Olefin polymerization cocatalysts: Development, applications, and prospects

Abstract

<p indent="0mm">The annual global production of polyolefin products exceeds 100 million tons, and they are widely used in various fields. In contrast to heterogeneous polymerization that produces polyolefins with broad dispersity, single-site transition-metal-catalyzed homogeneous olefin polymerization enables the synthesis of polyolefins with narrow molecular weight distribution, controllable chain segment length, and uniform insertion ratio. It can flexibly tune the molecular structure of polyolefins to provide polyolefin materials with the required properties, which can be applied to high-end applications such as medical packaging and medical equipment. It is important to choose a suitable combination of a catalyst and cocatalyst to obtain tailor-made polyolefins through high-efficiency homogeneous polymerization. The main function of the cocatalyst is to react with the catalyst precursor to generate and stabilize active metal cations, the latter being the center of catalyzing olefin polymerizations. This review summarizes the most important work regarding cocatalysts in the past two decades, including novel structures: Mono/dinuclear boron-, aluminum-, and polymer-based promoters. This review illustrates the importance of the weakly coordinative nature of the counter anion, providing the design principle for the development of a highly efficient cocatalyst. Activation mechanism: The use of an industrial catalysis-related pyridylamido hafnium complex is an example that showcases the complicated reactions of this catalyst with different cocatalysts, shedding light on the importance of choosing the pair of catalyst and cocatalyst. Applications in polymerization: Binuclear cocatalysts and the cocatalysts combined with coordination chain transfer polymerizations are used to synthesize novel structural polyolefins, indicating that the cocatalyst plays a crucial role in alternating polyolefin structures. Cocatalyst-involved catalyst deactivation reactions: This part emphasizes the reaction pathways of cocatalysts that lead to catalyst deactivation, including the replacement of metal and ligand. Further, this review proposes the possible future development of cocatalysts by investigating the reported compounds with high Lewis acidity and the development of novel functionalized cocatalysts. For instance, the development of external stimuli-responsive cocatalyst structures may be one of the future development directions. Combining cocatalysts with existing catalytic methods, such as the coordinative chain transfer polymerization for synthesizing polyolefins with a novel structure, may be the second development direction in the future. With the advancement of computer hardware and algorithms together with experimental data, computational chemistry helps understand the effect of cocatalysts on polymerization. Predicting the structure of new and efficient cocatalysts through big data and artificial intelligence is the third possible future development direction.