Catalysts alumoxane

The metallocene catalyst is used m combination with a promoter usually methyl alumoxane (MAO)... 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

Ewen was the first to report the synthesis of stereoregular propene polymers with soluble Group 4 metal complexes and alumoxane as the co-catalyst [13], He found that Cp2TiPh2 with alumoxane and propene gives isotactic polypropene. This catalyst does not contain an asymmetric site that would be able to control the stereoregularity. A stereo-block-polymer is obtained, see Figure 10.6. Formation of this sequence of regular blocks is taken as a proof for the chain-end control mechanism. 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

Above we mentioned the results reported by Ewen [13] who found that Cp2TiPh2/alumoxane gives a polypropene with isotactic stereoblocks. Naturally, this achiral catalyst can only give chain-end control as it lacks the necessary chiral centre for site control. In the 13C NMR the stereoblocks can be clearly observed as they lead to the typical 1 1 ratio of mmmr and mmrm absorptions in addition to the main peak of mmmm pentads. These are two simple examples showing how the analysis of the 13C NMR spectra can be used for the determination of the most likely mechanism of control of the stereochemistry. Obviously, further details can be obtained from the statistical analysis of the spectra and very neat examples are known [18],... 

MacKenzie (4) prepared poly(ethylcne-co-carbon monoxide) at low pressure using a nickel-based catalyst, (IV), with methyl alumoxane. 

The synthesis of several other catalysts has been described in detail in the literature (25). An aluminoxane can be prepared by the reaction of Al2(S04)3 x 14 H2O and trimethylaluminum in toluene at 0°C (4). The alumoxane acts as an activating co-catalyst to form an alkylmetallocene cation. 

Examples of alumoxanes suitable as activating co-catalysts in the catalysts system are methylalumoxane, isobutylalumoxane, 2,4,4-trimethyl-pentylalumoxane, and 2-methyl-pentylalumoxane. Mixtures of different alumoxanes can also be used (25). Alumoxanes have a core structure analogous to boehmite, i.e., a sequence of -(Al-O)n-, either linear or also as rings. 

On the other hand, instead of an alumoxane compound as activator, N,N-dimethylanilinium tetrakis-perfluorophenylboron has been used with a metallocene catalysts (29). 

Figure 9.5-2. Metallocene catalyst (left) and alumoxane co-catalyst (right).

Recently, Ewen 1291 has found that the soluble Cp2Ti(Ph)2 (Ph = phenyl)/methyl-alumoxane catalyst produces isotactic polypropylene ([m] = 0.83-0.85) at temperatures below —30 °C. The polymerization was carried out in the temperature range of —85 to 50 °C. The highest activity was achieved at —45 °C, and the isotactic (meso) dyad fraction [m] of the produced polypropylene decreased from 0.85 to 0.50 with an increase in the polymerization temperature. Figure 28 shows the time dependence of polymer yield, Mn, number of polymer chain produced per titanium atom, [N], and mjmn, obtained at —60 °C. The yield of polymer is almost proportional to time, but Rln increases to a constant value. The number of polymer chains [N] increases with time, and the value of Mw/Mn increases toward 2.0, indicating that chain transfer... 

The usual cocatalysts for the activation of Nd-alcoholates comprise common aluminum alkyls, alumoxanes and magnesium alkyls which have already been described for the activation of the Nd halides (Sect. 2.1.1.1) and Nd carboxylates (Sect. 2.1.1.2) AlMe3 (TMA) [185,234], TIBA [224,225, 229,230], DIBAH [226,227,232], MAO [232,246], modified methyl alumox-ane (MMAO) [231] and MgR2 [235]. The ratios of cocatalyst/Nd-alcoholate are comparable with those described for the activation of Nd carboxylates. Table 4 gives a selection of catalyst systems based on neodymium alcoholates. 

BR with narrow MMDs (Mw/Mn > 3.5) and a low solution viscosity can also be obtained by the use of a multi-component catalyst system which comprises the following six components (1) Nd-salt, (2) additive for the improvement of Nd-solubility, (3) aluminum-based halide donor, (4) alumoxane, (5) aluminum (hydrido) alkyl, and (6) diene. The solubility of the Nd-salt is improved by acetylacetone, tetrahydrofuran, pyridine, N,N-dimethylformamide, thiophene, diphenylether, triethylamine, organo-phosphoric compounds and mono- or bivalent alcohols (component 2). The catalyst components are prereacted for at least 30 seconds at 20 - 80 °C. Catalyst aging is preferably performed in the presence of a small amount of diene [397,398 ]. As the additives employed for the increase of the solubility of Nd salts exhibit electron-donating properties it can be equally well speculated that poisoning of selective catalyst sites favors the formation of polymers with a low PDI. 

Additives are also used to improve the solubility of halide donors [382, 383]. Metal(II) halides such as magnesium chloride, calcium chloride, barium chloride, manganese chloride, zinc chloride and copper chloride etc. are used as halide sources. In order to increase the solubility of the halides they are reacted with electron donors which have been previously described for the increase of solubility of Nd-components [338,339]. The number of catalyst components is further increased if two Al-compounds (alumoxane + aluminum (hydrido) alkyl) are used. In addition, a small amount of diene can also be present during the preformation of the different catalyst components as described by JSR. In some catalyst systems the total number of components reaches up to eight [338,339]. Such complex catalyst systems are also referred to in other JSR patents [384,385] (Sect. 2.2.6). 

It might be speculated that water reacts with the aluminum alkyl cocatalyst and forms alumoxanes which might also contribute to overall catalyst activity. 

Unbridged, bridged, substituted, and half-sandwich complexes have been used as metallocenes for ethylene polymerization (Figs. 1 and 2). To compare the activities and molecular masses, the polymerizations are carried out under the same conditions (30°C, 2 bar ethylene pressure, with toluene as a solvent) (105). Table IV shows the polymerization behavior of various met-allocene/alumoxane catalysts. Generally, zirconium-containing catalysts are... 

Figure 5.18. Molecular structure of [Me2A10]5, and schematic of the cage-opening mechanism of the alumoxane co-catalyst during metallocene-catalyzed polymerization.

---