Methylaluminoxane’s Molecular Cousin: A Well-defined and “Complete” Al-Activator for Molecular Olefin Polymerization Catalysts
Francesco Zaccaria, Cristiano Zuccaccia, Roberta Cipullo, Peter H. M. Budzelaar, Antonio Vittoria, Alceo Macchioni, Vincenzo Busico, Christian Ehm
Abstract
Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger "packages." Along with binary mixtures containing Lewis or Brønsted acids and Al-alkyl systems, methylaluminoxane (MAO), a still ill-defined oligomeric compound, is the only single-component cocatalyst known to fulfill all three roles effectively and simultaneously. Herein, we report a simple molecular alternative, Al-H-Al+[B(C6F5)4]−, an unusual borate salt containing a homodinuclear Al-cation (Al-H-Al+ = [iBu2(DMA)Al]2(μ-H)+). Unlike the simpler [AliBu2]+[B(C6F5)4]−, this species is easily synthesizable and stable at room temperature. Importantly, Al-H-Al+[B(C6F5)4]− can be used as a stand-alone cocatalyst for molecular olefin polymerization catalysis, representing an unprecedented molecular activator able to completely activate dichloride metallocene and prototypical post-metallocene precatalysts. Furthermore, spectroscopic and polymerization studies suggest that Al-H-Al+ is the true activating species formed in situ in the binary cocatalyst [PhMe2NH]+[B(C6F5)4]−/AliBu3. As little as 50 equiv of Al-H-Al+[B(C6F5)4]− are required for efficient catalyst activation and impurity scavenging, orders of magnitude below the amounts usually required with MAO or AliBu3. The high, yet "tamed," Lewis acidity of cationic Al-H-Al+ is likely responsible for the increased scavenging ability. Unlike MAO, the well-defined structure of Al-H-Al+[B(C6F5)4]− offers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.