Organo-soluble polyimides: Synthesis and polymerization of 2,2'-disubstituted-4,4',5,5'-biphenyltetracarboxylic dianhydrides

The objective of this research was to investigate the effects of introducing pendant groups in the 2- and 2'-positions of 4,4/,5,5'-biphenyltetracarboxylic dianhydrides on the properties of polyimides produced from the dianhydrides. Thus, the work was begun with the syntheses of 2,2'-dibromo-4,4',5,5'-biphenyltetracarboxylic dianhydride (DBBPDA) and 2,2/-diphenyl-4,4',5,5'-biphenyltetracarboxylic dianhydride (DPBPDA). The two new dianhydrides were polymerized with several substituted 4,4'-diaminobiphenyls including 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB) in refluxing m-cresol containing isoquinoline to afford two new series of soluble polyimides. The polyimides had intrinsic viscosities that ranged from 1.80 to 5.58 dl g(-1) (N-methyl-2-pyrrolidinone or m-cresol at 30 degrees C). Their glass transition temperatures (T(g)s) ranged from 322 to 351 degrees C (thermal mechanical analysis). Several of the polymers obtained from the new dianhydrides were soluble in acetone and tetrahydrofuran. The polymers formed water-white, tough films that were transparent above 350 nm. The films displayed negative birefringence, i.e. their in-plane refractive indices (n(parallel to)) were higher than their out-of-plane refractive indices (n(perpendicular to) ). For example, the DBBPDA/PFMB polymer, which was soluble in acetone and had a T-g of 330 degrees C, formed films with n(parallel to) = 1.637 and n(perpendicular to) = 1.564. The films had coefficients of thermal expansion that ranged from 1.05 x 10(-5) to 2.12 x 10(-5)degrees C-1. The films may be useful as retardation layers in liquid crystal displays. Copyright (C) 1996 Elsevier Science Ltd.