Solid-state polymerization induced by radiation

While the radiation-induced polymerization of monomers in the liquid or gas phase or in solution has been thoroughly investigated over a period of years, polymerization in the solid state under the influence of high-energy radiation has only recently attracted attention. The reactions involved appear to be quite different from those in the corresponding liquid monomers, being largely dominated by the crystalline environment and orientation of the monomer units. No single pattern of behaviour emerges from the very numerous monomeric systems studied. In one group the polymerization rate is considerably enhanced by monomer orientation within the crystal lattice, and oriented polymer may be formed directly in suitable crystals. In another group of monomers crystallinity impedes the reaction, which then tends to be initiated at crystalline defects to give amorphous polymer within the original crystalline lattice. Very different reaction rates may be observed in the same monomer, depending on its crystalline or amorphous (glass-like) environment. Other experiments emphasize the influence of crystallite size and perfection, of internal and external stresses, and of non-reactive groups incorporated within the solid. In suitable crystals, the orientation of each monomer unit, at the precise moment of its addition to the growing chain, may be deduced from the physical properties of the resultant polymer. In many monomers the reaction continues after the cessation of radiation; in others the initiating centres formed by radiation only become effective subsequently when the temperature is raised. Several explanations of these post-radiation effects have been suggested. Solid-state polymerization induced by radiation promises to become a valuable method for investigating the mobility of organic molecules within crystalline and amorphous solids, the nature and distribution of defects and dislocations, and the influence of temperature and pressure on their behaviour.