Alternatively, the catalyst components containing the transition metal, magnesium and the halogen may be prepared by reacting a liquid halogenated compound of the transition metal with a magnesium oxygenated compound such as, for instance, MgO, Mg(OH)Cl, magnesium carbonate, or MgX(OR), in which X is halogen and R is an alkyl or aryl radical containing from 1 to 15 carbon atoms, inclusive.