Ziegler-Natta catalysts are mixtures of titanium compounds like titanium(III) chloride, TiCl3, or titanium(IV) chloride, TiCl4, and compounds of aluminium like aluminium triethyl, Al(C2H5)3. There are all sorts of other catalysts constantly being developed.
These catalysts are used to make low density poly(ethene). The chains grow in a much more controlled - much less random - way.