The conformational characteristics of ethylene-norbornene copolymers and their influence on the (13)C NMR spectra

A rotational-isomeric-state treatment of ethylene-norbornene copolymers has been worked out for the purpose of establishing a correlation between conformation and (13)C NMR chemical shifts of this new class of polymers and of clarifying their microstructure. A comparison of conformer populations computed for the stereoregular alternating (N-E)(x) chains and for copolymers including such defects as N-N, E-E, E-E-E, and N-N-N indicates the need for accounting for the tacticity of the chain, besides its composition, in the interpretation of the spectra. Some stereochemical shifts were suggested by the calculations. In particular, conformational analysis predicted and evidenced the origin of distinct meso/racemic signals for methines C2 and C1 and for the ethylene carbon of the regularly alternating copolymers. This prediction induced a more thorough analysis of the (13)C NMR spectra of mainly alternating (N-E)(x) copolymers, allowing us to recognize and distinguish isotactic and syndiotactic N-E-N sequences. Then, a detailed and unambiguous assignment of the spectra of stereoregular and irregular alternating E-N copolymers was achieved. The strong effect of ring distortions in the presence of N-N dyads and triads is also discussed.