Polyene structures poly

Scheme 1. Chemical Structures of some important linear n-conjugated polymers. Polyenes 1, poly-ynes 2, polyenynes 3, poly(p-phenylene)s (PPP) 4, polyphenylenevinylenes (PPV) 5, polyphenyleneethynylenes (PPE) 6.

A polyaldolic condensation of acetone can be induced by triflic acid to give a solid resin with a polyenic structure resembling poly(methylacetylene) having some functional groups946 (289). 

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... 

Similarly, dehydrochlorination of poly(l,2,3-trichlorobutadiene) should bring about the formation of the cumulene form of carbyne as it seems to be predetermined by chemical structure of intermediate polyene, viz. poly(2,3-dichloro-2-butene-l,4-diylidene) (Scheme 12.3). 

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... 

Coloration of poly(vinyl chloride) due to formation of polyene structures... 

The estimation of the concentration of polyenes from their absorption spectra requires prior empirical calculation of the absorption coefficients [512]. A very sensitive method for the registration of polyene structures in degraded poly(vinyl chloride) is second derivative spectroscopy (cf. section 10.16) [1792, 1888]. 

In poly(vinyl chloride) containing polyene structures photodehydrochlorination is initiated by intramolecular energy transfer from polyene excited singlet states to the allylic C—Cl bond (reaction 3.326) and/or eventually to the allylic C—H bond (reaction 3.327). Triplet states are less likely to be involved because of a very efficient intersystem crosslinking in polyenes [564]. 

The rate of dehydrochlorination is higher in photolysed thermally degraded poly(vinyl chloride) films which contains large amounts of polyene structures [1677]. 

Polyene structures formed during the photodehydrochlorination of poly(vinyl chloride) samples are bleached in the dark as well under UV/VIS irradiation. Bleaching in the dark is a much slower process that in the presence of light. Oxygen plays an important role in bleaching processes in the dark and during light irradiation [771, 1674, 1791, 1881, 1882]. 

Polyene structures formed during the thermal- and/or photo-oxidative degradation of poly(vinyl chloride) react easily with a number of different solvents, which cause their discoloration (Table 3.16). This process is accelerated by irradiation with light (Fig. 3.39) or heating in the presence of various radical sources (Fig. 3.40 and Fig. 3.41). The decoloration mechanism depends upon the type of solvent used [1426, 1673] ... 

Formation of polyene structures causes yellow colouring of a polymer sample. Chain scission and crosslinking occur simultaneously. Rates of crosslinking are greater than those of polystyrene and this is attributable to the participation of the substituted phenyl radicals (formed by phenyl-alkyl group fission) in addition reactions. However, crosslinking is inhibited sterically, to some extent, in the poly(o-alkylstyrenes). 

The normal absorption spectra of photoirradiated poly(vinyl chloride) (Fig. 3.32a) and their corresponding second derivative spectra (Fig. 3.32b) show the formation of polyene structures in a polymer sample [1792,1888]. 

Figure 10.79 shows a typical photoacoustic spectrum obtained for poly(vinyl chloride) samples exposed to different periods of UV irradiation. This spectrum shows the formation of polyene structures during the photodegradation process. 

Poly(phenylacetylene) and poly(diphenylacetylene) consist of polyaromatic conjugated fragments side by side with polyene chains. The photosensitivity strongly depends from acceptor concentration, supermolecular structure, the synthesis procedure [177], The acceptor molecules inclusion increased the photosensitivity, the optimum of which was obtained for heterogeneous phases with a large interface area. The transition of the amorphous structure into the crystal one promoted the photosensitivity increase. The maximum quantum yield of 10 3 was at the energy 3 eV, mobilities were varied from 10-9 to 10-6 m2 V-1 s-1. 

Cobalt complexes find various applications as additives for polymers. Thus cobalt phthalocyanine acts as a smoke retardant for styrene polymers,31 and the same effect in poly(vinyl chloride) is achieved with Co(acac)2, Co(acac)3, Co203 and CoC03.5 Co(acac)2 in presence of triphenyl phosphite or tri(4-methyl-6- f-butylphenyl) phosphite has been found to act as an antioxidant for polyenes.29 Both cobalt acetate and cobalt naphthenate stabilize polyesters against degradation,73 and the cobalt complex of the benzoic acid derivative (12) (see Section 66.4) acts as an antioxidant for butadiene polymers.46 Stabilization of poly(vinyl chloride)-polybutadiene rubber blends against UV light is provided by cobalt dicyclohexyldithiophosphinate (19).74 Here again, the precise structure does not appear to be known. 

Other hyperbranched polymers showed similar absorption and luminescence properties. Upon photoexcitation, the hb-PA solutions emitted deep-blue to blue-green lights, whose intensities were higher than that of poly(l-phenyl-l-octyne), a well-known highly emissive polyene. The PL efficiencies of the polymers varied with their molecular structures. Polymers hb-P(38-VI), hfo-P(45-V), hb-P(48-VI), M>-P(50-VI), fcfo-P(50-VII) and hb-P(59-VI) exhibited (P values higher than 70%, with hfc-P(50-VII) giving the highest value of 98%. 

It has been shown that the dihalocarbenes (CX2) react with macro-molecular polyenes to give polydihalocyclopropane type products (17, 18). The transformations effected may be either partial or total on poly-isoprenes and polybutadienes. The reactivity of the polychloroprenes is slighter, and they undergo marked conversion only when they react with CClo. A study was made of the structures obtained and in particular their reduction to polycyclopropane hydrocarbons. 

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