Late-Metal Olefin Polymerization Catalysts

Theoretical studies have been carried out on all the late transition metal catalysts la [10-13], lb [14] and lc [15] in Figure 1. It is not the objective here to review all the computational results. We shall instead describe the general mechanistic insight that has been gained from the theoretical studies with the main emphasis on Brookhart s bis-imine catalysts. The experimental work on late transition metal olefin polymerization catalysts has been reviewed recently by Ittel [16] et al. 

Strong interest in late transition metal olefin polymerization catalysts resulted in the development of new five-coordinate Fe and Co systems (69) that afford highly linear, crystalline, high-density polyethylene.587-589 A new class of single-component, neutral Ni catalysts based on salicylaldimine ligands (70) was reported to be active in the polymerization of ethylene 590,591... 

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... 

The use of late transition metals as olefin polymerization catalyst requires the suppression of chain transfer while at the same time a high chain growth rate should be maintained. These new catalysts have an electron-deficient, in most cases, 14-electron and cationic metal center with a vacant coordination site. The most... 

The polymerization of olefins has long been one of the most industrially important applications of organometaiiic complexes.The use of transition metals as catalysts ranges from early metal (e.g., Ti, Zr) Ziegler-Natta type polymerizations to late-metal single site catalysts as well as atom transfer radical polymerizations (ATRPs)... 

The majority of polyolefins is produced with titanium (Zeigler catalysts) and zirconium (metallocene catalysts) or by a fiee radical process Oow-density polyethylene (LDPE)). Recently, late transition metals (LTMs), in particular nickel and palladium [10, 11], and iron and cobalt, are seeing a renewed interest as olefin polymerization catalysts [12, 13]. 

Late-transition metal (Ni, Pd, Fe, and Co) olefin polymerization catalysts are particularly stable to Lewis bases and water in contrast to the vast majority of olefin polymerization catalysts. Miilhaupt et al. used both Pd- and Ni-based catalysts, [ iPr2C6H3N=C(Me)C(Me)=NC6H3iPr2-2,6 Pd(CH3)(N=CCH3)]dBArJ- and N,N-bis(2,6-diisopropylphenyl)- , 4-diaza-2,3-dimethyl-l,3-butadiene nickel dibromide, for preparing highly branched PE-clay nanocomposite. They found that they were much less sensitive to the addition of non-modified and modified layered silicates than metallocene catalysts. 

Recent Progress in Late Transition Metal a-Diimine Catalysts for Olefin Polymerization... 

Fig. 1 Early examples of late transition metal olefin oligomerization and polymerization catalysts...

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

Figure 1. Late transition metal catalysts for olefin polymerization of current interest...

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

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