Interplay of Chiral Side Chains and Helical Main Chains in Polyisocyanates

Abstract

Here we describe the synthesis of seven new copolymer series prepared by copolymerization of hexyl isocyanate with seven new chiral azo chromophores with an isocyano functionality. The resulting copolyisocyanates (Nylon 1) possess a helical polymer conformation. In the cases in which the chiral group and polymer chain are linked to the same phenyl ring (copolymer series VI−IX) a transfer of chirality to the polymer chain is possible. I.e. one helical twist sense is preferred in this case. By variation of the chiral center, polymers with preferable right- or left-handed helical conformation are accessible. The photochemical isomerization of the azo chromophore (trans → cis) is possible in high yields (>90%). This leads, depending on the nature of the chiral center, either to an increase of the chiral interaction (further increase of the amount of already preferred helix conformation) or to a decrease of the chiral interaction (more racemic situation). Based on these results, a picture for the interplay of chiral side chains and helical polymer chain is developed. Measurements of circular dichroism (CD) spectra in the range of the azo-chromophore absorption prove the dominant role of the helical polymer chain. They show, in addition, that the chromophores can form a chiral superstructure mediated by the polymer chain. Some of these polymers show strong changes of their optical rotation during photoisomerization. This can be caused either by a shift of the equilibrium between right- and left-handed helical segments or by Cotton effects of the azo chromophores.