Radiation-induced cationic experimental

There is a sequence of experimental techniques ranging from the open beaker on the bench to the extremely rigorous h.v.t. used for studying the kinetics of radiation-induced cationic polymerisations. Each technique in the series has its characteristic level of extraneous materials, and as the level of cleanliness is raised, there is a concurrent loss of flexibility. 

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. 

When the 2-methyltetrahydrofuran and n-butylchloride glasses with about 10 mole-% of styrene in them are irradiated to the dose of 1 X 107 rad, polystyrene is obtained only from the latter glass, though the polymer yield is low (1 2%). This indicates that radiation-induced polymerization occurs in the glass where the cation radicals are formed by the radiation. The experimental results of the polymerization in glass matrices are consistent with the previous report that the radiation-... 

These ideas have been illustrated in a recent study of the co-crystalline complex of l-MethylCytosine 5-FluoroUracil [34], Using model calculations it was shown how the hydrogen bonding network of the crystal is able to sustain a proton shuttle which leads to the selective formation of certain radicals. Calculations were able to predict that the site of reduction would be the cytosine base (yielding the N3 protonated cytosine anion C(N3+H) ), while the uracil base would be the site of oxidation (yielding the N1 deprotonated uracil cation U(N1-H) ). These are indeed the primary radiation induced species observed experimentally [34, 88], The results also nicely agree with the model proposed for radical trapping by Bernhard [11],... 

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. 

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. 

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