Energy chain branching

Chain reactions can lead to thermal explosions when the energy liberated by the reaction cannot be transferred to the surroundings at a sufficiently fast rate. An explosion may also occur when chain branching processes cause a rapid increase in the number of chains being propagated. This section treats the branched chain reactions that can lead to nonthermal explosions and the physical phenomena that are responsible for both branched chain and thermal explosions. 

If the system is one of both chain branching and propagating steps, a could equal 1.01, which would indicate that one out of a hundred reactions in the system is chain branching. Moreover, hidden in this assumption is the effect of the ordinary activation energy in that not all collisions cause reaction. Nevertheless, this point does not invalidate the effect of a small amount... 

The recombination [reaction (4.64)] increases with decreasing temperature and increasing concentration of the third body M. Thus, the more diluent added, the faster this reaction is compared to the chain branching step [reaction (4.62)]. This aspect is also reflected in the overall activation energy found for rich systems compared to lean systems. Rich systems have a much higher overall activation energy and therefore a greater temperature sensitivity. 

Several mechanisms were proposed to interpret bond shift isomerization, each associated with some unique feature of the reacting alkane or the metal. Palladium, for example, is unreactive in the isomerization of neopentane, whereas neopentane readily undergoes isomerization on platinum and iridium. Kinetic studies also revealed that the activation energy for chain branching and the reverse process is higher than that of methyl shift and isomerization of neopentane. 

Promotion occurs because these reactions provide an extra mode of chain propagation and chain branching. Reactions 1, 2, and 3 would constitute promoting steps when their activation energies are lower than those of the corresponding reactions ... 

Figure 5.19 Photoluminescence spectrum of (PhSiMe) (1) and (n-pentylSiPh) at 20 K. The broad low-energy peak for (PhSiMe)n is due to chain branching. Reprinted with permission from reference 57. Copyright 2000 Kluwer Academic Publishers.

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