Polymerization technique:
The polymerization reaction may be carried out in the solid phase, liquid phase and gas phase (most of the commercial polymers are prepared in the liquid phase). The liquid phase polymerization may be further subdivided into four categories:
1. Bulk polymerization
2. Solution polymerization
3. Suspension polymerization
4. Emulsion polymerization
1. Bulk polymerization:
This is simplest process & is widely used for synthesis of condensation polymers. In this process the monomer is taken which is dissolved in monomer. The monomer is kept in a reactor & heated to suitable temperature. The chain transfer agent whenever used for controlling the molecular weight is also dissolved in the monomer.
Monomer dissolved in a suitable solvent + Initiator is used in solvent + catalyst + chain transfer agent.
Advantages:
- It is simple process compared to others.
- Initiator formed with transfer agent.
- Polymer is found in a pure state.
- The product obtained is of high purity.
- high molecular weight is also dissolved in the monomer.
Disadvantages:
- Heat is produced
- Agitation is required
- Viscosity rises
- Explosion may occur.
Uses:
Free radical polymerization of methyl-methacrylate or styrene to obtain transparent moulding powders.
2. Solution polymerization:
The monomer is dissolved in a suitable inert solvent along with the chain transfer agents. Free radical initiator is also dissolved in the solvent medium. Inert solvent medium helps to control viscosity increase.
Advantages:
- The technique is advantageous when the polymer is to be used in its solution form in case of adhesives & coating.
- It can be easily isolated
- Heat transfer easy.
Disadvantages:- Difficult to get very high molecular weight compound.
- Generally low molecular weight polymers are formed.
- Pure product can't be obtained.
Uses:
poly-iso-butylene, poly-acrylo-nitrile.
3. Suspension polymerization:
By using this technique only water insoluble monomer can be polymerized. In this process the monomer is suspended as discrete droplets (0.1mm to 1.0mm diameter). The product is obtained by filtration or spray drying.
Advantages:
- 100% monomer transfer into polymer.
- Pure polymer can be obtained.
- High quality product formed
- Heat transfer is easy
- No chain transfer agent is required.
- Economical technique.
Disadvantages:
- Only water soluble monomer can be polymerised.
Uses:
Poly-Vinyl-Chloride, Poly-Vinyl-Acetate.
4. Emulsion polymerization:
Emulsion polymerization is similar to suspension polymerization. But in this process a monomer dispersed in water in presence of a surface active agent. The surfactants helps to lower the tension at the monomer water interface & facilitate emulsification of the monomer in water. Emulsifier are added into the water, because of their low solubility.
Water insoluble monomer + Surface active agent + protective colloid + water water soluble initiator.
Advantages:
- Temperature control easy
- Heat transfer easy
- Polymers are dispersed uniformly in the aqueous solution
- Rate of reaction high
- Fine particles of polymer are found.
Disadvantages:
- Need soft water, hard water
- Costly
- There may be contamination
- The polymer have to be coagulated & dried to obtain a solid polymer.
Uses:
This technique is extensively used for the free radical polymerization of vinyl monomers. Containing water soluble initiators. It can be used for- PVC,PVA, Poly-butadiene, Poly-chloroprene.