Ziegler-Natta catalysis: 50 years after the Nobel Prize
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Introduction In 1963, the Nobel Prize for chemistry was awarded to Karl Ziegler and Giulio Natta “for their discoveries in the field of chemistry and technology of high polymers.”1,2 The economic access to polymers with desirable properties enabled by their discoveries paved the way for the widespread introduction of polymeric materials into modern society and was heralded to mark the beginning of the “polymer age” of mankind.
By hard work, not (only) by luck Often claimed as a typical example of serendipity in science, the development of what became known as “Ziegler catalysts”* was in fact a combination of careful observation and intensive work. Ziegler was working since 1949 at the Max-PlanckInstitut in Mülheim, Germany, on the “Aufbaureaktion” (growth reaction), the slow, stepwise oligomerization of ethylene with lithium and aluminum hydrides to give mixtures of metal alkyl species. When Ziegler’s PhD student, Erhard Holzkamp, obtained 1-butene in a nearly quantitative yield, this precipitated
* Aware of differing preferences for naming conventions, we use the term “Ziegler catalysts” if applied solely to ethylene polymerization, “Ziegler-Natta catalysts” with respect to α-olefin polymerization, and “Ziegler-Natta catalysis” as a general term, which recognizes the two fundamental aspects of coordination-insertion polymerization: the catalytic activity of the transition metal and the impact of stereocontrol on polymer microstructure and properties.
what Ziegler described later as a “strenuous investigation” in his laboratory.3–7 Contamination of the autoclave with traces of nickel, either from a previous hydrogenation experiment or from aggressive cleaning, turned out to be responsible for this outcome. The presence of nickel thus greatly increased the speed of the insertion reaction. This “nickel effect” prompted Ziegler to systematically investigate the effects of transition metals on the “Aufbaureaktion,” in other words, the reaction of ethylene (and in some cases propylene) with aluminum alkyls.3–8 In investigating V, Cr, Mo, Mn, Fe, Ru, Os, Co, Pt, Cu, Ag, and Au metal salts in combination with trialkyl aluminum, they observed either no influence on the growth reaction, catalytic di- or oligomerization, or even production of small amounts of solid “paraffin” products. In 1953, Ziegler’s student Heinz Breil investigated the reaction of ethylene with a mixture of triethylaluminum and zirconium acetylacetonate under typical conditions (100°C and 10 MPa) to obtain a linear polyethylene, very different from the highly branched, low-density polyethylene (LDPE) obtained by radical polymerization9 at that time (see Figure 1 for the structure of polyethylene). Shortly afterward in the same year, postdoctoral student Heinz Martin was able to polymerize ethylene at room temperature and atmospheric pressure with a mixture of (CH3CH2)2AlCl and TiCl4. Ethylene polymerization under atmospheric pressure was unprecedented and caused Martin to burst into Ziegler’s office announcing “Es geht in Glas!” (It works in glassware!) T
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