Polymerization Techniques: Chapter V

Polymerization Techniques

• Polymer synthesis can take place under a variety of conditions, with reaction temperatures ranging from -100°C (rubber polymerization) to 300°C (ethylene polymerization).

Mass Polymerization

• Polymerization takes place in the presence of the monomer alone, which acts as both reagent and solvent.
• If the polymer formed is soluble in its monomer, polymerization takes place in a homogeneous medium, and viscosity increases as the medium becomes increasingly solid (e.g., polystyrene).
• If the polymer formed is not soluble in its monomer, the polymer precipitates after a certain degree of polymerization (e.g., polyvinyl chloride).
• Advantages: relatively pure polymers, high molar mass, high degree of polymerization, and fast kinetics.
• Disadvantages: viscosity of the reaction mixture increases rapidly, making it difficult to ensure uniform mixing and heating in the reactor.

Emulsion Polymerization

• The monomer is dispersed as droplets in an immiscible liquid (e.g., water) and an emulsifier (a soap) forms micelles with suitable agitation.
• The initiator is water-soluble, not monomer-soluble.
• Micellar aggregates of emulsifier molecules containing monomers in their hydrophobic part are formed in water.
• Polymerization initiation occurs in water, and the radicals formed penetrate and activate the micelles.
• Each droplet or micelle is a polymerization microreactor.
• The resulting product is a latex, a continuous phase (water) in which polymer particles are suspended.
• Advantages: high-molecular-weight polymers, high polymerization rates, low-viscosity medium, ease of heat exchange, and emulsion stability.
• Disadvantages: contamination of the polymer by the emulsifier, small polymer particles, and risk of explosion.

Solution Polymerization

• Solution polymerization differs from bulk polymerization by using a solvent for the polymerization reaction.
• The solvent may or may not dissolve the catalyst and initiator.
• The solvent may be inert or act as a transfer agent (e.g., CCl4 in radical polymerization).
• The role of the solvent is to control the temperature of the reaction medium and thus control the chemical reaction.
• Advantages: better thermal control, lower viscosity, and easier mixing.
• Disadvantages: solvent is often toxic and expensive, slow reaction, larger reactor volume, and production of polymers with lower molecular weights.