Radiation-induced cationic initiation

The kinetics of radiation-induced polymerization of bulk nitroethylene was also studied at 10° C by the use of hydrogen bromide as an anion scavenger (27). The value of Gt (yield of the initiation by 100 eV energy absorbed) was found to be about 3, which was much larger than the value obtained for many radiation-induced cationic polymerizations. The propagation rate constant, kp, was estimated to be 4 x 107 M-1 sec-1. The large kp value was attributed to the concept that the propagating chain ends were free ions in contrast to the existence of counter ions in catalytic polymerization. 

The cation radical is believed to be the primary cationic intermediate involving the monomer molecule in the initiation process of radiation-induced cationic polymerization. Now, the question is how the car-bonium ions are formed from the cation radicals. 

Initiation of graft copolymerization by radiation-induced cationic mechanism was recently reviewed by Stannett (160). This method is especially useful for cationic graft copolymerization from inert polymer supports. 

In 1982, Crlvello and co-workers published a report on the UV Initiated cationic polymerization of vinyl ether monomers using onlum salt catalysts(14). Vinyl ethers are among the most reactive monomers which polymerize by a cationic mechanism. The radiation Induced cationic curing of vinyl ethers occurs much faster than the cationic curing of epoxy coatings. In fact, cure rates that are at least as fast as the free radical polymerization of acrylates can be achleved(8,14). A recent report Indicates that the cationic polymerization of vinyl ethers can occur even in the presence of certain polar functional groups(15). 

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. 

Despite improvements in experimental techniques, the fundamental processes in radiation-induced cationic polymerizations remain largely hypothetical. Pulse-radiolysis studies - on styrene solutions have led to the conclusion that charge transfer from the solvent produces a styrene cation-radical which then dimerizes to form both associated dimer cation-radicals and bonded dimer cation-radicals. These initial steps are thought to be sev al orders of magnitude faster than the subsequrat prop tion reactions. The presence of trace impurities can dictate the course of polymerization, and rate studies provide circumstantial evidence for the theory that nucleophiles can neutralize the cations in these systems and allow free-radical polymerization to occur alone. 

Solid-state polymerization of TOX can be initiated by different kinds of radiation, including y-rays, X-rays, electron beams, or a-particles.The mechanism of initiation is not well understood. It is, however, generally accepted that radiation induces cationic polymerization. Ions or radical ions are generated by electron transfer, the loss of hydrogen ions, or the heterolytic cleavage of TOX rings. 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

An analogous mechanism should also produce polymers on irradiation of epoxies. Crivello s recent mechanistic suggestions [29] are consistent with the mechanisms given above. One can conclude that radiation-induced polymerization of epoxies can proceed via several mechanisms. However, further work is needed to determine the relative contributions of the different mechanisms, which might vary from one epoxy to another. As part of the Interfacial Properties of Electron Beam Cured Composites CRADA [37], an in-depth study of the curing mechanism for the cationic-initiated epoxy polymerization is being undertaken. 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

So far as vinyl monomers are concerned, ionic propagation proceeds with carbonium ions (cationic polymerization) or carbanions (anionic polymerization) at the chain ends. The study of the initiation process of radiation-induced ionic polymerization seeks to elucidate how these ions are formed from the primary ionic intermediates. Possible reactions... 

These results indicate that n-butylvinylether forms the cation radicals through positive charge transfer rather than by capturing an electron to form the anion radical and suggests that the ionization potential of n-butylvinylether is lower than that of 3-methylpentane (according to the measurements by the present authors, this is the case) and its electron affinity is negative. The observed behavior of n-butylvinylether seems to coincide with its cationic nature in the radiation-induced polymerization. Though the formation of carbonium ions from the cation radicals has not yet been elucidated, the cation radicals may play an important role in the initiation process of polymerization. 

According to the studies of monomers in the organic glass matrices mentioned so far, the ion radicals formed from solute monomers relate their radiation-induced ionic polymerization to the primary effect of ionizing radiations on matter. It is concievable that the initiating species in the anionic polymerization (caxbanions) are formed by the addition of the monomer molecules to the anion radicals which result from electron transfer from the matrices to the solute monomer. The formation of the cation radicals is necessary also to initiate the cationic polymerization. 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

Application of pulse radiolysis to polymers and polymerization was motivated at first by the success of radiation-induced polymerization as a novel technique for polymer synthesis. It turned out that a variety of monomers could be polymerized by means of radiolysis, but only a little was known about the reaction mechanisms. Early studies were, therefore, devoted to searching for initiators of radiation-induced polymerization such as radicals, anions and cations derived from monomers or solvents. Transient absorption spectra of those reactive intermediates were assigned with the aid of matrix isolation technique. Thus the initiation mechanisms were successfully elucidated by this method. Propagating species also were searched for enthusiastically in some polymerization systems, but the results were rather negative, because of the low steady state concentration of the species of interest. 

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

Radiation-induced polymerization, which generally occurs in liquid or solid phase, is essentially conventional chain growth polymerization of a monomer, which is initiated by the initiators formed by the irradiation of the monomer i.e., ion radicals. An ion radical (cation radical or anion radical) initiates polymerization by free radical and ionic polymerization of the respective ion. In principle, therefore, radiation polymerization could proceed via free radical polymerization, anionic polymerization, and cationic polymerization of the monomer that created the initiator. However, which polymerization dominates in an actual polymerization depends on the reactivity of double bond and the concentration of impurity because ionic polymerization, particularly cationic polymerization, is extremely sensitive to the trace amount of water and other impurities. 

With the objective of oxidizing the fullerene core, radiolysis of any chlorinated hydrocarbon solvent provides the means of forming strongly oxidizing radical species [71]. For example, the radiation-induced ionization of dichloroethane (DCE) yields the short-lived and highly reactive solvent radical cation. In general, the electron affinity of [DCE] + is sufficient to initiate one-electron oxidation of the fullerene moiety (Eq. 6) [72-76]. 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

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