Emulsion polymerization
Radical polymerization in aqueous dispersion, producing latex particles.
Emulsion polymerization is a radical polymerization process that begins with an emulsion of water, monomer, and surfactant. The term "emulsion polymerization" is historically misleading, as the actual polymerization does not occur within the emulsified monomer droplets. Instead, within the first few minutes, latex or colloid particles—typically around 100 nanometers in size and composed of many individual polymer chains—form spontaneously. These particles are stabilized by surfactant molecules, which create an electrostatic repulsion that prevents coagulation. Alternatively, water-soluble polymers like certain polyvinyl alcohols or hydroxyethyl cellulose can serve as stabilizers, forming a "hairy layer" around each particle that repels others through steric hindrance.
The process yields dispersions often called latexes or emulsions, which are used directly in adhesives, paints, paper coatings, and textile coatings, offering an advantage over solvent-based products by containing no volatile organic compounds. Key advantages include the ability to achieve high molecular weights at fast rates—unlike bulk or solution polymerization, where rate and molecular weight are traded off—and excellent heat transfer through the water phase, enabling temperature control. The reaction medium’s viscosity remains low, similar to water, regardless of polymer molecular weight. Disadvantages include the retention of surfactants and other adjuvants in the final product, the energy cost of removing water for dry polymers, and the potential for significant chain transfer to polymer at high conversions. The method is not generally applicable to condensation, ionic, or Ziegler-Natta polymerizations.
Historically, emulsion polymerization originated at Bayer before World War I, inspired by natural rubber’s formation in dispersed particles. Early attempts used natural polymers like gelatin or starch, but these were suspension polymerizations. True emulsion polymerizations, employing surfactants and initiators, emerged in the 1920s for isoprene. Development accelerated through World War II for synthetic rubber production, though much was kept secret. After the war, the technique expanded to plastics and latex paints. The first successful theoretical framework was the Smith-Ewart-Harkins model from the 1940s, which described three intervals: monomer emulsified into droplets, micelle for
- field
- Polymer chemistry
- known_for
- Radical polymerization in aqueous dispersion; production of synthetic rubber and latexes
- first_conceived_at
- Bayer, before World War I
- first_true_emulsion_polymerizations
- 1920s, for polymerizing isoprene
- key_theory
- Smith-Ewart-Harkins theory (1940s)
Lore & Background
The early history of emulsion polymerization is connected with synthetic rubber. The idea of using an emulsified monomer in an aqueous suspension was first conceived at Bayer before World War I, inspired by natural rubber's formation in dispersed particles stabilized by colloidal polymers. Early workers used naturally occurring polymers like gelatin, ovalbumin, and starch as stabilizers, though these were suspension polymerizations by today's definition. The first true emulsion polymerizations, using a surfactant and polymerization initiator, were conducted in the 1920s to polymerize isoprene. Over the next twenty years, through the end of World War II, efficient methods for producing several forms of synthetic rubber by emulsion polymerization were developed, but most disclosures were confined to patents or kept secret due to wartime needs.
Reader's Guide
Emulsion polymerization is significant because it allows high molecular weight polymers to be made at fast rates, overcoming the tradeoff between molecular weight and rate seen in bulk and solution polymerization. The continuous water phase conducts heat well, enabling fast rates without loss of temperature control, and the viscosity of the reaction medium remains low regardless of molecular weight. The final dispersion can often be used directly without further processing. However, surfactants and other adjuvants remain in the polymer and are difficult to remove, and water removal for dry polymers is energy-intensive. The process cannot be used for condensation, ionic, or Ziegler-Natta polymerization, with some exceptions. After World War II, emulsion polymerization was extended to plastics and latex paints, replacing solvent-based materials and reducing volatile organic compounds. The Smith-Ewart-Harkins theory, developed in the 1940s, provided the first successful explanation of the mechanism, though it does not predict behavior for water-soluble monomers like methyl methacrylate or vinyl acetate, where homogeneous nucleation occurs.
Did You Know?
- The name 'emulsion polymerization' is a misnomer arising from a historical misconception; polymerization actually takes place in latex particles, not in emulsion droplets.
- The first true emulsion polymerizations, using a surfactant and initiator, were conducted in the 1920s to polymerize isoprene.
- Smith-Ewart theory does not predict polymerization behavior for monomers that are somewhat water-soluble, such as methyl methacrylate or vinyl acetate.
- Emulsion polymerization cannot be used for condensation, ionic, or Ziegler-Natta polymerization, although some exceptions are known.
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