Styrene (S)/acrylonitrile [AN; initial acrylonitrile molar feed compositions (fAN,0's) = 0.100.86) and tert-butyl methacrylate (tBMA)/AN (fAN,0 = 0.100.80) copolymers were synthesized at 90 degrees C in 50 wt % 1,4-dioxane solutions with a unimolecular initiator, N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-O-(2-carboxylprop-2-yl) hydroxylamine [BlocBuilder (BB)]. In the tBMA/AN copolymerizations, 8.08.5 mol % N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) free nitroxide relative to BB was added. The S/AN copolymers exhibited narrow, monomodal molecular weight distributions (MWDs) with low polydispersities [weight-average molecular weight (Mw)/number-average molecular weight (Mn) = 1.141.26], and the Mn versus monomer conversion (X) plots were relatively linear (Mn = 18.1 kg/mol, X approximate to 0.7); this suggested that pseudo-living behavior was approached. AN proved to be an effective controlling comonomer for tBMA because the tBMA/AN copolymers exhibited narrow monomodal MWDs with Mw/Mn = 1.171.50 and relatively linear Mn versus X plots to reasonably high X values (Mn = 15.6 kg/mol, X approximate to 0.6). The AN and S monomer reactivity ratios were rAN = 0.07 +/- 0.01 and rS = 0.27 +/- 0.02 (FinemanRoss) and rAN = 0.10 +/- 0.01 and rS = 0.28 +/- 0.02 (KelenTudos), respectively; these values were in good agreement with conventional free-radical polymerization. Error-in-variables model (EVM) analysis indicated that the use of cumulative composition S/AN data was more effective than typical approaches using low-X data with the MayoLewis model. The AN and tBMA reactivity ratios [rAN = 0.07 +/- 0.01 and rtBMA = 1.24 +/- 0.20 (FinemanRoss) and rAN = 0.14 +/- 0.01 and rtBMA = 0.89 +/- 0.19 (KelenTudos)] were similar to those reported for related alkyl methacrylate/AN conventional radical copolymerizations. EVM analysis suggested significant experimental error was associated with the tBMA/AN system, and this warrants further investigation. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012