Synthesis and properties of liquid-crystalline-conjugated polymers

We have synthesized novel liquid-crystalline (LC)-conducting polymers by introducing LC groups into acetylene monomers and polymerizing them with Ziegler-Natta, metathesis, and rhodium-based catalysts. All polymers prepared were soluble in organic solvents and had a thermotropic liquid crystallinity characterized with fan-shaped texture in polarizing optical microscopy. Phase transitions and the corresponding enthalpy changes were also evaluated by means of differential scanning calorimeter (DSC). High-order structures of the polymers were investigated by means of X-ray diffraction (XRD) analyses. XRD measurements showed that the polymers had layered structures in the LC state to give smectic A phases, which is in agreement with the results from the polarizing optical microscope. We found that the LC side chains alternate on both sides of the polyene chain, giving rise to a stereoregular sequence, such as head-head-tail-tail linkage. Macroscopic alignments of the polymers were performed in the liquid-crystalline phase by shear stress or a magnetic force field of 0.7-1.0 Tesla, which resulted in an enhancement by two orders of magnitude in the electrical conductivity of iodine-doped cast films. Orientation behaviors of the polymers as well as the monomers were investigated using fused-state C-13 NMR measurements with proton dipolar decoupling. Analysis of chemical shift tensors was also carried out to evaluate the order parameter and shielding anisotropy in the LC phase. As a result, we demonstrated that LC-conjugated polymers uniaxially aligned due to the magnetically forced alignment of the LC side-chain, giving rise to a monodomain structure.