Studies on Ring-Opening Metathesis Polymerization of Norborene Catalyzed by Hydride Containing Mononuclear Ruthenium Complex

Abstract

Ring-opening metathesis polymerization (ROMP) has emerged as an important tool for the preparation of architecturally unique macromolecules from cyclic olefins. A wide variety of catalytic systems, ranging from simple metal salts to highly sophisticated alkylidene metal complexes, have been used for the achievement of this synthetically useful transformation. Especially, tremendous efforts have been directed toward the ruthenium-based catalytic systems as well as the understanding of their reactivity patterns. And Grubbs catalysts represent the ultimate in terms of activity, but they are not cost effective due to the sophisticated ligand. Therefore, alternative catalyst precursors that are more readily accessible have been and are still being actively developed. Therefore, a hydride ligand containing mononuclear ruthenium complex Ru(p-cymene)HClPCy3 reported here was designed to catalyze ring-opening metathesis polymerization of norbornene with high activity. The hydride ligand is critical based on its high trans effect (for ligand dissociation) and the ability to generate carbene species through plausible migratory insertion and α-elimination steps. This catalytic system bearing hydride ligand could be an alternative for the well-defined Ru-based initiators which rules out the need of installation of complex ancillary ligands like NHC or O-Ligand in the coordination sphere or the need of activation with diazo compounds in the search for novel active catalytic species. Further study on the catalytic polymerization activity of Ru(p-cymene)H2PCy3 compared to the Ru(p-cymene)HClPCy3 system revealed that chloride ligand was also crucial to the hydride containing mononuclear ruthenium system, which contains easily dissociated ligand p-cymene and large phosphine ligand that stabilizes the metal complex. And the possible mechanism for the reaction was proposed.