Chlorides, photodehydrochlorination

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

Studies with model compounds have demonstrated that photodehydrochlorination is sensitized by jj-cresol triplets via a charge-transfer exciplex intermediate in which the alkyl chloride is the electron acceptor (15). The detailed mechanism suggested for this process (15) is outlined in Equations 11 and 12. 

Wavelengths of 500 and 600 nm caused photobleaching rather than photo-yellowing of the polymer. The chemistry of this phenomenon, also reported by others [29], is not fully understood. Quenching of polyenyl radicals by oxygen [84,85] and possible reaction of polyenes with hydrogen chloride formed during photodehydrochlorination, have been proposed [86,87] as possible mechanisms. 

In poly(vinyl chloride) containing polyene structures photodehydrochlorination is initiated by intramolecular energy transfer from polyene excited singlet states to the allylic C—Cl bond (reaction 3.326) and/or eventually to the allylic C—H bond (reaction 3.327). Triplet states are less likely to be involved because of a very efficient intersystem crosslinking in polyenes [564]. 

Oxygen in general accelerates dehydrochlorination of poly(vinyl chloride), especially in thermal degradation [551, 552, 890]. Activation energies of photodehydrochlorination in air and nitrogen are a(air) = 19.6 kJ mol and (nitrogen) = 32.1 kJ mol respectively. 

Polyene structures formed during the photodehydrochlorination of poly(vinyl chloride) samples are bleached in the dark as well under UV/VIS irradiation. Bleaching in the dark is a much slower process that in the presence of light. Oxygen plays an important role in bleaching processes in the dark and during light irradiation [771, 1674, 1791, 1881, 1882]. 

Fig. 3.43. Absorption spectra of photodehydrochlorinated chlorinated poly(vinyl chloride) (C-PVC) and after UV irradiation (15 min) and after laser irradiation (488 nm) in air for 0.1 s [554]. (Reproduced with permission from [554] published by John Wiley Sons, 1971.)...

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