Effect of Ionic Liquid on Structure and Properties of Polysquaraines

Abstract

Polycondensation of 4-bis[2-(1-methylpyrrol-2-yl)vinyl]-2,5-didodecyloxybenzene and squaric acid yielded the polysquaraine SQ with two isomeric subunits-1,3-addition (zwitterionic) and 1,2-addition (diketonic) moieties in the main chain structure. The former featured a C-O/C=O infrared (IR) absorption frequency at 1622 cm(-1); the latter, a C=O signal at 1716 cm(-1). Traditional synthesis yielded SQ as a powder with metallic luster that could not be cast as a polymer film from solution. When the ionic liquid [Oct(3)NMe][TfO] (IL) was present in the cosolvent of BuOH and benzene (1:3), however, the resulting SQI(x) polymers (x = 0.01-5 wt %) did not precipitate from solution, making it possible to cast continuous free-standing films with a large area (>1 X 1 cm(2)). A greater content of IL in the solution favored the formation of the 1,3-addition zwitterionic subunits in the SQI(x) polymer main chains, thereby changing the physical and optical properties of the polysquaraine, as evidenced in IR and optical absorption spectra. The features in the UV-Vis-NIR absorption spectra of SQ and SQL(x) were dependent on the IL concentration and the nature of the solvent. Among our synthesized SQI(x) polymers, SQI(0.01) and SQI(0.1) formed flexible free-standing films with metallic luster, smooth surfaces, and good semiconductivities (2.27 x 10(-5) and 4.74 x 10(-5) S/cm, respectively). X-ray diffraction patterns revealed that the presence of IL in the polymerization medium increased the SQI(x) interchain packing distance. SQI(0.01), and SQI(0.1) possessed thermal stabilities comparable with that of SQ. Our successful use of IL in the preparation of SQI(x) polymers appears to have great potential for application in ionic liquid-related organic or polymeric preparation and processing.