Light in the ultraviolet and visible range can disrupt selected bonds forming free
radicals. Such disruption occurs as we are exposed to sunlight. Suntan treatments often
contain certain compounds that can accept this damaging radiation. Related compounds are
also used in foods to give them longer shelf life. They are generally known as antioxidants.
Synthetic antioxidants include benzophenones, benzils, and certain organic ketones . Thus, diphenylketone decomposes on exposure of ultraviolet radiation of the
appropriate wavelength forming two free radicals.
The advantage of using such photochemical initiations is that polymerization can be carried
out at room temperature.
When molecules are exposed to light of higher energy, shorter wavelength or higher
frequency, electrons can be removed or added depending on the specific conditions. Usual
forms of ionizing radiation employed industrially and experimentally include neutrons,
X-rays, protons, and alpha and beta particles. A simplified scheme illustrating free radical
formation is given following.
In truth, the precise mechanism is generally more complex because various radicals, cations,
and anions will be formed. Ionizing radiation induced polymerizations are generally
carried out at room temperature and higher. Here, the reactants are selected so that the
free radicals are more stable than the cations and anions allowing a largely free radical
polymerization to occur.
Oxidation-reduction, redox, reactions are also often employed to initiate free radical
polymerizations in solution or heterogeneous organic–aqueous systems.
Free radicals can be created by passing a current through a reaction system sufficient
to initiate free radical polymerizations. Thus, solutions containing salts of carboxylic acids
oxidize at the anode of an electrochemical cell forming free radicals when current is
applied. Such free radicals can be used to initiate free radical polymerizations at, below,
or above room temperature.