Titanium Catalyst Systems

Mono-Cp titanium catalyst systems are also suitable for the polymerization of polar and non-polar olefinic monomers. The reduction of a mixture of Cp TiMe3 and Ph3C[B(C6F5)4] with zinc produces a catalyst for the syndiotactic homopolymerization of styrene. The same catalyst mediates the polymerization of methyl methacrylate to poly(methyl methacrylate) (PMMA) with >65% of syndiotacticity. This system is also effective for the co-polymerization of styrene/methyl methacrylate upon optimal conditions. A new polymerization mechanism to explain the characteristics of the polymers is proposed based on sequential conjugate addition steps.541... 

STUDIES IN SUPPORTED TITANIUM CATALYST SYSTEM USING MAGNESIUM DICHLORIDE-ALCOHOL ADDUCT... 

Studies in Supported Titanium Catalyst System 573 Synthesis of MgCl2-Alcohol Adduct... 

FIGURE 17.16 Possible orientations of the coordinated monomer and the allylic end group of the growing polymer chain around the metal center in diene polymerization with monocyclopentadienyl titanium catalyst systems (a) endo-endo (prone-prone) (b) endo-exo (prone-supine) (c) exo-endo (supine-prone) (d) exo-exo (supine-supine). (Reprinted with permission from Peluso, A. Improta, R. Zambelli, A. Organometallics 2000,19, 411-419. Copyright 2000 American Chemical Society.)... 

SCHEME 2.12 Polymerization of 1,5-hexadiene with bis(phenoxyimine) titanium catalyst system. 

The research into titanium catalyst systems went up to 75% in the period 1982-1987. Recently, however, they have been reduced to 70% (1988-June 1992) and then to 55% (July 1992-June 1993). In contrast, while studies in metallocene catalyst systems comprised 10% in 1982-1987, they have increased to 14 and 20% in the later two periods, respectively. These data show that studies into metallocene catalyst systems are growing rapidly these days. 

Catalysts. Iodine and its compounds ate very active catalysts for many reactions (133). The principal use is in the production of synthetic mbber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-83-4], are employed for producing stereospecific polymers, such as polybutadiene mbber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymeri2a tion (66) (see RUBBER CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabiH2ation of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

This catalyst system is used in about 70% of the -xylene oxidations, and the percentage is increasing as new plants almost invariably employ it. Process conditions are highly corrosive owing to the acetic acid and bromine, and titanium must be used in contact with some parts of the process. 

Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

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. 

Polymers containing 90-98% of a c 5-1,4-structure can be produced using Ziegler-Natta catalyst systems based on titanium, cobalt or nickel compounds in conjuction with reducing agents such as aluminium alkyls or alkyl halides. Useful rubbers may also be obtained by using lithium alkyl catalysts but in which the cis content is as low as 44%. 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

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