Water-Soluble Free Radical Addition Polymerizations

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

WATER-SOLUBLE FREE RADICAL ADDITION POLYMERIZATIONS... 

One of the most important parameters in the S-E theory is the rate coefficient for radical entry. When a water-soluble initiator such as potassium persulfate (KPS) is used in emulsion polymerization, the initiating free radicals are generated entirely in the aqueous phase. Since the polymerization proceeds exclusively inside the polymer particles, the free radical activity must be transferred from the aqueous phase into the interiors of the polymer particles, which are the major loci of polymerization. Radical entry is defined as the transfer of free radical activity from the aqueous phase into the interiors of the polymer particles, whatever the mechanism is. It is beheved that the radical entry event consists of several chemical and physical steps. In order for an initiator-derived radical to enter a particle, it must first become hydrophobic by the addition of several monomer units in the aqueous phase. The hydrophobic ohgomer radical produced in this way arrives at the surface of a polymer particle by molecular diffusion. It can then diffuse (enter) into the polymer particle, or its radical activity can be transferred into the polymer particle via a propagation reaction at its penetrated active site with monomer in the particle surface layer, while it stays adsorbed on the particle surface. A number of entry models have been proposed (1) the surfactant displacement model (2) the colhsional model (3) the diffusion-controlled model (4) the colloidal entry model, and (5) the propagation-controlled model. The dependence of each entry model on particle diameter is shown in Table 1 [12]. 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

Emulsion pol)m erization is a complex process in which the radical addition polymerization proceeds in a heterogeneous system. This process involves emulsification of the relatively hydrophobic monomer in water by an oil-in-water emulsifier, followed by the initiation reaction with either a water-soluble or an oil-soluble free radical initiator. At the end of the pol)nnerization, a milky fluid called "latex", "synthetic latex" or "pol)rmer dispersion" is obtained. Latex is defined as "colloidal dispersion of polymer particles in an aqueous medium". The pol)nner may be organic or inorganic. In general, latexes contain 40-60 % pol)nner solids and comprise a large population of polymer particles dispersed in the continuous aqueous phase (about lO particles per mL of latex). The particles are within the size range 10 nm to 1000 run in a diameter and are generally spherical. A typical of particle is composed of 1-10000 macromolecules, and each macromolecule contains about lOO-lO " monomer units [10-16]. 

Commercially, PFA is polymerized by a free-radical polymerization mechanism, usually in an aqueous medium via addition polymerization of TFE and PPVE. The initiator for the polymerization is usually a water-soluble peroxide such as ammonium persulfate. Chain transfer agents such as methanol, acetone and others are used to control the molecular weight of the resin. Generally, the polymerization regime resembles that used to produce PTFE by emulsion polymerization. Polymerization temperature and pressure usually range between 15 and 95°C and 0.5-3.5 MPa. 

A polymeric composition for reducing fluid loss in drilling muds and well cement compositions is obtained by the free radical-initiated polymerization of a water-soluble vinyl monomer in an aqueous suspension of lignin, modified lignins, lignite, brown coal, and modified brown coal [705,1847]. The vinyl monomers can be methacrylic acid, methacrylamide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinylacetate, methyl vinyl ether, ethyl vinyl ether, N-methylmethacrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, and additional AMPS. In this process a grafting process to the coals by chain transfer may occur. 

Coatings emulsions are generally formed by addition polymerization of common, highly available monomers, using free radical initiators to create polymers having molecular weights from a few thousand up to millions. The polymerization is most often stabilized by non-ionic and/or anionic surfactants, which emulsify the insoluble monomer droplets, and then stabilize the resulting particles, usually in the shape of a sphere. In addition to surfactants, emulsions are sometimes stabilized with water-soluble poly-... 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

Since the principal hazard of contamination of acrolein is base-catalyzed polymerization, a "buffer" solution to shortstop such a polymerization is often employed for emergency addition to a reacting tank. A typical composition of this solution is 78% acetic acid, 15% water, and 7% liydroquinone. The acetic acid is the primary active ingredient. Water is added to depress the freezing point and to increase the solubility of liydroquinone. Hydroquinone (HQ) prevents free-radical polymerization. Such polymerization is not expected to be a safety hazard, but there is no reason to exclude HQ from the formulation. Sodium acetate may be included as well to stop polymerization by very strong acids. There is, however, a temperature rise when it is added to acrolein due to catalysis of the acetic acid-acrolein addition reaction. 

PTFE is produced by free-radical polymerization mechanism in an aqueous media via addition polymerization of tetrafluoroethylene in a batch process. The initiator for the polymerization is usually a water-soluble peroxide, such as ammonium persulfate or disuccinic peroxide. A redox catalyst is used for low temperature polymerization. PTFE is produced by suspension (or slurry) polymerization without a surfactant to obtain granular resins or with a perfluori-nated surfactant emulsion polymerization) to produce fine powder and dispersion products. Polymerization temperature and pressure usually range from 0 to 100°C and 0.7 to 3.5 MPa. 

A monomer, typically 2-bydroxyetbyl methacrylate, HEMA (Fig. 7.1), which is water soluble and capable of undergoing rapid free radical addition polymerization to create part of the matrix of the set cement-polymeric matrix. 

---