Free Radical Polymerization in Aqueous Solution of Acrylic Acid Mediated by Nitroxides Originated from NaNO2

A series of investigations was carried out focusing on the confinement of the Trommsdorf (gel) effect in radical polymerization of acrylic acid, AA, in its Na-form. Sodium nitrite, NaNO2, functioning at room temperature as an inhibitor for water-soluble monomers, proved to be an effective mediator in polymerizations carried out at t = 90 °C. As a result it was possible to polymerize AA at a high initial monomer concentration, i.e. [M]0 = 40 g/dl. Therefore, concentrated solutions of NaPAA in water were obtainable in a one-step process. It was found that NaNO 2 retards the onset of polymerization up to 60 °C at which temperature only a small degree of conversion α= 30% was reached after 40 hours. On the other hand, without added NaNO2 a less concentrated monomer solution, having [M]0 = 10 g/dl was polymerized to α= 96% at 60 °C after 40 minutes. At 90°C, the polymerization of NaAA no longer appeared to be retarded by sodium nitrite, since in 40 g/dl solutions high degrees of conversion were obtainable within reasonable time periods, without any sign of autoacceleration. As NaNO2 generates nitroxides, it seemed reasonable to analyze the progress of the mediated polymerizations as an approach to “livingness”. In fact, some increase inover time of polymerization, τ, was observed, but generally the criteria for a “living polymerization” remained unfulfilled. Relatively high coefficients of polydispersityranging between 6.50 and 9.62 were found for all the NaNO2 mediated poly(acrylic acid)s. A value of the same order resulted for the non-mediated poly(acrylic acid).