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Thermal Initiator Chemistry Tutorial

Thermal initiators, also referred to as free radicals, serve a useful function in polymer-processing reactions used to create plastics for materials ranging from PVC pipes to DVD cases. Chemistry students should be able to grasp the fundamental properties of ideal thermal initiators and identify their compositions. It is also important for students to identify ideal free radical and reaction solvent combinations, as thermal initiators often function more effectively in certain solvents than in others.

Temperature
- The ideal thermal initiator maintains relative stability at room temperature; however, it should decompose rapidly at the temperature required for polymer processing in order to achieve a practical reaction rate. Polymerization must occur at temperatures in excess of 20 degrees C for the free radical to effectively initiate decomposition of the polymer. Most thermal initiators reach optimum temperature in benzene-based solvents, although acetone, water and toluene are also used for polymerization conducted at temperatures betwen 70 and 90 degrees C.
Half-Life and Decomposition
- Most commonly-used thermal initiators require an activation energy between 120 and 150 kiloJoules for decomposition. Free radicals must possess the ability to overcome the "cage effect," which causes regeneration of the initiator as the viscosity of the material increases. The half-life of the thermal initiator must be higher than the regeneration rate caused by the cage effect for a successful reaction. Half-lives are typically influenced by the polymer-processing decomposition temperature and the solubility of the initiator.
Stabilizer Removal Alternative
- Removing a stabilizer prior to the polymerization project must usually be accomplished by column chromatography, a difficult and time-consuming task. In most cases, adding a thermal initiator heated to the polymerization temperature will overwhelm the effect of the stabilizer. If this method is unsuccessful, adding excess initiator often raises the energy level enough to initiate decomposition. Once polymerization begins, the stabilizer is rendered ineffective and is absorbed by the solution as the polymers separate out.
Removal After Polymerization
- Fractional precipitation using a solvent/co-solvent combination such as toluene/methanol after polymerization purges the finished plastic product from residual initiators, as well as stabilizers and low-weight polymers. Removing initiators from the polymer is of equal importance to the process as adding them. This precipitation causes the polymer molecules to form a solid structure, typically in the form of powder, gum or fibers and tighten into a narrower molecular weight distribution that renders it useful in a wide range of plastic products.

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