Investigation of the microstructure of metallocene-catalyzed norbornene-ethylene copolymers using NMR spectroscopy

Norbornene-ethylene copolymers were prepared using the metallocene catalyst ethylene bis (indenyl) zirconium dichloride with MAO, and their microstructure was characterized with H-1-NMR and C-13-NMR methods. From a Cosy H-1-NMR spectrum it was found that all norbornene units are enchained in the exo-configuration. The sequence distribution of norbornene units was investigated using C-13-H-1 correlations, hmqc for one-bond correlations, and hmbc for two- or three-bond correlations. It was shown that norbornene diads were formed at a high norbornene content (45 mol %). When further increasing the norbornene incorporation (66 mol %) a number of new signals were obtained. A Cosy H-1-NMR spectrum revealed a new crosspeak which, according to the corresponding C-13-NMR shifts (hmqc), correlated well with a terminal unit of a trimer of norbornene. This means that at very high norbornene contents, norbornene triads can be formed. Because the formation of isotactic norbornene triads is very difficult to understand from a sterical point of view, an epimerization process causing stereoirregularities in the norbornene triad is proposed. (C) 1998 John Wiley & Sons, Inc.