Polymer stabilization photooxidation

A further effect of sterically hindered amines that could contribute to polymer stabilization has been described by Allan / McKellar31i-lft-41. They reported a conversion of a,3 to 3,Y unsaturated ketones in polyolefins and assumed that this played a role in the photooxidation of the polymers. The conversion... 

The variability in chemiluminescence from the photooxidation of solid polymer films prevented measurements of /3-values in solid media, where the results would be most pertinent to the general question of polymer stability. We might expect a close correspondence of /3-values to those determined in solution in this work, since the lifetimes of singlet oxygen in benzene and cyclohexane, which should bracket the properties of the polymers in this study, are nearly identical (22). 

U.V. Stabilizers.—Many of the papers discussed in preceding sections contain discussion of the nature and mechanism of u.v. stabilizers. A brief survey of antioxidants and stabilizers used in the plastics industry has appeared,28 and some new photostabilizers, including a polymeric u.v. absorber and a surface-grafted antioxidant, have been proposed.280 The mechanism of nickel chelate stabilizers has been further discussed,281 and the diffusion of and loss of light stabilizers in poly(olefins) described.282 As part of an attempt to understand the transformations of stabilizers during the ageing of polymers, the photooxidation of 2,6-di-t-butyl-4-methylphenol sensitized by Methylene Blue has been studied.283 U.v. light protection by sunscreens, with mechanisms of interest to the polymer field, has been described.284... 

Additional chemical stability can be given to PPVs by substitution at the vinyl-ene carbons. Thus, CN-PPV and PPV-DP are more stable than their parent polymers [173]. Carter et al. [172] showed that a random copolymer of PPV containing non-conjugated segments is considerably more stable to photooxidation than the fully conjugated polymer. Of course, the electrical and optical properties are also altered by these substitutions. 

A number of recent studies have been concerned with the effects of commercial heat stabilizers on the photodegradation of PVC. During irradiation at room temperature under air with 253.7-nm light, several dialkyltin dicarboxylates were found to increase the rates of the photooxidation and cross-linking of the polymer (62). However, at 0°C under air in a sunshine weatherometer, photooxidation was shown to be retarded by certain dibutyltin dicarboxylates (63). The latter result was also obtained in experiments involving the use of dibutyltin maleate with irradiation in the (280-400)-nm wavelength region at 38°C under air (50,51,64, 5,66). 

Poly(pyrido[3,4- ]pyrazine vinylene) 693 has been synthesized via condensation of 3,4-diamino-2,5-dibtomopyridine 691 with l,2-bis[3-(2 -ethylhexyloxy)phenyl]-ethane-l,2-dione 692 followed by coupling with l,2-bis(tri- -butyl-stannyl)ethylene in DMF at 110°C in the presence of tetrakis(triphenylphosphine)palladium. The vinylene polymer 693 showed improved stability toward photooxidation compared with similar polymers with purely aliphatic side chains and also had smaller band gaps (Equation 58) <2002SM(131)53>. 

Nickel complexes are of considerable importance as stabilizers and antioxidants for polymers of various kinds. The nickel(II) complex of the benzoic acid derivative (12) (see Section 66.4) acts as a stabilizer against oxidation of polybutadiene,46 but is less effective in this respect than the manganese and cobalt complexes. Complex (20) is effective in decreasing the rate of photooxidation of two-phase poly(vinyl chloride)-polybutadiene rubbers 74... 

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. 

In the preceding sections, discoloration and brightness reversion were discussed. Chemical constituents in modem papers exhibit a wide range of susceptibilities to oxidation that can be explained in part by differences in structures. Stabilization of color against thermal and photooxidation can be enhanced by modification of the polymer structure to eliminate the more reactive sites or chromophoric functional groups and to introduce groups that contribute to color stabilization. 

On the more commercial front, the photooxidation of polymers continues to attract attention with a continued special interest in natural materials. Bio- and photodegradable plastics are important for agricultural usage. The same applies to polymer stabilisation where commercial applications dominate significantly with much emphasis on the synergistic behaviour of stabilisers. For dyes and pigments stability continues to be of major concern. 

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