Organo-soluble polyimides: synthesis and polymerization of 2,2′-disubstituted-4,4′,5,5′-biphenyltetracarboxylic dianhydrides

Abstract

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 tetracarboxylic 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-rmcresol at 30°C). Their glass transition temperatures ( T gs) ranged from 322 to 351°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 ∥) were higher than their out-of-plane refractive indices ( n ⊥). For example, the DBBPDA/PFMB polymer, which was soluble in acetone and had a T g of 330°C, formed films with n ∥ = 1.637 and n ⊥ = 1.564. The films had coefficients of thermal expansion that ranged from 1.05 × 10 −5 to 2.12 × 10 −5°C −1. The films may be useful as retardation layers in liquid crystal displays.