Nickel catalysts for the preparation of functionalized polyolefin materials

Abstract

<p indent="0mm">As the largest class of thermoplastic polymers, polyolefin materials have wide applications and huge annual production. The introduction of even a small amount of polar functional groups into the polyolefins could excise great control over important material properties. As the most direct and economic strategy, the coordination-insertion copolymerization of olefin with polar-functionalized monomers can enable molecular-level control of the copolymer microstructures. This represents one of the biggest challenges in this field, and is recognized as one of the last “holy grails” in this field. Late transition metal catalysts have attracted much attention in the direct copolymerization of olefin with polar monomers for the synthesis of functionalized polyolefin materials. In this field, research efforts have been devoted to palladium catalyst systems, which have been investigated thoroughly in the past few decades. In contrast, the utilization of less-expensive and earth-abundant nickel catalysts has received much less attention and is much more challenging, mainly due to the higher oxophilicity and lower polar group tolerance of the nickel metal versus palladium metal. However, the field of nickel-catalyzed copolymerization of olefins with polar monomers has witnessed rapid developments in recent years. This perspective mainly focuses on the work concerning nickel-catalyzed olefin copolymerization and summarizes the following subjects: First, we summarize the design and development of different nickel catalyst systems for the copolymerization of olefins with polar monomers, including α-diimine nickel, salicylaldimine nickel, various [N,O] nickel, phosphine-sulfonate nickel, phosphine-phenolate nickel and [P,C/N/P=O] nickel systems. We focus on their ligand design and catalytic properties (activity, copolymer molecular weight, comonomer incorporation, etc.) in these copolymerization reactions. In addition to the above-mentioned homogeneous nickel systems, we also summarize various heterogeneous nickel catalysts for the copolymerization of ethylene with polar monomers. Heterogeneous catalysts offer many distinct advantages, such as controlling the morphology of the copolymer product and preventing reactor fouling in the copolymerization processes. Second, we discuss some of the unique properties of the polar-functionalized polyolefin materials bearing various polar functional groups. These polar-functionalized polyolefin materials show great mechanical properties along with excellent antioxidant, self-healing, flame retardant properties, etc. Notably, there have been very few studies on the properties of functionalized polyolefins prepared by nickel catalysts, due to the inferior properties of nickel catalysts in the copolymerization reactions. Considering the recent developments of various high-performance nickel catalyst systems, more efforts should be directed towards this research field. Finally, we discuss problems and challenges in this field, and propose some promising research directions: (1) The design of new nickel catalysts with further improved catalytic properties, with special focus on expanding the scope of applicable olefins and polar monomers (ethylene, propylene, carbon monoxide, various polar monomers); (2) the development of new heterogeneous nickel catalysts and new supporting methods that could achieve product morphology control and enable continuous polymerization process by preventing reactor fouling; (3) the systematic studies on the properties of functionalized polyolefin materials with the aim of developing high-end applications. Although the research history of nickel-catalyzed copolymerization to access functionalized polyolefin materials is relatively short, we believe that it will become an important leading direction in the field of polyolefins.