Developments in late transition metal catalysts with high thermal stability for ethylene polymerization: A crucial aspect from laboratory to industrialization

Since the pioneering work of highly active α-diimine palladium/nickel catalysts in 1995 by Brookhart, late transition metal catalysts have been extensively and deeply investigated. Although late transition metal catalysts continue to emerge with unique catalytic properties, they still encounter many formidable challenges in the process of industrialization. One of the biggest obstacles is that the traditional late transition metal catalysts typically show poor thermal stability and thus are inapplicable to industrial polymerization conditions. In recent years, numerous thermally robust late transition metal catalysts have been constantly developed to enhance the possibility of their industrial application. This review focuses on representative works involving Ni-, Pd-, Fe-, and Co-based catalysts with high thermal stability for ethylene polymerization in the past five years. Classified by the ligand structure, polymerization performance parameters under elevated temperatures including catalytic activity and polyethylene properties are summarized and discussed. It reveals that thermally stable α-diimine palladium/nickel catalysts may be the most promising candidate for industrialization. This is due to their good balance between catalytic activities and molecular weights of produced polyethylene. Furthermore, primary approaches to develop thermally robust late transition metal catalysts are outlined, which sheds light on the rational design and application of catalysts with desirable polymerization performance. As one of the most useful methods for improving the thermal stability of catalysts, steric effect can effectively suppress chain transfer and catalyst decomposition and/or deactivation process by blocking the axial coordination sites of the active metal center. Noted that most previously reported thermal stable catalysts have been developed by trial-and-error method, thus a regular understanding of the structure–property relationship of catalysts is required. To achieve the objective of industrialization, the thermal stability of late transition metal catalysts deserves more attention and is expected to achieve a further breakthrough.