Cationic diimine palladium complexes

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

In 1995 Brookhart and co-workers reported that cationic diimine-substituted palladium complexes of type 5 [Eq. (5)] can polymerize ethylene to high molecular-weight, highly branched material in organic solvents such as methylene chloride. 

Highly branched ethene-methyl acrylate polymers. The cationic palladium diimine complexes are remarkably tolerant towards functional groups, although the rates decrease somewhat when polar molecules are added. In ETM catalysis addition of polar molecules or monomers kills the catalyst and therefore it was very interesting to see what the new palladium catalysts would do in the presence of polar monomers. Indeed, using methyl acrylate a copolymerisation... 

Yet another era of organometallic polymer chemistry appears to arise from new cationic nickel(II)- and palladium(II) complexes 41 of Al,A-chelate (diimine) ligands (cf. eq. (19)). According to Brookhardt et al., both the homo- and copolymerization of a-olefins proceed with activities that compare in the case of... 

Syntheses of discrete cationic a-diimine complexes of palladium and nickel are well developed. DAB complexes containing halo ligands can be reacted with a variety of salts of noncoordinating anions to yield cationic organometallic species. For instance, the reaction in eq 2 illustrates chloride abstraction with NaBAF. " The resulting complexes are... 

It is possible to monitor ethylene chain growth at cationic a-diimine nickel and palladium centers by low-temperature NMR spectroscopy. ° ° Under these conditions, the alkyl ethylene complex on the left of Scheme 2 is the catalyst resting state. (This is... 

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... 

The active species for the PBI complexes is not as well characterized as in the nickel and palladium systems. It is assumed to be a cationic alkyl complex formed by reaction of the dihalo precatalyst with a cocatalyst such as methylaluminoxane (MAO). The resulting active species polymerizes ethylene at unusually high rates to form linear high-density polyethylene. Even at ethylene pressures as low as 1 atm, the polymerization is extremely exothermic and the crystalline polymer product rapidly precipitates from solution. Computational chemistry is proving to be of utility in understanding the mechanistic aspects of this chemistry. - Lower barriers to insertion, relative to the nickel a-diimine complexes, support the higher activity. 

Late transition metal catalysts that are highly active and produce high molecular weight polyolefins were recently reported [198,199]. For example, nickel and palladium-diimine catalysts 67 produce highly branched polyethylene that is totally different from those produced by conventional or homogeneous Ziegler-Natta catalysts [200]. On the other hand, iron and cobalt 2,6-pyridine bis(imine) complexes 68 give linear polyethylene [201,202]. These catalysts are used with co-catalysts such as MAO, and the active species are cationic. Neu-... 

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