Nattas Violet Titanium Trichloride

The important contribution made by Natta was the modification of the Ziegler catalyst for the polymerization of propj lene. Pasquon, one of Natta s colleagues, reported that, 

In this way, Natta was able to develop a stereospecific catalyst based on a-titanium trichloride, produced by reducing titanium tetrachloride in hydrogen at 500°-800°C, before combining it with the triethyl aluminum co-catalyst. 

The catalyst was further improved by producing a stereospecific form of a-titanium trichloride. The metastable yS-titanium trichloride which also contained co-crystallized aluminum trichloride was heated to temperatures of 300°-400°C. The transformation proceeded via intermediate y-titanium trichloride, which could also produce stereoregular polymer at temperatures of 100°-200°C. The a- and y-crystalline modifications of titanium trichloride have layered hexagonal and cubic crystal stractures, respectively, and are both active catalysts. 

Early commercial catalysts were supplied to a strict specificatioa The average particle size was usually in the range 10-100 pm with an activity exceeding 1000 g polypropylene per gram of catalyst and an isotactic index exceeding 88%. Continuous, rather than batch, operation was possible with the new, externally formed eatalyst but the efficiency was still low and the polypropylene still contained unacceptably high levels of both catalyst residues and atactic polymer. 

TABLE 8.2. First-Generation Ziegler-Natta Catalysts (1957-1960). 

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A number of models for catalytic active sites have been proposed to explain the isospecific polymerisation of propylene in the presence of TiC -based catalysts [68]. It was Natta who proposed [366], for the first time, that the steric control in isospecific propylene polymerisation is caused by the structure of the active species located on the borders of crystal layers of violet titanium trichloride. Arlman and Cossee [277] suggested that the isospecific active sites are located on lateral crystal surfaces, which in violet titanium trichloride such as oc-TiCh correspond to (110) planes. Titanium atoms present on the above-defined fracture surface have a vacant octahedral site bonded to five chlorine atoms (Figure 3.26) [1,68]. 

Ziegler restricted his studies to the polyethylene catalyst, and the next major discovery was made by one of his hcensees. Professor Guiho Natta, who worked with Montedison in Italy, investigated the polymerization of propylene and other a-olefms to produce solid crystalline polymers. He found that polymerization did not proceed when using the brown titanium trichloride used by Ziegler. He showed that crystalline stereoregular polypropylene could be produced when using violet titanium trichloride. 

The most widely used transition metal salts have been the relatively cheap titanium or vanadium halides, reduced with aluminum alkyls. In Ziegler s work, titanium tetrachloride was reduced to brown yS-titanium trichloride, which was able to polymerize ethylene. However, when yS-titanium trichloride catalyst was used in the polymerization of propylene, the product contained a high proportion of the gum like atactic polymer, which was not viable for commercial use. In contrast, Natta, in his work on the polymerization of propylene and other a-olefins, showed that violet a-titanium trichloride could polymerize propylene to the useful, crystalline, isotactic polymer. Nevertheless, a relatively large quantity of atactic polypropylene still had to be separated from the commercial isotactic product. 

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