Thorium polymerization catalysts

At this point Ziegler and his coworkers carried out experiments on the effects of adding various other metal compounds to triethylaluminum. In one of these experiments with zirconium acetylacetonate, ethylene, and triethylaluminum, they found, to their surprise, an autoclave filled with a solid cake of snow-white polyethylene (1. ) Further work revealed that aluminum alkyls in conjunction with certain transition metal compounds of Groups IV-VI, as well as uranium and thorium, were active ethylene polymerization catalysts. Ultimately, Ziegler catalysts were described to be the product of reaction of metal alkyls, aryls, or hydrides of Groups I-IV and certain transition metal compounds of Groups IV-VIII (Reaction 4). The choice of a particular catalyst and experimental conditions is dictated by the structure of the monomer to be polymerized. 

For example, the compound [Cp 2ThMe]2[(C2B9Hn)2Fe] is completely inactive in ethylene polymerization owing to strong coordination of the anion to each thorium center (the zirconium analog is an active catalyst) but [Cp 2ThMe][B(C6F5)4] rapidly polymerizes ethylene. " ... 

Some support materials can be rendered Lewis acidic enough to ionize dialkyl metallocenes. Marks and co-workers have reported (33) that alnmina dried at very high temperatures can react at least to some small degree with both thorium-and zirconium-based metallocene dimethyl species to yield active catalysts for polyethylene. The resulting cationic metal center is believed to remain coordinated to the surface through an Al-O-M Lewis acid/base linkage, at least prior to exposure to ethylene. Hybrid surface/cocatalyst systems based on aluminum alkyl-treated clays have been developed (34) in which the solid substrate appears to play some role in promoting polymerization activity far beyond that expected for non-methyl aluminoxane- or trialkylaluminum-activated catalysts. 

Actinide, lanthanide, and yttrium-based catalyst systems showing characteristics of reversible chain transfer in ethylene polymerization are summarized in Table 3. Samsel and Eisenberg claimed to observe the characteristics in ethylene polymerization with several metallocenes of actinides, such as the bis(pentamethylcyclopentadienyl) thorium complex 5 in combination with aluminum alkyl reagents. These systems catalyze the production of aluminum alkyl chain growth products at lower temperatures than those required by the uncatalyzed Ziegler process. The systems were limited to production of low-molecular-weight PE oligomers. 

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