Propene Polymerization with Bis(phenoxy–imine) Group 4 Transition Metal Complexes

Olefin polymerizations with group 4 transition metal complexes having two phenoxy–imine ligands (FI Catalysts) are reviewed with an emphasis on the characteristics and mechanisms of propene polymerization. The structures and properties of FI Catalysts can be easily modified by changing phenoxy–imine ligand structures. Such catalysts can be synthesized straightforwardly from readily available chemicals. An enormous library of FI Catalysts can be built up by combining a group 4 metal center and a variety of cocatalysts. The diversified library makes FI Catalysts so versatile that polyolefins with desired properties (molecular weight, molecular weight distribution, chain-end structure, tacticity, and so on) can be obtained sometimes predictably, by selecting the appropriate combination of ligand, metal, and cocatalyst. In propene polymerization, it remains a challenge to control the stereo- and regiochemistry of monomer enchainment in order to obtain commercially valuable products. FI Catalysts can produce syndiotactic and isotactic polypropene by Ti–FI catalysts/MAO and Zr– and Hf–FI catalysts/iBu3Al/Ph3CB(C6F5)4, respectively. Because of the well-defined and tunable nature of the catalysts, the coordination environment around the metal center can be controlled by the ligand structures to achieve extremely high stereoregularities, comparable to those of heterogeneous Ziegler–Natta catalysts and metallocene catalysts. These syndio- and isospecific FI Catalysts have contrasting reaction mechanisms, i.e., the syndiospecific polymerization is mediated via 2,1-insertion under chain-end control, while isospecificity arises from 1,2-insertion and a site control mechanism. The observed syndiospecificity can originate from the inherent fluxionality of FI Catalysts between configurational isomers.