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 Keq,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.