High-energy-radiation-induced cationic

High-Energy-Radiation-Induced Cationic Polymerization of Vinyl Ethers in the Presence of Onium Salt Initiators... 

The present paper reports a study of the initiation mechanism for high energy radiation-induced cationic polymerization of divinyl ethers in the presence of various onium salts. Although there is a great difference in the dose rates of y-radiation and electron beam, the radiation chemistry is essentially the same. 

Scheme 5.7 High-energy-radiation-induced cationic polymerization of tetrahydrofuran [21].

The radlatlon-lnduced cationic polymerization of vinyl and unsaturated monomers In the liquid state has been studied for over 25 years, and the essential features of this type of polymerization appear to be well established (1, ). In contrast to cationic polymerization by catalysts where the propagating species Is usually described as a solvated Ion pair, the distinctive characteristic of cationic polymerization Induced by high energy radiation Is that propagation occurs by free Ions with very large rate constants, the range of kp values for observable polymerization being from 10 ... 

As compared to vinyl monomers, relatively few studies of ringopening polymerization Induced by high energy radiation have been reported In the liquid state ( 7). Easily the best documented example Is the polymerization of 1,2-cyclohexene oxide described by Cordlschl, Ifele, and their co-workers (8-10). These authors found that the polymerization of this epoxide displays many of the characteristics previously observed for the radlatlon-lnduced cationic polymerization of unsaturated monomers, Including the great sensitivity to water (.3) and the strongly retarding effect of ammonia (11). 

The development of solid-state polymerization was largely due to the use of high-energy radiation such as y-rays 115,123), x-rays 124>, electron beams 125,126) or a-partic-les ll6). y-Rays are used most frequently. It is generally accepted that y-ray-induced polymerization of TXN proceeds by a cationic mechanism. Ions or radical-ions are formed by electron transfer from TXN, the loss of hydrogen atoms or the heterolytic cleavage of the ring 127>. 

In many cases, the initiation of cationic polymerizations is severely impeded or totally prevented by the diffusion-controlled neutralization of radical cations by secondary electrons. In the case of tetrahydrofuran (THE) the respective rate constant, as determined by pulse radiolysis measurements, is kneut = 2 x 10 1mol s (see Scheme 5.7, reaction (c) [21]. Therefore, high-energy radiation does not induce the polymerization of neat epoxides and vinyl ethers. 

When high-energy radiation is directly absorbed by polysaccharides, electronically excited moieties, radical cations and electrons are generated. To date, little is known of the mechanism of any subsequent processes leading to the formation of radiolysis products, but results obtained with low-molar-mass models have helped to provide an understanding to some extent of the radiation-induced chemical processes in polysaccharides [80,83]. Apparently, free-radical processes play an important role, as confirmed by ESR studies [83,95]. It is assumed that radical cations generated initially are rapidly transformed to free radicals by deprotonation, as illustrated in Scheme 5.20. 

Ionization of atoms or molecules is the main primary event induced by the interaction of radiations with condensed matter. The charged species produced by ionization, if not removed from the irradiated system, will naturally tend to recombine. The conventional theories of recombination treat the transport and reactions of charged species only after the electrons ejected from atoms or molecules become thermalized by dissipating their initially high kinetic energies to the surrounding medium and form a spatial distribution around their parent cations. The thermalization in condensed phases is fast and is usually... 

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