Vinyl ketone polymers, quantum

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. 

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. 

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,... 

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. 

FIGURE 1. Quantum yield of chain scission, 5, due to Norrish Type II reaction in styrene-phenyl vinyl ketone co-polymer as function of temperature. Xex=313 nm. Tg= glass transition temperature. [After Fig. 1, Macromolecules, 230 (1973)]. 

A reviewer of the original manuscript has suggested that a Norrish type II process (y-hydrogen abstraction) may be anticipated, as contrasted to a type I process (a-cleavage), since the photochemistry of esters is very similar to that of aliphatic ketones. It has been shown (14) that, at reaction temperatures above Tg (as in this present study), the major photochemical reactions of vinyl ketone copolymers are derived from the type II process, with quantum yields in the polymer similar to those... 

Another promising method for the preparation of oligomers of MMA uses the photolysis of high polymers containing a limited number of units bearing carbonyl groups. These units can be introduced by using methyl vinyl ketone as a comonomer. It is well established that absorption of a UV quantum by... 

The Norrish type II reaction of vinyl ketone copolymers also leads to scission of the backbone of polymer chain. Typical temperature dependence of the quantum yield for chain scission due to the Norrish type II reaction is shown in Fig. 27 for the copolymer of styrene and phenyl vinyl ketone There is a relatively small ittcrease in the quantum yield below T owing to the presence of local mode rdaxation in the main chain, but a drastic increase tak place in the region of glass transition temperature. Above T the quantum yield for the type II process is identical to that observed in solution. 

For copolymers of styrene containing minor amounts of phenyl vinyl ketone, photolysed in the solid phase, the quantum yields are low at temperatures below 100°C they rise abruptly to the value obtained in solution at temperatures above 100°C [510]. This temperature corresponds to the glass transition temperature (Tg) for the copolymer. It is characteristic of the polymer structure, and represents the temperature at which there is sufficient free volume in the polymer so that extensive motion may occur. For amorphous polymers, the macromolecules motion is sufficient to facilitate the formation of the cyclic intermediate required for the Type II process to occur, and for the separation of radical species by the Type I process. Above Tg, molecular mobility is sufficiently great that the rate of formation of the... 

The quantum yields of the Norrish t5q>e I and type II reactions have been measured for a variety of aliphatic ketones (6, 87) the type I process seems to be far less important than type II. As already seen (Section II.2.2.2.), also in the photooxidation of polymer systems containing carbonyl groups the Norrish type II process is considered the main cause of chain scission (82). This assumption is further supported by several photolysis studies (80, 83, 84) of polyethylene, where the IR investigations indicate the formation of vinyl groups in the oxidized polymers. 

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