Polymerization catalysts

As the cost of separation of catalysts from the products is high, polymerization catalysts are usually not recovered at the end of the process. Hence it is essential that catalysts should be non-detrimental to the product quality. The catalysts should also have high activity so that minimum amount of catalyst is needed for the process.  This will minimize the amount of catalyst retained within the products.

Commercial polymerization catalysts can be broadly classified as follows:

1. Oxygen containing initiators such as peroxides for free radical polymerization.
   
   
2. Speciality acids, bases, metal ion compounds and organometallic complexes for step (condensation) polymerization.
   
   
3. Ziegler Natta, metallocene or supported metal oxide catalysts for coordinative and stereo specific polymerization.

Sometime catalyst additives are used which improve chain transfer, production rate and stereoselectivity in free radical and coordinative polymerization. For example ethyl benzoate is used as additive to improve the catalyst stereo selectivity.

Ziegler – Natta catalysts


Ziegler – Natta catalysts are prepared from transition metal halides such as chloride or iodide of Ti, V, Zr, Cr, W, Co and aluminum (Mg or Li) alkyl. The titanium catalysts are prepared by the interaction of TiCl₄, and alkyl aluminium compounds in hydrocarbon solvent. Titanium supported on magnesium salts are also used. For production of polyethylene, poly propylene and polydienes Ti-Al or Ti-Mg complexes are typically used. The homogeneous vanadium based catalysts such as VOCl₃, VCl₄, or VO (OR)₃, with aluminum alkyls such as RAlCl₂, are used for production of polymers by copolymerization. The Ziegler – Natta catalysts are capable of stereoregulation during polymerization reaction and thereby increases  selectivity of a particular product. For example Ziegler – Natta catalyzed process is highly selective for linear polyethylene production.