Dinuclear Metal Cooperativity Effect And Second Coordination Effect In Olefin Polymerization

Since Brookhart’s seminal work on nickel and palladium complexes bearing α-diimine ligands, late transition metal olefin polymerization catalysts have attracted tremendous interest. Among them, many methods were invented to control the olefin polymerization process such as ligands modification, cocatalyst selection, the change of polymerization condition. In this work, dinuclear olefin catalyst and second coordination effect were selected to shed light on these issues. We mainly focus on the following three parts:1. First, the synthesis, characterization and ethylene polymerization properties of a series of xanthene bridged dinuclear a-diimine Ni(Ⅱ) complexes were described. The dinucleating a-diimine ligands were prepared from the xanthene bridged di-anilines, which were synthesized and isolated on multi-gram scale without using column chromatography. The di-anilines could potentially be used to prepare various dinucleating imine-containing olefin polymerization ligands. In ethylene polymerization, the dinuclear a-diimine Ni(Ⅱ) complexes showed good thermal stability and high catalytic activity. In addition, the polyethylene generated by these dinuclear a-diimine Ni(Ⅱ) complexes showed high molecular weight, narrow polydispersity and very low branching density. Metal-metal cooperativity effect was hypothesized to be able to slow down the P-hydride elimination and correspondingly the chain walking process.2. Then, a series of dinucleating a-diimine ligands and the corresponding nickel and palladium complexes were synthesized and characterized. The dinucleating a-diimine ligands are designed to incorporate naphthalene, biphenylene and xanthene bridged structures, which may lead to different metal-metal distances. These dinuclear complexes are highly active in ethylene polymerization and copolymerization with methyl acrylate. Higher catalytic activity, higher polyethylene molecular weight and much lower polyethylene branching density were observed for the dinuclear Ni complexes comparing with the mononuclear analogue in ethylene polymerization. For the dinuclear palladium complexes, much more dramatic differences in activity and polyethylene molecular weight were observed. Most interestingly, similar catalytic activities were observed for the dinuclear and mononuclear palladium complexes in ethylene-methyl acrylate copolymerization, while only the mononuclear complex was able to incorporate appreciable amount of the methyl acrylate monomer.3. Finally, a series of α-diimine ligands and the corresponding palladium complexes were synthesized and characterized. These a-diimine ligands possess unique properties, which could lead to metal complexes with very similar electronic and steric environment around the palladium center. For example, the CO stretching frequencies of the palladium carbonyl complexes are exactly the same. At a remote position on the second coordination sphere of the ligand structure, substituents (Ph, CF3, NO2 and OMe) are installed to probe their potential influence in olefin polymerization processes. The properties of these palladium complexes in ethylene polymerization, ethylene/methyl acrylate copolymerization, ethylene/norbornene copolymerization and ethylene/5-norbornene-2-yl acetate copolymerization are investigated in detail. Interestingly, most the metal complexes behaved very similarly with each other except for the complex bearing Ph substituent, which showed much better activity and generated polymers with much higher molecular weight.Our work explained to a cetain extent how the dinulear catalyst control and second coordination effect influence the polymerization process, especially the change of the polymers’ properties. Our future work mainly center on the dinuclear catalyst’s effect on the copolymerization of polar monomer with other olefins, additionally we may undertake some industrial applications about polyolefin product.