Free Radical Polymerization - Reaction Methods

Bulk and solution polymerization

A typical recipe:

  • 1 mole monomer
  • 0.01 mole initiator
  • 1 L solvent (optional)
  • 0.01 mole chain transfer agent (optional)

Purge system with inert gas to remove dissolved molecular oxygen. Heat the mixture for 4 - 48 h at 70 °C. Isolate product.

Advantages Disadvantages
Bulk Simple, few ingredients, cheap. Make polymer and use it. Autoacceration will occur. Possible problems with heat transfer during polymerization.
Solution Solvent will absorb exotherm of polymerization. Can avoid autoacceleration. Polymer ends up in solution, maybe use directly. Must purchase and recover solvent. Must separate polymer from solvent at some point. Possibly environmentally unfriendly.

Two Phase Free Radical Polymerizations

Suspension polymerization:

  • Water insoluble monomer.
  • Water insoluble initiator.
  • Suspending agent (optional).
  • Droplets are 100 to 10-3 mm diameter.
  • "Mini reactors."

Emulsion polymerization:

  • Water insoluble monomer.
  • Water soluble initiator.
  • Surfactant (except in special cases).
  • Droplets are 10-5 to 10-6 mm diameter.
  • Complicated mechanism.
Advantages Disadvantages
Suspension Simple, few ingredients, cheap. Reaction medium is mostly water, which absorbs the hear of polymerization. Produces beads that have technological uses (xerographic toner, catalyst carriers, ion exchange resins, substrates for combinatorial synthesis, etc.) Autoacceration will still occur. Isolation of the polymer can be laborious if you didn't want beads. May need to purify polymer from suspending agent.
Emulsion Makes very high MW polymer quickly. Reaction medium is mostly water, which absorbs the hear of polymerization. Creates very tiny particles of polymer that have technological uses (paint, coatings, drug delivery, etc.). Isolation of the polymer can be laborious if you didn't want viny particles. May need to purify polymer from surfactant.

MW Distribution from an Actual Emulsion Polymerization

Emulsion polym. MWD

Mechanism of Emulsion Polymerization

The mechanism is actually very complicated and continues to be studied to this day. The reaction actually takes place in several stages, but, to a first approximation, the following description suffices:

The surfactant molecules surround small amounts of monomer molecules, creating micelles. However, the usual recipe contains much more monomer than can be accomodated in micelles, so there are also large droplets of monomer that are stabilized by small amounts of surfactant. Depending on the monomer, there may also be a small amount of monomer dissolved in the water.
Initiator forms free radicals in the water, where they may find a few monomers to react with. In any case, the radicals diffuse into the micelles, where they find lots of monomer but no other growing chains to cause termination (at least, for a while). The growing chain is then protected from termination until a second radical diffuses into the micelle. This is why the MW can be so high in emulsion polymerization without slowing the rate of conversion.
No polymerization seems to occur in the large monomer droplets. Why? The explanation lies in simple statistics. Compared to the droplets, there are a large number of micelles, with much higher surface area (estimated 1000 times more). It is simply more likely for a radical to diffuse into a micelle than a droplet.

Emulsion polymerization