Start by High-Energy Radiation

Excited electrons with insufficient energy to produce further ionization are called thermal electrons. Thermal electrons emanate phonons on returning to the ground state, where they finally recombine with previously formed cations. These low-energy phonons can also start the polymerization of suitable monomers. Ions can recombine to produce new excited compounds. However, they can also react with other molecules and then start an ionic polymerization. Such cationic polymerizations, however, occur only at low temperatures with ultrapure monomers in solvents of high dielectric constants. Ultrapure monomers contain less than 10 % impurities. 

In the majority of cases, high-energy irradiation starts free radical polymerizations. The steady-state concentration of ions or free radicals is given by 

Here Vi is the free radical formation rate or ion formation rate and fe, is the rate constant for termination or recombination. 

The rate of formation of ions is a factor of approximately 10-100 times smaller than that for free radicals. Conversely, the recombination constants are about 100 times larger for ions than for free radicals. Thus, it follows that the steady-state concentration of ions is about 100 times smaller than that for free radicals. Consequently, the majority of radiation-initiated polymerizations proceed by a free radical mechanism. 

Free radicals are produced by the dissociation of excited molecules. High-energy-irradiation-induced polymerization is especially important for graft polymerization and polymerization in the solid state. Because of high investment costs, high-energy-radiation-initiated polymerization of gaseous, liquid, or dissolved monomers has not become established. However, to a small extent ( 2000 t/a), the polymerization of methyl methacrylate is radiation initiated. 

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Radiation-Initiated Polymerizations 2L2.1, Start by High-Energy Radiation... 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

To form a polymer, the initial monomers must be activated in some way to start the reaction, a step called initiation. This can be accomplished by heat or high-energy radiation such as ultraviolet light. These processes, which are not reproducible enough for industrial production, contribute to the degradation of polymers in use. Industrially, the initiation stage is achieved by mixing a wide variety of active molecules with the monomers. 

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