Ziegler-Natta Polymerization of Ethylene

Ziegler-Natta polymerization is used extensively for the polymerization of simple olefins (such as ethylene, propene, and 1-butene) and is the focus of much academic attention, as even small improvements to a commercial process operated on 

Late Transition Metai-cataiyzed Polymerization of Ethylene 

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Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

In many ways, TiCl4 behaves as a covalent compound of a nonmetal. It is a strong Lewis acid that forms complexes with many types of Lewis bases, and it hydrolyzes in water. It also reacts with alcohols to yield compounds having the formula Ti(OR)4. However, it is the behavior of TiCl4 (reacting with [A1(C2H5)3]2) as a catalyst in the Ziegler-Natta polymerization of ethylene that is the most important use of the compound (see Chapter 22). 

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. 

An ESI mass spectrometer coupled online to a microreactor was used to intercept the catalytically active cationic intermediates of the Ziegler-Natta polymerization of ethylene with the homogeneous catalyst system [Cp2Zr(Me)Cl]-MAO (MAO = methylaluminoxane). For the first time these intermediates were studied directly in the solution and their catalytic activity proved.60... 

The Ziegler-Natta polymerization of ethylene can be adapted to make molecules of only modest size (C6--C20) and containing certain functional groups. If, for example, the mctal-alkyls initially obtained are heated (in the presence of ethylene and a nickel catalyst), the hydrocarbon groups are displaced as straight-chain 1-alkenes of even carbon number. Large quantities of such alkenes in the C12-C20 range are... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Problem 9.9 The kinetic rate behavior of Ziegler-Natta polymerization of ethylene and propylene is given by the empirical equation... 

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

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

Jin, Y.-H., Park, H.-J., Im, S.-S., Kwak, S.-Y, and Kwak, S. 2002. Polyethylene/clay nanocomposite by in situ exfoliation of montmorillonite during Ziegler-Natta polymerization of ethylene. Macromolecular Rapid Communications 23 135-140. 

As reported by Santos and Metzger [50], Ziegler-Natta polymerization can be carried out in a microflow system coupled directly to the ESI source of a Q-TOF mass spectrometer (Figure 14.12). In the first micromixer, catalyst (Cp2ZrCl2-MAO) and monomer solutions are mixed continuously to initiate the polymerization. The polymerization occurs in the microtube reactor. The solution thus obtained is introduced into the second micromixer M2, where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species can be characterized by mass spectrometry. This is the first case where an alkyl zirconium cation intermediate in the homogeneous Ziegler-Natta polymerization of ethylene is detected directly. 

Burger, B. J. Thompson, M. E. Cotter, W. D. Bercaw, J. E. Ethylene insertion and -hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene. J. Am. Chem. Soc. 1990,112, 1566-1577. 

If reaction had proceeded in the same manner as Ziegler-Natta polymerization of ethylene and substituted ethylenes (Section 29.6B), a 1,2-addition polymer would have been formed. What is formed, however, is an unsaturated polymer in which the number of double bonds in the polymer is the same as that in the monomers polymerized. This process is called ring-opening metathesis polymerization, or ROMP, after the olefin metathesis involving reaction of acyclic alkenes and nucleophilic carbene catalysts described in Section 24.6. 

Use as Ligand in Ziegler-Natta Polymerization of Ethylene in Solution. A medium pressure process for the polymerization of ethylene has been developed in the presence of a catalytic system involving a monocyclopentadienyl titanium species, containing TIPSthiolate as the heteroligand and two activable ligands (2C1 or 2Me), associated with an ionic activator such as triphenyl-carbenium tetrakis(pentafluorophenyl)borate. ... 

Many examples of Ziegler-Natta polymerizations of ethylene as well as of propylene had been examined (13). The kinetic... 

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