Applications of metal N-heterocyclic carbene complexes in olefin polymerizations

Polyolefins play an important role in modern society and are applied as both commodity plastics and high-performance polymers. Nowadays, they are widely used in large-scale industrial production and considered one among the most important polymers in the world. Since the 1950s, the exploration of Ziegler-Natta catalysts has not just brought continuous breakthroughs in the industrialization of polyolefins, but has also promoted the development of research in organometallic chemistry. Obviously, a key and long-standing focus in the polyolefin industry is the investigation of new metal catalysts that possess customizable catalytic performances. Over the past few decades, metal complexes with N-heterocyclic carbene (NHC) ligands have achieved great improvements and found vast applications in diverse research fields, and definitely, they have been also demonstrated to show excellent catalytic performances in olefin polymerization. Numerous reviews have been published on the catalytic performances of NHC-chelated metal complexes; however, a comprehensive overview of them with the central metals across the periodic table in olefin polymerization is currently lacking. This review aims to bridge this gap by providing a comprehensive summary of NHC catalysts and their corresponding structure-activity relationships in the polymerization and copolymerization of different olefins. This review summarizes the progress in recent decades regarding the structure-activity relationships of various well-defined metal-NHC catalysts in the polymerization of olefins, particularly alpha-olefins, styrene, norbornene, and others.