Decamethylferrocene (FeCp2*) was found to be an effective cocatalyst for metal-catalyzed living radical polymerization with FeBr2/n-Bu4NBr as the main catalyst. For polymerization of methyl methacrylate (MMA) with a bromine-type initiator, the conversion was limited in the absente of FeCp2*, whereas the polymerization was apparently promoted up to higher conversion (similar to 90%) upon combination with FeCp2*. Despite the acceleration, molecular weights and its distributions (MWDs) were fairly controlled (M-w/M-n < 1.1, conversion = 90%). The "concerted" catalysis with the two iron complexes allowed superior control of the polymerization to those without FeCp2* to give high halogen functionalities of obtained polymers. Importantly, the two iron complexes can perform the catalysis helping each other, independently of electron support from ligands (e.g., phosphine). Therefore, ligand dissociation by polar groups, i.e., "catalyst poison", does not matter in the FeCp2*-concerted system, leading to control over (co)polymerizations with functional monomers with polar pendant groups, such as poly(ethylene glycol) (PEGMA), hydroxyl (HEMA), and carboxyl (MAA). The robust catalysis with the sustainable and safe metal, iron, would be promising toward actual applications with metal-catalyzed living radical polymerization.