Isocyanates nco oh
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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。 PowerPoint PPT Presentation


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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 。

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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。

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Isocyanates nco oh

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。


Isocyanates nco oh

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。


Isocyanates nco oh

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

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.


Isocyanates nco oh

Synthesis

N-methylanilinemethod

50 ℃ 2hr

PTHF

N2

N2

1hr

70 ℃ 3hr

40 ℃

MDI

Blocked agent


Isocyanates nco oh

Synthesis


Isocyanates nco oh

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.


Isocyanates nco oh

Results and discussion

Urea:153.69 ppm

urethane:154.39 ppm


Isocyanates nco oh

Results and discussion

Polyisocyanate 2


Isocyanates nco oh

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。


Isocyanates nco oh

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.


Isocyanates nco oh

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↑


Isocyanates nco oh

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℃.


Isocyanates nco oh

Results and discussion

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


Isocyanates nco oh

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.


Isocyanates nco oh

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.


Isocyanates nco oh

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。


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