Selective preparation of poly(p-oxybenzoyl) (POB) in the copolymerization system of p-acetoxybenzoic acid (p-ABA) and m-acetoxybenzoic acid (m-ABA) was examined by using reaction-induced crystallization of oligomers. Polymer crystals mainly composed of p-oxybenzoyl moiety were precipitated when. the content of m-ABA in the feed was 30 mol %. The formation of the polymer crystals was attributed to both the reactivity of monomer and the phase-separation behavior of oligomer. Reactivity of p-ABA was twice higher than that of m-ABA, and thereby, the homo-oligomers of p-oxybenzoyl moiety were more rapidly formed in solution than do co-oligomers at the early stage in polymerization. They were selectively precipitated by crystallization to form crystals because of low miscibility. Co-oligomers containing m-oxybenzoyl moiety were also formed in solution, but they were unable to be phase-separated because of higher miscibility. Further polycondensation occurred between oligomers in the precipitated crystals, leading to the formation of POB. This polymerization proceeded with selecting certain monomers by crystallization and afforded a new methodology for fractional polycondensation. (c) 2006 Wiley Periodicals, Inc.