Styrene/tert-butyl acrylate (S/t-BuA) mixtures were copolymerized using nitroxide-mediated polymerization in bulk at 115 degrees C using an alkoxyamine unimolecular initiator, 2-[N-tert-butyl-2,2-(dimethylpropyl)aminooxy]propionic acid (BlocBuilder) to observe the effect of additional N-tert-butyl-N-[1-diethylphosphono(2.2-dimethylpropyl)] nitroxide (SG1) on kinetics and copolymer composition. Polymerizations with 4.5 mol % SG I relative to BlocBuilder were more controlled with lower polydispersities (from 1.14 to 1.22), and the number-average molecular weight (M) over bar (n) exhibited linear behavior with conversion up to similar to 50%. In contrast, without SG1, apparent rate constants were greater but higher polydispersities were observed, and (M) over bar (n) vs conversion deviated from the ideal linear relationship at low conversions similar to 20%. The value of k(p)K (k(p) = propagation rate constant, K = equilibrium constant) for t-BuA at 115 degrees C was 3.0 x 10(-6) s(-1) and is lower than other acrylic monomers. The copolymers were richer in t-BuA than the feed composition at low initial t-BuA. feed compositions f(t) (- BuA.0) (20 mol %) but became richer in styrene when f(t - BuA.0)> 40 mol %. Copolymer reactivity ratios ranged from r(t - BuA) = 0.09 to 0.12 and r(S) = 0.40 to 0.49 using Fineman -Ross, Kelen - Tudos, and nonlinear least-squares fitting methods.