Monomers water-insoluble

The fourth and most interesting of the polymerization techniques we shall consider is called emulsion polymerization. It is important to distinguish between suspension and emulsion polymerization, since there is a superficial resemblance between the two and their terminology has potential for confusion A suspension of oil drops in water is called an emulsion. Water-insoluble monomers are used in the emulsion process also, and the polymerization is carried out in the presence of water however, the following significant differences also exist ... 

Superabsorbents. Water-sweUable polymers are used extensively in consumer articles and for industrial appUcations. Most of these polymers are cross-linked acryUc copolymers of metal salts of acryUc acid and acrylamide or other monomers such as 2-acrylamido-2-methylpropanesulfonic acid. These hydrogel forming systems can have high gel strength as measured by the shear modulus (134). Sometimes inorganic water-insoluble powder is blended with the polymer to increase gel strength (135). Patents describe processes for making cross-linked polyurethane foams which contain superabsorbent polymers (136,137). 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

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). 

Suspension polymerization of water-insoluble monomers (e.g., styrene and divinylbenzene) involves the formation of an oil droplet suspension of the monomer in water with direct conversions of individual monomer droplets into the corresponding polymer beads. Preparation of beaded polymers from water-soluble monomers (e.g., acrylamide) is similar, except that an aqueous solution of monomers is dispersed in oil to form a water-in-oil (w/o) droplet suspension. Subsequent polymerization of the monomer droplets produces the corresponding swollen hydrophilic polyacrylamide beads. These processes are often referred to as inverse suspension polymerization. 

The second step in the production of monodispersed polymer particles involves the swelling of activated particles with a monomer or a mixture of monomers, diluents, and porogens, and the shape of the swollen oil droplets must be maintained in the continuous aqueous phase. The monomer or the mixture of monomers may be added in bulk form, preferably as an aqueous dispersion to increase the rate of swelling, especially in the case of relatively water-insoluble monomers. 

Emulsion polymerization is widely used to produce polymers in the form of emulsions, such as paints and floor polishes. It also used to polymerize many water insoluble vinyl monomers, such as styrene and vinyl chloride. In emulsion polymerization, an agent emulsifies the monomers. Emulsifying agents should have a finite solubility. They are either ionic, as in the case of alkylbenzene sulfonates, or nonionic, like polyvinyl alcohol. 

As described above, the enzymatic polymerization of phenols was often carried out in a mixture of a water-miscible organic solvent and a buffer. By adding 2,6-di-0-methyl-(3-cyclodextrin (DM-(3-CD), the enzymatic polymerization of water-insoluble m-substituted phenols proceeded in buffer. The water-soluble complex of the monomer and DM-(3-CD was formed and was polymerized by HRP to give a soluble polymer. In the case of phenol, the polymerization took place in the presence of 2,6-di-O-methyl-a-cyclodextrin (DM-a-CD) in a buffer. Only a catalytic amount of DM-a-CD was necessary to induce the polymerization efficiently. Coniferyl alcohol was oxidatively polymerized in the presence of a-CD in an aqueous solution. ... 

By using this technique only water insoluble monomers can be polymerised. In this process, the monomer is suspended as discrete droplets (0.1 to 1.0 mm diameter) in dilute aqueous solution containing protective colloids like polyvinyl alcohol and surfactants, etc. The droplets have large surface area and can readily transfer heat to water. Suspension is brought about by agitating the suspension. Protective colloids prevent coalescence of the droplets. A monomer soluble initiator is used. The product is obtained by filtration or spray drying. This process cannot be carried out yet in a continuous process hence batch processing has to be used. 

Figure 10.3 A water-insoluble monomer is stabilized by a surfactant and polymerized to give a polymer latex...

In this study, methylated -cyclodextrins were found to accelerate the polymerization of aryl group-containing water-insoluble monomers using water-soluble initiators in a two phase water/chloroform system. The results are consistent with transport of monomer from the organic phase to the aqueous phase, where initiation of the polymerization begins, followed by... 

Water insoluble monomers such as vinyl chloride may be polymerized as suspended droplets (10-1000 nm in diameter) in a process called suspension (pearl) polymerizations. Coalescence of droplets is prevented by the use of small amounts of water-soluble polymers, such as PVA. The suspension process is characterized by good heat control and ease of removal of the discrete polymer particles. 

As shown in Figure 6.4, the water-insoluble monomer (M) is attracted to the lyophilic ends in the micelles, causing the micelles to swell. The number of swollen micelles per milliliter of water is on the order of 10. However, at the initial stages of polymerization (phase I) most of the monomer is present as globules that resemble those observed in suspension polymerization. 

Polymerization in P-cyclodextrin (CD) complexes with monomer offers a route to polymerization, as well as other organic reactions, in water without the need for organic solvents [Ritter and Tabatabai, 2002]. P-Cyclodextrins are toms-shaped, cyclic oligosaccharides obtained by degradation of starch. The hydroxyl groups of the glucose repeat unit of CD are located on the outer surface. This makes the outer surface hydrophilic, whereas the inner surface and cavity are hydrophobic. Water-insoluble monomers become solubilized in water when mixed with CD or CD derivatives because the monomers are absorbed into the cavity. This allows polymerization in aqueous, not organic media, with water-soluble initiators. 

The usual procedures of fractional, azeotropic, or extractive distillation under inert gases, crystallization, sublimation, and column chromatography, must be carried out very carefully. For liquid, water-insoluble monomers (e.g., styrene, Example 3-1), it is recommended that phenols or amines which may be present as stabilizers, should first be removed by shaking with dilute alkali or acid, respectively the relatively high volatility of many of these kinds of stabilizers often makes it difficult to achieve their complete removal by distillation. Gaseous monomers (e.g., lower olefins, butadiene, ethylene oxide) can be purified and stored over molecular sieves in order to remove, for example, water or CO2. 

Very recently a new method was developed that opens the possibility to polymerize even hydrophobic monomers in aqueous solution. This method is based on the finding that hydrophobic monomers can be made water-soluble by incorporation in the cavities of cyclodextrins. It has to be mentioned that no covalent bonds are formed by the interaction of the cyclodextrin host and the water-insoluble guest molecule. Obviously only hydrogen bonds or hydrophobic interactions are responsible for the spontaneous formation and the stability of these host-guest complexes. X-ray diffraction pattern support this hypothesis. Radical polymerization then occurs via these host-guest complexes using water-soluble initiators. Only after a few percent conversion the homogeneous solution becomes turbid and the polymer precipitates. 

As mentioned earlier, the necessary components for emulsion polymerization are monomer, emulsifier, initiator, and water. The monomer is insoluble in water in principle, although a small amount of water-soluble monomer can be used as a comonomer in some practical cases. On the other hand, the initiator is usually water soluble. These relations mean that the loci for radical formation and radical polymerization are different, that is, in aqueous and organic phases, respectively. This might look irrational, but the unique advantages of emulsion polymerization lie in it. When radicals are formed from a water-soluble initiator in an aqueous medium in Figure 11.1.2, top, left, each radical undergoes one of the following reactions ... 

The homogeneous nucleation theory may suggest that the dependence of particle number on the concentrations of emulsifier and initiator changes from monomer to monomer. In fact, z in Eq. (4) is 0.6 for a water-insoluble monomer such as styrene (as predicted by Smith-Ewert) but decreases with increasing solubility of monomer in water or increasing transfer of radicals out of particles (12) ... 

Suspension Polymerization. When water is the solvent and the monomer is insoluble in water, suspension polymerization is often employed. The monomer and initiator form small droplets within the aqueous phase, and polymerization proceeds via bulk polymerization. Droplet sizes are typically between 10 and 1000 p,m... 

Suspension polymerizations are among the most convenient laboratory procedures as well as plant procedures for the preparation of polymers. The advantages of this method include wide applicability (it may be used with most water-insoluble or partially water-soluble monomers), rapid reaction, ease of temperature control, ease of preparing copolymers, ease of handling the final product, and control of particle size. 

The idea of the preparation of porous polymers from high internal phase emulsions had been reported prior to the publication of the PolyHIPE patent [128]. About twenty years previously, Bartl and von Bonin [148,149] described the polymerisation of water-insoluble vinyl monomers, such as styrene and methyl methacrylate, in w/o HIPEs, stabilised by styrene-ethyleneoxide graft copolymers. In this way, HIPEs of approximately 85% internal phase volume could be prepared. On polymerisation, solid, closed-cell monolithic polymers were obtained. Similarly, Riess and coworkers [150] had described the preparation of closed-cell porous polystyrene from HIPEs of water in styrene, stabilised by poly(styrene-ethyleneoxide) block copolymer surfactants, with internal phase volumes of up to 80%. 

It is assumed that the water-insoluble lignin is a polymerization and oxidation product of a Cio monomer, C9Hn03(0CH3). The brown color may be the result of loss of 0.17 methyl groups and hydrogen atoms during the formation of a quinonoid structure and loss of 0.7 moles of water. 

The water-insoluble lignin amounted to about one-third, and the hexane-dioxane soluble lignins (which were mostly monomers) to about 20%, of the lignins dissolved. 

In order to overcome the reactivity ratio problem of AA, the use of acrylic monomers, such as -butyl acrylate, 2-ethylhexyl acrylate, ethyl acrylate, N-methylol acrylamide, and acrylamide have been suggested (14,15). Also, the use of water insoluble comonomers based on acrylamide has been described (16). 

Polymerization Methods. Acrylonitrile and its comonomers can he polymerized by any of the well-known free-radical methods. Bulk polyntenzaiion is the most fundamental of these, but its commercial use is limned by ns aulocutalyiie nature. Aqueous dispersion polymerization is the itiosi common commercial method, whereas solution polymerization is used in cases where the spinning dope can he prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacrylic compositions where a high level of a water insoluble monomer is used or where the monomer mixture is relatively slow reacting. 

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