The role of bulky substituents in the polymerization of ethylene using late transition metal catalysts: A comparative study of nickel and iron catalyst systems
A series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M=Ni) and tridentate (for M=Fe) heterocycle-imine ligands L have been synthesized and characterized. Compared to the well-known α-diimine nickel and bis(imino)pyridine iron catalysts, these systems contain a bulky imine substituent on one side and a non-bulky N-heterocycle on the other. Depending on the ligand and the conditions used, either four- or five-coordinate complexes are obtained in the case of nickel. Iron complexes are generally five-coordinate, even with potentially tetradentate ligands. Activation of these precatalysts with MAO affords active catalyst systems for the oligomerization/polymerization of ethylene. Compared to α-diimine nickel and bis(imino)pyridine iron catalysts, both metal systems provide only half of the steric protection and consequently the catalytic activities and the degree of polymerization are significantly lower. Lower activities are attributed to a reduced stability of the active species under polymerization conditions, whereas the lower molecular weights are a result of increased β-H transfer rates. Variations within the heterocyclic component of the ligand reveal that both steric and electronic factors influence the polymerization behavior of these catalysts.
Britovsek, George J.P.,Baugh, Simon P.D.,Hoarau, Olivier,Gibson, Vernon C.,Wass, Duncan F.,White, Andrew J.P.,Williams, David J.
p. 279 - 291
(2008/10/08)
Substituted 1-(2-pyridyl)-2-azaethene-(N,N)-nickel dibromide complexes as catalyst precursors for homogeneous and heterogeneous ethylene polymerization
The complex 1-(2-pyridyl)-2-azaethene-(N,N)-nickel dibromide and its derivatives can be used for the catalytic polymerization of ethylene after activation with methylaluminoxane (MAO). The polyethylenes made with these catalysts have a high degree of short-chain-branching with up to 240 branches per 1000 carbon atoms. Homogeneous ethylene polymerization reactions produce highly viscous oligomers with low molecular weights. Polymers with significantly higher molecular weights and lower amounts of branching are obtained when the ethylene polymerization reactions with these catalyst systems are carried out as heterogeneous reactions. The activities of these paramagnetic complexes are significantly lower than the activities of metallocene catalysts. (C) 2000 Elsevier Science B.V.
Koeppl, Alexander,Alt, Helmut G.
p. 45 - 54
(2008/10/08)
More Articles about upstream products of 215594-94-8