Ru-aryloxide metathesis catalysts with enhanced lability: Assessing the efficiency and homogeneity of initiation via ring-opening metathesis polymerization studies

New Ru-aryloxide catalysts for olefin metathesis are obtained in high yields by treating RuCl2(IMes)(3-bromopyridine)(double bond; length as m-dashCHPh) with TlOC6X5 (X = F, Br). The lability of the 3-bromopyridine ligand increases reactivity, relative to the corresponding pyridine complexes, in ROMP of cyclooctene and norbornene monomers. Initiation efficiency and rates of initiation and propagation were determined: in all cases, propagation is faster than initiation. Initiatiors 5a/b enable a level of control over molecular weight and polydispersity intermediate between that possible with the second-generation catalyst 2, and its pyridine derivative 3a/b.