Bernadette Charleux

Controlled/living radical polymerization in aqueous media: homogeneous and heterogeneous systems

Abstract Controlled/living radical polymerizations carried out in the presence of water have been examined. These aqueous systems include both the homogeneous solutions and the various heterogeneous media, namely dispersion, suspension, emulsion and miniemulsion. Among them, the most common methods allowing control of the radical polymerization, such as nitroxide-mediated polymerization, atom transfer radical polymerization and reversible transfer, are presented in detail.

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 °C, 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, Mn, increased linear...

Preparation of Block copolymers of polystyrene and poly (t-butyl acrylate) of various molecular weights and architectures by atom transfer radical polymerization

Block copolymers of polystyrene and poly(t-butyl acrylate) were prepared using atom transfer radical polymerization techniques. These polymers were synthesized with a CuBr/N,N,N′,N″,N″-pentamethyldiethylenetriamine catalyst system and had predictable molecular weights based on the degree of polymerization, as calculated from the initial ratio of monomer to initiator. The final polydispersities were low (1.10 < Mw /Mn < 1.3) for all the homopolymers and block copolymers. Polymers of various chain architectures were prepared, ranging from linear AB diblocks to three-armed stars composed of AB diblocks on each arm. The key to controlled synthesis with this catalyst system was the choice of the solvent, temperature, and concentrations of catalyst and deactivator.

Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of n-Butyl Acrylate

Chain transfer to polymer (CTP) in conventional free-radical polymerizations (FRPs) and controlled radical polymerizations (ATRP, RAFT and NMP) of n-butyl acrylate (BA) has been investigated using (13) C NMR measurements of branching in the poly(n-butyl acrylate) produced. The mol-% branches are reduced significantly in the controlled radical polymerizations as compared to conventional FRPs. Several possible explanations for this observation are discussed critically and all except one refuted. The observations are explained in terms of differences in the concentration of highly reactive short-chain radicals which can be expected to undergo both intra- and inter-molecular CTP at much higher rates than long-chain radicals. In conventional FRP, the distribution of radical concentrations is broad and there always is present a significant proportion of short-chain radicals, whereas in controlled radical polymerizations, the distribution is narrow with only a small proportion of short-chain radicals which diminishes as the living chains grow. Hence, irrespective of the type of control, controlled radical polymerizations give rise to lower levels of branching, when performed under otherwise similar conditions to conventional FRP. Similar observations are expected for other acrylates and monomers that undergo chain transfer to polymer during radical polymerization.

Elaboration of Monodisperse Spherical Hollow Particles with Ordered Mesoporous Silica Shells via Dual Latex/Surfactant Templating: Radial Orientation of Mesopore Channels

Monodisperse spherical hollow nanoparticles of mesoporous silica featuring mesopores with a radial orientation in the silica shell were synthesized via a dual-templating method. Specifically designed polystyrene latexes with anionic or cationic surface charges acted as the core templates, while cetyltrimethylammonium bromide served as a co-template to structure the mesopore formation during tetraethoxysilane hydrolysis/condensation. The particles were well-separated and presented homogeneous mesoporous silica shells. Average particle diameters were less than 200 nm, and the particles displayed high values of specific surface area and pore volume. The shell thickness and the hollow core diameter could be tuned independently while the radial pore structure was preserved. A detailed analysis of the nitrogen adsorption-desorption isotherms proved that the central cavity was completely isolated from the external medium, that is, only accessible through the radial mesopores of the shell. Consequently, our particles gather the advantages of a well-defined structure, straight penetrating channels across the silica shell, and a high accessible porous volume of the central core. These properties make them far better candidates than simple mesoporous particles for any storage and/or controlled release applications.

Tetrathiafulvalene End-Functionalized Poly(N-isopropylacrylamide): A New Class of Amphiphilic Polymer for the Creation of Multistimuli Responsive Micelles

In this article, we report the formation of micelles from a tetrathiafulvalene (TTF) end-functionalized poly(N-isopropylacrylamide) (poly(NIPAM)) derivative (1). We have determined the critical aggregation concentration (CAC) and average diameter of the micelles using fluorescence spectroscopy and dynamic light scattering experiments, respectively. We have exploited the NIPAM backbone of the polymer to thermally transform the swollen hydrophilic poly(NIPAM) derivative to a more globular hydrophobic state at the lower critical solution temperature (LCST). Finally, we have shown that we can exploit the chemical oxidation and complexation properties of the TTF unit to disrupt the micelle architecture to release the hydrophobic dye Nile Red from the interior of the micelle.

Effect of the solvent composition on the morphology of nano-objects synthesized via RAFT polymerization of benzyl methacrylate in dispersed systems

A hydrophilic macromolecular RAFT (reversible addition–fragmentation chain transfer) agent (macroRAFT agent) composed of 50 mol% methacrylic acid and 50 mol% poly(ethylene oxide) monomethyl ether methacrylate end-capped by a reactive trithiocarbonate group (P(MAA-co-PEOMA)) was used in the polymerization of benzyl methacrylate (BzMA) in different media, ethanol–water and 1,4-dioxane–water mixtures. Depending on the solvent composition, the polymerization showed features of either a dispersion polymerization (monomer soluble in the initial medium) or an emulsion polymerization (monomer insoluble in the initial medium). In all cases, the RAFT mechanism led to the in situ formation of well-defined amphiphilic P(MAA-co-PEOMA)-b-PBzMA block copolymers that self-assembled during the growth step into self-stabilized nano-objects, according to a polymerization-induced micellization process. For a given composition of the block copolymer, the final morphology depended strongly on the solvent composition. The presence of the organic co-solvent was favorable to the formation of fibers while an increased amount of water favored the formation of spherical particles. Compared to the ethanol–water system, in which the non-spherical objects existed only above 77–80 vol% of ethanol, in 1,4-dioxane–water mixtures the morphological transition was observed at a lower proportion of organic co-solvent (close to 20 vol%). For a given molar mass of the macroRAFT agent and an increased molar mass of the PBzMA block in a given solvent composition (ethanol–water, 95/5, v/v), the morphology changed from spheres to fibers and then to large spheres or vesicles. The molar mass window in which fibers were obtained was wider than that observed in pure water at pH 5 using the same macroRAFT agent [X. Zhang et al., Macromolecules, 2011, 44, 4149].

Atom transfer radical polymerization of n‐butyl methacrylate in an aqueous dispersed system: A miniemulsion approach

Ultrasonication was applied in combination with a hydrophobe for the copper-mediated atom transfer radical polymerization of n-butyl methacrylate in an aqueous dispersed system. A controlled polymerization was successfully achieved, as demonstrated by a linear correlation between the molecular weights and the monomer conversion. The polydispersities of the polymers were small (weight-average molecular weight/number-average molecular weight < 1.5). The influence of several factors, including ultrasonication, the amount of the surfactant, and the nature of the initiator, on the polymerization kinetics, molecular weight, and particle size was studied. The polymerization rate and molecular weights were independent of the number of particles and only depended on the atom transfer equilibrium. The final particle size, however, was a function of all the parameters.

Effect of comonomer composition on the controlled free-radical copolymerization of styrene and maleic anhydride by reversible addition-fragmentation chain transfer (RAFT)

Controlled free-radical copolymerization of styrene and maleic anhydride was performed in 1,4-dioxane or tetrahydrofurane solution at 60 °C using the RAFT technique. The effect of monomer feed ratio on copolymerization kinetics and on control over molar mass distribution was examined. It was shown that polymerization was faster and quality of control was poorer when the proportion of maleic anhydride in the monomer feed was larger. These features were assigned to a decrease in the chain transfer constant of the polymeric RAFT agent, most probably due to an increase in the apparent rate constant of propagation with the proportion of maleic anhydride.

Amphiphilic block copolymers from a direct and one-pot RAFT synthesis in water.

The syntheses of amphiphilic block copolymers are successfully performed in water by chain extension of hydrophilic macromolecules with styrene at 80 °C. The employed strategy is a one-pot procedure in which poly(acrylic acid), poly(methacrylic acid) or poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate) macroRAFTs are first formed in water using 4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA) as a chain transfer agent. The resulting macroRAFTs are then directly used without further purification for the RAFT polymerization of styrene in water in the same reactor. This simple and straightforward strategy leads to a very good control of the resulting amphiphilic block copolymers.