RADIATION-INDUCED POLYMERIZATION

High energy radiations may initiate either free radical or ionic polymerizations depending on experimental conditions and on the monomer under consideration. In the free radical reaction the initiation step is temperature independent and over all activation energies are much smaller than in the chemically initiated process. This property is of practical value if the reaction involves a semi-solid mixture. At high conversions the polymer itself becomes activated by the radiation and the monomer is then homografted" on its own polymer. In the presence of a solvent the polymerization is initiated by radicals resulting from the radiolysis of monomer and solvent. In such binary mixtures either "sensitization" or "deactivation" may follow from energy transfer processes. Cationic polymerizations are observed at very low temperatures and in "ultra-dry" monomers. Very long kinetic chains occur in these systems on behalf of the very high propagation rates caracteristic of free ion propagation. Solid-state polymerization was reported to proceed faster in crystalline monomers than just above their melting points. This effect appears to be caused by a rigorous dehydration in the crystal which favors a cationic mechanism. The specific features of radiation initiated polymerization which may be of practical value are stressed in greater detail. © 1979."