Neutral, Noncoordinating, and Hydrocarbon-Soluble Protic Cocatalyst for Olefin Polymerization

Active species for coordination polymerization usually consist of a transition-metal cation and a noncoordinating counteranion. Such species are often generated in situ from neutral metal precursors and cocatalysts, such as fluoroaryl-substituted borate salts. However, these salts are scarcely soluble in solvents with low dielectric constants, which are often necessary for the highly stereospecific polymerization of olefins. Here, we have prepared a neutral fluoroarylborane that is converted into a boratabenzene anion in the presence of a base due to its highly protic C–H bond at the 10-position. This borane served both as a conventional Lewis acid and a Bro̷nsted acid when reacted with Cp2ZrMe2 to give cationic zirconocene species. Although its Lewis acidity was lower than that of B­(C6F5)3, this species successfully activated the catalyst Me2Si­(Flu)­(NtBu)­TiMe2 and promoted the polymerization of propylene in both toluene and heptane to give polypropylene with a ultrahigh molecular weight (>106).