Synthesis and Properties of Soluble 3,3′,4,4′-Benzophenonetetracarboxylic Dianhydride Copolyimides Based on 1,1-Bis[4-(4-aminophenoxy)phenyl]-1-phenylethane and Commercial Dianhydrides

1,1-Bis[4-(4-aminophenoxy)phenyl]-1-phenylethane (BAPPE) is a useful monomer for preparing soluble polyimides, but polyimide synthesized from BAPPE and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) shows poor solubility. Insoluble BTDA/BAPPE polyimide can copolymerize with soluble polyimides at certain molar ratios (m2/m1) to produce arbitrarily soluble copolyimides. To prepare soluble copolyimides, the smallest m2/m1 ratio changes due to the matching dianhydrides, depending on the structure of the dianhydride component. When polymers contain the same m2/m1 ratio, copolyimides formed through thermal imidization show poorer solubility than these formed through chemical imidization, and copolyimide cyclized at 250 °C has a better solubility than that cyclized at 300 °C; this is due to the formation of intermolecular linkages during the high-temperature thermal cyclization. Additional, these copolyimides are characterized by good mechanical properties together with good thermal stability, and a comparative study on the properties of the homopolyimide and copolyimide are also presented.