Preparation photodegradation

A few reports of pyrazine monomers have appeared in the literature (70Millloo). The most extensive study of pyrazine monomers involves their use to prepare photodegradable polymers <76MI11104). 2-Vinylpyrazine (145) copolymers and poly(2,5-distyrylpyrazine)... 

Irradiation of a solid film prepared from the polymer 11 in air afforded photodegradation products which are soluble in 2-ethoxy-ethanol. IR spectra of the products show strong absorption bands attributed to Si-OH and Si-O-Si stretching frequencies. In contrast to the products from 10, these products show absorption due to the Si-H bond. This result indicates that some of the silenes would be formed in this system. The intense absorption at 254 nm in the UV spectrum again disappeared after UV-irradiation. 

Quantum yields and lifetimes of emission (fluorescence) as well as other principal rates of deactivation have been measured on 2-hydroxy benzophenone and 2-hydroxyphenyl benzotriazole derivatives. Polymerizable UV screening agents have been prepared and copolymerized with acrylics in order to obtain transparent films containing nonfugitive UV screening agents. Preliminary results of studies of photodegradation on these copolymers are also reported here. 

The antidepressant protriptyline (116) causes skin photosensitization in man. Jones and Sharpies irradiated an aqueous solution of the hydrochloride with a medium-pressure mercury lamp for 16 h and separated the products by preparative TLC. First formed was the epoxide (117) which photohydrated to the diol (118). Also isolated was the enol (119) [84], Earlier, Gasparro and Kochevar had shown that only the hydrochloride was photodegraded under nitrogen in water or ethanol. Three products were isolated and all lysed erythrocytes, but the structure of only one was suggested. This was a cyclobutyl dimer as shown by its mass spectrum and its photolysis back to protriptyline by light of 254 nm. Presumably, a [2 + 2] cycloaddition of the olefine bonds had occurred [85]. 

Degradation of DDT, dieldrin and lindane by the flavoprotein preparation was almost more efficient in the absence than in the presence of FMN (e.g. Figure 3). On the contrary, photodegradation of mexacarbate was greatly enhanced by FMN and other flavin cofactors. It is well known that flavin cofactors, such as FMN, are active photosensitizers. Hence it is possible that the mechanisms or pathways involved for the photodegradation of DDT, dieldrin and lindane and that for mexacarbate are different... 

Nemutlu, E., Ozaltin, N., and Altinoz, S. (2004). Determination of rofecoxib, in the presence of its photodegradation product, in pharmaceutical preparations by micellar electrokinetic capillary chromatography. Anal. Bioanal. Chem. 378, 504—509. 

The ionisation state of molecules in the solution state appears to be an important variable in photodegradation mechanisms. A recent pubhcation on riboflavin oral liquid preparations shows that the formulation is most photostable at pHs between 5 and 6, where the non-ionised form predominates [78]. The rate of photolysis increase 80-fold at pH 10.0, owing to increased redox potential. Conversely, at pH 3.0, the increased photolysis is associated with the excited singlet state, in addition to the triplet state. 

Isocyanates that are produced fi om aliphatic amines are utilized in a limited range of polyurethane products, mainly in weatherable coatings and specialty applications where the yellowing and photodegradation of the aromatic polyurethanes are undesirable [5]. The aliphatic isocyanates are not used more widely in the industry due to the remarkably slow reaction kinetics of aliphatic isocyanates compared to their aromatic counterparts [6]. Due to the slow reactivity of aliphatic isocyanates, it is not practical to use them in the preparation of flexible or rigid foams, which are the main commercial applications for polyurethane chemistry. 

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