Synthesis and characterization of liquid crystalline (norbornene-block-acetylene) block copolymers

We report the synthesis and characterization of block copolymers of mesogenic acetylene and norbornene moieties. As monomers, a norbornene derivative with cyanostilbenyl side groups and an acetylene derivative with a cholesteryl group were used. The almost quantitative polymerization is initiated with the well-defined Schrock-type molybdenum carbene Mo(CH-t-Bu)(NAr)(O-t-Bu)(2) with NAr = 2,6-diisopropylaniline, t-Bu = tert-butyl. Polydispersity indices (PDIs) in the range of 2 for the block copolymers are obtained. The thermal and morphological behaviour of the polymers is characterized by means of differential scanning calorimetry (DSC) and X-ray diffraction. Both homo- and copolymers form thermotropic mesophases, the acetylene polymer exhibiting a smectic A phase, while the polynorbornene is nematic. The block copolymer shows microphase separation retaining the homopolymers' mesophases. To our knowledge, the materials presented here are the first example of phase-separated block copolymers of two side chain liquid crystalline polymers.