Photooxidative degradation

Structure 20 was proposed for thalibrunine in 1974 (JO). This assignment rested mainly on degradation (photooxidation-Zn/HCl reduction) to 21 and cleavage (Na/NH3) to (S)- V-methylcoclaurine (22). At that time no fragments... 

Spiropyran molecules have also been utilized as reverse wettable surfaces, whereby, a hydrophilic surface may be photomodulated to a hydrophobic surface and vice versa. Lygeraki et al. utilized the polar (mercocyanine, zwitterionic open-form) and nonpolar (spiropyran closed-form) properties of the spiropyran-merocyanine photo-chromic transformation to photomodulate surface wettability. 6-N02 BIPS (10 wt %) was added to a poly(methy 1 methacrylate) host matrix and patterned to produce reverse wettable surfaces. However, reversibility is limited to 6-8 irradiation cycles because of degradative photooxidation [22], Reverse wettable surfaces are commercially important as they may enhance drug delivery or function as a self-cleaning surface. 

PROBABLE FATE photolysis, direct photolysis is improbable, indirect photolysis is too slow to be important, aqueous photolytic half-life 100 yrs oxidation could occur, but could probably not compete with degradation, photooxidation half-life in air 2.5 days hydrolysis too slow to be important, half-life is greater than 100 days volatilization not expected to be a likely transport process, will volatilize under windy conditions or from shallow waters sorption sorption onto particulates and complexation with organic substances are dominant transport processes, expected to adsorb if released to soil biological processes bioaccumulated and metabo-... 

These results suggest the possibihty of using degraded (photooxidized) polyethylene, from recycling of Qms for greenhouses, as a functionalized polymer to obtain compatibihzed nylon/polyethylene blends. 

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

Finishing. AH acetal resins contain various stabilizers introduced by the suppHer in a finishing extmsion (compounding) step. The particular stabilizers used and the exact method of their incorporation are generally not revealed. Thermal oxidative and photooxidative stabilizers have already been mentioned. These must be carefully chosen and tested so that they do not aggravate more degradation (eg, by acidolysis) than they mitigate. 

Cleavage of Carbon—Carbon Bonds. Under appropriate conditions, the propanoid side chain in lignin maybe mptured to form three-, two-, or one-carbon fragments. This carbon—carbon fragmentation occurs in a variety of laboratory treatments and technical processes such as in bleaching of chemical pulps with CI2, CIO2, and O2, in microbial degradation (15), and in photooxidation (16). 

Thermal, Thermooxidative, and Photooxidative Degradation. LLDPE is relatively stable to heat. Thermal degradation starts at temperatures above 250°C and results in a gradual decrease of molecular weight and the formation of double bonds in polymer chains. At temperatures above 450°C, LLDPE is pyrolyzed with the formation of isoalkanes and olefins. 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

Fig. 6. The initial degradation pathway for thermooxidation and photooxidation. The free radical X is generated by the effect of heat or light on impurities,...

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Similarly, photooxidation of dihydrocoralyne (108) in hot methanol at pH 8, subsequent addition of sodium methoxide and additional irradiation yielded 6,7-dimethoxyisoquinolone and 3-methyl-3,5,6-trimetho-xyphthalide via the betainic intermediate 109 (77H45) (Scheme 39). It was demonstrated earlier that dihydrocoralyne is oxidized to this betaine in quantitative yields under physiological conditions (76H153). The autoox-idative degradation of the mesomeric betaine was rationalized by the addition of singlet oxygen. 

There has been an emphasis on recovery and recycling of trichloroethylene to reduce emissions of this photoreactive chemical to the atmosphere (CMR 1986 McNeill 1979). Photooxidative destruction has been successfully used in conjunction with air-stripping techniques to volatilize trichloroethylene from water and degrade it to nontoxic products (Bhowmick and Semmens 1994). If possible, recycling should be used instead of disposal. 

Figure 5 shows the plots of the two major photooxidative processes versus photolysis time and temperature. Table IV summarizes the two major photooxidative degradation rate constants and activation energies. 

This analytical methodology deduces the four photooxidative processes. The data on the total CO2 evolved from the samples were measured by gas chromatography and the isotopic C02 ( 6C02 and 4°C02) data by GC/mass spectroscopy. The rate constants of the two major photooxidative degradation processes at 25°C were deduced from h 18o2 per -(C H NO)- (i.e., to produce C02 product, t = 42 hr) at 196°C and the other from 02 per -(C H NO)- (i.e., to 2produce C02 product, t = 8 min) at 196°C. The rate constants of the former process was estimated as 1.10 x 10- 1 mole sec-- - and the latter as 1.03 x lO-- - 1 mole-- - sec-- -. The activation energies of these two processes were deduced as 10.8 kcal/mole for the former and 15.7 kcal/mole for the latter. 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

Ketone photolysis in an inert atmosphere has been widely studied--ft 5-Q. Apart from polymer photooxidation studies, however, little work has been done on their degradative irradiation in an oxidizing medium51 53. 

From the foregoing degradation scheme for DiPK in the presence of nitroxide, as well as from what we know about DBK photooxidation, it would be expected that the two radicals a and b primarily formed would be captured by oxygen and then give rise to the isobutyryl and isopropyl peroxy radicals c and d. Theoretically, these could interact to form isobutyric acid and acetone (reaction (8)) ... 

Pigment-Sensitized Dye Photooxidation. Dye degradation sensitized by titanium dioxide is a recognized phenomenon (16). 

On the other hand, carbonyl quenching by polyenyl radicals in PVC has been invoked in order to account for autoinhibition during photooxidative degradation (52). Some workers have mentioned the possibility of singlet oxygen formation in PVC via the quenching of excited polyenes (25,29) or cyclohexadienes (53). 

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