Quantum yields photodegradation polymers

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. 

Photolytic. Dalapon (free acid) is subject to photodegradation. When an aqueous solution (0.25 M) was irradiated with UV light at 253.7 nm at 49 °C, 70% degraded in 7 h. Pyruvic acid is formed which is subsequently decarboxylated to acetaldehyde, carbon dioxide, and small quantities of 1,1-dichloroethane (2-4%) and a water-insoluble polymer (Kenaga, 1974). The photolysis of an aqueous solution of dalapon (free acid) by UV light (X = 2537 A) yielded chloride ions, carbon dioxide, carbon monoxide, and methyl chloride at quantum yields of 0.29, 0.10, 0.02, and 0.02, respectively (Baxter and Johnston, 1968). 

Photolysis of polyphenylvinylketone at 365 nm in benzene solution or in the solid state results in a decrease in the molecular weight of the polymer [6], The quantum yield of this process (irradiation at 313 nm) has been estimated to be 0.25 [Ref. 60] or 0.22 [Ref. 61] in solution. Since no carbon monoxide resulting from the Norrish type I split could be detected, the Norrish type II scission is considered to be responsible for the photodegradation of polyphenylvinylketone [6] namely,... 

Although polymethylmethacrylate is remarkably stable to sunlight, the vacuum photodegradation of this polymer has received considerable attention. Films of polymethylmethacrylate exposed to ultraviolet radiation of 253.7 nm undergo a rapid decrease in molecular weight. The quantum yield of the main chain scission process has been estimated by several authors. Various values have been proposed 1.2 x 10 2 [Ref. 69],... 

As mentioned above, the M-M-bonded polymers are of interest as photochemically labile materials (Scheme 20). The quantum yields for the photodegradation of these polymers and model complexes decreased as the chain lengths increased. This effect has been observed in other polymer systems, and several explanations are possible. One invokes the theory that radiationless decay is faster in molecules with more vibrational modes and would thus make the quantum yield for the formation of requisite radical cage pair lower as the chains get longer. ... 

The expression in Eq. (5), generally used in photobiology, implies the absorbance spectrum of the substrate to be highly correlated to the quantum yield spectrum. While Martin et al. [18] have suggested that the expression at Eq. (4) is perhap.s more appropriate for polymers containing photostabilizers and impurities, its practical superiority over the simpler form in Eq. (5) has not been demonstrated. It is feasible that in many polymer photodegradation processes the absorption factor merely scales the spectral quantum yield. The activation spectrum is then expressed as [18]... 

---