Nickel and Palladium a-Diimine Catalysts

Also using palladium, Gottfried and Brookharthave demonstrated conditions that allow for living polymerizations of 1-hexene and 1-octadecene at 0 °C where quenching with Et3SiH is 

It was discovered that addition of carbon monoxide (CO) in the presence of 1-hexene and 72c afforded an alternating copolymer (Borkar et al., 2007). Taking advantage of this, a poly(l-hexene)-/7focfe-poly(l-hexene-flft-CO) diblock copolymer was made through sequential monomer addition. Similarly, a PE-fofock-poly(ethylene-a/f-CO) diblock copolymer was also synthesized through sequential monomer addition. 

Brookhart and coworkers demonstrated the living nature of nickel catalysts 65 and 66 with the synthesis of well-defined di- and tri-block copolymers of a-olefins (Killian et al, 1996). Using 66/MAO at —15 °C, a PP-Wock-poly(l-hexene) was produced with monomodal, narrow molecular-weight distributions (Af,/Af = 1.11-1.13), which exhibited less branching than predicted, due to partial chain straightening. Additionally, a poly(l-octadecene)-c -PP-co-poly( 1-octadecene) was prepared by activation of 65 or 66 with MM AO at — 10°C 

Guan and coworkers have extended the study of hindered diimine catalysts even further with cyclophane complex 69 (Camacho et al, 2004). Upon activation with MMAO, 69 is highly active for production of branched PEs (66-97 branches/1000 carbons) with relatively narrow polydispersities (M M as low as 1.23 at 50 °C). Most significantly, these catalysts exhibit impressive thermal stability, with good activities even up to 90 °C. However, the polydispersity increases, and the activities decrease somewhat at higher temperatures. Interestingly, a related alkyl cyclophane Ni complex demonstrated almost no activity for ethylene polymerization (Camacho et al., 2005). 

To achieve living ethylene polymerization with palladium catalysts 72a,b, Brookhart and coworkers demonstrated that specific reaction conditions, particularly quenching reactions with Et3SiH to prevent chain coupling, were crucial (Gottfried and Brookhart, 2001). At 5 °C, highly branched ( lOO branches/1000 carbons), amorphous PEs with very narrow 

---

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

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... 

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... 

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... 

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... 

Three key features of the original a-diimine polymerization catalysts are (1) highly electrophilic, cationic nickel and palladium metal centers (2) the use of sterically bulky a-diimine ligands and (3) the use of noncoordinating counterions or the use of reagents thought to produce noncoordinating counterions. The electrophilicity of the late metal center... 

The simplest catalyst precursors are the metal dihalides. A number of routes have been developed into these and other catalyst precursors. These methods and the routes to particular complexes of nickel and palladium bearing a-diimine ligands are summarized in Table 1.91107-112 easiest involve... 

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 detailed mechanistic picture developed around the cationic a-diimine nickel and palladium catalysts provides a mechanistic rationale for the above choice of physical attributes. 

Catalyst libraries for combinatorial screening of diimine-based nickel and palladium catalysts can be developed by attaching a diketone to a Merrifield resin, reacting with a variety of anilines in the presence of a dehydrating catalyst, and complexing with the metal (Scheme 25). After each variant is activated with MAO or borate salt, the 96-well microtiter plate is exposed to ethylene and polymerization activity is qualitatively determined by infrared imaging. 

McLain et polymerized cyclopentene by late transition metal catalysts using MAO and borate-activated nickel and palladium diimine complexes. The nickel diimine complexes produce crystalline materials showing a ds-1,3 enchainment with a melting point of 240-330 °C. The hydroligomers were mainly atactic. Palladium catalysts gave pure atactic polymers. It is also possible to polymerize substituted cyclopentenes such as 3-methyl- or 3-ethyl-cyclopentene. 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

---