Ketone polymers quantum yield

The quantum yields of the Norrish Type II process depend on the chain length and structure of the ketone carbonyl group (Table 3.7) [859]. For a methyl or phenyl ketone, the quantum yield remains almost independent of chain length, regardless of whether it is in a polymer or a small molecule whereas, if it is in the centre of a chain, the quantum yield decreases with chain length. A side chain ketone shows a considerably higher efficiency, which is more or less independent of molecular weight. 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

A similar variation in the quantum yield of the Norrish type I process is illustrated in Figure 3 for solid copolymers of ethylene containing three different ketone structures. The ketone groups in the backbone of the polymer chain in ethylene- copolymers show much lower quantum yields than those from the secondary or tertiary structures induced by copolymerization of methyl vinyl ketone and methyl isopropenyl ketone with ethylene. (See Table I, structures I, II and III.) In the latter two cases, the Norrish type I cleavage produces a small radical and a polymer radical, and it seems likely that the small radical has a much greater probability of escaping the cage than when the radicals produced are both polymeric, as in the case of structure I. 

Quantum yields of the Norrish reactions of both types depends on the structure of the ketone polymer in the following ways. 

It is important to point out that the quantum yield for the Norrish II chain scission reaction ( Cs) is highly affected by the mobility of polymer chains. For example, the photolysis CS for a film of the copolymer poly(styrene-co-phenyl vinyl ketone) irradiated at 313 nm in the solid state was shown to be low (0.04—0.09) at temperatures below the copolymer Tg (glass transition temperature) but increased dramatically at,... 

Figure 5. Quantum yield for carbon monoxide evolution (< >co) function of chain length temperature, 120°C solvent, paraffin oil. Reprinted with permission from G. H. Hartley and J. E. Guillet, Photochemistry of Ketone Polymers II. Studies of Model Compounds, Macromolecules, 1, 415 (1968).

Additional information regarding structural effects in model polymer ketones was reported by Plooaid and Guillet (19), who compared the photochemical quantum yields in solution for the methyl esters of ketone diacids with polyesters ... 

When the ketone is in the backbone of the polymer, the excitation of the ketone produces two polymeric radicals which must separate from each other within a short period of time in order to produce chemical products. If, however, the ketone group is in a side chain, as in structure B, then a polymeric radical is formed simultaneously with a small radical fragment. This second fragment can diffuse relatively rapidly through the polymer solid, and the quantum yields are increased by at least one order of magnitude (26-28). 

Further confirmation of the important effect of solid-phase transitions in polymer photochemistry was reported by Dan and Guillet (29). They studied the quantum yields of chain scission, c >s, as a function of temperature in thin solid films of vinyl ketone homo- and copolymers. For polymers where the Norrish type-II mechanism was possibici large increases in n were observed at and above the glass transition T. Figure 8 shows this effect in a styrene copolymer containing about 5% phenyl vinyl ketone (PVK). Below Tg, )s is about 0.07, but at Tg it rises to about 0.3, a value similar to that observed for photolysis in solution at 2S°C. A similar effect was observed with poly (methyl methacrylate-co-methylvinyl ketone) (PMMA-MVK) and PVK homopolymer. 

The quantum yields in the styrene ketone copolymers are highly dependent on the structure of the ketone group included in the polymer. For example, the quantum yield for the type-I process is 0.09 in MVK copolymers where the substituent on the ketone group is a methyl group, but increases to 0.45 where the substituent is tertiary butyl (30) ... 

The polyacrylophenones represent another important category of photosensitive polymers. Substitution on the phenyl ring can alter both the efficiency and mechanistic pathway to reaction products (38). Early work (39) showed that the photochemistry could be related to that of small-molecule analogs. Work at the Slovak Academy of Sciences (40) has extended to a very large number of substituted derivatives. In common with other ketone systems, the quantum yields for chain scision are reduced significantly in the solid phase. Some of this is due to the restrictions on molecular mobility, which reduce the quantum yields of type-II photoprocesses. Another important factor is the extensive triplet migration, which, in the solid phase, leads to quenching by reaction products (41), such as the olefin produced by the type-II photoprocess. 

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