• 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。

Introduction

• 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.

Materials

PTHFPTHF

MDIMDI

N2 N2

1hr

50 2hr℃

70 3hr℃

40 ℃

Blocked agent

N-methylanilinemethod

N-methylanilinemethod

Synthesis

Sample Blocked agent

1 Polyurethane prepolymer ( the terminal –NCO )

2 N-Methylaniline-Blocked Polyisocyanate

3 N-Methyl-o-toludine-Blocked Polyisocyanate

4 N-Methyl-p-toludine-Blocked Polyisocyanate

5 N-Methyl-o-anisidine-Blocked Polyisocyanate

6 N-Methyl-p-anisidine-Blocked Polyisocyanate

7 2-Chloro-N-methylaniline-Blocked Polyisocyanate

8 4-Chloro-N-methylaniline-Blocked Polyisocyanate

9 Methyl 2-Methylaminobenzoate Blocked Polyisocyanate

10 Methyl 4-Methylaminobenzoate Blocked Polyisocyanate

11 N-Methyl-4-nitroaniline-Blocked Polyisocyanate

12 Preparation of N-Methylaniline-Blocked Phenyl Isocyanate

Synthesis

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 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.

有內子分氫鍵

Results and discussion

Urea:153.69 ppm urethane:154.39 ppm

Results and discussion

Polyisocyanate 2

Results and discussion

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。

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。

Results and discussion

Variable-temperature 1H NMR spectra of blocked polyisocyanate 2, showing a frequency shift of (a) hydrogen-bonded urethane and (b) urea protons in CDCl3.

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.

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。

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。

Results and discussion

BH 的 N上電子密度↑BH 的 N上電子密度↑

在鄰位有極化的取代基，降低氫鍵的自催化反應在鄰位有極化的取代基，降低氫鍵的自催化反應

Results and discussion

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 .℃

Urea : 1685 cm-1

Ureathane : 1685 cm-1

Results and discussion

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

Results and discussion

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.℃

(A)

(B)

(C)

Results and discussion

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.

The entropies of activation are highly negative because of the formation of a hydrogen-bonded, four-centered, rigid complex in the transition state.

Results and discussion

• 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。

Conclusions