Urea group

Another useful reaction is the reaction of water with isocyanate to generate CO2 and urea groups which modify the polymeric stmcture. This vigorous reaction is also a prime source of exothermic heat to drive equation 3 to completion. 

Further reaction of the active hydrogens on nitrogen in the urethane groups (3) can occur with additional isocyanate (1) at higher temperatures to cause formation of aHophanate stmctures. The active hydrogens in urea groups can also react with additional isocyanate to form disubstituted ureas which can stiU further react with isocyanate to form biurets (13). 

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

Reactions of urethane and urea groups to form allophanates and biurets. 

Polyurethanes (PURs) are usually described as being prepared by the reaction of diols with diisocyanates. However, this is an oversimplification because often water is deliberately added in the production of flexible polyurethane foams. Unreacted isocyanate groups react with water to form carbon dioxide and urea groups in the polymer chain. The carbon dioxide acts as a blowing agent in the production of PUR foams. Also, polyurethanes can be formed by the reaction of bischloroformates with diamines. 

Apart from the Important reaction leading to the formation of urethane groups, carbon dioxide can be released during curing by hydrolysis of the Isocyanate group, leading to the formation of urea groups (10). 

In transition metal complexes, two main coordination modes have been described for urea ligands W-monohapto, or N,0-chelates. In the case of molecules containing more than one urea function, the molecules act as Ar,AT-chelates, so that one of the urea functions always behaves as monohapto-ligand. For example, complexes of Co(III) with N-(2-pyridylmethyl)urea and ethylenediamine have been characterised by X-ray crystallographic analysis (Scheme 7). The urea group is coordinated through only one of its N... 

In reactions (3) and (4) the isocyanate is capable of reacting with the active hydrogen in a urethane, or urea group, to give branching, or crosslinking by the formation of an allophonate or a biuret group. 

A non-covalently functionalized dendrimer was also applied in a continuous allylic amination reaction.[33] PPI dendrimers functionalized with urea adamantyl groups can act as host molecules for phosphorus ligands equipped with acetyl urea groups (Figure 4.18). The so formed supramolecular complex was reacted with a palladium precursor... 

Bottom Pictorial representation of (a) the achiral, S8 symmetric homodimer 16-16 and of (b) the two enantiomeric forms of a calixarene capsule heterodimer. The head-to-tail directionality of the urea groups (arrow) defines the chirality. 

The Photopolymer Plate, a water developable photopolymer relief printing master plate made by modification of polyvinyl alcohol with urea group oligomer having a functional polyvinyl base, for making paper mold and matrix for printing master plates, has characteristics which are very close to those of the conventionally etched metal relief printing plates and has some superior points to them. 

Replacement of the carbamate group with isosteric functionalities such as an IV-methyl carbamate, urea, or amide group clearly confirmed the favorable qualities of the carbamate group [57], While the introduction of a urea group, as in case of iV-9-(tert-butylcarbamoyl)-9-desoxy-9-aminoquinine selector, instead of carbamate functionality turned out to be virtually equivalent in terms of enantiorecognition capabilities [57,58], the enantiomer separation potential was severely lost on iV-methylation of the carbamate group, like in 0-9-(N-me hy -N-tert-butylcarbamoyl)quinine [32,58], or its replacement by an amide, such as in case of Af-9-(pivaloyl)-9-desoxy-9-aminoquinine selector [57,58], For example, enantioselectivities dropped for DNB-alanine from 8.1 for the carbamate-type CSP, over 6.6 for thein-ea-type CSP, to 1.7 for the amide-type CSP, and 1.3 for the A -methyl... 

As previously discussed, heterocyclic polyamines containing methylenediamine functionality are usually unstable if unprotected. In contrast, the presence of a urea group stabilizes this functionality and allows the isolation of a number of heterocyclic amines. These are usually synthesized via a condensation reaction and isolated as the hydrochloride salt. The A-nitration of the 2,5,7,9-tetraazabicyclo[4.3.0]nonan-8-one and 2,4,6,8-tetraazabicyclo[3.3.0]octan-3-one ° ring systems has been investigated and serve as valuable examples. 

In crosslinking with water, pairs of isocyanate end groups in the chain-extended polymer OCN-X-NCO first react with a molecule of water this results in a linear coupling through urea groupings, with simultaneous elimination of CO2 ... 

The subsequent crosslinking probably occurs by reaction of the hydrogen atoms of the resulting urea groups with isocyanate groups still present in the starting polymer or the chain-extended polymer, with the formation of biuret groups ... 

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