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What are the Mechanism Of Polymerization?

Answer:

Mechanism Of Polymerization:

Techniques of polymerization : The conversion of a monomer into a polymer is an exothermic
reaction, if this heat is not dissipated or properly controlled, serious explosions will result. The main
manufacturing processes are given below :

1. Bulk polymerization:

It is used in producing polystyrene, PVC and polymethyl methacrylate. The polymerization is carried out in two stages. In the first stage, the monomer is taken in the liquid state and the initiator is  dissolved in the monomer. The reaction mass is heated for initiating the polymerization. As the proportion of the polymer increases, the temperature also increases and the reaction mixture is cooled by circulating water through cooling coils. In the second stage, the highly viscous polymerization mixture is fed into the upper end of a vertical reactor. The reaction is carried out at 110- 2000 C. Almost 100% polymerization is achieved.

2. Suspension polymerization :

This process is largely used to manufacture PVC and polystyrene.
Only water insoluble monomer can be polymerized by this technique, The monomer is dispersed
as fine droplets in water containing gelatin as suspending agent to stabilise suspension, i.e. to prevent the droplets from coalescing. The polymerization is initiated by a monomer soluble initiator (benzoyl peroxide for vinyl chloride). Polymerization proceeds to 100% conversion and the product is obtained as spherical beads or pearls.

3. Emulsion polymerization :

It is widely used to prepare polyvinyl chloride, polyvinyl acetate, etc. the monomer is dispersed in water as droplets which are stabilized by the addition of soap. Initiators are either water soluble or monomer soluble. Normally, persulphates are used as initiators. The polymer obtained is in the form of latex. The latex can be coagulated with an acid to get the solid polymer.

4. Solution polymerization :

It is the polymerization of one or more monomers in solution. Vinyl chloride, acrylonitrile and vinyl acetate are polymerized by this method. It is used to create polymers and copolymers by dissolving a monomer and a catalyst in a non‐reactive solvent. The disadvantages of this method are that the polymer produced is of low molecular weight and is always contaminated with traces of solvent.