Quantum yield of dehydrochlorination

It has been found that the quantum yield of dehydrochlorination of solid poly(vinyl chloride) films at 254-400 nm, 0hci = 0.011 in N2, and 0hci = 0.015 in pure oxygen [552, 559, 561, 562] and at 514 nm (ion argon laser), / Hci = 0.005 in N2 and (/)hci = 0.009 in O2 [560], remains initially constant. This means that incident photons, which are increasingly absorbed by the polyenes formed, initiate the dehydrochlorination of poly(vinyl chloride) with an efficiency that remains constant throughout the photolysis. 

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]. 

Determination of the quantum yield of the different processes involved in the photo-oxidation of poly(vinyl chloride) indicates that for each scission of the polymer backbone 11 molecules of HCl are evolved, whilst 3 carbonyl groups, 2 hydroperoxides and 0.4 intermolecular crosslinks appear on the polymer chain [552, 559]. 

The low (j)iici value of 1.1 molecules of HCl evolved per 100 photons absorbed indicates tht most of the excitation energy is dissipated by other routes radiationless internal conversion, fluorescence, cis-trans isomerization and cyclization. 

Quantum efficiency of the dehydrochlorination was found to be 10 times larger when poly(vinyl chloride) was photolysed in solution [169] than as a film [561]. 

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The quantum yield of dehydrochlorination under an inert atmosphere increases linearly with increasing isotatic content [403]. 

In the process, the chlorine radicals abstract hydrogens and form HCl. The quantum yield of hydrogen chloride from this reaction is hci = 0.11 [556]. This indicates that only 1 in every 100 photons that are absorbed causes the dehydrochlorination and is followed by formation of polyenes. It led to the conclusion that a photochemical process must take place between the polyene sequences and HCl [556, 557]. 

The dehydrochlorination chain reaction is 10 times more eflBcient in chlorinated poly(vinyl chloride) than in non-chlorinated polymer, with a quantum yield of 0.12mol/photon in the absence of oxygen [564], There may be two possible explanations for this ... 

These quantum yield values appear to be much higher than unity and therefore demonstrate that carbonization occurs by a chain reaction process. The mechanism of the laser-induced dehydrochlorination of photodegraded C-PVC can be schematically represented by the follo-... 

The stationary rate of carbonyl buildup decreases and the dehydrochlorination quantum yield increases as the content of isotactic triads increases [406, 407]. 

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