Biuret groups

If an isocyanate group reacts with a hydrogen within the polyurea structure, a branching point is formed, a biuret group ... 

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. 

One can observe positive deviations in the region of rH < 1 (excess of isocyanate groups) which are due to side reactions (allophanate, urea and biuret groups). In the region of rH > 1 the agreement of wg values is good. In the case of i e, the predicted curves depend not only on the results of the branching theory but also... 

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 ... 

In the preparation of a prepolymer, every effort is made to prevent the formation of any unplanned branching such as biuret groups. The prepolymer is essentially linear except when some cross-link sites have been introduced by using a multifunctional isocyanate or triol. 

The isocyanate (2270 cm"1) uretedinedione ring carbonyl (1780 cm"1) and urea carbonyl (1660 cm"1) groups can usually be identified. Carbonyls from ester, urethane, allophanate, isocyanuric acid ring and Biuret groups all absorb near 1730 cm 1 and are difficult to distinguish. Hydrogen bonds which can function as physical crosslinks in PU have been... 

Of course allophanate and biuret groups are hydrolysed and glycolysed in a similar manner to urea groups, the first reaction takes place at the allophanate or biuret group. These reactions are presented in the Figure 20.1. 

At least 15 processes have been proposed for the synthesis of polyureas with the repeat unit —R—NH—CO—NH—. Most of these are unsuitable for commercial production because of side reactions. For example, in the conversion of diisocyanates with diamines, biuret groups readily occur, and therefore, cross-linked polymers (see also Section 28.4). Polymers from the reaction of diamines with COS cannot be obtained free of sulfur, etc. 

Biuret n (allophanamide, carbamylurea) NH2CONHCONH2 H2O. A white crystalline material derived from urea by heat or by reaction with an isocyanate. It is used primarily in analytical chemistry, but the biuret group is formed during some polymerization reactions, such as primary bonding in urethane elastomers. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

The processing of these fibers is generally carried out by dry spinning from a collodion in either dimethylformamide or dimethylacetamide. Simultaneously, a chain extension proceeds by reaction with a bivalent molecule. The latter can be either a diol or a diamine in this latter case, a biuret group is formed ... 

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