Emulsion and Suspension Polymerization

Emulsion and suspension polymerizations are heterogeneous, water-based processes in which monomer is dispersed as micelles or droplets; emulsion uniquely lets rate and molar mass be high together by compartmentalizing radicals, while suspension yields polymer beads.

Definition

Emulsion polymerization is a heterogeneous radical process in which monomer is emulsified in water with surfactant and polymerizes inside surfactant-stabilized particles; suspension polymerization is a heterogeneous process in which monomer droplets stabilized by a suspending agent each polymerize like a small bulk reactor to give polymer beads.

Scope

This topic covers the two principal aqueous heterogeneous processes. For emulsion polymerization it includes surfactant micelles, particle nucleation, the three intervals of the Smith-Ewart description, and radical compartmentalization. For suspension polymerization it includes monomer droplet stabilization by suspending agents, bead formation, and the essentially bulk-like kinetics within each droplet. Water as continuous phase provides excellent heat transfer in both.

Core questions

• How does compartmentalizing radicals in particles let emulsion polymerization achieve high rate and high molar mass at once?
• Where are particles nucleated and how does the Smith-Ewart picture describe their growth?
• How do suspending agents and stirring set the bead size in suspension polymerization?
• Why does water serve so well as the continuous phase in both processes?

Key theories

Smith-Ewart theory of emulsion polymerization

Polymerization proceeds in many small monomer-swollen particles, each containing on average about half a radical, so a radical that enters a particle propagates until another enters and terminates it; this compartmentalization decouples rate from termination and yields simultaneously high rate and high molar mass.

Mechanisms

In emulsion polymerization, surfactant above its critical micelle concentration forms micelles where water-soluble initiator radicals enter and begin polymerizing; monomer diffuses from droplet reservoirs through the water to feed growing particles. Because each particle holds at most a few radicals and entering radicals terminate one another only intermittently, the effective radical lifetime per particle is long, giving high molar mass at high rate. In suspension polymerization, mechanical agitation breaks oil-soluble monomer into droplets stabilized by a protective colloid; each droplet polymerizes by bulk kinetics to a solid bead, with size set by stirring and stabilizer.

Clinical relevance

Emulsion polymerization produces the latexes used in water-based paints, adhesives, paper and textile coatings, and synthetic rubber such as styrene-butadiene rubber, delivering polymer in a directly usable, low-VOC dispersion. Suspension polymerization produces poly(vinyl chloride) and polystyrene beads, expandable polystyrene, and the crosslinked beads used for ion-exchange and chromatography resins.

History

Emulsion polymerization was scaled up during the Second World War to make synthetic rubber, and its mechanism was clarified by Harkins's qualitative picture of micellar nucleation and then by the quantitative Smith-Ewart kinetic theory published in 1948, which remains the standard framework.

Key figures

• Wendell Smith
• Roswell Ewart
• William Harkins

Seminal works

• odian2004
• young2011

Frequently asked questions

Why can emulsion polymerization achieve high rate and high molar mass at the same time?

Radicals are isolated in separate particles, so a growing radical is not quickly terminated by another. Each chain therefore grows for a long time (high molar mass) while many particles polymerize in parallel (high rate)—a combination impossible in a single homogeneous phase.

What is the difference between emulsion and suspension polymerization?

Emulsion uses surfactant micelles and water-soluble initiator, producing sub-micron latex particles. Suspension uses a protective colloid and oil-soluble initiator, producing much larger beads, each behaving like a tiny bulk reactor.