13C NMR studies of ethene-norbornene copolymers: assignment of sequence distributions using13C-enriched monomers and determination of the copolymerization parameters

Abstract

Ethene and norbornene were copolymerized using metallocene catalysts that produce copolymers having isolated norbornene units or microblocks with a maximum of two norbornene units. The resonances of the norbornene C5/6 and the ethene carbon atoms, which overlap extensively in the 13C NMR spectrum, were differentiated and assigned by comparing the 13C NMR spectra of the copolymers obtained from monomers having 13C at natural abundance with those prepared from feedstocks containing 13C1-enriched ethene or 13C5/6-enriched norbornene. The NMR analysis revealed that the chemical shifts of the norbornene C5/6 carbon atoms are triad sensitive and those of the ethene carbon atoms are pentad sensitive. 13C NMR analysis of copolymers containing isolated norbornene units in various proportions allowed the resonances of the norbornene C5/6 and the ethene carbon atoms to be assigned to the respective triads and pentads. The complete triad distributions of these copolymers determined in this way were used to calculate the copolymerization parameters for a representative metallocene catalyst.