On the tacticity of polynorbornenes with 5,6-endo pendant groups that contain substituted aryl chromophores

Two dimers and a series of polymers with 5,6-endo pendant aryl groups that contain different substituents at the para positions were synthesized. The conformation and stereochemistry of the dimers and polymers were determined by nonlinear optical analysis (EFISH) as well as UV/Vis and C-13 NMR spectroscopy. The chemical shifts of C7 for the polymers appeared as two peaks in the C-13 NMR spectra when the substituents are electron -withdrawing groups. The percentage decrease in the relative extinction coefficient of the polymers, ed, was linearly related to the Hammett constant sigma. Polynorbornenes with electron-with- drawing substituents may adopt isotactic stereochemistry with all pendant groups aligned in one direction. The nature of the interactions between neighboring chromophores may be one of the most important factors in directing the stereoregularity and conformation of these polymers. The corresponding polymers derived from the exo isomers appeared to be less stereo-regular.