Synthesis and characterization of alternating copolyimides containing two different diimide units

Abstract

AbstractSoluble and stable precursors of aromatic imidodiamines were prepared and employed in polymerization reactions with aromatic dianhydrides to yield poly(amic acid ester)s. Thermal imidization of the polymers produced strictly aromatic copolyimides. The reaction of pyromellitic dianhydride (PMDA, 1c), 3,3′,4,4′‐ biphenylteracarboxylic dianhydride (BPDA, 1a), or 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA, 1b) with isopropyl alcohol produced a mixture of isomeric esters. These isomer mixtures were employed in amidation reactions without further separation. Direct amidation with ethyl chloroformate was employed. The acids were reacted with ethyl chloroformate in tetrahydrofuran and the resulting solutions were added slowly to solution of excess 1,4‐ or 1,3‐phenylendediamine in tetrahydrofuran. The reaction of diamines 5a–f with dianhydrides yielded poly(amic acid ester)s. The poly(amic acid ester)s were more stable against depolymerization during storage in solution compared with the corresponding poly(amic acid)s. Alternating copolyimides 6–11 had two different diimide units derived from PMDA (1c), BPDA (1a), or BTDA (1b). In differential scanning calorimetry (DSC) analyses, alternating copolyimides 7,9 and 11 containing 1,3‐phenylenediamine units exhibited distinct and reproducible glass transition temperatures, which were a little lower than the average values of corresponding homopolyimides and those of random copolyimides. But alternating copolyimides 6, 8, and 10, which contain 1,4‐phenylenediamine units, showed very weak glass transitions.