Ziegler-Natta ethylene polymerization

Cyclic Polyolefins (GPO) and Gycloolefin Copolymers (GOG). Japanese and European companies are developing amorphous cycHc polyolefins as substrate materials for optical data storage (213—217). The materials are based on dicyclopentadiene and/or tetracyclododecene (10), where R = H, alkyl, or COOCH. Products are formed by Ziegler-Natta polymerization with addition of ethylene or propylene (11) or so-called metathesis polymerization and hydrogenation (12), (101,216). These products may stiU contain about 10% of the dicycHc stmcture (216). 

Section 14.15 Coordination polymerization of ethylene and propene has the biggest economic impact of any organic chemical process. Ziegler-Natta polymerization is canied out using catalysts derived from transition metals such as titanium and zirconium. ir-Bonded and a-bonded organometallic compounds aie intennediates in coordination polymerization. 

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... 

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. °°... 

Thus, in the presence of methylaluminoxane (MAO) at 23°C, (C5H5B-N(/-Pr)2)2 ZrCl2 polymerizes ethylene with an activity of 105 kg ofpolyethylene/(h [Zr] mol), similar to that observed with well-studied Cp2ZrCl2 as the catalyst. It is believed that MAO is functioning in its usual role in these Ziegler-Natta polymerizations (methylation of Zr and abstraction of methyl to form a highly reactive Zr cation).41... 

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). 

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. 

Research Focus Development of Ziegler-Natta polymerization catalysts containing magnesium and titanium for polymerizing ethylene. 

Traditionally, UHMWPE is produced by Ziegler-Natta polymerization. This process requires exceptionally pure ethylene and other raw materials. 

When ethylene reacts with triethyl- or tripropylaluminum, multiple carbometa-lation takes place, resulting in the formation of oligomers.509 Oxidation of the products followed by hydrolysis yields alcohols, whereas displacement reaction produces terminal alkenes that are of commercial importance.510 Transition-metal compounds promote the addition to form polymers (Ziegler-Natta polymerization see Section 13.2.4). 

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... 

Another important use of aluminum alkyls is in the Ziegler-Natta polymerization process. This process for the polymerization of ethylene can be represented by the overall reaction... 

The electronic configuration of titanium is [Ar] 3d24s2, which means that Ti(IV) compounds are d° species with free coordination sites 1-27,28). H-NMR and 13C-NMR data are known and have been occasionally discussed in terms of bond polarity 19), but such interpretations are obviously of limited value. The electronic structure of methyltitanium trichloride 17 and other reagents have been considered qualitatively 52) and quantitatively S3 56> using molecular orbital procedures. It is problematical to compare these calculations in a quantitative way with those that have been carried out for methyllithium 57> since different methods, basis sets and assumptions are involved, but the extreme polar nature of the C—Li bond does not appear to apply to the C—Ti analog. Several MO calculations of the w-interaction between ethylene and methyltitanium trichloride 17 (models for Ziegler-Natta polymerization) clearly emphasize the role of vacant coordination sites at titanium 58). 

Fig. 27. Reaction scheme of a Ziegler—Natta polymerization process Cat, catalytic active centre ethylene molecule w, polymer chain w, polymer chain with a vinyl group, al, 1/3 Al R, alkyl group (for example, C2H5) H, H2, hydrogen atom or hydrogen molecule 8S). By permission of Butter-worth Scientific Ltd.

Control of Distribution in Polyolefins with Catalytic Systems Table 2. Kinetic scheme for Ziegler-Natta polymerization (ethylene)... 

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... 

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