Poly(2,5-dialkoxy-p-phenylenebutadiynylene)s -: Novel rigid conjugated polymers with liquid crystalline property

Novel rigid conjugated poly(p-phenylenebutadiynylene)s having long alkoxy side chains, octyloxy (C8), decyloxy (C10), and dodecyloxy (C12) at 2,5-positions of a phenylene unit, are synthesized by oxidative polycondensation of corresponding 2,5-dialkoxy-1,4-diethynylbenzenes with Hay catalyst. The polymers are soluble in hot solvents such as CHCl3 and fusible upon heating. UV-Vis spectra of the polymers show an absorption maximum (lambda(max)) around 420 nm due to the pi-conjugation along the main chain. An intense fluorescence exhibiting the lambda(max) around 475 nm is observed in all of the polymers. Cyclic voltammetry of the polymer shows two Li ion-participated redox reactions occurred at the dialkoxyphenylene moiety and in the conjugated polymer chain due to an n-doping process. Electrical conductivity is an order of 10(-8) S cm(-1) which increases to an order of 10(-4) S cm(-1) upon vapor phase doping by H2SO4. Thermochemical investigations by DSC and polarized optical microscopy suggest that the polymers have a nematic Liquid crystalline phase. The molecular arrangements in the phase is evaluated by XRD and a molecular dynamics simulation. A nematic phase is shown to arise from a side-by-side accretion of the rigid rodlike main chains. A lamellar structure is suggested in the crystalline phase. (C) 1999 Elsevier Science S.A. All rights reserved.