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Polymerization of ethylene was carried out in toluene using the homogeneous catalytic system Cr[OC(CH3 h ]4 /Al(GH, hC1, combined with various metal chlorides (MtCS,). The polymerization activity was found to be strongly dependent upon the MtCL, used. A clear correlation was found between the activity and the elctronegativity X(MtX+ ) of the metal ion in MtCS, . MtCS, containing metal ions with X(MtX+ ) < 8,5 (electronegativity of the active C?+ ) show a markedly increased activity, whereas those with X(MtX+ ) > 8,5 have a rather decreased activity. Thus, for example, the catalyst combined with MgCb shows very high activity to give a linear polyethylene with a remarkably high molecular weight and narrow molecular weight distribution. A plausible mechanism for the enhancement of the activity by suitable MtCJ is proposed on the basis of these results.