Recent advancements in N-ligated group 4 molecular catalysts for the (co)polymerization of ethylene

Group 4 metal (Zr, Ti, Hf) catalysts for olefin polymerization and specifically those based on non-metallocene complexes have continued to be a subject of intense study in homogeneous catalysis. With a view to forming new or improved polyolefinic materials, complexes bearing N-donor anionic ligands such as β-diketiminate, amidinate, guanidinate, amido, imido as well as mixed N-donor ligands including N,C,C-azaallyl and N,O-phenoxy-imine, have been central to many key developments; high catalytic activities for homo- and copolymerization of ethylene have been a highlight of their catalysis. The fine tuning of these nitrogen-containing ligands significantly controls the catalytic performances of their metal catalysts as well as the structural properties of the resulting polymers with high molecular weight or even ultra-high molecular weight materials accessible. In this review the focus is on more recent publications in the field, in which we correlate the influence of ligand structure with the catalytic performance and microstructure of the polyethylenes. Furthermore, we examine the effects of co-catalyst on activity and thermostability of the precatalyst while efforts directed towards the copolymerization of ethylene with 1-hexene are also summarized. Overall, this work presents an overview of current knowledge pertaining to catalyst design and especially with regard to how the modulation of steric and electronic properties impact on the (co)polymerization process.