Free Radical Initiators

Dr. S. S. Newaz

Polyorganix Inc., Houston, TX

Free Radical Initiator/Stabilizer FAQs

Q. How does one choose the appropriate initiator?

In a free radical addition polymerization, the choice of polymerization initiator depends mainly on two factors: a) its solubility and b) its decomposition temperature. If the polymerization is performed in an organic solvent, then the initiator should be soluble in that solvent and the decomposition temperature of the initiator must be at or below the boiling point of the solvent. Commonly, 2,2'-Azobis(2-methylpropionitrile) (441090) and Benzoyl peroxide (179981) suit these requirements. If the desired polymerization occurs at or below 20 °C, then special, low temperature free radical initiators need to be used. Various AZO- type initiators can be chosen for range of decomposition temperature requirement: see Thermal Initiators Selection Table.

For emulsion polymerization or polymerization in an aqueous system, a water soluble initiator like potassium persulfate (216224) or an organic, water-soluble initiator 4,4'-Azobis(4-cyanovaleric acid) (118168) would be suitable.

Q. How does one determine the reactivity of monomer?

Determination of monomer reactivity is not always obvious or straightforward. Researchers rely on their experience and published data on individual monomers. In general, extent of conjugation in the molecular structure can be viewed as indicative of its tendency to form the initial free radical required for propagating a free radical polymerization. Usually, a more conjugated system is more likely to undergo the free radical polymerization.

Q. When is it necessary to remove a stabilizer prior to polymerization and how does one do so?

Usually, it is not necessary to remove stabilizers. They are typically present in ppm level and the use of a free radical initiator at the polymerization temperature will overwhelm the effect of the stabilizer. In worst-case scenarios, one may need to add extra amounts of initiator to sustain an acceptable polymerization rate. In most cases, once a polymerization initiates, the rate can be sustained without much difficulty. If, however, it is absolutely necessary to remove the stabilizer, column chromatography is the preferred method. Easy-to-use, pre-packed inhibitors removal columns (306312 and 311340) are available to simplify this task.

Q. How does one remove residual initiator, stabilizer, and/or unreacted monomer after polymerization?

It is a common practice to dissolve the polymers in a solvent prior to end use, followed by precipitating the polymer using a co-solvent. Usually the residual initiators and stabilizers will remain in solution and the polymers will separate out as a solid (powder, gum or fibers). This process may be repeated until desirable polymer characteristics are obtained. This fractional precipitation is also effective in removing lower molecular weight polymers, resulting in narrower molecular weight distribution – of course accompanied with a loss of yield. Typical solvent/co-solvent pairs could be: Toluene/hexane, toluene/methanol, THF/water, etc.