Reactivity of isocyanate groups

The reaction rates of diisocyanates are strongly influenced by their molecular structure. The reactivity of isocyanate groups is enhanced by adjacent electron-withdrawing substituents. Aromatic rings are very effective electron withdrawing groups, and it is for this reason that the majority of commercial diisocyanates are aromatic. Many of the diisocyanates used commercially consist of mixtures of isomers. Some of the more important commercial diisocyanates are illustrated in Fig. 25.6. Diisocyanates must be handled carefully to avoid exposing workers to their hazardous vapors. 

Ono, H.-K., Jones, F. N., and Pappas, S. P., Relative Reactivity of Isocyanate Groups of Isophorone Diisocyanate. Unexpected High Reactivity of the Secondary Isocyanate Group, J. Polym. Sci., Polym. Lett. Ed., 23, 509-515 (1985). 

TABLE 2-3 Reactivity of Isocyanate Group with -C4H90H ... 

In the one-shot method, factors like the different reactivity of isocyanate groups and the different hydroxyl groups of a polyol and a chain extender may affect the distribution of HS in the chain or even produce a mixture (oligomer) of diisocyanate-chain extender and diisocyanate-macrodiol homopolymers [4,24,25]. 

Lastly, of course, the main reaction of interest is the formation of urethane groups by reaction of isocyanate groups and hydroxy-groups of the polyester or polyether. Even these reactions do not exhaust the possibilities available to the highly reactive isocyanate group. It will then go on to react with the urethane links to form a structure known as an allophanate (see Reaction 4.13). 

The silyl ether derivative of the alcohol is used in Eq. 2-195 since the corresponding alcohol OCN—R—CONH—R —OH cannot be isolated because of the high degree of reactivity of isocyanate and alcohol groups toward each other. 

The reagent ratio in the reactive mixture is selected so that there is a small surplus of isocyanate groups. 

Reactivities of isocyanates depend on their structure. Table 2.6 gives the main isocyanates used for polymer network synthesis. Conjugation with aromatic nuclei makes ArNCO particularly reactive. The reactivity of diisocyanates is well documented in the literature. For symmetric diisocyanates such as diphenylmethane 4,4 -diisocyanate (MDI) or para-phenylene 4,4 -diisocyanate (PPDI), both NCO groups have initially the same reactivity. But as the NCO group itself exhibits an activating effect on isocyanate reactivity, the fact that one NCO group has reacted introduces a substitution effect that usually decreases the reactivity of the second NCO group. 

Luda, M.P. Costa, L. Bracco, P. Levchik, S.V. Relevant factors in scorch generation in fire retarded flexible polyurethane foams II Reactivity of isocyanate, urea and urethane groups. Polym. Degrad. Stab. 2004, 86, 43-50. 

Like other, 8-dicarbonyl compounds, the substituted keto amides (X)—(XV) are very reactive and represent the key intermediates in a complex set of side reactions [95]. A considerable amount of research has been devoted to the elucidation of the reactions of these compounds under polymerization conditions [118—120,124,125,127,128, 130,131, 134—143]. With respect to the high reactivity of carbonyl groups and the adjacent —CH2— or i CH- groups, the structures like (X)—(XV) are unstable at elevated temperatures and even less stable in the presence of a strong base. Bukac and Sebenda [137] showed that keto amides with one hydrogen atom at the nitrogen decompose very easily in the presence of base into isocyanate which takes part in many subsequent reactions, scheme (45). The most important consequence of side reactions is the fact that water is present even if the anionic polymerization is started under... 

The reactivity of unhindered isocyanates with primary amines is considerably higher than that with primary alcohols. The reactivity of amines increases with the basicity of the amine, and aliphatic amines are much faster than aromatic amines.Polar solvents can increase the reactivity of isocyanates through the stabilization and polarization of the isocyanate group. ... 

The reactivity of the model phenols and benzyl alcohols with phenyl isocyanate was determined in the presence of a tertiary amine (DMCHA) and a tin catalyst (DBTDL) by measurement of the reaction kinetics. The experimental results based on initial equal concentrations of phenyl isocyanate and protic reactants showed that the catalyzed reactions followed second order reaction with respect to the disappearance of isocyanate groups (see Figure 1). It was also found that a linear relationship exists between the experimental rate constant kexp, and the initial concentration of the amine catalyst (see Figure 2). In the case of the tin catalyst, the reaction with respect to catalyst concentration was found to be one-half order (see Figures 3-4). A similar relationship for the tin catalyzed urethane reaction was found by Borkent... 

It was found that the reactivity of isocyanates in the cyclotrimerization reactions increased with the presence of the electron withdrawing groups in the vicinity of the isocyanate group, with the increased nucleophili-city of a catalyst and relative permitivity of the solvent system. 

The reactivity of isocyanates toward active hydrogen compounds is a much more complex problem. As a general rule, the -NCO groups of a diisocyanate have different reactivities, in spite of the perfect symmetry of the molecule. The explanation of this effect is simple after the reaction of the first molecule of the hydrogen active compound (an alcohol for... 

This big difference in the reactivity of isocyanates, with primary amines and with hydroxyl groups results in the polyurea being formed preferentially - only a small part of the hydroxyl groups being reacted. 

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