Ethylene-Norbornene Copolymers by Cs-symmetric Metallocenes: Determination of the Copolymerization Parameters and Mechanistic Considerations on the Basis of Tetrad Analysis

The copolymerization of ethylene and norbornene by catalytic systems composed of i-Pr-[(Cp)(Flu)]ZrCl2 (1) and methylaluminoxane was investigated. Ethylene-norbornene (E-N) copolymers with 40.2 mol % of norbornene are highly alternating (NENE 50 mol %) and contain a significant amount of racemic ENNE (8 mol %) and no ENNN sequences. The microstructural analysis by 13C NMR of such copolymers was completely obtained at the tetrad level by a methodology that exploits all the peak areas of the spectra and accounts for the stoichoimetric requirements of the copolymer chain. The analysis at the tetrad level allowed us to test the statistical model best describing E-N copolymerization with Cs-symmetric catalyst 1 and to study the polymerization mechanism. The root-mean-square deviations between experimental and calculated tetrads demonstrate that the first-order Markov model is sufficient to describe the microstructure of E-N copolymers with 1. It is concluded that in E-N copolymerizations with this catalyst both N and E are inserted according to a Cossee's migratory insertion, and backskips of the copolymer chain to its original position occur, causing the formation of both meso and racemic NEN sequences. The probability of chain backskip is relatively high with respect to that observed in syndiotactic propylene polymerization under the same polymerization conditions. This effect seems to be due to norbornene strong coordinating ability which can influence the competition between site epimerization and chain propagation