Aluminum alkyl halides

In the past, several aluminum-alkyl, halide, and alkoxide complexes supported by multidentate ligands were examined for their catalytic lactide polymerization activities. To this end, monomeric aluminum complexes 148a, b (Fig. 21) were synthesized in our laboratory for producing polyesters with thiolate end groups [137]. These complexes initiated polymerizations under reflux condition in toluene and xylene forming PLAs with narrow molecular weight distributions (PDIs 1.15-1.25). 

Sidgwick, ChemElems( 1950), 414-15 Aluminum Alkyl Halides of the general formulae AlRXaand AlR X were prepd by Grignard and other investigators by treating Al with alkyl halides 2 Al + 3RX = AlRXa + AlR,X. 

Numerous binary and ternary diene polymerization initiator systems with neodymium as the rare-earth metal component have been designed empirically and investigated since the early discoveries in the 1960s. Commercially used neodymium-based catalysts mostly comprise Nd(III) carboxylates, aluminum alkyl halides, and aluminum alkyls or aluminum alkyl hydrides [43, 48,50-52]. Typically, the carboxylic acids, which are provided as mixtures of isomers from petrochemical plants carry solubilizing aliphatic substituents R. They are treated with the alkylaluminum reagents to generate the active catalysts in situ (Scheme 11). 

Aluminum alkyl halides of the types R2A1X and RA1X2 are halide-bridged dimers in which, unlike Al2Me6, Al is surrounded by an electron octet. They are stronger Lewis acids than the trialkyls. The sterically very crowded compounds mes MX2 are monomeric they fail to give complexes with Et20 (M = Al, Ga, In X = Cl, Br).27... 

A continuous solution process [61] has been developed and is operated in the USSR (now CIS). Copolymerization is performed in 10-12 wt% comonomers solution in isopentane or other C5-C7 hydrocarbons, at temperatures between -90 and -50° C, initiated by an aluminum alkyl halide/water catalyst. The catalyst is eliminated by adding a low amount of methanol to the reactor effluent stream, and the polymer is extracted by contacting the solution with hot water and steam. Further operations are similar to those of slurry process. 

Alkyl halides are widely used as cocatalysts in combination with aluminum alkyl halides or aluminum halide Lewis acids. Tlie reaction scheme in Fig. 9-2 illustrates the complicated equilibria which may affect the initiation process. Each carbenium ion can initiate polymerization or remove an ethyl group from the counterion to produce a saturated hydrocarbon, REt, and a new more acidic Lewis acid. The propagating macrocarbenium ions can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the initiator is ostensibly dielhylaluminum chloride there may be major contributions to the polymerization from ethyl aluminum dichloride or aluminum chloride. 

The purpose of this study was to investigate the mechanism of cationic olefin polymerizations by model experiments using alkyl-aluminum/alkyl halide initiator systems and to correlate the results of model experiments with corresponding polymerization reactions. 

Effect of the third component. In the ternary system formed using SAAC Nd(4), the activity was very different for different kinds of aluminum alkyl halides as the third component. The following order of activity was found A1(C2H5)2C1 > Al(C2H5)Cl2 > Al(C2H5)2Br. When we used alkyl chloride in place of aluminum alkyl halide, the catalytic activity depended on the type of alkyl chloride as well as the solvent of catalyst preparation, as shown in Table 13. Triphenylchloromethane is the best of the alkyl chlorides. 

This technique was particularly successful for the introduction of aluminum alkyl halides into co-condensates of first-row transition metal vapors and butadiene/arene mixtures to catalyze butadiene telomerization reactions. The oligomers produced were significantly different from the products produced by heterogeneous catalyst slurry systems. 

An alternative solution process, developed in Russia, uses a C5-C7 hydrocarbon as solvent and an aluminum alkyl halide as the initiator. The polymerization is conducted in scraped surface reactors at -90 to —50°C. The solution process avoids the use of methyl chloride, which is an advantage when butyl rubber is to be halogenated. However, the energy costs are higher than for the slurry process because of the higher viscosity of the polymer solution. Consequently, it is imlikely that the well-established slurry process will be displaced. 

Natta and Ziegler found early on that vanadium halides could be treated with aluminum alkyls to form catalysts competent for alkene polymerizations. A variety of simple precursors in various oxidation states, for example, VCI3, VCI4, and VOCI3. can be treated with aluminum alkyl halides to produce active catalysts. Unlike the titanium and chromium systems, the vanadium catalysts can be single sited with narrow MWDs and more importantly narrow composition distributions. Also unlike the other systems, vanadium incorporates a-olefms at rates somewhat slower but comparable to ethylene. For this reason, vanadium catalysts are used commercially to make EPMs and EPDMs that can have at least 50 mol.% ethylene with the balance being predominantly other olefins and much smaller amounts of diolefins (Figure 9). 

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