The supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) of butyl acrylate (nBA) and styrene (Sty) using miniemulsion is reported for the first time. The polymerization was carried out in the presence of Cu(II)Br-2-based catalytic systems using EHA(6)TREN or BPMODA* and sodium dithionite (Na2S2O4) as SARA agent. Kinetic data revealed a controlled polymerization for both monomers, with a very stringent control over the molecular weight distribution (D1.2) but low monomer conversion. Reaction conditions were optimized in an attempt to understand the kinetics of polymerization, aiming to increase the final monomer conversion while maintaining the control over the polymerization. Self-chain extension reactions revealed low chain end fidelity, which corroborate the impossibility of increasing monomer conversion even after the judicious variation of the main polymerization parameters (monomers, surfactant, deactivator, and Na2S2O4 concentrations, and its method of addition). The data presented suggest a particular feature of Na2S2O4 in these polymerization systems involving the significant formation of dead chains ends, which has never been observed for any other reported system using this SARA agent. In contrast, control ARGET miniemulsion experiments under the same reaction conditions and using ascorbic acid as reducing agent allowed to achieve high monomer conversions. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 879-888