Kinetic Study of Living Ring-Opening Metathesis Polymerization with Third-Generation Grubbs Catalysts

The rate of living ring-opening metathesis polymerization (ROMP) of N-hexyl-exo-norbornene-5,6-dicarboximide initiated by Grubbs third-generation catalyst precursors [(H₂IMes)(py)₂(Cl)₂Ru═CHPh] and [(H₂IMes)(3-Br-py)₂(Cl)₂Ru═CHPh] is measured to be independent of catalyst concentration. This result led to the development of a rate law describing living ROMP initiated by a Grubbs third-generation catalyst that includes an inverse first-order dependency in pyridine. Additionally, it is demonstrated that one of the two pyridines coordinated to the solid catalyst is fully dissociated in solution. The monopyridine adduct formation is confirmed in solution by ¹H DOSY (diffusion-ordered NMR spectroscopy), and a Van't Hoff analysis of the equilibrium between mono- and dipyridine adducts (extrapolated K_eq,0 ∼ 0.5 at 25 °C). Finally, the difference in polymerization rates between two catalyst precursors is demonstrated to correspond to the difference in coordination strength between the two pyridines, suggesting that the catalytic species involved in the polymerization's rate-determining step is not coordinated to pyridine.