Chromates, metal alkyl

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

The mechanisms by which transition-metal oxidizing agents convert alcohols to aldehydes and ketones are complicated with respect to their inorganic chemistry. The organic chemistry is clearer and one possible mechanism is outlined in Figure 15.4. The key intennediate is an alkyl chromate, an ester of an alcohol and chromic acid. 

Separation. — The separation of thorium from the rare earth metals with which it is still mixed may be accomplished by three methods (1) the carbonate separation depends on the fact that thorium carbonate is much more soluble in sodium carbonate than the carbonates of the rare earth metals (2) by the fractional crystallization of the mixed sulfates at 15°-20°, crystals of Th(S04)2 8 H20 are obtained at the insoluble end of the series (3) thorium oxalate forms a soluble double salt with ammonium oxalate, while the rare earth oxalates are almost insoluble in this reagent. Some other methods which have been suggested are fractionation of the chromates,4 of the hydrogen alkyl sulfates,5 of the acetates, by the use of sebacic add 6 and hydrogen peroxide. 

Emulsion systems for anticorrosive coatings are usually copolymers of vinylidene chloride, vinyl chloride, and an alkyl acrylate or methacrylate. Relatively small amormts of surfactant and colloid are used in order to minimize water sensitivity in the dried coating. Flash rusting inhibitors such as ammonium chromate/dichromate or alkali metal nitrate/nitrites may also be added. 

Aluminum surfaces are painted for both protective and decorative purposes. Almost any type of paint for metals, such as acrylic, alkyl, polyester, vinyl, etc., may be used. Surface preparation is extremely important. Aluminum alloy surfaces are best prepared by the application of a conversion coating, either chromate or phosphate. 

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