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Olefin Polymerization
contributed by
Dr. Robert E. LaPointe1
Novomer

General Information

Mechanism

Catalysts

    There have been many generations of olefin polymerization catalysts:
     
  1. "Black Box" heterogeneous catalysts consisting of a high valent transition metal halide, oxide or oxo-halide with an alkylating co-catalyst such as an alkyl aluminum were first described by Karl Ziegler and Guilio Natta.  Most of these catalyst systems are prepared on supports. Classic examples, many of which are still used today, include TiCl4/MgCl2/AlEt3 (LLDPE, Dow), CrO3/Al2O3/AlEt3 (HDPE, Phillips) and VOCl3/AlEt3 (EPDM, Shell).

  2. While these catalysts are exceedingly active, they have an exceedingly low tolerance for functional groups because of their Lewis acidic nature.  Typically they contain multiple active sites with varying, but generally poor, reactivity ratios for α-olefins relative to ethylene. Little is known about the nature of the actual catalytic species in these systems.
     

  3. "Black Box" homogeneous catalysts consisting of the high valent transition metal complex Cp2ZrCl2 in combination with methylalumoxane ([MeAlO]n, MAO, a hydrolysis product of AlMe3) was first described by Walter Kaminsky (Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99) and was active for the homopolymerization of ethylene to high density polyethylene. This class of catalysts came to include rationally designed systems, such as Ewen's Et(Ind)2ZrCl2/MAO (isotactic polypropylene: Ewen, J. A., J. Am. Chem Soc. 1984, 106, 6355-6364), and iPr(Cp)(Flr)ZrCl2/MAO (syndiotactic polypropylene: Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D., J. Am. Chem. Soc. 1988, 110, 6255-6256). These catalysts rejuvenated olefin polymerization catalysis research in the early 1980's. Despite a quarter century of intense academic and industrial research, the nature of MAO is still largly unknown. Active site counting studies and chromatographic analysis suggest that the MAO exists as clusters with ~10-15 aluminum atoms and acts as both a methyl transfer agent and a Lewis acid to form anions which are weakly coordinating to the cationic metal methyl complexes which are the active catalysts.

  4. Kaminsky Catalyst

    These catalysts have good activity and remarkable tunability via ligand changes in the pre-catalyst complex. For example, the chiral active site of the Kaminsky-Brintzinger catalyst system (Kaminsky, W.; Kulper, K.; Brintzinger, H.H.; Wild, F.R.W.P. Angew. Chem. Int. Ed. Eng.1985, 24, 507.) depicted above generates isotactic polypropylene. However, the use of large amounts of MAO (~ 1000 equivalents, presumably to drive the above equilibrium to the right) results in high catalyst cost and catalyst residue loading of the polymer.
     

  5. Cationic homogeneous catalysts with weakly coordinating anions were first described by Richard Jordan (Jordan, R.F.; LaPointe, R.E.; Bajgur, C.S.; Echols, S.F.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4111.) and prepared via the reaction of early transition metal alkyl complexes, such as Cp'2ZrMe2 with oxidizing tetraphenylborate salts including AgBPh4 and (Cp2Fe)BPh4.

  6. Jordan Catalyst

    Like the Kaminsky type catalysts, the Jordan type catalysts have exceptional tunability via the metal ligands. The early versions of these catalysts showed only modest activity, due to competition for the active site between the olefin and the ether.
     

  7. After the identification of the active site as an alkyl metal cation, rapid advances in pre-catalyst ligand design and activator systems were made by both industrial and academic researchers. New ligands produced catalyst systems with improved comonomer incorporation (1 and 2, Dow, for ethylene/1-octene and ethylene styrene, respectively), tacticity, molecular weight, and even variable tacticity (3, Waymouth, rotation of the ligands switches the active site symmetry allowing production of block atactic/isotactic polypropylene elastomer). New anions (4, Exxon; 5 Marks; 6, Dow) and activators give catalyst systems with astonishing activities (approaching 50,000,000 grams of polymer/gram of metal) in industrial reactors.


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Further Reading and References

dividing line

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This page was last updated Tuesday, March 31, 2015
This document and associated figures are copyright 1996-2016 by Rob Toreki or the contributing author (if any) noted above. Send comments, kudos and suggestions to us by email. All rights reserved.