Laser dehydrochlorination

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

In the present work, a somewhat different approach was chosen in order to produce conducting polymers the basic idea was to start with a cheap material, like PVC, and try to remove all the hydrogen and chlorine atoms from the polymer chain. The purely carbon material thus obtained was expected to exhibit the electrical conductivity of a semimetal, while being insensitive to the atmospheric oxygen. In this paper, we report for the first time how PVC can be completely dehydrochlorinated by simple exposure to a powerful laser beam that combines both the photochemical and the thermal effects. 

These quantum yield values appear to be much higher than unity and therefore demonstrate that carbonization occurs by a chain reaction process. The mechanism of the laser-induced dehydrochlorination of photodegraded C-PVC can be schematically represented by the follo-... 

Since the UV degraded C-PVC still contains substantial amounts of the initial CHC1-CHC1 structure, one can expect the chlorine radicals evolved to also initiate the zip-dehydrochlorination of these structures. The resulting chlorinated polyenes will then be further destroyed by the laser irradiation, so that finally all the C-PVC polymer is converted into a purely carbon material within a fraction of a second. 

Fig. 4.3 Data of Raman spectroscopy (He-Ne laser, A,=632,8 nm) for the following samples 1 - polyvinyl chloride-polyvinylidene chloride composition after chemical dehydrochlorination (80°C, KOH in dimethyl sulfoxide -propan-2-ol 1 1, w/w) 2 - chemically dehydrochlorinated polymer carbonized in CO at 350°C 3 - carbonized product activated in CO at 950°C...

Most polymers can be analysed as received, as pellets, powders, films, fibres, in solution, or even as whole articles such as mouldings. Fine fibres can present some difficulties if a Raman microscope is not available. Raman spectroscopy has found applications in the identification of polymers in which additives obscure the polymer peaks in the IR spectrum. Reclaimed polymer is more prone to fluorescence than virgin material, causing problems for Raman analysis [394], Laser-Raman spectroscopy often allows polymer identification (e.g. in recycled material) only in conjunction with IR spectroscopy. Raman spectroscopy has been used to examine weathered PVC plasticised with DOP, DOA and BBP for dehydrochlorination [395], Laser-Raman spectroscopy has also been proposed as a suitable method for precise detection of ageing deterioration of vinyl chloride resins containing plasticisers and fillers used as electrical wire and cable coatings [396]. 

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. 

Samples which contain chlorinated polyenes (3.73) can be further completely dehydrochlorinated by laser radiation (488 nm from an argon ion laser) by the zip-dehydrochlorination mechanism induced by chlorine (Cl radicals) [552, 554-557, 1965] ... 

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