Abstract: Catalysts for the polymerization of olefins, comprising the product of the reaction between:A) a cyclopentadienyl compound of Ti, Zr or Hf;B) an alumoxane compound of formula: ##STR1## in which R.sup.1, R.sup.2, R.sup.3, R.sup.4, the same or different from each other, are alkyl, alkenyl or alkylaryl radicals having 2 to 20 carbon atoms.

Abstract: Catalyst components for the polymerization of olefins comprising a titanium halide or alkoxyhalide and an electron-donor compound selected from ethers having two or more ether groups and containing at least one heteroatom selected from the group consisting of N, S, P, Si, non-ether O and halogen atoms or at least one double bond, or both at least one heteroatom and at least one double bond, supported on a magnesium halide in active form.Catalysts obtained from said catalyst components and an Al-alkyl compound, as well as catalysts obtained by the reaction of an Al-alkyl compound and an ether having the above defined characteristics with a solid catalyst component comprising a titanium halide or alkoxyhalide and an electron-donor compound having particular characteristics of extractability with Al-triethyl, supported on activated magnesium halide.