Ziegler Natta Polymerization (ZNP)

Ziegler Natta Polymerization: Polymerization reactions especially of olefins and dienes catalysed by organometallic compounds is known as coordination polymerization. The first step in polymerization is the formation of a monomer – catalyst complex between the organometallic compound and the monomer. Here Mt indicates metals like Ti, Mo, Cr, Ni. 

In the formation of monomer – catalyst complex, a coordination bond is involved in between a carbon atom of the monomer and the metal of the catalyst. Hence the polymerization effected by such catalyst systems is called coordination polymerization.

Ziegler Natta catalysts are such type of catalyst as existed in coordination polymerization.  It comprises of two components as against single component organo metallic component and other consisting of halides of IV-VIII group  elements having transition valences. The co-catalysts are organo-metallic compound such as alkyls, aryls and hydrides of I-IV metals.  The commonly used catalysts and co-catalysts are Titanium chlorides (both tri and tetrachlorides) and triethyl aluminium i.e. Al(C2H5)3, diethyl aluminium chloride Al (C2H5)2Cl. Aluminium alkyls acts as the electron acceptor and the titanium halide acts as electron donor. Therefore these two forms a coordination complex which is necessary for coordination polymerization. The formed complex is insoluble in the solvent .

2.) Many structures are proposed for these complexes. From the active centre, the chain reaction propogates and form a solid surface of catalyst complex phase and the monomer is complexed with metal ion of the active centre before it inserts into growing chain. 

3.) When catalyst and co-catalyst components are mixed , there occurs a chemisorption of the aluminium alkyl (electro positive in nature) on the Titanium Chloride solid surface, resulting in the formation of an electron deficient bridge complex as.

4.) The monomer is attracted towards Ti-C bond (C from alkyl group R)in the active centre. When it forms a π -complex with Titanium ion. The rate of reaction is influenced by the electrons present in the active centre.

5.) The bond between R and Ti opens up producing an electron deficient Ti and a carbanion at R.

6.) The Titanium ion attracts the π electron pair of monomer and forms a sigma bond. While the counter ion attracts electron-deficient centre of the monomer. The monomer is then inserted into a transition state ring structure.

7.) This transition state now gives rise to the chain growth at the metal carbon bond regenerating the active centre. Repeating the whole sequence with addition of a second monomer the structure of resultant chain growth as.

8.) The monomer insertion is repeated in this manner and orientation of the substituent group of monomer is always taken from the metal ion end resulting a stereo regular polymer.

Flow  Diagram: