Unit 3- Class 8- High Polymers (Methods of polymerization - Emulsion polymerization)

Emulsion polymerization: In this method emulsion of water insoluble monomer and water is prepared and is stabilized by the addition of surface acting agents (surfactants) such as soap. Polymerization is initiated by the addition of water-soluble initiator such as potassium persulphate. After adding the initiator, the system is kept agitated in the absence of oxygen at 70oc. 

Mechanism: The surfactant has hydrophilic head and hydrophobic tail. The water- soluble initiator links to the hydrophilic end whereas the monomer is linked to the hydrophobic end. At a little higher concentration it gets dispersed. When the concentration of surfactant exceeds critical micelle concentration (cmc), the soap molecule form micelle (aggregation of 50-100 molecules) oriented with tails inwards and head outwards. Now, an initiator molecule at the polar end diffuses into the micelle ti initiate the polymerization process. As the polymerization progresses, there will be depletion in the number of monomers within the micelle. They are replenished by the medium. This continues till the polymer formed is big enough to come out, the process is terminated by combination. The pure polymer is isolated from the emulsion by the addition of de-emulsifier.

Advantages:
1.    The rate of polymerization is high.
2.    Easy heat control.
3.    Avery high molecular weight polymer is obtained
4.    Molecular weight control is possible.
5.    Viscosity build up is low.

Disadvantages: Polymer needs purification.

Application: This method is used for the production of PVC, PVA etc.

These are the 3 methods of polymerization.We will discuss about glass transition temperature in our next class.