Oxygenates, properties water tolerance

MTO proves to be an efficient and effective catalyst in this reaction when the dienophile is an a, -unsaturated ketone or aldehyde. It is especially active in water, usually with isolated yields >90%. Kinetic studies show that the reaction rate is proportional to the catalyst concentration. The desirability of 1 as a Diels-Alder catalyst stems from a combination of favorable properties the tolerance for many substrates, the inertness to air and oxygen, the use of aqueous medium, and the absence of product inhibition. The initial step appears to be the coordination of the carbonyl oxygen to the rhenium center. Steric crowding around rhenium inhibits reactions of the larger dienophiles [27]. 

ETBE is made from com ethanol, a domestic renewable oxygenate. This could drastically cut down the dependence on oil imports. As an ether, ETBE displays superior water tolerance properties and can be shipped by pipeline. 

The choice between the various oxygenates is being influenced by cost, availability, and their impact on fuel properties. All have high octane, but alcohols (especially methanol) can have an effect on volatility, water tolerance, and elastomer compatibility. 

Reiser reports that in order not to terminate the reaction and hence inhibit propagation, the counter anion must have very low nucleophilicity, since strong nucleophiles or bases will terminate the reaction immediately. Nevertheless, the polymerization can tolerate a small amount of water (1-2%), which is important for the practical usefulness of the system. Oxygen, which acts as a biradical, shows no effect on cationic polymerization—quite an important practical advantage. Characteristically, the epoxy polymers that are the result of the curing process tend to have excellent mechanical properties, including thermal and dimensional stability, nontoxicity, and chemical inertness. 

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