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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。
• 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
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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
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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-methylaniline
method
N2
PTHF
50 ℃ 2hr
N2
1hr
70 ℃ 3hr
MDI
40 ℃
Blocked
agent
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
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
The electron density around the proton is reduced,
and this deshielding moves the proton signal to a
鍵有關，而在解封閉反應中會形成氫
higher frequency.
鍵及四級化過渡態的錯合物。
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。