Polymer stabilization photolysis

Synthetic polymers stabilize metal colloids as important catalysts for multi-electron reactions. Polynuclear metal complexes are also efficient catalysts for multielectron processes allowing water photolysis. 

Scavenging mechanism has traditionally been considered as the principal contribution of HAS to polymer stabilization. It is more important in polymers where the photoinitiation is mainly due to ROOH photolysis, e.g. in PP [134]. Scavenging of C- and O-centred radicals participating in the initiation and propagation steps of the chain oxidation of polymers has been envisaged for the original structures of HAS and their active transformation products (nitroxides, O-alkylhydroxylamines, hydroxylamines). 

Terpolymer 194 having a higher content of the sensitizing moiety but a comparable level of the HAS units imparted a higher stability to PP, due to the formation of smaller photolytic fragments migrating through the polymer matrix. Photolysis of 194 occurs before the start of PP photo-oxidation [301]. 

The incorporation of lignins in poly(olefins) and rubbers has attracted much attention within the very important context of polymer stabilization against degradation mechanisms involving free radicals, viz. oxidation, photolysis... 

Also in the case of a polymer therefore, provided the acyl peroxy radicals are formed by ketone photolysis in the presence of oxygen, the oxidation of amines by these radicals would make a significantly greater contribution to stabilization than the nit-roxide. The latter is in any case present in only very small amount as secondary producti - -. 

Since photoinduced charge separation occurs easily at the semiconductor-water interface, its stabilization with polymer coating is one of the most promising approaches to water photolysis by visible irradiation. 

If small bandgap semiconductors could be stabilized in water for much more prolonged time by polymer coating, it must lead to developments of efficient photochemical cell and water photolysis system. 

Efforts towards stabilization of metaphosphonimidates with bulky substituents failed. Photolysis of 5 at low temperature did not allow us to stabilize or ]. Irradiation of y in benzene gave rise to dimers and polymers but also to the hydrogen abstraction product. Moreover we did not succeed at the present time in the preparation of the phosphorus azide 2 from 8 certainly because of steric hindrance. 

The Ni and Pt complexes can also be incorporated into polymer films of quaternized poly(vinylpyridine) (PVP) and deposited onto the transparent electrode (84). Photocurrents are enhanced to microamps (pA), an increase that may be attributed to either the effect of immobilization of the complexes near the electrode surface or an increase of the excited-state lifetimes in the polymer matrix. However, the effective concentrations of the complexes in this study were much greater than for the acetonitrile solutions in their earlier work. The polymer films are not stable to continuous photolysis, and voltammograms of the films are quite sensitive to anions used in the supporting electrolyte. The system can be stabilized by using a polymer blend of PVP and a copolymer containing quaternary ammonium ion and including [Fe(CN)6]4- in the electrolyte solution (85). Upon irradiation of the visible MLCT bands of [M(mnt)2]2 (M = Ni, Pt), photocurrents are produced. The mechanism (Scheme 4) is believed to involve photooxidation of the metal bis(dithiolene) triplet state by the Sn02 electrode, followed by [Fe(CN)6]4 reduction of the monoanion, with completion of the ET cycle as ferricyanide, Fe(CN)6 3, diffuses to the other electrode and is reduced. 

The colorless, air-stable, volatile (61), usually abbreviated as MTO, has an outstanding thermal stability (dec > 300 °C) and is freely soluble in virtually any solvent from water to pentane. It is converted to Re04 in basic aqueous solution. In water at 70 °C, it forms an electrical conducting polymer of formula [Ho.5 Re(CH3)o.9203 ]oo. Photolysis homolyzes the Me-Re bond. However, the primary reaction brought about by irradiation into the UV absorption near 260 nm in a solid Ar matrix involves tautomerization of MTO to the methylidene hydroxo derivative H2C=Re(0)20H (62). ... 

This is similar to the mechanism postulated by Kwei for the production of the P chloro ketones. It is also possible that this absorption is due to the acetyl fragments split off by type I photolysis. We have observed that the carbonyl absorption at 1770 cm can be removed by reprecipitation from THF solution by methanol. This indicates that the groi ) responsible for this absorption is not chenoically attached to the molecule. The acetyl radical produced by the type I can abstract hydrogen, or chlorine, either from the polymer or the stabilizer to form either acetaldehyde or acetyl chloride which may hydrolyze to the acid. 

Fulgides and related diarylethenes have been investigated extensively because of the long-term thermal stability of their photocyclized colored forms, which could lead to this application in erasable optical recording materials and photoswitchable optical elements. The entrapment of these photochromic molecules in polymer films is necessary for these practical applications. Picosecond laser photolysis was employed to study the electrocyclic reaction of a furylfulgide (18, Figure 8) in polymer solids and revealed that the colored structure was formed with a time constant of ca. lOps irrespective of the nature of the polymer matrix.49... 

UV absorbers have been shown to undergo photolysis reactions in polymer materials, the loss being quite critical depending upon the nature and form of the material. Spectroscopic methods have been developed to design more effective UV absorbers which function via intra-molecular proton transfer in the excited state. Other effective stabilizers for polymers includes thioureas for PMMA, aryl nitrenes for PP, diphenylnitrone for polyethylene, zinc glycerolate in PVC o and hexazoclanes for cellulosics and nylons. ... 

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