Benzophenone ionic hydrogenation

The addition of thiols to olefins (thiolene reaction), to form thioethers, is a well-known reaction. The process can occur by either free-radical or ionic mechanisms. The free-radical reaction can be initiated thermally via a peroxide or by UV irradiation with benzophenone. The initiation step involves the formation of a thiyl radical by hydrogen atom abstraction. Both of these species are capable of starting polymer chains (Table 2.30). 

Photopolymerization is traditionally initiated by direct photolysis of a precursor to provide free radicals via bond homolysis. Examples of such initiators include benzoin, and benzoin ethers, disulfides, and azoalkanes or dialkylperoxides. Hydrogen abstraction chemistry, typified by benzophenone photochemistry, is also recognized as extremely useful. However, a number of viable commercial photopolymer imaging systems are based upon ionic (especially cationic) polymerization. These systems will be discussed next. 

The photoinduced hydrogen abstraction by benzophenone and thio-benzophenone from thiophenol in several ionic liquids has been studied by... 

Unlike benzophenone, the 2-hydroxybenzophenones which show similar UV absorption constitute one of the most important classes of UV deactivators (absorbers) presently used. The effectiveness of ortho-hydroxybenzophenones in degrading electromagnetic energy without giving rise to free radicals or to other activation forms is associated with the possible formation, through a six-membered cycle, of an internal hydrogen bond between hydroxyl and carboxyl (109). The presence of this bond would allow energy transfer from the triplet state and hence formation of enolic quinone in the ionic form (III) [Eq. (47)] by emission... 

Recently, benzophenone-based initiators with hydrogen donating amine moieties covalently attached via an alkyl spacer were introduced as photoinitiators for vinyl polymerization [101,126-130] (see 1, Table 10). Although also following the general scheme of lype II initiators, the initiation is a monomolecular reaction, as both reactive sites are at the same molecule. Hydrogen transfer is suspected to be an intramolecular reaction. The ionic derivatives (2 and 3) shown in Table 10 are used for polymerization in the aqueous phase [131-133]. With 4,4 -diphenoxybenzophenone (4 in Table 10) in conjunction with tertiary amines, polymerization rates that are by factor of 8 higher than for benzophenone were obtained [134]. 

Muldoon, M.J., McLean, A.J., Gordon, C.M., and Dunkin, I.R, Hydrogen abstraction from ionic Hquids by benzophenone triplet excited states, /. Chem. Soc., Chem. Commun., 2364, 2001. 

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