Oil-soluble initiators

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 type and concentration of oil-soluble initiator are effective both on the polymerization rate and on the average size of the final product. The polymerization rate and the average size of the final product usually increase with the increasing initiator concentration. 

Step I Preparation of aqueous emulsion of 1-chlorododec-ane containing benzoyl peroxide as the oil soluble initiator... 

The uniform polymeric microspheres in submicron-or micron-size range can also be prepared as seed particles by the soapless emulsion or dispersion polymerization of a hydrophobic monomer like styrene. The uniform seed particles are swollen with the organic phase including functional comonomer, monomer, and oil-soluble initiator at a low temperature in an aqueous... 

The appearance of polymerized monomer droplets indicates that polymerization is initiated both in the monomer droplets and in monomer-containing micelles. This result is completely different from that obtained in the EP of styrene under identical conditions, where no monomer droplets polymerize. Similar experiments with 1,3,5-trivinylbenzene also yielded polymerized monomer droplets as by-products [77]. The amount of polymerized 1,4-DVB droplets further increased when PPS was replaced by an oil soluble initiator, such as, AIBN [83] or, when the EP was thermally initiated [84]. Figure 5 compares electron micrographs of the polymers formed by thermally (90 °C) initiated EP of 1,4-DVB and S. 

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). 

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. 

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]. 

In inverse polymerization, water-soluble monomers are emulsified with low HLB (hydrophilic-lipophilic balance) surfactants in an organic medium and the reaction is initiated with water-soluble or oil-soluble initiators. A review of the subject can be found in a recent publication of Greenshields [46]. 

For miniemulsion polymerization, the initiator can be either oil- or water-soluble. In the case of an oil-soluble initiator, the initiator is dissolved in the monomeric phase prior to miniemulsification. Then the reaction starts within the droplets. This is comparable to suspension polymerization where the initia-... 

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. 

Recently, carboxyl- and amino-functionalized polystyrene latex particles were synthesized by the miniemulsion copolymerization of styrene and acrylic acid or 2-aminoethyl methacrylate hydrochloride (AEMH) [70, 71]. The reaction was started by using an oil-soluble initiator, 2,2 -azobis(2-methylbutyronitrile) (V-59). Two types of surfactant, i.e., ionic negatively charged SDS or positively charged CTMA-Cl, and nonionic Lutensol AT50 (which is a PEO hexadecyl ether with an EO block length of about 50 units) were used to stabilize the initial droplets and final particles. 

Several researchers have carried out experimental and/or theoretical investigations on emulsion polymerizations initiated with oil-soluble initiators and reported that the kinetics of the emulsion polymerizations is basically similar to that initiated with water-soluble initiators [193-202]. Breitenbach et al. [193] carried out the emulsion polymerization of St initiated by BPO at 50 and 60 °C. The authors interpreted the experimental results by assuming a relatively rapid exchange of low molecular weight radicals between the micelle-polymer... 

---