Coordination polymerization
The polymerization catalyzed by transition metal complex such as Zieglar-Natta catalysts or metallocene catalysts is also known as coordination polymerization. The Ziegler –Natta catalysts system may be heterogeneous (some titanium based system) or soluble (most vanadium containing species). The best known are derived from TiCl4 or TiCl3 and aluminium trialkyl. These catalysts are highly stereospecific and can orient the monomer in specific direction before addition to the chain. The Ziegler-Natta and metallocene initiators are considered as coordination initiators that perform stereoselectivity by co-ordination. The olefin polymerization is carried out in presence of Ziegler–Natta catalyst (TiCl4 supported on MgCl2).
Mechanism and rate
Radical chain polymerization involves initiation, propagation, termination, chain transfer and inhabitation. For free radical polymerization the mechanism of formation of polymer using peroxide catalysts can be represented as follows:
Assuming that (a) overall rate of reaction is determined by rate of propagation and (b) rate of initiation of free radical is equal to rate of their termination, the overall rate equation can be derived as:
The coordination polymerization on Ziegler–Natta catalyst is assumed to be initiated by adsorption of monomer at an electron deficient surface vacant site on octahedral structure of titanium metal alkyl complex. A transition complex is formed by opening of the double bond. The complex is then rearranged by insertion of the monomer into the growing chain. When the insertion occurs at the original chain growing site with respect to metal ion and original vacant site is retained then the growth corresponds to isotactic growth. However if the chain growth site and vacant site interchange, then the chain growth corresponds to syndiotactic growth. The mechanism is shown in Fig. 7.
Fig. 7. Polymerization of ethylene with Ziegler-Natta catalysts