Direct polymerizations of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) in the presence of boronic anhydride were examined by using crystallization under non-stoichiometric condition. The polymerizations were carried out with the addition of monofunctional compounds such as an aromatic carboxylic acid and a phenol. High molecular weight poly(p-oxybenzoyl) (PUB) and poly(2-oxy-6-naphthoyl) (PUN) were obtained in the form of precipitates, and their molecular weights were much higher than those calculated according to the stoichiometric imbalance. Oligomers were formed by the direct polymerization with eliminating water in the solution, and then they were precipitated to form the crystals at the initial stage of the polymerization. Oligomers end-capped by monofunctional compounds were also formed in the solution, but the polymerization continued with the elimination of monofunctional compounds by the ester exchange reaction. The end-capped oligomers were precipitated in the middle of the polymerization, and the polymerization proceeded efficiently even in the precipitated crystals by not only the direct polymerization but also the ester exchange polymerization, resulting in the increase in molecular weight. (C) 2015 Elsevier Ltd. All rights reserved.