Generation of Radicals by Hydrogen Abstraction

The best known compound in this class is benzophenone. In its triplet excited state it readily abstracts a hydrogen atom from a suitable donor  

The ketyl radical does not initiate polymerization in an efficient way but the R radical generally does. Frequently used hydrogen donors are tertiary amines. They have been reported to be more efficient donors than alcohols, ethers or thiols With amines, the a-hydrogen transfer may occur either directly or via the intermediate formation of an exciplex, electron transfer and finally proton transfer  

An additional advantage of the use of ketone/amine combinations is the fact that the amine radicals are not only very reactive towards oxygen but the formed 

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