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Polymerisation Techniques the Four Different Types of Polymerization

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Polymerisation Techniques the Four Different Types of Polymerization Polymerisation techniques The four different types of polymerization techniques which are commonly used for the commercial production of polymers are 1. Bulk Polymerization 2. Solution Polymerization 3. Suspension Polymerization 4. Emulsion Polymerization Bulk Polymerization It is used in the free radical polymerization of ethylene, styrene and methyl methacrylate. The reaction system is homogeneous and the monomer is taken in the liquid state. The initiator is dissolved in the monomer. The chain transfer agent is also dissolved in the monomer. Chain transfer agent is used to control the molecular weight of the polymer. The reaction system is heated or exposed to radiation for initiating the polymerization. The reaction system is agitated for mass and heat transfers. The bulk polymerisation process is described in the following figure. monomer initiator Advantages 1. It is simple technique to obtain the polymer 2. Polymer obtained has a high purity 3. Polymer obtained can be used as such without purification. Disadvantages 1. Polymers with very broad molecular weight distribution are obtained due to the increase in viscosity of the medium. 2. Auto acceleration( enormous increase in the rate of polymerization) takes place. Solution Polymerization It is a useful technique for the preparation of polymers used in solution form- adhesives and coating compositions. It is used in the industrial production of poly- acrylonitrile by free- radical polymerization and polyisobutylene by cationic polymerization. In this technique, monomer is dissolved in a suitable inert solvent. Chain transfer agent is dissolved in the inert solvent. Free- radical initiator is also dissolved in the solvent medium. Ionic and coordination catalysts are either dissolved or suspended in the solvent medium. Inert solvent is used to control the viscosity increase and to promote a proper heat transfer. The solution polymerisation process is described in the following figure. monomer - initiator Inert solvent Advantages 1. Thermal control is easier than in the bulk polymerization. 2. Solvent acts as a diluent and aids in removal of heat of polymerization. 3. Solvent reduces viscosity, making processing easier. Disadvantages 1. Due to the chain transfer to the solvent it is difficult to get very high molecular weight polymers. 2. Polymer has to be separated from the reaction system. Suspension Polymerization In this polymerisation techniques the polymer is obtained as spherical beads and hence this polymerisation technique is also known as Bead or Pearl polymerisation. It is used for the polymerization of styrene, methyl methacrylate, vinyl chloride, vinyl acetate. Insoluble monomers can be polymerised by this technique In this technique, monomer is water-insoluble and is suspended in water in the form of fine droplets (0.001-1 cm in diameter). The monomer droplets are stabilised and prevented from coalescing by using suspending agents, surface active agents and by stirring. Size of the monomer droplets depends on the monomer- water ratio, type and concentration of stabilising agent and speed of agitation. Initiators are soluble in monomers. Each monomer droplet acts as an independent bulk polymerization nucleus. The continuous aqueous phase separating the monomer droplets acts as an efficient heat transfer medium. The suspension polymerisation process is described in the following figure. monomer - water-soluble initiator- Suspending agent (Inorganic salt)- water Polymer beads The monomer droplet is shown in the following figure. monomer droplet suspending agent monomer molecule free radical initiator Advantages 1. The technique is economical as water is used as the heat transfer medium. 2. The polymer obtained will have narrow molecular weight distribution 3. Isolation of the product is easy Disadvantages 1. Cannot be used for polymers whose glass transition temperature is less than the polymerization temperature 2. Polymer obtained must be separated and purified. Emulsion Polymerization It is a predominant process for the preparation of vinyl acetate, chloroprene, butadiene/ styrene/acrylonitrile copolymers, various acrylates. The polymer is obtained as milky white dispersion known as latex. The latex can be used as such for making adhesives and water- soluble paints. Polymer can be extracted from the latex by destabilizing the emulsion. Monomer is water insoluble and is dispersed in the aqueous phase as a uniform emulsion. The emulsion is stabilized by surfactants (surface active reagents). Surfactants lower the surface tension at the monomer-water interface and facilitate the emulsification of the monomer in water. Surfactant may be cationic, anionic and nonionic. Every surfactant molecule has a hydrophilic head and hydrophobic tail. The surfactant molecules form micelles beyond CMC (critical micelle Concentration). Micelles are the aggregates of surfactant molecules. Hydrophilic head-loves water Hydrophobic tail-hates water The emulsion polymerisation process is described in the following figure. monomer - water-soluble initiator- surfactant water Two types of particles exist during the polymerization process: 1. monomer droplets 2. micelles (in which polymerization is occurring) Monomers are present in monomer droplets and in micelles. Polymérisation process occurs inside the micelle. The monomers present in the monomer droplets enter into the empty space in the micelle. empty micelle monomer droplet monomer molecule monomer filled micelle micelle with growing polymer monomer chain molecule Advantages 1. Thermal and viscosity problems are minimized due to the high heat capacity and ease of stirring of the continuous aqueous phase. 2. Molecular weight may be increased without decreasing rate of polymerization 3. The latex may be used directly without purification Disadvantages 1. The polymer is contaminated with emulsifier and demulsifier 2. This technique can be adopted only for water insoluble monomer .
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