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On the mechanism of olefin polymerization by titanocene/MAO catalysts: Relationships between metathesis and addition polymerization
Author(s) -
Tritto Incoronata,
Li San Xi,
Boggioni Laura,
Sacchi Maria Carmela,
Locatelli Paolo,
O'Neill Aideen
Publication year - 1997
Publication title -
macromolecular chemistry and physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.57
H-Index - 112
eISSN - 1521-3935
pISSN - 1022-1352
DOI - 10.1002/macp.1997.021980505
Subject(s) - norbornene , methylaluminoxane , cationic polymerization , polymer chemistry , polymerization , polyethylene , chemistry , carbene , ethylene , metallocene , catalysis , organic chemistry , polymer
Ethylene polymerizations and norbornene oligomerizations catalysed by Cp 2 Ti 13 CH 3 Cl/MAO (Cp: cyclopentadienyl; MAO: methylaluminoxane) mixtures have been carried out at different temperatures (from ‐20°C to 20°C), in order to test the validity of carbene mechanisms in α‐olefin polymerizations. Depending on the temperature, different ratios of the cationic species [Cp 2 Ti 13 CH 3 ] + [Cl · MAO] − and precursors of the alkylidene Cp 2 Ti = 13 CH 2 exist. The in situ polymerization of 13 C enriched ethylene was monitored by 13 C NMR spectroscopy. Moreover, catalytic activity was determined and polyethylene samples were analyzed by 13 C NMR and gel permeation chromatography (GPC). The following evidence has been provided against the carbene mechanism in the α‐olefin polymerization with titanocene based catalysts: (a) in the in situ ethylene polymerization experiments the appearance of polyethylene signals is concurrent with the decrease of cationic [Cp 2 Ti 13 CH 3 ] + [Cl · MAO] − signals and is not related to the intensity of the alkylidene Cp 2 Ti = 13 CH 2 signals; (b) from the 13 C NMR analysis of polyethylene chain‐end groups the 13 C enrichment of Cp 2 Ti 13 CH 3 Cl has only been found in the methyl chain‐end group and not in the methylene of the propyl chain‐end group, as should have been the case if the carbene mechanism had been valid; (c) from norbornene oligomerization (at 0°C) the addition product 2‐ 13 C enriched methyl‐norbornane has been identified. Moreover, the identification of a 13 C enriched methylidene‐norbornane dimer at higher temperatures has revealed the possibility of norbornene addition to titanium carbenes through the formation of titanacyclobutane without the opening of the norbornene ring. However, this process requires higher energies with respect to the Cossee type insertion.