Poly(2-oxy-6-naphthoyl) (PON) was obtained as precipitates by direct polymerization of 2-hydroxy-6-naphthoic acid (HNA) in the presence of boronic anhydrides such as 2,4,6-tris(3,4,5-trifluorophenyl)-1,3,5,2,4,6-trioxatriborinane (TFBA) and 2,4,6-tri(biphenyl-4-yl)-1,3,5,2,4,6-trioxatriborinane (BPBA). The polymerizations were carried out in an aromatic solvent at 300 degrees C for 24 h. Both morphology and molecular weight of the PON precipitates were considerably influenced by not only the structure of the boronic anhydride but also its concentration (c(B)). Spheres with needles on their surface were formed in the polymerization with TFBA at c(B) of 50-70 mol%, and those having smooth surface were obtained at c(B) of 100 mol%. The M(n) increased with the value of c(B) in the range from 2.7 x 10(3) to 9.0 x 10(3). Aggregates of cone-like crystals were prepared in the polymerization with BPBA at c(B) of 10-30 mol%, and spheres having rugged surface were formed at c(B) of 50-100 mol%. The M(n) also increased with the value of c(B) in the range from 5.4 x 10(3) to 12.9 x 10(3). These PON precipitates possessed high crystallinity in spite of the morphology. The copolymerization of HNA and 4-hydroxybenzoic acid (HBA) was also examined in the presence of BPBA at c(B) of 100 mol% with varying the content of HBA in feed (r(f)). Copolymers were obtained as spheres of which the content of 4-oxybenzoyl moiety was lower than the value of r(f) due to the difference in the solubility of the oligomers.