The first and dominant class of titanium-based catalysts (and some vanadium-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses:
* catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene copolymerization reactions leading to copolymers with a low 1-alkene content, 2–4 mol% (LLDPE resins), and
* catalysts suitable for the synthesis of isotactic 1-alkenes.
The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely.
Commercial catalysts are supported by being bound to a solid with a high surface area. Both TiCl and TiCl give active catalysts. The support in the majority of the catalysts is MgCl. A third component of most catalysts is a carrier, a material that determines the size and the shape of catalyst particles. The preferred carrier is microporous spheres of amorphous silica with a diameter of 30–40 mm. During the catalyst synthesis, both the titanium compounds and MgCl are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as Al(CH).
All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with TiCl as the active ingredient and MgCl as a support. Another component of all such catalysts is an organic modifier, usually an ester of an aromatic diacid or a diether. The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts. These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers.