Radiation-induced cationic propagation

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 behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

The very straightforward results concerning the mechanism of propagation and cyclization in the polymerization of cyclosiloxanes were obtained by studying the radiation-induced cationic polymerization of 6-, 8-, and 10-membered cyclic siloxanes (D3, D4, Ds) [252,253]. 

The radiation-induced cationic polymerization in the presence of onium salts has attained practical importance for the EB curing of systems containing epoxides or vinyl ethers [22,23]. The chemical structures of typical compounds were presented in Table 3.23. THE does not play a role in this context, because of its very low propagation rate constant (kp 4 x 10 lmol s ). A reaction mechanism for the polymerization of vinyl ethers in the presence of an iodonium salt, as proposed by Crivello [22], is shown in Scheme 5.8. 

We emphasise that the strongest single argument against the view that the pseudocationic reactions are propagated by ions is the nil-effect of water on the rate. The strongly inhibiting effect of water on radiation-induced true cationic polymerisations is too well known to require further elaboration here. 

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... 

Vinylethers are known to polymerize in a cationic mechanism in the conventional homopolymerization system (35). Williams et al. (36) and Naruse et al. (37) studied the radiation-induced polymerization of vinylethers and found that they polymerize in the cationic mechanism by ionizing radiations, too. The propagation rate constant of the polymerization of isobutylvinylether in bulk at 30° C was estimated to be 3 x 105 M l sec 1, much higher than that of the polymerization by... 

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. 

One final piece of experimental evidence is also of relevance, i.e. tl data reported on the radiation induced polymerization of 1,2 cyclohexer oxide [97]. Under very dry conditions the mechanism appears to be a fr( cationic one and apparently the ratio of the rate coefficient for termi ation to that for propagation, kt/k, is 2.4. If termination is assum( to be diffusion controlled, then kp would be of the same ordi (10 —10 ° 1 mole sec ). The authors have pointed out that this merely a rough estimate and represents an upper limit. However, even this data is in error by 3 or 4 orders of magnitude, such a measure ( reactivity is considerably higher than any reported from chemical i itiation of similar monomers. 

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 ... 

Polymerizations induced by y radiation proceed by both cationic and free-radical mechanisms. Predominance of the cationic process can be assured by exhaustive purification of monomers and solvents or by employing high radiation dose rates (from electron beam sources). The rate of cationic polymerization has been shown to be strongly influenced by the dielectric constant of the medium, giving rise to discrepancies between radiation-induced bulk polymerizations and chemically initiated solution polymerizations. These discrepancies have been attributed to specific solvation of propagating ions by monomer and polymer. 

The equilibrium between ions and ion pairs is not maintained in all polymerizing systems. For example, the cationic polymerization induced by ionizing radiation produces the positive and negative ions, the latter initiating a free carbonium ion propagation which is terminated by their collision with negative ions. Such a collision destroys the free ions, and hence their stationary, but not the equilibrium, concentration is determined by their rate of formation and destruction. 

Prof. Szwarc also contributed to the understanding of mechanisms of cationic polymerizations. In addition to the previously mentioned study of the cationic polymerization of styrene initiated by trifluoroacetic acid, he developed novel methods of initiation of cationic polymerization, e.g., the initiation of cationic polymerization by transfer of Cl " and N02 ions, and initiation by electron-transfer. In cooperation with deSorgo and David Pepper, he carried out the first stop-flow study of cationic polymerization that demonstrated the formation of the positive polystyryl cation and allowed its spectrum to be recorded. This work was published in J.C.S. Chem. Comm. 419, (1973). He was the first to point out that cationic polymerization induced by ionizing radiation is propagated by free cations [Makromol. Chem. 35a. 123 (I960)]. 

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