Precise Coordination Polymerization Methods Oriented by the Design of Counteranion of the Catalyst Active Species

In coordination polymerization catalyzed by cationic transition-metal species, a counter anion exists close to the metal cation in a polymerization medium. Therefore, the counteranions with a designed structures may show unusual polymerization behaviors compared with conventional catalyst systems, although most of polymerization behaviors are determined by the steric and electronic effect from the ligand. Recently, we have found many examples that boron and aluminum-based activators play a crucial role in improving the activity and regio/stereospecificity, and this review summarizes these achievements in detail. Especially, modifications of methylaluminoxane (MAO), which is the most common activator for coordination olefin polymerization catalysts, often promoted unique polymerizations such as living polymerization, after the appropriate modifications. Besides, we newly designed a fluoroarylborane cocatalyst that successfully increased the propagation rate of olefin polymerization using a titanium catalyst, and the final molecular weight reached millions. This neutral fluoroarylborane has a strong Brønsted acidity and the resulting borataanthracenide counteranion would show an excellent non-coordinative character. The application of this borane to the cationic polymerization of beta-pinene, including the microstructure analyses, is also mentioned.