Zirconium, electronic factors

The (m) fraction is in the range 0.6-0.7 for polymerization at high monomer concentration in the temperature range —25 to 25°C [Resconi et al., 2000]. Substituents on the phenyl groups of the 2-phenylindene ligands, especially meta substituents, slow down the conformational interconversion, and this increases the isotacticity of the polymerization. Both steric and electronic factors operate to affect the interconversion of conformers. Bis(2-phenylin-dene)zirconium dichloride/MAO yields polypropene with (mmmm) = 0.33 for polymerization of liquid propene at 20°C. 3,5-Di-f-butyl and 3,5-difluoromethyl substituents increase (mmmm) above 0.70 [Lin and Waymouth, 2002 Wilmes et al., 2002a,b]. 

The reasons why alkynes 14 and 19 having aryl or vinyl groups, respectively, insert into the zirconium-silicon bond of 3, whereas the alkyne 17 having alkyl groups inserts into the carbon-zirconium bond of 3, are still not clear. Presumably, electronic factors are important for the insertion reaction. 

In the 1980s, reports from Corbett s and Simon s laboratories presented a then-new chemistry based on interstitially stabilized clusters of zirconium and the rare-earth elements. " Coincident with those discoveries, molecular orbital schemes were offered to aid in understanding the electronic factors that exert influence on the stability of these cluster-based compounds. Nevertheless, direct physical measurements that supported the schemes were relatively scarce, and were almost entirely restricted to magnetic susceptibilities. " " ... 

The interesting feature of 20 is that the Si-H interaction occurs for the set of electron-donating ancillary ligands Cp2/PMe3. Thus, the only factor that can, in principle, account for the different behavior of titanium and its heavier analogs in these reactions is the contracted nature of the titanium d-orbitals and hence the less effective backdonation from metal as discussed in Section II.B. The nonclassical nature of the zirconium complex 19 compared with neutral 18 can be then attributed to the presence of a positive charge. 

Many attempts have been made to quantify SIMS data by using theoretical models of the ionization process. One of the early ones was the local thermal equilibrium model of Andersen and Hinthome [36-38] mentioned in the Introduction. The hypothesis for this model states that the majority of sputtered ions, atoms, molecules, and electrons are in thermal equilibrium with each other and that these equilibrium concentrations can be calculated by using the proper Saha equations. Andersen and Hinthome developed a computer model, C ARISMA, to quantify SIMS data, using these assumptions and the Saha-Eggert ionization equation [39-41]. They reported results within 10% error for most elements with the use of oxygen bombardment on mineralogical samples. Some elements such as zirconium, niobium, and molybdenum, however, were underestimated by factors of 2 to 6. With two internal standards, CARISMA calculated a plasma temperature and electron density to be used in the ionization equation. For similar matrices, temperature and pressure could be entered and the ion intensities quantified without standards. Subsequent research has shown that the temperature and electron densities derived by this method were not realistic and the establishment of a true thermal equilibrium is unlikely under SIMS ion bombardment. With too many failures in other matrices, the method has fallen into disuse. 

Aluminium oxide (AI2O3) or zirconium oxide (Zr02) are also used as supports of reticulated deposits based upon polymers of butadiene or styrene-divinylbenzene or hydroxymethylstyrene. Porous graphite, in the form of spheres whose surface is 100 per cent carbon and therefore completely hydrophobic, has been used in applications with compounds possessing atoms carrying lone pairs of electrons thus having high retention factors. 

---