Polyene length

The relative comparison between reaction rate constants in step 3 gives = 20itj and when kz> k, the stabilizer HCl complex accumulates. The stabilizer is efficient when k > ki. The distribution of polyene lengths depends on the ratio of the speed of evolution of HCl to its speed of readdition. 

Polyene structures and the maximum polyene length have been studied... 

This implies that the element FaV of the eigenvector relative to the Ath normal mode rules the intensity distribution among the Raman-active lines. In particular it is possible to predict the intensity ratios between the 3 and t l bands from the ratio (L3H /L n)-. The variation of this intensity ratio with increasing polyene length has been predicted for /m-butyl end-capped polyenes, in nice agreement with experimental findings (991. 

As HCl is released from PVC the discolouration increases with a colour shift towards red. This is attributed to the build up of polyene sequences, which can be studied by UV spectroscopy (Figure 12). It appears that the average polyene length is fairly short, reaching only 6-10 double bonds in conjugation. During dehydrochlorination there is also an increase in MW due to crosslinking, and some cyclization occurs. 

Thiols, in general, are well-known to imdergo additions to alkene double bonds by a free-radical mechanism or in an ionic process that is promoted by strong acids such as HCI. During PVC thermolysis, such additions of ester thiols would tend to inhibit coloration by shortening polyene lengths. They also would tend to inhibit polyene growth if they occurred, for example, as in equation 2,... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

The Tz orbitals of butadiene (Scheme 18) qualitatively obtained from the orbitals of ethylenes are also supported by the electronic spectra of polyenes. The HOMO of butadiene is higher that the HOMO of ethylene since the former is the out-of-phase combination of the latter. The LUMO of butadiene is the in-phase combination of the LUMOs of ethylene and lies lower than the LUMO of ethylene. The energy gap between the HOMO and the LUMO is smaller in butadiene. In fact, the wavelength ( m ) is longer for butadiene (217 nm) than for ethylene (165 nm). The wavelength increases with the chain length of the polyenes. 

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