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 Pr-i) and methyl aluminoxane 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 C-13 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 C-1 symmetric catalysts and to study the influence of ligand substitution of these catalysts on the polymerization mechanism. The root-mean-square deviations between experimental and calculated pentads demonstrate that the two-site alternating mechanism is not valid. Penultimate (second-order Markov) effects play a decisive role in E-N copolymerizations with i-Pr[(3-Pr-i-Cp)(Flu)]-ZrCl2 (2). The first-order Markov model is sufficient to describe the microstructure of E-N copolymers with i-Pr[(3-Me-Cp)(Flu)]ZrCl2 (3) at least at not too high N/E feed ratios. It is concluded that in E-N copolymerizations with these catalysts, both N and E are inserted at the same open site. Norbornene undergoes a Cossee's migratory insertion and after every insertion the copolymer chain backskips to its original position. The synthesis of alternating E-N copolymers, possible only at very high N/E feed ratios, derives from the impossibility of having two consecutive norbornene insertions. The isotacticity is a consequence of norbornene insertion always at the same site with the same face. The steric interactions between the growing polymer chain and the methyl Cp substituent of 3 and the isopropyl Cp substituent of 2 appear to be important to determine the copolymerization statistics.