Investigations of Polybenzimidazole Synthesis

To investigate the suitability of a sample of 3,3,4,4- tetraaminodiphenyl, from a new commercial source, for the synthesis of polybenzimidazoles PBI, two standard procedures were used. The first procedure, melt phase reaction with diphenyl isophthalate, afforded a hydrated product of low viscosity. The second procedure, reaction with isophthaldehyde bis bisulfite adduct, in refluxing N,N-dimethylacetamide, did not give meaningful results because of difficulties in purifying the bis bisulfite adduct. A new method of PBI synthesis was then developed in which the tetraamine was reacted with diphenyl isophthalate in refluxing sulfolane. The product obtained by this method had a satisfactory inherent viscosity and showed no evidence of any appreciable cross-linking or hydration. The sample of tetraamine was therefore concluded to be of suitable quality for PBI synthesis. A critical comparison is given of the three synthetic methods used. The scope of the sulfolane solvent system for PBI-type syntheses was also explored by examining other tetraamine-diphenyl ester combinations. For cases where low reactivity of monomers or low solubility of products precluded the use of sulfolane, phenyl sulfone proved to be a satisfactory alternative solvent.