Polyimides can be classified into two types: condensation polymerization and addition polymerization.
Condensation Polyimides
Condensation-type aromatic polyimides are produced by reacting aromatic diamines with aromatic dianhydrides, aromatic tetracarboxylic acids, or aromatic tetracarboxylic acid dialkyl esters. A drawback of condensation-type polyimides is that the synthesis reaction is carried out in high-boiling-point proton-sensitive solvents such as dimethylformamide and N-methylpyrrolidone. Polyimide composites are typically produced using prepreg molding processes. These high-boiling-point proton-sensitive solvents are difficult to completely evaporate during prepreg preparation. Furthermore, volatiles are released during the cyclization (imidization) of polyamic acid, which easily creates pores in the composite material, making it difficult to obtain high-quality, pore-free composites. Therefore, polyimides are rarely used as the base resin for composite materials and are mainly used to manufacture polyimide films and coatings.
Addition Polyimides
Condensation polyimides have the disadvantages mentioned above. To overcome these disadvantages, addition polyimides have been developed. Currently, the most widely used are polybismaleimide and norbornene-terminated polyimides. These resins are usually low molecular weight polyimides with unsaturated end groups, and are polymerized through the unsaturated end groups during application.
① Polybismaleimide
Polybismaleimide is formed by the condensation polymerization of maleic anhydride and aromatic diamine. Compared with polyimides, its performance is comparable, but its synthesis process is simpler, post-processing is easier, and cost is lower, making it convenient to manufacture various composite materials. However, the cured product is relatively brittle.
② Norbornene-terminated polyimide resins: Among these, the most important is a type of PMR (for in-situ polymerization of monomer reactants) developed by the NASA Alves Research Center. RMR-type polyimide resins are produced by dissolving monomers such as dialkyl esters of aromatic tetracarboxylic acids, aromatic diamines, and monoalkyl esters of 5-norbornene-2,3-dicarboxylic acids in an alcohol (e.g., methanol or ethanol), creating a solution that can be directly used to impregnate fibers.
