Ethylene-Norbornene Copolymers by ansa Fluorenyl Metallocenes: Mechanistic Considerations on the Basis of Tetrad and Pentad Analysis

The microstructure of ethylene (E)-norbornene (N) copolymers obtained by C s-symmetric i-Pr[(Cp)(9-Flu)]ZrCl2 (1) and C 1-symmetric i-Pr[(3-R-Cp)(9-Flu)]ZrCl2 (R = Me (2); R = i-Pr (3)) was determined by 13C NMR at tetrad (1) and pentad level (2, 3), respectively. This has allowed to study the influence of Cp substitutions on the statistical model of copolymerizations: Markov 1 statistics for 1 and 2; Markov 2 for 3, where the penultimate unit influences next insertion owing to steric interactions between i-Pr and copolymer chain. The presence of only racemic ENNE sequences in E-N copolymers by 1 shows that copolymerizations occur through chain migratory mechanism. Chain backskips account for the formation of both meso and rac NEN sequences with 1 and for the insertion of N and E at the same site of 2 and 3.