Significant increases in the productivity of iron-, chromium-, and titanium-based MgCl2-supported catalysts for ethylene polymerization have been obtained by incorporation of a limited amount of a nickel diimine catalyst giving branched polyethylene. Formation of the latter during the early stages of polymerization reduces the monomer diffusion limitation inherent in ethylene homopolymerization, thereby increasing the productivity of the main catalyst component. The final products are essentially linear, high-density polyethylenes containing very small amounts of branched polymer. As well being effective for catalysts prepared by coimmobilization of the Ni and Fe, Cr, or Ti component on the MgCl2 support, it is also shown that the productivity of preformed Ziegler-Natta catalysts can be improved significantly by impregnation with the nickel diimine component. In the latter case, more rapid increases in rate throughout the polymerization indicated easier fragmentation of the support in the presence of the nickel complex.