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Platinum Metals Rev., 1959, 3, (3), 99

Synthesis of Fluoroaromatic Polymers

Intermediates for High-Temperature Plastics

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A research programme on the synthesis of fluorocarbon polymers for possible use in high-speed aircraft, ordnance equipment and other types of high-temperature application has recently been carried out at the National Bureau of Standards. Most plastic materials at present available are not capable of withstanding high-temperature conditions but it is thought that polymers derived from completely fluorinated aromatic compounds should have considerably improved resistance to temperature and also to radiation.

Hexafluorobenzene provides a convenient starting point for the synthesis of such polymers and the discovery of a method for its synthesis in reasonable yields and quantities (M. Hellman, E. Peters, W. J. Pummer and L. A. Wall, J. Amer. Chem. Soc., 1957, 79, 5654–5656) has greatly accelerated their development. Hexafluorobenzene is obtained by pyrolysis of tribromofluoromethane in a platinum tube 80 cm long, 1 cm wide and about 1 mm thick packed with platinum gauze. The best yields, 55 per cent, were obtained at 540–550°C under a nitrogen pressure of 4.5 atm.

Pentafluorophenol, one of the most useful compounds obtained from hexafluorobenzene, is prepared by the addition of solid potassium hydroxide to its pyridine solution. The hydrogenation of hexafluorobenzene using a platinum-on-carbon catalyst at 300°C gives a high yield, 40 per cent, of the monohydro derivative, pentafluorobenzene, together with 10 per cent tetrafluorobenzenes (R. E. Florin, W. J. Pummer and L. A. Wall, J. Res. Nat. Bureau Standards, 1959, 62, (March), 119–121). The catalyst maintains its activity for long periods. The main products of the reduction can be brominated to pentafluoro-bromobenzene and dibromotetrafluorobenzene or iodinated to the analogous iodides. The presence of the reactive bromine or iodine atoms makes such compounds invaluable intermediates for the preparation of further fluorocarbons and their polymers.

So far successful attempts to prepare polymers have been made using sodium pentafluorophenolate, diiodotetrafluoro-benzene and pentafluorobromobenzene as starting materials. It is expected that such polymers will possess the combination of properties necessary to give both high thermal stability and plasticity.

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