at least one iron compound of the formula I ##EQU2## in which a is 1 or 2 and q is 1, 2 or 3, L is a divalent to heptavalent metal or non-metal, Q is a halogen atom, m is an integer corresponding to the sum of the valencies of L and q, R1 is a ??-arene and R2 is the anion of a ??-arene, and b) at least one electron acceptor as an oxidising agent, and wherein, if desired, the polymerisation reacti