Radical-chain reactions, inhibition polymerization

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 aryl radicals produced from the photolysis of the diaryliodonium salt abstract a hydrogen atom from THF producing the aryl hydrocarbon and the THF radical. The THF radical is further oxidized by the diaryliodonium salt, resulting in the formation of a stabilized THF cation which may initiate cationic polymerization. Simultaneously, the aryl radical which is the principal chain carrier is regenerated. Because such free radical chain reactions are inhibited by oxygen, this process is most efficiently carried out in the absence of air. 

Anything that breaks the chain by converting the active chain-carrying species into an ordinary uncreactive molecule inhibits the reaction, and since the chains are often long an inhibitor may be effective in very small traces. The chain-starting catalysts may also be effective in very small amounts provided that no inhibitor is also present. The fact that a reaction is a chain reaction sensitive to small amounts of catalysts and inhibitors does not necesssarily mean that it is a radical chain, but the nature of the substances effective as catalysts or inhibitors will usually differentiate a radical chain from an ionic one. An example of an ionic chain reaction is the polymerization of an olefin-Lewis acid system when water is added as a co-catalyst. Water is so very effective that it is suspected that the polymerization observed in some cases with the driest obtainable reaction mixtures is due to the presence of minute and unavoidable amounts of water. 

Inhibition [77]. An inhibitor is itself being consumed as it traps free radicals. To be effective, it must therefore be present in an excess over the initiator. In practice, this limits effective inhibition to chain reactions apt to be set off by small amounts of an initiator other than the bulk reactant. The most common application of inhibition is for protection of sensitive chemicals whose decomposition or polymerization by chain mechanisms may easily be triggered. 

The cw-isomers were formed in excess. Styrene inhibited the formation of the carbenoid and polymerized under the reaction conditions. This indicates that a free radical chain mechanism is involved in the formation of the carbenoid reagent. ... 

Although the presence of water is generally not an issue in free-radical chain polymerization (indeed water may be a suitable medium for polymerization as in Protocols 5-7) unlike, for example, chain-growth polymerization initiated by anionic species, it is always advisable to use solvents of the highest purity and this will generally include some element of predrying. In general, solvents should be distilled, particularly as a number of suitable solvents for polymerization reactions contain stabilizers which usually serve to mop up free radicals and therefore inhibit the polymerization... 

That polymerization is observed in this system even after irradiation has ceased is evidence of the occurrence of a chain reaction mechanism. This postirradia-tive polymerization leads to the gradual enlargement of features in the imaged resist, which degrades the resolution. The use of radical scavengers such as l,l-diphenyl-2-picrylhydrazyl (DPPH) has been demonstrated as an effective way of inhibiting this reaction. ... 

Oxygen inhibits free radical polymerization through two pathways. First, an oxygen molecule can quench the active triplet state of the photoinitiator and be excited to singlet state itself The seeond path is scavenging of the photoinitiator and polymer radicals through oxidation with the yield of peroxy radicals. The overall chain reactions of photopolymerization with the presence of O2 is schematically shown as follows... 

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