A series of extended 6-substituted quinoxaline AB monomer mixtures, 2(4-fluorophenyl)-3-[4-(4-hydroxyphenoxy)phenyl]-6-substituted quinoxaline and 3-(4-fluorophenyl)-2-[4-(4-hydroxyphenoxy)phenyl]-6-substituted quinoxaline, were prepared and polymerized to afford phenylquinoxaline oligomers. High-molecular-weight polymers could not be obtained because of the formation of cyclic oligomers. On the basis of matrix-assisted laser desorption/ionization time-of-flight analysis and molecular modeling results, the formation of a cyclic dimer could be a favorable process resulting in lowmolecular-weight ohgomers. They were completely soluble and amorphous, with glasstransition temperatures varying from 165 to 266 degrees C, and they had thermooxidative stability, with samples displaying 5% weight loss temperatures of 419-511 degrees C in nitrogen. The thermal properties of the monomers and resultant polymers dramatically depended on the polarity of the substituents. The monomers and resultant oligomers displayed high fluorescence in tetrahydrofuran solutions and N-methyl-2-pyrrolidinone solutions, respectively. (c) 2005 Wiley Periodicals, Inc.