Chemisorption Pathways and Catalytic Olefin Polymerization

Organometallics 2009, 28, 2053–2061

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Chemisorption Pathways and Catalytic Olefin Polymerization Properties of Group 4 Mono- and Binuclear Constrained Geometry Complexes on Highly Acidic Sulfated Alumina Linda A. Williams and Tobin J. Marks* Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113 ReceiVed NoVember 19, 2008

Mono- and binuclear “constrained-geometry catalyst” (CGC) group 4 hydrocarbyls Me2Si(Me5C5)(tBuN)ZrMe2 [CGCZrMe2, 1], 1-Me2Si(3-ethylindenyl)(tBuN)ZrMe2 [EICGCZrMe2; Zr1, 2], (µ-CH2CH23,3′){(η5-indenyl)[1-Me2Si-(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2, Zr2, 3], and (µ-CH2CH2-3,3′){(η5indenyl)[1-Me2Si-(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2, Ti2, 4] undergo rapid chemisorption on highly Brønsted acidic sulfated alumina (AlS) surfaces. 13C CPMAS NMR spectroscopy of the chemisorbed 13 CRH3-enriched complexes EICGCZr13Me2/AlS (2*/AlS) and EBICGC(Zr13Me2)2/AlS (3*/AlS) reveals that chemisorption involves two processes, M-C σ-bond protonolysis at the strong surface Brønsted acid sites and heterolytic M-C scission with methide transfer to strong surface Lewis acid sites, forming similar “cation-like” electrophilic organo-group 4 complexes such as EICGCM13Me+. Relative rates of ethylene homopolymerization mediated by the catalysts prepared via chemisorption on AlS are 4/AlS > 2/AlS > 3/AlS > 1/AlS, for ethylene polymerization at 75 psi ethylene and 25 °C. Ethylene/1-hexene copolymerizations mediated by the same set of catalysts display relative polymerization rates of 4/AlS > 3/AlS > 2/AlS > 1/AlS, for copolymerizations at 75 psi ethylene, 0.8 M 1-hexene, and 25 °C. Introduction Single-site heterogeneous catalysts have been studied extensively in recent years as olefin polymerization agents, since they combine the advantages of both traditional hetero- and homogeneous catalysts.1 Supporting discrete metal-organic complexes on inorganic oxide supports, such as silica or alumina, yield olefin polymerization catalysts that are suitable for use in existing reactors, have reasonably well-defined active sites, and exhibit far greater control over polymer molecular weight, comonomer incorporation, density, chain branching, and tacticity than many traditional Ziegler-Natta heterogeneous catalysts.1b,2 * Corresponding author. E-mail: [email protected]. (1) For recent reviews see: (a) Thomas, J. M.; Raja, R. Top. Catal. 2006, 40, 3–17. (b) Severn, J. R.; Chadwick, J. C.; Duchateau, R.; Friederichs, N. Chem. ReV. 2005, 105, 4073–4147. (c) Thomas, J. M.; Raja, R.; Lewis, D. W. Angew. Chem., Int. Ed. 2005, 44, 6456–6482. (d) Hlatky, G. C. Chem. ReV. 2000, 100, 1347–1376. (e) Marks, T. J. Acc. Chem. Res. 1992, 25, 57–65. (2) (a) Coates, G. W. Chem. ReV. 2000, 100, 1223–1252. (b) Bhaduri, S., Mukesh, D., Eds. Homogenous Catalysis: Mechanisms and Industrial Applications; John Wiley & Sons: New York, 2000; Vol. 1, pp 105126. (3) For recent reviews on single-site polymerization catalysts see: (a) Amin, S. B.; Marks, T. J. Angew. Chem., Int. Ed. 2008, 47, 2006–2025. (b) Kim, S. H.; Somorjai, G. A. Proc. Nat. Acad. Sci. U.S.A. 2006, 103, 15289–15294. (c) Kaminsky, W. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3911–3921. (d) Gladysz, J. A., Ed. Chem. ReV. 2000, 100 (special issue on “Frontiers in Metal-Catalyzed Polymerization”). (e) Marks, T. J.; Stevens, J. C. Top. Catal 1999, 7, 1 (special volume on “Advances in Polymerization Catalysis. Catalysts and Processes”). (f) Kaminsky, W. Metalorganic Catalysts for Synthesis and Polymerization: Recent Results by Ziegler-Natta and Metallocene InVestigations; Springer-Verlag: Berlin, 1999. (g) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428–447. (h) Jordan, R. F. J. Mol. Catal. 1998, 128 (special issue on “Metallocene and Single Site Olefin Catalysis”), and references therein. (i) Kaminsky, W.; Arndt, M. AdV. Polym. Sci. 1997, 127, 144–187. (j) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255–270. (k) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143–1170. (l) Soga, K., Terano, M., Eds. Catalyst Design for Tailor-Made Polyolefins; Elsevier: Tokyo, 1994.

Such supported catalysts have been shown to exhibit very high activity for R-olefin polymerizations3 as well as for catalytic hydrocarbon transformations.4 This research group has previously studied the chemisorptive reaction pathways of 13C-enriched organoactinides and group 4 metal hydrocarbyls on various metal oxide surfaces.5,6 It was demonstrated by 13C CPMAS and MAS NMR spectroscopy that, depending on the surface acidity of the support, three distinct chemisorptive pathways are operative. For Lewis acidic surfaces, including highly dehydroxylated alumina, partially dehydroxylated alumina, or MgCl2, the metal hydrocarbyl species is activated via heterolytic M-C bond scission, transferring an alkide group to surface acid sites and forming “cationic” adsorbate structures (e.g., structure A).5 This reaction finds strong chemical analogy (and chemical/spectroscopic similarity) to analogous heterolysis processes effected in solution by organo-Lewis acids such as perfluoroaryl boranes and MAO (e.g., eq 1).7 It was also established that metal hydrocarbyl chemisorption on weakly Brønsted acidic surfaces, such as partially dehydroxylated silica and MgO, yields catalytically less active “µ-oxo” species via M-CH3 protonolysis (e.g., structure B).5b,d,g,6 This process also has close homogeneous phase analogies (e.g., eq 2).8In these sepcies, the strong oxo conjugate (4) (a) Candy, J. P.; Coperet, C.; Basset, J. M. Top. Organomet. Chem. 2005, 16, 151–210. (b) Coperet, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J. M. Angew. Chem., Int. Ed. 2003, 42, 156–181. (c) Sheldon, R. A., van Bekkum, H., Eds. Fine Chemicals through Heterogeneous Catalysis; Wiley-VCH: Weinheim, 2001. (d) Lindner, E.; Auer, F.; Baumann, A.; Wegner, P.; Mayer, H. A.; Bertagnolli, H.; Reinohl, U.; Ertel, T. S.; Weber, A. J. Mol. Catal. A: Chem. 2000, 157, 97–109. (e) Lefebvre, F.; Basset, J.-M. J. Mol. Catal. A: Chem. 1999, 146, 3–12. (f) Scott, S. L.; Basset, J.-M.; Niccolai, G. P.; Santini, C. C.; Candy, J. P.; Lecuyer, C.; Quignard, F.; Choplin, A. New J. Chem. 1994, 18, 115–122. (g) Iwasawa, Y.; Gates, B. C. CHEMTECH 1989, 3, 173–181, and references therein. (h) Basset, J.-M. et al., Eds. Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis; Kluwer: Dordrecht, 1988. (i) Iwasawa, Y. AdV. Catal. 1987, 35, 187–264.

10.1021/om801106c CCC: $40.75  2009 American Chemical Society Publication on Web 03/05/2009

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base of the weak surface Brønsted acid site coordinates strongly to the metal center and both saturates the coordination sphere and depresses the electrophilicity.

In contrast to the above reactivity patterns, recent studies of metal hydrocarbyl chemisorption on highly Brønsted acidic sulfated metal oxides including sulfated zirconia9 and sulfated alumina6b,10 reveal that organo-group 4 complexes undergo facile M-C σ-bond protonolysis at the strong Brønsted acidic sites. Again, this reactivity mode finds analogies in protonlytic processes such as in eq 3.11 The result of this chemisorptive process is a highly reactive “cation-like” organometallic electrophile (e.g., structure C), with nearly 100% of the surface sites displaying catalytic activity for ethylene polymerization and arene hydrogenation.6b All indications are that the organometallic cation-oxo anion interaction in C is nondirectional and largely electrostatic.6c One of the great advantages of using a sulfated metal oxide to support a catalyst is the low cost and ease of preparation of sulfated metal oxides, which have sufficient acidity to both activate and immobilize the catalyst. This immobilization-based activation in principle eliminates the need for expensive cocatalysts such as MAO or noncoordinating (5) (a) Motta, A.; Fragala´, I. L.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 16533–16546. (b) Eisen, M. S.; Marks, T. J. J. Mol. Catal. 1994, 86, 23–50. (c) Marks, T. J. Acc. Chem. Res. 1992, 25, 57–65. (d) Eisen, M. S.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 10358–10368. (e) Finch, W. C.; Gillespie, R. D.; Hedden, D.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 6221–6232. (f) Gillespie, R. D.; Burwell, R. L., Jr.; Marks, T. J. Langmuir 1990, 6, 1465–1477. (g) Dahmen, K. H.; Hedden, D.; Burwell, R. L., Jr.; Marks, T. J. Langmuir 1988, 4, 1212–1214. (h) Toscano, P. J.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 653–659. (i) He, M.-Y.; Xiong, G.; Toscano, P. J.; Burwell, R. L., Jr.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 641– 652. (6) (a) Nicholas, C. P.; Marks, T. J. Langmuir 2004, 20, 9456–9462. (b) Nicholas, C. P.; Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 4325–4331. (c) Ahn, H.; Nicholas, C. P.; Marks, T. J. Organometallics 2002, 21, 1788–1806. (d) Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 7103–7110. (e) Toscano, P. J.; Marks, T. J. Langmuir 1986, 2, 820– 823. (7) (a) Conroy, K. D.; Hayes, P. G.; Piers, W. E.; Parvez, M. Organometallics 2007, 26, 4464–4470. (b) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1–77. (c) Stahl, N. G.; Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 10898–10909. (d) Metz, M. V.; Sun, Y.; Stern, C. L.; Marks, T. J. Organometallics 2002, 21, 3691–3702. (e) Chen, Y.-X.; Kruper, W. J.; Roof, G.; Wilson, D. R. J. Am. Chem. Soc. 2001, 123, 745–746. (f) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391–1434, and references therein. (g) Kristen, M. O. Top. Catal. 1999, 7, 89–95. (h) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623–3625. (i) Kaminsky, W.; Kulper, K.; Brintzinger, H. H. Angew. Chem., Int. Ed. Engl. 1985, 24, 507–509. (8) Benetollo, F.; Cavinato, G.; Crosara, L.; Milani, F.; Rossetto, G.; Scelza, C.; Zarella, P. J. Organomet. Chem. 1998, 55, 177–185. (9) For recent reviews on solid acids, see: (a) Song, X.; Sayari, A. Catal. ReV.-Sci. Eng. 1996, 38, 329–412, and references therein. (b) Corma, A. Chem. ReV. 1995, 95, 559–614. (c) Yamaguchih, T. Appl. Catal. 1990, 61, 1–25. (10) (a) Matsuhashi, H.; Motoi, H.; Arata, K. Catal. Lett. 1994, 26, 325– 328. (b) Arata, K. AdV. Catal. 1990, 37, 165–211. (11) Lin, Z.; Marechal, J.-F. L.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1987, 109, 4127–4129.

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anions such as tetrakisperfluorophenyl borate, while reducing problems such as catalyst leaching, associated with such cocatalysts in heterogeneous systems.12 Constrained-geometry catalysts (CGCs) offer many attractions versus conventional metallocene-based polymerization catalysts, such as thermal stability and greater enchainment activity owing to the more open coordination spheres.13 The openness of the CGC active site favors increased comonomer incorporation and subsequent long chain branching in the polymeric products.13c Previously, our group studied the homogeneous polymerization properties of both mononuclear and binuclear CGC-type group 4 catalysts.14 In the homogeneous systems, it is found that the binuclear complexes Zr2 and Ti2 exhibit a “binuclear effect”, which substantially increases the selectivity for long chain branching and comonomer enchainment versus the mononuclear analogues, Zr1, and Ti1. It is proposed that this binuclear effect reflects cooperation between the two proximate electrophilic metal centers in which one metal center assists R-olefin insertion into a second metal-polymeryl species, probably via an agostic interaction with a hydrogen-carbon bond of the R-olefin monomer (e.g., structure D).14e Heretofore, the synthesis of supported CGC-type catalysts focused on three principal immobilization tactics. Early work on CGC-type catalysts can be found in the patent literature.15 CGC-type catalysts were first immobilized on modified surfaces. Dow Chemical Company15a,b and Mitsubishi Petrochemical Company15c,d each reported reacting a support such as Al2O3 or SiO2 with an organoaluminum cocatalyst, MAO or trialkylaluminum, prior to treating the support with a solution of CGC catalyst. Catalysts synthesized in this manner are claimed to exhibit high activity, resistance to poisoning, and long catalyst lifetimes. To activate their catalysts, Exxon Chemicals used N,Ndimethylanilinium tetrakis(pentafluorophenyl)borate in the presence of triethylaluminum.15e,f The use of MAO is undesirable due to high cost and propensity for increased catalyst leaching from the support.12 The most widely used immobilization strategy for CGC-type complexes is to introduce a covalent linkage from the support (12) (a) Kaminsky, W.; Winkelbach, H. Top. Catal. 1999, 7, 61–67. (b) Kaminsky, W.; Strubel, C. J. J. Mol. Catal. A 1998, 128, 191–200. (c) Semikolenova, N. V.; Zakharov, V. A. Macromol. Chem. Phys. 1997, 198, 2889–2897. (d) Mulhaupt, R.; Calabrese, J.; Ittel, S. D. Organometallics 1991, 10, 3403–3406. (13) (a) Cano, J.; Kunz, K. J. Organomet. Chem. 2007, 692, 4411– 4423. (b) Kaminsky, W. Catal. Today 2000, 62, 23–34. (c) McKnight, A. L.; Waymouth, R. M. Chem. ReV. 1998, 98, 2587–2598. (14) (a) Li, H.; Marks, T. J. Proc. Nat. Acad. Sci. U.S.A. 2006, 103, 15295–15302. (b) Li, H.; Stern, C. L.; Marks, T. J. Macromolecules 2005, 38, 9015–9027. (c) Li, H.; Li, L.; Schwartz, D. J.; Metz, M. V.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 14756–14768. (d) Guo, N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542–6543. (e) Li, L.; Metz, M. V.; Li, H.; Chen, M.-C.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 12725– 12741. (f) Li, L.; Marks, T. J. Organometallics 1998, 17, 3996–4003. (15) (a) Jacobsen, G. B.; Spencer, L.; Wateraerts, P. L. (Dow Chemical Co.) PCT Int. Appl. WO 96-16092. (b) Kolthammer, B. Tracy, J. C.; Cardwell, R. S.; Rosen, R. K. (Dow Chemical Co.) PCT Int. Appl. WO 94-07928. (c) Toshihito, S.; Kazuhiro, Y. (Mitsubishi Chemical Co.) Eur. Pat. Appl. EP-728773. (d) Saito, J.; Tsutsui, T.; Matsui, N.; Kawai, K.; Fujita, T.; Ishida, T.; Takahashi, M. (Mitsuibishi Petrochemical Ind.) Jpn Kokai Tokkio Ind. JP07216010. (e) Canich, J. M.; Licciardi, C. F. (Exxon Chemical Co.), U.S. Patent No. 5057475. (f) Upton, P. J.; Canich, J. M.; Hlalky, G. G.; Turner H. W. (Exxon Chemical Co.) PCT Int. Appl. WO9403506. (g) Lynch, J. Fisher D. Langhauser, F. Goertz H. H. Kerth, J. Schweier, G. (BASF A.G.), Eur. Pat. Appl. EP-700934. (h) Stevens, J C.; Nerthamer, D. R. U.S. Patent No. 5064802.

Group 4 Complexes on Sulfated Alumina

to the ancillary ligand, encompassing either part16,17 (Scheme 1, pathway A) or all18 (Scheme 1, pathway B) of the ligand structure, followed by metalation of the ligand to afford the desired complex tethered to the support. This method is less than ideal owing to the difficulty of characterizing the immobilized products during the various synthetic steps, inability to separate undesired byproducts formed on the support during the synthesis, and the need for additional cocatalyst, typically MAO, to activate the final catalyst. Furthermore, the catalytic properties of complexes synthesized in this manner depend on many factors, including metalation procedure16a and surface preparation.16b,c The polymeric products of these catalysts exhibit polydispersity indices (PDI) in the range 2-5, suggesting the presence of a number of kinetically distinct active sites.16c,18a The third common method employed to immobilize CGCtype complexes on metal oxide supports is to synthesize a CGCtype complex containing a reactive alkylsiloxyl tether (Scheme 2).19 Upon reaction of the discrete CGC-type catalyst with the surface, multiple active sites are formed since the complex may react with the surface either through the tether or directly with the metal center.19 The supported catalyst then requires activation with MAO to yield the active catalyst. The polymeric products of these supported catalysts were shown to be bimodal, indicating the presence of multiple active sites.19 In this contribution, we report the chemisorption of mononuclear and binuclear CGC-type organo-group 4 complexes on sulfated alumina (AlS), characterize the adsorbed species by solid-state 13C CPMAS NMR spectroscopy, and investigate their catalytic activities for ethylene homopolymerizations as well as ethylene/1-hexene copolymerizations. We demonstrate the first example of a discrete binuclear CGC-type organometallic polymerization catalyst immobilized on a metal oxide support. We also report that highly electrophilic, “cation-like” organogroup 4 species are formed on AlS and that such species exhibit moderate olefin polymerization activities.

Experimental Section All procedures for air- and moisture-sensitive compounds were carried out with rigorous exclusion of O2 and moisture in flame(16) (a) McKittrick, M. W.; Jones, C. W. Chem. Mater. 2005, 17, 4758– 4761. (b) McKittrick, M. W.; Yu, K.; Jones, C. W. J. Mol. Catal. A.: Chem. 2005, 237, 26–35. (c) Yu, K.; McKittrick, M. W.; Jones, C. W. Organometallics 2004, 23, 4089–4096. (d) McKittrick, M. W.; Jones, C. W. J. Catal. 2004, 227, 186–201. (17) (a) Burkett, S. L.; Soukasene, S.; Milton, K. L.; Welch, R.; Little, A. J.; Kasi, R. M.; Coughlin, E. B. Chem. Mater. 2005, 17, 2716–2723. (b) Kasi, R. M.; Coughlin, E. B. Organometallics 2003, 22, 1534–1539. (18) (a) Juvaste, H. S.; Pakkanen, T. T.; Iiskola, E. I. Organometallics 2000, 19, 4834–4839. (b) Juvaste, H. S.; Pakkanen, T. T.; Iiskola, E. I. Organometallics 2000, 19, 1729–1733. (c) Juvaste, H. S.; Pakkanen, T. T.; Iiskola, E. I. J. Organomet. Chem. 2000, 606, 169–175. (19) Galan-Fereres, M.; Koch, T.; Hey-Hawkins, E.; Eisen, M. S. J. Organomet. Chem. 1999, 580, 145–155.

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or oven-dried Schlenk-type glassware interfaced to a dual-manifold Schlenk line or a high-vacuum (10-5-10-6 Torr) line, or in a nitrogen-filled M-Braun glovebox with a high capacity recirculator (99 atom % D), dried over Na/K alloy, and stored in resealable flasks. The comonomer 1-hexene (Aldrich) was dried over CaH2 and vacuum-transferred into a storage tube containing activated 4 Å molecular sieves. The organometallic complexes Me2Si(Me5C5)(tBuN)ZrMe2 (1),20 1-Me2Si(3-ethylindenyl)(tBuN)ZrMe2 (2),14e (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si-(tBuN)](ZrMe2)}2 (3),14e and (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 (4)21 were prepared by literature procedures. The 13 C-labeled complexes 1-Me2Si(3-ethylindenyl)(tBuN)Zr(13Me)2 (2*) and (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si-(tBuN)](Zr(13Me)2)}2 (3*) were synthesized from 13CH3Li · LiI prepared from 13 CH3I (99% 13C, Cambridge Isotope Laboratories) using analogous methods. The support, sulfated alumina (AlS, Alfa Aesar γ-alumina, 99.97% metal basis), was prepared as described previously.6b Physical and Analytical Measurements. Solution NMR spectra were recorded on either a Varian Inova-400 (FT, 400 MHz 1H, 100 MHz 13C) or an Inova-500 (FT, 500 MHz, 1H; 125 MHz,13C) spectrometer. 1H and 13C NMR experiments on air-sensitive solution samples were conducted in Teflon valve sealed sample tubes (JYoung). 13C CPMAS solid-state NMR spectra were recorded on a Varian VXR400 (FT, 100 MHz, 13C). For 13C CPMAS solid-state NMR spectroscopy, air-sensitive samples were loaded into cylindrical zirconia rotors in the glovebox and capped with a solid Teflon cap. For routine spectra of organozirconium adsorbates, the crosspolarization contact time was 2 ms, and the recycle time was 5 s. For adsorbed complexes, 16 000 scans were required to obtain satisfactory signal:noise ratios, with a spinning rate of 5-10 kHz, depending on peak location versus the location of spinning sidebands. BET surface area measurements were performed on a Micromeritics ASAP 2010 instrument. ICP analyses were performed at Galbraith Laboratories Inc. (Knoxville, TN). Melting temperatures of polymers were measured by DSC (DSC 2920, TA Instruments, Inc.) from the second scan with a heating rate of 10 °C/min. (20) (a) Canich, J. M.; Hlatky, G. G.; Turner, H. W. PCT Appl. WO9200333, 1992. Canich, J. M. (Exxon Chemical Co.) Eur. Patent Appl. EP 420 436-A1, 1991. (b) Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.; Nickias, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. (Dow Chemical Co.) Eur. Patent Appl. EP 416 815-A2, 1991. (21) Li, H.; Li, l.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 10788–10789.


Williams and Marks

Scheme 1. Synthesis of Covalently Tethered CGC-Type Supported Catalysts16c,18a

Scheme 2. Synthesis of Covalently Linked CGC-Type Supported Catalyst on SiO2 or Al2O3 Employing Reactive Tether Method19

Chemisorption of Mononuclear CGC-Type Zirconium Complexes on Sulfated Alumina. In a two-sided fritted reaction vessel, 25 mL of pentane was condensed onto carefully measured quantities of the mononuclear CGC-type zirconium complex and sulfated alumina support. The resulting slurry was stirred at 25 °C for 1 h, then filtered. The impregnated support was collected on the frit and washed five times with pentane, then dried in Vacuo for 1 h. The prepared catalysts were stored in a sealed container under a dry N2 atmosphere at -40 °C until used. If more than the saturation coverage of the organozirconium complex was used, excess complex could be seen on the opposite side of the fritted vessel after washing. When a sample synthesized in this manner was analyzed by ICP spectroscopy following digestion with 48% HF at 100 °C for 12 h, the maximum loadings of organozirconium complexes and sulfur present were 0.79 atoms Zr/nm2 (9.24 × 10-8 mol Zr/mg AlS) and 11.85 atoms S/nm2, respectively. Chemisorption of Binuclear CGC-Type Group 4 Complexes on Sulfated Alumina. In a two-sided fritted reaction vessel, 25 mL of toluene was condensed onto carefully measured amounts of binuclear CGC-type organo-group 4 complex and sulfated alumina. The resulting slurry was stirred at 25 °C for 2 h and filtered. The resulting impregnated support was collected on the frit, washed four times with toluene and twice with pentane, then

dried in Vacuo for 1 h. If more than the saturation coverage of the organo-group 4 complex was used, excess complex could be seen on the opposite side of the fritted vessel after washing. When a sample synthesized in this manner was analyzed by ICP spectroscopy following digestion with 48% HF at 100 °C for 12 h, the maximum loadings of organozirconium complexes and sulfur present were 0.58 atoms Zr/nm2 (6.8 × 10-8 mol Zr/mg AlS) and 7.38 atoms S/nm2, respectively. The maximum loading of Ti was found to be 0.28 atoms Ti/nm2 (4.3 × 10-8 mol Ti/mg AlS). Ethylene Homopolymerizations. In a typical experiment, a 300 mL medium-pressure reaction vessel, dried overnight in a 160 °C oven, was charged in the glovebox with 50 mg of supported catalyst and 10 mL of dry toluene. The reactor was removed from the glovebox, interfaced to the high-pressure line behind a blast shield, and freeze-pump-thaw degassed at -78 °C. The reactor was warmed to the desired temperature using an external bath and equilibrated for 5 min, charged to 75 psi ethylene pressure, and stirred rapidly (>1500 rpm) to minimize mass transport effects. After 3 h the polymerization was quenched with methanol, and the polymeric product was collected by filtration, dried overnight in Vacuo at 60 °C, and weighed. Ethylene + 1-Hexene Copolymerizations. In a typical experiment, a 300 mL medium-pressure reaction vessel, dried overnight


Figure 1. 13C CPMAS NMR spectra (100 MHz) of (A) Zr1Me2 (2; 64 scans, repetition time ) 5 s, contact time ) 2 ms, spinning speed 7.5 kHz); (B) Zr1Me/AlS (2/AlS; 16 000 scans, repetition time ) 5.0 s, contact time ) 2 ms, spinning speed 7.5 kHz); (C) Zr113Me2 (2*; 64 scans, repetition time ) 5.0 s, contact time ) 2 ms, spinning speed 7.5 kHz); (D) 13C CPMAS NMR spectra of Zr113Me/AlS (2*/AlS; 16 000 scans, repetition time ) 5.0 s, contact time ) 2 ms, spinning speed 7.5 kHz). Spinning sidebands are denoted by ×. in a 160 °C oven, was charged in the glovebox with 50 mg of supported catalyst and 10 mL of dry toluene. The reactor was removed from the glovebox, interfaced to the high-pressure line behind a blast shield, freeze-pump-thaw degassed at -78 °C, and warmed to the desired temperature by external bath. Ethylene was added to the reactor and maintained at 1.0 atm, while 1.0 mL of 1-hexene comonomer was immediately added to the reactor by gastight syringe, with rapid stirring. The reactor was then pressurized to 75 psi of ethylene and stirred rapidly (>1500 rpm), to minimize mass transport effects. After 3 h, the polymerization was quenched with methanol, and the polymeric product was collected by filtration, dried overnight in Vacuo at 60 °C, and weighed. Active Site Counting Experiments. A 9.92 × 10-4 M solution of H2O in toluene was prepared by thoroughly drying 165.09 g of toluene over Na/K alloy, then vacuum transferring to a separate flask where 3.4 µL of H2O was syringed in under argon flush. Additionally, to investigate the effects of other poisons, a 1.16 × 10-3 M solution of neopentyl alcohol (NpOH) was prepared by addition of 20.9 mg of NpOH to thoroughly dried toluene (177.63 g). Both of the solutions were used in a batch poisoning study of the olefin polymerization active sites of Zr1/AlS. Batch poisoning experiments using ethylene polymerization as the test reaction were carried out in a similar manner to the ethylene polymerizations described above. In a typical experiment, a 300 mL medium-pressure reaction vessel, dried overnight in a 160 °C oven, was charged in the glovebox with 50 mg of supported catalyst. In place of the 10 mL of toluene, substoichiometric amounts of poison solution (either H2O or NpOH) were added with enough toluene to maintain the solvent volume at a constant 10 mL. The usual ethylene polymerization procedure was then followed. The percentage of active sites was determined by measuring the activity of the catalyst, then plotting the activities as a function of added poison, calculating the linear least-squared best-fit line to the data, and extrapolating to the zero activity point. In all cases, active site calculations are based on the assumption that each molecule of either H2O or NpOH reacts with or poisons one active catalytic site. In previous studies with other catalysts, H2O and NpOH poisoning results were found to be indistinguishable.6


Figure 2. 13C CPMAS NMR spectra (100 MHz) of (A) Zr2Me4 (3; 64 scans, repetition time ) 5.0 s, contact time ) 2 ms, spinning speed 5.0 kHz); (B) Zr2Me4/AlS (3/AlS; 16 000 scans, repetition time ) 5 s, contact time ) 2 ms, spinning speed ) 5.0 kHz); (C) Zr213Me4 (3*; 64 scans, repetition time ) 5.0 s, contact time ) 2 ms, spinning speed 10 kHz); (D) Zr213Me4/AlS (3*/AlS; 16 000 scans, repetition time ) 5 s, contact time ) 2 ms, spinning speed ) 10.0 kHz). Spinning sidebands are denoted by ×.

Results This section begins with a discussion of the chemisorption of CGC-type organo-group 4 precatalysts on sulfated alumina, to synthesize supported olefin polymerization catalysts. We then discuss the structural characteristics of EICGCZr13Me2 (2*) and EBICGC(Zr13Me2)2 (3*) supported on AlS as deduced by 13C CPMAS NMR spectroscopy. Third, ethylene homopolymerizations mediated by 1/AlS, 2/AlS, 3/AlS, and 4/AlS will be discussed. Finally, we investigate the ethylene + 1-hexene copolymerization properties of 1/AlS, 2/AlS, 3/AlS, and 4/AlS. Catalyst Syntheses. All organozirconium catalyst precursors used in this study were synthesized as described elsewhere.14e,20,21 To determine the effects of catalyst nuclearity on olefin polymerization properties for the supported CGC catalysts, mononuclear (1, 2) and binuclear (3, 4) CGC-type group 4 catalysts were employed as adsorbates on sulfated alumina. AlS was synthesized as described elsewhere,6b and the surface area was determined to be 70.2 m2/g by BET measurements (see Experimental Section for details). Structural Characterization of Adsorbate Species. CGCtype organozirconium molecule chemisorption on AlS supports was characterized by solid-state 13C CPMAS NMR spectroscopy using the labeled probe complexes 2* and 3* and tracking the fate of the 13C labeled methyl groups originally coordinated to Zr. In Figure 1D, the 13C CPMAS NMR spectrum of 2*/AlS exhibits major resonances at δ 124.3, 43.3, and -11.7. Data and assignments are summarized in Table 1. The resonance at δ ∼124 is readily assigned to the indenyl ring carbon atoms and is analogous to that in the neat complex (Figure 1A). The broad upfield resonance at δ -11.7 can be assigned to a surface Al-13CH3 functionality, produced by formal methide transfer to a surface Lewis acidic site (structure A above). Such surface 27 Al quadrupole-broadened resonances have also been observed for a variety of organozirconium complexes chemisorbed on dehydroxylated alumina.5,6b The remaining intense resonance at δ 43.3 is assignable to a “cation-like” electron-deficient EICGCZr13CH3+ species, as evidenced by the marked downfield shift of the resonance from the field position of the neutral molecule (δ 40.9; Figure 1C, Table 1). This assignment is


Williams and Marks

Table 1. Solid-State 13C CPMAS NMR Data for Neat and Supported Organozirconium Complexesa complex

indenyl rings

NCMe3

M-CRd

NCMe3

CH2CH2

SiMe2

EICGCZrMe2 (2) 2/AlSc EICGCZr13Me2 (2*) 2*/AlSb EBICGC[Zr(Me2)2]2 (3) 3/AlSc EBICGC[Zr(13Me2)2]2 (3*) 3*/AlSb

133.9-125.6, 86.2 123.9 132.4-125.9, 86.0 133.0 133.6-123.1, 86.9 123.2 128.2-123.1 125.8

56.0 57.0 54.7

40.9, 38.9

35.0 32.3

23.0, 17.7 21.2, 13.6

3.5-2.7 1.9 3.5-2.7

36.1 32.4

32.0

3.3 1.8 3.6

a

55.9 56.7 57.8 13

40.9, 38.9 43.3 40.6 40.1 43.4

other

-11.7 (Al-CH3)

-9.1 (Al-CH3)

b

In ppm downfield from Me4Si; referenced to the solid-state C spectrum of adamantane. Some resonances for these samples are buried within the M-13CR-enriched resonance. c AlS ) sulfated alumina. d Without 13C enrichment, the M-CR and Al-CH3 resonances are too broad and dilute, to distinguish from the background.

supported by the (δ Zr-CH3) downfield shift in the analogous homogeneous catalyst EICGCZr13CH3+Q- at δ 47.6 for Q- ) 13 CH3B(C6F5)3-.22 The remaining ligand resonances of the supported complex are obscured by the broadened 13CH3enriched resonances. When non-13CH3-enriched samples are used, the ligand resonances of the supported complex, including the NCMe3 resonance, are clearly distinguishable (Figures 1A, 1B), and those data are summarized in Table 1. The resonances at δ 56 for 2 and δ 57 for 2/AlS are assignable to the bound NCMe3 group. If the ligand were protonated by the acidic support to form NHCMe3 and detached from the metal center, the resonance would be expected at δ ∼50, as in the free ligand before metalation.14e As can be seen in Figure 2D, the 13C NMR spectrum of 3*/ AlS exhibits resonances at δ 125.6, 57.6, 43.1, and -10.6. Similar to the mononuclear analogue 2*/AlS described above, the resonances at δ 125.8 and -9.1 can be readily assigned to the indenyl ring carbon atoms and surface Al-CH3 moieties, respectively. The feature at δ 43.5 is assignable to a “cationlike” electron-deficient EBICGCZr13Me+ species, as evidenced by the downfield shift relative to the neutral molecule (Table 1). The field position is analogous to that in the mononuclear complex above. When non-13CH3-enriched samples are used, the ligand resonances of the supported complex, including the NCMe3 resonance, are distinguishable in the spectrum (Figures 2A, 2B), and data are summarized in Table 1. The resonance at δ 55.9 for 3 and 56.8 for 3/AlS is assignable to the NCMe3 group, as in the mononuclear case above. The chemisorption of 1, 2, and 3 on AlS was also monitored by 1H NMR spectroscopy of an NMR-scale slurry phase reaction in C6D6. During chemisorption, catalyst peaks disappear due to catalyst immobilization on the surface, and methane is produced via M-C σ-bond protonlysis by the strong surface Brønsted acid sites, evidenced by a methane resonance at δ ) 0.16 in C6D6. During chemisorption of the organozirconium catalyst precursors onto sulfated alumina, a distinct color change from colorless (22) NMR-scale activation of Zr113Me2 with B(C6F5)3 in C6D6.

to yellow occurs on the support, while chemisorption of organotitanium precursors onto sulfated alumina yields a color change from colorless to orange. These color changes are indicative of cation formation during chemisorption and are analogous to the color changes observed for the homogeneous activation of organo-group 4 complexes (e.g., structure E, where A ) organo-Lewis acid, such as B(C6F5)3).23

Olefin Polymerization Experiments. Ethylene homopolymerization studies employing catalysts 1, 2, 3, and 4 supported on AlS were carried out as slurries in a high-pressure batch reactor at 75 psi ethylene and 25 or 60 °C, with rapid stirring (>1500 rpm) to minimize mass transport effects. Results are summarized in Table 2. Ethylene homopolymerization activities follow the order 4/AlS > 2/AlS > 3/AlS > 1/AlS at 25 and 60 °C. Efforts to separate the polymer from the support were unsuccessful, even by extraction with hot 1,2,4-trichlorobenzene at 140 °C. The formation of such high molecular weight polyethylene has also been reported for ethylene polymerizations mediated by related catalyst systems, such as organozirconium adsorbates on alumina,24 on sulfated zirconia,6c and on sulfated alumina.6b Melting point characterization of the present polymeric products by DSC shows that all catalysts studied form polyethylenes with a melting temperatures, Tm, between 138 (23) (a) Chen, Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 6287–6305. (b) Deck, P. A.; Beswick, P. A.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1772–1784. (c) Sun, Y.; Spence, R. E. V. H.; Piers, W. E.; Parvez, M.; Yap, G. P. A. J. Am. Chem. Soc. 1997, 199, 5132–5143, and references therein. (d) Baumann, R.; Davis, W. M.; Schrock, R. R. J. Am. Chem. Soc. 1997, 119, 3830–3831. (e) Wang, Q.; Gillis, D. J.; Quyoum, R.; Jeremic, D.; Tidoret, M.-J.; Baird, M. C. J. Organomet. Chem. 1997, 527, 7–14. (f) Chen, Y.-X.; Stern, C. L.; Yang, X.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 12451–12452.



Table 2. Summary of Ethylene Homopolymerization Data for CGC-Type Supported and Nonsupported Group 4 Complexes on Sulfated Alumina entry

catalyst

temp (°C)

1b,c 2a 3b,d 4a 5a 6b,d 7a 8a 10a 11a

CGCZrMe + PNB CGCZrMe/AlS (1) Zr1 + B1 Zr1/AlS (2) Zr1/AlS (2) Zr2 + 2B1 Zr2/AlS (3) Zr2/AlS (3) Ti2/AlS (4) Ti2/AlS (4)

25 25 25 25 60 25 25 60 25 60

e

[M] (µmol)

reaction time (h)

PE yield (mg)

activityf (×103)

15 4.46 10 4.02 2.85 5.0 5.60 4.46 2.36 2.19

1.17 3 0.75 3 3 1.25 3 3 3 3

65 84 950 217 105 1090 137 77 462 667

3.7 1.21 127 3.51 4.93 87 1.66 1.19 14.4 20.6

Tm (°C)

Mn 590 000

138 610 138 137 760 139 137 136 137

a Carried out at 75 psi ethylene, 10.0 mL of toluene. b Homogeneous polymerization experiment carried out at 1.0 atm ethylene in 100 mL of toluene. Ref 14f. d Ref 14e. e AlS ) sulfated alumina, PNB ) tri(β-perfluoronapthyl)borane; B(C10F7)3, B1 ) Ph3C+ -B(C6F5)3. f Units: grams of polymer/(mol M · h · atm).

c

Table 3. Summary of Ethylene/1-Hexene Copolymerization Data for CGC-Type Supported and Nonsupported Group 4 Complexes on Sulfated Alumina

c

entry

catalyste

temp (°C)

[M] (µmol)

[1-hexene] (M)

reaction time (h)

yield (mg)

activityf (×103)

Tm (°C)

1a 2b,c 3a 4a 5b,c 6a 7a 8b,d 9a 10a

CGCZrMe/AlS (1) Zr1 + B1 Zr1/AlS (2) Zr1/AlS (2) Zr2 + 2B1 Zr2/AlS (3) Zr2/AlS (3) Ti2 Ti2/AlS (4) Ti2/AlS (4)

25 25 25 60 25 25 60 24 25 60

3.86 10 3.26 3.16 5 4.74 4.28 5 2.40 2.37

0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.64 0.8 0.8

3 0.5 3 3 0.75 3 3 0.08 3 3

31 960 60 87 980 89 29 4300 654 875

0.536 192 1.21 1.78 131 1.24 0.45 5100 16.4 18.3

133

Mn 640

135 133 840 135 133 136 137

a Carried out at 75 psi ethylene, 10.0 mL of toluene. b Homogeneous polymerization experiment carried out at 1.0 atm ethylene in 100 mL of toluene. Ref 14e. d ref 21. e AlS ) sulfated alumina, B1 ) Ph3C+ -B(C6F5)3. f Units: grams of polymer/(mol M · h · atm).

Figure 3. Computed energy-minimized structure of Cp2ZrMe+ ion coordination to a µ3-O alumina surface site, showing steric crowding around the supported metal center.5a

and 139 °C, indicating that the polyethylene is highly crystalline, with low branching content.25 Ethylene + 1-hexene copolymerizations employing catalysts 1, 2, 3, and 4 supported on AlS were performed under identical conditions as the ethylene homopolymerizations previously discussed, with the addition of 1-hexene comonomer (0.8 M). The results of the copolymerizations can be seen in Table 3, and activities follow the trend 4/AlS > 3/AlS g 2/AlS > 1/AlS at 25 °C and 4/AlS > 2/AlS g 3/AlS at 60 °C. Efforts to separate the polymer from the support were again unsuccessful, even by extracting with hot 1,2,4-trichlorobenzene at 140 °C. DSC melting point characterization of the polymeric products (24) Jezequel, M.; Dufaud, V.; Ruiz-Garcia, M. J.; Carrillo-Hermosilla, F.; Neugwbauer, U.; Niccolai, G. P.; Lefebvre, F.; Bayard, F.; Corker, J.; Fiddy, S.; Evans, J.; Broyer, J.-P.; Malinge, J.; Basset, J.-M. J. Am. Chem. Soc. 2001, 123, 3520-3540. (25) Crist, B. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 3231– 3236.

shows that all catalysts studied produce copolymers with melting temperatures, Tm, between 133 and 137 °C. Active Site Assays. Attempts to determine the density of catalytically significant sites for 2/AlS by incremental batch poisoning of the ethylene homopolymerizations with either water or neopentyl alcohol aliquots, in substoichiometric amounts, were inconclusive. For the ethylene polymerizations, the activity decrease was minimal, and some polymerization activity was maintained at greater than stoichiometric amounts of poison. Therefore, the number of active sites could not be accurately determined, and we speculate that the ligation is too sterically encumbered to allow poison molecule access to the catalytic center or that the poison reacts preferentially with surface Al-CH3 moieties. For the purposes of this contribution, it is assumed that essentially all of the Zr and Ti sites on AlS are active for olefin polymerization, as found in previous Cp′2ZrMe2/ AlS studies.6b The activities reported are to be taken as the lower limit possible if all the metal sites were catalytically significant. These results indicate that the present supported CGC-type catalysts are remarkably tolerant to small amounts of water in the reaction solvent, with minimal poisoning of activity.

Discussion Chemisorption of CGC Organometallic Precursors on AlS. The exact structures of the acidic centers in sulfated alumina are currently unresolved; however, adsorption/desorption studies employing NH3,26 pyridine,26 and CH3CN27 suggest that sulfonation enhances the acidity of the Lewis acidic sites, increases the Lewis acid site population density, and also creates strong Brønsted acidic sites on the alumina surface. From these observations, two chemisorption reaction pathways for the organozirconium precursor on AlS can be proposed (eqs 4 and


Williams and Marks

5). The chemisorption of the CGC-type catalysts employed in this study was probed using 13C solid-state NMR and solution 1 H NMR spectroscopy. Saturation coverage of the organo-group 4 complexes was used to minimize exposed Brønsted acidic sites on the surface and prevent side reactions at those sites during polymerization.

A protonolytic chemisorption pathway (eq 4) is evidenced by observation of a methane signal (δ ) 0.16 in C6D6) in the 1 H NMR spectrum during in situ slurry phase chemisorption of 1, 2, 3, and 4 on AlS, coupled with a downfield shift of the remaining Zr-13Me signal in the 13C solid-state NMR spectra (Figures 1D, 2D). Second, a significant amount of the catalyst also reacts with the AlS via methide transfer to the surface Lewis acid sites (eq 5), as evidenced by a broad 27Al-CH3 resonance (δ ∼ -9 to -11) in the 13C solid-state NMR spectrum (Figures 1D, 2D). The absence of a Zr-CH3 resonance at δ ∼31 indicates that the formation of the catalytically less active “µ-oxo” species (structure B) is at most a minor pathway. Spectroscopic evidence also suggests that the ligand is not protonated by the surface and remains attached to metal through chemisorption. The NCMe3 resonance at δ ∼57 in the supported complexes is unchanged from the free complex. If the ligand underwent protonation, the 1H resonance is expected to be at δ ∼50, the resonance position in the free ligand before metalation.14e The chemisorption of the binuclear catalysts 3 and 4 could in principle occur via several different binding modes, depending on the relative positions of acidic surface sites. If the local environment is rich in strong Brønsted acidic sites, both metal centers could in principle be activated by the protonolytic chemisorption pathway (e.g., structure F). When the local environment has a high density of Lewis acidic surface sites, both metal centers in the binuclear complex might be activated by methide transfer to the surface (e.g., structure G). Furthermore, if the local environment contains both strong Brønsted acidic sites and Lewis acidic sites, one metal center might be activated by the protonolytic chemisorption pathway while the other might be activated by methide transfer to the surface (e.g., eq 6, structure H). During the chemisorption process, the supported binuclear catalysts, regardless of chemisorption mode, are activated by formation of similar cationic metal centers, which are then potentially active with respect to olefin polymerization. Owing to the fact that the adsorbate structures are quite similar, it is unlikely that they would be rigorously distinguishable by 13C CPMAS NMR. It is also in principle conceivable that only one metal center of the dimer is activated; however, integration of the 13C (26) Yang, T.; Chang, T.; Yeh, C. J. Mol. Catal. A: Chem. 1997, 123, 163–169. (27) Escalona, P. E.; Penarroya, M. M.; Morterra, C. Langmuir 1999, 15, 2079–5087.

CPMAS NMR resonances in Figure 2B suggests that for the majority of molecules both metal centers were activated. If only one metal center were activated by the support, the expected relative integration of the resonances would be ∼3:1 (Zr-Me: Al-Me), whereas if both metal centers were activated by methide transfer to the support, the relative integration would be ∼1:1. For the chemisorption of 3* on AlS (Figure 2B) the relative integration of the resonances is found to be 1:0.6, coupled with additional metal centers being activated by the protonolytic chemisorption pathway, as evidenced by methane evolution in slurry phase solution NMR experiments. We propose that in the majority of molecules both metal centers are activated. For these catalysts, it is estimated that ∼60% of the chemisorption occurs by methide transfer to the surface (eq 5), while the remaining ∼40% occurs by protonolytic chemisorption (eq 4). It is also possible to rule out chemisorption which includes activation at one metal center coupled with rearrangement to form a µ-methyl fragment (e.g., structure I), since a characteristic resonance at δ 15-2228 is not detectable.

Ethylene Homopolymerizations. In all cases, ethylene homopolymerizations mediated by the supported catalysts are significantly less active than the homogeneous analogues in toluene solution (see Table 2). It is proposed that this depressed activity is largely due to steric hindrance from the ancillary ligand-support interaction, blocking ethylene approach to the active site (e.g., Figure 3), as well as mass transfer effects. The supported catalysts studied here produce ultrahigh molecular weight polyethylene with Tm ) 136-139 °C. This result represents a striking increase over the homogeneous Zr system, where the analogous catalysts form low molecular weight polyethylene with Mn ) 600-800 (Table 2, entries 3 and 6),14e and is comparable to the molecular weight increase seen in previous studies of supported catalysts.6b In the present study, binuclear Ti complex Ti2/AlS is found to be significantly more active than any of the zirconium complexes examined. This parallels the homogeneous systems where Ti complexes are more active than analogous Zr systems.14e,21 Among the Zr complexes investigated here, mononuclear Zr1/AlS is found to be the most active for ethylene homopolymerization, followed by Zr2/AlS and CGCZrMe/AlS. The lower activity reported for Zr2/AlS versus Zr1/AlS is tentatively attributed to the (28) Chen, X.-Y.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 12451–12452.



increased steric bulk of the bimetallic complex limiting ethylene from reaching the active site. The supported catalysts examined in this report are found to be less active than the more sterically open catalysts previously studied such as Cp*ZrMe3/AlS and Cp*2ZrMe2/AlS and are in agreement with observations presented in that study14e indicating that activity is roughly inversely proportional to coordinative saturation around the metal center. Mass transfer effects may also play a role in observed activity for the supported catalysts, since diffusional resistance of the active sites to the monomer presumably occurs from both the propagating polymer chain and the support itself.29 Polymerizations conducted at higher temperatures in the present study yield similar results. Ti2/AlS is found to be the most active catalyst, followed by Zr1/AlS and Zr2/AlS for polymerizations run at 60 °C. It should be noted that as the reaction temperature is increased, the activity of Ti2/AlS and Zr1/AlS increases, while the activity of Zr2/AlS decreases. These differences may reflect a combination of catalyst thermal stability, mass transfer effects, and steric effects. Increased polymerization temperature was found to have minimal effect on the polyethylene melting temperature (1-2 °C) and no effect on the solubility of the polymeric products. Ethylene/1-Hexene Copolymerizations. As in the ethylene homopolymerization experiments, all supported CGC-type catalysts exhibit lower copolymerization activity than their homogeneous analogues (see Table 3). Furthermore, the present supported catalysts are also less active with respect to ethylene/ 1-hexene copolymerization than toward ethylene homopolymerization. For ethylene/1-hexene copolymerizations at 25 °C, binuclear complexes Ti2/AlS and Zr2/AlS are found to be the most active, followed by Zr1/AlS and CGCZrMe/AlS, while for copolymerizations conducted at 60 °C, Ti2/AlS is the most active, followed by Zr1/AlS and Zr2/AlS. Again, it should be noted that as the reaction temperature is increased, the activity of Ti2/AlS and Zr1/AlS increases, while the activity of Zr2/ AlS decreases. These differences may reflect a combination of mass transfer effects, steric effects, and catalyst thermal stability. The enhanced comonomer incorporation selectivity observed in the homogeneous copolymerizations mediated by Zr214e and Ti221 is minimal in the copolymerizations mediated by Zr2/

AlS and Ti2/AlS. The similar Tm values of the present ethylene/ 1-hexene copolymers suggest that the 1-hexene incorporation level is approximately the same for each supported catalyst investigated, with each depressing Tm 3-5 °C from that of the respective homopolymer.30 It is suggested that the steric bulk of the bimetallic complex limits 1-hexene approach to the active sites, thus countering the binuclear effect displayed by Ti2 and Zr2 in homogeneous solution.14e Mass transfer effects may also play a role in 1-hexene incorporation into the polymer chain, since diffusional resistance of the active sites to the comonomer presumably occurs from both the propagating polymer chain and the support itself.29 Increasing the polymerization temperature has a minimal effect on copolymer melting temperature (1-2 °C) or solubility.

(29) Czaja, K.; Novoksonova, L. A.; Kovaleva, N. J. Macromol. Chem. Phys. 1999, 200, 983–988.

(30) Chen, H. Y.; Chum, S. P.; Hiltner, A.; Baer, E. J. Polym. Sci., Part B: Polym. Phys. 2001, 37, 1578–1593.

Conclusions This contribution demonstrates via 13C CPMAS NMR spectroscopy employing 13CH3-enriched mono- and binuclear CGC-type organo-zirconium complexes that electrophilic, “cationlike” organo-group 4 species are formed by two chemisorptive pathways on highly acidic sulfated alumina. Chemisorption occurs via either a protonolytic M-CH3 scission pathway or methide transfer to the surface, depending on the type of surface site acidity (Brønsted or Lewis acidic). In each case, similar polymerization-active cationic species are formed. This contribution also presents the first example of binuclear CGC-type Ti and Zr complexes supported on a metal oxide. Polymerization studies of these CGC-type group 4 complexes supported on AlS show moderate activities for ethylene homopolymerization and ethylene/1-hexene copolymerizations, with negligible catalyst nuclearity effects detected. The Ti catalyst studied display an order of magnitude higher polymerization activity than any of the Zr catalysts; however all catalysts display lower activity than their homogeneous analogues.

Acknowledgment. We are grateful to the Department of Energy, for the support of this research under grant 86ER1311, and to Mr. H. F. Yuen and Dr. S. B. Amin for helpful discussions. OM801106C

Chemisorption Pathways and Catalytic Olefin Polymerization

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