The addition of the pi-pi stacking agent octafluorotoluene (OFT) resulted in up to a 50% reduction in monomer conversion after 24 h for atom transfer radical polymerization (ATRP) reactions of styrene, when performed at 85 degrees C with 1 eq of OFT compared with styrene in the initial reaction mixture. Monitoring the progress showed that the ATRP of styrene in the presence of either OFT or hexafluorobenzene (HFB) maintained a linear relationship between monomer conversion and number average molecular weights, while showing a first order rate dependence on monomer. The effects of pi-pi stacking on the K(ATRP) could be overcome by using adjusting the redox activity of the metal-ligand complex while maintaining reaction temperatures of 85 degrees C. Further experiments showed that nitroxide-mediated polymerizations of St were affected to an identical extent by the presence of the pi-pi stacking agent HFB. The ATRP of pentafluorostyrene (PFSt) in the presence of pi-pi stackers benzene or toluene showed an increase in monomer conversion compared with reactions in their absence, consistent with M(n) pi-pi stacking increasing the stability of the active radical. Interactions between the pi-pi stacking agents OFT and HFB and the aromatic groups in the ATRP of St or PFSt were verified by (1)H NMR analysis. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 156-165, 2012