Phenoxyl radicals polymerization processes

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

The mechanism of this polymerization reaction has been investigated thoroughly [15, 16]. However, even after more than 50 years since its commercial development, the mechanism of PPO formation is not fully unraveled [17,18]. In fact, the nature of the initial active species is still a matter of debate. Indeed, while some researchers consider that the C-O coupling reaction occurs via phenoxyl radicals (Figure 7.2b) [21], others believe that phenoxonium cations are involved in the polymerization process (Figure 7.2a) [22]. 

The use of phenols such as ferf-butylcatechol as free-radical scavengers is based on the fact that phenolic hydrogens are readily abstracted by radicals, producing relatively stable phenoxyl radicals that interrupt chain processes of oxidation and polymerization. Alcohols such as cyclohexanol, on the other hand, do not function as radical scavengers. Explain why the two types of molecules differ in their abilities to donate a hydrogen atom to a radical, R. ... 

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