On the Mechanism of Olefin Polimerization by Titanocene / MAO Catalysts: Relationship between Methatesis and Addition Polymerization

Ethylene polymerizations and norbornene oligomerizations catalysed by (Cp2TiCH3Cl)-C-13/MAO (Cp: cyclopentadienyl; MAO: methylaluminoxane) mixtures have been carried out at different temperatures (from -20 degrees C to 20 degrees C), in order to test the validity of carbene mechanisms in alpha-olefin polymerizations. Depending on the temperature, different ratios of the cationic species [(Cp2TiCH3)-C-13](+)[Cl . MAO](-) and precursors of the alkylidene Cp2Ti = (CH2)-C-13 exist. The in situ polymerization of C-13 enriched ethylene was monitored by C-13 NMR spectroscopy. Moreover, catalytic activity was determined and polyethylene samples were analyzed by C-13 NMR and gel permeation chromatography (GPC). The following evidence has been provided against the carbene mechanism in the alpha-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 [(Cp2TiCH3)-C-13](+)[Cl . MAO](-) signals and is not related to the intensity of the alkylidene Cp2Ti = (CH2)-C-13 signals; b) from the C-13 NMR analysis of polyethylene chain-end groups the C-13 enrichment of (Cp2TiCH3Cl)-C-13 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 degrees C) the addition product 2-C-13 enriched methyl-norbornane has been identified. Moreover, the identification of a C-13 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.