Polymeric surfactants complex formation

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

At surfactant concentrations above the equivalent point (>5 x 10 4 mol dm 3) the A curves in the presence of the polyion resemble the behavior of the curve in pine surfactant solutions. They show a break at the so-called apparent critical micellization concentration, cmc [16,42], This term is usually used for characterizing the formation of free surfactant micelles in the presence of the polymeric component. The cmc is higher than the ordinary cmc due to the formation of polyelectrolyte-surfactant complex at lower surfactant concentrations. The bound surfactant ions are not available for... 

In copolymerization, the more reactive monomer may be added to the reactor over time to produce a more uniform copolymer composition distribution. This may be done by feeding comonomer at fixed rates, by adding various comonomers at predetermined times, or by following a complex monomer addition policy determined by off-line optimization of a mathematical model of the polymerization process. If copolymer composition is measured or estimated on-line, the reactive monomer can be added in a closed-loop fashion [35]. In emulsion polymerization, surfactant may be added over time to control the formation of new particles, and hence the particle size distribution (PSD) [36]. 

The complex formation of PVP with sodium dodecyl sulfate (SDS) should be of interest not only in its own right but also in connection with the use of mixtures of PVP and SDS or other surfactants in emulsion or suspension polymerizations [95-97]. 

The ether oxygen atoms in poly(ethylene oxide) chains have a strong hydrogen-bonding affinity that results in the formation of association or interpolymer complexes when the polymer chain interacts with polymeric acids such as poly(acrylic acid) and poly(methacrylic acid) in solution (104) and with compounds such as urea and thiourea in bulk (105-107). The subject of association complexes has been reviewed with a particular emphasis on the polyoxyethylene complexes (108) and on polyoxyethylene-based surfactant complexes (109). 

Surfactant—polymer systems have additional technological significance since surfactants are normally used in the emulsion polymerization of many materials, often involving the solubilization of monomer in micelles prior to polymerization and particle formation. Surfactants have also been shown to increase the solubility of some polymers in aqueous solution. The combined actions of the surfactant as a locus for latex particle formation (the micelle) in some cases, particle stabilization by adsorbed surfactant, and as a solubilizer for monomer permit us to expect quite complex relationships between the nature of the surfactant and that of the resulting latex. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

The procedure chosen for the preparation of lipid complexes of AmB was nanoprecipitation. This procedure has been developed in our laboratory for a number of years and can be applied to the formulation of a number of different colloidal systems liposomes, microemulsions, polymeric nanoparticles (nanospheres and nanocapsules), complexes, and pure drug particles (14-16). Briefly, the substances of interest are dissolved in a solvent A and this solution is poured into a nonsolvent B of the substance that is miscible with the solvent A. As the solvent diffuses, the dissolved material is stranded as small particles, typically 100 to 400 nm in diameter. The solvent is usually an alcohol, acetone, or tetrahydrofuran and the nonsolvent A is usually water or aqueous buffer, with or without a hydrophilic surfactant to improve colloid stability after formation. Solvent A can be removed by evaporation under vacuum, which can also be used to concentrate the suspension. The concentration of the substance of interest in the organic solvent and the proportions of the two solvents are the main parameters influencing the final size of the particles. For liposomes, this method is similar to the ethanol injection technique proposed by Batzii and Korn in 1973 (17), which is however limited to 40 mM of lipids in ethanol and 10% of ethanol in final aqueous suspension. 

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

The mechanism of particle formation at submicellar surfactant concentrations was established several years ago. New insight was gained into how the structure of surfactants influences the outcome of the reaction. The gap between suspension and emulsion polymerization was bridged. The mode of popularly used redox catalysts was clarified, and completely novel catalyst systems were developed. For non-styrene-like monomers, such as vinyl chloride and vinyl acetate, the kinetic picture was elucidated. Advances were made in determining the mechanism of copolymerization, in particular the effects of water-soluble monomers and of difunctional monomers. The reaction mechanism in flow-through reactors became as well understood as in batch reactors. Computer techniques clarified complex mechanisms. The study of emulsion polymerization in nonaqueous media opened new vistas. 

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