We achieved metal-catalyzed living radical polymerization (LRP) through "unique" catalyst transformation of iron (Fe) complex in situ. A dicarbonyl iron complex bearing a pentaphenylcyclopentadiene [(Cp-Ph)Fe(CO)(2)Br: Cp-Ph = eta-C5Ph5] is too stable itself to catalyze LRP of methyl methacrylate (MMA) in conjunction with a bromide initiator [H-(MMA)(2)-Br]. However, an addition of catalytic amount of triphenylphosphine (PPh3) for the system led to a smooth consumption of MMA giving "controlled" polymers with narrow molecular weight distributions (similar to 90% conversion within 24 h; M-w/M-n = 1.2). FTIR and P-31 NMR analyses of the complex in the model reaction with H-(MMA)(2)-Br and PPh3 demonstrated that the two carbonyl ligands were irreversibly eliminated and instead the added phosphine was ligated to give some phosphorous complexes. The ligand exchange was characteristic to the Cp-Ph complex: the exchange was much smoother than other cyclopentadiene-based complexes [i.e., CpFe(CO)(2)Br: Cp = C5H5; Cp*Fe(CO)(2)Br, Cp* = C5Me5]. The smooth transformation via the ligand exchange would certainly contribute to the controllability at the earlier stage in the polymerization as well as at the latter. The catalytic activity was enough high, as demonstrated by the successful monomer addition experiment and precise control even for higher molecular weight polymer (M-w/M-n < 1.2 under 1000-mer condition). Such an in situ transformation from a stable complex would be advantageous to practical applications. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 537-544, 2011