A process for polymerizing ethylene comprising contacting under polymerization condition ethylene and optionally one or more olefins with a catalyst system obtainable by contacting: a bis amido compound of formula (I) wherein Y is Si, Ge and Sn; X is hydrogen, halogen or an hydrocarbon radical; R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6, are hydrocarbon radical; Q is a neutral Lewis base; and m is 0 2; one or more boron activating cocatalysts and one or more aluminum alkyls or alumoxanes.