Styrene/Acrylic Acid Random Copolymers Synthesized by Nitroxide-Mediated Polymerization: Effect of Free Nitroxide on Kinetics and Copolymer Composition

Abstract

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 fAA,0 > 40 mol %, the apparent rate constants increased sharply, noticeable exotherms were observed, and polydispersities increased from 1.20 at fAA,0 = 0 mol % to 1.48 at fAA,0 = 80 mol %. With 4.5 mol % [SG1]/[BlocBuilder], polymerization rates were slower and not as strongly affected by fAA,0 although exotherms were still noticeable at high fAA,0. Polydispersities remained ∼1.3 and only increased to >1.4 at high conversions for fAA,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 kpK values for comonomer mixtures bracketed between those of styrene and acrylic acid homopolymerizations at 120 °C (kp = propagation rate constant, K = equilibrium constant). Copolymer reactivity ratios estimated for samples produced using 9 mol % [SG1]/[BlocBuilder] by nonlinear least-squares minimization were rAA = 0.25 ± 0.11 and rS = 0.93 ± 0.37, in agreement with previous literature.