Monomers in water

Soap. A critical ingredient for emulsion polymerization is the soap (qv), which performs a number of key roles, including production of oil (monomer) in water emulsion, provision of the loci for polymerization (micelle), stabilization of the latex particle, and impartation of characteristics to the finished polymer. 

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. 

This is the same order of the solubility of the monomer in water. The increase in acrylonitrile concentration increases the percent grafting and grafting efficiency. 

The values of these ratios change appreciably by passing from the heterogeneous (suspension) to the homogeneous (DMF) system. In the case of copolymerization in suspension in the presence of the K2S208—AgN03 oxidation-reduction system at 30—40 °C, the ratios were found to be ry = 0,77 0,2 and r2 = 1,09 0,04, whereas in the case of the copolymerization in solution they are = 0,52 and r2 = 1,7. The difference in these values seems to be the result of the different solubility of the monomers in water and of the different rate of diffusion of the monomers to the surface of the precipitated copolymer20. From this it follows that 4 is the more reactive monomer in this binary system. 

Latex or emulsion polymers are prepared by emulsification of monomers in water by adding a surfactant. A water-soluble initiator is added, e.g., persulfate or hydrogen peroxide (with a metallic ion as catalyst), that polymerises the monomer yielding polymer particles, which have diameters of about 0.1 pm. The higher the concentration of surfactant added, the smaller the polymer particles. 

The surface active agents (surfactants) may be cationic, anionic or non-ionic. Surfactants commonly used are cetyltrimethyl ammonium bromide (CTABr), sodium lauryl sulphate (NaLS) and triton-X, etc. The surfactants help to lower the surface tension at the monomer-water interface and also facilitate emulsification of the monomer in water. Because of their low solubility surfactants get fully dissolved or molecularly dispersed only at low concentrations and at higher concentrations micelles are formed. The highest concentration where in all the molecules are in dispersed state is known as critical micelle concentration (CMC). The CMC values of some surfactants are listed in table below. 

Figure 10. Solubilities of vinyl monomers at 20°C. Numbers indicate the solubilities of the monomers in water (g dm 3) (BA) butyl acrylate (BMA) butyl methacrylate (EA) ethyl acrylate (MA) methyl acrylate. (, 27) (, 28)...

Working in emulsion is essentially limited to radical polymerization in water. Similar to suspension polymerization, the basic principle is to disperse a sparingly water-soluble monomer in water and bring about polymerization in this state. There are, however, some essential differences between the two procedures ... 

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) ... 

Polymerization in suspension (bead or pearl polymerization) under normal pressure in the range from 60 to 80°C operates with a suspension of globules of an oil-soluble monomer in water and uses a monomer soluble catalyst. Substantial quantities of polystyrene and poly vinyl acetate are made by this method. 

Polymerization in emulsion under normal pressure and in the temperature range from —20 C to 60°C uses a fine emulsion of oil-soluble monomers in water and initiates the reaction with a system of water-soluble catalysts. This method is probably the most important of all, because it is used in very large scale in die copolyiuerization of butadiene and styrene and in the polymerization of many other monomers, such as chloroprene and vinyl chloride, to produce latices of the various synthetic rubbers. 

Aqueous systems have been studied by a very large number of investigators. Economy, safety, convenience and quality of product have combined to make this the method of choice for commercial production of copolymers. The industrial importance of such end products as elastomers and acrylic fibers has been a special incentive to related fundamental studies. Furthermore, the relatively high solubility of acrylonitrile monomer in water coupled with insolubility of the polymer make it a convenient test monomer for studies of initiation by redox systems (6, 25, 102). Large numbers of homogeneous chemical initiators and some heterogeneous initiators have been studied as well as initiation by photochemical means, by ultrasonics and by ionizing radiation. It will not be possible here to review the enormous world literature. Several publications (/, 92, 117) refer in some detail to the older papers, and we shall restrict our comments to recent interpretations that have received support from several quarters. 

The experimental factors that affect the stability of the latex during and after polymerization are the recipe used for the polymerization, the type and intensity of agitation during and after the polymerization, the temperature of polymerization and storage, and the age and storage conditions of the latex. The recipe used in the polymerization included the mode of polymerization, the monomer-water ratio, the solubility of the monomer in water, the emulsifier type and concentration, initiator type and concentration, total electrolyte concentration, and impurities present in the system. 

Because of its heterophase nature, emulsion polymerization is generally more complicated than simple solution polymerization in which monomers and polymers are soluble in a suitably chosen solvent. In emulsion polymerization the different relative solubilities of monomers in water and in the polymer particles lead to different reaction locales and to different particle structures. Another complicating factor is the need to achieve and maintain colloidal stability throughout the polymerization and subsequent handling of the dispersions. Emulsion polymers can properly be called products by process since the process details exert such a powerful effect on the properties of the particles and resultant films. Consequently, an emulsion polymer is far more than a product defined by a simple polymer composition. 

Standing Committee of Analysts, HM Stationary Office, London. Methods for the Examination of Waters and Associated Materials 1988. Determination of Acrylamide Monomer in Waters and Polymers (1987). 

Polyvinyl chloride. Organic peroxides are used to catalyze the free radical polymerization of vinyl chloride monomer in water. The organic peroxide is selected to generate free radicals thermally at the temperature of polymerization. 

What is the molecular structure of quinine in ether, if quinine exists as monomer in water ... 

Waterborne paints are usually emulsions of polymer-based binder particles. They are made by making an emulsion of droplets of monomers in water, after which the monomers are polymerized to form solid particles. When applied, the water and possibly other solvents evaporate, and the binder particles fuse to form a solid layer. 

The hydrophobic part of the aggregate molecules forms the core of the micelle while the polar head groups are located at the micelle-water interface in contact with the water molecules. Such micelles usually have average radii of 2... 4 nm and contain 50... 100 monomers in water. Their geometric structure is usually roughly spherical or ellipsoidal. In non-aqueous nonpolar solvents, the micellar structures are generally the inverse of those formed in water. In these solvents, the polar head groups form the interior of the micelle while the hydrocarbon chains of the ions are in contact with the nonpolar solvent. 

Brandstetter et al. [25] produced similar particles without the intensive shearing step by polymerizing acrylate monomer in water in the presence of a long-chain alcohol, an emulsifier and a water-insoluble initiator. The particles have a mean particle size of 0.2-6 p,m. After their preparation the particles can be grafted with SAN and then used as impact modifiers for ASA or as delustering agents for thermoplastics [26]. 

---