On the Ethylene-Norbornene Copolymerization Mechanism

Series of propene(norbornene (P-N) copolymers were synthesized in the presence of two C2 symmetric zirconocenes, rac-Et(Indenyl)2ZrCl2 (1) and rac-Me2Si(Indenyl)2ZrCl2 (2), and of the Cs symmetric zirconocene Me2C(Cp)(Fluorenyl)ZrCl2 (3) with MAO as cocatalyst. P-N copolymers were characterized by 13C NMR spectroscopy, SEC, and DSC. A first assignment of the main 13C NMR signals of P-N copolymers obtained with 1 and 2 containing isolated N units was obtained on the basis of DEPT experiments and by comparison with isotactic polypropene (i-PP) and E-N copolymer spectra. Ab initio theoretical 13C NMR chemical shifts, combined with R.I.S. statistics of the P-N chain, gave important detailed indications for the assignment of the complex 13C spectra of these copolymers. Such assignments were used to estimate the N copolymer content. The comparison of N content values obtained from the areas of different regions of the spectra reveals that 1,3 propene misinsertions are formed in greater amount as the [N]/[P] ratio of the feed increases. This may be due to the steric hindrance of the Mt-tertiary carbon bond when N is the last inserted unit, which makes difficult the next propene insertion and causes the low polymerization activities, molecular masses and Tg. The spectra of P-N copolymers obtained with 3 revealed to be even more complex. A limited assignment of the spectrum has been achieved. This allowed us to evaluate the N content in the copolymers and to evidence the low tendency of the Cs symmetric 3 to give 1,3 propene misinsertions in P-N copolymers. This is in agreement with the results observed with this catalyst in propene homopolymerization.