Chromium oxidants, polymer attachment

A novel polysiloxane, containing the isocyanide group pendent to the backbone, has been synthesized. It is observed to react with the metal vapors of chromium, iron and nickel to afford binary metal complexes of the type M(CN-[P])n, where n = 6, 5, 4 respectively, in which the polymer-attached isocyanide group provides the stabilization for the metal center. The product obtained from the reaction with Fe was found to be photosensitive yielding the Fe2(CN-[P])q species and extensive cross-linking of the polymer. The Cr and Ni products were able to be oxidized on exposure of thin films to the air, or electrochemically in the presence of an electron relay. The availability of different oxidation states for the metals in these new materials gives hope that novel redox-active polymers may be accessible. 

The data in Figure 185 allow a comparison of the MW distributions of polymers made from the 250 and 400 °C silicas, with polymer produced by chromium oxide on silica, as shown in Figure 19 or 24, for example. They are quite similar. This comparison supports the argument that both types of catalyst contain the same, or at least similar, active species. In both cases, this may be interpreted as the di-attached species shown in Scheme 38. 

The second peak in Figure 197 is a very-high-MW peak centered near 6.0 on the logarithmic MW scale (MW = 106 g mol-1). Because of its very high MW, and the low temperature used to calcine the alumina, this peak is probably the result of a di-attached Al-associated site. A similar peak, also assigned to an oxidic chromium species, was observed (Figure 194) by heat treatment of another diarenechromium(O) catalyst. The MW of that peak is consistent with the GPC of polymer made with chromium oxide on alumina. 

For example, the trimethylsilylmethyl derivative of chromium(II) is well suited to this purpose. Although it produces a highly active catalyst on aluminophosphate or fluoride-treated alumina supports, it is barely active on silica by itself. Nevertheless, when added to silica-supported Cr(II) oxide, the result is a highly active catalyst that produces branched polymer. In addition to reacting with silanol groups, the chromium alkyl may also react with chromium oxide to again produce mono-attached species, such as is shown in Scheme 44. Coordination between one Cr atom and its chromium or oxide neighbor also seems likely. 

The active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2% Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain. 

The data of Table 55 show how the polymer composition varied with activation temperature. Such observations have been reported from this and other laboratories for catalysts made with several different organo-chromium compounds [301,640,644,654], and most recently by Bade et al. [311], who used chromium allyl to make their catalyst. Presumably, the calcination temperature of the silica resulted in the formation of two very different species. Cr(DMPD)2 reacted with silica treated at 250 and at 400 °C to yield di-attached or coordinated species, whereas it reacted differently with silica treated at 600 °C, because on that support only a single oxide attachment can form. Clearly, the higher OH group population has a major effect on the behavior of the site. 

Alternatively, one could also argue that the coordinated mono-attached species shown in Scheme 38 is identical to the allyl species shown in Scheme 11 after the rearrangement shown in Scheme 12. These ideas are illustrated in Scheme 39, which is one interpretation that produces three main families of sites (a) a mono-attached chromium species that produces very-low-MW polymer, (b) a di-attached chromium species that produces high-MW polymer, and (c) a mono-attached and oxide-coordinated chromium species that produces mid-high-MW polymer. Possibly, there is little distinction between species 2 and 3. Other, similar, mechanistic pathways can also be put forward, as in Section 5. 

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