Fast and Controlled Aqueous Ring-Opening Metathesis Polymerization with a Ruthenium Alkylidene Initiator Bearing a pH-Responsive N-Heterocyclic Carbene Ligand

A pentacoordinate ruthenium benzylidene complex was synthesized and characterized, bearing a sterically demanding NHC ligand containing two β-dimethylaminoethoxy groups, two chloride ligands, and a pH-responsive DMAP ligand. The NHC ligand requires only one additional step in the standard synthesis from commercially available precursors, which makes it a highly attractive specialized ligand for aqueous applications. The catalyst was employed in acidic aqueous media, exhibiting unprecedented high activity and efficiency in the aqueous ROMP of various water-soluble (7-oxa)norbornene derivatives. In addition to the elevated activity, the aqueous polymerization also proceeds with first-order kinetics with catalyst loadings between 0.1% and 1%, indicating the potential for controlled living ROMP. Aqueous RCM was performed with diallylammonium chloride in aqueous HCl. The metathesis reaction was active for several minutes only, but conversions into the predicted 2,5-dihydropyrrole of up to 30% were accomplished with 1% catalyst loading. Reaction times past 20 min did not afford additional RCM product. The unprecedented metathesis reactivity, in particular in aqueous ROMP, makes this catalyst the first representative of a new and highly active generation of olefin metathesis catalysts for homogeneous aqueous applications.