As an alternative to strong acid reaction media for the Friedel-Crafts acylation for a polymer-forming reaction, a mild polyphosphoric acid (PPA) with optimized amount of phosphorous pentoxide (P2O5) has been tested for the polymerization of AB monomers 4-(2-phenoxyethoxy)benzoic acid and 3-(2-phenoxyethoxy) benzoic acid, and an AB(2) monomer 3,5-bis(2-phenoxyethoxy)benzoic acid. The reaction progress of AB2 monomer was conveniently traced by FTIR spectroscopy monitoring aromatic ketone (C=O) stretching bands arisen from carboxylic acid groups at the chain ends and carbonyl groups in the backbone as a function of reaction time at 110 CC. The resultant linear and hyperbranched polymers containing flexible oxyethylene spacers, which were prone to be hydrolyzed in strong acids at elevated temperature, displayed high intrinsic viscosities. Thus, the reaction medium PPA/P2O5 mixture as an electrophilic substitution reaction was indeed benign not to depolymerize growing polymer molecules but strong enough for the direct generation of carbonium ion from carboxylic acid to promote efficient polymerization. The resultant hyperbranched poly(etherketone) (PEK) displayed the best solubility among samples. All PEKs showed good thermal stability; glass transition temperatures were in the range of 90-117 degrees C 5% weight loss generally occurred at greater than 345 degrees C in air. (c) 2007 Wiley Periodicals, Inc.