Cocatalyst, aluminum alkyl

From the early 1960s onwards, the use of lanthanide (Ln) based catalysts for the polymerization of conjugated dienes came to be the focus of fundamental studies [31]. The first patent on the use of lanthanides for diene polymerization originates from 1964 and was submitted by Union Carbide Corporation (UCC) [32,33]. In this patent the use of binary lanthanum and cerium catalysts is claimed. Soon after this discovery by UCC, Throckmorton (Goodyear) revealed the superiority of ternary lanthanide catalyst systems over binary catalyst systems. The ternary systems introduced by Throckmorton comprise a lanthanide compound, an aluminum alkyl cocatalyst and a halide donor [34], Out of the whole series of lanthanides Throckmorton... 

Various cocatalysts are used in Nd-carboxylate-based systems. Most commercially available aluminum alkyls were studied in detail AlMe3 (TMA) [ 174, 184-186], AlEt3 (TEA) [159,187], APBu3 (TIBA) [175,179,188] and A10ct3 [ 189,190]. One of the most referenced cocatalysts is Al Bu2H (DIBAH), e.g. in [178,179,187,191]. Some of the aluminum alkyl cocatalysts were studied comparatively [49,174,179,189,190,192,193]. Some of these studies report results and trends which seem to be contradictory. Since there are so many factors which have an influence on polymerization characteristics and on polymer properties, the discrepancies between the results of different research groups in many cases can be reconciled on the basis of different experimental conditions. 

In the whole study the highest catalyst activity (TOF = 808200 h1) is observed for NdV/DIBAH/EASC at DiBAH/ Ndv = 30. It is interesting to note that the NdP-based catalyst system exhibits its highest activity (TOF = 722 200 hr1) at a DiBAH/ Ndp-ratio as low as 5. This performance is unique as the NdV-based and the NdA-based systems are completely inactive at this low DiBAn/ttNd-ralio. Since the costs for the aluminum alkyl cocatalyst are a major factor in total catalyst cost the high activity of the NdP system at DiBAH/ NdP = 5 has to be particularly emphasized from an economic point of view [268,269]. 

Beside the activating effect aluminum alkyl cocatalysts are also efficient molar mass control agents. Control of molar mass is achieved by the adjustment of the molar ratio of nAi/nN(j (Sects. 2.1.4, 2.2.8 and 4.5). An increase in the amount of cocatalyst results in a decrease of molar mass. A change of the nAi/ Nd-ratio also influences the rate of the polymerization reaction which is a major shortcoming in the large-scale production of Nd-BR, particularly in continuous processes. Detailed discussions of this issue are found in Sect. 2.2.8. Because of this disadvantage research on Nd-BR still strikes out to find efficient non Al-based molar mass control agents which do not influence the rate of polymerization. 

The combination of Nd alcoholates with Mg alkyls also yields catalyst systems with a high trans- 1,4-selectivity. In Nd-alcoholate-based catalyst systems magnesium alkyls are applied in considerably smaller amounts than aluminum alkyl cocatalysts (nMg/nNd > 1) [235]. 

Variations of the amount of cocatalyst which are usually expressed by the molar ratio W Nd have a significant influence on polymerization rates, molar masses, MMDs and on the microstructures of the resulting polymers. These aspects are addressed in the following sections with a special emphasis on ternary catalyst systems. For ternary systems it has to be emphasized, however, that in many reports the ratio Ai/ Nd only accounts for the amount of aluminum alkyl cocatalyst and not for other Al-sources such as alkyl aluminum halides. Variations of the Ai/ Nd-ratios are also used for defined control of molar mass. This aspect is addressed in separate sections (Sects. 2.2.8 and 4.5). 

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

In contrast to these observations, no influence on molar mass was found when the amount of aluminum alkyl cocatalyst was varied for two allyl Nd-based catalyst systems (1) Nd(tj3- CsHs Cl 1.5 THF and (2) Nd( y3- C3H5)C12- 2 THF [292]. 

Why do Nd-phosphates require less aluminum alkyl cocatalyst for activation It is not clear how many equivalents of aluminum alkyls are required in order to generate one active species from Nd-phosphates. Clarification is needed why Nd phosphates require less aluminum alkyl for activation than for example Nd carboxylates do. 

There are few data in the literature as to the influence of aluminum alkyl cocatalyst on polymer MWD obtained with high yield catalytic systems. 

Reaction of R Mg with a transition metal compound produces a reduced transition metal composition co-precipitated with an inorganic magnesium compound. In this respect, dialkylmagnesium compounds are functioning in much the same way as aluminum alkyls described in section 4.2.2. As before, additional aluminum alkyl cocatalyst must be introduced in the polymerization reactor to alkylate the transition metal and create active centers. 

Metal cyclopentadienyl complexes can also be used as cocatalysts, with the intent of creating chromocene-like structures on the surface of the catalyst, as shown in Scheme 46. Chromocene catalysts, which contain mono-attached chromium species incorporating one cyclopentadienyl ligand, are noted for their sensitivity to H2. It is believed that Cr/silica catalysts can be modified to make this species by the addition of metal cyclopentadienyls to the reactor, such as LiCp or MgCp2 [695],or by use of a combination of cyclopentadiene or indene with an aluminum alkyl cocatalyst [696]. When these modified catalysts are allowed to polymerize ethylene in the presence of a remarkable broadening of the polymer MW distribution is observed, mainly as a result of a shift of the low-MW part of the MW distribution. The chromocene surface species is known for its ability to incorporate H2 (thus lowering the polymer MW) and also to reject 1-hexene. Thus, these unusual cocatalysts have the potential to reverse the normal branch profile of polymers made with Cr/silica catalysts (i.e., to put more branches into the longer chains). 

Shortly thereafter, yet another transition-metal catalyst (Ziegler catalyst) capable of polymerizing ethylene at low pressure was discovered in Germany (18). This approach used a transitional metal halide, or other complex, activated by an aluminum alkyl cocatalyst. Transition-metal compoimds of Groups IVa through Via (Ti preferred) were claimed. 

The transition-metal compoimd, usually a titanium(III) or titanium(IV) chloride, is transformed into the active catalytic species upon reaction with an aluminum alkyl cocatalyst. During this reaction the active metal becomes alkylated, and ethylene is inserted into the metal-alkyl bond. Often the active site is considered to be a bridging complex between the transition metal and the alkyl aluminum compoimd, in which one or two ligands are shared between the two metals. 

Metallocenes and Other Single-Site Cataiysts. One type of Ziegler catalyst, based on cyclopentadienyl titanium or zirconium halides (which provided only marginal activity using aluminum alkyls as cocatalysts) received an extreme enhancement in activity in the mid-1970s with the discovery of methylalumi-noxane (MAO) cocatalyst (68,69). Unlike traditional aluminum alkyl cocatalysts, MAO is far more capable of ionizing the transitional metal compound (57,70-73). 

Vanadium catalysts have also been developed for polyethylene and ethylene-based copolymers, particularly ethylene-propylene-diene rubbers (EPDM). Homogeneous (soluble) vanadium catalysts produce relatively narrow MWD polyethylene, whereas supported vandium catalysts give broad MWD (36). Polymerization activity is strongly enhanced by the use of a halogenated hydrocarbon as promoter in combination with a vanadium catalyst and aluminum alkyl cocatalyst (36,37). 

Vanadium-based catalytic systems for EP(D)M synthesis are comprised of a vanadium compound ( catalyst precursor ), a chlorinated aluminum alkyl ( cocatalyst ), and a chlorinated ester ( promoter ). Typical components of a vanadium-based catalytic system are the following ... 

Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used in Ziegler systems, are indeed capable of polymerising ethene, but only at a very low activity. Only with the discovery and application of methylaluminoxane (MAO) it was possible to enhance the activity, surprisingly, by a factor of 10000 [113]. Therefore, MAO plays a crucial part in the catalysis with metallocenes. 

Unlike Ziegler-type catalysts, based mostly on titanium, where an aluminum alkyl cocatalyst is responsible for the formation of a Ti-alkyl bond and initiation of the ethylene polymerization process, chromium-based catalysts do not require such a cocatalyst. 

DuPont introduced a commercial solution process in the 1960s with the tradename of Sclair that reportedly used a Ziegler-type catalyst based on both a vanadium compound (VCl or VOCI3) and a titanium compormd (TiCl ) in the presence of an aluminum alkyl cocatalyst. Operating conditions were above 200°C and 1,000 psi [42]. 

Xia W, Tonosaki K, Taniike T, et al Copolymerization of ethylene and cyclopentene with the Phillips CrOx/Si02 catalyst in the presence of an aluminum alkyl cocatalyst, J Appl Polym Sci 111(4) 1869-1877, 2009. 

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