Diimine ligand catalysts

The first examples of highly active olefin polymerization catalysts based on late transition metals were nickel and palladium complexes containing bulky diimine ligands.310 312 For example, complex (120) was found to polymerize ethylene with an activity of ll,000gmmol h bar A range of PE materials with molecular weights up to 106 and... 

Catalysts Derived from Cyclic a-Diimine Ligands. 206... 

Numerous modifications to the bis(aryl)-a-diimine ligand motif have been reported, especially in the patent literature. The review by Ittel includes an index of diimine and related ligands in proprietary publications up to the year 2000 [26]. Instead, this discussion will focus on academic reports of Ni(II) and Pd(II) polymerization catalysts bearing a-diimine or other closely related chelating neutral nitrogen ligands. 

Fig. 8 Teraryl-substituted a-diimine ligand-based Ni(II) polymerization catalysts...

A specific goal we have in mind is to design cyclophane-based a-diimine ligands for overcoming the low thermal stability of the regular a-diimine catalyst systems. 

Fig. 11 Late transition metal polymerization catalysts from cyclic a-diimine ligands...

Since their discovery over a decade ago, late transition metal a-diimine polymerization catalysts have offered new opportunities in the development of novel materials. The Ni(II) catalysts are highly active and attractive for industrial polyolefin production, while the Pd(II) catalysts exhibit unparalleled functional group tolerance and a propensity to form unusually branched polymers from simple monomers. Much of the success of these catalysts derives from the properties of the a-diimine ligands, whose steric bulk is necessary to accelerate the insertion process and inhibit chain transfer. 

Gibson described the synthesis of four-coordinate cationic aluminium alkyls 1 which were reported to be well-defined aluminium polymerization catalysts [12]. However, the polydispersities of the products obtained were high (2.9-6.3), showing that there is not a single well-defined active species. The experiments were carried out in metal autoclaves, and Fe and Co complexes of pyridine-diimine ligands are extremely active in ethene polymerization [34], so a transition-metal impurity does not seem an unreasonable explanation. 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

Polymerisation and Copolymerisation of Olefins in the Presence of Catalysts Containing Ni or Pd and u-Diimine Ligands... 

Very recently, McLain et al. [560] and Brookhart et al. [561] from du Pont de Nemours and Company invented catalysts that consist of transition metal compounds and a-diimine ligands, and sometimes other cocatalysts. Processes... 

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