Well-defined zwitterionic nickel initiators for the oligomerization of ethylene: Applications and mechanistic aspects.

摘要

Boratabenzene complexes of zirconium oligomerize ethylene when activated with methylaluminoxane (MAO). Variation of the exocyclic substituent on boron affects the reactivity and product selectivity of these catalyst systems. 11B NMR spectroscopy can be used to observe exchange reactions between the boron exocyclic group and MAO, which complicate the catalytic process by producing a mixture of species in the reaction solution. An alternative ethylene oligomerization catalyst is prepared by addition of tris(pentafluorophenyl) borane (TPFPB) to the known single-component polymerization catalyst, 2-diphenylphosphinobenzoate-κ2P,O-η 3-methallyl nickel. The resulting Zwitterionic species rapidly consumes ethylene at room temperature and atmospheric pressure, producing mainly 1-butene. Substitution of the η3-methallyl fragment for the isoelectronic η 3-benzyl results in faster initiation and more efficient use of the complex.; The inexpensive Lewis acid, BF₃, can be used in place of TPFPB to make 2-diphenylphosphinobenzoate(trifluoroborate)-κ2P,O-η 3-benzyl nickel. This complex is also active for ethylene consumption, producing a distribution of oligomers. BF₃ activation can also be carried out with other nickel complexes.; The ethylene oligomer distributions produced by these Zwitterionic nickel catalysts are independent of monomer pressure, but are affected by reaction temperature. At high temperature, the products are of higher average molecular weight than of low temperature reactions. Analysis of the oligomer distributions produced over a range of temperatures allows for determination of relative activation parameters for propagation and termination mechanisms. This analysis is applied to a series of Zwitterionic nickel complexes to determine the effects of steric and electronic influences on the activation parameters.; The well-defined, single-component nature of Zwitterionic ethylene oligomerization catalysts based on nickel make them ideal for dual catalyst systems. When these catalysts are used in conjuction with a well-defined olefin polymerization catalyst, branched polyethylene is produced from ethylene alone. The amount of branching incorporated into the polymer backbone is controlled by the relative amounts of oligomerization and polymerization catalysts.; Use of Zwitterionic nickel complexes is expanded beyond ethylene oligomerization to include cyclization of α,ω-dienes. The tendency for the catalysts studied to favor chain transfer over propagation gives good selectivity for ring-closed products rather than oligomeric materials. Several aliphatic dienes are successfully cyclized including the novel ring-closure of 1,4-pentadiene to cyclopentene.