Introduction. 異氰酸鹽 (Isocyanates, -NCO ) 和羥基 (-OH ) 化合物的 反應性極高 ，但在有些應用上需要延遲異氰酸鹽與羥基的反應，並在適當的步驟下適放異氰酸鹽官能基。 封閉型異氰酸鹽是 異氰酸鹽 和 帶有活性氫的封閉劑 (Blocked agegnt BH) 反應生成一個 弱鍵結 的化合物；在一定溫度下釋放出 (-NCO) 官能基。. Introduction. 封閉型聚異氰酸鹽， 對於水氣有良好的抵抗性 及 較佳的儲存性 。
50 ℃ 2hr
70 ℃ 3hr
The electron-donating substituents rendered the nitrogen atom of N-methylaniline more basic for its easy attack on the partially positive carbon atom of the -NCO group, thereby increasing the rate of the blocking reaction. The slow blocking reaction of N-methylaniline substituted with electron-donating substituents at the ortho position may be attributed to the steric factor.
The use of phenol as a blocking agent for isocyanates is understandable because phenol is less nucleophilic toward isocyanate groups; as a result, the bond that forms between the carbonyl carbon of isocyanate and the oxygen atom of phenol is labile.
At low temperatures, the hydrogen-bonded proton has a dipolar attraction between the positively polarized hydrogen and the negatively polarized nitrogen of the blocking agent, which leads to intramolecular association with consequent lengthening and weakening of the original -NH bond.
The electron density around the proton is reduced, and this deshielding moves the proton signal to a higher frequency.
As the temperature increases, the hydrogen bond becomes weak, and the original -NH bond is shortened; this leads to increased electron density around the proton, and this shielding move the proton to a lower frequency.
Variable-temperature 1H NMR spectra of blocked polyisocyanate 2, showing a frequency shift of (a) hydrogen-bonded urethane and (b) urea protons in CDCl3.
Urea : 1685 cm-1
Ureathane : 1685 cm-1
FTIR spectrum of (A) blocked isocyanate 12, (B) blocked polyisocyanate 2 recorded at 140℃, and (C) blocked polyisocyanate 2 recorded after 120 min at 140℃.
Carbonyl regions of FTIR spectra of blocked polyisocyanate 2 recorded at 140 ℃ for different time intervals.
Changes in the intensities of (A) NCO, (B) urea C=O, and (C) allophanate C=O absorption of blocked polyisocyanate 2 at 125 ℃ with respect to time.
The entropies of activation are highly negative because of the formation of a hydrogen-bonded, four-centered, rigid complex in the transition state.
In the case of blocked polyisocyanate 10, the peak of urea carbonyl is partly merged with the adjacent allophanate carbonyl absorption; thus, there was difficulty in the calculation of the kinetic parameters.