Polymerization in Monomer Droplets

While the extent of polymerization in monomer droplets is often very small, the following reaction conditions and/or recipes can promote polymer reactions in the droplets. 

In summary, formation of particle nuclei from emulsified monomer droplets is almost certain to occur in any emulsion polymerization system in which these droplets are present. As mentioned earlier, however, monomer droplets containing polymer will primarily serve as reservoirs to provide monomer to the much more numerous and smaller latex particles formed by other particle nucleation mechanisms. Polymerization in monomer droplets can be eliminated or at least minimized by using seed polymer particles and slowly adding monomer (neat or as an emulsion) to supply the growing seed particles (i.e., seeded semibatch emulsion polymerization under the monomer-starved condition). 

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Ugelstadt, J., El-Aasser, M.S. and Vanderhoff, J.W. (1973) Emulsion polymerization initiation of polymerization in monomer droplets. Journal of Polymer Science Polymer Chemistry Edition, 11, 503-513. 

Sadurm, N., Solans, C., Azemar, N. and Garci a-Celma, M.J. (2005) Studies on the formation of O/W nano-emulsions, by low-energy emulsification methods, suitable for pharmaceutical aplications. Emulsion polymerization initiation of polymerization in monomer droplets., 26, 438-445. 

In linear EP of bifunctional monomers, such as S, with water soluble initiators, the monomer droplets do not compete with micelles in capturing radicals from the aqueous phase because the total surface area of the droplets is much smaller than that of micelles and growing particles. Nevertheless, if some radicals enter monomer droplets, rapid termination takes place. Therefore, polymerization in monomer droplets is negligible [88]. However, if in the crosslinking EP of 1,4-DVB a few radicals are captured by monomer droplets, they can polymerize completely because the recombination of radicals is suppressed by the gel effect. Moreover, in thermal initiation or in initiation by hydrophobic initiators, such as AIBN, radicals are formed predominantly in the hydrophobic phase, i.e. in monomer droplets and in micelles, and crosslinking EP is initiated in the organic phase. 

Isopar-M SMO AIBN 1 1 6 30-50 47 Baade and Reichert [49] Inverse micelles not detected. Polymerization in monomer droplets. Kinetic latex stability. Solution-like kinetics, with interfacial reactions Inverse- Suspension... 

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. 

Due to the reduced absorption of monomers and the low rate of polymerization in the micelles, the diffusion of monomer molecules from droplets to the growing particles is limited. Correspondingly, the probability of polymerization in the droplets increases. 

In EP of bifunctional vinyl monomers, the reaction rate increases with the emulsifier concentration because the number of particles increases. However, in the crosslinking EP of divinyl monomers, the reaction rate is inversely proportional to the emulsifier concentration. This unusual behavior is due to nucleation taking place in both micelles and monomer droplets. In monomer droplets, the kinetics resembles that of bulk polymerization and therefore the reaction rates... 

The mechanism of crosslinking emulsion polymerization and copolymerization differs significantly from linear polymerization. Due to the gel effect and, in the case of oil-soluble initiators, monomer droplets polymerize preferentially thus reducing the yield of microgels. In microemulsion polymerization, no monomer droplets exist. Therefore this method is very suitable to form microgels with high yields and a narrow size distribution, especially if oil-soluble initiators are used. 

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-... 

Fig. 1.5.8 Scanning electron micrograph of poly(p-fer-butylstytene) particles obtained by polymerization of monomer droplets in the aerosol phase. The monomer and initiator flow rates were 1.2 dm3 min-1 and 40 cm5 min-1 and the boiler and initiator reservoir temperatures were 50°C and 25°C, respectively. The initiator was injected into the flowing aerosol at two positions. The modal diameter of these particles is 1.8 [xm. (From Ref. 67.)...

Suspension polymerization may be the most important particle-forming polymerization from an industrial viewpoint. The system is very simple, composed of monomer, initiator, stabilizer, and medium (water in most cases). The monomer droplets with dissolving initiator are dispersed in water and the stabilizer exists at the interface. But suspension polymerization is regarded as a kind of homogeneous polymerization because the polymerization occurs only in monomer droplets and water does not affect the polymerization. Water contributes only to dividing the polymerization locus into small droplets and absorbing the heat evolved by polymerization. On the contrary, in emulsion polymerization, which is another type of polymerization performed in water and as practically important as suspension polymerization, water affects the polymerization significantly. In this section, emulsion polymerization is first discussed, and then some modified emulsion polymerizations such as soap-free emulsion polymerization and micro and mini emulsion polymerizations are described. 

Unlike in conventional emulsion polymerization, no monomer droplets exist in a microemulsion polymerization system, and hence, oil-soluble initiators partition into the monomer-swollen micelles, the resultant polymer particles and the water phase. Therefore, in microemulsion polymerization, the polymerization only proceeds in the monomer-swollen micelles and the resultant polymer particles over the entire course of polymerization. Pairs of radicals produced in volumes as small as monomer-swollen micelles and polymer particles may terminate as soon as they are generated. If so, it is expected that the radicals responsible for the polymerization in the monomer-swollen micelles and the resultant polymer particles would usually be those generated from the fraction of the initiator dissolved in the water phase. In order to examine whether this expectation is correct, oil-in-water (O/W) microemulsion polymerizations of St were carried out using four kinds of oil-soluble azo-type initiators with widely different water-solubilities [209]. It was found that the rates of polymerization with these oil-soluble initiators were almost the same irrespective of their water-solubilities, when the polymerizations were carried out with the same rate of radical production for the whole system for all of the oil-soluble initiators used. Moreoever, the rate of polymerization with any of these oil-soluble initiators was only about 1/3 of that with KPS at the same rate of radical production. Considering that the rate of polymerization was pro-... 

In Fig. 4, theoretical values of n calculated by Eq. (30) are plotted against the value of C = pJkfNj, Experimental values of n obtained at the conversion point hr vinyl acetate and vinyl chloride emulsion polymerizations where monomer droplets disappear in the water phase (20 4( ... 

A. Polymerization with Initiation in Monomer Droplets Formed by the Diffusion Process. . 396... 

Another, more specific method for the preparation of emulsions of Z, involves the addition of Z to a preformed mixture of an ionic emulsifier, a long-chain fatty alcohol, and water. In this way, the rapid formation of a stable emulsion may he obtained at ordinary stirring with relatively modest amounts of emulsifier. The mechanism of Ais process is still not satisfactorily explained. Also, subsequent polymerization (in the case where Zi is a monomer) may lead to polymerization with initiation in monomer droplets. 

If fatty alcohols with chain length equal to Or less than 16 carbon atmns are applied, the emulsion formed is relatively unstable. In order to get initiation in monomer droplets, the polymerization should then be carded out immadiately after preparation of the monomer emulsion. 

I hcse unusual variations of N with emulsifier concentration and temperature suggest that the aqueous monomer droplets may furnish loci for polymerization initiation. In conventional emulsion polymerizations, the monomer droplets usually range in diameter from 1 to 10 microns therefore their total surface area is small relative to that of the micelles, and they serve only as reservoirs in the particle initiation process. However, if these monomer droplets were smaller by one or two orders of magnitude, they would compete effectively with the monomer-swollen micelles for the available radicals and therefore would serve as loci for particle initiation. 

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