Conjugated double bond sequences

The first two reactions yield unsaturated polymeric products that are more or less colored, depending on the extent of reaction. The color of the degraded macromolecular compounds is related to the length of the resulting conjugated double bond sequences. 

To prevent or to limit the formation of conjugated double bond sequences by promoting addition reactions or by functioning, as an oxidation catalyst to stabilize the color of PVC. 

The intense discoloration which developed rapidly upon UV exposure reveals the high photosensitivity of C-PVC that is even more pronounced than for PVC itself, as shown by the UV-visible absorption spectra of figure 5. After 15 minutes of irradiation, large amounts of polyenes have already accumulated in C-PVC, with sequence lengths up to 20 conjugated double bonds, while PVC is hardly affected after that short exposure. 

Acetylation of puupehenone (63) does not afford the expected monoacetyl derivative (64). Instead, the triacetyl derivative (65) was obtained exclusively, indicating the addition of the acetyl group to the conjugated double-bond system of puupehenone. The monoacetyl derivative of puupehenone (64) was obtained as a side product of the addition-elimination reaction sequence of HBr/HCl with puupehenone followed by acetylation which resulted in 66. 

The stabilization by conjugated double bonds makes this sequence energetically attractive. It can be easily understood, however, that the activated butene will also have a tendency to isomerize. Consequently, oxidation products are often found together with isomerization products. 

The chief avason for utilizing the reaction sequence described is the fact that formation of the a,p-unsaturated ketone in II makes it possible to dilTea ntiate between the two double bonds, because the copper hydride complex [PPh CuH], reduces only the conjugated double bond... 

In degrading pure PVC, relatively short polyene sequences containing up to about 10-12 conjugated double bonds are formed primarily. The share of longer polyene sequences is small, the upper limit being about 25-30 conjugated double bonds. 

Polyenes exhibit two strong Raman bands between 1600 and 1500 cm and between 1200 and 1100 cm , respectively. These are attributed to in-phase vibrations of the C=C and C-C bonds, respectively, throughout the entire chain. Examples include all-trans-retinol (Fig. 4.1-6B), /3-carotene, and carotene-containing proteins, obtained, for instance, from lobster shells (Rimai et al., 1973 Oseroff and Callender, 1974). Polyene sequences with approximately twenty conjugated double bonds, formed by HCl elimination from polyvinyl chloride as a result of aging, also show these Raman bands at 1495 and 1115 cm (Peitscher and Holtrup, 1975). These bands are enhanced as a consequence of the resonance Raman effect, which makes it possible to detect such groups in polymers and natural materials, even at low concentration. 

Production and use of PVC occur in the presence of air, i. e. in the presence of oxygen. Therefore, it is surprising that the mechanistic details of thermooxidative degradation of PVC are still not fiilly revealed. The major reactions of this process are shown in Scheme 1. As indicated in this Scheme, thermal dehydrochlorination yields HCl and simultaneously sequences of conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation of radicals. Subsequent chain reactions result in additional initiation of HCl loss and further oxidative processes (/, 8). 

---