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 °C, using an alkoxyamine initiator based on the N-tert-butyl-N-(1-diethyl phosphono-2,2dimethyl 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 Mn), a deviation from linearity was observed in the Mn 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.

This article follows a previous study (Macromolecules 2005, 38, 5485) demonstrating that the nitroxide SG1-mediated polymerization of methyl methacrylate can be achieved at 90 °C with high conversion and high quality of control by introducing a small amount of styrene. In this work, the resulting polymer was characterized and the presence of SG1-based alkoxyamine at the polymer chain-end was identified, supporting the livingness of the macromolecules. In particular, it was shown that the alkoxyamine end group was connected to a single styrene terminal unit and that the methyl methacrylate penultimate unit had a strong effect on the temperature of dissociation. Consequently, the copolymerization of methyl methacrylate with a low molar proportion of styrene could be performed at temperatures below 90 °C. The polymer was also used as an efficient macroinitiator in the polymerization of styrene and n-butyl acrylate, to form methyl methacrylate-based block copolymers.

Water-soluble macroalkoxyamines are shown to be particularly well-suited initiators for nitroxide-mediated emulsion polymerization. They lead to the synthesis of amphiphilic block copolymers that self-assemble in situ into well-defined nanoobject morphologies, in agreement with the principles of polymerization-induced micellization. Depending on the molar mass of the hydrophobic block, the formed nanoparticles are hairy spherical micelles, nanofibers, or vesicles. The nanofibers are the most intriguing and spectacular structure and strongly affect the physicochemical properties of the aqueous dispersions.

No abstract