Titanium placement

Fortunately, a wide variety of functionality is compatible with the titanium tartrate catalyst (see Table 1), but the judicious placement of functional groups relative to the allylic alcohol can lead to further desirable reactions following epoxidation. For example, in (40), asymmetric epoxidation of the allylic alcohol is followed by intramolecular cyclization un r the reaction conditions to give the tetrahydrofuran... 

Samples weighing 2-5 mg are then dissolved in 5-molar nitric acid. The strontium fraction is purified using ion-specific resin and eluted with nitric acid followed by water. This solution is loaded onto a titanium filament for placement in the instrument (Fig. 4.20). Isotopic compositions are obtained on the strontium fraction thermal ionization mass spectrometer (TIMS). This is a single focusing, magnetic sector instrument equipped with multiple Faraday collectors. Strontium is placed on a thin filament and measured. Sr/ Sr ratios are corrected for mass fractionation using an exponential mass fractionation law. Sr/ Sr ratios are reported relative to a value of 0.710250 for the NIST 987 standard (e.g., if the Sr/ Sr ratios for the standards analyzed with the samples average 0.710260, a value of 0.000010 is subtracted from the ratio for each sample). 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

Four different stereoisomers are possible for polymer XLII, poly (cyclobutane-1,2-diyl) (Sec. 8-lf). Cis and trans isomers are possible for pol3mier XLin, poly (but-1-ene-1,4-diyl). (XLni is the same polymer obtained by the l,4-pol3fmerization of 1,3-butadiene— Sec. 8.10). Traditional Ziegler-Natta initiators based on vanadium and metallocene initiators yield polymerizations almost exclusively through the double bond. Titanium, tungsten, and ruthenium initiators yield predominantly ROMP with varying amounts of cis and trans placements. 

The placement of permanent bends of 135 in the nickel-titanium archwires failed to yield definitive increases (Afl" values) for the low-temperature martensite peaks on the TMDSC plots, compared to those for the as-received wires [35,37], It was assumed that bends with acute angles would be needed to increase the amount of work-hardened martensite sufficiently to cause significant increases in the enthalpy changes associated with these low-temperature peaks. 

Use of Ziegler-Natta catalysts, as seen from Table 5.8, can yield an almost all-cw-l,4-polyiso-prene or an almost all-franj- 1,4-polyisoprene. The microstructure depends upon the ratio of titanium to aluminum. Ratios of Ti A1 between 0.5 1 and 1.5 1 yield the cis isomer. A1 1 ratio is the optimum. At the same time, ratios of fi Al between 1.5 1 to 3 1 yield the trans structures. The titanium-to-aluminum ratios affect the yields of the polymers as well as the microstructures. There also is some influence on the molecular weight of the product. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only cis and trans arrangements are affected. In addition, the molecular weights of the polymers and the microstructures are relatively insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the rates, the molecular weights, and the microstructures. 

The internal donors restrict the placement of titanium on the (110) face of MgCla. 

Transformation of a nximber of multi-component titanium-base alloys into their Al- and Mo-equivalent formats provides a rationalization for their placement into one or another of the previously discussed phase-stability classifications (Table 2.6). 

The catalyst complex of the TiCls/AlRs system essentially acts as a template for the successive orientation and isotactic placement of the incoming monomer units. Though a number of structures have been proposed for the active species, they fall into either of two general categories monometallic and bimetallic, depending on the number of metal centers. The two types can be illustrated by the structures (I) and (II) for the active species from titanium chloride (TiCU or TiCls) and alkylaluminum (AIR3 or AIR2CI). 

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