A series of new thermotropic siloxane backbone liquid crystalline polymers with two symmetric mesogens were prepared by hydrosilation using $Rh(PPh_3)_3Cl$ catalyst, and side chain liquid crystalline polysiloxane polymers containing two symmetric chiral center also bave been synthesized by the same method.
The number average molecular weights ($\barMn$) of the polymers which were determined by GPC were in the range of 1 000 ~ 5 000. They exhibited good solubility in common organic solvents such as tetrahydrofuran (THF) and chloroform. The structures of the monomers and polymers were confirmed by IR and $^1H$-NMR spectra. As the hydrosilation reaction proceeded, allyl proton peaks at about 4.5×6.3 ppm disappeared and a new strong peak due to the methyl protons of siloxane of the polymers appeared at 0 ppm. Thermal properties of monomers and polymers were analyzed by differential scanning calorimeter (DSC), thermogravimetric analyzer (TGA) and polarized optical microscope.
Monomers showed no liquid crystalline phase but the polymers exhibited nematic or smectic phases according to their mesogen moiety. Glass transition temperature($T_g$), melting temperature($T_m$), and isotropization temperature(Ti) of the polymers were decreased with increasing the flexible spacer length of the liquid crystal mesogen.
Introduction of a siloxane spacer into the backbone and of mesogen containing two symmetric chiral groups as side chane is an effective molecular design of chemical structure of the polymer to give a better organized mesophase of lower transition temperature.