Selective preparation of poly(p-oxybenzoyl) (POB) crystals was examined from the viewpoint of a dimer effect on fractional polycondensation. Four different copolymerization systems were chosen as the combinations of p-acetoxybenzoic acid (p-ABA), m-acetoxybenzoic acid (m-ABA), and their dimers. The crystals obtained from the copolymerization of the dimer of p-ABA (p-ABAD) and m-ABA contained only 3.1 mol % of m-oxybenzoyl moiety even at high content of m-oxybenzoyl moiety in feed (chi(f)) of 40 mol %. p-Oxybenzoyl homo-oligomers were more rapidly formed from p-ABAD in the solution than from p-ABA, and they were crystallized to form the crystals with segregating co-oligomers. While co-oligomers containing more m-oxybenzoyl moiety were formed in the solution, afterward they were unable to be phase-separated because of higher miscibility. The further polycondensation proceeded in the precipitated crystal, and finally the POB crystal was selectively formed. Lower polymerization temperature and concentration enhanced the fractionability, and the POB crystals containing less than 1 mol % m-oxybenzoyl moiety were prepared at chi(f) of 30 mol %, 270 degrees C, and a concentration of 0.5%. The dimer effect on the fractional polycondensation was clearly observed. (C) 2008 Wiley Periodicals, Inc.