Alternating Isotactic Ethylene−Norbornene Copolymers by C1-Symmetric Metallocenes: Determination of the Copolymerization Parameters and Mechanistic Considerations on the Basis of Pentad Analysis

The copolymerization of ethylene and norbornene by catalytic systems composed of i-Pr[(3-R−Cp)(Flu)]ZrCl2 (R = Me or Pri) and methylaluminoxane was investigated. Isotactic, alternating ethylene−norbornene (E−N) copolymers with percentages of pentads NENEN up to 21% and norbornene incorporation up to 40% were obtained. NEENE sequences were observed in copolymers synthesized with both metallocenes. The microstructural analysis by 13C NMR of such alternating isotactic copolymers was completely obtained at pentad level by a methodology that exploits all the peak areas of the spectra and accounts for the stoichoimetric requirements of the copolymer chain. Such a methodology has allowed us to reconsider some controversial assignments of ethylene CH2 signals. The present understanding of E−N copolymer NMR spectra at the pentad level has allowed us to test the statistical model best describing E−N copolymerization with C1 symmetric catalysts and to study the influence of ligand substitution of these catalysts on ...