Ferrocene Cocatalysis in Metal-Catalyzed Living Radical Polymerization: Concerted Redox for Highly Active Catalysis.

Abstract

Ferrocene (FeCp2), despite its high stability, was found, for the first time, to cocatalyze living radical polymerization in concert with a ruthenium main catalyst (RuII) that is directly responsible for generating growing radicals. FeCp2 turned out to promote the following key reactions: regeneration of RuII through a reduction of XRuIII (FeIICp2 + XRuIII → FeIIICp2+X- + RuII; X: halogen); halogen-capping reaction, or regeneration of dormant species ∼∼∼C-X, by the resultant trivalent ferrocenium cation FeIIICp2+ (∼∼∼C• + FeIIICp2+X- → ∼∼∼C-X + FeIICp2). The cocatalysis was further improved by the addition of n-Bu4NCl to allow a dramatic decrease in the initial RuII concentration without any loss of the high controllability. For example, in conjunction with FeCp2/n-Bu4NCl, only 50 ppm (for monomer) of RuII can catalyze living radical polymerization to give controlled polymers with high molecular weights and narrow molecular weight distributions (Mn ∼ 1.0 × 105; Mw/Mn ∼ 1.3). Such a concerted catalysis with ferrocene would open the door to practical applications of living radical polymerization.