128 ELEMENTARY CHEMICAL PROCESSES 



with a final chain-termination step which involves another radical. Only 

 the simplest type has been indicated. Polymer chains involving two or 

 more kinds of molecules are also possible. In general, such free-radical 

 chain reactions can proceed readily at room temperature and the chains 

 may be 1000 or more molecules long. An interesting example of radia- 

 tion-induced polymerization is afforded by the work of Dainton (26) on 

 alpha-ray-induced polymerization of acrylonitrile in deaerated water. 

 In that work he gave good evidence that the chain starter is free OH. 

 The possibility of such chain-propagated effects of radiation-produced 

 free radicals certainly may be quite important for radiobiological 

 processes. 



A converse phenomenon, perhaps equally important for radiobiology, 

 is suggested by the work of Grassie and Melville (27), who report free- 

 radical-induced depolymerization. A simple illustration of such a 

 phenomenon is afforded by unpublished work of Sworski, Gordon, and 

 Burton which has led to speculation that acetylene production in benzene 

 radiolysis proceeds via the steps 



CeHe + H -^ — CH2=CH2— CHa^CHa— CH2— CH3 



/ 

 CH2=CH2 + — CH2=CH2— CH2— CH3 



/ 

 CH2=CH2 + — CH2— CH3, etc. 



in a sort of "peeling-off" reaction. The first step may involve some con- 

 siderable activation energy, but the interesting feature is that links in a 

 chain of similarly linked units break successively so that a large molecule 

 is degraded, as the result of one primary step, into a number of smaller 

 ones. Such a process may also be operative in radiobiology and could 

 account for sensitivity of biological particles to energetic free radicals. 



DIRECT HITS 



We have seen that radiation may act on biological material indirectly 

 through its action on ambient water. Even when the primary effect of 

 the radiation is on the biological material, free H and resultant IIO2 

 necessarily formed in the ambient layer (via reaction 3 or 19) may have 

 an important chemical effect. However, direct action on such material 

 is not necessarily precluded. Indeed, on desiccated biological material 

 radiation must act directly. Consequently, it is important to note that 

 the remarks concerning the effects of radiation on organic compounds 

 may have considerable significance for radiobiology. Two classes of 

 effects may occur. Either the primary chemical effect occurs at or near 



