Ethylene Oligomerisation Catalysts: Synthesis, Evaluation and Mechanistic Studies

The oligomerisation of ethylene to linear alpha olefins is an important industrial process for the production of polyethylene co-monomers, plasticizers, surfactants and waxes. With the ever increasing market demand for short chain linear alpha ole fins (1-butene, 1-hexene and 1-octene) there has been intensive research in both industry and academia on methods to selectively oligomerise ethylene to form only the short chain fractions. As such, a range of novel catalysts based on titanium and chromium have been synthesised and characterised. Their behaviour in ethylene oligomerisation and polymerisation has been studied. In most cases, the mechanism of oligomer formation has been investigated. A range of aryloxyand alkoxyligands, both monodentate and chelating, have been reacted with Ti(IV) to give complexes of the form [Ti(OArhClz], [Ti(O"OR)Cb] and [Ti(0"0R)2C}z] (R = aryl, alkyl). X-ray crystallography of selected samples has revealed that the complexes vary in the Lewis base solvation and/or aggregation state. The complexes have been evaluated as ethylene oligomerisation catalysts in conjunction with alkylaluminium activators such as triethylaluminium and methylaluminoxane. Polymerisation is the major process, however, ethylene oligomers such as dimers and higher oligomers also result. The bimodal distribution of products indicates a number of active species arise when the catalyst is activated. Deuterium labelling studies are consistent with a metallacycle mechanism for oligomer formation.