Post-metallocene catalysis

Beginning of the post-metallocene catalysis - the new ones often have imine ligand, but not Cp. They are characterized by high activity, a possibility of adjusting polymer MW and MWD as well as copolymerization of olefins with polar monomeric and macromeric species. A wide spectrum of catalysts based on fight and heavy transition metals have been published in open literature and patents. There are also several types of catalysts with intermediate stmcture - pseudo metallocene with a post-metallocene imine ligand. Unfortunately, the product shows poor thermal stability and is not ready for commercialization. 

PE mixtures constitute an important part of commercial blends. The information in Appendix, Table 18.12, suggests five major steps in the production of commercial PE, which result in different types of PE, commercially available at present, viz., 1935 - LDPE with LCB 1950 - LLDPE or HOPE catalyzed by, e.g., CraOg, NiO, M02O3, or CoO 1953 - the Ziegler-Natta (Z-N) catalysis for HOPE, UHMWPE, and LLDPE with a broad MWD and heterogeneous comonomer placement 1975 - the metallocene catalysis producing narrow MWD and homogeneously distributed SCB and 1997 - post-metallocene catalysis that leads to PE copolymers with adjustable MW and MWD as well as copolymerization of olefins with polar monomers and macromers. 

S.D. Ittel, L.K. Johnson, M. Brookhart, Late-metal catalysts for ethylene homo- and copolymerization. Chem. Rev. 100, 1169-1203 (2000) K.P. Bryliakov, Post-metallocene catalysis for olefin polymerization, Russian Chem. Rev. 76(3), 253-277 (2007)... 

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

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