Emulsion polymerization

Emulsion polymerization is a type of polymerization that takes place in an emulsion typically incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. If the polymerization is carried out in the absence of surfactants, it is generally known as a suspension polymerization.

Advantages of emulsion polymerization include:

• The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate.
• Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight.
• The final product can be used as is and does not generally need to be altered or processed.

Most emulsion polymerizations use a free-radical polymerization method. Emulsion poymerization can be carried out as a batch reaction, but in many cases is performed as a starve-fed reaction to insure a good distribution of monomers into the polymer backbone chain.

The leading theory for the mechanism of starve-fed, free-radical emulsion polymerization is summarized by the following:

• Surfactants emulsify the monomer in a water continuous phase.
• Excess surfactant creates micelles in the water.
• Small amounts of monomer diffuse through the water to the micelle.
• Initiator (water-soluble and introduced into the water phase) reacts with monomer in the micelles
  • The micelles in total, comprise a much larger surface area in the system than the fewer, larger monomer droplets, which is why the initiator typically reacts with the micelle and not the monomer droplet.
• Monomer in the micelle quickly polymerizes and the growing chain terminates.
• More monomer from the droplets diffuses to the growing micelle/particle, where more initiators will eventually react.
• Monomer droplets and initiator are continuously, and slowly added to maintain their levels in the system as the particles grow.
• When the monomer droplets have been completely consumed, the initiator is typically added in for a little while longer to consume any residual monomer.
• The final product is an emulsion of polymer particles in water.