A Chemically Functionalized Carboxylate-Alumoxane Nanoparticle Support for Olefin Polymerization Catalysts Stephen J. Obrey 1a and Andrew R. Barron* ,1a,b Department of Chemistry, Rice University, Houston, Texas 77005, and Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005 Received February 22, 2001; Revised Manuscript Received November 12, 2001 ABSTRACT: A metallocene/MAO-based solid olefin polymerization catalyst has been developed using chemically functionalized nanoparticles (carboxylate-alumoxanes) as a well-defined substrate. Reaction of p-hydroxybenzoate-alumoxane (p-HB-A) nanoparticles, formed from the acid with boehmite, with methylalumoxane (MAO) results in a solid nanoparticle-based MAO (n-MAO), which in combination with zirconocenes [including Cp 2ZrCl2, Cp2ZrMe2, and ( n BuCp)2ZrCl2] produces an active solid catalyst for olefin polymerization. The catalytic activity of the n-MAO-based catalyst is comparable to the homogeneous analogue and a traditional silica-supported catalyst under identical reaction conditions and with the same Al (MAO):Zr ratio. The n-MAO approach offers the potential of the MAO being easily chemically modified. Introduction Although it was known since the 1950s that com- pounds of aluminum react with water to give species containing aluminum-oxygen bonds, commonly termed alumoxanes, 2 it was not until the work of Manyik et al. 3 that their application to olefin catalysis was first appreciated. Subsequently, it was shown that the ad- dition of water to the soluble metallocene/alkylalumi- num catalyst systems resulted in a large increase in catalyst activity. 4 The high catalytic activity of a met- allocene in combination with preformed methylalumox- ane (MAO) was shown by the pioneering work of Kaminsky and Sinn. 5 Subsequently, much of the re- search in the field has been to make these catalysts industrially feasible, engineering the specific polymer properties, understanding the roles of the catalyst and cocatalyst play in the polymerization process, and developing alternative cocatalysts to MAO. 6 Industrially, a supported catalyst is desirable to allow for drop-in replacement of the metallocene catalysts in slurry or gas-phase plants. 7 Supported metallocene/ MAO catalyst systems were first suggested by Kamin- sky and co-workers, 8,9 and a wide range of supports, both inorganic (e.g., silica, alumina, magnesium chlo- ride, and zeolites) and organic (e.g., polystyrene, pol- ysiloxanes, methyl acrylate), have been investigated. Supported metallocene/MAO catalysts fall into three general classes: (a) supporting the MAO (or other activator) followed by reaction with the metallocene; (b) supporting the metallocene and then reacting with the MAO; (c) reacting the metallocene/MAO mixture with the support. 10 Most examples of supporting the metal- locene component involve attaching the metallocene through a specific functional group or “tether”. The activity of the metallocene/MAO catalyst has been shown to be dependent on the identity of this tether. 11 In contrast, direct support of MAO has been limited to the reaction of MAO with the surface hydroxides of partially hydrate silica. 12 There has been little effort to tether the MAO through a well-defined linkage group. A further issue with the supports tried to date is that, while it is clear that MAO is more active per Al when reacted with a surface, it is unclear why this occurs. Is the increased activity solely a function of minimizing bimetallic activation of the metallocene, does the MAO undergo a structural rearrangement to a more active form, or are the inactive components of MAO removed? It is desirable to be able to tailor the surface to enhance the cocatalyst activity of MAO, since the activity of MAO is related to the identity of the surface. One last caveat to any new support is that it should ideally form a uniform dispersion on the nanometer range in the final polymer so as not to have a detrimental effect on the transparency of blown films. Thus, a nanoparticle support with a surface that can be chemically modified with a variety of reactive moieties should be ideal. As a result, we have focused our attention on a class of chemically modified alumina nanoparticles, carboxy- late-alumoxanes. 13 Carboxylate-alumoxanes are inexpensive aluminum oxide nanoparticles prepared by the reaction of the mineral boehmite with carboxylic acids, 14,15 whose size (10 to >100 nm) may be controlled by changing the nature of the carboxylic acid and reaction conditions used in the synthesis. 16 Most importantly, these materi- als may be prepared with an almost limitless variety of functional groups, allowing alteration of the chemical characteristics of the surface as well as the ability to covalently bond to MAO. 17 Results and Discussion Synthesis, Characterization, and Reactivity of n-MAO. For our initial studies we have concentrated on a single functional group that readily simulates a hydroxylated surface of an inorganic oxide: p-hydroxy- benzoate-alumoxane (p-HB-A). We have previously reported that p-HB-A may be prepared from the reaction of p-hydroxybenzoic acid and boehmite. 18 Furthermore, contact angle measurements of p-hydroxybenzoate- modified alumina surface show that the p-phenol group has a higher acidity than the native oxide. 19 Reaction of p-HB-A with MAO solution yields MAO- substituted alumina nanoparticles, n-MAO (Scheme 1). It is interesting that there is no apparent reaction at * To whom correspondence should be addressed: e-mail: arb@ rice.edu; URL www.rice.edu/barron. 1499 Macromolecules 2002, 35, 1499-1503 10.1021/ma010314m CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002