Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts Hans H. Brintzinger, * David Fischer, Rolf Miilhaupt, Bernhard Rieger, and Robert M. Waymouth Dedicated to all those excellent graduate students who are the true heroes of this story Current studies on novel, metallocene­ type of metal center with a defined coor­ the homo- and copolymerization of based catalysts for the polymerization of dination environment. This makes it cyclic olefins, the cyclopolymerization ex-olefins have far-reaching implications possible to correlate metallocene struc­ of dienes, and access even to functional­ for the development of new materials as tures with polymer properties such as ized polyolefins, these catalysts greatly well as for the understanding of basic molecular weight, stereochemical mi­ expand the range and versatility of tech­ reaction mechanisms responsible for the crostructure, crystallization behavior, nically feasible types of polyolefin mate­ growth of a polymer chain at a catalyst and mechanical properties. Homoge­ rials. center and the control of its stereoregu­ neous catalyst systems now afford effi­ larity. In contrast to heterogeneous cient control of regio- and stereo regular­ Keywords: alkenes . catalysis . metal­ Ziegler-Natta catalysts, polymerization ities, molecular weights and molecular locenes . polymerizations by a homogeneous, metallocene-based weight distributions, and comonomer catalyst occurs principally at a single incorporation. By providing a means for 1. Introduction Sawhorse projection: Modified Fischer projection: Forty years after Karl Ziegler's invention of transition metal ~ CH. CH. 1-- CH • catalyzed polyinsertion and Giulio Natta's discovery of the .." ... ~ .. ~ ...... r:- atactic ----. I----- CH. CH. stereoselective polymerization of ex-olefins'£! -4] we are witness­ I ing the evolution of new generations of catalysts and polyolefin materials, which originate from studies on homogeneous, metal­ ~ CH. ~ CH. ~ locene-based polymerization catalysts. In the following, we will .. ' .. ~ ......~ ... synd iotactic ----. ----- CH. CH. CH. attempt to review some of these recent developmentsP - 7] I I Research on metallocene-catalyzed olefin polymerization has derived much of its impetus from the desire to model the reac­ CH. CH. CH. CH. CH • ----. I I I----- tion mechanisms of heterogeneous polymerization catalysts. In .... ~ ...~ ...~ .. ~ .. r:- isotactic the evolution of Ziegler-Natta catalysis, an empirical approach has proven highly successful. Modern MgClz-supported cata­ lysts have tremendous activities for the production of polypropene and other polyolefins; at the same time, they are Conformation of chains in crystalline isotactic polypropene: ['J Prof. Dr. H. H. Brintzinger Fakultat fur Chemie der Universitiit D· 78434 Konstanz (Germany) Telefax: ]nt. code +(7531) 883137 Dr. D. Fischer BASF AG. Ahteilung ZKP Ludwigshafen (Germany) Prof. Dr. R. Mulhaupt Institut fur Makromolekulare Chemie der Universitat Freiburg (Germany) Dr. B. Rieger Institut fUr Anorganische Chemie der U niversitiit Tubingen (Germany) Scheme 1. Structure of isotactic. syndiotactic, and atactic polypropene in sawhorse Prof. Dr. R. M. Waymouth and modified Fischer projections [107] (top), and conformation of the chains in Department of Chemistry. Stanford University (USA) crystalline isotactic polypropene, determined by Natta and Corradini [2] (bottom). 1143 highly stereoselective, forming practically only the isotactic, The relationship between the properties of a particular cata­ most useful stereoisomer of the polymer[8] (Scheme 1). Still un­ lyst and the coordination geometry of its reaction centers, how­ satisfactory, however, is our understanding of the reaction ever, leaves many questions open, [8g. h) due to the notorious mechanisms that are the basis of this advanced catalyst technol­ nonuniformity of active sites in these heterogeneous catalysts ogy. Plausible hypotheses concerning these mechanisms and the limited experimental access to their structural details. In have certainly been advanced, most notably Cossee's model a review written in 1980, Pino and Miilhaupt summarized this of polymer chain growth by cis-insertion of ct-olefins into a shortcoming: [4b] "Up to now, there is no direct proof for the Ti-C bond on the surface of crystalline TiCI 3 .[9.10] Based on structures proposed; most of them are the result of speculations Natta's early ideas about the role of chiral surface sites in the or derived from indirect experimental indications." formation of isotactic polyolefins,[6. 11] very adequate models In this situation, related developments in other fields of catal­ were proposed to explain the induction of stereoregular polymer ysis nourished the hope that homogeneous organometallic cata­ growth by the chiral environment of the catalyst centers[9b. 12] lysts capable of stereoselective olefin polymerization might (Scheme 2). eventually allow more direct observations on the catalyst species H.-H. Brintzinger D. Fischer R. Miilhaupt B. Rieger R. M. Waymouth Hans-Herbert Brintzinger, born in Jena, Germany, in 1935, studied chemistry in Basel, where he received his Ph.D. in 1960 with Hans Erlenmeyer and his Habilitation in 1964. Afier teaching chemistry at the University of Michigan for several years, he moved to the University of Constance in 1972, where he holds a professorship for General Chemistry and Coordination Chemistry. His fields of interest are organometallic chemistry and reaction mechanisms in homogeneous catalysis. David Fischer, born 1963 in Cologne, Germany, studied chemistry at the University of Cologne. After earning his diploma in physical chemistry, he joined Rolf Miilhaupt 's group at the University 0/ Freiburg as a graduate student. He received his Ph.D. in 1992for studies on propene polymerization with homogeneous zirconocene catalysts. After a postdoctoral period with Robert Waymouth at Stanford, he joined the plastics laboratory of BASF AG in 1993, where he is still working on stereospecific propene polymerization with metallocene catalysts. Rolf Miilhaupt was born in Waldshut-Tiengen, Germany, in 1954. From 1973 to 1978 he studied chemistry in Freiburg; he received his Ph.D. in 1981 with Professor Pino at the Laboratory for Technical Chemistry at the ETH in Ziirich. Following industrial research positions at the Central Research and Development Experimental Station of Dupont in Wilmington, Delaware,from 1981 to 1985 and at Ciby-Geigy AG in Marly, Switzerland,from 1985 to 1989, he was appointed professor of Macromolecular Chemistry at the University of Freiburg in 1989. Since 1992 he has been director of the Material Research Center in Freiburg. His research interests include, besides transition metal catalyzed polymerization, new polymer structural materials, material compounds, dispersions and specialty polymers. Bernhard Rieger was born in Augsburg, Germany, in 1959. He studied chemistry at the University of Munich and received his Ph.D. in 1988 for studies on enantioselective hydrogenations with rhodium-phosphane catalysts. After research appointments at the Institutefor Polymer Science and Engineering of the University of M assachuselts at Amherst and in the plastics laboratory of BASF AG, he received his Habilitation in 1995 at the University of Tiibingen, It'here he presently holds a faculty position in chemistry. Robert M. Waymouth was born in Warner Robins, Georgia (USA) , in 1960. He received bachelor's degrees in mathematics and chemistry at Washington and Lee University and his Ph.D. from California Institute of Technology in 1987, where he worked u'ith Professor R. Grubbs. Following a year's postdoctoral appointment at the ETH in Switzerland with Professor Pino, he joined thefaculty at Stanford in 1988, where he is now an Associate Professor of Chemistry. His research interests are in stereoselective catalysis. 1144 Scheme 3. Polymerization of ethene by cis-insertion into the Ti-C bond of an alkylaluminum halide activated titanocene complex, as proposed by Breslow, New­ burg, and Long [17]. Iy metallocene studies contributed to the ideas put forth by Scheme 2. Model for the stereospecific polymerization of propene at a chiral Ti Cossee[9al with regard to the mechanisms of heterogeneous center on the edge of a TiCl, crystal. as proposed by Corradini and co-workers Ziegler-Natta catalysis. [12 bJ. The growing polymer chain occupies the open sector of the chiral coordina­ tion sphere; the olefin adopts that enantiofacial orientation which places the olefin substituent lrans to the polymer chain at the incipient C-C bond (left); a cis orien­ tation of olefin substituent and polymer chain is disfavored (right). 2.1. Metallocene Cations in the Polymerization of Ethene An interesting question remained unresolved by this early research: does olefin insertion occur in a bimetallic species, in involved and, hence, on the mechanisms of polymer growth and which an alkyl group or a halogen bridges the titanium and its stereochemical control,l13] This sentiment was expressed, for aluminum centers, as advocated by Natta and his group,(19] by example, by Boor in the concluding chapter of his book on Patat and Sinn,[ZO] by Reichert and co-workers,[ZI] and by Hen­ Ziegler-Natta catalysts.[3c] rici-Olive and Olive?[Z2] Or does it require the formation of a Particularly attractive as model systems of this kind ap­ truly ionic species [CpzTiR]+ by abstraction of a halide anion peared alkylaluminum-activated metallocene complexes of the and its incorporation into an anion