Synthesis of polybenzimidazoles via aromatic nucleophilic substitution reactions of self-polymerizable (A-B) monomers

Abstract Self-polymerizable (A-B) polybenzimidazole (PBI) monomers have been prepared and converted to PBIs via aromatic nucleophilic substitution reactions. Thus, 2(4-fluorophenyl)-5(6)-hydroxy-benzimidazole (FHB) and 2-(4-fluorophenyl)-5-hydroxy-1-phenylbenzimidazole (PFHB) have been prepared and polymerized at 230°C to 250°C in N-cyclohexyl-2-pyrrolidinone (CHP) containing potassium carbonate. The imidazole ring in these monomers activated the fluorine atom for nucleophilic displacement by the phenate ion. The resulting polymers were soluble in N-methyl-2-pyrrolidinone (NMP) and had intrinsic viscosities that ranged from 0.6 to 2.6 dl g−1 (NMP at 30°C). The PBI obtained from FHB was semicrystalline with a glass transition temperature (Tg) of 365°C, while the poly(N-phenylbenzimidazole) (PPBI) obtained from PFHB was amorphous with a Tg of 278°C. Thin films of the PPBI polymer were tough and flexible, having tensile strengths as high as 100 MPa, while those of the PBI polymer were brittle. The PBI retained 95% of its weight to 460°C when subjected to thermogravimetric analysis (t.g.a.) in air, while the PPBI retained 95% of its weight to 535°C under the same conditions. In order to lower the Tg and also to improve the mechanical properties of the PBI, PFHB was copolymerized with FHB. The Tg values of the copolymers decreased from 342°C to 296°C as their PFHB content increased from 25 to 75 mol%, while the tensile strengths of thin films of the copolymers increased with increasing PFHB content. Random copolymers were also prepared from a self-polymerizable poly(phenylquinoxaline) (PPQ) monomer and FHB.