Dehydrochlorination of poly vinyl

In the presence of hydroquinone the rate of dehydrochlorination of poly (vinyl chloride) in oxygen is reduced considerably (20, 24). The presence of an oxygen scavenger in lieu of a radical scavenger or antioxidant would be expected to have an even greater effect. 

T. Yoshioka, T. Akama, M. Uchida, and A. Okuwaki, Analysis of two stages dehydrochlorination of poly(vinyl chloride) using TG-MS, Chem. Lett., 322 (2000). 

Molecular Orbital Theory of Polyenes Implicated in the Dehydrochlorination of Poly(vinyl chloride)... 

Ion-Pair Mechanism for the Thermal Dehydrochlorination of Poly (vinyl chloride). Clearly there is nothing in our theoretical analysis of the electronic structures and energetics of the polyenyl cations which would preclude their participation in Reaction 2 as either transition states or intermediates. However, it remains to establish the values of n at which the pathway of Reaction 2 will be competitive with other reaction processes. On the basis of our comparison (see above) with the ionization of tropenyl chloride, it is apparent that kinetic amounts of contact and perhaps even solvent-separated ion pairs will be available at n values (Equation 2) of about 1 or 2. However, in view of our previous arguments, it seems unlikely that free ions will ever achieve a high concentration in PVC. Given the fact that proton abstraction by chloride ion (Equation 2b) is necessary to complete the elimination... 

However the material obtained was an unprocessable powder. Unlike substituted polyacetylenes, polyacetylene is insoluble, infusible and unstable in air. The discovery of a technique to synthesize the polymer in the form of a free-standing film and the use of electron donors and acceptors to dope it to have metallic conductivity produced intense interest in the polymer in the last two decades. A wide variety of catalyst systems has been described for the polymerization of acetylene. Besides the route via acetylene polymerization, polyacetylene can also be obtained by a two-step route which involves the synthesis of soluble polymer precursors, which are converted to polyacetylene via thermal elimination and transition-metal-catalysed isomerization, as well as by polymerization of cyclooctatetraene , by dehydrochlorination of poly(vinyl chloride) and by dehydration of poly(vinyl alcohol)... 

Among the indirect methods, dehydrochlorination of poly(vinyl chloride) and dehydration of poly (vinyl alcohol) will not be described further in this chapter, because the products may have relatively short conjugated sequences with many irregular structures. 

Nonchain scission refers to reactions involving pendant groups that do not break the backbone. Typical of such reactions are dehydrochlorination of poly( vinyl chloride) (Equation 1.57), elimination of acid from poly(vinyl esters)—for example, poly(vinyl acetate) (Equation 1.58)—and elimination of alkene from poly(alkyl acrylate)s (Equation 1.59). 

The dehydrochlorination of poly(vinyl chloride) has been the subject of much investigation, particularly with the view of developing greater stability in PVC... 

For PNCs, the layered double hydroxides (LDHs) constitute a new class of mainly synthetic, reinforcing nanoparticles. During synthesis, LDHs may be preintercalated with organic anions. Unfortunately, most LDH platelets are thin, f=0.6 0.1nm, small in diameter, d=30 to 40 nm [Wu et al., 2007]), and decompose thermally at T IOTC [Camino et al., 2001]) consequently, their main application has been for absorption of HCl during dehydrochlorination of poly(vinyl chloride) (PVC) and halogenated polymers, or as flame retardants. The flow behavior of polyethylene (PE) with LDHs was reported to be similar to that of CPNCs [Costa et al., 2006]. There are no commercial PNCs with LDHs on the market, but experimental products such as Perkalite, a preintercalated aluminum-magnesium (Al-Mg) LDH, have been... 

Scheme 7.16 Mechanism of the light-induced dehydrochlorination of poly(vinyl chloride).

Example 5.18 Suppression of the Thermal Dehydrochlorination of Poly (Vinyl Chloride) by Addition of Stabilizers... 

The dehydrochlorination of poly(vinyl chloride) has been the subject of much investigation, particularly with the view of developing greater stability in PVC polymers and copolymers. Like many polymeric reactions, dehydrochlorination is a complex process. The vinylene groups, created by the elimination of HCl from adjacent carbon atoms in the chain. 

Dehydrochlorination of poly(vinyl chloride) and photFrie s reaction in polycarbonates. 

Perhaps the most common such reaction in carbon-chain polymers is elimination of the substituent, with the formation of a double bond. The classic, and most important, example is the dehydrochlorination of poly(vinyl chloride)(PVC), which can be written formally as... 

The alternative suggestion that dehydrochlorination of poly(vinyl chloride) involves unimolecular elimination has been made by analogy with low molecular weight alkyl halides. It has been found that the homogeneous gas phase pyrolysis of many alkyl halides to hydrogen halide and olefin follows first order kinetics, which is consistent with a one-step mechanism proceeding through a cyclic transition state ... 

It will be noticed that dehydrochlorination of poly(vinyl chloride) by an unzipping process results in long sequences of conjugated double bonds. (In degraded poly(vinyl chloride) there are probably 5—20 double bonds per conjugated sequence.) It is widely held that this formation of conjugated polyenes is responsible for the development of colour in degraded poly(vinyl chloride) but, so far, it has not been possible to relate the various colours which may be obtained to actual chemical species. 

It has also been suggested that dehydrochlorination of poly(vinyl chloride) may occur through intermolecular condensation. This type of reaction leads to... 

Oxygen in general accelerates dehydrochlorination of poly(vinyl chloride), especially in thermal degradation [551, 552, 890]. Activation energies of photodehydrochlorination in air and nitrogen are a(air) = 19.6 kJ mol and (nitrogen) = 32.1 kJ mol respectively. 

It has been found that the quantum yield of dehydrochlorination of solid poly(vinyl chloride) films at 254-400 nm, 0hci = 0.011 in N2, and 0hci = 0.015 in pure oxygen [552, 559, 561, 562] and at 514 nm (ion argon laser), Hci = 0.005 in N2 and (/)hci = 0.009 in O2 [560], remains initially constant. This means that incident photons, which are increasingly absorbed by the polyenes formed, initiate the dehydrochlorination of poly(vinyl chloride) with an efficiency that remains constant throughout the photolysis. 

Commercially used plasticizers such as di n-octyl adipate, di -octyl sebacate, di(2-ethylhexyl)phthalate and butyl benzyl phthalate can contribute to the photo-dehydrochlorination of poly(vinyl chloride) and this is attributed to a free radical mechanism, whereby a radical is first formed in the plasticizer and then transferred to the polymer [255, 297, 298, 688, 1636, 2270]. In the case ofdi(2-ethylhexyl)phthalate and tri(2-ethylhexyl)trimellitate the observed effects are even more drastic under conditions of solar irradiation [554]. This effect has been attributed to the fact that the trimellitate exhibits a much greater absorption in the wavelength range of the solar spectrum and hence is more likely to form free radicals. 

The side effects due to degradation can be alleviated through a precise knowledge of the mechanism involved. For instance, the autocatalytic thermal dehydrochlorination of poly(vinyl chloride) and poly(vinylidene chloride) suggests that basic additives can weU stabilize them and prevent their degradation by neutralizing the HCl gradually formed. 

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