Sensitization process photoinitiated reactions

In the continuous process the solution of sensitizer and monomer forms a thin surface layer on the polymer in which the photoinitiated reaction takes place without affecting the bulk phase of the polymer. In the vapor phase process, the grafting reaction starts at the... 

With respect to photoinitiation, generally, it is important to be very careful in one s choice of sensitizers. For example, attempts to initiate the cyclization of homobenzylic ethers failed if 1,4-dicyanobenzene was used as a sensitizer. Rapid regeneration of the starting material by back-electron transfer from the dicyanobenzene anion-radical to the substrate cation-radical was the cause of cyclization inefficiency. To slow this unproductive process, a mixture of A-methylquinolinium hexafluorophosphate (sensitizer), solid sodium acetate (buffer), and tert-butylbenzene (cosensitizer) in 1,2-dichloroethane was employed. This dramatically increased the efficiency of the reaction, providing cyclic product yields of more than 90% in only 20 min (Kumar and Floreancig 2001, Floreancig 2007). 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

Chloro-l-hydroxynaphthalene is converted into the sulphonate 246 on eosin-sensitized irradiation in the presence of sodium sulphite. A study of the chain substitution of the chloro group in 4-chloro-l-hydroxynaphthalene by aqueous sodium sulphite has shown that two mechanisms for the photoinitiation have been identified and two intermediates have been detected a radical anion of 4-chloro-l-naphthoxide and the sulphite radical anion. Thus, an SjjajI mechanism is suggested and is one that involves reaction with the radical anion of sulphite. An example of the S n 1 process between a phenol and the (2-cyanoaryl)azo-f-butylsulphides has been reported. The 1 reactivity of several compounds (Scheme 27) have demonstrated that 247 is a product however, this is also photochemically reactive and is converted into the cyclic ether 248 . 

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