Mechanism of Ziegler-Natta Polymerization

Despite passage of more than 57 years since the basic discoveries, the mechanism of Ziegler-Natta polymerization is still not fully understood. As in all chain-growth polymerizations (12), the basic steps are initiation, propagation and termination (chain transfer). 

Cossee and Arlman (13,14) were among the first to propose a comprehensive mechanism for Ziegler-Natta catalysis and supported their proposals with molecular orbital calculations. The Cossee-Arlman proposal involves a migratory alkyl transfer (15) and, with some refinements, remains the most widely cited mechanism for Ziegler-Natta catalysis. A summary is presented below. (For more details, see references 5,12,16 and 17.) 

The reduced form of titanium is octahedral and contains open coordination sites ( ) and chloride ligands on crystallite edges. Initiation begins by formation of an active center, believed to be a titanium alkyl. Alkylation by TEAL cocatalyst produces an active center  

The alkyl migrates (rearranges) such that an open coordination site moves to a crystallite edge position. Coordination of an ethylene monomer occurs to create a Ji-complex as in eq 3.5. Subsequent addition across ethylene results in the propagating species  

Because titanium-carbon a-bonds are known to be unstable, a different mechanism that invokes coordination of the aluminum alkyl to the titanium alkyl has been postulated. It is suggested that the titanium alkyl is stabilized by association with the aluminum alkyl. Coordination also accommodates the well-known propensity of aluminum alkyls to associate (18). This is known as the bimetallic 

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Cossee, P, J. Catalysis, 3, 80 (1964) The Mechanism of Ziegler-Natta Polymerization. II. Quantum-Chemical and Crystal-Chemical Aspects, Chap. 3 in The Stereochemistry of Macromolecules, Vol. 1, A. D. Ketley, ed., Marcel Dekker, New York, 1967. 

T. Keii, A Kinetic Approach to Elucidate the Mechanism of Ziegler-Natta Polymerization, in Ref. 9, p. 263. 

Cossee P (1967) The mechanism of Ziegler-Natta polymerization. II. Quantum chemical and crystal-chemical aspects. In Ketley AD (ed) The stereochemistry of macromolecules, vol. 1, Chap. 3. Marcel Dekker, New York, pp 145-175... 

Scheme 11.37 General mechanism of Ziegler-Natta polymerization of ethylene.

The insertion of ethylene into a carbon-cobalt bond was carefully studied as part of work on the mechanism of Ziegler-Natta polymerization (Scheme 1.10). ... 

The mechanisms of Ziegler-Natta polymerization are not fully understood, but the heterogeneous nature of the catalysts appears to play an important role in controlling the stereochemistry of the polymer. The mechanism suggested in Scheme 5 therefore possibly reflects the process taking place on the (modified) surface of a titanium chloride crystal. 

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