Solubilities mutual monomer-water

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

Model II. Some monomers have high monomer-water mutual solubilities compared to styrene (Table I) (5). For the swelling of these systems, neglect of the effect of water dissolved in swollen particles as well as in the monomer phase could lead to significant errors. Therefore, free energy terms describing the water-monomer and water-polymer interactions should be included in the equilibrium equations to cover a wide range of monomers. 

Monomer-Water Mutual Solubilities at Room Temperature (5 )... 

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 Interface surface tension was calculated from the data on mutual solubility (20)of monomer and water determined by means of refractometer or Interferometer. 

This problem was first treated in detail by Haward (1949). He considered the case of a bulk polymerization that has been compartmentalized by subdividing the reaction system into a large number of separate droplets, each of volume v. Radicals are generated exclusively within the droplets and always in pairs. An example would be the polymerizatiim of styrene in emulsified droplets dispersed in water initiated the thermal decomposition of an oil-soluble initiator which partitions almost exclusively within the monomer droplets. In the model considered by Haward, radicals are unable to exit from the droplets into the external phase. The only radical-loss process is in fact bimolecular mutual termination. It therefore follows that all the droplets must always contain an even number (including zero) of propagating radicals, and that the state of radical occupancy will change in increments of 2. The conclusion reached by Haward is that in this case the effect of compartmentalization is to reduce the overall rate of polymerization per unit volume of disperse phase. The f ysical reason for this is that, as the volume of the droplets is reduced, so are the opportunities for a radical to escape from the others—and hence to avoid mutual... 

An emulsion polymerization requires the mechanism of polymer particle nudeation to reside outside the monomer droplets. This physical-chemical process involves a series of radical reactions in the continuous phase followed by homogeneous or micellar particle formation. Either of these mechanisms require the initiator to be insoluble in the monomer phase, such as a water soluble initiator and an organically soluble monomer, or vice versa. If, in contrast, the initiator is soluble in the monomer phase, all the components of the reaction are contained in the dispersed phase and the continuous phase serves only to decrease the viscosity and dissipate heat Such polymerizations are categorized as suspensions. The second definition, however, makes no statement as to the magnitude of n and therefore the two criteria are mutually exclusive. 

Hydrophobically associating polymers consist primarily of water-soluble monomer units with a small number of water-insoluble monomer units. Synthesis of high-molecular-weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization (2-4) because of the mutual immiscibility of the water-soluble and hydrophobic monomers. The use of surfactant micelles enabled solubilization of the hydrophobic monomer (alkylacrylamide [R]) into the aqueous phase containing the water-soluble monomer (acrylamide [AM]). The resulting RAM polymer after isolation provided homogeneous aqueous solutions. 

Harkins calculated from the solubility of styrene in water (0.00368 mol dm at 50 °C [50]) that there are 4 x 10 molecules dm . In a 3% solution of potassium dodecanoate there are about 1 x 10 micelles dm , but with 61 molecules per micelle with an unswollen radius of 2.1 nm the cross-sectional area of the monomer-swollen micelles exceeds that of the styrene molecules by a factor of at least 12. Hence the micelles are more likely to capture initiator radicals produced in the aqueous phase. Polymerization within the micelles must be much faster than in the water because the concentration of styrene will be much the same as in bulk (8.5 mol dm ). The molar mass of the polystyrene produced is much larger than the molar mass of all the styrene molecules solubilized in a micelle thus, the monomer must be able to diffuse through the aqueous phase from other micelles and monomer droplets to allow the polymer radical to continue to grow until it is finally terminated by the entry of another initiator radical from the aqueous phase. Under the standard conditions of the mutual recipe (Table 4.1) there is 180 g water to 100 g styrene taking the emulsion droplets to have a radius of 1 pm, the ratio of the total cross-sectional areas of droplets to micelles to monomer molecules is about 1 30 2.5. The ratio of total surface areas would be even more heavily biased in favour of micelles. Hence it is probable that many more radicals will be captured from the aqueous phase by the micelles than by the emulsion droplets or than react with the monomer molecules in aqueous solution. 

PTFE is manufactured primarily by free-radical methods in aqueous media. DeSimone et al. developed a C02/aqueous hybrid process that allows for the safer handling of the TFE, resulting in high-molecular-weight PTFE resins [44]. This system represents a substantial deviation from traditional systems, as CO2 and water exhibit low mutual solubilities, allowing for compartmentalization of monomer, polymer, and initiator based on their solubility characteristics. 

A different interpretation suggested an alternative mechanism based on the formation of complexes between proteins and surfactants within the detergent formulation, which produce larger mixed micelles and consequent lowering of the CMC of the system (119,120). This model was initially supported by the observation that some insoluble dyes, which were solubilized in tenside solutions above the CMC of the surfactant, could gain water solubility abundantly below the CMC when small amounts of protein substances were added to the system (121), and is therefore strictly linked to the hypothesis that only monomers of the surfactants can penetrate tlie skin keratin matrix and induce adverse effects on the skin. These two interpretations are not mutually exclusive and there could reasonably be contemporary cooperative occurrence of the two mechanisms. 

There are some requirements for monomers used for the emulsion polymerization. The primary requirement for monomers is that they must have a limited water solubility and be soluble in the formed polymer. However, the solubility in water should not be too high, otherwise this monomer would tend to polymerize in the water phase. In the mechanism for emulsion polymerization one of the driving forces is the absorption of monomer into the polymeric particles if the monomer and polymer are not mutually soluble then this process will not be efficient. Many different vinyl monomers are currently used in practical emulsion polymerization, including acrylates, methacrylates, St, AN (in copolymers), VAc, isoprene, and 1,3-butadiene. In addition, the monomers would not react with water, surfactants and other additives. Table 11.6 presents the propagation rate coefficients of various monomers examined in emulsion polymerization. 

The relatively large monomer droplets (generally 2-5ym in diameter) have too small a surface area to capture radicals from the aqueous phase and therefore serve as reservoirs for the diffusion of monomer through the aqueous phase to the pol3onerizing oligomeric radicals, micelles, or polymer particles. Despite the unfavorable statistical probabilities, however, some monomer droplets capture radicals and polymerize to form microscopic or near-microscopic particles (14), and some of these particles which are entirely separate from the main particle size distribution are formed in most batch polymerizations. Polymerization in monomer droplets becomes much more significant when the size of the emulsion droplets is decreased. The use of ionic emulsifier-fatty alcohol mixtures (13) and, later, ionic emulsifier-alkane mixtures (15), allows the preparation of 0.1-0.2ym size styrene monomer droplets, which compete favorably with initiation in micelles and in the aqueous phase as the mechanism of particle nucleation. The mechanism of formation of these "mini-emulsions" has been attributed to the very low solubility of the fatty alcohols and alkanes in water (16) or to the formation of crystalline complexes between the ionic emulsifiers and fatty alcohols (17) the two mechanisms are not mutually exclusive. Thus this mechanism pertains only to special systems. 

Table 1. Mutual solubilities of water and various monomers (M).

An important characteristics of emulsion polymerization is that, unlike other polymerization processes, the polymerization rate and molecular weight can be increased at the same time. This is due to the compartmentalization of the system that reduces the probability of mutual termination of propagating radicals. The behavior of this compartmentalized system dqiends on the rate of exchange of species between the elements of the system. The main ingredients of an emulsion polymerization system include monomer, dispersant, emulsifier, and initiator. Water is commonly used as the dispergant. A water-insoluble monomer can be dispersed in water by means of an oil-in-water emulsifier and polymerized with a water-soluble initiator. 

Use of mutual solvents (e.g. alcohols) or solvent mixtures to dissolve both the water and oil-soluble monomers also has some serious limitations [4]. For example, even though a common solvent can be found for the monomers, it is unlikely that the polymer will also be soluble in such a medium. Thus, the polymer will precipitate before it has built up a sufficient molecular weight for use as a viscosifier. In addition, most of the possible water miscible solvents (e.g. alcohols, ether, acetone) are chain transfer agents for free radical polymerization, and their presence leads to low molecular weight products. 

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