Catalyst formation in the system Ni(acac)2, C3H4, RnAlX3−n was studied. Polymerization experiments showed that, by replacing ionic groups such as acac−, Br−, Cl− with alkyl or hydride groups, an active catalyst is obtained. Electrolysis of Ni(acac)2 in tetrahydrofuran also gives an active catalyst. Lewis acids like (iBu)3Al and Et3Al increase the polymerization rate, while Lewis bases like pyridine and triphenylphosphine not only decrease the rate but also change selectivity. The selectivity is not changed if different transition metals (e.g. Co, Pd, Ni) are used. Kinetic measurements show a first order dependence on Ni. The dependence on (iBu)3Al changes from first to zero order with increasing Al/Ni ratio. This can be explained by assuming that the very active catalyst is formed via an equilibrium between a nickel complex and (iBu)3Al. A first order deactivation of the nickel catalyst is observed; it is faster during polymerization than during ageing of the catalyst.