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 C-13 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 C-13 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 C-13 NMR spectra of a large number of E-N copolymers, prepared with catalyst precursors rac-Et(indenyl)(2)ZrCl2 (1), rac-Me2Si(2-Me-benz[e]indenyl)(2)ZrCl2 (2), Me2Si(Me4Cp)((NBu)-Bu-t)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.