Polymerizations of 4-(4-acetoxybenzylideneamino)-benzoic acid were performed in dibenzyltoluene (DBT) and a mixture of DBT and liquid paraffin at 350 degrees C for 6 h. Fibrillar crystals of poly[4-(4-oxybenzylideneamino)benzoyl] (POAB) having the width of 50-450 nm and the length of over 15 pm were obtained by the crystallization during the polymerization. The fibrillar crystals possessed high crystallinity and the molecular chains aligned perpendicular to the long axis of the fibrillar crystals. Plate-like crystals were initially formed by the crystallization of oligomers, and then they changed to the fibrillar crystals via the formation of bundle-like crystals after 1 h. Molecular weight increased by the further polymerization in the crystals. Based on these results, one-pot preparation of the fibrillar POAB crystals was examined by the polymerization of 4-acetoxybenzaldehyde and 4-aminobenzoic acid. The polymerization at 180 degrees C for 2 h and then at 350 degrees C for 6 h afforded the fibrillar crystals with a small amount of the ribbon-like crystals. Although the side-reaction to generate the p-benzamide sequences was not completely depressed, the sequence of heating in which 180 degrees C for the formation of the azomethine linkage and then 350 degrees C for the formation of the ester linkage was preferable to prepare the fibrillar POAB crystals. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 127-137, 2011