Copolymerizations of p-dichlorobenzene (DCB)/4-bromophenyl ether (BPE), DCB/4,4'-dibromobiphenyl (DBB), and DBB/BPE pairs with sodium sulfide under high temperature (270-290 degrees C) utilizing N-methyl-2-pyrrolidinone (NMP) as solvent were carried out to give C(DCB/BPE), C(DCB/DBB), and C(DBB/BPE) copolymers, respectively. The reactivity of dihaloaromatic monomers toward thiolate anion in the polycondensation reaction followed the order DBB > DCB > BPE. The reactivity gap between DBB and DCB toward thiolate anion seemed to be smaller than that between BPE and DCB, resulting in both high yield and high molecular weight in the C(DCB/DBB) copolymers compared to C(DCB/BPE) copolymers. The copolymerization of DBB/BPE pair with sodium sulfide, which has larger reactivity gap than the DCB/ DBB or DCB/BPE pair, gave mixtures of PBS and PPSE homopolymers especially in the range of 50-80 mol % BPE in the feed. The C(DCB/DBB) and C(DCB/BPE) copolymers, however, exhibited random copolymer character in all comonomer ratios in the feed as evidenced by copolymer composition and DSC data.