Polyvinylchloride dehydrochlorination

On the other hand dehydrochlorinated polyvinylchloride li > and polimethyl-jS-chlorvinyl-ketone 74> catalyze the autoxidation of hydrocarbons, and the activities are related to the semiconductive properties of the catalysts. Recently it has been shown that entirely inert polymers like polyethylene, polypropylene and polyftetrafluoro) ethylene are rather efficient catalysts for the oxidation of te-tralin 75>. 

A series of studies dealing with the process integrated dehydrochlorination of plastic mixtures (polyvinylchloride, polystyrene and polyolefines) has been done by Homung and Bockhorn [33-35]. By varying the temperamre in distinct steps it was possible to evolve 99.6% of the HCl before the main pyrolysis of polystyrene and polyolefines appeared. 

At temperatures below 240 °C, there is essentially no gas evolution, with the exception of some HjO after long heating time. In the range 250-300 °C, small amounts of low hydrocarbons (AN, MAN, propionitrile) evolve. These compounds are supposed to proceed from depolymerization of uncyclized parts of the polymer. Hydrogen cyanide starts to evolve after 70 min at 250 °C and after 30 min at 300 °C. The early evolution of HCN has frequently been ascribed to dehydrocyanation (analog to dehydrochlorination from polyvinylchloride), either within a molecule, leading to unsaturation ... 

Vinyl chloride monomer, the basic building block of polyvinylchloride (PVC), is commercially manufactured by dehydrochlorination of 1,2-dichloroethane. The modern process for producing 1,2-dichloroethane involves oxychlorination of ethylene in a fluidized bed catalytic reactor ... 

Although it is well known that when polyvinylchloride (PVC) is irradiated with UV light, discoloration, dehydrochlorination, decomposition and crosslinking occur, few ESR studies on PVC irradiated with UV light have been reported. Nishijima et al. (86) observed an ESR spectrum after UV irradiation of a polyvinylchloride film with light of 185 nm at —196° C under vacuum. The sample was heated at various temperatures for 10 min after UV irradiation at —196° C. The changes in a spectral shape, radical concentration, A Hmsl and A H are shown in Figs. 9 and 10. It is apparent that a broad component decays out at about —100° C and a narrow component at about 80° C. The former could be attributed to some kinds of alkyl radicals and the latter to... 

This chlorine atom attack accounts for the decrease in the molecular weight of polymethylmethacrylate, monomer production at abnormally low temperature and the delay in the dehydrochlorination of polyvinylchloride. This delay in dehydrochlorination is general for blends of polyvinylchloride and any polymer containing hydrogen atoms that can be abstracted by chlorine atoms. 

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds to the elimination of HC1 and the second to that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded styrene evolution, in fact, does not occur below 350°C. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 300° C. According to McNeill et al. [323], structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of Cl to the aromatic ring, yielding a cyclohexadienyl-type radical which is unable to induce depolymerization of the styrene chain. 

The structure —CHC1—CH2—CO—CH2 — was found by Kwei [99] in polyvinylchloride after photo-oxidation. Such j3 chloroketones decompose by the Norrish type I mechanism without loss of chlorine atoms. Hydrogen chloride is obtained only when polyvinylchloride is photo-oxidized above 30°C [98]. It seems that zipper dehydrochlorination plays little role in the reaction occurring on exposure to ultraviolet light at temperatures below 150°C in the presence of air [97], and that hydrogen chloride is mainly a product of thermal decomposition rather than photolysis [98], The following mechanism can be proposed which takes into account the experimental results namely, that chain scission and crosslinking occur simultaneously on irradiation at 253.7 nm [100] and that carbon dioxide is evolved, while an absorption band at 1775 cm-1 (ascribed to peracids) is detected in the infrared spectrum [98]. 

The method of production has been found to have a striking influence on the thermal stability of polyvinylchloride (PVC) over a temperature range up to 340 °C [1, 2]. Thus, PVC obtained as a result of y-irradiation and benzoyl peroxide (BP) initiation has approximately the same stability, while PCV obtained by initiation with azobisisobutyronitrile (AZBN) is noticeably less thermally stable over the temperature range of 220-270 °C. However, stabilisation towards further thermal degradation of all PVC samples tested is observed at about 60% weight loss, possibly due to the considerable dehydrochlorination of the polymer to form polyene and crosslinked structures. 

Hollander, A., Kobryanskij, V. M., Zimmerman, H., and Behnisch, J., Preparation of soluble polyacetylene by chemical dehydrochlorination of polyvinylchloride, Ho-uptjahrestag. Gera, November 16-18, Kurzref. Chem. Ges. DDR, Friedrich-Schiller-Univ, Jena Sekt. Chem., 1988, p. 63. 

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