Alternating Isotactic Ethyleneƒ{Norbornene Copolymers by C1-symmetric Metallocenes: Determination of the Copolymerization Parameters and Mechanistic Considerations on the Basis of Pentad Analysis (Articolo in rivista)

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  • Alternating Isotactic Ethyleneƒ{Norbornene Copolymers by C1-symmetric Metallocenes: Determination of the Copolymerization Parameters and Mechanistic Considerations on the Basis of Pentad Analysis (Articolo in rivista) (literal)
Anno
  • 2004-01-01T00:00:00+01:00 (literal)
Alternative label
  • Tritto I.; Boggioni L.; Ferro D.R. (2004)
    Alternating Isotactic Ethyleneƒ{Norbornene Copolymers by C1-symmetric Metallocenes: Determination of the Copolymerization Parameters and Mechanistic Considerations on the Basis of Pentad Analysis
    in Macromolecules (Print)
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Tritto I.; Boggioni L.; Ferro D.R. (literal)
Pagina inizio
  • 9681 (literal)
Pagina fine
  • 9690 (literal)
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  • http://pubs.acs.org/doi/pdfplus/10.1021/ma048502a (literal)
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  • 37 (literal)
Rivista
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#numeroFascicolo
  • 26 (literal)
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  • Scopu (literal)
  • ISI Web of Science (WOS) (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • ISMAC-CNR (literal)
Titolo
  • Alternating Isotactic Ethyleneƒ{Norbornene Copolymers by C1-symmetric Metallocenes: Determination of the Copolymerization Parameters and Mechanistic Considerations on the Basis of Pentad Analysis (literal)
Abstract
  • 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. (literal)
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