Titanium alkyl

The monometallic mechanism is illustrated in Fig. 7.13a. It involves the monomer coordinating with an alkylated titanium atom. The insertion of the monomer into the titanium-carbon bond propagates the chain. As shown in... 

Figure 7.14a illustrates the insertion of a propylene monomer into an edge vacancy in a crystal adjacent to an alkylated titanium atom. In Fig. 7.14b a cross-sectional view of the same site shows how the preferential orientation of the coordinated monomer is dictated by constraints imposed by the protuberances on the crystal surface. 

In the foUowiag cases, only those reactions ia which there is no chain growth, or at most dimerisation, are considered (see Olefin polymers). Alkyl titanium haUdes can be prepared from alkyl aluminum derivatives. The ring stmcture imparts regiospecificity to the ensuing carbometalation (216) ... 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

RjAl, R = alkyl) titanium compounds aluminum = 1.9 4.2, weight ratio) ... 

A number of authors have studied the reductive termination step. The classical work by Hermann and Nelson (121) showed that the reduction of the titanium in alkyltitanates occurred readily with the more alkylated species. The same authors (122) showed that the stability of the alkyl metal decreased markedly when the number of alkyls per metal atom increased. This is parallel to increased anionicity of the alkyl. Vanheerden (123) pointed out that the disproportionation of alkyl-titanium was bimolecular. Cotton (124) showed that the coupling occurs when phenyl Grignard reagents are mixed with various transition metal salts. This is through disproportionation or combination of the alkyl radicals. 

There are numerous alkyl titaniums, and many of their reactions resemble those of alkyllithiums and alkylmagnesium halides. They are protolyzed by water and alcohols, R Ti(R/)3 + HA —> RH + A-Ti(R )3 they insert oxygen, R-TiR + 02 — ROTiR and they add to a carbonyl group ... 

The type of cleavage of the alkyl titanium bond is certainly dependent upon the titanium valence state. Tetravalent compounds will tend to cleave homolytically, but heterolytic cleavage will become more favorable at the lower valence states because of higher bond polarity (290). Ethylene polymerization takes place more readily on alkyl vanadium compounds than on alkyl titanium compounds and yields higher molecular weight linear polymer (340). This is attributable to the fact that... 

Benzenesulfenyl chloride alkene adducts may be transformed to many useful molecules. Intermediates, such as (3), can be treated with base to pi uce vinyl or allyl sulfi s (equation 2). Alternatively, the adducts can be oxidized and treated with base to yield vinyl sulfones in high overall yield (equation 3). The thiirane intermediates or adducts, i.e. (3), may be alkylated with alkyl-titanium and -aluminum reagents which replace the chloride substituent with retention of configuration. 

G g = 1,99 — Ti3+ compounds with titanium-alkyl < titanium-hydride bonds... 

Natta, G., L. Porri, A. Carbonaro, and G. Stoppa, Polymerization of conjugated diolefins by homogeneous aluminum alkyl-titanium alkoxide catalyst systems. I. Cis-1,4 isotactic poly-l,3-pentadiene, Makromolekulare Chemie, 77, 114-125, 1964. 

Soluble single species catalysts are also known, such as the bis(7r-allyl nickel halides) [7]. These can be prepared separately or in situ by reacting bis-allyl nickel (which is an ologomerization catalyst for butadiene but does not give high molecular weight polymer) with an equimolar quantity of nickel halide, and thus bears some resemblance to the catalysts from titanium subhalides and alkyl titanium halides. It is of interest to note that the active species is the monomeric form of the initiator as TT-complex with butadiene (XII) [61]. 

Dialkyltitanium complexes supported by aminotroponiminato ligands have been described. The treatment of TiCl2(LL)2 (LL = N,N -dimethylaminotroponiminato) with Grignard reagents under different conditions and molar ratios leads to the synthesis of dialkyl or chloro alkyl titanium complexes (Scheme 55). The solid-state structure of some of these complexes have been established by X-ray diffraction. Their reactivity has been widely studied (Section 4.05.2.3).-123-125... 

Alkyl titanium reagents in stereoselective addition to aldehydes and dialkylamino titanium compounds as protecting groups of aldehydes in the presence of ketones (see 1st edition). 

Conventionally, HAS are blended with PO during processing. 2-(Diethy-lamino)-4,6-bis[butyl(l,2,2,6,6-pentamethyl-4-piperidyl) amino]-l,3,5-triazine may be fed with an olefin directly into the low pressure polymerization process catalyzed with a modified MgCl2 supported Ziegler-Natta catalyst [142]. The catalytic activity was not impaired [143], Tetramethylpiperidine was reported to be a useful component in MgC -supported Ziegler-Natta catalysts as well. Very high stereospecificity of the synthesised PO was achieved. A complex of HAS with the alkyl aluminium activator was envisaged without interaction with the catalytically active alkyl titanium compound [144],... 

The reaction of an alkylalumlnum compound with titanium tetrachloride produces at room temperatures an almost Instantaneous precipitation of lower valence titanium chloride (B-TiClg) and evolution of gaseous hydrocarbons (36). This process (Reactions 5 and 6) represents a ligand exchange reaction of an alkyl and a chlorine group followed by decomposition of the unstable o-bonded alkyl titanium trichloride. These reactions are typical for numerous Ziegler-type catalysts ... 

A model olefin polymerization catalyst previously characterized as [(C5H5)2TiAlEt2]2 has been studied by X-ray diffraction, H n.m.r., and mass spectral techniques. The compound contains (l- -CjHj) and/i(l- dimeric titanium-aluminium hydride structure (3) has been suggested. Alkyl exchange between a polymeric alkyl-titanium compound and alkylaluminium compound, present in excess, is... 

For the monomer to be dispersible in the suspension system, it must be immiscible or fairly insoluble in the reaction medium. In some instances, partially polymerized monomers or prepolymers are used to decrease the solubility and also increase the particle size of the monomer. The initiators employed in the polymerization reaction are mainly of the peroxide type and, in some cases, are azo and ionic compounds. Examples include benzoyl, diacetyl, lauroyl, and /-butyl-peroxides. Azo-bis-isobutyronitrile (AIBN) is one of the most frequently used azo initiators, while aluminum and antimory alkyls, titanium chloride, and chromium oxides are typical ionic initiators. The amount of catalyst used depends on the reactivity of the monomer and the degree of polymerization, varying from 0.1 to 0.5% of the weight of the monomer. 

Biscylcopentadienyl alkyls, titanium(III), thermal stability Blscylcopentadlenyl alkyls, titanium(III), chemical reactivity1 Biscylcopentadienyl titanium chloride Biscyclopentadienyl aryls, tltanium(II)... 

Disproportionation reactions, titanium complexes Biscyclopentadienyl allyls, titanium Dlhaptoacyl complexes, titanium Allyl elimination reactions, titanium complexes Biscyclopentadienyl alkyls, titanium, pyridine metalatlon reactions... 

Bis(pentamethylcyclopentadlenyl) alkyls, titanium Bis(pentamethylcyclopentadienyl) hydrocarbyls, titanium Bis(pentamethylcyclopentadlenyl)... 

Bis(pentamethylcyclopentadienyl) titanium(III) hydride BisCpentamethylcyclopentadienyl) titanium(II), reactions BisCpentamethylcyclopentadienyl) alkyls, titanium, ring metalatlon... 

The mechanism of Ziegler-Natta catalysis is quite complex. A key step in the chain growth involves an alkyl-titanium bond and coordination of the monomer to the metal. The coordinated monomer then inserts into the carbon-titanium bond, and the process continues. 

Sources of traces of metals in polymers are neutralising chemicals added to the final stages of manufacture to eliminate the effects of acidic catalyst remnants on polymer processing properties (e.g., hygroscopicity due to residual chloride ion). A case in point is high-density polyethylene (HOPE) and PP produced by the aluminium alkyl-titanium halide route which is treated with sodium hydroxide in the final stages of manufacture. 

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