Methylaluminoxane's Molecular Cousin: A Well-defined and "Complete" Al-Activator for Molecular Olefin Polymerization Catalysts

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.