The alpha-diimine iron complexes (R',R")[NN]FeCl2 ((R',R")[N,N] = R'-N = CR"-CR"= N-R') are efficient catalysts for the atom-transfer radical polymerization (ATRP) of styrene when R' and R" are electrondonating substituents and favor catalytic chain transfer (CCT) when electron-withdrawing substituents are employed. An organometallic pathway, alongside a halogen-atom-transfer equilibrium, is proposed to mediate the observed reactivity. The model alkyl complexes, (R',R")[N,N]FeCl2(R), where R = PhCH2 and Ph(Me)CH, were generated via treatment of (R',R'')[N,N]FeCl3 with RMgCl. The alkyl derivatives obtained from intermediate spin-state (R',R'')[NN]FeCl3 complexes were found to be stable to ca. -30 degrees C and favor CCT, whereas the alkyl derivatives derived from high-spin-state trichloride precursors are unstable above -78 degrees C and favor ATRP. Azo-initiated polymerizations of styrene are moderately controlled by alpha-diimine iron catalysts. The role of organometallic-mediated radical polymerization (OMRP) in the controlled polymerization of styrene is discussed: an analysis of the radical concentrations generated by the competing OMRP and ATRP equilibria indicates that the halogenophilicity of the Fe(II) catalyst dominates the carbophilic alkyl radical-trapping capacity of the Fe(II) species in this alpha-diimine catalyst system.