Ethylene−Norbornene Copolymer Microstructure. Assessment and Advances Based on Assignments of 13C NMR Spectra

A best-fitting procedure for the quantitative determination of the molar fractions of the stereosequences that define the microstructure of an ethylene−norbornene (E−N) copolymer from 13 C NMR spectra has been set up. The quantitative determination of copolymer microstructure will allow one to clarify the E−N copolymerization mechanism. This method utilizes the observed peak areas of the 13 C signals and takes into account the consistency between peak areas and the stoichiometry of the copolymer chain. Thus, a further extension of signal assignments is made possible by guessing assignments of unknown signals and by discarding inconsistent hypotheses. This procedure has been applied to the analysis of the 13 C NMR spectra of a large number of E−N copolymers, prepared with catalyst precursors rac -Et(indenyl) 2 ZrCl 2 ( 1 ), rac- Me 2 Si(2-Me-benz[ e ]indenyl) 2 ZrCl 2 ( 2 ), Me 2 Si(Me 4 Cp)(N t Bu)TiCl 2 ( 3 ), and Me 2 C(Flu)(Cp)ZrCl 2 ( 4 ). An estimate of the molar fractions of the various stereosequences with a standard deviation on the order of 1−2% has been obtained. The comparison between controversial assignments existing in the literature for a number of ethylene signals has confirmed our previous assignments. New signals such as those of the C2/C3 carbons of EENNEE meso sequences ( M ) and of the external carbons C5 of MM and MR triads in ENNNE sequences have been assigned.