Polyreactions in emulsion

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 ingredients for an emulsion polymerization consist essentially of four components  

The size of the micelles is significantly increased by the addition of monomer up to a diameter of 4.5-5 nm. However, the size of the monomer droplets is stilt very much larger than that of the micelles (diameters up to 1 pm). In emulsion polymerization, one generally uses 0.5-5 wt% of emulsifier relative to monomer. With the usual oil-in-water emulsions, the water content varies from half to four times the amount of monomer. 

The micelles are present at a concentration of about lO per ml of liquor and each micelle contains around 100 monomer molecules. In contrast, the number of monomer droplets is only about 10 ° per ml. Thus, despite the larger volume of monomer droplets, the micelles offer a very much larger surface area. A radical formed in the aqueous phase will thus encounter a monomer-filled micelle much more often than a monomer droplet. Therefore, the polymerization takes place practically only in the micelles and not in the monomer droplets. The monomer consumed in the micelles is replaced by diffusion from the monomer droplets through the aqueous phase. According to the theories of Harkins and of Smith and Ewart, the kinetic course of an emulsion polymerization is divided into three intervals At first some of the micelles increase rapidly in size as the polymerization advances and are transformed into so-called latex particles, containing both monomer and polymer. These are still very much smaller than the monomer droplets and have an initial diameter of about 20-40 pm, corresponding to about 

10 particles per ml of liquor. The monomer used up is continuously replaced from the monomer droplets via diffusion through the aqueous phase. More and more emulsifier molecules are adsorbed from the aqueous phase onto the surface of the growing latex particles, assisting their stabilization the micelles that still contain no polymer thereby slowly disappear. The concentration of free emulsifier finally falls below the critical micelle concentration at this point, the surface tension increases significantly. From then on, practically no new latex particles can be formed. The first phase of the emulsion polymerization, the so-called particle-formation period, is completed after about 10-20% conversion. 

Working in emulsion is essentially limited to radical polymerization in water. 

Similar to suspension polymerization, the basic principle is to disperse a sparingly 

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