Ethylene is polymerized with 1,4-bis(2,6-disopropylphenyl)acenaphthenediiminenickel (II) dichloride dissolved in the ionic liquid 1-butyl-3-methylimidazolium organochloraluminate under mild reaction conditions(pressure of 1050 mbar, reaction temperature between -10 degreesC and +10 degreesC). The catalyst solution can be reused in successive ethylene polymerization cycles upon addition of an alkylaluminum cocatalyst, showing enhanced activities and producing polyethylenes with bimodal molecular weight distributions suggesting the formation of non-uniform active centers.