Correlations between the properties and structures of some polyhetero-arylenes

Outstanding amongst the heat-resistant organic polymers synthesized in the last decade is the polydiphenylenoxypyromellitimide (I), which is capable of very large stretching deformation (manufactured by DuPont as film PM, or Kapton). The elongation at break, ϵ20°C, of the polypyromellitimide film can be up to 200% [1] while remaining flexible at 4°K [2]. Attempts were made to explain the large deformation capacity of the PM film on the basis of the specific characteristics of the imide ring [3, 4], but comparisons of the mechanical properties of other polyimides with those of polyhetero-arylenes (PHA) of similar structure, especially the polybenzoxazoles, decidedly proved them to be identical (Table 1). Although one can synthesize the complete oxazole analogue of polymer I having a molecular weight sufficient for the formation of strong films, due to the known specificity of low temperature polycondensation, strong films have not been produced so far [7]. High polymers II and III, in which the pyromellitimide groups were partly replaced by benzobisoxazole and benzobisimidazole rings [10] were produced recently.