A series of pentamethylcyclopentadienyliron(II) complexes, ligated by one carbonyl (CO) and one phosphine [Cp*Fe(CO)(L-phos)Br; Cp* = C5Me5; L-Phos = PPh3, PMePh2, PMe2Ph, P(m-tol)(3), and P(p-tol)(3)], were employed for living radical polymerization. In conjunction with it bromide initiator [H-(MMA)(2)-Br], these Cp*Fe complexes catalyzed living radical polymerization of methyl methacrylate (MMA) better controlled than those with the corresponding cyclopentadienyl (Cp) complexes [CpFe(CO)(L-phos)Br; Cp = C5H5]. The finer control was demonstrated by successful monomer-addition experiments, a wider range of controllable molecular weight (M-n = 10(4)-10(5) or DPn = 100-1000), and narrower molecular weight distributions (M-w/M-n similar to 1.2). FT-IR analysis of initiator-catalyst model reactions showed that an efficient carbonyl release from the original coordinatively saturated 18e complex into the unsaturated 16e form is important in the catalysis to generate a growing radical from the initiator. The higher catalytic activity allowed controlled polymerizations of other monomers that are not available for the Cp catalysts, such as methyl acrylate and a functional methacrylate with poly(ethylene glycol) pendent group.