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The mechanism of atom transfer radical polymerization (ATRP) mediated by sodium dithionite (Na 2 S 2 O 4 ), with Cu II Br 2 /Me 6 TREN as catalyst (Me 6 TREN: tris[2-(dimethylamino)ethyl]amine) in ethanol/water mixtures, was investigated experimentally and by kinetic simulations. A kinetic model was proposed and the rate coefficients of the relevant reactions were measured. The kinetic model was validated by the agreement between experimental and simulated results. The results indicated that the polymerization followed the SARA ATRP mechanism, with a SO 2 ˙ − radical anion derived from Na 2 S 2 O 4 , acting as both supplemental activator (SA) of alkyl halides and reducing agent (RA) for Cu II /L to regenerate the main activator Cu I /L. This is similar to the reversible-deactivation radical polymerization (RDRP) procedure conducted in the presence of Cu 0 . The electron transfer from SO 2 ˙ − , to either Cu II Br 2 /Me 6 TREN or R–Br initiator, appears to follow an outer sphere electron transfer (OSET) process. The developed kinetic model was used to study the influence of targeted degree of polymerization, concentration of Cu II Br 2 /Me 6 TREN and solubility of Na 2 S 2 O 4 on the level of polymerization control. The presence of small amounts of water in the polymerization mixtures slightly increased the reactivity of the Cu I /L complex, but markedly increased the reactivity of sulfites.