Hyperbranched polyimides for gas separation applications. 1. Synthesis and characterization

A series of wholly aromatic hyperbranched polyimides were successfully prepared by condensation polymerization of a triamine monomer, tris(4-aminophenyl)amine (TAPA), and a series of commercially available dianhydride monomers such as 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA), and pyromellitic anhydride (PMDA). The synthesis involves the formation of hyperbranched polyamic acid precursors in the first step and the thermal or chemical imidization in the subsequent step, which is similar to the conventional two-step method for the synthesis of linear polyimides. Different monomer addition orders resulted in different hyperbranched polymers even if the same kinds of monomers but different molar ratios were used. When a dianhydride solution was added to the TAPA solution (manner 1), an amine-terminated hyperbranched polyimide was obtained, whereas the addition of TAPA solution to a dianhydride solution (manner 2) yielded an anhydride-terminated hyperbranched polyimide. The appropriate molar ratios of TAPA to a dianhydride are 1:1 for manner 1 and 1:2 for manner 2. From H-1 NMR spectra the branching degrees of the amine-terminated hyperbranched polyimides were estimated to be 0.64, 0.72, and 0.68 for 6FDA-TAPA, DSDA-TAPA, and PMDA-TAPA, respectively. All the anhydride-terminated polyimides were found wholly branched. Gel permeation chromatography (GPC) measurement revealed that both the amine-terminated and anhydride-terminated 6FDA-TAPA polyimides had moderate number-averaged molecular weights, but the latter had very broad molecular weight distribution.