Propene-Norbornene Copolymers. Toward a Description of Microstructure at Triad Level Based on Assignments of 13C NMR Spectra

A procedure for the quantitative determination of the molar fractions of the stereosequences defining the microstructure of a propene-norbornene (P-N) copolymer from C-13 NMR spectra has been set up. 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. With the support of homonuclear H-1-H-1 and heteronuclear H-1-C-13 NMR techniques, by guessing assignments of unknown signals, and by discarding inconsistent hypotheses, an extension of signal assignments is made possible. This procedure, which allows for a quantitative analysis of copolymer sequences as accurate as possible, has been applied to the analysis of the C-13 NMR spectra of a number of P-N copolymers prepared with catalyst precursors rac-Et(Indenyl)(2)ZrCl2 (1) and rac-Me2Si(2-Me-Inclenyl)(2)ZrCl2 (2). A complete description of the microstructure at triad level, including 1,3- and 2,1-propene insertions, has been attempted. New signals have been assigned such as those of the carbons of propene in the alternating triad NP(12)A, and of norbornene in tetrad NP12NP12, as well as the signals of P-beta methyl groups in triad NP12P12 adjacent to a variable number of P-12 units all in isotactic relationship and those of the S-xy methylene of a 1,3 propene inserted unit in the NP13P12 and of the methyl carbon atom of central monomer in P21P12N and NP21P12. Although the goal of determining all the triads has not been achieved, an estimate of the molar fractions of the major sequences with a standard deviation on the order of 2-4% has been obtained. The quantitative determination of copolymer microstructure will allow one to clarify the P-N copolymerization mechanism.