is a transition metal selected from groups 4 or 5, each Q is the same or different and is a hydride or a hydrocarbyl group having from 1 to 20 carbon atoms or an alkenyl group having from 2 to 20 carbon atoms or a halogen, m=1-3, n=0 or 1, and k is such that the sum of m+k is equal to the oxidation state of M, and (ii) an alumoxane cocatalyst, characterized in that said alumoxane is provided unde