Abstract: The invention relates to a catalyst system and a process suited for the preparation of a rubbery copolymer of ethylene, one or more .alpha.-olefins and optionally one or more polyunsaturated compounds, in which the catalyst system comprises a vanadium compound, an organometal compound and optionally a promoter, characterized in that the organometal complex in the catalyst system is an imido complex that satisfies the general formula(R.sub.u -X.sup.1.sub.v -Me.sup.1).sub.n =NR.sup.1 (I)where each individual Me.sup.1 is a metal chosen from group 1, 2, 12 or 13 of the Periodic System of Elements, each individual R is hydrogen or a hydrocarbon group having 1-20 C atoms, N--R.sup.1 is an imido group where R.sup.1 is hydrogen or a hydrocarbon group having 1-20 C atoms or a group with the general formula MR.sup.3.sub.3, where M is an element chosen from group 14 of the Periodic System of Elements, each individual R.sup.3 is hydrogen, a hydrocarbon group having 1-20 C atoms or a heteroatom-containing group, X.sup.