Metallocene/aluminoxane

Kaminsky, W. and Noll, A., Polymerization of Phenyl Substituted Cyclic Olefins with Metallocene/Aluminoxane Catalysts , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 149-158. 

Production problems with current metallocene-aluminoxane catalysts reflect the high aluminoxane/metal ratio needed for good catalyst productivity and stereochemical control. Aluminoxane production is slow and relatively inefficient and the large amounts used make post-polymerization catalyst removal processes necessary. These deficiencies are expected to be remedied by different cocatalysts, which are being disclosed in the patent literature. 

Evolution of methane is observed simultaneously with the mixing of metallocene and methylalumoxane. This is caused by the formation of species containing Zr-CH2-Al bonds. The process is repeatable, as shown by the fact that up to 60 mol methane are formed per mol metallocene. Aluminoxane probably reactivates the centers by alkyl transfer [30]. 

Industrially used catalysts for the propylene polymerization are based mainly on Ti compounds (Table 1) , but there are some systems that use MoOj/CaO pretreated with H2 and Cr03 supported on Al203/Si02 or metallocene/aluminoxane systems. 

These metallocene/aluminoxane catalysts are considered "single site" catalysts and therefore produce PE with veiy narrow polydispersily index (close to Stockmayer s distribution where the polydispeisity index is 2) and narrow chemical composition distribution, CCD. They ate usefiil in producing HDPE, and LLDPE. However, one drawback to the metallocene/aluminoxane catalysts is that significant quantities of aluminoxane by-products must be removed after production, due to the high Al/Zr ratios required for optimum performance. The primary role of the aluminoxane is in formation of the active site species. (The initial stracture in the catalytic cycle shown in Figure 5.) The series of complexation and fast alkylation... 

The same level of control over polydispersity index and chemical composition distribution is also realized when metallocene/aluminoxane catalysts are used to produce olefin copolymers. 

Using only a single metallocene/aluminoxane catalyst describe how you could prepare three HDPEs each with different M . 

Ethylene was the first olefin to be polymerized using metallocene/ aluminoxane catalysts. Ethylene polymerization with metallocenes has been extensively reviewed elsewhere [1-6]. The metallocenes commonly used for ethylene polymerization, such as nonchiral cyclopentadienyl derivatives, are also able to polymerize propylene with high productivities, but only atactic chains are produced. 

Soares, J.B.R and Hamielec, A.E. (1995) Metallocene/aluminoxane catalysts for olefin polymerization. A review. Polymer React. Engng, 3(2), 131-200. 

Many other catalysts capable of polymerizing olefins have become available since the original Ziegler-Natta catalyst based on crystalline titanium chloride was introduced. More recently, the discovery of soluble metallocene/aluminoxane catalysts opened the doors to a new revolution in the production of polyolefins. These catalyst systems are able to make polyolefins in very high yields and with a degree of microstructural control not possible to achieve using conventional Ziegler-Natta catalysts. 

A recent new discovery is the fact that the hydrolysis of branched /3-alkyl-substituted aluminoxanes are, in some cases, as effective as co-catalysts in olefin polymerization as MAO.63,64 For example, when combined with the the metallocenes, Cp 2ZrCl2, the hydrolysis products (Al/HzO = 2) of R3A1 (R = Bu and Oct) produced akylated ion pairs with high polymerization activities.65 The same combinations with Cp2ZrCl2 did not produce active catalysts, a result interpreted as due to the inhibition of /3-hydride elimination in the substituted metallocene derivatives. 

It is now well recognised that the active species is a cationic complex, or more precisely a solvent-separated or tight ion pair, the structure of which depends on the mode of catalyst activation. Early spectroscopic and synthetic studies on metallocene dimethyl precursors helped to outline the principal reaction pathways, these have been reviewed [16, 21, 23]. Some of this chemistry is briefly summarised here since it presents the background for the understanding of later studies on methyl-aluminoxane (MAO) systems. 

The metallocene catalysts must be first activated by an aluminoxane co-catalyst, e.g., tetramethylaluminoxane (MAO) which is an oligomer, n being 10-15 (Fig. 9.5-2, right). Because a high excess of MAO is required for activation, a binary co-catalyst system was developed consisting of an organoborate and tri-isobutylaluminium (TiBA). Organoborate can be used in a stoichiometric ratio which reduces the costs and the residue of activator products in the polymer [5]. 

Solid-state hydroxyisobutylaluminoxane co-catalysts prepared by Wu [4] were as effective in activating metallocenes in olefin polymerization as the corresponding alkyl aluminoxanes but at a lower aluminum/metal ratio. 

CP2M— R] [XfAlCCHj)—O)J is involved. Direct involvement of the aluminoxane is implicated since hydrogenation and polymerization reactions that use aluminoxane/ zirconocene catalysts are influenced by the nature of X. The latter results contrast with solution XPS studies that suggest similar species are formed from the dialkyl and dichloride derivatives of metallocenes in the presence of aluminoxane . Interestingly, the enantioface of 1-pentene, styrene, and 2-phenyl-1-butene that is hydrogenated by the 1/aluminoxane catalyst is opposite of that which is polymerized " for 1-pentene and propene. 

If zirconocene compounds or other metallocenes are mixed with aluminoxanes (produced by the controlled reaction of H2O and AIR3) in hydrocarbon solvents, soluble catalysts are formed with extremely high polymerization activities . ... 

Ewen s model can explain the experimental results that show polymerization rates vary linearly with the product of the monomer [M], metallocene [Mt], and aluminoxane concentrations [C] at low monomer conversions with [Al] over a certain range. At constant [M], [Mt], [C], and temperature, the experimental results (Ewen, 1984) show a linear relationship between polymerization rate and polymerization time (t) according to equation... 

Several important homogeneous catalytic reactions (e.g. hydroformylations) have been accomplished in water by use of water-soluble catalysts in some instances water can act as a solvent and as a reactant for hydroformylation. In addition, formation of aluminoxanes by partial hydrolysis of alkylaluminum halides results in very high activity bimetallic Al/Ti or Al/Zr metallocene catalysts for ethene polymerization which would be otherwise inactive. Polymerization of aryl diiodides and acetylene gas has recently been achieved in water with palladium catalysts. Finally, nickel-containing enzymes, such as carbon monoxide dehydrogenase (CODH) and acetyl-CoA synthase, operate in water with reaction mechanisms comparable with those of the WGSR or of the Monsanto methanol-to-acetic-acid process. ... 

Conventional MAO has very low solubility in aliphatic solvents as well as poor storage stability in solution, which considerably limits its utility. Other more soluble and commonly used aluminoxanes are ethylaluminoxane and isobutylaluminoxane, which are synthesized by the partial hydrolysis of triethyl-aluminum (TEA) and triisobutylaluminum (TIBA), respectively. However, these alkylaluminoxanes do not perform as well as MAO in metallocene-mediated olefin polymerization. " It was reported, however, that tetrakis(isooctyl) alumoxane [(i-octyl)2—O—Al-(i-octyl)2], prepared by reaction of Al(i-octyl)3 with 0.5 equiv of water, exhibits remarkable cocatalytic activity, comparable to or even greater than that obtained with MAO, for ethylene polymerization catalyzed by racemic an5a-bis(indenyl)-type zir-conocene dichlorides. Furthermore, commercial modified methylaluminoxanes (MMAO) available from... 

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