Chiral polyisocyanates from an azomonomer with a very high chiral induction

The synthesis of new chiral polyisocyanates is described. For this purpose a new chiral azobenzene containing monomer with the chiral center in alpha -position to the isocyanate group was synthesized. The anionic copolymerization was carried out in THF as solvent with potassium cyanide that was complexed by 18-crown-6 as initiator. This allowed a better control of the reaction and thereby the synthesis of polyisocyanates with a fairly low polydispersity. The copolymers show an extremely high transfer of chirality from the chiral side groups to the helical backbone in dilute solution. Copolymers with only 1.6 mol % of chiral side groups show nearly the full optical rotation and exist predominantly in the P-helical conformation. It was found that the isomerization of the azobenzene groups hardly has any influence on the conformation of the helix. Lyotropic liquid crystalline phases were formed from the polymers and 1,2-dichlorobenzene. It could be shown that even polymers with a very low content of chiral side groups (1.6 mol %) form cholesteric phases with a selective reflection in the visible range. The pitch of the cholesteric phase and thereby the wavelength of the selective reflection bands could be reversibly altered by the isomerization of the azobenzene groups.