In the cases of suspension- and solution-polymerizations, the catalyst is introduced into a reactor together with a polymerization medium including an aliphatic hydrocarbon such as hexane or heptane; an aromatic hydrocarbon such as benzene, toluene or xylene; or an alicyclic hydrocarbon such as cyclohexane or methylcyclohexane, and, an ??-olefin is added under a pressure of about 1 to about 200 Kg