S. DILLI AND J. H. GREEN 



the primary process is again the formation of chlorine atoms and the final 

 result is side-chain substitution exclusively. With ionizing radiation it is 

 apparent that ionization and excitation of the toluene molecule is at least 

 as important as attack on the chlorine molecule and ionization effects — 

 charge transfer and electron capture — play an important part. The final 

 products are mainly the result of nuclear sul:)stitution leading again to 

 1,2,3,4,5,6,-hexachlorocyclohexane, as well as side-chain substitution leading 

 to benzyl chloride and benzotrichloride. 



(r) There are conditions in which the required chain reaction can be 

 ' catalysed ' by ionizing radiation at much lower pressures and temperatures 

 than by orthodox catalysts. There is a real cost advantage here. The process 

 which has attracted the greatest attention up to the present is the polymeriza- 

 tion of ethylene to polyethylene. The normal process for the production of 

 polyethylene requires pressures of the order of 1000 atmospheres and 

 temperatures of 300-400°C. There would be a reduction in operating costs, 

 if the reaction were carried out under normal conditions of temperatures and 

 pressure, but raw materials cost the same (about 80 per cent of total costs for 

 the polymerization) and the economic advantage of using ionizing radiation 

 is debatable. If the radiation source cost nothing, as it may in the future 

 disposal of fission-products, there would probably be a slight advantage in 

 using ionizing radiation. 



Perhaps the real advantage in using y radiation for polymerization will 

 be in the ease of control of the final properties of the polymer. For example, 

 the chain length of polyacrylonitrile and polyvinylpyrrolidone can be varied 

 readily by changing the dose rate and monomer concentration. In the latter 

 polymer an additional advantage is that the radiation product has a much 

 narrower range of molecular weights than the catalytic product^^--". 



We have referred already to the formation of polymers in the solid state 

 but it is worth while to add some further comments on this topic. A very 

 interesting possibility is that, because the reaction occurs in an ordered 

 lattice, it may be possible to alter the nature of addition products by changing 

 the order. Such addition products would not normally be formed in the 

 disoidered liquid or gas phase. Some progress has already been made in 

 the formation of controlled graft and block co-polymeis by the effect of 

 y-rays on solid solutions of appropriate monomers. Irradiation at a later 

 stage, i.e.^ after a solid polymer is formed, has been extensively studied and 

 the technicjue is used industrially to modify polymers for special purposes. 

 For example, where a higher heat resistance is required, extensive radiation 

 produced cross-linking gives a polymer which is much less fusible than the 

 parent polymer. 



[d) In circumstances where an orthodox catalyst must be removed at 

 great expense from the final product, the use of y radiation is attractive. 

 It is not easy to find an industrial chemical process which might be quoted 

 in this section, but an illustration of the possibilities is given by the oxidation 

 of toluene to benzoic acid. Radiolytic oxidations offer some promise 

 generally-^. The orthodox process is to use a cobalt naphthenate catalyst 

 which becomes mixed with the product in such a way that a laborious and 

 costly purification procedure is requiied. Radiation catalysis offers advan- 

 tages in such circumstances. 



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