Titanium trichloride, Ziegler-Natta catalysts

The primary use of TiCl is as a catalyst for the polymerisa tion of hydrocarbons (125—129). In particular, the Ziegler-Natta catalysts used to produce stereoregular polymers of several olefins and dienes, eg, polypropylene, are based on a-TiCl and A1(C2H3)3. The mechanism of this reaction has been described (130). SuppHers of titanium trichloride iaclude Akso America and Phillips Petroleum ia the United States, and Mitsubishi ia Japan. 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

Heterogeneous Ziegler-Natta catalysts composed of titanium trichloride and alkylaluminum have been used to prepare block copolymers of ethylene with a-olefins 44-46), even though there is no known example of such a catalyst meeting the requirement for a living polymerization. The produced block copolymers have broad molecular weight distributions (Mw/Mn = 4 20) and are present in small concentrations... 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Studies on Ziegler Natta Catalysts J. Polymer Sci. A-1 (1966). — Rodriguez, L. A. M., H. M. VAN Looy, and J. A. Gabant Part I. Reaction between Trimethylaluminium and a-Titanium Trichloride, p. 1905 — Part II. Reaction between a-or jS-TiClj and AlMe, AlMejCl, or AlEtj at Various Temperatures, p. 1917. — van Looy, H. M., L. A. M. Rodriguez, and J. A. Gabant Part III. Composition of the nonvolatile product of the reaction between titanium trichloride and trimethylaluminum or di-methylaluminum chloride, p. 1927. — Rodriguez, L. A. M., and H. M. VAN Looy Part IV. Chemical nature of the active site, p. 1951. Part V. Stereospecificity of the active center, p. 1971. 

These Ziegler-Natta catalysts are complexes of transition metal halides with organometallic compounds typically, triethylaluminum-titanium trichloride. 

In 1953, Ziegler [38] first employed aluminum trialkyl-titanium tetrachloride (RsAl-TiCU) catalyst to prepare stereo-irregular polypropylene. Later in 1954, Natta [39,40] refined the Ziegler catalyst with aluminum trialkyl-titanium trichloride (RsAl-TiCls) catalyst and successfully prepared highly stereoregular polypropylene. The new catalyst rendered polypropylene the possibility of practical usefulness. In 1957, it led to the construction of the first production facility for polypropylene by Montecatini Co. in Italy. 

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

PP, a semi-crystalline thermoplastic, is made in its homopol5mier form by pol5mierising propylene monomer using stereospecific Ziegler-Natta catalysts. The catalyst system is termed stereospecific because it controls the position of the side (methyl) group in each propylene unit in the polymeric chain. A t3 ical catalyst system may be prepared by combining titanium trichloride with tributyl aluminium or its variants. Most commercial PP is isotactic. 

Rodriguez, L. A., Van Looy, H. M., and Gabant, J. A., Studies on Ziegler-Natta catalysts. Part I. Reaction between triethyl-aluminum and a-titanium trichloride, J. Polym. Sci., Al, 4, 1905, 1966. 

A typical example is the reaction product of an aluminum alkyl with titanium tetrachloride or titanium trichloride. These catalysts were first discovered by the German chemist K. Ziegler (late 1940s) for the low-pressure polymerization of ethylene. Subsequent work by G. Natta (early 1950s) showed that these and similar catalysts are useful for preparing stereoregular polyolefins thus, the family of catalysts is sometimes called Ziegler-Natta catalysts. (Odian, G. C., Principles of Polymerization John Wiley and Sons, Inc., New York, 2004 Elias, H. G., Macromolecules Vol 1-2, Plenum Press, New York, 1977). 

Poly(4-methyl-l-pentene) was first introduced in 1965 by Imperial Chemical Industries Ltd (UK) but since 1975 the polymer has been manufactured solely by Mitsui Petrochemical Industries Ltd. Polymerization is carried out using a Ziegler-Natta catalyst such as titanium trichloride/diethylaluminium chloride in a hydrocarbon diluent at atmospheric pressure and 30-60°C ... 

Refinements to the Ziegler-Natta catalysts continued, with several attempts to convert brown >8-titanimn trichloride to the active purple y-titanium trichloride at a lower temperature. The objective of this work was an increase in the activity of the catalyst by avoiding the formation of the aluminium trichloride that was isomorphous with the y-titanium trichloride. High temperatures were normally required to reduce the aluminium chloride content, and this led to a reduction in the surface area, and hence, activity. In a development by Solvay, titanium tetrachloride was reduced with aluminum alkyl at about 1°C, before extraction of the co-ciystallized aluminum chloride with either dibutyl- or di-isoamyl ether. The porous -titanium trichloride was then heated with excess titanium tetrachloride at 65-100°C to remove excess ether and to produce high-surface-area (<75 m g ) purple a-titanium trichloride under controlled conditions. ... 

Natta A process for polymerizing propylene and other higher olefins, catalyzed by crystalline titanium trichloride and an alkyl aluminum compound such as triethyl aluminum. The polymer can exhibit various types of stereoregularity, depending on the catalyst and the conditions. Invented in 1954 by G. Natta at the Istituto de Chimica Industrial del Politecnico di Milano, Italy, and commercialized in 1957. Now used widely, worldwide. See also Ziegler, Ziegler-Natta. 

PP is produced by a variety of processes, most of them by a diluent phase propylene polymerization utihzing a Ziegler-Natta-activated titanium trichloride catalyst in the presence of low- to high-boiling hydrocarbons. Residual catalyst removal followed by hydrocarbon slurry centrifugation is the immediate upstream operation prior to thermal drying. Hexane is the solvent used in the major PP processes in operation today. As a result these polymers are solvent wet. 

Scheme 6.34. A representation of the polymerization of ethene (ethylene, CH2=CH2) by a catalyst composed of the Lewis acid mixture of titanium tetrachloride (TiCL,) and triethyl-aluminum [(CH3CH2)3A1] a Natta-Ziegler Catalyst. The actual catalytic species is (presumably) an alkyl surface-substituted octahedral titanium trichloride (TlCh) derivative with a...

Shortly after the discovery of the polymerization catalyst by Ziegler [1], in 1957 Breslow [2] and Natta [3] used the newly synthesized titanocene as a transition metal component in combination with aluminum alkyls for the polymerization of ethene. Compared with the heterogeneous Ziegler catalyst based on titanium tetrachloride or titanium trichloride and triethylaluminum, the titanocene/Al(C2H5)2Cl catalyst is homogeneous and soluble in hydrocarbons. It therefore was preferentially studied in order to understand the elementary steps of the polymerization, which is simpler in homogeneous than in heterogeneous systems. On the other hand, there was... 

PP is produced by a variety of processes, most of them by a diluent phase propylene polymerization utilizing a Ziegler-Natta-activated titanium trichloride catalyst in the presence of low- to high-boiling hydrocarbons. 

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. 

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