Anionic polymerization of methyl methacrylate (MMA) initiated with late transition-metal halides [manganese chloride (MnCl2), iron dichloride (FeCl2), iron trichloride (FeCl3), cobalt chloride (CoCl2), or nickel bromide (NiBr2)]/organolithium [nButyllithium (nBuLi) or phenyllithium (PhLi)]/triisobutylaluminum (iBu(3)Al) systems is described. Except for the system with NiBr2, the polymerizations of MMA afforded narrow molecular weight distribution poly(methyl methacrylate)s (PMMAs) with high molecular weights in quantitative yields at 0 degreesC in toluene. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analyses of the PMMAs obtained by the systems with FeCl2, FeCl3, and CoCl2 revealed that the polymers had hydrogen (H) at both chain ends. Accordingly, the reaction of the transition-metal halides with the organolithium in the presence of iBu(3)Al should result in the formation of transition-metal hydride [M-H](-) species, which was nucleophilic enough to initiate the MMA polymerization. Because the presence of a six-membered cyclic structure resulting from backbiting was confirmed from the MALDI-TOF MS analyses of the PMMA obtained with the metal halide (FeCl2, FeCl3, or CoCl2)/organolithium systems in the absence of iBu(3)Al, the introduction of H at the omega-chain end indicated that iBu(3)Al should prevent the backbiting. However, the MnCl2/nBuLi/iBu(3)Al initiating system gave PMMAs bearing H at the a chain end and six-membered cyclic structure at the omega chain end. (C) 2003 Wiley Periodicals, Inc.