Methylaluminoxane's Molecular Cousins: Well-Defined Aluminum-Alkyl Borate Salts as Cocatalysts for Olefin Polymerization.

The success story of molecular olefin polymerization has two protagonists, group 4 metal precatalysts and group 13 cocatalysts, which play equally important roles but have evolved at very different paces. Precatalyst design has shown tremendous progress over the past decades. Conversely, cocatalyst development has almost reached a standstill after methylaluminoxane (MAO) and organic borate salts/tri-iso-butylaluminum (TIBAL) binary systems were introduced in the 1980s-1990s. The complexity of innovating cocatalysts arises from the multifaceted roles that they fulfill and the numerous subtleties presiding over their reactivity. Major limitations persist, mostly related to the complex structure of MAO, hampering rational tuning, and to borate salts being "incomplete" activators, since they function only as alkyl abstractors. Recently, we identified an innovative cocatalyst providing comparable performance to these established activators while combining their distinctive advantages. This system is based on a borate salt of an unusual dinuclear aluminum-alkyl cation, {[iBu2(DMA)Al]2(μ-H)}[B(C6F5)4] (AlHAl_DMA; DMA = N,N-dimethylaniline), which behaves as a single-component "complete" cocatalyst like MAO, while featuring a well-defined molecular structure like traditional organic borate salts. This species can activate both dialkyl and dichloride precatalysts even in stoichiometric amounts, that is, orders of magnitude lower than those required with MAO. Moreover, it efficiently scavenges impurities, enabling olefin polymerization at catalyst concentrations as low as 10-7 M. Its well-defined structure provides opportunities for reactivity modulation. This Account summarizes the discovery of AlHAl_DMA, contextualizing it in the chemistry of olefin polymerization catalysis and organoaluminum compounds. The main features of this novel cocatalyst are discussed by comparing its polymerization performance to those of established cocatalysts. Representative examples also illustrate avenues for polymer microstructure modulation upon substitution of MAO and borate salt/TIBAL with AlHAl_DMA. Structural tuning approaches are presented, showing that a novel class of innovative aluminum alkylborate (AAB) salts can be accessed. Some of these novel AAB salts also served as model species to trace the fundamental reactivity of AlHAl_DMA. Finally, this Account highlights future challenges in the cocatalyst development for olefin polymerization.