Urethans and amines

The chemistry of the glycolysis of polyurethanes is complicated by the fact that there are additional groups in the polymer such as ureas, allophanates, and biurets, and die PURs may be crosslinked. In die presence of the appropriate glycols and at about 200°C, PURs undergo transesterification to form polyols. Under the same conditions, ureas undergo glycolysis to form urethanes and amines (Fig. 10.5). 

Aziridines represent another group of cyclic monomers that are capable of copolymerizing with C02 to potentially provide useful polymeric materials, namely polyurethanes. Early studies of the reactions of aziridines with C02 led to the production of cyclic urethanes [72] and polymers [73, 74], but the polymeric product was shown to consist of both urethane and amine linkages, as depicted in Equation 8.7. However, because the rate of homopolymerization of aziridines is similar to that of the copolymerization of aziridines and C02, the urethane linkages were limited to -30%. 

Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence... 

The Curtius, Hofmann (see p. 267), and Schmidt (see p. 307) reactions are in that order decreasingly mild, decreasingly flexible, and increasingly expeditious. The last-named quality varies somewhat with the available starting material, whether the free acid or the ester. The Curtius reaction lends itself to the preparation of isocyanates, sym-and 08-ureas, amides, urethans, and amines at will, and provides a wide choice of experimental conditions. For synthetic purposes the Hofmann reaction can be used only to prepare sym-ureas, urethans, and amines directly, and halting the reaction at a desired intermediate is often not possible. The variety of experimental conditions is narrower and more limited. The Schmidt reaction on carboxylic acids or derivatives has been applied as a preparative method only to the production of amines although urethans and isocyanates have been prepared occasionally by this reaction, it can hardly be considered a preparative method for them. Amides can be prepared by the Schmidt reaction only from ketones. The choice of experimental conditions employable in the Schmidt reaction is narrow. 

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