Living polymerizations are powerful synthetic tools that have enabled the access to a wide variety of well-defined macromolecules with varying length, composition, and architectures.1,2 The precision of these polymerization methods relies exclusively on controlling the rates of each elementary polymerization step (initiation, propagation, transfer, and termination).2 Within these techniques, ring-opening metathesis polymerization (ROMP) of high ring strain cyclic monomers by Grubbs third-generation (G3) catalysts offers a rare combination of narrow molecular weight distributions at very high monomer conversions, fast polymerization rates, and good functional group tolerance.3 The precision of this method is a result of the fast and quantitative initiation step, enabled by the lability of the pyridine ligand coordinated to the ruthenium center, and the absence of any undesired chain transfer/termination reactions. ROMP catalyzed by G3 has been widely implemented for the synthesis of a multitude of polymer architectures that have found applications in drug delivery, self-assembly materials, and sequence controlled polymers. Despite this ubiquity, the polymerization mechanism of ROMP initiated by six-coordinate third-generation Grubbs complexes remains relatively unexplored in comparison to the depth of knowledge gained over the years for olefin metathesis reactions catalyzed by five-coordinate ruthenium-based olefin metathesis catalysts.4,5 Here, we will present an in-depth analysis of the kinetics of living ROMP catalyzed by Grubbs third-generation catalysts. A combination of 2H and 13C kinetic isotope studies led us to identify the rate determining step of the polymerization. Furthermore, we demonstrate that the ability of the polymer to form a chelated species is highly dependent on the linker attached to the norbornene group.

Mechanistic and kinetic studies of ring-opening metathesis polymerization with third-generation Grubbs catalysts

2019 Fall National Meeting

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