Polyene formation

There is one report of polyene formation during photooxidation with no concurrent evolution of HC1 (52) This observation can perhaps be attributed to insensitivity of the method used for HC1 detection. 

This is a result of polyene formation from PVC and PVDC, as shown in Scheme 4.1 ... 

A broad absorption band centered at about 340 nm Is also readily observed In an Irradiated film, and this band has been attributed to a benzalacetophenone-llke moiety (10). Absorption bands at 280 and 310 nm In films of PS Irradiated under 600 Torr of oxygen may be the result of polyene formation (. We have observed absorption bands In the same spectral regions In PS solutions In halogenated solvents Irradiated In the absence of air (5)(11). 

Poly(vinyl halides) - Photodehydrochlorination occurs in PVC with a foam backing as determined by Raman spectroscopy. For PVC stabilised with Zn/Ca stearates phooxidation indicates that dehydrochlorination dominates over oxidation at the inner layers beyond 100 microns " . The effect of polyene formation also appears to be a function of light intensity as well as oxygen diffusion rate. 

Figure 3.30. Successive Raman spectra collected during the thermal degradation of a PVC PVP blend (90 10) at 120 C, showing the rapid increase in the bands at 1100 and 1500cm due to polyene formation. Adapted from Kaynak et al. (2001).

When PVC was irradiated at room temperature in the presence of air, a singlet spectrum was observed, which could be attributed to poly-enyl radicals. In this case, however, these free radicals could not be correlated with polyene formation. 

Another type of argument to support a free radical mechanism was advanced by Palma and Carenza [172]. Thermal and 7-initiated dehydrochlorination between 80 and 130°C were compared. In view of the resemblance shown by the kinetic data for polyene formation, the same mechanism was thought to be operative in both cases. Since according to the authors, a free radical mechanism is clearly established for 7-initiated processes, this is also operative in thermal degradation. Salovey and Bair [171] reported that the thermal degradation of PVC at 155°C is enhanced by irradiation with 1 MeV electrons. Since later stages of isothermal weight loss for thermal and radiolytic decomposition follow 3/2 order kinetics, a free radical mechanism is also postulated by these workers. 

Vinyl polymers are particularly susceptible to thermal degradation. A typical example is rigid PVC, which is impossible to process under commercially acceptable conditions without the use of thermal stabilizers. Unstabilized PVC imdeigoes dehydrochlorination near the melt processing temperature. This involves liberation of hydrochloric acid and the formation of conjugated double bonds (polyene formation). The intense coloration of the degradation products is due to polyene formation. A second example of a polymer that undergoes nonchain-scission reaction is poly(viriyl acetate) or PVAc. When heated at elevated temperatures, PVAc can liberate acetic acid, which is followed by polyene formation. 

The solar light-induced dehydrochlorination of PVC plasticized with phtha-lates has been reported to be sensitized by the plasticizer [38, 39]. In marked contrast, more recent work has revealed a weak protective effect of phthalates with respect to C-Cl bond cleavage and polyene formation. Phthalates are likely to quench electronically excited states of impurity chromophores [40]. 

Polyene formation is not necessarily stopped altogether by a heat stabiliser, but it can be restricted to short sequences of a few (say five) carbon-carbon bonds, so that the polymer does not absorb too much light in the visible region of the spectrum, causing colour changes. This also discourages crosslinking and/or chain scission. 

In addition to polyene formation, dehydrochlorination may also lead to cross-linking. Thus it has been found that when poly(vinyl chloride) is heated in nitrogen there is a continuous increase in molecular weight and the polymer becomes insoluble [11]. 

Propagation reactions are responsible for the development of polymer chain microstructure that determines polymer material properties. For example, PVC chains can have head-to-tail and head-to-head structures. The head-to-tail structure is favored for its low-energy state. However, the head-to-head structure can also be formed particularly at high temperature. PVC materials with significant head-to-head structures have poor quality in application, are not stable, and are susceptible to polyene formation. 

Scheme 3.20 Elimination of HCI with polyene formation at the exposure of poly(vinyl chloride) to...

Commercial PVC products commonly contain plasticizers (up to 40%) such as phthalates or mellitates. In the past, phthalates were found to exert a weak protective effect with respect to C—Cl bond cleavage and polyene formation by quenching the electronically excited states of impurity chromophores [101]. 

Kinetically, the dehydrochlorination is a chain reaction, since chlorine radicals generated during double-bond formation can abstract hydrogens, thus forming new lateral macroradicals. Dehydrochlorination causes an intense discoloration which becomes more pronounced as the absorbed dose is increased, because of the increasing content of conjugated double bonds (polyene formation). 

---