1090 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



The peroxidic radicals are also reactive entities, but their afl&nity is for 

 hydrogen atoms and they tend to abstract hydrogen from some other 

 molecule of substrate, thus: 



ROO- + RH -> ROOH + R- 



(These equations were written by Backstrom for the oxidation of benzalde- 

 hyde'° and have been adopted by many others.)^^- ^' ^^ The latter reaction 

 results from molecular collision with formation of intermediate additive 

 complexes which decompose into the indicated products and in general 

 many ineffective colHsions will occur before reaction takes place. The 

 molecule of substrate which loses hydrogen in this way is now a free radical 

 and it repeats the process, reacting first with oxygen and then with another 

 molecule of substrate. This linear chain reaction continues until two radicals 

 unite by coUision with each other, thus terminating two chains. The word 

 hnear is itaUcized in the previous sentence to emphasize that this part of 

 the reaction is not in itself autocatalytic. The autocatalytic nature of the 

 oxidation stems from the fact that the product of the reaction as outlined 

 is a hydroperoxide, ROOH. Such compounds are relatively unstable and 

 slowly decompose into free radicals which -initiate new chains. This might 

 go as follows: 



ROOH -^ RO- + OH 

 RO- + RH -^ ROH + R- and OH + RH ^ HOH + R- 



Thus, though the original rate of generation of free radicals from cracking 

 might have been very low, the combined rate increases quite rapidly since 

 each molecule of peroxide produced in the chain reaction becomes a po- 

 tential source of new radicals. Eventually the rate reaches what appears to 

 be a steady state and finally levels off. A typical oxygen absorption curve 

 for a liquid hydrocarbon is shown in Fig. 11. The region of fast reaction has 

 received attention from those interested in the oxidation of small molecules 

 but it is unimportant to people interested in polymers because it has been 

 shown by various workers that only shght oxidation is required to destroy 

 the useful properties of a polymer.^^ By the time oxidation has proceeded 

 far enough to be getting into a rapid rate it has already resulted in enough 

 chain scissions to have lowered the molecular weight below useful levels. (A 

 simple calculation will illustrate this point. Suppose a polymer molecule 

 whose molecular weight is 32,000 reacts with one molecule of oxygen (mol. 

 wt. 32) and a chain scission results. The molecular weight of the polymer 

 molecule will have been halved by reaction with .1% of its weight of oxygen. 

 Not every reaction with oxygen results in chain scission of course;^ but, 

 even so, the amount of oxygen required to ruin the polymer is very small.) 



