Polymerization of the cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) by the methods of controlled radical polymerization AGET ATRP and SARA ATRP is reported, demonstrating successful synthesis of well-defined cationic polymers in aqueous solutions. A modification of SARA system (ISARA) for aqueous solutions was proposed based on the use of supplemental redox intermediates (SRI) mediating electron transfer between zerovalent metal and the deactivator Cu-II/L, ensuring steady generation of the activator in situ. Ascorbic acid and hydroquinone in their oxidized form, and iron(III) chloride are between preferable SRI. A mechanism of generation of the activator Cu-I/L in the presence of zerovalent metals and SRI in aqueous solutions of ISARA ATRP was proposed. The main advantages of ISARA compared to AGET ATRP are simpler technical implementation, better reproducibility of the results and good control over polymerization up to high conversions of the monomers.