Carboxylic Acid-Terminated Hyperbranched Polybenzoxazole and Its Polyarm-Star Block Copolymers

A new AB2 monomer, 5-amino-4-hydroxyisophthalic acid hydrochloride, was synthesized and polymerized in poly(phosphoric acid) (PPA) to afford a carboxylic acid-terminated hyperbranched polybenzoxazole (HPBO). Taking advantage of the large number of peripheral carboxylic acid groups on HPBO, polyarm-star block copolymers, HPBO-b-mPEK and HPBO-b-pPEK, were conveniently prepared in the same reaction medium with additional amounts of phosphorus pentoxide (P2O5) via “direct” Friedel−Crafts acylation reaction with 3- and 4-phenoxybenzoic acids as AB monomers. All the resulting polymers displayed polyelectrolyte behaviors in solution due to the large number of carboxylic acid termini. The UV−vis absorption intensities, which increased as polymer concentration was increased, showed quasi-linear dependence in all sample solutions. The emission intensity drastically decreased as the concentration of HPBO core polymer was increased, while the block copolymers were not much affected by concentration variation. The emission behaviors of HPBO-b-mPEK and HPBO-b-pPEK should be inherent to the chromophoric inner HPBO cores, which were shielded by the outer linear PEK shells. The morphology study suggested that both proton conductivity and optical behaviors of HPBO might be greatly influenced by the globular core−shell architecture.