Components and catalysts for the polymerization of olefins

Catalysts for the polymerization of alpha-olefins CH.sub.2 .dbd.CHR, wherein R is an alkyl radical with 1-4 C atoms or an aryl radical, comprising the product of the reaction between at least the following components:

• (a) an Al-alkyl compound;
• (b) an electron-donor compound (or Lewis base) which, under the standard conditions of measurement, reacts with MgCl.sub.2, but results not to be completely complexed with Al-triethyl at the equivalent point of a potentiometric titration;
• (c) a solid comprising a Ti compound containing at least a Ti-halogen bond, said Ti compound being supported on an anhydrous Mg halide, and at least one of the following activators in an amount not lower than 5% by mols with respect to the Mg halide:
• (1) an Al, Fe, Mn, Sb or Zn-compound containing at least a metal-halogen bond;
• (2) a complex between an electron-donor compound and a Lewis acid selected from the halides of metals of Groups II to VIII of the Periodic System other than Ti;
• (3) a compound or composition inert towards the Mg halide, having a dielectric constant at 20.degree. C. higher than 2;
• compounds (1), (2) and (3) being extractable from the solid for at least 30% by mols by treatment with toluene at 80.degree. C. for 2 hours, and the surface area of the solid after extraction being higher than 20 m.sup.2 /g.