Page 238 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Free Radical Chain Polymerization 201

TABLE 6.5
Types of Polymerization Systems
Monomer Location
Monomer–Polymer Phase
Relationship Continuous Dispersed
Homogeneous (same phase) Bulk, solid state, solution Suspension
Heterogeneous (different phase) Bulk with polymer Emulsion; suspension with polymer
Precipitating precipitating

TABLE 6.6
Summary of Popular Polymerization Techniques
Bulk
Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for
molecular weight control. Characterized, on the positive side, by high-polymer yield per volume of reaction, easy
polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of

polymers produced by bulk polymerization include poly(methyl methacrylate), PS, and low-density (high pressure)
polyethylene
Solution
Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics,
boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready
removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and
control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower
than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic
polymerizations are carried out utilizing solution polymerization, including water-soluble polymers prepared in aqueous
solution (namely poly(acrylic acid), polyacrylamide, and poly(N-vinylpyrrolidinone). PS, poly(methyl methacrylate),
poly(vinyl chloride) (PVC), and polybutadiene are prepared from organic solution polymerizations

Suspension
A water-insoluble monomer and initiator are used. Again, a chain-transfer agent may be used to control chain size.
Stirring is usual. Droplets of monomer-containing initiator and chain-transfer agent are formed. A protective colloidal
agent, often poly(vinyl alcohol) (PVA), is added to prevent coalescence of the droplets. Near the end, the particles
become hard and are recovered by filtration. Because the liquid is water based, solvent recovery and treatment problems

are minimal. The products may contain a number of impurities, including any of the agents added to assist in the
polymerization process. Polymers produced by suspension polymerization include poly(vinyl chloride), PS resins, and
copolymers such as poly(styrene-coacrylonitrile), SAN, and poly(vinyl chloride-co-vinylidene chloride)

Emulsion
The system usually contains a water-soluble initiator (in contrast to the requirement that the initiator must not be water
soluble in suspension polymerizations), chain-transfer agent, and a surfactant. The hydrophobic monomer forms large
droplets that are stabilized by the surfactant. At a certain surfactant concentration, the surfactant molecules form micelles
that contain 50–100 surfactant molecules. During the polymerization, the monomer, that has a small but real water
solubility, migrate from the monomer droplets through the water and into these micelles. Polymerization begins when the
water-soluble initiator enters into the monomer-containing micelle. Because the concentration of micelles (about
10 /L) is high compared with the concentration of monomer droplets (about 10 /L), the initiator is more likely to enter
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a micelle than a monomer droplet. As polymerization continues, monomer is transferred to the growing micelles. At
about 50%–80% conversion the monomer droplets disappear and the micells become large polymer-containing droplets.
This suspension is called a latex. The latex is stable and can be used as is or the polymer recovered by coagulation. In
inverse emulsion polymerization, the monomer, which is hydrophillic, is dispersed in an organic liquid. Here, the
monomer is usually contained in an aqueous solution.

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