LEWIS-ACID COORDINATION-COMPLEXES OF POLYMERS .3. POLY(BENZOBISIMIDAZOBENZOPHENANTHROLINE) LADDER AND SEMILADDER POLYMERS

The structures, solution properties and solid state properties of complexes of poly(benzobisimidazobenzo-phenanthroline) ladder (BBL) and its semiladder analogue (BBB) with the Lewis acid aluminium(III) chloride (AlCl) and gallium(III) chloride (GaCl3) are reported. At a 4:1 mole ratio of Lewis acid to polymer (BBB or BBL) repeat unit, electron donor-acceptor complexes of Lewis acid-base type, which are highly soluble in organic solvents, are formed. A detailed spectroscopic investigation shows that complexation of both polymers involves coordination of the Lewis acids to the electron-rich carbonyl oxygens and the imine nitrogens in the polymer backbones. The complexes of BBL exhibit the characteristic rigid-rod behaviour of the pristine ladder polymer. BBL complexes have rigid, rod-like chain conformations in dilute solution as evidenced by their solvent independent intrinsic viscosities. The complexes form liquid crystalline solutions above 8 wt% BBL in AlCl3/nitromethane and above 9-9.5 wt% in GaCl3/nitromethane. The solid complexes of BBL have slightly smaller optical absorption edges than BBL and similar to 50% larger absorption coefficients at lambda(max). D.s.c. revealed a glass transition (T-g) of the 4:1 GaCl3:BBL complex at 15 degrees C. Dynamic mechanical experiments indicated the T-g, measured at the loss tangent peak, was in the range 29-37 degrees C depending on frequency, with an activation energy of 448 kJ mol(-1). The complexes of BBB by contrast exhibit flexible-coil behaviour due to the polymer topology. In solution, BBB complexes have intrinsic viscosities which depend on the solvent and Lewis acid used, due to differing degrees of coil expansion in different solvent media. Coil expansion in solution leads to improved conjugation of BBB complexes as evidenced by comparison of the electronic absorption spectra with those of BBL. BBB complexes, like the pure polymer, are apparently not liquid crystalline at high concentrations in solution. The solid 4:1 GaCl3:BBB complex has a T-g at 30 degrees C, and a dynamic mechanical analysis T-g in the range 12-24 degrees C. The T-g activation energy, at 293 kJ mol(-1), is lower than that of the BBL/GaCl3 complex. The overall results of the present studies provide a basis for understanding the structure and properties of these polymers in terms of intermolecular interactions and also have implications for the processing of the polymers for diverse applications.