Dimanganese decacarbonyl (Mn-2(CO)(10)) initiates the radical polymerization of methyl methacrylate (MMA) in the presence of a variety of organic halides including C6H5CH2Br, Me2C(Br)CO2Et, BrCH2C(Br)=CH2, and ClCH2C(cl) CH2 in toluene at 60-90 degreesC. When using allylic halides (e.g., BrCH2C(Br)=CH2 or BrCH2C(CO2Me)=CH2), the number-average molecular weights and molecular weight distributions of the PMMA were both low and relatively narrow. The production of relatively low molecular weight polymers when using allylic halides could be explained by an addition-fragmentation chain-transfer mechanism and this is supported by end-group analysis.