40.How does the structure of isocyanate affect the reactivity of the NCO group?
Electronegativeness of the R group: If the R group is an electron-absorbing group, the electron cloud density of the C atom in the-NCO group is even lower, and it is more vulnerable to attack by nucleophilic reagents, that is, it is easier to perform nucleophilic reactions with alcohol, amine and other compounds.If R is the electron-supplying group and is transmitted through the electron cloud, the electron cloud density of the C atom in the-NCO group will increase, making it less susceptible to attack by nucleophiles, and its ability to react with active hydrogen-containing compounds will decrease.
Inducing effect: Since the aromatic diisocyanate contains two NCO groups, when the first-NCO group participates in the reaction, due to the conjugated effect of the aromatic ring, the-NCO group that does not participate in the reaction will act as an electron-absorbing group, which enhances the reactivity of the first NCO group. This effect is the inducing effect.
Steric hindrance effect: In aromatic diisocyanate molecules, if two-NCO groups are on an aromatic ring at the same time, then the influence of one NCO group on the reactivity of the other NCO group is often more significant.However, when two NCO groups are on different aromatic rings in the same molecule, or they are separated by a hydrocarbon chain or aromatic ring, the interaction between them is not large, and it decreases with the increase of the length of the chain hydrocarbon or the increase of the number of aromatic rings.
41.Reactive activity of active hydrogen compounds and NCO
Aliphatic NH2>Aromatic NH2>Primary OH>water> secondary OH> phenol OH> carboxyl> substituted urea>amide> carbamate.(If the density of the electron cloud in the nucleophilic center is greater, the stronger its electronegativeness, the higher its reactivity with isocyanate, and the faster the reaction speed; conversely, the lower the activity.)
42.Effect of hydroxyl compounds on their reactivity with isocyanates
The reactivity of an active hydrogen compound (ROH or RNH2) is related to the nature of R. When R is an electron-absorbing group (low electronegativeness), it is difficult to transfer hydrogen atoms, and the reaction between the active hydrogen compound and the NCO is more difficult; if R is an electron-supplying substitution group, the reactivity of the active hydrogen compound and the NCO can be improved.
43.What is the use of the reaction of isocyanate with water
It is one of the basic reactions for preparing polyurethane foams.The reaction between them first produces unstable carbamate, and then decomposes into CO2 and amine. If the isocyanate is excessive, the resulting amine will react with the isocyanate to form urea.
44.When preparing polyurethane elastomers, the water content of polymer polyols should be strictly controlled
For elastomers, coatings, and fibers, no air bubbles are required, so the water content in the raw materials must be strictly controlled, usually less than 0.05%.
45.The difference in the catalytic effect of amine and tin catalysts on the reaction of isocyanate
Tertiary amine catalysts have high catalytic efficiency for the reaction of isocyanate and water, while tin catalysts have high catalytic efficiency for the reaction of isocyanate and hydroxyl group.