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Preface There's no gainsaying that metallocene catalysts have had a stunning influence on the mature field of olefin polymerization. Compared to poorly understood heterogeneous conventional Ziegler-Natta systems that produce polymers of broad molecular weight distribution, metallocene catalysts exhibit a truly single-site nature, have extraordinarily high activities, and are amenable to rational modification and even to comprehensive characterization. Metallocene polymerization catalysts were discovered almost at the same time as conventional Ziegler-Natta catalysts, but their low activities relegated them to playthings of the laboratory until they were joined with methylalumoxane to afford systems of much higher activity. The discovery that changes in substituents on the cyclopentadienyl ligand could influence activity, molecular weight, comonomer incorporation, and stereospecificity gave a further boost to their commercial applications. But what is so special about the cyclopentadienyl group as a ligand for metallocene catalysts? It and the substituent groups on it serve to stabilize the active catalytic site and to exert a steric and electronic influence that affects catalyst performance and the characteristics of the polymer produced. B y acting as stable, non-reactive ligands, the cyclopentadienyl groups prevent the active catalytic species from forming other active sites, leading to polymers with broad molecular weight and composition distributions. A n increasing recognition exists that these roles can more than adequately be filled by many other stable ancillary ligands. This and the already crowded, murky patent picture in metallocene catalysis have impelled investigators both in academia and industry to seek new singlesite catalysts that share the advantages of metallocene systems, but that
xi Patil and Hlatky; Beyond Metallocenes ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
do not contain cyclopentadienyl groups. We term this new family of catalysts next-generation single-site olefin polymerization catalysts. Broadly speaking, the stabilizing ancillary ligands used in these catalysts can be broken down into three major categories: •
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• •
isolobal equivalents to the cyclopentadienyl ligand, such as boratabenzenes, azaborolinyls and phospholes neutral ligands, such as the family of alpha-diimines anionic chelating ligands such as the phenoxyimines
The combined effect of these discoveries has been to broaden enormously our previous concepts of what can be done with olefin polymerization catalysts. Up to now, Group 4 metals (Ziegler-Natta or metallocene) produced linear high polymers and metals such as nickel were suitable only for oligomerization of olefins. Some next-generation single-site catalysts of the Group 4 metals already exhibit activities as high as their metallocene cousins and can produce living and stereoregular polyolefins. Nickel-based next-generation catalysts polymerize ethylene to high molecular weight polymer with unusual hyper-branched structures. The portfolio of metals complexes capable of polymerizing ethylene has even been extended to copper. And next-generation catalysts are assisting in the assault of that most formidable of obstacles, the copolymerization of alpha-olefins with polar comonomers. A symposium was organized by the Division of Polymeric Materials: Science and Engineering, Inc. at the 223 American Chemical Society (ACS) National Meeting in Orlando, Florida on April 7-8, 2002 to review the scope of progress in this rapidly expanding field. The chapters in this volume are a result of presentations made at this symposium. To our knowledge, this is the first compilation of progress in nextgeneration non-metallocene polymerization catalysts. The reader will note the broad sweep in the metal complexes (from titanium to copper) used. New ligand synthesis, new metal compounds, new polymer structures, and new combinations of monomers are all featured. In line with the single-site nature of these catalysts, chapters on the analysis and prediction of catalyst behavior using spectroscopy and molecular modeling are also included. rd
xii Patil and Hlatky; Beyond Metallocenes ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
The editors thank the Division of Polymeric Materials: Science and Engineering, Inc., ExxonMobil Corporation, and Equistar Chemicals L P for their support of the symposium on which this book is based.
Abhimanyu O. Patil
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ExxonMobil Research and Engineering Company 1545 Route 22 East Annandale, NJ 08801 Telephone: 908-730-2639 Fax: 908-730-2539 Email:
[email protected] Gregory G. Hlatky Equistar Chemicals LP 11530 Northlake Drive Cincinnati, OH 45249 Telephone: 513-530-4004 Fax: 513-530-4206 Email:
[email protected] xiii Patil and Hlatky; Beyond Metallocenes ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
