The rate of living ring-opening metathesis polymerization (ROMP) of N-hexyl-exo-norbomene-5,6-dicarboximide initiated by Grubbs third-generation catalyst precursors [(H(2)IMes)(py)(2)(Cl)(2)Ru=CHPh] and [(H(2)IMes)(3-Br-py)(2)(Cl)(2)Ru=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-ordet 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 H-1 DOSY. (diffusion-ordered NMR spectroscopy), and a Van't Hoff analysis of the equilibrium between mono- and dipyridine adducts (extrapolated K-eq,K-0 similar to 0.5 at 25 degrees 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.