REVIEWS The Search for New-Generation Olefin Polymerization Catalysts: Life beyond Metallocenes George J. P. Britovsek, Vernon C. Gibson,* and Duncan F. Wass The introduction of well-defined, sin- generally acknowledged that no single record of being applied in this way, has gle-site organotransition metal olefin class of catalyst will be capable of signposted the way for further techno- polymerization catalysts in the early controlling all of the macromolecular logically significant advances in the 1980s highlighted the possibilities for parameters relevant to a wide and field. In this review, we highlight the controlling and dramatically improving varied range of polyolefinic products. key advances that have occurred in the the properties of commodity polymer Over the past few years, an intense discovery and development of non- products such as polyethylene and search has therefore developed, in Group 4 metallocene catalysts, amply polypropylene. Group 4 metallocenes both academic and industrial research demonstrating that there are signifi- and half-sandwich titanium ± amide laboratories, for new-generation cata- cant signs of life beyond the Group 4 complexes (constrained-geometry cat- lysts. Some of the most significant metallocenes. alysts) have been at the forefront of recent developments have occurred these developments, and, as we ap- with late transition metal systems. Keywords: homogeneous catalysis ´ proach the late 1990s, these catalysts Particularly, the discovery of excep- olefins ´ polymerizations ´ transition are increasingly finding their way into tionally active catalysts based on iron, metals commercial operations. However, it is a metal that had no previous track 1. Introduction 1.1. Background X X M M X Me Si X The past 15 years have witnessed tremendous advances in 2 N the design and application of organometallic complexes as a- tBu olefin polymerization catalysts; many are now reaching the early stages of commercialization. These developments have A B grown out of an increased understanding of the factors that Scheme 1. Group 4 metallocenes (A) and constrained-geometry catalysts are important for stabilizing polymerization-active metal (B). centers and controlling their activity and selectivity, combined with the industrially important discovery that methylalumox- ane (MAO) cocatalysts afford highly active and long-lived greater control over the properties of the resultant polymers catalyst systems. To date, Group 4 metallocenes (A, and to extend the family of products to new monomer Scheme 1) and related catalyst systems such as the half- combinations, the search is gathering apace for yet new highly sandwich amide or constrained-geometry catalysts (B) have active, selective catalyst families that tolerate a variety of been at the forefront of these developments. functional groups. In this review we shall highlight recent However, the search for new catalysts would appear to be advances in the search for new catalysts, focusing primarily on far from over. Driven by industrys desire to obtain ever ligand ± metal complex design and catalyst activity rather than the properties of the resultant polymeric materials. A simple [*] Prof. Dr. V. C. Gibson, Dr. G. J. P. Britovsek, D. F. Wass classification scheme is outlined that allows ligand ± metal Department of Chemistry combinations for active catalysts to be charted. Variations on Imperial College of Science, Technology and Medicine Group 4 metallocene catalyst systems will not be discussed in Exhibition Road, South Kensington, London SW7 2AY (UK) Fax: (44)171-5945810 any detail here since they have been reviewed extensively E-mail: [email protected] elsewhere.[1±7] Angew. Chem. Int. Ed. 1999, 38, 428 ± 447 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 1433-7851/99/3804-0429 $ 17.50+.50/0 429 REVIEWS V. C. Gibson et al. [8] 1.2. The Active Site Na[B{3,5-(CF3)2C6H3}4] or silver salts such as AgBF4 or AgOSO2CF3 (AgOTf) for the later transition metals. It is generally agreed that the catalytically active species in Route B involves the abstraction of an alkyl ligand or, more olefin polymerization is a coordinatively unsaturated cationic strictly, an alkyl anion. Reagents used for these ligands are, alkyl complex [LnMR] that is stabilized by several ligands L for example, [Ph3C][B(C6F5)4], [PhNHMe2][B(C6F5)4], [8] (Scheme 2). To generate such species several methods can be [H(OEt2)2][B{3,5-(CF3)2C6H3}4] or B(C6F5)3. Whereas the employed; three different routes (A, B, and C) are shown in trityl reagent is an abstracting agent, the anilinium salt and the Scheme 2. acid remove the alkyl ligand by protonation. In the case of B(C6F5)3 the alkyl ligand is only partly abstracted leading to ªcation-likeº catalytic species.[9, 10] The applications of per- fluorophenyl-substituted boranes and borates as cocatalysts R R have been recently reviewed.[11] LnM L M n Route C is a combined alkylation and abstraction process, X R which can be achieved by treating a dihalide procatalyst A B + first with an alkylating species and then with one of the R aforementioned alkyl-abstracting agents, for example a tri- L M n alkylaluminum compound followed by B(C6F5)3 . Some re- agents can perform both processes, for example, alkylalumi- num halides and especially alumoxanes such as methylalumi- C noxane (MAO). Albeit the structure of MAO is still largely X unknown, the high activities and long catalyst lifetimes it affords are well documented and still the subject of ongoing LnM study.[12±15] X Scheme 2. Three different routes to the catalytically active species Besides these classical activation methods, an alternative strategy for the generation of a catalytic active species has [LnMR] (see text for details). & represents the site of coordinative unsaturation. [16] been introduced by Erker et al. Treatment of a [Cp2Zr(bu- tadiene)] complex (C) with B(C6F5)3 results in a metallocene- borate-betaine system (D), which is highly active in olefin Route A involves the abstraction of an anionic ligand (e.g. polymerization (Scheme 3).[17, 18] In these systems the cationic a halide) and its substitution for a ªnoncoordinatingº and anionic parts are combined within the same molecules, anion by a salt elimination. Common reagents are ªzwitterionic metallocenesº.[19] Vernon C. Gibson, born in 1958 in Grantham, England, studied chem- istry at the University of Sheffield before moving to the University of Oxford, where he was awarded a D. Phil. in 1983 for work on the coordination and organometallic chemistry of the early transition metals carried out in the group of M. L. H. Green. He then spent two years as a NATO postdoctoral researcher at the California Insti- V. C. Gibson G. J. P. Britovsek D. F. Wass tute of Technology with J. E. Ber- caw before returning to England to take up a lectureship in chemistry at Durham University in 1986. He was appointed to a Chair of Chemistry at Durham sponsored by BP Chemicals in 1993. In 1995 he and his group moved to Imperial College, London, where he is Head of the Centre for Catalysis and Advanced Materials. George J. P. Britovsek, born in 1966 in Heerlen, The Netherlands, studied chemistry at the Technische Universität Aachen (Germany). In 1993 he earned his doctorate from this university under the direction of W. Keim. He then spent two years as a postdoctoral researcher at the University of Tasmania (Australia) with K. J. Cavell, before joining the group of V. C. Gibson at Imperial College in London in 1996. Duncan F. Wass was born in Leicester, England, in 1973. After studying chemistry at Durham University, he moved to Imperial College in 1995 where he is working for his Ph.D. under the supervision of V. C. Gibson. 430 Angew. Chem. Int. Ed. 1999, 38, 428 ± 447 Olefin Polymerization Catalysts REVIEWS Ar Cp2Zr ( ) C – B(C6F5)3 Ar N + Zr+ O N X X F F M N N M X X N M X D tBu N B F Ar N tBu X 3 Ar F F G [N–,N–] H trig-[N–,O,N–] I planar-[N–,N,N–] Scheme 4. Examples of zirconium(iv) procatalysts to illustrate the ligand classifications. PPh Me Ti Zr 2 Me2Si N site of the trigonal bipyramid, while in the case of I the N,N,N R tridentate chelating ligand binds in a planar arrangement, E F thereby leaving the chloro ligands to occupy the equatorial Scheme 3. The zwitterionic approach to active polymerization catalysts. sites of the trigonal bipyramid. The ligands present in G, H, and I may be represented as [N,N], trig-[N,O,N], and Similarly Devore and co-workers independently demon- planar-[N,N,N], respectively. A description of this kind will strated the same principles for constrained-geometry titanium be found adjacent to the pictorial representations of the complexes E.[20] Piers and co-workers have shown recently procatalysts under discussion here, but will not be elaborated that simple alkenes, for example coordinated ethylene in upon. II [Cp2Zr (C2H4)(PPh2Me)] (F), also react with B(C6F5)3 to form a zwitterionic metallocene capable of initiating olefin polymerization.[21] 1.4. Catalyst Activities In this review, except for those cases where well-defined catalysts are reported, we have not considered in any detail The main focus of this review is catalyst performance for the method of activation employed. By far the most common ethylene polymerization, though reference to other a-olefins approach has been activation of a dialkyl or dihalide is included wherever they have been reported. A ªhealth precursor with MAO. In these cases the alkyl or halide group warningº has to be applied when comparing catalyst activities are referred to as a generic group X. reported by different groups of researchers, since experimen- tally determined values are highly dependent upon the precise reaction conditions, including stirring rate and the configu- ration of the reactor.