IV. Summary

Free-radical processes can be divided into chain and nonchain reac­tions. Chain reactions consist of ­initi­ation, propagation, and termination phases. A similar set of reac­tions (i.e., radical for­mation, transformation of one radical into another, and radical dis­appear­ance) also occurs in nonchain processes. The difference in these two types of reaction is that in a chain reaction the transformation of one radical into another also creates the radical needed to start the transformation process anew, but for nonchain reactions each radical formed causes only one “trip” through the transformation cycle. The most widely used initiator in chain reaction is 2,2'-azo­bis(iso­butyro­nitrile), a compound that provides the continuous supply of radicals needed to sustain a typical reaction; that is, a reaction that takes place over a period of several hours at 80-110 ^oC. Peroxides also are thermal initiators, but they are less commonly used because they produce reactive radicals that can cause undesired side reactions. Triethyl­boron–oxygen, ultrasound, and light all initiate radical reactions and have the added advantage that they can be used in reactions that are conducted at or below room temperature.

At the core of a chain reaction is the propagation phase, the part of the reaction where reactant molecules are converted into products. Each pro­pa­ga­tion sequence consists of a group of ele­mentary reactions. Successful propa­ga­tion depends upon the ability of each participating radical to react selec­tively with only one type of mole­cule present in the reaction mixture.

The final phase in a chain reaction is termination. Chain reactions are terminated by any pro­cess, such as radical combination, that removes a par­tic­ipating radical from the propagation sequence. Reaction efficiency is a measure of how long a typical chain reaction con­tin­ues before termination takes place. An efficient reaction is generally regarded as one with a chain length greater than 100.