Abstract
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 13C 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 13C 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 13C NMR spectra of a large number of E−N copolymers, prepared with catalyst precursors rac-Et(indenyl)2ZrCl2 (1), rac-Me2Si(2-Me-benz[e]indenyl)2ZrCl2 (2), Me2Si(Me4Cp)(NtBu)TiCl2 (3), and Me2C(Flu)(Cp)ZrCl2 (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.
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