Abstract
The novel AB2 and A2B type monomers for hyperbranched polybenzoxazoles (HBPBOs) containing one (or two) hydroxy amine and two (or one) carboxylic acid groups in the aromatic backbones were prepared starting from 5-methoxyresorcinol and 4-fluorobenzonitrile for AB2 and 3,5-dihydroxybenzoic acid and 3-fluoro-6-nitrophenol for A2B monomer. Hyperbranched poly(o-hydroxyamide)s (HBPHAs) as precursors of HBPBOs were synthesized by self-polycondensation of the ABx type monomers in the presence of (2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonic acid diphenyl ester (DBOP) as a condensing agent at room temperature. The terminal groups of free carboxylic acid or amine were chemically modified with 2-amino-4-tert-butylphenol or 3,5-dimethylbenzoic acid. End-capped HBPHAs had good solubility in aprotic solvents and tetrahydrofuran (THF) having the weight average molecular weight (Mw) in the range of 27000–131000. The conversion of HBPHA to HBPBO was carried out by thermal and chemical cyclization methods. HBPBOs prepared by chemical cyclization of HBPHAs in polyphosphoric acid (PPA) were soluble in aprotic solvents such as N-methyl-2-pyrrolidinone (NMP), N,N-Dimethylacetamide (DMAc), Dimethyl Formamine (DMF), and THF, whereas thermally cyclized ones were insoluble in any organic solvents. 1H NMR and IR analyses indicated that the chemical cyclization proceeded quantitatively. The weight-average molecular weights (Mw) of soluble HBPBOs were found to be 23000–97000 by gel permeation chromatography (GPC) measurement with standard polystyrene calibration. Both of HBPBOs prepared by thermal and chemical cyclization methods showed excellent thermal stability having the 10wt% weight loss (T10) in the range of 488–537°C under nitrogen.
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