related to salen ligands. and DuPont. The concept of bulky bis-imine ligands was extended to iron complexes Representative catalysts feature diiminopyridine ligands. These catalysts are highly active but do not promote chain walking. The give very linear high-density polyethylene when bulky and when the steric bulk is removed, they are very active for ethylene oligomerization to linear alpha-olefins. A salicylimine catalyst system based on zirconium exhibits high activity for ethylene polymerization. The catalysts can also produce some novel polypropylene structures. Despite intensive efforts, few catalysts have been successfully commercialized for the copolymerization of polar monomers. References Catalysts Coordination chemistry Polymer chemistry