Polymerization of 4-acetoxybenzoic acid (ABA) with 3,5-diacetoxybenzoic acid (DABA) was examined to control the morphology of poly(p-oxybenzoyl) (POB). Polymerizations were carried out at a concentration of 1.0% in an aromatic solvent Therm S-1000 (R) (mixture of dibenzyltoluene) at 320 degrees C. Polymerization of ABA yielded the POB fibrillar crystals, but the polymerization with DABA at a concentration in the feed (chi(f)) of 0.10-0.15 afforded novel network structures comprised of spheres connected by fibrillar crystals. The diameter of the spheres prepared at chi(f) of 0. 15, which were 0. 7 and 5.0 mu m, showed bimodality. The network distance, fibril length, and fibril width were 6.1, 2.6, and 0.1 mu m, respectively. They possessed high crystallinity. The network structure was formed as follows. Co-oligomers were first precipitated in the beginning of the polymerization by liquid-liquid phase separation to form the microdroplets. The fibrillar crystals were formed in the coalesced spheres by the crystallization of oligomers induced by the increase of molecular weight. The fibrillar crystals connecting the spheres gradually appeared owing to the shrinkage of the spheres. The fibrillar crystals grew from the surface of the spheres with the crystallization of homo-oligomers of 4-oxybenzoyl units, and finally the network structure was completed. (c) 2005 Wiley Periodicals, Inc.