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。

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。 • 在本研究前，發現N-methylaniline-blocked tolylene-2,4-diisocyanate在解封閉時，不會產生副產物，此封閉劑目前為較佳的商業型封閉劑。 • 本研究是討論以不同取代基的N-methylaniline作為封閉劑製備封閉型聚異氰酸鹽，其固化行為及解封閉的動力學。

Materials • N-Methylaniline (Lancaster) • Methyl 2-methylaminobenzoate(Lancaster) • Methyl 4-methylaminobenzoate(Lancaster) • N-methyl-o-anisidine (Aldrich) • Nmethyl-p-anisidine (Aldrich) • N-methyl-o-toludine(Aldrich) • N-methyl-p-toludine (Aldrich) • 2-chloro-N-methylaniline (Aldrich) • 4-chloro-N-methylaniline(Aldrich) • N-methyl-4-nitroaniline (Aldrich)

Materials • 4,4-Methylenebis(phenylisocyanate) (MDI; Lancaster) • Phenyl isocyanate(Aldrich) • Poly(tetrahydrofuran) (Terathane; Mn=2000;Aldrich) • Dibutylamine (Fluka) • Hydroxyl-terminated polybutadiene(HTPB; Mn=2500), obtained from Vikram SarabhaiSpace Centre, was used after drying for 2 hat 80 ℃ in vacuo. • Toluene (Merck), methanol(Merck), and chloroform (Merck) were purifiedaccording to standard procedures.

Synthesis N-methylanilinemethod 50 ℃ 2hr PTHF N2 N2 1hr 70 ℃ 3hr 40 ℃ MDI Blocked agent

Synthesis

Results and discussion 有內子分氫鍵 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.

Results and discussion Urea:153.69 ppm urethane:154.39 ppm

Results and discussion Polyisocyanate 2

Results and discussion 經由實驗證實，並非如此，而是四級化過渡態上的氫鍵，會產生自催化效應，使C=O和N的鍵結變更弱。 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. NMA上氮的親核性較phenolic system高，和羰基的碳形成的鍵結較穩定，因此，可從電荷差異來看鍵的強弱。

Results and discussion 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. 由此可證實，Urea的質子與分子內氫鍵有關，而在解封閉反應中會形成氫鍵及四級化過渡態的錯合物。 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.

Results and discussion 在鄰位有極化的取代基，降低氫鍵的自催化反應 BH的N上電子密度↑

Results and discussion 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℃.

Results and discussion Carbonyl regions of FTIR spectra of blocked polyisocyanate 2 recorded at 140 ℃ for different time intervals.

Results and discussion (A) (C) (B) 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.

Results and discussion 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.

Conclusions • 一系列不同取代基的N-methylanilines封閉劑與異氰酸酯被合成出來，且解封閉的溫度、固化時間及解封閉動力學在這篇研究報告出來。 • N-methylanilines經過解封閉後殘留在系統中，與酚類化合物相比，其腐蝕性較低，較適合用來製作熱固型聚胺酯。

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。