Syhthesis Of Isomeric BiphenyltetracarboxylicDianhydrides

Biphenyl type polyimide(PI) derived from biphenyltetracarboxylic dianhydrides(BPDAs) is the most important and representative varieties of PI. There three isomeric of BPDAs: 2,2’,3,3’-biphenyltetracarboxylic dianhydride(3,3’-BPDA), 2,3,3’,4’-biphenyltetracarboxylic dianhydride(3,4’-BPDA), 3,3’,4,4’-biphenyltetracarboxylic dianhydride(4,4’-BPDA). Althrough there are many different synthetic routes to BPDAs, the methods suffer from disadvantages of harsh conditions, and the starting materials are unavailable. The cost of the synthesis of BPDAs was still high. So it is important to develop moderate synthetic methods for the preparation of these isomers with low cost, high purity and high yield.To address these limitations, we set out a new approach synthesis of BPDAs by three steps. Firstly, Grignard reagents was synthesized by using cholo-o-xylene with Mg and 2-THF as the solvent, the optimum conditions for this reaction were obtained as follow: n(cholo-o-xylene): n(Mg): n(2-THF)=2: 1: 2, the optimum reaction temperature was 95℃, and the yield of Grignard reagents was more than 99%. The couping of Grignard reagents with cholo-o-xylene affords tetramethyl biphenyl isomers, employed triphenylphospgine-nickel chloride complex as the catalyst, and cholo-o-xylene as the solvent. The optimum conditions for this reaction were obtained as follow: n(cholo-o-xylene): n(Grignard reagents): n(Ni)=4: 1: 0.001, 110℃, and the yield of the product was more than 86%, 3,3’-: 3,4’-: 4,4’-tetramethyl biphenyl=22.8: 50.8: 26.5. Then themixture was separated by rectification, the GC tests showd that each product has a purity of greater than 98%, and the boiling points are for 3,3’-tetramethyl biphenyl 268℃, for 3,4’- tetramethyl biphenyl 288℃, and for 4,4’-tetramethyl biphenyl >310℃, the structures of the products were characterized 1H NMR and 13 C NMR respectively. And at last, the synthesis of biphenyl tetracarboxylic acid by liquid-phase axidation of tetramethyl biphenyl was studied, the conditions for this reaction were obtained as follow: n(Co): n(Mn): n(Br): n(tetramethyl biphenyl): n(HOAc)=2.1 : 1.3: 4.4: 75: 1080, the optimum reaction temperature was 200℃, the reaction pressure was 1 MPa, and the yield of the target product was more than 92%. The biphenyl tetracarboxylic acid afforded BPDA by sublimation, the melting points are for 3,3’-BPDA 268-270℃, for 3,4’-BPDA 194-196℃, and for 4,4’-BPDA 298-300℃.Two PIs were prepare from 3,4’-BPDA and 4,4’-BPDA with 4,4’-oxydianiline through the conventional two step process. The solubility of PIs and the viscisity of the polyimide acid and PIs were investigated. PIs thermal property were analyzed by DSC and TGA, PIs exhibited excellent thermal stability with the Tg for 3,4’-BPDA/ODA-PI 307℃, and Td5 545.5℃; Tg for 4,4’-BPDA/ODA-PI 275℃, and Td5 566.3℃. The structures of 3,4’-BPDA/ODA-PI was characterized by FT-IR respectively.