Rosebrugh, L.E.; Marx, V.M.; Keitz, B.K.; Grubbs, R.H. “Synthesis of Highly Cis, Syndiotactic Polymers via Ring-Opening Metathesis Polymerization Using Ruthenium Metathesis Catalysts” J. Am. Chem. Soc. 2013, 135, 10032-10035.
Controlling the stereochemistry of ring opening metathesis polymerization (ROMP) via the selection of initiator can be a challenge. Previously, catalysts based on metals such as tungsten and molybdenum have been demonstrated to yield polymers with high cis content; however, until now, ruthenium-based catalysts were known to produce trans polymers. Over the past year, the Grubbs group has capitalized on their recent success of ruthenium-based cis selective catalysts for cross metathesis (CM) and began to study selectivity control in polymerizations. In a couple of recent JACS communications, Grubbs and co-workers illustrate the use of a ruthenium catalyst to produce polymers with high cis content and also disclose new ruthenium catalysts for the synthesis of syndiotactic polymers.
Initial investigations with catalysts designed for cis selective CM showed no significant effect on polymerization control. However, the newly discovered more stable nitrato complex 2 when treated with a variety of norbornene based monomers produced polymers with high cis content ranging from 61% to 93%. Additionally, a 6-15% increase in cis content up to 91% could be achieved by lowering the reaction temperature from room temperature to -20 °C. Catalyst 2 repeatedly yielded polymers with higher cis content when compared to the Hoveyda-Grubbs catalyst (1).
Figure 1. Ruthenium catalysts
Grubbs and co-workers also investigated the use of catalyst 2 for the polymerization of low ring strain carbocycles such as cis-cyclooctadiene, cyclopentene and trans-cyclooctene. To their delight, catalyst 2 yielded polymers with increased cis content compared to similar polymers prepared with catalyst 1. Furthermore, the polymers from these low ring strain monomers had significantly higher molecular weights compared to similar polymers gained from catalyst 1. This increase in molecular weight is hypothesized to be due to the suppression of secondary metathesis reactions such as chain backbiting or intermolecular chain transfer reactions.
Building on their success of the synthesizing polymers with high cis content via a ruthenium-based initiator; the Grubbs group focused efforts on obtaining syndiotactic polymers. Recent advances with the use of sodium pivalate for the needed C-H activation in the catalyst synthesis allowed for the synthesis of catalysts 3-5, which contain a less sterically encumbered N-heterocyclic carbene (NHC). Although the group’s earlier work with catalyst 2 produced atactic polymers, catalysts 3-5 all yielded polymers with high syndiotacticity when reacted with norbornene-based monomers. Furthermore, as expected the polymers with increased tacticity displayed a higher Tg, presumably due to the increased packing order. Finally, the increased tacticity of the polymers is most likely due to the introduction of the smaller N-tert-butyl group on the NHC ligand of the catalyst; however, the exact nature of the substituents effect is still underway.
These couple of communications from the Grubbs group nicely increases the variety of specific microstructures that can be accomplished utilizing a ruthenium-based initiator for polymerization. Furthermore, the discovery of new ruthenium catalysts for polymer stereocontrol bodes well for a vibrant and exciting future in the development of materials with new properties.