Metal alkyls catalysts

The results of the isotactic polymerization of styrene and the polymerization of alpha-methyl styrene with transition metal alkyl catalysts have been summarized in Fig. 6. Alphamethylstyrene is polymerized by... 

Concerning metal alkyl catalysts such as diethylzinc for the copolymerisation, it was established that they formed zinc carboxylate species owing to the reaction with maleic anhydride [192],... 

Note Added in Proof Coordinated anionic polymerization of dienes by alkali metal alkyl catalysts and the effect of cation solvation on mechnism have been discussed in a similar manner by Medvedev and Gantmakher (231a). 

Influence of the Catalyst. Several different types of aromatic reactions are catalyzed by Lewis acids, including alkylation, polymerization, isomerization, acylation, and halogenation. The dependence of these reactions on various Lewis acids is shown in Table 12.11. This table shows a few common Lewis acids and the type of reaction(s) for which each is best suited. The relative strength of Lewis acids was discussed in Section 2.3. Olah gives a comprehensive list of Lewis acid catalysts, and includes several typical synthetic applications. Metal alkyl catalysts are also effective catalysts, but the aromatic substrate is usually converted to a mono-, di-, or trialkyl derivative. Both metal halides and their alkyl derivatives are effective Friedel-Crafts catalysts, as shown in Table 12.11,m as are common inorganic and organic acids. Triflate derivatives are trifluorosulfonate ester (—S02CF3 OTf) can be prepared with various metal counterions. Triflates are very effective catalyst in Friedel Crafts reactions. Some of the more common catalysts are B(OTf)3, Al(OTf)3, and Ga(OTf)3.ll3... 

Stuart, A. P., Sun Oil Co., Polymerization of Ethylene with Cerium Acetylacetonate-Metal Alkyl Catalyst, U.S. 2921060 [I960] C.A. 1960 12657. 

The elimination of alcohol from P-alkoxypropionates can also be carried out by passing the alkyl P-alkoxypropionate at 200—400°C over metal phosphates, sihcates, metal oxide catalysts (99), or base-treated zeoHtes (98). In addition to the route via oxidative carbonylation of ethylene, alkyl P-alkoxypropionates can be prepared by reaction of dialkoxy methane and ketene (100). 

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

Catalytic alkylation of aniline with diethyl ether, in the presence of mixed metal oxide catalysts, preferably titanium dioxide in combination with molybdenum oxide and/or ferric oxide, gives 63% V/-alkylation and 12% ring alkylation (14). 

High Density Polyethylene. High density polyethylene (HDPE), 0.94—0.97 g/cm, is a thermoplastic prepared commercially by two catalytic methods. In one, coordination catalysts are prepared from an aluminum alkyl and titanium tetrachloride in heptane. The other method uses metal oxide catalysts supported on a carrier (see Catalysis). 

Using a solution process, the choice of catalyst system is determined, among other things, by the nature of the third monomer and factors such as the width of the mol wt distribution to be realised in the product. A number of articles review the induence of catalyst systems on the stmctural features of the products obtained (3,5—7). The catalyst comprises two main components first, a transition-metal haHde, such as TiCl, VCl, VOCl, etc, of which VOCl is the most widely used second, a metal alkyl component such as (C2H )2A1C1 diethylalurninum chloride, or monoethyl aluminum dichloride, (C2H )AlCl2, or most commonly a mixture of the two, ie, ethyl aluminum sesquichloride, [(C2H )2Al2Cl2]. 

The process of anionic polymerisation was first used some 60 or more years ago in the sodium-catalysed production of polybutadiene (Buna Rubbers). Typical catalysts include alkali metals, alkali metal alkyls and sodium naphthalene, and these may be used for opening either a double bond or a ring structure to bring about polymerisation. Although the process is not of major importance with the production of plastics materials, it is very important in the production of synthetic rubbers. In addition the method has certain special features that make it of particular interest. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Finally, chain polymerisation can occur via coordination, as is the case for polymerisation involving Ziegler-Natta catalysts. These catalysts are complexes formed between main-group metal alkyls and transition metal salts. Typical components are shown in Table 2.1. 

Initiation presumably involves metal alkyls as the primary source of carbanions. These are immediately available from the Grignard reagents, organosodium compounds, or sodium amide used as catalysts when the alkali metal itself or its solution in liquid ammonia is used, addition to the monomer may precede actual initiation. ... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

II. Soluble Transition Metal Alkyl Compounds as Polymerization Catalysts. 266... 

Wilke s allyl compounds were found to be very poor catalysts indeed, e.g., Ti(2 Me-allyl)4, was found only to have an activity equal to 0.5 gm/m.M Ti/atm/C2H4/hr. For this reason there has been considerable dispute that transition metal alkyls can be the intermediates in Ziegler polymerization. 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

IV. Heterogeneous Polymerization Catalysts Derived from Transition Metal Alkyl Compounds... 

The surfaces of some types of silica and alumina freed from adsorbed water contain acidic -OH groups. Ballard et al. (15) showed that these -OH groups react readily with transition metal alkyls giving stable compounds that are highly active polymerization catalysts for olefins. These systems are best described with reference to silica. 

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