Micelles initial reaction

There are two stages involved in a typical emulsion polymerization. In the seed stage, a mixture of water, surfactant, and colloid is first heated to the reaction temperature (85-90°C). Next, 5-10% of the monomer mixture with a portion of the initiator is added. At this point the reaction mixture contains monomer droplets stabilized by surfactant, some dissolved monomei the initiator, and surfactant (in solution and in micelles). The initiator breaks down to produce radicals, when heated and these initiate the polymerization of the dissolved monomers. Growing polymer chains eventually enter a micelle, initiating reaction of the monomer inside. If a second growing polymer enters the micelle, termination can occur. 

The function of emulsifier in the emulsion polymerization process may be summarized as follows [45] (1) the insolubilized part of the monomer is dispersed and stabilized within the water phase in the form of fine droplets, (2) a part of monomer is taken into the micel structure by solubilization, (3) the forming latex particles are protected from the coagulation by the adsorption of monomer onto the surface of the particles, (4) the emulsifier makes it easier the solubilize the oligomeric chains within the micelles, (5) the emulsifier catalyzes the initiation reaction, and (6) it may act as a transfer agent or retarder leading to chemical binding of emulsifier molecules to the polymer. 

The problem of slow turnover has been noted. Initial reaction of a nucleophilic group, e.g. imidazole or oximate, in a functional micelle, with a carboxylic or phosphoric ester, for example, gives an acylated or phosphory-lated imidazole or oxime, and these derivatives hydrolyze slowly to regenerate the nucleophile. Kunitake and Shinkai (1980) discuss a number of reactions in micelles which contain both nucleophilic and basic groups which are potentially capable of acting as bifunctional reagents (Tonellato, 1979, Kunitake and Shinkai, 1980 Bunton, 1984)... 

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-... 

Not surprisingly, it is rather difficult to separate the different contributions of the different interactions as they occur in the micellar Stern region. In an attempt to solve this problem, the group of Engberts used a series of hydrolysis reactions of activated esters and amides to probe the reaction environment offered by micelles. The reactions initially involved the water-catalyzed pH-independent hydrolysis reactions of i-methoxy-phenyl dichloroacetate 4 and l-benzoyl-3-phenyl-l,2,4-triazole 5, as extensive information on the rate retarding effects of added cosolutes on this reaction was available. ... 

Lipases generally show low hydrolytic activity when their ester substrates are dissolved in aqueous media and present in imimeric form. A pronounced increase in activity is observed when the substrate concentration reaches the solubiUty limit and a separate phase is formed. In the case of surfactants this impUes that a possible increase in activity can be expected above the CMC. Attempts to investigate how the hydrolysis is affected by micelUzation were made for the linear surfactant 1 of Fig. 4. The CMC of this surfactant is 10 mM, and a marked change in the activity of the MML is indeed observed when this concentration is exceeded, see Fig. 6. The initial reaction is faster (steeper slope) above the CMC. When CALB was used to catalyze the reaction, no increase of the reaction rate was observed above the CMC. It was also found that the rate, expressed in moles of surfactant consumed per minute, was independent of the start concentration (same slope). A tentative explanation to the fact that the MML but not the CALB-catalyzed hydrolysis is accelerated by the presence of micelles may be that MML but not CALB is able... 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

Kinetic investigations of the effects of urea and similar denaturing agents on rates and thermodynamic parameters of micelle catalyzed reactions have been suggested to be more sensitive probes for the nature and extent of hydrophobic interactions than CMC determinations. Thus, Monger and Portnoy (1968) have taken advantage of the base catalyzed hydrolysis of micellar -nitrophenyl dodecanoate. The rate constant for the hydrolysis of -nitrophenyl dodecanoate decreases rapidly with increasing initial concentration of the ester (10 to 10 m), and the second-order rate constant for the base-catalyzed hydrolysis of this ester... 

The effects of emulsifiers in emulsion polymerization systems may be enumerated as follows (l stabilization of the monomer in emulsion, (2) solubilization of monomer in micelles, (3) stabilization of polymer latex particles, (4) solubilization of polymer, (5) catalysis of the initiation reaction, and (6) action as transfer agents or retarders which leads to diemical binding of emulsifier residues in the polymer obtained. 

Two theories were held before the intensive research (56,57) in America during the recent war firmly established the idea that the initial reaction occurred within the soap micelles. 

During the inhibited self-initiated autoxidation of methyl linoleate by a-Toc in solution, Niki and coworkers made the interesting observation that a-Toc acts as an antioxidant at low concentrations, but high concentrations (up to 18.3 mM) actually increased hydroperoxide formation due to a pro-oxidant effect. The pro-oxidant effect of a-Toc was observed earlier by Cillard and coworkers in aqueous micellar systems and they found that the presence of co-antioxidants such as cysteine, BHT, hydroquinone or ascor-byl palmitate inverted the reaction into antioxidant activity, apparently by reduction of a-To" to a-Toc . Liu and coworkers ° found that a mixture of linoleic acid and linoleate hydroperoxides and a-Toc in SDS micelles exhibited oxygen uptake after the addition of a-Toc. The typical ESR spectrum of the a-To" radical was observed from the mixture. They attributed the rapid oxidation to decomposition of linoleate hydroperoxides, resulting in the formation of linoleate oxy radicals which initiated reactions on the lipid in the high concentration of the micellar micro-environment. Niki and coworkers reported pro-oxidant activity of a-Toc when it was added with metal ions, Fe3+25i Qj. jjj (jjg oxidation of phosphatidyl choline liposomes. a-Toc was found... 

The growth of polymer particles constitutes the driving force not only for mass transport of monomer to the main reaction site, but also for adsorption of surfactant onto the growing surface of the particles. Hence, micelles (if present) disaggregate and their concentration diminishes with time until they eventually disappear, that is, when the surfactant concentration falls below CMC at this point, micellar nucleation ceases. Only about 1/lOOOth of the micelles initially present act as nucleation sites, and the rest disaggregate to stabilize the growing particles. 

Oxidant and reductant present as perchlorates, in various concentration ranges, up to 25 x 10- M for details see the original reference. The same data recalculated by a procedure in which ion atmosphere effects are considered by inclusion of the Debye-Huckel limiting law in the calculation . Mononuclear, solvated species reaction product is Aga+. "2/ Decomposition of Ag2 in anionic micelles. Rapid reaction (stopped flow). Ligand is pyrazine-2,3-dicarboxylic acid. Reaction product is a strongly absorbing yellow species kinetics not simple , but no details recorded. Autocatalytic reaction, specific rate calculated from the initial rate. 

The addition of dyes in the initial reaction mixture affords dye-doped silica cores. According to their solubility, in fact, they partitimi between water and hydrophobic micelles, the latter fraction remaining physically entrapped in the silica network. Derivatizing the dye with a trialkoxysilane group leads to its co-condensati(Mi with TEOS, resulting in robust luminescent systems. Thus, this method allows the physical or covalent entrapment of dozens of molecules to a small silica core, providing very bright nanosystems. 

The photoprotolytic reactions in micellar solutions show the usual values of the deuteration isotope effect [122] which gives evidence that the photoprotolytic dissociation is not controlled by the exit of the excited molecules from the micelles. The product of the dissociation A can be formed within a micelle initially and then can leave the micelle. The increase of the number of the carbon atoms in surfactant molecule causes an increase of the exit rate constant. 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

As the quinone stabilizer is consumed, the peroxy radicals initiate the addition chain propagation reactions through the formation of styryl radicals. In dilute solutions, the reaction between styrene and fumarate ester foUows an alternating sequence. However, in concentrated resin solutions, the alternating addition reaction is impeded at the onset of the physical gel. The Hquid resin forms an intractable gel when only 2% of the fumarate unsaturation is cross-linked with styrene. The gel is initiated through small micelles (12) that form the nuclei for the expansion of the cross-linked network. 

The reaction involves the nucleophilic attack of a peracid anion on the unionized peracid giving a tetrahedral diperoxy intermediate that then eliminates oxygen giving the parent acids. The observed rate of the reaction depends on the initial concentration of the peracid as expected in a second-order process. The reaction also depends on the stmcture of the peracid (specifically whether the peracid can micellize) (4). MiceUization increases the effective second-order concentration of the peracid because of the proximity of one peracid to another. This effect can be mitigated by the addition of an appropriate surfactant, which when incorporated into the peracid micelle, effectively dilutes the peracid, reducing the rate of decomposition (4,90). 

The reaction is considerably modified if the so-called emulsion polymerisation technique is used. In this process the reaction mixture contains about 5% soap and a water-soluble initiator system. The monomer, water, initiator, soap and other ingredients are stirred in the reaction vessel. The monomer forms into droplets which are emulsified by some of the soap molecules. Excess soap aggregates into micelles, of about 100 molecules, in which the polar ends of the soap molecules are turned outwards towards the water whilst the non-polar hydrocarbon ends are turned inwards (Figure 2.17). 

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

In suspension polymerization, the monomer is agitated in a solvent to form droplets, and then stabilized through the use of surfactants to form micelles. The added initiator is soluble in the solvent such that the reaction is initiated at the skin of the micelle. Polymerization starts at the interface and proceeds towards the center of the droplet. Polystyrene and polyvinyl chloride are often produced via suspension polymerization processes. 

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