Methylaluminoxane'S Molecular Cousin: A Well-Defined And "Complete" Al-Activator For Molecular Olefin Polymerization Catalysts

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 Bronsted 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+ = [iBu(2)(DMA)Al](2)(mu-H)(+)). Unlike the simpler [ AliBu(2)](+)[ 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 postmetallocene 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)](-)/AliBu(3). 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 AliBu(3). 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)](-) ffers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.