Correlation between the Isomerization of Side Groups and the Helical Main Chain in Chiral Polyisocyanates

In polyisocyanates with chiral azo chromophores a change of the helical conformation can be triggered photochemically. In this investigation we studied the correlation between the isomerization of the main chain and the photoisomerizable side groups by CD measurements. A few minutes of irradiation are enough to reach a new helix equilibrium of the polymer chain. Also during slow thermal back-relaxation, the helical conformation of the polymer chain follows the isomerization of the side groups reversibly. The relation between the ratio of cis and trans isomer and the preference for one helical conformation can be linear or nonlinear. For low concentrations/inductions of the chiral side groups a linear relation is found. For higher concentrations/inductions the stronger inducting species dictates the helical conformation in accordance with the “sergeants and soldiers” principle.