Conjugated polyenes structure

This method of diene formation with definite E and Z structures has wide synthetic applications [518], particularly for the syntheses of natural products with conjugated polyene structures. Bombykol and its isomers (650 and 651) have been prepared by this method[5l9]. The synthesis of chlorothricolide is... 

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

While initial aging did cause some loss in volume resistivity, it was no more severe in unstabilized than in stabilized compositions. The most startling effect was the marked increase in volume resistivity under severe aging conditions. This may be due to volatilization loss of DOP or cross-linking of polyvinyl chloride, both of which could reduce migration of ionic impurities, or it may involve adsorption of ionic impurities by the dark-colored, conjugated, polyene structure which forms by loss of hydrogen chloride. In any case it deserves further study. 

The dehydrochlorination reaction causes a color change. Marvel and co-workers (17, 19) attributed the discoloration to a conjugated polyene structure. They suggested that random dehydrochlorination to form an olefinic bond would be followed by allylically activated removal of successive hydrogen chloride units to form the polyene structure. 

Virtually all previously published opinions on thermally degraded poly (vinyl chloride) regarded the dehydrochlorination process as being in the first instance, a monomolecular process resulting in conjugated polyene structures, and that all oxidation and cross-linking reactions... 

The fire-retardant mechanism associated with nanoclays has recently been studied and is likely to involve the formation of a ceramic skin which catalyzes char formation by thermal dehydrogenation of the host polymer to produce a conjugated polyene structure. " The nanocomposite structure present in the resulting char appears to enhance the performance of the char through reinforcement of the char layer. These effects would explain the apparent fire-retardant synergy observed when nanoclays are incorporated into polymer formulations containing condensed phase fire-retardant systems, including coated fillers. 

The presence of oxygen in the environment of degrading PVC generally accelerates the loss of HCl. This result is believed to be due to attack by oxygen at the conjugated polyene structures to bleach them and simultaneously form oxygenated structures that further labilize adjacent chlorine atoms. 

Usami et al (1990) [214] studied the thermal degradation of PAN fibers (Nikkiso AN/ MA/ITA) using gas chromatography, NMR and FTIR, confirming that the main stabilization reaction was the formation of a ladder polymer and that the contribution of dehydrogenation and/or dehydrocyanide reactions was to form conjugated polyene structures. Scheme XXXVI shows the competitive thermal reactions ... 

Another type of degradation (dehydrohalogenation) also occurs with chlorine containing polymers such as PVC. PVC may lose hydrogen chloride when heated and form a chromophoric conjugated polyene structure. Since the allylic chlorides produced are very unstable, the degradation continues as an unzipping type of chain reaction, see Reaction 6.13. 

As with any prospective new application we reasoned that optimization of physical and chemical properties would be required in order to generate practically useful electrically conductive polymers. We were concerned about mechanical properties, flexibility, conductivity levels, solubility, processability, oxidative stability, etc. Based upon the perceived requirement of a conjugated polyene structure, substituted polyacetylenes were the obvious way to introduce substituents for the purpose of tailoring these characteristics. Unfortunately the literature provided ample evidence of the sluggish nature of substituted polyacetylenes toward polymerization. 

When poly(vinyl chloride) is exposed to UV irradiation it turns yellow and subsequently a deep red-brown colour. This discoloration occurs rapidly, due to the formation of conjugated polyene structures. Simultaneously, large... 

In the second step, the degradation of the polymer (which has already become the dehydrochlorinated product) continues with cracking and pyrolysis to low hydrocarbons of linear or cyclic structure (more than 170 Cj-Cy products have been identified) [a.39]. Loss of HCl leaves a residue with a conjugated polyene structure having both cis and trans arrangements. Polyenes undergo aromatisation and crosslinking and form a wide variety... 

The carotenoids generally found in foods are linear zll-trans E form) polyenes formed from eight isoprenoid units. The structures of common carotenoids are shown in Figure 8.11. The linear conjugated polyene structure has the ability to delocalize an unpaired electron and hence the capacity to act to terminate free radical reactions with the production of resonance stabilized free radical structures. Thus, carotenoids may potentially (a) provide retinol and (b) act as antioxidants. 

POLYENE STRUCTURE. Treatment of PVA with heat results in a formation of conjugated polyene structures. Consequently PVA is colored yellow. The conjugated polyene structure has usually been studied... 

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