Synthesis, structure and properties of TAP-PMDA hyperbranched polyimides with different terminated groups

Three kinds of hyperbranched polyimides with different terminated groups were prepared from A2+B′B2 monomers based on pyromellitic anhydride (PMDA) and 2,4,6-triaminopyrimidine (TAP) via a two-step polymerization, with molar ratios of TAP: PMDA being 1:2, 2:3 and 1:1. The amino group still existed in all of the products indicated by FTIR and 1H NMR. The absolute molecular weights were measured by SEC/MALLS, which suggested the polyimides with the higher molecular weight would be obtained when the molar ratio of amine groups to anhydride groups of the monomers was 1:1. The degree of branching of polymers indicated by 1H NMR was 62%, 49% and 46% respectively, which suggested that the anhydride-terminated polyimides have better branched structures than the amine-terminated ones. The d-spacing values increased from 4.5495 to 5.0696 Å when the molar ratios of PMDA:TAP increase from 1:1 to 2:1. The intermolecular packing in anhydride-terminated hyperbranched polyimides was more incompact than the amine-terminated ones. The inherent viscosities and thermal properties were measured. Glass transition temperature and 5% weight loss temperature of the obtained hyperbranched polyimides increased with DB decreasing.