Polymer-Supported Aluminum Chloride

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13], 

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Methylenebicyclo[2.2.1]hept-2-ene has been polymerized with Lewis acid-type catalysts to yield soluble polymers 132). Aluminum chloride and boron trifiuoride were used in specific examples. Apparently, the other types of initiation yield cross-linked products from this monomer (however, see Table II.1). Infrared analysis supported structure [111] for the polymer 74). The molecular weight of the polymer appeared to be low based on the viscosity data that were reported, but it could be molded and spun into fibers. 

In another procedure, the preparation of the polymer-supported scandium catalyst was performed according to Scheme 8.17 [70], Polystyrene, cross-linked with divinylbenzene, was treated with 5-phenylvaleryl chloride in carbon disulfide in the presence of aluminum trichloride. The carbonyl groups were then reduced using aluminum trichloride-lithium aluminum hydride in diethyl ether to afford double spacer resin. After sulfonation (chlorosulfonic acid/acetic acid), resin was treated with scandium(III) chloride in acetonitrile at room temperature to give the polymer-supported scandium chloride. Finally, it was treated with trifluo-romethanesulfonic acid to afford the immobilized triflate. 

In order to confirm that the film thicknesses monitored on quartz were the same as those on aluminum under identical coating conditions, the following experiment was performed. Sample films on aluminum, 10 cm2, were obtained for each polymer under coating conditions established on quartz. The polymer films were dissolved off the aluminum support with methylene chloride into 10 mL volumetric flasks. The... 

There are also examples for which there is no need to separate a catalyst, because it can be left in the product without adverse effects. Magnesium chloride-supported catalysts for the polymerization of propylene attain such high mileage that they can be left in the polymer. Earlier less-efficient catalysts had to be removed by an acidic extraction process that produced titanium- and aluminum-containing wastes. The earlier processes also produced heptane-soluble polymer that had to be removed, and, sometimes, ended up as waste. The newer processes produce so little that it can be left in the product. Thus, improved catalysts have eliminated waste. 

The ionic liquid process has a number of significant advantages over the industrial Cosden process. This system uses a supported or liquid phase aluminum(lll) chloride catalyst. Using the ionic liquid process, the polymer forms a separate layer, which is substantially free of catalyst and ionic liquid solvent. This effect greatly enhances the degree of control available to reduce undesirable secondary reactions (i.e., isomerization) without requiring alkali quenching of the reaction. 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

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