Polymerizations using Oil-soluble Initiators

Asua, J.M., Rodriguez, V.S., Sudol,E.D.,andEl-Aasser, M.S. 1989. The free radical distribution in emulsion polymerization using oil-soluble initiators. J. Polym. Sci. A 27 3569-3587. 

General. Aqueous solutions of hydrophilic monomers were emulsified in xylene using water-in-oil emulsifiers, and polymerized using oil-soluble initiators. Typical hydrophilic monomers were sodium p-vinylbenzene sulfonate, sodium vinylbenzyl sulfonate, 2-sulfoethyl acrylate, acrylic acid, acrylamide, vinylbenzyl-trimethylammonium chloride, and 2-aminoethyl methacrylate hydrochloride. Typical oil-soluble initiators were benzoyl and lauroyl peroxides. In some cases, water-soluble potassium persulfate was used, both separately and in mixtures with oil-soluble peroxides. Of the water-in-oil emulsifiers, one of the most effective was Span 60 (technical sorbitan monostearate. Atlas Chemical Industries, Inc.). 

Alduncin, J.A., Forcada, J., and Asua, JAl. (1994) Miniemulsion polymerization using oil-soluble initiators. Macromolecules, 27,2256 2261. 

Choi et al. [53] have successfully used both water-soluble and oil-soluble initiators in the miniemulsion polymerization of styrene. Alducin and Asua [119] have studied the MWD of polystyrene miniemulsion polymerized with oil-soluble initiators. Rodriguez et al. [61] have developed a mathematical model of seeded miniemulsion polymerization with oil-soluble initiator. Blythe et al. [ 120] have successfully carried out miniemulsion polymerization of styrene with AMBN (oil-soluble). Ghazaly et al. [117] have used AIBN for the miniemulsion copolymerization of a hydrophobic bifunctional macromer. The polymerization progressed much faster when KPS was used than when AIBN was used. This may be due to the tendency of oil-soluble initiator radicals to recombine before initiating polymerization, as discussed by Luo. 

Polymerizations of the monomer emulsions were carried out with oil-soluble initiators. Oil-soluble initiators have often been employed in emulsion polymerization recipes and are generally used in suspension polymerization. Whereas in the latter case the initiation naturally takes place in the monomer droplets, the locus of initiation and growth of particles in emulsion polymerization with oil-soluble initiators has been open to some doubt. However, the fact that the particle size and size distribution is not very different from the results with water-soluble initiators and that the particles are generally much smaller than the droplets in the monomer emulsions indicates that with... 

Emulsion polymerization of styrene using oil-soluble initiators (25)... 

Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 pm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same [Co et al., 2001 de Vries et al., 2001 Lopez et al., 2000 Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with oil-soluble initiators. Nucleation in emulsion polymerization occurs almost exclusively in the early portion of the process (interval I). Nucleation occurs over a larger portion of the process in microemulsion polymerization because of the large amount of surfactant present. Nucleation... 

A priori, it seems logical to apply the accepted concepts of conventional emulsion polymerization (with water-soluble initiators) to inverse emulsions using oil-soluble initiators. In fact, only few attempts have been made to apply the Smith-Ewart theory [26,36-38]. The determination of n is difficult here because of the ill-defined stages of the reaction, the unusual kinetics and the broad particle size distribution. The kinetic studies of Vanderhoff et al. [26,29] and Visioli [37] are examples of applying the Smith-Ewart theory to the polymerization of acrylamide and p-vinylbenzenesulfonate in xylene initiated with benzoyl peroxide. The data unexpectedly followed Smith-Ewart Case 1 (n 0.S). It was postulated that radicals were generated in, or enter particles pairwise due in the enhanced water solubility of the benzoyl peroxide by the presence of monomo-. 

Most of the minionulsion polymerization processes have been carried out by using water-solnble initiators following the conventional mnulsion polymerization, althongh a number of researchers have looked at the possibilities of using oil-soluble initiators. Refiners and Schork used lauroyl peroxide as both the initiator and also as the costabilizer. Alduncin et al. used other oil-soluble... 

Colloidal dispersants such as cellulose derivatives or vinyl alcohol are generally used in suspension polymerization of VDF to prevent coalescence and agglomeration of particles during polymerization. The pol5unerization pressure is similar to that of emulsion polymerization. Organic oil-soluble initiators such as peroxides, peroxycarbonates, or peroxypivalates are used depending upon... 

In this process, an aqueous solution of a water-soluble monomer is dispersed in an organic continuous phase using a nonionic surfactant of low hydrophilic-lipophilic balance (HLB=4-6). The polymerization is initiated using oil-soluble initiators and the mechanisms involved are similar to those occurring in emulsion polymerization [47,48]. 

Initia.tors, The initiators most commonly used in emulsion polymerization are water soluble although partially soluble and oil-soluble initiators have also been used (57). Normally only one initiator type is used for a given polymerization. In some cases a finishing initiator is used (58). At high conversion the concentration of monomer in the aqueous phase is very low, leading to much radical—radical termination. An oil-soluble initiator makes its way more readily into the polymer particles, promoting conversion of monomer to polymer more effectively. 

Emulsion Polymerization. Emulsion and suspension reactions are doubly heterogeneous the polymer is insoluble in the monomer and both are insoluble in water. Suspension reactions are similar in behavior to slurry reactors. Oil-soluble initiators are used, so the monomer—polymer droplet is like a small mass reaction. Emulsion polymerizations are more complex. Because the monomer is insoluble in the polymer particle, the simple Smith-Ewart theory does not apply (34). 

Various novel applications in biotechnology, biomedical engineering, information industry, and microelectronics involve the use of polymeric microspheres with controlled size and surface properties [1-31. Traditionally, the polymer microspheres larger than 100 /urn with a certain size distribution have been produced by the suspension polymerization process, where the monomer droplets are broken into micron-size in the existence of a stabilizer and are subsequently polymerized within a continuous medium by using an oil-soluble initiator. Suspension polymerization is usually preferred for the production of polymeric particles in the size range of 50-1000 /Ltm. But, there is a wide size distribution in the product due to the inherent size distribution of the mechanical homogenization and due to the coalescence problem. The size distribution is measured with the standard deviation or the coefficient of variation (CV) and the suspension polymerization provides polymeric microspheres with CVs varying from 15-30%. 

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. 

The initiators used in suspension polymerization are soluble in the monomer droplets. Such initiators are often referred to as oil-soluble initiators. Each monomer droplet in a suspension polymerization is considered to be a miniature bulk polymerization system. The kinetics of polymerization within each droplet are the same as those for the corresponding bulk polymerization. 

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. 

In the conventional emulsion polymerization, a hydrophobic monomer is emulsified in water and polymerization initiated with a water-soluble initiator. Emulson polymerization can also be carried out as an inverse emulsion polymerization [Poehlein, 1986]. Here, an aqueous solution of a hydrophilic monomer is emulsified in a nonpolar organic solvent such as xylene or paraffin and polymerization initiated with an oil-soluble initiator. The two types of emulsion polymerizations are referred to as oil-in-water (o/w) and water-in-oil (w/o) emulsions, respectively. Inverse emulsion polymerization is used in various commerical polymerizations and copolymerizations of acrylamide as well as other water-soluble monomers. The end use of the reverse latices often involves their addition to water at the point of application. The polymer dissolves readily in water, and the aqueous solution is used in applications such as secondary oil recovery and flocculation (clarification of wastewater, metal recovery). 

The most commonly used water-soluble initiator is the potassium, ammonium, or sodium salt of peroxodisulfates. Redox initiators (Fe2+salt/peroxodisul-fate, etc.) are used for polymerization at low temperatures. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in emulsion polymerization. They are, however, less efficient than water-soluble peroxodisulfates. This results from the immobilization of oil-soluble initiator in polymer matrix, the cage effect, the induced decomposition of initiator in the particle interior, and the deactivation of radicals during des orption/re-entry events [14, 15]. 

The miniemulsion polymerization also allows the use of oil-soluble initiators which is the preferential choice for monomers with either high water solubility (e.g., MMA in order to prevent secondary nucleation in the water phase) or for monomers with an extremely low water solubility (e.g., LMA) where the monomer concentration in the water phase is not high enough to create oligo-radicals which can enter the droplets. 

The polymerization of more hydrophilic monomers is also possible, as shown for MMA and vinyl acetate [36, 56, 73]. In the case of monomers with a pronounced water solubility, the nucleation in water should be efficiently suppressed in order to avoid secondary nucleation in the water phase. This can be achieved, e.g., by using an oil-soluble initiator and the polymerization of acrylonitrile or by adding a termination agent to the continuous phase. A typical calorimetric curve of MMA polymerization using a hydrophobic initiator shows a fast conversion. 

Starting from those two dispersion situations, the locus of initiation is expected to have a great influence on the reaction products and the quality of the obtained copolymers. Therefore three different initiators were used, an oil-soluble initiator (e.g., 2,2 azobis(2,4-dimethylvaleronitrile (ADVN)), an interfacial active initiator (e.g., PEGA200), and a water-soluble initiator (e.g., potassium peroxodisulfate (KPS)) in order to initiate the polymerization selectively in one of the phases or at the interface. 

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