Well-defined poly[(ethylene oxide)-block-(sodium 2-acrylamido-2-methyl-1-propane sulfonate)] diblock copolymers [P(EOm-b-AMPS(n))], have been obtained by water-based ATRP using alpha-methoxy-omega-(2-methylbromoisobutyrate) poly(ethylene oxide)s (MeO-P[EO](m)-(BrB)-B-i with m ranging from 12 to 113) and CuBr center dot 2Bpy (Bpy for 2,2'-bipyridyl) as macroinitiator and catalytic complex, respectively. Compared to direct polymerization in water, it has been demonstrated that the water/methanol (3:1, v/v) mixture is better suited for predicting the final number-average molar mass from the initial monomer-to-macroinitiator molar ratio and achieving a quite narrow polydispersity, even at high monomer conversion ((M) over bar (w)/(M) over bar (n) approximate to 1.4 at 80% conversion). The effect of temperature, solvent mixture composition and addition of NaCl salt on the polymerization rate and extent of control over the copolymer molecular parameters have been highlighted as well.