Bisphosphine Monoxide-Ligated Ruthenium Catalysts: Active, Versatile, Removable, and Cocatalyst-Free in Living Radical Polymerization

As potentially bidentate ligands, bisphosphine monoxides [BPMOs; Ph<sub>2</sub>P(O)(CH<sub>2</sub>)<sub><i>n</i></sub>PPh<sub>2</sub>; <i>n</i> = 1, 2; Ph = C<sub>6</sub>H<sub>5</sub>] were found to be effective for pentamethylcyclopentadiene ruthenium chloride catalysts [Cp*Ru<sup>II</sup>Cl(BPMO)<sub><i>m</i></sub>; <i>m</i> = 1,2] in living radical polymerization: active, versatile, cocatalyst-free, and removable. The complexes catalyzed living radical polymerizations of a variety of monomers and their functionalized derivatives: methyl acrylate, methyl methacrylate (MMA), styrene, 2-hydroxyethyl methacrylate, and poly(ethylene glycol) methacrylate. The controllability and activity were high enough even with a small amount of catalyst ([Ru]<sub>0</sub>/[initiator]<sub>0</sub> = 1/200; 50 ppm for monomer), to give high molecular weight PMMA with narrow MWD (<i>M</i><sub>n</sub> = 103 000: <i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> = 1.19) and block copolymers. Such an activity and a wide applicability in terms of monomers have been found for few Ru catalysts thus far. Importantly, they did not necessarily need a cocatalyst (aluminum alkoxide, amine, etc.) for their catalysis, in contrast to most of the other ruthenium catalysts that are effective only with a cocatalyst. The cocatalyst-free catalysis is concluded to be derived from the phosphine oxide moiety in BPMO, whose hemilabile coordination promotes the deactivation process [∼∼∼C• (growth active) → ---C−Cl (dormant)] and, in turn, accelerates the whole catalytic cycle (radical ↔ dormant; Ru<sup>II</sup> ↔ Ru<sup>III</sup>). Furthermore, the high polarity of BPMO ligands effectively helped near perfect removal of the catalyst residue (>99.7% for PMMA) just by single reprecipitation into methanol.