Cationic Cp*–Ruthenium Catalysts for Metal-Catalyzed Living Radical Polymerization: Cocatalyst-Independent Catalysis Tuned by Counteranion

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• 作者：Kojiro Fujimura ; Makoto Ouchi ; Junya Tsujita ; Mitsuo Sawamoto
• 刊名：Macromolecules
• 出版年：2016
• 出版时间：April 26, 2016
• 年：2016
• 卷：49
• 期：8
• 页码：2962-2970
• 全文大小：542K
• 年卷期：0
• ISSN：1520-5835

文摘

We present the cationic Cp*–ruthenium complexes [Cp*Ru(CH₃CN)(PPh₃)₂A; A^–: the counteranion; abbreviated Cp*Ru⁺A^–] as the active, cocatalyst-free, and tunable catalysts for metal-catalyzed living radical polymerization (Mt-LRP). A PF₆-based cationic complex in conjunction with a alkyl halide initiator led to controlled polymerization of methyl methacrylate (MMA) giving well-controlled PMMAs with narrow molecular weight distribution (M_w/M_n < 1.10) at appropriate polymerization rate (conversion >80% in 15 h), although cocatalyst was not combined. The catalytic feature was clearly different from the neutral counterpart [Cp*RuCl(PPh₃)₂: Cp*Ru], resulting in retarded polymerization under the cocatalyst-free condition. Interestingly, the catalytic activity was influenced by the counteranions. For example, the PF₆-based complex did not afford catalysis for controlled polymerization of methyl acrylate (MA), but an Sb₆-based [Cp*Ru(CH₃CN)(PPh₃)₂SbF₆: Cp*Ru⁺SbF₆^–] allowed control of the polymerization. Model reaction analysis revealed the unique transformation of cationic complex through activation process for carbon–halogen bond: weakly coordinated solvent (e.g., CH₃CN) is likely to be eliminated to give a 16e complex instead of the neutral phosphine ligand (e.g., PPh₃). Indeed, bipyridine (bipy), which could coordinate tighter than PPh₃, was acceptable as ligand for the cationic complex to control radical polymerization, in sharp contrast to with the neutral counterpart requiring ligand elimination for the activation process.