Ring‐opening polymerization of norbornene initiated by tungsten alkylidene complexes. Activation by AlCl 3

Addition of aluminium compounds to the stable metal‐carbene complex tetracarbonyl[4–5‐η‐1‐methoxy‐4‐pentenylidene]tungsten ( 1 ), showing an intramolecular coordination of double bond to the transition metal, leads to highly active catalytic systems in the polymerization of norbornene. It is shown that, in optimal conditions, AlCl 3 is more efficient than AlEt 3 for cyclo‐olefin polymerization. From spectroscopic studies (UV‐visible, 1 H and 13 C NMR) of the activated system 1 ‐AlCl 3 , a mechanism of activation by AlCl 3 is proposed involving a modification of the nature of carbene 1 by the Lewis acid.