Chain reaction, free-radically

As with other chain reactions, free radical polymerization is a rapid reaction which consists of the characteristic steps of initiation, propagation, and termination. Free radical initiators are produced by the homolytic cleavage of covalent bonds as well as numerous radiation-associated methods. 

Fig. 56. Directed graph of a rooted tree from a linear chain polymerized by a chain reaction (free radical polymerization). I denotes the initiator, E the terminated chain end106)...

CHAIN REACTION (Free Radical). Sec Free Radical. 

Most transformations studied so far in this field are considered to be free radical reactions, usually chain reactions. Free radical alkylation reactions have been reviewed recently, (40, 70, 78) and therefore will not be treated here. We shall discuss the free radical aspects as far as they are related to the problems with which we are concerned. 

Now it s time to learn more about the chemistry of chain-growth reactions. To begin, let s consider the most prevalent type of chain reaction, free radical polymerization. A free radical is defined as a species having one unpaired electron. For example, the methyl free radical would look like this ... 

Free-Radical Chain Reactions. Free-radical chain reactions are sensitive to two types of catalysis a catalyst may increase the rate of initiation of chains by introducing an additional initiation pathway or may lead to new chain propagation steps. For example a chain reaction involving S(IV) and 02 can be initiated by a number of alternative pathways ... 

Oxidation. Atmospheric oxygen can oxidize polymer chains through the well known peroxide and hydroperoxide chain reaction. Free radicals have to be initiated to start this chain, and these are often formed by UV light. In practice it is usually necessary to incorporate some form of UV absorber and antioxidant in a polymeric material for service out of doors. 

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