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)diphen yl ether-3',4'-dicarboxylic acid monomethyl ester (6), was synthesized star ting from 3,5-dimethoxyphenol. A hyperbranched polyamic acid methyl eater p recursor was prepared from self-polycondensation of the AB2 type monomer in the presence of (2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonic acid diphe nyl ester (DBOP) as a condensing agent. End-capping reactions of free amine end groups of the precursor with acetyl, n-heptanoyl chloride, and 4-methy lphthalic anhydride were carried out. By chemical imidization of these prec ursors in the presence of acetic anhydride and pyridine, hyperbranched arom atic polyimides which were soluble in NMP, DMF, and DMSO were successfully prepared. By gel permeation chromatography measurement with a laser light s cattering 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 a nalyses, it was found that both end-capping and subsequent chemical imidiza tion reactions proceeded quantitatively. The degree of branching (DB) of hy perbranched aromatic polyimides were around 0.50. By thermogravimetric (TG) and differential scanning calorimetry (DSC) measurements of the hyperbranc hed aromatic polyimides, 5 wt % thermal loss temperatures (T-5) were observ ed 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-methylphthalimid e, respectively. The densities of the transparent films of the hyperbranche d 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 f rom 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).