Styrene/acrylic acid (S/AA) mixtures were copolymerized in concentrated 50 wt % 1,4-dioxane solutions at 120 degrees C at two ratios of additional free nitroxide mediator, N,N-tert-butyl-N-[1'-diethylphosphono2,2'-dimethylpropyl] nitroxide (SG1), relative to 2-[N-tert-butyl-2,2-(dimethylpropyl)aminooxy]propionic acid (BlocBuilder, Arkema) alkoxyamine unimolecular initiator (4.5 and 9 mol % [SG1]/[BlocBuilder]). Without SG1 at initial acrylic acid monomer feed concentrations f(AA,0) > 40 mol %, the apparent rate constants increased sharply, noticeable exotherms were observed, and polydispersities increased from 1.20 at f(AA,0) = 0 MOl % to 1.48 at f(AA,0) = 80 mol %. With 4.5 mol % [SG1]/[BlocBuilder], polymerization rates were slower and not as strongly affected by f(AA,0) although exotherms were still noticeable at high f(AA,0). Polydispersities remained similar to 1.3 and only increased to > 1.4 at high conversions for f(AA,0) = 80 mol %. Exotherms were rendered nearly negligible when 9 mol % [SG1]/[BlocBuilder] was used, leading to copolymers with much narrower molecular weight distributions and k(p)K values for comonomer mixtures bracketed between those of styrene and acrylic acid homopolymerizations at 120 degrees C (k(p) = propagation rate constant, K = equilibrium constant). Copolymer reactivity ratios estimated for samples produced using 9 mol % [SG1]/[BlocBuilder] by nonlinear least-squares minintization were r(AA) = 0.25 +/- 0.11 and r(s) = 0.93 +/- 0.37, in agreement with previous literature.