Designing Sequence Selectivity into a Ring-Opening Metathesis Polymerization Catalyst

Abstract

The development of a chemoselective catalyst for the sequence-selective copolymerization of two cycloolefins by ring-opening metathesis polymerization is described, starting with the mechanistic work that established the structure of the key metallacyclobutane intermediate. Experimental and computational investigations converged to a conclusion that the lowest energy metallacyclobutane intermediate in the ruthenium carbene-catalyzed metathesis reaction had the four-membered ring trans to the phosphine or NHC ligand. The trans-metallacyclobutane structure, for the case of a degenerate metathesis reaction catalyzed by a Grubbs first-generation complex, necessitated a rotation of the 3-fold symmetric tricyclohexylphosphine ligand, with respect to the 2-fold symmetric metallacyclobutane substructure. The degeneracy could be lifted by constraining the rotation. Lifting the degeneracy created the possibility of chemoselectivity. This mechanistic work led to a concept for the "tick-tock" catalyst for a chemoselective, alternating copolymerization of cyclooctene and norbornene from a mixture of the two monomers. The design concept could be post facto elaborated in terms of stereochemistry and topological theory, both viewpoints providing deeper insight into the design of selectivity into the catalytic reaction. The iterative interaction of theory and experiment provided the basis for the rational design and optimization of a new selectivity into an existing catalytic system with decidedly modest structural modifications of the original carbene complex.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…Although ring-opening metathesis polymerization of olefins (ROMP) has been studied intensively and has found application even in industrial use, examples of ring-opening alkyne metathesis polymerization are more exceptional. [3,164] Well-defined catalysts, which controllably initiate metathesis polymerization, while being inert towardsC Cb onds of the polymer chain, were unknown for many years. In the late 1980s, Schrock's group reported the first ring-opening alkyne metathesisp olymerization (ROAMP) of cyclooctyne with molybdenumc omplex [tBuCMo(OtBu) 3 ]r esulting in polymeric material with PDIs of more than 4, which suggested the occurrence of backbiting.…”

New ROMP Synthesis of Ferrocenyl Dendronized Polymers

“…This strategy provides G1 monomer 7 and G2 monomer 11 that contain three and nine Fc moieties, respectively, whereas the monomer 4 bearing a single Fc unit was synthesized for comparison ( Scheme ). All the polymerization reactions used Grubbs' third generation catalyst 1 and are controlled, and the polymerization rates are highly dependent on the bulk of the dendritic macromonomers.…”

Remote control grubbs catalysts that modulate ring‐opening metathesis polymerizations

“…30 Steric modulation of the coordination environment in Grubbstype Ru complexes has also been shown to produce latent ROMP catalysts that undergo activation at elevated temperatures. For example, in 2011, Grubbs and coworkers reported a Ru complex that featured iodide ligands in lieu of the prototypical chlorides (5). 31 While the increased steric bulk hindered monomer coordination and resulted in a Ru complex that was inactive towards the ROMP of functionalized norbornene derivatives at room temperature, elevated temperatures (85 8C) resulted in nearly complete conversion of monomer to polymer.…”

“…Over the past several decades, there has been a surge in the use of ring‐opening metathesis polymerization (ROMP) chemistry to synthesize macromolecular materials . The increasing attention is due, in part, to the advent and development of Grubbs‐type catalysts .…”

“…INTRODUCTION Over the past several decades, there has been a surge in the use of ring-opening metathesis polymerization (ROMP) chemistry to synthesize macromolecular materials. [1][2][3][4][5][6] The increasing attention is due, in part, to the advent and development of Grubbs-type catalysts. 7 In addition to their broad functional group tolerance, these complexes are robust and amenable to extensive modification.…”