Fire‐ and heat‐resistant adhesive resins based on maleimido and citraconimido substituted 1‐[(dialkoxyphosphinyl)methyl]‐2,4‐ and ‐2,6‐diaminobenzenes

Abstract

A novel class of fire‐ and heat‐resistant bisimide resins was prepared by thermal polymerization of maleimido or citraconimido derivatives of 1‐[(dialkoxyphosphinyl)methyl]‐2,4‐ and ‐2,6‐diaminobenzenes (1). The neat bisimide resin prepared by curing 1‐[di(2‐chloroethoxyphosphinyl)methyl]‐2,4‐ and ‐2,6‐bismaleimidobenzene exhibited a limiting oxygen index 75% higher and smoke evolution about 30 times lower compared with the parent polymer obtained by curing m‐phenylenebismaleimide. The char yield of cured bisimide resins at 700°C was 58–70% in a nitrogen atmosphere and 35–60% in air. An increase in formula weight between the imide groups slightly reduced the char yield. The polymer precursors were synthesized by reacting the phosphorus‐containing diamines (1) (1 mol) with maleic anhydride/citraconic anhydride (2 mol) or by reacting the monomaleimido derivative of (1) with benzophenone tetracarboxylic dianhydride/methylenebis(4‐phenylisocyanate) in a 2:1 mole ratio. The monomers were characterized by elemental analysis, Fourier‐transform–infrared (FT‐IR), proton nuclear magnetic resonance (¹H‐NMR) spectroscopy, and gas chromatography–mass spectroscopy (GC‐MS). Direct cleavage of the PC bond and inversion of the synthesis reaction may occur during their pyrolysis. The thermal polymerization of the monomers was investigated by differential scanning calorimetry (DSC). Biscitraconimides are thermally polymerized at a relatively lower temperature than the corresponding bismaleimides.