High-energy-radiation-induced cationic polymerization

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. 

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