Polyimide is composed of tetracid Dianhydride and diamine in polymerization，The synthesis of the main One step method、two step method、three step method and Gas phase precipitation method.
(1)One step method. One step method is Dianhydride and diamine directly polymerized polyimide in high boiling point solvent,Monomer Not by polyamic acid Direct synthesis of poly imide.The reaction conditions of this method was much more moderate than heat treatment,The key is to select a suitable solvent.As the relative molecular mass of polymer increased, should try to remove moisture.Usually with water to remove azeotropic water generated,Or the polyamic acid salt with isocyanate to replace diamine and generated under high temperature and high pressure polymerization.the control process of this method are yet to be perfected, and the practical step forward.
(2)Two step method. The two step method is the Dianhydride and diamine obtained polyamic acid precursor,Then by heating or chemical methods,Intramolecular cyclodehydration of polyimide forming.Chemical imidization method,The dehydrating agent polyamide acid,Polyimide chemical cyclization generated contains a large number of different imides.The prepared polyimide and polyimide prepared heating method,The physical and chemical properties are different,Especially the isoimide rings has low thermal stability and high chemical reactivity. different dehydration agent,Different cyclization product imine / isoimide ratio,Can be thought of as tautomeric highly unstable caused.The two step process is mature, but the polyamic acid solution is not stable, it’s very sensitive to moisture, decomposition occurs during storage, so appeared polyamic acid alkyl ester method, polyamide acid silyl ester method improvement method.Another precursor of polyimide polyamide ester, is a relatively stable polymer, solid or solution to form long-term storage.High relative molecular mass of polyamide ester is usually composed of aromatic diacid diester acyl chloride, solution polycondensation or condensation polymerization was prepared with aromatic diamine.Polyamide ester heat or imidized polyimide occurred in the reaction catalyzed by organic bases, but the small molecular compound off alcohol or alpha olefin rather than water.Soluble polyamide ester intermediates in the polyamic acid, soluble in common organic solvents with low boiling point, such as dichloromethane, four oxygen furan, and can obtain high concentration solution, and can be changed by the ester structure makes the polyamide ester with different properties, and can be used for preparing high strength and high modulus materials, is a typical method for the synthesis of polyimide. But the imidization reaction activity and low, complex process, high manufacturing cost, further optimization.
Three step method. The three step method is polyisoimide obtain polyimide method via.Polyisoimide stable structure, as the first polyimide matrix, because the heat treatment does not emit water and other low molecular substances, easily heterogeneous into imide, and can make excellent performance of polyimide.Polyisoimide is composed of polyamide acid in dehydration agent, dehydration and cyclization and into, and then in the role of acid or alkali isomerization catalysts such as polyimide, the isomerization reaction easily at a high temperature.Polyisoimide