First nitroxide-mediated controlled free-radical polymerization of acrylic acid

Controlled poly(acrylic acid) homopolymers were synthesized for the first time by direct nitroxide-mediated polymerization of acrylic acid. The polymerizations were performed in 1,4-dioxane solution at 120 degreesC, using an alkoxyamine initiator based on the N-tert-butyl-N-(1-diethyl phosphono-2,2-dimethyl propyl) nitroxide, SG1. The kinetics were controlled by the addition of free nitroxide at the beginning of the polymerization and the optimal amount was 9 mol % with respect to the initiator. In this case, whatever the initiator concentration, all polymerizations exhibited the same rate and conversion reached 85-90% within 5 h. Although the rate constant of propagation of acrylic acid is very large, its reactivity is moderated by a low activation-deactivation equilibrium constant between active macroradicals and SG1-capped dormant chains. Various alkoxyamine concentrations were investigated to target different molar masses. At high initiator concentrations, the number-average molar mass, M-n, increased linearly with monomer conversion and followed the theoretical values; the polydispersity indexes ranged between 1.3 and 1.5. At low initiator concentration (high target M.), a deviation from linearity was observed in the M-n vs conversion plot and was clearly assigned to chain transfer to 1,4-dioxane. From these results, the best experimental conditions to obtain well-defined homopolymers with the minimum amount of dead chains were identified.