Synthesis of thermoplastic–thermosetting merged polyimides by use of the Diels–Alder reaction

Abstract

Novel thermoplastic–thermosetting merged polyimides (PIs) have been developed by Diels–Alder (DA) intermolecular reaction of the bisfuran 2,5-bis(furan-2-ylmethylcarbamoyl)terephthalic acid A with a series of bismaleimides, viz., m,m′-dimethyl p,p′-dimaleimidobiphenyl (B 1 ), m,m′-dimethoxy p,p′-dimaleimidobiphenyl (B 2 ), 1,1′-bis(4-maleimidophenyl)cyclohexane (B 3 ), and 2,2′-bis-[4-(4-maleimidephenoxy)phenyl]propane (B 4 ). The intermediate DA adducts obtained, C 1–4 , were aromatized and imidized (i.e. cyclized) through the carboxyl and amide groups to afford thermoplastic–thermosetting merged PIs D 1–4 . Bisfuran A was prepared by condensation of pyromellitic dianhydride with furan-2-ylmethanamine and duly characterized. Synthesized DA adducts and PIs were characterized by use of elemental analysis, spectral features, number average molecular weight, degree of polymerization, and thermal analysis. To facilitate the correct structure assignment and to verify the identities of the DA adducts and PIs, a model compound 4 was prepared in a similar way from phthalic anhydride and furan-2-ylmethanamine. FT-IR spectral features of PIs D 1–4 were compared with those of model compound 4 and found to be identical. Glass fiber-reinforced composites GFRC 1–4 were prepared from this system and characterized by chemical, mechanical, and electrical analysis. All the composites had good mechanical, electrical, and thermal properties and good resistance to organic solvents and mineral acids.