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.

Highly active molecular catalysts for olefin polymerization are extremely difficult to run in High Throughput Experimentation (HTE) platforms. With common activators like e.g. methylalumoxane (MAO) or a combination of tri-iso-butylaluminum and trityl tetrakis(perfluorophenyl)borate (TIBA/TTB), the necessary downscaling ends up with (sub)nanomole precatalyst loadings and poorly reproducible results, due to the presence of adventitious impurities in similar amounts. Unexpectedly, we have now discovered that a convenient solution to this problem is provided by TIBA/AB (AB = N,N-dimethylanilinium tetrakis(perfluorophenyl)borate), a long-known but relatively uncommon protic activator. Indeed, with a proper operating protocol, a tunable precatalyst activation delay (minutes to hours) can be achieved, and even at high (≥ 10 nmol) catalyst loadings a transient phase of well-controlled activity can be maintained long enough to produce the polymer amounts necessary for the characterizations under highly reproducible conditions. Importantly, polymer properties were not affected by the choice of activator, provided that the polymerization was kinetically controlled, which makes TIBA/AB the best option for HTE screenings of industrially relevant catalysts.