Dehydrochlorination photo

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

In PVC, alkaline-earth-metal stearates prevent dehydrochlorination quite effectively by reducing the energy of activation of photo-oxidation. ... 

One of the most interesting recent topics in ruthenium chemistry is undoubted ) C—H bond activation in which the generation of coordinatively unsaturated specie> may play an important role. These species are usually produced by thermal or photo-mediated reductive elimination of dihydrogen, alkanes, alkenes or arenes. Recently, dehydrochlorination from RuHCI(CO) (P BuTMe) is reported to give a 7C-allyl complex via C—H activation of propylene (eq (42)) [144]. 

The structure —CHC1—CH2—CO—CH2 — was found by Kwei [99] in polyvinylchloride after photo-oxidation. Such j3 chloroketones decompose by the Norrish type I mechanism without loss of chlorine atoms. Hydrogen chloride is obtained only when polyvinylchloride is photo-oxidized above 30°C [98]. It seems that zipper dehydrochlorination plays little role in the reaction occurring on exposure to ultraviolet light at temperatures below 150°C in the presence of air [97], and that hydrogen chloride is mainly a product of thermal decomposition rather than photolysis [98], The following mechanism can be proposed which takes into account the experimental results namely, that chain scission and crosslinking occur simultaneously on irradiation at 253.7 nm [100] and that carbon dioxide is evolved, while an absorption band at 1775 cm-1 (ascribed to peracids) is detected in the infrared spectrum [98]. 

In the presence of FeClj, photo-oxidation of trisubstituted cyclic olefins in pyridine-benzene has been reported to give gem-dichloroketones from which methyl ketones possessing a terminal triple bond can be obtained by dehydrochlorination. For example, under these conditions 1-methylcyclohexene has been converted into (43), an important intermediate in the preparation of brevi-comin (44). These reactions have been interpreted in terms of a long-range... 

Other studies of interest on PVC photo-oxidation include the influence of light intensity,chlorination, solvent diffusion,and inhibition of dehydrochlorination at the surface layers.The chlorination process destabilized the PVC. 

Poly(vinyl halides) (PVC etc.) and PoIy(vinyl alcohol) (PVA).—A series of papers has appeared concerned with the photo-dehydrochlorination, discoloration, and sensitized photolysis of model compounds of PVC,819 and its copolymers.220 The photolysis of poly(vinyl bromide)221 and a trifluorochloroethylene-vinylidene fluoride copolymer228 has been described. The photo-oxidative destruction of PVA has been reported in a series of papers.223... 

The discoloration is due to a dehydrochlorination process restilting in the formation of long conjugated polyene sequences in the polymer chain [Eq. (7-10)]. Polyenes can give rise to photo-cross-linking reactions. 

Blending methyl methacrylate-butadiene-styrene copolymer with poly(vinyl chloride) for instance was shown to decelerate the dehydrochlorination (leading to discoloration). The gel content, surface energy, and the spectroscopic characteristics of the blend was altered by the presence of the seccHid polymer [158]. In ethylene-propylene-diene rubber EPDM where the third monomer is ethylene-2-norbomene (NB), the photo-oxidation rate as measured by the accumulation of typical products such as hydroperoxides, varied linearly with the NB content [159]. The same held true for peroxide-crosslinked compounds of the same EPDM except that the linear relationship was found between the relative carbonyl absorbance on photoxidation and the amoiuit of peroxide used to crosslink the material... 

The zip dehydrochlorination basically proceeds in a cage reaction, which explains why long polyenes are mostly found in poly(vinyl chloride) photo-lyzed in the solid state. In solution, where the chlorine radical (Cl ) can more easily diffuse out of the cage, growth of polyene sequences is less favoured and discoloration is much less pronounced [169]. 

The constant quantum yield measured at room temperature implies that the degradation quantum efficiency is not dependent on the amount of these unsaturations in the commercial sample [407]. When the double bonds initially present in the poly(vinyl chloride) sample were removed by catalytic hydrogenation, before they induced photo-oxidation, the quantum yield of dehydrochlorination remained the same, (/>hci = 0.015, whereas the rates of degradation and discoloration were sharply reduced [552]. 

Commercially used plasticizers such as di n-octyl adipate, di -octyl sebacate, di(2-ethylhexyl)phthalate and butyl benzyl phthalate can contribute to the photo-dehydrochlorination of poly(vinyl chloride) and this is attributed to a free radical mechanism, whereby a radical is first formed in the plasticizer and then transferred to the polymer [255, 297, 298, 688, 1636, 2270]. In the case ofdi(2-ethylhexyl)phthalate and tri(2-ethylhexyl)trimellitate the observed effects are even more drastic under conditions of solar irradiation [554]. This effect has been attributed to the fact that the trimellitate exhibits a much greater absorption in the wavelength range of the solar spectrum and hence is more likely to form free radicals. 

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