Titanium, trichloride, active form

The active component of the catalyst mixture is a complex which gets formed between titanium trichloride and triethylaluminium. The structure of the complex may be put as follows ... 

It has also been found that the polymer formed from the beginning of the reaction is already prevaiUngly isotactic. This means that, from the start of the reaction, there exist a certain number of active centers on the solid a-TiCU surface which immediately yield iso tactic polymer consequently, it can be excluded that, at least for the active centers present on the initial free surface of a-titanium trichloride, there is an initial activation process, whose rate is slow enough to be observed even when operating at low temperature (30°). 

There is some evidence that the faces on which titanium atoms may exist, e.g., lateral faces (thus, not the 001 faces supposed free from fault), show a greater catalytic activity. From adsorption measurements of radioactive triethylaluminum on an a-titanium trichloride sample having well developed crystals (sample B, see Fig. 7), one may observe that the total amount of alkylaluminum which can be adsorbed (Table X, last line) is remarkably greater than the one sufficient to form a monomolecular layer on the lateral faces of the crystals (38). It is most likely that the alkyl-... 

It is widely accepted that the formation of isotactic polypropylene is brought about by the chiral structure, d or 1, of the titanium species in the catalyst system. The concept of the steric control in propylene polymerization is the same as that in the methyloxirane polymerizations which have been discussed in the foregoing sections. An active center having d - or 1 -chirality is formed along the crystal surface of 6 -titanium trichloride. 

A controlled molar excess of the aluminum alkyl cocatalyst was added to the reduced titanium ( 2 1) before the catalyst entered the reaction zone. All the components were carefully purified and handled in a dry, inert atmosphere. Despite these precautions, the catalyst had a low surface area and was not very active. Most of the titanium was inaccessible due to the formation of relatively large crystals and relatively few active sites were formed at the crystal edges. Furthermore, 6-titanium trichloride was not stereospecifrc for the polymerization of higher olefins. 

The catalyst was further improved by producing a stereospecific form of a-titanium trichloride. The metastable yS-titanium trichloride which also contained co-crystallized aluminum trichloride was heated to temperatures of 300°-400°C. The transformation proceeded via intermediate y-titanium trichloride, which could also produce stereoregular polymer at temperatures of 100°-200°C. The a- and y-crystalline modifications of titanium trichloride have layered hexagonal and cubic crystal stractures, respectively, and are both active catalysts. 

By 1960, the original preparation of brown -titanium trichloride in situ was no longer used by most producers. It was replaced by commercial y-titanium trichloride, which contained an isomorphous form of aluminum chloride in the molar ratio (STiCb.AlCls). This was ball milled to form < -titanium trichloride, which increased activity and led to the formation of a higher proportion of isotactic polymer. However, even after these improvements in catalyst performance, there was still a limitation to the eatalyst activity because the aluminum trichloride in the catalyst reacted with the triethyl aluminum co-catalyst to form quantities of ethyl aluminum dichloride, which is detrimental to the polymerization reactions. Titanium trichloride ciystals were also relatively large despite the milling treatment to reduce particle size. 

In catalytic processes with enzymes such as D-oxynitrilase and (R) xynitrilase (mandelonitrilase) or synthetic peptides such as cyclo[(5)-phenylalanyl-(5)-histidyl], or in reaction with TMS-CN pro-mot by chiral titanium(IV) reagents or with lanthanide trichlorides, hydrogen cyanide adds to numerous aldehydes to form optically active cyanohydrins. The optically active Lewis acids (8) can also be used as a catalyst. Cyanation of chiral cyclic acetals with TMS-CN in the presence of titanium(IV) chloride gives cyanohydrin ethers, which on hydrolysis lead to optically active cyanohydrins. An optically active cyanohyrMn can also be prepared from racemic RR C(OH)CN by complexation with bru-... 

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