A zirconium complex having two phenoxy-imine chelate ligands, bis[N-(3-tert-butylsalicylidene)anilinato]zirconium(IV)dichloride (1), was found to display a very high ethylene polymerization activity of 550 kg of polymer/mmol of cat‚h with a viscosity average molecular weight (M v ) value of 0.9 × 10 4 at 25 °C at atmospheric pressure using methylalumoxane (MAO) as a cocatalyst. This activity is 1 order of magnitude larger than that exhibited by Cp 2 ZrCl 2 under the same polymerization conditions. The use of Ph 3 CB(C 6 F 5 ) 4 / i-Bu 3 Al in place of MAO as a cocatalyst resulted in extremely high molecular weight polyethylene, M v 505 × 10 4 , with an activity of 11 kg of polymer/mmol of cat‚h at 50 °C. This M v value is one of the highest values displayed by homogeneous olefin polymerization catalysts. Complex 1, using Ph 3 CB(C 6 F 5 ) 4 /i-Bu 3 Al as a cocatalyst, provided a high molecular weight ethylene-propylene copolymer, M v 109 × 10 4 , with 8 kg of polymer/mmol of cat‚h activity at a propylene content of 20.7 mol %. X-ray analysis revealed that complex 1 adopts a distorted octahedral coordination structure around the zirconium metal and that two oxygen atoms are situated in trans position while two nitrogen atoms and two chlorine atoms are situated in cis position. DFT calculations suggest that the active species derived from complex 1 possesses two available cis-located sites for efficient ethylene polymerization. Changing the tert-butyl group in the phenoxy benzene ring enhanced the polymerization activity. Bis[N-(3-cumyl-5-methylsalicylidene)cyclohexylaminato]zirconium(IV)dichloride ( 7) with MAO displayed an ethylene polymerization activity of 4315 kg of polymer/mmol of cat‚h at 25 °C at atmospheric pressure. This activity corresponds to a catalyst turnover frequency (TOF) value of 42 900/s‚ atm. This TOF value is one of the largest not only for olefin polymerization but also for any known catalytic reaction. Ligands with additional steric congestion near the polymerization reaction center gave increased M v values. The maximum M v value, 220 × 10 4 using MAO, was obtained with bis[N-(3,5-dicumylsalicylidene)-2′-isopropylanilinato]zirconium(IV)dichloride (15). Thus, polyethylenes ranging from low to exceptionally high molecular weights can be obtained from these zirconium complexes by changing the ligand structure and the choice of cocatalyst.

This contribution reports the discovery and application of phenoxy-imine-based catalysts for olefin polymerization. Ligand-oriented catalyst design research has led to the discovery of remarkably active ethylene polymerization catalysts (FI Catalysts), which are based on electronically flexible phenoxy-imine chelate ligands combined with early transition metals. Upon activation with appropriate cocatalysts, FI Catalysts can exhibit unique polymerization catalysis (e.g., precise control of product molecular weights, highly isospecific and syndiospecific propylene polymerization, regio-irregular polymerization of higher alpha-olefins, highly controlled living polymerization of both ethylene and propylene at elevated temperatures, and precise control over polymer morphology) and thus provide extraordinary opportunities for the syntheses of value-added polymers with distinctive architectural characteristics. Many of the polymers that are available via the use of FI Catalysts were previously inaccessible through other means of polymerization. For example, FI Catalysts can form vinyl-terminated low molecular weight polyethylenes, ultra-high molecular weight amorphous ethylene-propylene copolymers and atactic polypropylenes, highly isotactic and syndiotactic polypropylenes with exceptionally high peak melting temperatures, well-defined and controlled multimodal polyethylenes, and high molecular weight regio-irregular poly(higher alpha-olefin)s. In addition, FI Catalysts combined with MgCl(2)-based compounds can produce polymers that exhibit desirable morphological features (e.g., very high bulk density polyethylenes and highly controlled particle-size polyethylenes) that are difficult to obtain with conventionally supported catalysts. In addition, FI Catalysts are capable of creating a large variety of living-polymerization-based polymers, including terminally functionalized polymers and block copolymers from ethylene, propylene, and higher alpha-olefins. Furthermore, some of the FI Catalysts can furnish living-polymerization-based polymers catalytically by combination with appropriate chain transfer agents. Therefore, the development of FI Catalysts has enabled some crucial advances in the fields of polymerization catalysis and polymer syntheses.

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