Ruthenium alkylidenes: a new class of catalysts for Kharasch addition and controlled radical polymerisation of vinyl monomers

Air-stable and readily available ruthenium benzylidene complexes of the general type (RuCl2
(=CHPh)(L)(L')) (L, L' = P(cC5H9)3, PCy3 and/or a N-heterocyclic carbene) constitute a new
class of catalyst precursors for atom transfer radical addition (ATRA, also called Kharasch
addition) and 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 addition of carbon tetrachloride to methyl methacrylate and
styrene in moderate to high yield (Table 1). They also promote the polymerisation of methyl
methacrylate and styrene in a controlled way with good to excellent yields (Table 2). The
ligands L (P(cC5H9)3, PCy3 and/or a N-heterocyclic carbene) play a particularly important
role in determining the rate of the polymerisation. A similarly pronounced influence is exerted
by the substituents on the N-heterocyclic carbene. Our results indicate that: 􀀀 The catalysts
decompose quickly under ATRA and ATRP conditions; 􀀀 Polymerisations are mediated by
both (RuCl2(=CHPh)(L)(L')) complexes and ruthenium species bereft of the benzylidene
moiety.