Polyimide synthesis, microwave-assisted

Imai Y (1996) A new facile and rapid synthesis of polyamides and polyimides by microwave-assisted polycondensation. In Hedrick JL, Labadie JW (eds) Step-growth polymers for high-performance materials. Am Chem Soc Washington, p421... 

A New Facile and Rapid Synthesis of Polyamides and Polyimides by Microwave-Assisted Polycondensation... 

In a typical experiment, a monomer or its salt (2 g) in a polar high boiling solvent (1-2 ml) was irradiated under nitrogen atmosphere. The microwave assisted polycondensation proceeded rapidly and was completed within 5 min (inherent viscosity around 0.5 dL/g) for the polyamide synthesis [68] and within 2 min for the polyimides (inherent viscosity above 0.5 dL/g) [69]. The rate of polycondensation of the salt monomers under various conditions decreased in the following series the microwave induced polycondensation>solid-state thermal polymeriza-tion>high-pressure thermal polycondensation [71]. 

Step-Growth Polymerization Microwave-assisted step-growth polymerizations -that is, polycondensation and polyaddition reactions - have been studied extensively. According to a review by Wiesbrock et ol. [10], a plethora of reports has been devoted to the microwave-assisted synthesis of polyamides, polyimides, polyethers, and polyesters. For the majority of polymerizations, the reaction rates were significantly increased under microwave irradiation as compared to conventional heating, whilst in many cases the reaction times were shortened from hours, and sometimes days, to about 10 minutes. Moreover, the product purity was improved and the polymers exhibited superior properties, most likely due to a reduction in... 

A new facile method for the rapid synthesis of aliphatic polyamides and polyimides was developed by using a domestic microwave oven to facilitate the polycondensation of both w-amino acids and nylon salts as well as of the salt monomers composed of aliphatic diamines and pyromellitic acid or its diethyl ester in the presence of a small amount of a polar organic medium. Suitable organic media for the polyamide synthesis were tetramethylene sulfone, amide-type solvents such as A -cyclohexyl-2-pyrrolidone (CHP) and 13-dimethyl-2-imidazolidone (DMI), and phenolic solvents like m-cresol and c)-chlorophenol, and for the polyimide synthesis amide-type solvents such as A-methyl-2-pyrrolidone, CHP, and DMI. In the case of the polyamide synthesis, the polycondensation was almost complete within 5 min, producing a series of polyamides with inherent viscosities around 0.5 dL/g, whereas the polyimides having the viscosity values above 0.5 dL/g were obtained quite rapidly by the microwave-assisted polycondensation for only 2 min. 

Polyimide Synthesis from Nylon-Salt-Type Monomers. The microwave-assisted polycondensation was further applicable to the effective, rapid synthesis of aliphatic polypyromellitimides from the salt monomers XPMA and XPME composed of aliphatic diamines and both pyromellitic acid PMA and its diethyl ester PME (Eqs 3 and 4). 

The solvent effect on the microwave-assisted polycondensation of two salt monomers 12PMA and 12PME is summarized in Table 3. The best polymerization medium for this polycondensation was DMI, which gave the polyimides with inherent viscosities about 0.7 dL/g or higher. Other amide-type solvents such as NMP and CHP were also effective for producing the polyimides with high viscosity values. Sulfolane was the next suitable solvent. However, m-cresol, which was found to be one of the best solvent for the microwave-assisted polyamide synthesis, was inadequite for the polyimide synthesis because of lower solubility of these pyromellitic acid-based salt monomers. 

Table 4. Microwave-Assisted Synthesis of A Series of Polyimides from Nylon-Salt-Type Monomers ...

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