Urea oligomer, synthesis

The effect of structural variations in siloxane oligomers in the synthesis and properties of the resulting siloxane-urea copolymers have also been investigated 161). In these studies aminopropyl-terminated poly(dimethyl-diphenyl)siloxane and poly-... 

Table 14. Synthesis and characteristics of siloxane-urea segmented copolymers based on MDI and aminopropyl terminated PDMS oligomers 1571...

The synthesis of the oligomers involved the known reaction of isocyanates and cyanamide (Nl CN). For example, N-cyano-N -phenyl urea has been synthesized from phenyl isocyanate and an aqueous alkaline solution of cyanamide in high yield.(3) Recently, similar reactions were used to prepare various di-N-cyanourea compounds from diisocyanates. ( 1) These monomers were also synthesized directly by reacting diisocyanates with cyanamide at melt temperatures. 

Some of these oligomers have later been used in the synthesis of various segmented urea or imide type copolymers and in the modification of epoxy networks, which have been discussed elsewhere (5,11). ... 

The alkyl-, alky laryl- and diarylcarbodiimides are the diimides derived from carbon dioxide, however, no direct formation of carbodiimides from amines and carbon dioxide is known. Interestingly, carbodiimides can be obtained from amines and carbon dioxide via a switteri-onic titanium complex (see Section 2.2.8). The major starting materials for the synthesis of carbodiimides are isocyanates, 1,3-disubstituted ureas or 1,3-disubstituted thioureas. The synthesis of isocyanates requires the use of the toxic carbonyl chloride or its oligomers. A book on the synthesis and reactions of isocyanates appeared in 1996. ... 

The approaches used for the synthesis of ribonucleotide oligomers follow the general lines of the method used for the synthesis of mononucleotides. Thus, oligonucleotides can be obtained by heating for several days uridine, urea and ammonium dihydrogen phosphate at temperatures between 85 and 100°C under ambient humidity or dry conditions. Some of... 

Hydrogen cyanide can also be converted to cyanoacetylene and hydrogen cyanate, both precursors of pyrimidines. These reactions were reproduced in the laboratory. In fact, in 1828, F. Wohler made urea from hydrogen cyanate and ammonia, the first synthesis of an animal substance from inorganic materials. Very likely, all these processes occurred primarily in an aqueous environment where and OH" ions acted as specific-acid or specific-base catalysts. It is particularly impressive that the three major classes of nitrogen containing biomolecules, purines, pyrimidines, and amino acids are formed by the hydrolysis of the oligomers formed directly from... 

Polymer Synthesis and Characterization. Diamines or diols can be used as chain extenders in copolymerization reactions, but urea linkages formed from diamines often lead to polymer insolubility. Poor solubility is avoided by using diols, but this requires a two stage preparation (Scheme 1) because of the disparity in reactivity of alcohols and amines with isocyanates. To form urea and urethane linkages separately, the reaction between isophorone diisocyanate and benzene dimethanol was carried out first in bulk at 115 °C without catalyst (Equation 1). Secondly, reaction between aminopropyl endcapped dimethylsiloxane oligomers and diisocyanate intermediates was effected in THF at room temperature (Equation 2). Completion of the polymerization reaction was established by NMR and infrared spectroscopy. ... 

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