /g.