Examining the Effects of Monomer and Catalyst Structure on the Mechanism of Ruthenium-Catalyzed Ring-Opening Metathesis Polymerization

The
mechanism of Ru-catalyzed ring-opening metathesis polymerization
(ROMP) is studied in detail using a pair of third generation ruthenium
catalysts with varying sterics of the N-heterocyclic carbene (NHC)
ligand. Experimental evidence for polymer chelation to the Ru center
is presented in support of a monomer-dependent mechanism for polymerization
of norbornene monomers using these fast-initiating catalysts. A series
of kinetic experiments, including rate measurements for ROMP, rate
measurements for initiation, monomer-dependent kinetic isotope effects,
and activation parameters were useful for distinguishing chelating
and nonchelating monomers and determining the effect of chelation
on the polymerization mechanism. The formation of a chelated metallacycle
is enforced by both the steric bulk of the NHC and by the geometry
of the monomer, leading to a ground-state stabilization that slows
the rate of polymerization and also alters the reactivity of the propagating
Ru center toward different monomers in copolymerizations. The results
presented here add to the body of mechanistic work for olefin metathesis
and may inform the continued design of catalysts for ROMP to access
new polymer architectures and materials.