Ethylene slurry polymerizations were performed using supported nickel diimine catalysts. The effects of catalyst structure and polymerization conditions on catalyst activity, polymer particle morphology, and polymer properties were investigated. Covalently attached supported nickel diimine catalysts activated with ethyl aluminum sesquichloride (EASC) have high activities and made polymers with spherical morphology. Compared with the equivalent homogeneous catalysts, the covalently attached supported catalysts produced polyethylene with higher melting temperatures, higher molecular weights, and broader molecular weight distributions. Borates used as internal activator during the synthesis of these supported catalysts successfully activated the nickel diimine complexes.