Process for the stereoregular polymerization of alpha-olephins

Process for the stereoregular polymerization of alpha-olefins of the formula CH.sub.2 .dbd.CHR wherein R is an alkyl radical having 1-6 carbon atoms and mixtures thereof with ethylene, which process comprises polymerizing the olefins in contact with a catalyst the starting catalyst-forming components of which comprise the following components (A) and (B):

• (A) the addition and/or substitution reaction product of an electron-donor compound (or Lewis base) free from ester groups of oxygenated organic and inorganic acids with an Al-trialkyl compound or the addition reaction product of said electron-donor compound with an Al-alkyl compound containing two or more Al atoms linked together through an oxygen or a nitrogen atom, component (A) being characterized in that the amount of Al-alkyl compound present as addition compound with the electron-donor compound ranges from 0.01 to 1 mole per mole of the starting Al compound and in that the Al-compound present as substitution compound ranges from 0.01 to 0.9 moles per mole of the starting Al-compound;
• and
• (B) the product obtained by contacting a Ti compound selected from the group consisting of the halogenated bi-, tri-, and tetravalent Ti compounds and complexes of said Ti compounds with an electron-donor compound, with a support comprising an anhydrous Mg or Mn dihalide and from 30% to 90% of a co-support selected from the group consisting of LiCl, CaCO.sub.3, CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, Na.sub.2 SO.sub.4, Na.sub.2 CO.sub.3, Na.sub.2 B.sub.4 O.sub.7, Ca(PO.sub.4).sub.2, CaSO.sub.4, BaCO.sub.3, Al(SO.sub.4).sub.3, B.sub.2 O.sub.3, Al.sub.2 O.sub.3 and SiO.sub.2, the Mg or Mn dihalide being in an active state such that the X-rays powder spectrum of component (B) does not show the most intense diffraction lines as they appear in the X-rays powder spectrum of the normal non-activated Mg or Mn dihalides, the X-rays powder spectrum of component (B) showing a broadening of said most intense diffraction lines, component (B) being further characterized in that the Ti compound present therein, expressed as Ti metal, is less than 0.3 g atom per mole of the total amount of the electron-donor compound present in a combined form in the catalyst and the catalyst being additionally characterized in that the Al/Ti molar ratio is from 10 to 1,000.