Palladium a-diimine catalysts

The cationic palladium a-diimine catalysts, much more so than their nickel analogs, are tolerant of polar functional groups. Emulsion polymerization in aqueous solvents can take place to give polymer microspheres useful as adhesives. Monomers such as acrylates, unsaturated ethers, and... 

While a wide range of monomers have been incorporated in ethylene copolymerizations with the palladium a-diimine catalysts, there are also a variety of monomers which slow homopolymerization and are not incorporated into the polymer chain and others which inhibit all polymerization. There is a general trend that removing the polar functionality from the vicinity of the double bond lessens the rate penalty experienced in the copolymerizations. A superior approach is to place a group which blocks doublebond isomerization between the olefinic group and... 

With the advent of the nickel and palladium a-diimine catalyst systems, it was reasonable to extend the diimine chemistry to cobalt and iron. Low to moderate activity was observed in a limited number of cases, but the observation led to a broader search for catalytic activity. In addition to complexes of nickel and palladium, other late metal complexes that catalyze insertion polymerization of ole-hns include ruthenium, - 75,76,132,139,278-282... 

FIGURE 16.12 Palladium a-diimine catalysts used for ethylene/norbomene copolymerization (GLY = 1,2-ethanediimine = glycine-derivate BUD = 2,3-butanedirmine). 

FIGURE 16.13 Diagram for the copolymerization of ethylene and norbornene with palladium a-diimine catalysts (triangles = 19 squares = 20). 

Broadening this comparison to include copolymers prepared by both early and late transition metal catalysts, the results discussed immediately above show that Ci-symmetric zirconocenes such as 9/MAO produce only copolymers with isolated norbornene units or alternating structures (at 30 C), mainly with isotactic (meso) configurations. C2-symmetric zirconocenes such as 2/MAO readily produce norbornene dyads that are exclusively meso-linkcd (isotactic). In accordance with their catalyst structures, Q-symmetric zirconocenes such as 8/MAO produce norbornene dyads with a rac-linkage (syndiotactic), although with a generally lower stereoselectivity. Palladium a-diimine catalysts, despite the homotopic nature of their coordination sides (that would be expected to give a mixture of meso and racemic blocks), produce norbornene dyads that are solely rac-connected. This behavior can be attributed to a chain-end control type polymerization mechanism. 

Figure 9.9 Nickel and palladium a-diimine catalyst for living olefin polymerization.

Utilizing a triacrylate, Zhang et synthesized a trinuc-lear palladium a-diimine catalyst (110, Figure 28) for the production of star PE. At 5 °G in the presence of ethylene. 

J. Kiesewetter, B. Arikan, and W. Kaminsky, Copolymerization of eth-ene with norbomene using palladium(II) a-diimine catalysts Influence of feed composition, polymerization temperature, and ligand structure on copolymer properties and microstructure, Polymer, 47 (10) 3302-3314, May 2006. 

The cationic palladium a-diimine complexes are remarkably functional-group tolerant. Ethylene polymerizations can be carried out in the presence of ethers, organic esters, and acids, but nitriles tend to inhibit polymerizations. In addition, polymerizations have been carried out in the presence of air and in the presence of an aqueous phase.Aqueous emulsion and suspension polymerizations using these catalysts have been developed as a route to microspheres of polymer for adhesives as well as for other applications.2 ° 2 Preparation of elastomers is often complicated by difficult solvent removal, so polymerizations in supercritical CO2 have been investigated. It is also possible to combine the activity of the palladium catalysts with other polymerization techniques such as living-free-radical polymerizations. One interesting observation is that the... 

Kiesewetter, J. Kaminsky, W. Ethene/norbornene copolymerization with palladium(II) a-diimine catalysts. From ligand screening to discrete catalyst species. Chem. Eur. J. 2003, 9, 1750-1758. 

Gottfried AC, Brookhart M. Living and block copolymeriza-tion of ethylene and a-olefms using palladium(II)-a-diimine catalysts. Macromolecules 2003 36 3085-3100. 

By comparison of the a-diimine catalysts with the phosphine sulfonate-based palladium catalyst system, the change from a symmetric to an asymmetric ligand stmcture is apparent on first sight. Here (as well as in other ligand systems Section 3.24.4.1.4), it could be shown that this leads to major differences in reactivity. Despite the partially open catalyst stmcture, these complexes are of remarkable stability and activity in... 

One of the more extraordinary recent developments in nickel and palladium polyalkene catalysis has been the development of a-diimines with bulky substituents as ligands in nickel and palladium complexes. When bulky aryl groups are used (R = isopropyl), these catalysts polymerize ethylene with high activities to high molecular weight highly branched... 

The specific catalysts that work the best are characterized by cationic palladium centers with bulky a-diimine ligands and noncoordinating counterions. Generation of a specific catalyst is shown in equation (38). 

Triggered by the developments in late transition metal catalyzed polymerization, new catalyst systems were described very recently for the oligomerization of ethene. Nickel and palladium complexes based on a-diimine ligands 13 and imi-nophosphines 14 were reported to be very active and selective oligomerization catalysts [57, 58], Activation of the Ni(II) diimine halides with a large excess of MAO (210 equiv.) leads to oligomerization catalysts with activities of between... 

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