Polyimide is typically prepared by the polycondensation reaction of dianhydrides and diamines. The synthesis methods, depending on the reaction pathway and processing method, mainly fall into two categories: two-step and one-step methods.
Two-Step Method (Polyamic Acid Method)
The two-step method is currently the most widely used preparation method. First, in a polar solvent, an aromatic dianhydride reacts with a diamine to generate a soluble polyamic acid (PAA) precursor. Subsequently, through heat treatment or chemical dehydration, the polyamic acid undergoes a cyclization reaction (iminolation), transforming into an insoluble and infusible polyimide material. This method has advantages such as mild reaction conditions and ease of processing into films and coatings, making it the main route for the industrial production of polyimide films and coating materials.
One-Step Method (Direct Polycondensation Method)
The one-step method involves the direct polycondensation reaction of dianhydrides and diamines under high-boiling-point solvents or specific reaction conditions, simultaneously completing the cyclization to generate polyimide. This method has a relatively simple process flow, but it requires high precision in controlling reaction conditions and is typically used for the preparation of specific types of polyimides.
Copolymerization and Structural Regulation
In practical applications, to improve the processing performance, flexibility, or electrical properties of polyimides, different structural monomers are often introduced through copolymerization reactions to adjust the rigidity, flexibility, and polarity of the molecular chains. Furthermore, modification can be carried out during or after polymerization through methods such as filling and blending to obtain functionalized polyimide materials with specific properties.
