Olefin Polymerization

General Information

The transition metal catalyzed polymerizaion of olefins can be thought of as a reaction in which the carbon-carbon π-bond in α-olefins (1-alkenes) are cut and then remade as a carbon-carbon σ-bonds, linking the olefins together in long hydrocarbon chains.  This reaction is exothermic by ~ 20 Kcal/mol, the energy difference between the carbon-carbon π- and σ-bonds.  For typical early transition metal, d⁰ catalysts, the activation energy is ~ 10 Kcal/mole, making this a very facile reaction.

These long chain macromolecules are known as polymers (from the Greek, poly = many and mer = units) and are the principal constituents of many common plastics. Some familar plastics made via early transition metal catalyzed polymerization include: high density polyethylene (HDPE, R=H), linear low density polyethylene (LLDPE, R = mostly H with some Et, Bu or Hx), polypropylene (R = Me) and ethylene-propylene-diene-modified rubber (EPDM, R = H, Me and Alkenyl).

Mechanism

The commonly accepted mechanism for the olefin polymerization reaction is shown below. A severely electron deficient metal center coordinates the π-bond of an olefin to form a weakly bound olefin complex; the bond is weak because there are no d-electrons to form a π-bond. The bound olefin then inserts into a metal-carbon bond within the complex via a four center transition state, forming new metal-carbon and carbon-carbon bonds. This process then repeats, extending the chain indefinitely.

Chain growth can be terminated by a number of processes. One of the most impotant is β-hydride elimination. Since this is simply the reverse of insertion, the activation energy is ~ 30 Kcal/mole plus the energy difference between the metal-carbon and metal-hydrogen bonds. Catalysts with relatively strong M-C bonds give higher molecular weight polymers, while those with relatively strong M-H bonds give lower molecular weight polymers (waxes). Beta-hydride elimination leads to polymers with vinyl (R=H) or vinylidene (R=Me, Et, Pr...) end groups. The vinyl terminated polymers are, of course, α-olefins and may also be incorporated into the growing polymer chains giving "long chain branched" products under appropriate conditions.

Another termination process of great practical use is hydrogenolysis. This sigma-bond metathesis reaction leads to polymers with saturated end groups:

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).

   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.

2. "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.

   

   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.

3. 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.

   

   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.

4. 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.

   

   

Further Reading and References

• Review articles:
  • Hlatky, G.G. Coord. Chem. Rev. 2000, 199, 235-329.
  • Resconi, L.; Chadwick, J. C.; Cavallo, L. in Comprehensive Organometallic Chemistry III. Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier, 2007, 4, 1005-1166.
  • Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chemical Reviews 2000, 100(4), 1253-1346. (Selectivity in Propene Polymerization with Metallocene Catalysts)
• Books/Chapters:
  • Baugh, L.S.; Canich, J.A.M. Stereoselective Polymerization with Single-Site Catalysts; 2000, CRC Press: Boca Raton, 2007. (selected chapters)
  • Kuran, W. Principles of Coordination Polymerisation; Wiley, New York, 2001, p. 43.
  • Moore, E. P Polypropylene Handbook: Polymerization, Characterization, Properties, Processing, Applications; Hanser Gardner Pub. 1996, p 419.
  • Boor, J. Ziegler-Natta Catalysts and Polymerizations; Academic Press: New York, 1979; p 670.
  • Rieger, B.; Baugh, L. S.; Kacker, S. Late Transition Metal Polymerization Catalysis; Wiley-VCH, 2003./li>

¹The author and Rob Toreki acknowledge Larry Jones of DSM PTG for his insightful editorial comments and the reference list provided here.