Polyimide is one of organic polymer materials with the excellent comprehensive performance, and has the advantages of high-temperature resistance up to 400° C. above, long-term application ranging from −200° C. to 300° C., high insulation performance and the like, which is used widely in aerospace, aviation, microelectronics, nanotechnology, liquid crystals, separating membranes, laser and other fields.
In the prior art, polyimide is generally synthesized using dianhydride and diamine as raw materials, firstly by performing low-temperature polycondensation on dianhydride and diamine in an aprotic polar solvent such as N,N-dimethyl acetamide, N,N-dimethyl formamide, N-methyl pyrrolidone, to obtain soluble polyamic acid; using the polyamic acid solution to form a film or spinning the polyamic acid solution, followed by heating to about 300° C. to obtain a polyimide film or polyimide fibers by cyclodehydration; or adding acetic anhydride and a tertiary amine catalyst to the polyamic acid solution to obtain a polyimide solution or a polyimide powder by cyclodehydration. For example, U.S. Pat. No. 3,179,631 and Polym. Adv. Tech, 1998, 9: 202 both disclose a method of polymerizing a dianhydride with a diamine in N,N-dimethyl acetamide, N,N-dimethyl formamide or N-methyl pyrrolidone to obtain a polyamic acid solution, followed by adding a dehydrating agent acetic anhydride and a tertiary amine catalyst to obtain polyimide by cyclization; U.S. Pat. No. 3,249,588 and Polym. Adv. Tech, 1998,9: 202 also discloses a method of polymerizing a dianhydride with a diamine in N,N-dimethyl acetamide, N,N-dimethyl formamide or N-methyl pyrrolidone to obtain a polyamic acid solution, followed by directly heating the polyamic acid solution or adding triethylamine with heating to obtain polyimide by cyclization; in addition, U.S. Pat. No. 5,378,420 and Polym. Adv. Tech, 1993, 9: 202 also discloses a method of synthesizing polyimide directly from a tetracarboxylic acid or a dianhydride and a diamine at a high temperature with phenols as a solvent. However, when the polyimide powder is prepared using the above methods, complicated post-treatments such as multiple washing etc. are needed to obtain a polyimide powder with high purity; when the resulting polyimide is soluble in a solvent, it is necessary to carry out solvent removal, grinding and other steps to obtain the polyimide powder, which is not only cumbersome and costly, but also toxic because of the solvent involved, which is not favorable for the polyimide production. High Perf. Polym., 1999,11: 205; High Perf. Polym., 2003,15: 269; High Perf. Polym., 2006, 18: 31 and others also discloses a polyimide synthesis using water as a medium, but this method need to be performed under pressure, and the molecular weight of the resulting polyimide is lower.