The catalyst of claim 1 wherein TiCl4 is reduced with a compound of formula AlR'x X3-x by mixing TiCl4 and the organo-aluminium compound at a temperature below 20??? C, and, when the addition of the organo-aluminium compound has been completed, heating the reaction mixture containing the TiCl3 formed to a temperature in the range from 20 up to 80??? C, where R' is an alkyl group having 1 to 18 ca