The Ziegler-Natta catalyst used for polymerisation of ethene and styrene(active species) involved in the polymerisation is: 

Transition metal halides from groups IV to VIII of the current periodic table are often reacted with organometallic compounds from groups I to III to create or get Ziegler-Natta catalysts. A combination of trimethylaluminum ($\mathrm{Al}{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3$) and titanium tetrachloride (${\mathrm{TiCl}}_4$) is one such example. It has been established that catalysts are really helpful.