Synthesis of phenylquinoxaline oligomers containing pendant electron-donating and electron-withdrawing groups

Abstract

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 low-molecular-weight oligomers. They were completely soluble and amorphous, with glass-transition temperatures varying from 165 to 266°C, and they had thermooxidative stability, with samples displaying 5% weight loss temperatures of 419-511°C in nitrogen. The thermal properties of the monomers and resultant polymers dramatically depended on the polarity of the substituants. The monomers and resultant oligomers displayed high fluorescence in tetrahydrofuran solutions and N-methyl-2-pyrrolidinone solutions, respectively.