Traditionally Ziegler-Natta catalyst (TiCl3 or TiCl4 along with
aluminium based co-catalyst) was used for olefin
polymerization.
But in recent years a variety of other complexes containing
metals like aluminium, nickel, zirconium, ruthenium, iron, cobalt,
hafnium, yttrium, titanium, vanadium, etc have also been employed
as catalysts for olefin polymerization.