ON THE MECHANISM OF OLEFIN POLYMERIZATION BY TITANOCENE MAO CATALYSTS- RELATIONSHIPS BETWEEN METATHESIS AND ADDITION POLYMERIZATION/

Ethylene polymerizations and norbornene oligomerizations catalysed by (Cp2TiCH3Cl)-C-13/MAO (Cp: cyclopentadienyl; MAO: methylaluminoxane) m ixtures have been carried out at different temperatures (from -20 degr ees C to 20 degrees C), in order to test the validity of carbene mecha nisms in alpha-olefin polymerizations. Depending on the temperature, d ifferent 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 NM R spectroscopy. Moreover, catalytic activity was determined and polyet hylene samples were analyzed by C-13 NMR and gel permeation chromatogr aphy (GPC). The following evidence has been provided against the carbe ne mechanism in the alpha-olefin polymerization with titanocene based catalysts: a) in the in situ ethylene polymerization experiments the a ppearance of polyethylene signals is concurrent with the decrease of c ationic [(Cp2TiCH3)-C-13](+)[Cl . MAO](-) signals and is not related t o 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 enrich ment 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 shoul d have been the case if the carbene mechanism had been valid; c) from norbornene oligomerization (at 0 degrees C) the addition product 2-C-1 3 enriched methyl-norbornane has been identified. Moreover, the identi fication of a C-13 enriched methylidene-norbornane dimer at higher tem peratures has revealed the possibility of norbornene addition to titan ium carbenes through the formation of titanacyclobutane without the op ening of the norbornene ring. However, this process requires higher en ergies with respect to the Cossee type insertion.