Novel nematic bis-1,5-hexadiene monomers: Synthesis and photopolymerization to cholesteric polymer networks

The synthesis of two novel nematic monomers 1a and 1b with two polymerizabl e 1,5-hexadiene groups is described. Their photopolymerization in the nemat ic mesophase has been investigated by photo-DSC-measurements and IR-spectro scopy. The hexadienes photopolymerize within 30 min to a final conversion o f 87% and thus stower than the corresponding acrylates. Mixtures of hexadie nes and acrylates copolymerize and photopolymerization is finished after tw o minutes of UV-irradiation. By mixing a nematic bis-1,5-hexadiene compound with a cholesterol bisacrylate, enantiotropic cholesteric phases are obtai ned. Photopolymerization of such mixtures in the liquid crystalline phase l eads to crosslinking. Networks with reflection colors from red to blue are obtained by variation of the amount of the chiral component. The helical tw isting power (HTP) for networks of the bishexadiene 1a and the cholesterol derivative 3 is HTP/(n) over bar = 5.6 mu m(-1) ((n) over bar: average refr active index) which is almost by 50% larger than the value for the correspo nding bisacrylate network.