| STRUCTURE |
Andrew R. Barron, the Welch Chair of Chemistry and professor of material science at Rice University, has done much of the seminal work in elucidating structures for MAO. His work has led most chemists to think of MAO as a number of cage structures instead of linear polymer molecules as they had in the past. But, Barron notes, it is MAO's tendency to form many cage structures that makes it difficult to study. "If you picked up a pair of nanotweezers and plucked out a molecule of MAO, you would get a molecule," he says. "If you did it again, you would get a different molecule." Even though models of MAO are starting to emerge, no one has been able to crystallize it, which makes confirming its structure difficult. "Even if someone gets a structure, that's just one of the potential structures," Barron says. "We already have over 10 structures of tert-butylaluminoxane," he adds. EVOLUTION An early candidate to replace MAO, isobutylaluminoxane decamer is shown here with the isobutyl groups in gray and white and the core aluminum and oxygen in red and yellow, respectively. COURTESY OF ALBEMARLE MAO's elusiveness wouldn't matter much if it were the perfect activator, but it isn't. It is made through the hydrolysis of trimethylaluminum, an expensive raw material. Moreover, great excesses of MAO have to be used relative to the amount of metallocene catalyst, increasing the cost of using it. Rice University's Barron notes that much of the activator work has predicated itself on finding strong Lewis acids, even though MAO itself isn't exactly a Lewis acid. Instead, he says, MAO has latent Lewis acidity--it becomes a Lewis acid when its cage structure opens in the presence of a metallocene. Barron cautions that going for merely a strong Lewis acid creates activators that are sensitive to impurities. On the other hand, he adds, the strong Lewis acids are much more active than MAO. |
| UPDATE | 10.01 |
| AUTHOR | Rice Uni.'s Barron Andrew R. |
| LITERATURE REF. | This data is not available for free |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |