Titanium(IV) Chloride-Triethylaluminum1

TiCl4-AlEt3
(TiCl4)

[7550-45-0]  · Cl4Ti  · Titanium(IV) Chloride-Triethylaluminum  · (MW 189.68) (AlEt3)

[97-93-8]  · C6H15Al  · Titanium(IV) Chloride-Triethylaluminum  · (MW 114.19)

(alkene polymerization and/or oligomerization catalyst;1 alkyne polymerization, oligomerization, and/or trimerization catalyst;2 cationic polymerization catalyst for aldehydes, epoxides; alkylation of alkenes3)

Physical Data: see Titanium(IV) Chloride and Triethylaluminum.

Form Supplied in: TiCl4: colorless (occasionally pale yellow) liquid; 1.0 M solution in dichloromethane or toluene; widely available. AlEt3: colorless liquid; 1.0 M hexane solution; 25 wt % toluene solution; widely available.

Purification: reflux TiCl4 with mercury or a small amount of pure copper turnings to remove the last traces of light color (due to FeCl3 and VCl4), then distil under nitrogen in an all-glass system, taking precautions to exclude moisture.4 Distil AlEt3 under vacuum in a column containing a heated nichrome spiral.5

Handling, Storage, and Precautions: TiCl4 is moisture sensitive; fumes in air; an irritant to eyes and respiratory tract. AlEt3 is flammable in air; should be kept away from water, and stored under an inert atmosphere. Use in a fume hood.

Polymerization of Ethylene, Propylene or Other a-Alkenes.

The Ziegler-Natta catalyst6 is one of the most widely used organometallic catalysts. This type of catalyst has made it possible to obtain linear polyethylene and isotactic polypropylene under mild reaction conditions (eq 1).

The recent development of MgCl2-supported catalysts has been a major breakthrough in the production of polyalkenes.7 By the addition of other components (Lewis bases as internal and external electron donors), high isotacticity as well as high activity can be achieved.8 TiCl4/AlEt3 without the Lewis base may be used for polyethylene synthesis or the polymerization of other monomers, but not for the preparation of isotactic polypropylene because of their low stereospecificity. When stereospecific poly(a-alkene) is the desired product, the following five components are basically used as catalyst: (1) transition metal compounds, typically TiCl4; (2) a solid support (MgCl2 is the most commonly reported); (3) an internal Lewis base, typically aromatic esters such as ethyl benzoate and dialkyl phthalate; (4) organoaluminum compounds (so-called cocatalyst), typically AlEt3; and (5) an external Lewis base, such as aromatic esters, phenylalkoxysilanes, and 2,2,6,6-Tetramethylpiperidine. Although many variations are possible for this catalyst system, this article will describe only some typical examples.

Practical procedures for the preparation of a solid catalyst (MgCl2-supported TiCl4 with an internal Lewis base) can be found in a number of patents and in the literature. An outline of a typical procedure is as follows. Anhydrous MgCl2 is mechanically co-milled with ethyl benzoate for 10-100 h and rinsed out of the mill with heptane. The solid support is suspended in a heptane solution of TiCl4 and stirred for several hours at 80-100 °C. The solid is filtered and washed with hydrocarbons to remove unreacted TiCl4 and dried. The content of Ti in the solid catalyst is ca. 2-5 wt %.

Polymerization is often carried out under atmospheric pressure in propylene-saturated heptane at 40-60 °C. After an external Lewis base is added, polymerization is started by adding organoaluminum compounds such as AlEt3. The solvent and inert gas should be purified carefully since contamination by a slight amount of oxygen or water may affect not only the activity of the catalyst, but also the microstructure of polymers obtained. A variety of supports and their preparation methods have also been investigated. Some examples of the polymerization of propylene are shown in Table 1. Extensive investigations have also been performed for the polymerization of higher a-alkenes.9

g-TiCl3/AlEt2Cl, as well as TiCl4/AlEt3, is also known as a stereospecific catalyst. In the case of TiCl3, a less stereoregular polymer is obtained with AlEt3.17 Many other metals, such as vanadium,18 chromium,19 etc., are also used as catalysts. A comparison of the Ti catalyst with Cr and V is shown in Table 2.

In the copolymerization of ethylene and a-alkenes, the distribution and sequence of each unit in the polymer chain should be considered. It is known that the presence of an a-alkene enhances the activity of the catalyst for ethylene polymerization.21 Some examples of copolymerization are summarized in Table 3. It is commonly believed that the oxidation state of Ti at the active site is TiIII. It is important for copolymerization because TiIII is effective for polymerization both of ethylene and other a-alkenes, while TiII can polymerize only ethylene.22 The existence of TiII in the catalyst system causes a mixture of copolymer and ethylene homopolymer to be formed.

Polymerization of Conjugated Dienes.

TiCl4/AlEt3 with a nickel catalyst is effective for the preparation of cis-1,4-polybutadiene.29 trans-1,4-Polybutadiene can be obtained with the TiCl4/VCl4/AlEt3 system (eq 2).30

Various transition metal catalysts other than titanium catalysts can also be used. A cobalt catalyst with AlEt3 and AlEt2Cl gives 1,2-polybutadiene and cis-1,4-polybutadiene, respectively.31 For the polymerization of isoprene, the combination TiCl4/AlEt3 gives cis-1,4-polymer,32 while Vanadium(IV) Chloride/AlEt333 and V(acac)3/AlEt334 give trans-1,4-polymer and 3,4-polymer, respectively. Other substituted35 or cyclic36 dienes can also be polymerized with Ziegler-Natta catalysts. The TiCl4/AlEt3/phosgene system affords a random copolymer of butadiene and propylene.37 Copolymerization of ethylene-butadiene with titanium has also been investigated.38

Polymerization of Other Vinyl Compounds.

Titanium-catalyzed polymerization of other vinyl compounds, such as internal alkenes,39 cyclic alkenes, allenes,40 and those with functional groups, have been investigated. Three modes of polymerization are known for norbornenes41 and its derivatives:42 a vinyl polymerization and two types of ring-opening polymerization providing polymers with trans and cis double bonds (eq 3).

Cycloalkenes show a similar behavior in their polymerization.43 The mechanism of ring-opening polymerization is explained by metathesis at the catalyst metal.44 Some examples have been reported on vinyl monomers with functional groups such as alkyl acrylates,45 vinyl chloride,46 vinylidene fluoride,47 vinyl acetate,48 etc.

Polymerization and Oligomerization of Alkynes.2,49

Although a variety of methods for the preparation of conjugated polyacetylene is known, Ziegler-Natta catalysts are the most suitable for obtaining polyacetylene film (eq 4).

Ti(OBu)4/AlEt3 provides a good film of polyacetylene on the surface of a catalyst solution.50 In this method, a concentrated solution of AlEt3 is needed. Since it can ignite easily in air, the concentrated solution should be handled carefully under an inert atmosphere. Acetylene gas is also potentially explosive. Trimerization of acetylene often competes with polymerization to give benzene (eq 4).51 A combination of titanium compounds and n-Butyllithium is also a good catalyst system for the formation of polyene.52 It is known that the polymerization of mono- and disubstituted acetylene is catalyzed by Group 5 or 6 metal systems53 rather than titanium. Some substituted alkynes, such as phenylacetylene,54 phenoxyacetylene,55 trimethylsilylacetylene,56 and dihaloacetylene,57 can be polymerized or oligomerized by Ti catalysts.

Polymerization and Oligomerization of Other Compounds.

Various other kinds of compounds can be polymerized or oligomerized by the TiCl4/AlEt3 catalyst system. Ring-opening polymerization of cyclopropanes proceeds in the presence of TiCl4 as well as Tungsten(VI) Chloride or Tin(IV) Chloride.58 Cyclic ethers such as trioxane59 and propylene oxide60 also undergo ring-opening polymerization to afford polyethers. Some bifunctional compounds, such as o-phthalaldehyde,61 malealdehyde,62 and succinonitrile63 afford cyclopolymerization products.

Miscellaneous Reactions Yielding Small Molecules.

Some synthetic reactions which give small molecules are known. Cyclotrimerization of butadiene gives 1,5,9-cyclodecatriene.64 Cross-cyclodimerization of ethylene and butadiene proceeds catalytically to afford vinylcyclobutane (eq 5).65

Alkylation of alkenes with organoaluminum reagents is catalyzed by titanium salts to give organoaluminum compounds.3,66 AlEt2Cl rather than AlEt3 usually gives better results (eq 6).


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Tamotsu Takahashi & Noriyuki Suzuki

Institute for Molecular Science, Okazaki, Japan



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