Micelle initiation

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

The second important effect is the number of micelles initiated which is very much higher in the case of 1,4-DVB than in the case of styrene (see 4.1). Thus the total surface area increases faster in the case of... 

In polymerizations where desorption is very high, all micelles grow simultaneously. This is because radical absorption is followed almost iimiediately by desorption with only a brief micellar growth period. As a result, all micelles experience an equal but intermittant growth. The number of latex particles is determined solely by the number of micelles initially present. The number of micelles, m, is given by the expression... 

FIGURE 1. Oxygen uptake profiles for oxidation of 0.12 M methyl hnoleate in 0.5 M SDS micelles, initiated by 0.03 M of the thermal azo initiator di-tcrt-butylhyponitrite, comparing the effects of the retarder melatonin (R) with phenolic antioxidants U—uninhibited oxidation, R1— 8.72 X 10-5 melatonin, R2—87.2 x 10-5 melatonin, a-Toc—8.72 x 10-5 a-Toc, nib—8.72 x 10 5 M BHT [butylated hydroxytoluene (2,6-di-t-butyl-4-methylphenol)], Vc—8.72 x 10 5 M Trolox (2,5,6,7-tetramethyl-2-carboxy-5-hydroxychroman), Va—8.72 x 10-5 PMHC (2,2,5,6,7-pen-tamethyl-5-hydroxychroman). Reproduced by permission of Elsevier Science from Reference 283... 

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. 

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. 

Emulsion polymerization is similar to suspension polymerization in that water is the continuous phase, but the main difference is that a water-soluble initiator is used. The water-insoluble monomer is dispersed in the water using emulsifying agents, such that the latex is made up of droplets (1.5 pm diameter) and micelles (0.01 pm). Because of the enormous surface area of the micelles, initiation and polymerization takes place at this interface and the monomer is replenished by diffusion from the larger droplets to the micelle/growing polymer particle. A latex is obtained which is often used directly as such for paints, adhesives, etc. 

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. 

During the polymerization in inverse micelles initiators soluble in the organic phase, such as dinitryl asoisobutyric acid or organic peroxides, are used. 

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

Figure C2.3.9. Product distribution of dissymmetrical ketone photolysis as influenced by cefyltrimethylammonium chloride (CTAC) micelles. The initial ketone, A(CO)B is photolysed to lose the carbonyl group and to produce tliree products, AA, AB and BB. These data are for benzyl (A) 4-methylbenzyl (B) ketone. Product AA is 1,2-diphenylethane, product BB is 1,2-ditolylethane and product AB is l-phenyl-2-tolyl-ethane. At low CTAC concentration, in the absence of micelles, a random distribution of products is obtained. In the presence of micelles, however, the AB product is heavily favoured. Adapted with pennission from 1571.

The surfactant is initially distributed through three different locations dissolved as individual molecules or ions in the aqueous phase, at the surface of the monomer drops, and as micelles. The latter category holds most of the surfactant. Likewise, the monomer is located in three places. Some monomer is present as individual molecules dissolved in the water. Some monomer diffuses into the oily interior of the micelle, where its concentration is much greater than in the aqueous phase. This process is called solubilization. The third site of monomer is in the dispersed droplets themselves. Most of the monomer is located in the latter, since these drops are much larger, although far less abundant, than the micelles. Figure 6.10 is a schematic illustration of this state of affairs during emulsion polymerization. 

Polymerization begins in the aqueous phase with the decomposition of the initiator. The free radicals produced initiate polymerization by reacting with the monomers dissolved in the water. The resulting polymer radicals grow very slowly because of the low concentration of monomer, but as they grow they acquire surface active properties and eventually enter micelles. There is a possibility that they become adsorbed at the oil-water interface of the monomer... 

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. 

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

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. 

Styryl free radicals simultaneously initiate micelle nuclei at points of high fumarate concentration. The micelles continue to expand, interacting with free... 

Emulsion Polymerization. Emulsion polymerization takes place in a soap micelle where a small amount of monomer dissolves in the micelle. The initiator is water-soluble. Polymerization takes place when the radical enters the monomer-swollen micelle (91,92). Additional monomer is supphed by diffusion through the water phase. Termination takes place in the growing micelle by the usual radical-radical interactions. A theory for tme emulsion polymerization postulates that the rate is proportional to the number of particles [N. N depends on the 0.6 power of the soap concentration [S] and the 0.4 power of initiator concentration [i] the average number of radicals per particle is 0.5 (93). 

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). 

Emulsion Polymerization. In this method, polymerization is initiated by a water-soluble catalyst, eg, a persulfate or a redox system, within the micelles formed by an emulsifying agent (11). The choice of the emulsifier is important because acrylates are readily hydrolyzed under basic conditions (11). As a consequence, the commonly used salts of fatty acids (soaps) are preferably substituted by salts of long-chain sulfonic acids, since they operate well under neutral and acid conditions (12). After polymerization is complete the excess monomer is steam-stripped, and the polymer is coagulated with a salt solution the cmmbs are washed, dried, and finally baled. 

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). 

An increase in the rate of radical production in emulsion polymerisation will reduce the molecular weight since it will increase the frequency of termination. An increase in the number of particles will, however, reduce the rate of entry of radicals into a specific micelle and increase molecular weight. Thus at constant initiator concentration and temperature an increase in micelles (in effect in soap concentration) will lead to an increase in molecular weight and in rate of conversion. 

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