Ziegler-Natta polymerization mechanism

A linear mechanism similar to the accepted mechanisms for Ziegler-Natta polymerization, which has been proposed, e.g. by Marshall and Ridgewell (21), is also excluded by the tracer experiments. 

In this contribution, we review the mechanism of polymerization and oligomerization involving early transition metals, taking as our basis recent results in advanced organometallic chemistry. First of all, some recent examples of the previous reviews concerning the Ziegler-Natta polymerization are cited [1-10]. Then, relevant new reports are surveyed in a systematic fashion. 

These results at least demonstrate that ethylene can be polymerized by an alkylidene hydride catalyst, probably by forming a metallacyclobutane hydride intermediate. The extent to which this is relevant to the more classical Ziegler-Natta polymerization systems (27) is unknown. Recent results in lutetium chemistry (28), where alkylidene hydride complexes are thought to be unlikely, provide strong evidence for the classical mechanism. 

The examination of models for stereospecific Ziegler-Natta polymerizations indicate the importance of the nonbonded interactions for the stereoselectivity mechanisms. 

Ziegler-Natta polymerization leads to linear unbranched polyethylene, the so-called high density polyethylene (HDPE), which is denser, tougher and more crystalline. By copolymerization with other alkenes it is possible to obtain linear low density polyethylene (LEDPE) with better mechanical properties than LDPE. Blends of LLDPE and LDPE are used to combine the good final mechanical properties of LLDPE and the strength of LDPE in the molten state. 

Various mechanisms have been proposed to explain the stereoselectivity of Ziegler-Natta initiators [Boor, 1979 Carrick, 1973 Corradini et al., 1989 Cossee, 1967 Ketley, 1967a,b Tait and Watkins, 1989 Zambelli and Tosi, 1974]. Most mechanisms contain considerable details that distinguish them from each other but usually cannot be verified. In this section the mechanistic features of Ziegler-Natta polymerizations are considered with emphasis on those features that hold for most initiator systems. The major interest will be on the titanium-aluminum systems for isoselective polymerization, more specifically, TiCl3 with A1(C2H5)2C1 and TiCLt with A1(C2H5)3—probably the most widely studied systems, and certainly the most important systems for industrial polymerizations. 

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. 

The Ziegler-Natta polymerization of ethylene and propylene is among the most significant industrial processes. Current processes use heterogeneous catalysts formed from Ti(IH)Cl3 or MgCl2-supported Ti(IV)Cl4 and some otganoaluminum compounds. The widely accepted Cossee mechanism of ethylene polymerization is illustrated in Scheme 62. 

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

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

Figure 5.15. Mechanism involved in Ziegler-Natta polymerization for tacticity control over the growing polymer chain.

One such process is the Cossee-Arlman mechanism,proposed for the Ziegler-Natta polymerization of alkenes (also discussed in Section 14-4-1). According to this mechanism, a polymer chain can grow as a consequence of repeated 1,2 insertions into a vacant coordination site, as follows ... 

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cycliza-tion to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create in-termolecular cross-links that render the exposed areas insoluble. 

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). 

Richard F. Heck (bom 1931) was a student of Saul Winstein (UCLA) and Vladimir Prelog (ETH Zurich). He started mechanistic work on homogeneous catalysis in 1956 when he entered Hercules Inc. (Wilmington, Del., USA) as a research chemist. He pioneered the elucidation of reaction mechanisms of organometallic processes, e. g., hydro-formylation and Ziegler-Natta polymerization, and published a number of key papers about the chemical and mechanistic backgrounds of these reactions. He was a chemistry professor at the University of Delaware from 1971 until his retirement in 1989. For the Heck reaction the reader is referred to Section 3.1.6. 

The )9-alkyl elimination step can considered as analogous to the more familiar f-hydride elimination in which a C-C bond is broken instead of a C-H bond. Note that this reaction is the microscopic reverse of the propagation step in the mechanism of the Ziegler-Natta polymerization of olefins (eq. (I)). 

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