Fluorine Containing Aromatic Polymers
The high strength of the carbon-fluorine bonds and the shielding effect of the highly electronegative fluorine atoms improve heat stability. One such material was synthesized by heating perfluoroalkyl amidines above their melting points. Ammonia evolves and triazine ring containing polymers form [174]:
The thermal stability of these polymers in vacuum is about equivalent to polytetrafluoroethylene [174].
Among other fluorine containing materials reported in the literature are a group of polyesters [175]:
Their heat stability and other physical properties, however, do not appear to be superior to conventional materials. The Ullmann reaction can be used to prepare polyperfluorophenylene [176]. The products, however, are low in molecular weight:
Higher molecular weight materials, like poly (perfluoro phenylene ether), form from potassium pentafluoro phenoxide [177], where Mn is about 12,500:
Liu et al. reported improved process for synthesizing fluorinated polymers in supercritical carbon dioxide [177]. The improvement consisted of modifying the reaction system and designing and using sampling tubes. Low molecular weight fluorine containing polymers form from per fluoroaromatic compounds through a loss of aromaticity when they react with bis(fluoroxy)-difluoromethane [180]. More interesting is formation of poly(α, α, α , α -tetrafluoro-p-xylylene) by a polymerization technique that closely resembles the preparation of poly(p-xylylene) by vacuum pyrolysis of a dimer [181]:
The resultant polymers maintain useful mechanical properties for up to 3,000 h in air at 250C. Fluorinated epoxy resins are another group of materials that might potentially possess improved thermal stability. This, however, has not been demonstrated. It was shown [190] though that these materials can be key intermediates in organic coating and plastics that require special properties, like hydrophobicity, oleophobicity, light stability, and low friction.
