Later generation versions of these compounds can use magnesium or vanadium instead of two more common metals.
One scientist began research into these entities after examining his "Aufbau," or "Growth" reaction, leading him to note that a nickel contaminant unexpectedly led to the formation of 1-butene. The initial appeal of these entities was a drastic reduction in the thousands of atmospheres of pressure necessary for a certain process.  Substrates fill octahedral vacancies on their coordination spheres in a series of 1,2 insertions in the leading theory explaining these compounds, an alternative to Green-Mooney theory known as the (*) Cossee-Arlman mechanism. Later generation versions of these compounds can use magnesium or vanadium instead of two more common metals. Their namesake polymerization is especially useful because it can create unbranched polyethylene and isotactic polypropylene. Numerous Alpha-olefins compounds are synthesized by, for 10 points, what titanium-aluminum catalysts named after a German and an Italian chemist? 
One scientist began research into these entities after examining his "Aufbau," or "Growth" reaction, leading him to note that a nickel contaminant unexpectedly led to the formation of 1-butene.
Ziegler-Natta Catalyst [or reagent or whatever]