Preparation and properties of hyperbranched aromatic polyimides via polyamic acid methyl ester precursors

An AB(2) type monomer, an isomeric mixture of 3,5-bis(4-aminophenoxy)diphenyl ether-3',4'-dicarboxylic acid monomethyl ester (6), was synthesized starting from 3,5-dimethoxyphenol. A hyperbranched polyamic acid methyl ester precursor was prepared from self-polycondensation of the AB2 type monomer in the presence of (2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonic acid diphenyl ester (DBOP) as a condensing agent. End-capping reactions of free amine end groups of the precursor with acetyl, n-heptanoyl chloride, and 4-methylphthalic anhydride were carried out. By chemical imidization of these precursors in the presence of acetic anhydride and pyridine, hyperbranched aromatic polyimides which were soluble in NMP, DMF, and DMSO were successfully prepared. By gel permeation chromatography measurement with a laser light scattering detector, weight-average molecular weights (M-w) of hyperbranched aromatic polyimides were found to be 37 000-188 000. From IR and H-1 NMR analyses, it was found that both end-capping and subsequent chemical imidization reactions proceeded quantitatively. The degree of branching (DB) of hyperbranched aromatic polyimides were around 0.50. By thermogravimetric (TG) and differential scanning calorimetry (DSC) measurements of the hyperbranched aromatic polyimides, 5 wt % thermal loss temperatures (T-5) were observed to be above 395 degrees C, and T-g values were 189, 138, and 186 degrees C for the end groups of acetoamide, n-heptanoamide, and 4-methylphthalimide, respectively. The densities of the transparent films of the hyperbranched polyimides having acetoamide end groups were found to be less than 1.35, which were lower than those of their linear analogues. The film, prepared from the hyperbranched polyimide with 4-methylphthalimide end groups, had a lower dielectric constant and birefringence and a shorter cutoff wavelength than the film of poly(4,4'oxydiphenylene pyromellitimide).