A tricomponent system, constituting monomer (methyl methacrylate, MMA), higher oxidation state transitionmetal catalyst (FeBr3) and a ligand (triphenylphosphine, PPh3), MMA/FeBr3/PPh3 system without external initiator (alkyl halide) has been studied extensively with different spectroscopic analyses. To figure out the mechanism, a series of explicit model reactions were conducted with a molar ratio of [MMA](0)/[FeBr3](0)/[PPh3](0) = 200/1/n (n = 0.1-3.0) at 80 degrees C, and the corresponding polymerization behaviors were investigated. Combined with theoretical deduction and spectroscopic evidences, the composition of the in-situ generated initiators was gradually confirmed, which were redox products of FeBr3 and PPh3. (C) 2017 Wiley Periodicals, Inc.