The effect of pH and the ligand nature over the atom transfer radical polymerization (ATRP) of the anionic monomer sodium 2-acrylamido-2-methylpropanesulfonate (AMPSNa) was investigated in aqueous medium by using W-halogenated poly(ethylene oxide) and CuBr, as macroinitiator and catalyst, respectively. The stability of both catalytic complexes and macroinitiator was investigated in function of pH, that is, fixed between 7.5 and 12. UV-VIS spectroscopy confirmed a good catalytic complex stability in the studied conditions. Hydrolysis of the macroinitiator ester group at pH higher than 7.5 was detected by H-1 NMR and GPC, yielding ill-defined polymer samples when ATRP is performed in alkaline conditions. 2,2'-Bipyridyl (Bpy), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), and tris(2-methyl-aminoethyl)amine (Me-6-TREN)-based complexes were compared at the optimal pH (pH 7.5). When polymerization was carried out in the presence of CuBr . 2Me(6)-TREN complex block copolymers with narrow molecular weight distribution (1.1 <= (M)over bar>(W)/(M)over bar>n <= 1.3), and good agreement between theoretical and experimental molar masses was obtained. Moreover, increasing the PAMPSNa polymerization degrees (n) did not affect the control over the polymerization. Preliminary characterization of the diblock copolymers behavior in aqueous medium revealed a strong polyelectrolyte effect independently of n. Interestingly, occurrence of interactions between the PEO and PAMPSNa-blocks was also evidenced by differential scanning calorimetry and thermogravimetric analyses. (C) 2009 Wiley Periodicals, Inc. J Polyro Sci Part A: Polym Chem 47: 1108-1119, 2009