[en] Air-stable and readily available ruthenium benzylidene complexes of the general type [RuCl2(=CHPh)(L)(L’)] (L, L’ = PCy3 and/or N-heterocyclic carbene) constitute a new class of catalyst precursors for atom-transfer radical polymerisation (ATRP) of methyl methacrylate and styrene, and provide an unprecedented example for the involvement of ruthenium alkylidenes in radical reactions. They promote the polymerisation of various monomers with good to excellent yields, and in a controlled way with methyl methacrylate and styrene. The ligand L (PCy3 and/or N-heterocyclic carbene) turned out to play a particularly important role in determining the rate of the polymerisations. A similarly pronounced influence is exerted by the substituents on the N-heterocyclic carbene. Our results indicate that the catalysts decompose quickly under ATRP conditions, and polymerisations are mediated by both [RuCl2(=CHPh)-(L)(L’)] complexes and ruthenium species bereft of the benzylidene moiety. In addition, investigation on the reaction of metathesisable olefins such as styrene in the presence of carbon tetrachloride also exemplify the dual activity of ruthenium alkylidene complexes for radical reactions and olefin metathesis.
