1. How does the structure of isocyanate affect the reactivity of NCO groups?
Answer:
-A. Electronegative effect of the R group: If the R group is an electron-withdrawing group, the electron density on the carbon atom of the -NCO group is reduced, making it more susceptible to nucleophilic attack. This increases its reactivity with compounds such as alcohols and amines. Conversely, if the R group is an electron-donating group, the electron density on the carbon atom of the -NCO group is increased, reducing its reactivity with nucleophilic reagents.
-B. Inductive effect: In aromatic diisocyanates, the conjugation of the aromatic ring influences the reactivity. When one -NCO group reacts, the remaining -NCO group acts as an electron-withdrawing group due to the resonance effect of the aromatic ring, enhancing the reactivity of the first -NCO group. This phenomenon is known as the inductive effect.
-C. Steric hindrance: For aromatic diisocyanates, if both -NCO groups are attached to the same aromatic ring, their mutual influence on reactivity is significant. However, if the -NCO groups are located on different aromatic rings within the same molecule, or separated by hydrocarbon chains or rings, their interaction diminishes. The degree of influence decreases with longer chains or an increased number of aromatic rings.
2. Reactivity of NCO groups with various active hydrogen compounds
Answer: The reactivity order of active hydrogen compounds with isocyanates is as follows:
Aliphatic NH₂ > Aromatic NH₂ > Primary OH > Water > Secondary OH > Phenolic OH > Carboxylic acid > Substituted urea > Amide > Carbamate.
(The higher the electron density and electronegativity of the nucleophilic center, the higher its reactivity and reaction rate with isocyanates. Lower density corresponds to reduced reactivity.)
3. Influence of hydroxyl compound structure on reactivity with isocyanates
Answer: The reactivity of active hydrogen compounds (ROH or RNH₂) with NCO groups depends on the nature of the R group. If R is an electron-withdrawing group (low electronegativity), the transfer of the hydrogen atom becomes difficult, reducing reactivity. Conversely, if R is an electron-donating substituent, it increases the reactivity of the compound with NCO groups.
4. Applications of the reaction between isocyanates and water
Answer: This reaction is a fundamental step in producing polyurethane foam. Initially, it generates unstable carbamic acid, which decomposes into CO₂ and amines. If an excess of isocyanate is present, the resulting amines react further with isocyanates to form urea.
5. The importance of controlling water content in polymer polyols for polyurethane elastomer preparation
Answer: For applications such as elastomers, coatings, and fibers, the absence of bubbles is critical. Thus, the water content in raw materials must be strictly controlled, typically below 0.05%.
