Synthesis and characterization of the octaaminopropyl-silsesquioxane

Gao Jungang*, Zhang Xuejian, Wang Shichen, Run Mingtao
(College of Chemistry and Environmental Science, Hebei University, Baoding, 071002, China)

Absract  Octaaminopropylsilsesquioxane (OAPS) was synthesized with alkali-equilibrated mixtures of-aminopropyltriethoxysilane and quaternary ammonium hydrate. The molecular mass and distribution were determined by GPC and VPO. Element contents were obtained from Element analysis. The structure was characterized using FTIR, H NMR, 13C NMR, 29Si NMR. The thermal stability was measured by means of DSC. Melting point was observed with microscopic melting point apparatus.

In the field of inorganic-organic hybrid materials, there is growing interest in polyhedral silsesquioxane-based systems. The interest of these silica-like cage structures is driven by both their growing commercial availability and the relative ease of attaching functional groups to the cage, facilitating the incorporation of the organic component into a macromolecular system.
Polyhedral oligomeric silsesquioxane (POSS) are a class of cage-shaped nanoscale building blocks that can self-assemble to construct hybrid organic/inorganic nanomaterials with enhanced physical properties[1-3], POSS nanocubes consist of silicon and oxygen atoms linked into a well-defined, cubic inorganic framework with silicon atoms at the corners and oxygen atoms interspersed along the edges. Each of the silicon corners can be functionalized with a variety of organic substituents[4,5] to confer precise control over the topology and architecture. Hu and Yuan[6,7] had proved that the octahedral silsesquioxane can be synthesized in the presence of MeNOH. Scheme 1 illustrates the structural representation of the cubic silsesquioxane. In this paper, the hydrolyzation processing of the aliphatic aminosilsesquioxane was examined, and octaaminopropylsilsesquioxane (OAPS) was synthesized.
Scheme 1 The structural formula of the cubic silsesquioxane.

In cubic macromonomers, all eight vertices are functionalized and equally reactive. Upon processing, the size of the cube core and the functionalized arms define the size of the organic/inorganic components in the resulting nanocomposites where both components are distributed uniformly and periodically.

2.1 Materials
-aminopropyltriethoxysilane (chromatographic degree of purity 98%), (CHNOH·4HO was obtained from Beijing Fubide Fine Chemical Co. LTD. Methanol (99.5%) was obtained from Tianjin Chemical Reagent Co. of China.
2.2 Preparation of OAPS
Octaaminopropylsilsesquioxane (OAPS) was prepared from HNCHCHCHSi(OCHCH (0.25mol) and MeNOH (0.77mol) under N, the CHOH (500ml) was used as solvent. The reaction was first carried out at normal temperature for 24h, and then continued at 60ºCfor 48h. The CHOH was removed by distillation, then the product was kept at 60ºCunder vacuum for another 48h. the leavings was kept at 110ºCfor 24h to get rid of superfluous MeNOH and HO. Finally, the product was purified by 100ml n-hexane and 100ml toluene, yield was 92.7%. The molecular weight was determined by GPC and Vapor pressure osmometer (VPO).
2.3 Fourier transform IR spectra (FTIR)
FTIR was obtained using an Avatar series Nicolet 205 FTIR spectrometer; optical grade potassium bromide (KBr) was used as a background material. The resolution is±2cm-1, and the spectral range is from 4800 to 200cm-1
2.4 NMR Analyses
H NMR analyses were performed with a Bruker DSX-600 spectrometer at 600MHz with 10000Hz spectral width, a relaxation delay of 2s, a pulse width of 20ºC, and 30K data points. All the spectra were recorded in DO using the HO signal as an internal reference.
13C NMR analyses were performed at 300MHz with 30KHz spectral width, a relaxation delay of 5s, All the spectra were recorded in acetone medium.
Solid state 29Si NMR spectra were obtained at 300K using a Bruker DSX-300 spectrometer operating at 59.6MHz. Pulse delays were 5s. The sample temperature was 300K and TMS was used as reference.
2.5 Element analysis
Element analyses were performed with PE-2400.
2.6 Differential Scanning Calorimetry (DSC)
Calorimetry was performed on materials using a CDR-4P differential scanning calorimetry. The airflow rate was 60ml/min. Sample (9-10 mg) was placed in a pan and raised to 400ºC (10ºC/min/air) without capped. The heat flow differences between reference and sample were recorded.
2.7 Melting point mensuration
X-4 microscopic melting point apparatus (Beijing) was used to observe the melting point of Octaaminopropylsilsesquioxane (OAPS) at the heating rate of 2ºC/min.

The process of formation of the silsesquioxane compounds is described as a multi-step hydrolysis-condensation reaction (see Scheme 2). The first step is the hydrolysis of an-amino- propyltrioethoxysiloxane with water to form organic polysilanol compound. In the right environment, depending on the concentration of water, solvent and pH, these precursors can condense with each other, leading to the formation of oligosiloxanes. By means of thermo-dynamics[8,9], kinetics and solubility of the products, the resulting product mixture can be determined, ranging from lower oligosiloxane dimers or tetramers to polyhedral oligomeric silsesquioxanes like the tri- and tetrasilanol silsesquioxanes.
The hydrolysis of r-aminopropyltriethoxysilane in the presence of MeNOH was studied with various proportions. When the proportion is 3:1, crystal water of tetramethylammonium can provide adequate HO in hydrolyzation. The equal molar ratios of tetramethylammonium and ethoxy can provide hydrolytic condition completely and then the reaction selectively to form cube cage, while in otherproportions the cage cannot be formed or impurity product obtained. Less solvent could lead to ladder molecules (Mn -20000), while more solvent decreased pH value so that the cage could not be formed.
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Scheme 2 The process of formation of the silsesquioxane compounds

Molecular masses and distributions were measured by GPC and VPO as in Table 1. OAPS data are also shown as comparison. Because the hydrodynamic volumes of “spherical” OAPS are smaller than the linear polystyrenes used for GPC calibration, the measured molecular weights were smaller than the theoretical values. But the molecular mass is 810 determined by VPO, which is close to the theoretical value.

Table 1GPC data of OAPS

OAPS Calcd Mn Mw PDI Mn
880 714 1257 1.76 810

Theoretical molecular weight. Polydispersity index

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29Si NMR of OAPS
07704804.gif (33816 bytes)

The analysis of FTIR is shown in Figure 2 and Table 2. Although O-H band at 3300-3500cm-1 in the FTIR spectra is hardly distinguishable from the N-H peak at ∽3300cm-1, 29Si NMR spectra (Fig. 1) clearly shows Si peak at -68.0 ppm, which suggests that 3383.60 cm-1 in the FTIR is N-H peak (Fig. 2). The result of 29Si NMR spectra accord with the octaaminophenylsilsesquioxane characterized by Choi[10]

Table 2FTIR, NMR and Element Analysis of OAPS

Composite Element analysis (%) FTIR
NMR (ppm)
Theoretical value Measured value   13 29Si
[NH(CHSiO1.5   32.727 32.855 Si-O-Si 1037.66-1134.00
N-H 1576.57-3383.60
C-N 1488.68
C-Si 948.88
C-C 1194.03
C-H 2934.07
C-H 1287.92-1340.48
C-H 1389.87-1403.80



0.406- 0.476
long-chain alkyl
2.304- 2.475




  7.272 7.363
  12.727 12.851
  21.818 20.266

The correlative data can be seen from Table 2, the theoretical value is close to the measured value in element analysis. We can also obtain the conclusion that the molecular mass is close to theoretical value. The ethoxyl groups have been hydrolyzed completely in the H NMR and 13C NMR.
The thermal behaviors of OAPS have been characterized by means of DSC (Fig.3) and melting point apparatus,OAPS melts at 196.6ºCand begins to decompose at 242ºC.
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Fig.3 DSC of OAPS cube (10ºC/min/air, first run)

When the proportion of tetramethylammonium and r-aminopropyltriethoxysilane is 3:1, the OAPS can be synthesized in CHOH. OAPS begins melting at196.6ºC, which suggests it has high thermal stability.