Vinyl ethers, radiation ionic polymerization

Although radical cations are generated in some electron-irradiated monomers (e.g., vinyl ethers or epoxies), efficient cationic polymerization is not observed. Under certain conditions (addition of iodonium, sulfonium, or sulfoxonium salts, cationic polymerization with the use of electron beam irradiation can be induced. Several studies on radiation cross-linking of elastomers support the concept of ionic mechanism. ... 

The evidence in the case of styrene, where both modes of radiation-induced polymerization can be conveniently studied, is quite convincing that reduction of the concentration of water changes the predominating mode of propagation from purely free radical to essentially ionic. Evidence for an ionic propagation initiated by radiation has also been obtained in pure a-methylstyrene (3, 24), isobutylene (12, 32), cyclopenta-diene (5), / -pinene (2), 1,2-cyclohexene oxide (II), isobutyl vinyl ether (6), and nitroethylene (38), although the radical process in these monomers is extremely difficult, if not impossible, to study. 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

It is believed that the free positive and negative species annihilate each other immediately on contact. This is borne out by the strict square root relationship which is found between the rates of polymerization and the dose rate of the radiation. The growing chain ends are therefore free in nature, i.e., with no ion-pair component. This makes radiation initiated ionic polymerization an excellent method for studying free ion polymerization examples of the power of this method have been presented for p-methoxy styrene (1 9) and the vinyl ethers (16,20,21). 

The principal features of this radiation initiated ionic polymerization as understood at the present time has been recently presented and will not be repeated here. The unequivocal free ion nature of the polymerizations gives us an opportunity to study such reactions and compare the results with those obtained with, for example, chemical initiation with stable carbonium ion salts. This paper will describe the results of such studies with cationic polymerization and be limited to two vinyl ethers, ethyl vinyl ether, EVE, and isopropyl vinyl ether IPVE which behave in somewhat different ways. In particular, methods of estimating the rate constants for propagation will be presented and the results obtained discussed with particular emphasis on the effect of solvents on these values. 

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