Styrene/acrylic acid (S/AA) mixtures were copolymerized in concentrated 50 wt % 1,4-dioxane solutions at 120 °C at two ratios of additional free nitroxide mediator, N,N - tert -butyl- N -[1′-diethylphosphono-2,2′-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 ∼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 °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 minimization were r AA = 0.25 ± 0.11 and r S = 0.93 ± 0.37, in agreement with previous literature.
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Lessard et al. (2008) studied this question.
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