A precursor for the formation of catalysts for the (co)polymerization of α-olefins, comprising titanium, magnesium, at least one metal selected from hafnium and zirconium, aluminum and chlorine, obtained with a process comprising treatment with a siloxane compound. Said solid precursor, used in combination with a suitable co-catalyst in high-temperature (co)polymerization processes of α-olefins, shows an improved productivity, a high incorporation of co-monomers in the copolymerization of ethylene and an increased thermal stability with respect to the systems so far in use.