Styrene polymerization via generation of activators by monomer addition (GAMA) for atom transfer radical polymerization (ATRP) has been examined extensively with bulk FeX3 and FeX2 at 110 degrees C in conjunction with various phosphorus-bearing ligands. It was found that GAMA possesses advantages over normal ATRP. Most importantly, narrower polydispersity index (PDI) values were observed from the styrene polymerizations with Fe(III) over those with Fe(II). Every instance of 2-(diphenylphosphino)-N,N'-dimethyl-[1,1'-biphenyl]-2-amine and 2-(diphenylphosphino) pyridine with the Fe(III) system were controlled excellently without addition of any radical initiator or reducing agent additives. Initiator type was found to exert a significant factor to influence on the controllability of polymerization. The initiation of 1-phenylethyl chloride and methyl-2-chloropropionate gave rise to formation of polymers with narrow PDI (1.05-1.20), whereas those from 1-phenylethyl bromide increased to 1.35. The GAMA of bulk styrene exhibited the best performance in terms of both rate and controllability compared with toluene and anisole. Both formation of block copolymer from the macroinitiator and efficient perturbation of polymerization with 2,2,6,6-tetramethylpiperidine 1-oxyl provided firm evidence to support the living and radical characteristics for the GAMA of styrene. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polyrn Chem 48: 144-151, 2010