Difference between homogeneous and heterogeneous polymerization
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Liquid phase polymerization
Radical polymerizations can be carried out both by homogeneous and heterogeneous process depending on whether the initial reaction mixture is homogeneous or heterogeneous. Bulk polymerization and solution polymerization are homogeneous processes while suspension and emulsion polymerization are heterogeneous processes. By heterogeneous polymerization thermal and viscosity problems can be controlled more efficiently.
1. Bulk polymerization
Bulk polymerization of pure liquid monomer is the simplest process and carried out by using initiator in the absence of diluent or solvent. For this process reaction rate is high due to high monomer concentration and result in high yield per volume of reactor. Another advantage is that the relatively pure product is produced. However, control of the bulk polymerization, exothermic in nature, is difficult. The viscosity of the reaction system increases rapidly even at relatively low conversion. The heat removal is difficult due to high viscosity and low thermal conductivity of the polymer melt. Consequently local hot spots may occur resulting in degradation and discoloration of the polymer product. Bulk polymerization requires careful temperature control and strong elaborate stirring equipment. Though, bulk polymerization is commercially less used, polymerization of ethylene, styrene and methyl methcrylate are carried out by this method. The heat dissipation and viscosity problem are reduced by carrying out polymerization at low conversion. Bulk polymerization can be carried out in conventional stirred tank reactor, long tubular reactor with high surface to volume ratio and screw extruder reactors.
2. Solution polymerization
Solution polymerization of monomers is carried out with dissolved monomers and initiators in solvent. Typical solvents include aromatic and aliphatic hydrocarbons, esters, ethers, alcohol or water. The solvent acts as diluent and aids in transfer of the heat of polymerization. In presence of solvent the stirring becomes easier since the viscosity of the reaction mixture is decreased. Consequently controlling of process temperature is much easier in solution polymerization compared to bulk polymerization. However, in presence of solvent purity of the product is reduced particularly if there is a difficulty in removal of solvent. Vinyl acetate, acrylonitrile and ester of acrylic acid are polymerized in solution.
3. Suspension polymerization
Suspension polymerization is carried out by suspending relatively large droplets (10-1000µm) of insoluble monomers along with catalyst in water. The water to monomer weight ratio varies from 1:1 to 4:1 in most polymerization. The monomer droplets are prevented from coalescing by agitation and presence of stabilizers. The suspension stabilizers are typically used in less than 0.1 wt% of the aqueous phase. Two types of stabilizer are used:
Water soluble polymers such as poly vinyl alcohol, sodium poly styrene sulfonate, hydroxypropyl cellulose etc.
Water insoluble inorganic compounds such as talc, barium sulfate, kaolin, calcium phosphate etc.
Styrene, acrylic and methacrylic esters, vinyl chloride, vinyl acetate and tetrafluoro ethylene are polymerized by suspension method.
4. Emulsion polymerization
Emulsion polymerization involves finely divided droplets of insoluble monomers suspended in water. Hydrophobic monomer droplets, of diameter in the range of 0.5 -10 µm, are dispersed in water which also serves as heat transfer medium. In emulsion polymerization water soluble initiators such as persurphates are used. The difference between emulsion polymerization and suspension polymerization lies in the type and size of the particles in which polymerization occurs and kind of initiator employed. Many industrial polymers are produced by emulsion polymerization such as polybutadiene and PVC.
Gas phase polymerization
Large scale production of polyethylene and polypropylene from gaseous monomer is carried out using heterogeneous catalyst. Powdered catalysts are mixed with gaseous monomers at the reactor entrance. Reactors are fluidized bed or stirred reactors. The major advantage of this process is that monomers can be easily separated from polymers. Catalyst residues are not separated from polymers.
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