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Materials Properties Derived from INSITE Metallocene Catalysts

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Materials Properties Derived from INSITE Metallocene Catalysts REVIEW Materials Properties Derived from INSITE Metallocene Catalysts By P. Stephen Chum,* William J. Kruper, and Martin J. Guest Novel metallocene catalysts for the synthesis of ethylene/a-olefin copolymers are reviewed here. The technology usedÐsingle-site constrained geometry catalyst technologyÐis demonstrated to be useful for the preparation of a wide array of copolymers with unique materials properties, such as a high melt fracture resistance, as illustrated in the Figure. 1. General Aspects and Challenges of Catalysis procatalyst is inactive for olefin polymerization and may be activated through the use of Lewis acid catalysis with mixtures Over the past decade, the development of Dow's INSITE of modified methylalumoxane (MMAO) and electron-defi- (trademark of The Dow Chemical Co.) metallocene catalysts cient boranes such as tris-perfluorophenylborane (FAB). Al- has led to the launch of many new polyolefin product lines ternatively, the procatalyst may be activated through the use that had been previously unattainable from conventional of preformed, non-coordinating counterions, which are appro- Ziegler±Natta catalysis.[1] From a structure±activity perspec- priately ion-paired with protonated ammonium or trityl salts. IV tive, the catalyst ligand structures are readily tailored synthet- The nature of the catalytically active species derived from Ti ically from both an electronic and steric point of view. This analogues under polymerization conditions has recently been [4] alteration motif has led to the development and screening of reviewed. It is clear from our studies that variation of the several hundred ansa-cyclopentadienyl amido group IV metal Lewis acid components of the catalyst package can affect constrained-geometry catalysts (CGCs), which have been parameters such as efficiency, comonomer incorporation, Mw, evaluated for the preparation of a wide array of ethylene/ polydispersity, and more importantly, polymer microstructure a-olefin copolymers possessing unique materials properties.[2] and stereoregularity. These parameters are far more dramati- The catalyst precursor or procatalyst (see Scheme 1) con- cally affected by the structural variation of the base ligand [5] tains a group IV metal, preferably titanium, which can be acti- structure through different synthetic approaches. vated either as the TiIV dialkyl or TiII diene complex.[3] The Dow has focussed on catalyst structures that have outstand- ing efficiency or turnover number (typically in excess of 5±7 ´ 106 kg polymer/kg metal) at high temperature (140±180 C) as well as a high propensity for incorporating a-olefin comono- mer throughout the polyethylene backbone. The quest for high catalyst activity at higher temperature is economically driven since higher temperature activity relates to faster poly- merization kinetics and higher throughput of product in exist- ing plant facilities. Dow has recently developed a series of het- Scheme 1. Ansa-Cp-amido procatalyst. R,R¢ = alkyl, alkaryl, carbocyclic, NR2, OR, PR2; E = Si, C, B; R¢¢¢ = alkyl, alkaryl, carbocyclic n = 1,2; R = CH3, alkyl. eroatom-containing catalyst systems that show outstanding performance at higher temperatures with excellent efficiency [6] ± and Mw. [*] P. S. Chum, M. J. Guest At these high temperatures the active catalysts are not ªliv- The Dow Chemical Company ingº (i.e., one polymer chain per catalyst metal site) and as 2310 Brazosport Blvd., B-1470 Freeport, TX 77541 (USA) polymerization proceeds via monomer insertion into the met- E-mail: [email protected] al±carbon bond of the growing polymer chain, a termination W. J. Kruper event may occur via b-hydride elimination of the chain.[7] This The Dow Chemical Company 1776 Building event produces an a-olefinic macromer and a metal hydride Midland, MI 48674 (USA) catalyst species, the latter being capable of producing a new Adv. Mater. 2000, 12, No. 23, December 1 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/2312-1759 $ 17.50+.50/0 1759 P. S. Chum et al./INSITE Metallocene Catalysts polymer chain via the same monomer initiation/propagation the synthesis of catalyst structures that will not only provide REVIEW process. An important consequence of this repetitive process good a-olefin reactivity, but will allow the incorporation of is that numerous polymer chains, typically 3±5 ´ 103 per metal diene monomers (e.g., butadiene, isoprene, ethylidene nor- center, are generated from a single catalyst site. Another bornene) or styrene (S) into the polymer backbone. Diene sought after feature of these systems is their ability to re-in- monomers are frequently strong poisons for the activated me- corporate the a-olefinic macromer into another propagating tallocene species, but are important for the production of eth- polymer chain, much like a short chain a-olefin. This phenom- ylene propylene diene monomer (EPDM) rubbers and new enon leads to long-chain branching (LCB) in the polymer lines of extended thermoplastic olefins (TPOs). We have suc- microstructure, rendering the polymer highly processable cessfully identified several catalysts based upon indenyl struc- under melt-flow operations such as extrusion. The unique tures that are capable of producing these materials in a high- materials properties imparted by LCB to the polymer struc- temperature, solution process. We have likewise developed ture will be subsequently enumerated. catalysts capable of incorporating variable amounts of aro- Heterogeneous catalysts for Ziegler±Natta polymerizations matic olefins such as S into the ethylene (E) backbone (ethyl- have in general suffered from the inability to incorporate sub- ene styrene interpolymers, ESIs), giving rise to a new family stantial amounts of a-olefin under high-temperature, solution of materials.[8] conditions. A second and more demanding challenge involves The ultimate and largely unmet challenge in E/a-olefin Steve Chum obtained his B.S. from Hong Kong Baptist University in 1970 and his Ph.D. from Oregon State University in 1978 for work on organotitanium catalyst chemistry. Between 1978 and 1980, he was a research chemist at Owens-Corning Fiberglass Inc. In 1980, he joined the Central Research organization of The Dow Chemical Company in Midland. Since then, he has had several research assignments at Dow in the areas of foam, film, and polyolefin technologies. He is currently a research fellow at Dow and a society fellow of the Society of Plastics Engineers. He is the co-author of 42 journal publications and a book on metallocene polymer technology. He is also a co-inventor on 27 issued US patents. William J. Kruper received his B.Sc. in Chemistry in 1976 and his Ph.D. in Organic Chemistry from The University of Michigan (1981), where he worked in the area of olefin and hydrocarbon oxidation catalysis (Prof. J. T. Groves). He joined Dow in 1981 and has worked in a wide variety of programs, including discovery and process pharmaceuticals research and catalysis for poly- meric materials. He is currently a Senior Scientist in the Insite Catalysis group of the Chemical Sciences Capability, where he has responsibility for synthetic approaches to metallocene ligands/ activators and new applications of catalysis. He is the author of 16 external publications and 23 US Patents. Martin Guest obtained his B.Sc. from Loughborough University (UK) in 1975 and his Ph.D. from the same university in 1979 for research on the production, characterization, and surface behavior of homopolymers and block copolymers. He was elected to Fellow of the Institute of Materials (UK) in 1992, and served as Associate Editor of the Journal of Applied Polymer Science for three years from January 1996. Following materials R&D in the UK nuclear industry, he joined The Dow Chemical Company (The Netherlands) in 1985, conducting R&D for styre- nic and engineering polymers. He is currently a Scientist at Dow in Polyolefins Research (Free- port, TX, USA), responsible for the materials science and materials engineering of olefin poly- mers. He is a co-inventor on nine issued US patents and is the author or co-author of over 30 publications on subjects including the characterization, rheological, and mechanical properties of copolymers, thermoplastic blends, and composites. 1760 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/2312-1760 $ 17.50+.50/0 Adv. Mater. 2000, 12, No. 23, December 1 P. S. Chum et al./INSITE Metallocene Catalysts REVIEW copolymerization involves the discovery of industrially feasi- mer, which can be crystallized into a high-crystallinity, high- ble catalysts for the incorporation of heteroatom-func- density polymer, called high-density polyethylene (HDPE). tionalized or so-called polar comonomers into the polymer Copolymerization of E and a-olefin comonomer using the backbone.[9] These would include commercial monomers such Ziegler±Natta catalyst yields linear polymer with short as acrylate esters, acrylonitrile, vinyl acetate, and vinyl chlo- branches (from the a-olefin comonomer) along the polymer ride to name a few. The ability to copolymerize these mono- backbone. This class of polymer is called linear low-density mers with E or propylene could enable the production of low- polyethylene (LLDPE) because the short branches in the cost TPOs that could rival engineering thermoplastics and polymer backbone partially inhibit the crystallization of the condensation polymers in terms of materials performance at a polymer and result in a lower density, lower crystallinity poly- fraction of the cost. Clearly many group
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