On the mechanism of olefin polymerization by titanocene/MAO catalysts: Relationships between metathesis and addition polymerization

✍ Scribed by Incoronata Tritto; San Xi Li; Laura Boggioni; Maria Carmela Sacchi; Paolo Locatelli; Aideen O'Neill

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 747 KB
• Volume: 198
• Category: Article
• ISSN: 1022-1352
• DOI: 10.1002/macp.1997.021980505

✦ Synopsis

Abstract

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.