Mechanism photodegradation

See for example (a) Pospisil, J. and Klemchuk, P. P., Oxidation Inhibition in Organic Materials, Vol II, CRC Press, New York, 1990 (b) Rabek, J. F Polymer Photodegradation, Mechanisms and Experimental Methods, Chapman Hall, London, 1995 (c) Wypych, G., Handbook of Material Weathering, 2nd Edn, ChemTec Publishing, Ontario, Canada, 1995. 

The photodegradation mechanism of 2,4-xylidine to a mineralized form, oxalate was deduced from GC-MS trace detection of reaction intermediates. A summary of the proposed degradation path is shown in Fig. 16.2. The presence of some of these intermediates are supported by GC and HPLC data [2]. 

Makino, M., Kamiya, M., and Matsushita, H. Photodegradation mechanism of 2,3,7,8-tetrachlorodibenzo-jo-dioxin as studied by thePM3-MNDO method, Chemosphere, 24(3) 291-307, 1992. 

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. 

A similar photodegradation mechanism is considered to hold for ferric ni-trilotriacetate complexes at 365 nm in acidic medium [62], The quantum yield as a function of irradiation wavelength and pH is shown in Table 3 and major photoproducts shown below. 

SCHEME 14.1 The general stmcture of an acrylic polymer and the estahhshed photodegradation mechanism via Norrish I a-cleavage of the carhonyl side chain, leading to main-chain polymeric radical a and oxo-acyl radical b. The secondary P-scission rearrangement reaction leading to the propagating radical c is also shown. 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

Rabek, J. F. Polymer Photodegradation Mechanisms and Experimental Methods Chapman and Hall Cambridge, 1995. 

The appearance of many degradation products along with the phenol provided an opportunity to investigate the photodegradation mechanism. The structures of the early eluting species were therefore investigated by liquid chromatography/mass spectrometry (LC/MS) and UV spectroscopy. The UV... 

As reported in Table 15, the kinetic data clearly indicate that the photoinitiation activity of poly(BMOA-co-MtA) is not substantially affected by the content of BMOA co-units along the polymer chain and is of the same order of magnitude as that found for the model compound BMOAc. The absence of a polymer effect in the above photoinitiators has been interpreted [84] in terms of a photodegradation mechanism of the macromolecules involving the ftee radical species anchored to the main chain, even in the presence of acrylic monomers, analogous to what is reported in Scheme 18. Moreover, the induction period of the HDDA/BA photoinduced polymerization increases, on decreasing the content of... 

Photodegradation mechanisms an important topic for the practical use of photochromic compounds in variable-optical-transmission materials... 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

Neugebauer H., Brabec C., Hummelen J. C. and Sariciftci N. S. (2000), Stabihty and photodegradation mechanisms of conjngated polymer/fullerene plastic solar cells . Solar Energy Mat. Solar Cells 61, 35 2. 

Based on our observation, the proposed photodegradation mechanism of DR was shown in Scheme VI, where a monochloro-substituted azo-benzene is photoreduced in surfactant solution via photodechlorination, in which the surfactant acts as solvent to improve the solubility of DR in aqueous phase and as a hydrogen source to induce photoreduction. The photodechlorination is followed by photodecoloration, which is likely initiated by forming partly reduced hydrazobenzene (-N-NH-), then the colorless hydrazobenzene can be further reduced by prolonged irradiation into aniline, or substituted aniline. Similar results of chromophore reduction were also reported by Leaver I.H. (1980). 

Arce, R., Martinez, L., and Danielsen, E. (1993) The photochemistry of adenosine intermediates contributing to its photodegradation mechanism in aqueous solution at 298 K and characterization of the major product, Photochem. PhotobioL, 58, 318-328. 

Neugebauer, H., Brabec, C., Hummelen, J.C., Saricrftci, N.S., 2000. Stability and photodegradation mechanisms of conjugated polymer/fuUerene plastic solar cells. Sol. Energ. Mat Sol. Cells 61,35-42. 

Kato, M. andYoneshige, Y. (1973) Photodegradation Mechanism. Macromol. Chem., 164, 159. 

Reich, L. and Stivala, S.S. (1971) Photodegradation Mechanism. Elements of Polymer Degradation, (McGraw Hill, Inc., New York), 32-35. 

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