Aryliodonium cation-radicals

This Bronsted acid, which is generated by interaction of the aryliodonium cation radical with the solvent or monomer, is the true initiator of cationic polymerization of the vinyl ether monomers. 

In the first step of this mechanism, light is absorbed producing an electronically excited diaryliodoniiun salt. Rapid decay of the photoexcited species then occurs with resultant cleavage of a carbon-iodine bond giving an aryliodonium cation-radical and an aryl radical. 

Figure 8, Chain reaction photoinitiation. The efficiency of photoinitiation can be increased by chain reactions. Hydrogen abstraction by triplet excited benzophenone forms a THF free radical. Subsequent oxidation by the aryliodonium salt produces the THF cation capable of initiating polymerization, and a phenyl radical. Hydrogen abstraction by the phenyl radical produces the THF free radical completing the...

Ledwith and coworkers have studied the cationic polymerization of THF initiated by onium salts in combination with free radical proginators.(20-23) When 2-2-dimethoxy-2-phenylacetophenone (IRGACURE 651) was irradiated in THF, the experiment containing an aryliodonium salt had a seven fold greater polymerization rate than a comparable experiment containing an arylsulfonium salt (21). 

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