Water-soluble nitroxides with different structures were tested as mediators in the controlled free-radical polymerization of sodium 4-styrenesulfonate carried out in water at 130 °C. Nitroxides based on 1,1,3,3-tetramethylisoindolin-2-oxyl and 1,1,3,3-tetraethylisoindolin-2-oxyl with an ionic group on the aromatic ring (either a quaternary ammonium or a sulfonate substituent) exhibited better efficiency than water-soluble derivatives of 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO). The steric hindrance at the nitroxide site (ethyl or methyl substituent) had a much larger influence on the activation–deactivation equilibrium than the type of ionic group. With nitroxides bearing four ethyl substituents, the equilibrium constant was one order of magnitude larger than that obtained for nitroxides with four methyl substituents. Final molar masses matched the predicted values, with narrow distribution. “Livingness” of the poly(sodium 4-styrenesulfonate)s was confirmed by successful reinitiation of sodium acrylate in water at 130 °C. Water-soluble nitroxide radicals used for the synthesis of hydrophilic polymers.
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Huang et al. (2002) studied this question.