A Novel Green Synthesis Method of Poly (3-Glycidoxypropyltrimethoxysilane) Catalyzed by Treated Bentonite

The present work focuses on the preparation and characterization of poly(3Glycidoxypropyltrimethoxysilane) (PGPTMS) under mild conditions. Ring-opening polymerization of the 3-Glycidoxypropyltrimethoxysilane (GPTMS) is initiated with the bentonite of Maghnite-H+ (MagH+), an ecologic and low-cost catalyst. The evolution of epoxy ring-opening was studied in bulk and in solution using CH2Cl2 as solvent, as well as the influences of several factors such as the amount of Mag-H+, polymerization time and temperature on the yield of polymer were investigated. The best polymer yield (30 %) was obtained in bulk polymerization at room temperature (20 °C) for a reaction time 8 h, and it’s increases with time and reaches 68 % for 7 days. The structures of the obtained polymers (PGPTMS) were confirmed respectively by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR). The thermal properties of the prepared polymers were given by Differential Scanning Calorimetry (DSC) and thermogravimetric analysis (TGA), the Tg of PGPTMS is recorded at -31.27 °C, and it is thermally stable with a degradation start temperature greater than 300 °C, all decomposition stopped at 600 °C. Copyright © 2020 BCREC Group. All rights reserved


Introduction
As is known, the 3-Glycidoxypropyltrimethoxysilane (GPTMS) is one of the most used coupling agents in the field of surface conversion treatment, while respecting the environment [1][2][3]. Hybrid organic-inorganic compounds prepared by the opening of epoxy ring of GPTMS [4][5][6][7][8] are known for their improved mechanical strength and thermal stability [9]. ously with condensation with basic catalyst [23]. GPTMS derived hybrid materials were synthesized by TiCl4, in nonhydrolytic conditions, to achieve epoxy ring opening polymerization by a multistep reaction , linear compounds of different lengths and an inorganic network were formed by hydrolysis [24] and condensation reactions. Different amounts of BF3OEt2, used to catalyze the organic polymerization of GPTMS affect the degree of inorganic cross-linking and the length of the ethylene oxide chains [25,26]. Other catalysts were used by other reviews as triethylamine [27] to produce a three-dimensional cross-linked elastomer. In previous work [28], a linear polyGPTMS and transparence polymer with great thermal resistance has been obtained with the Multimetal cyanide (MMC) as a catalyst, under heating (130 °C).
On the other hand, the influence of temperature on the hydrolysis of GPTMS has been studied by several authors [29]. Hydrolysis and condensation reactions accompanied with the opening of epoxy ring are favored with increase in temperature. As mentioned in the literature, the cationic polymerization of GPTMS cannot start without heating [4][5][6][7][8]28] such as the radicalar polymerization, the main purpose of this work is to use a simple and safe processing method without using high temperature to prepare a linear polyGPTMS with unhydrolysis alkoxy group. The polymerization reaction is realized under mild conditions, using a cationic catalyst, Mag-H + , an Algerian proton exchanged montmorillonite clay, in way to be in the context of green chemistry.
In a previous work [30], it has been shown that there is a correlation between acid activation of maghnite with different concentrations of H2SO4 and its catalytic activity. It has been found that the best results are obtained by acid activation with H2SO4 (0.25M) solution because the cationic exchange of maghnite by protons (Mag-H + ) is complete without destroying its structure. This solid catalyst is widely used in the ring-opening polymerizations as an effected catalyst with demonstrated results published in many international reviews [31][32][33][34][35][36][37], and has many advantages such as very low cost, the easy removal of the reaction mixture, regenerated by heating to a temperature above 100 °C [31,32] compared to other catalysts.
In our work, we have explored a new method for a green synthesis of PGPTMS, by ring opening polymerization of GPTMS under mild conditions with the aim to respect the principles of green chemistry, without spend any en-ergies. The polymerization reaction of GPTMS is initiated with a green catalyst to replace electrophilic catalysts, such as: Brønsted and Lewis acids which are very noxious and corrosive. The best results in term of transparency and yield of the product are obtained by the synthesis method in bulk. The structure of the obtained polymer (PGPTMS) was confirmed respectively by FTIR, 1 H-NMR, and 29 Si-NMR. The thermal properties of the prepared polymers were given by TGA analysis. The effects of different synthesis parameters, such as the amount of Mag-H + , temperature and polymerization time, were discussed.

Structural Analysis
The chemical structure of the products was determined by Fourier transform infrared spectroscopy analysis (FTIR), converted from Perkin Elmer Two FT-IR with UATR sampling accessory. X-ray diffraction (XRD), conducted at room temperature by Bruker AXS D8 advanced X-ray diffractometer (40 kV, 30 mA) using Cu-K radiation ( = 0.154 nm) at the rate of 2° min -1 in the 2θ range of 2.0 -80°. The 1 H-NMR and 29 Si-MAS/NMR ( 29 Si-Magic angle spinning/nuclear magnetic resonance) measurements were carried, using on an Avance Bruker NMR Spectrometer at 300 and 99. 35 MHz, respectively. The spinning speed for cross-polarization magic angle spinning (CPMAS) was 4 MHz for 29 Si MAS NMR at room temperature. Chemical shift references are tetramethylsilane (TMS) and 4 mm zirconia rotor was used. Thermogravimetry (TGA) analysis, performed by PerkinElmer STA 6000 under air and nitrogen in the temperature range 20-800 °C with a heating rate of 10 °C/min, the mass of the sample to be analyzed is 5-7 mg. Differential Scanning Calorimetry (DSC) analysis performed by universal VA.5A TA instrument, site average scan be-tween -100 °C and 100 °C with heating rate of 10 °C/min, using 6 mg of PGPTMS.

Preparation of the Mag-H+ Catalyst
Mag-H+ was prepared by a method similar to that described in our previous study [31,32]. The raw Maghnite was activated with a sulfuric acid solution (0.25 M) to give a Maghnite exchanged with protons, called Mag-H + . In an Erlenmeyer flask, 30 g of crushed raw Maghnite was dispersed in 1 L of distilled water. The mixture was stirred well during 2 h at room temperature, then left to settle for about 8 hours in a volumetric test tube until the appearance of Montmorillonite supernatant. After that about 100 mL of supernatant is taken and replaced by 100 mL of distilled water, the mixture is stirred again and left to settle for a second 8 hours. This operation is repeated until we recover the maximum of montmorillonite. To exchange the cation ions present in the clay by H + ions, a solution of sulfuric acid H2SO4 (0.25 M) is added to the dried montmorillonite recovered. The mixture is maintained stirring for 48 hours, Then the catalyst (Mag-H+) formed, is filtered after being washed several times with distilled water up to pH 7. The Mag-H + was dried in an oven for 24 h at 105 °C and then crushed. The catalyst structure is characterized by FTIR and XRD.

Ring-opening Polymerization of GPTMS
The ring-opening polymerization of 3-Glycidoxypropyltrimethoxysilane (GPTMS) was carried out in bulk and in solution using CH2Cl2 as solvent at various temperature and polymerization time, using various amounts of Mag-H + . The procedure was identical in any case, involving the addition of catalyst to the mechanical stirred solution containing GPTMS and solvent. Before use, Mag-H + was dried in an oven at 120 °C overnight. The polymeriza-tion reaction of GPTMS is described in Scheme 1.

Detailed example of synthesis
In a flask containing 2 mL of GPTMS (0.0084 mol) mixed with 5 mL of dichloromethane, we added an amount of 20% by weight of dried Mag-H + . The mixture was stirred for 8h at room temperature (20 °C). For ending the reaction a drop distilled water is added, then the mixture is filtered to remove the catalyst and precipitated in 50 ml of cold methanol (CH3OH). The resulting polymer was extracted by filtration, washed for several times and dried in vacuum over night and weighed.

FT-IR Analysis
FT-IR spectrum of Mag-H+ compared to the raw-Maghnite Figure 1 shows that the hydrated structure of Magnhite-H + results in characteristic vibrations of hydroxyl groups around 3400 cm -1 and 3600 cm -1 . The intense band 1004 cm -1 corresponds to the valence vibration of Si-O in the tetrahedral layer. Various other absorption bands at 780 ; 520 and 500 cm -1 can be attributed to the montmorillonite clay structure. All these expected absorption bands confirm that the structure of the compound is thus in good agreement with vibration values obtained from the literature [38][39][40].

XRD Analysis
The X-ray diffractograms of the raw-Maghnite and the Mag-H + are shown in Figure  2. The increase of interlayer distance d (001) = 11.20 Å in raw-Maghnite, characteristic of a single water layer between the sheets to a d(001) = 15 Å , distance attributed to two interlamellar water layer in Mag-H+. This increase indicates that there is a cationic exchange of  Figure 3 (a), Figure 3 (b) and Figure 3 (c). The characteristic absorption peaks of the epoxy group observed at around 778-815 cm -1 and 900-910 cm -1 [41] in the spectrum of the monomer Figure 3  The FTIR spectrum of polymer PGPTMS synthesized in solution and in bulk was assigned as follows ref [4-8, 28 , 42]. Characteristic bands of GPTMS epoxy ring were located at : 500 cm -1 (Si-O in Si-O-CH3); 734 and 841 cm −1 (C-C), 1089 cm -1 (-CO in Si-O-CH3); 1190 cm -1 (-CO in C-O-C); 1016 cm -1 (-CO epoxy ring ); 1480 cm -1 (-CH); 2800-3000 cm -1 (-CH2 and -CH3) and 3360 cm -1 (-OH). According to the literature, the peak located at 907 cm −1 could describe the elongation of C-O of C-OH end groups [42][43][44][45]. The absorption band around 1016 cm -1 , is also attributed to asymmetric stretching of Si-O-Si bonds [5] is observed in both polymerization. This bond is formed by the epoxy group reacting with water in air or adsorbed by Mag-H+ during the PGPTMS reprocessing procedure.
It is observed that there is an increasement of the intensity of the absorption band corresponding to the group (C-O) at about 1017 cm -1 for the PGPTMS prepared in bulk Figure 3(c) to that prepared in solution Figure 3(b), which indicates the formation of poly (ethylene oxide) in bulk with a degree of polymerization greater than that in solution. As well as, a large difference in the intensity and width of the absorption band of hydroxyl group situated at about 3360 cm -1 , this band is more intense and wider in spectrum (c) than that observed in spectrum (b).

1 H-NMR analysis
The chemical displacement of the protons of the GPTMS and the PGPTMS are shown in    Figure 5. Comparing the two spectra, it can be seen that the change in chemical shift and coupling of the resonances of H6, H7 and H7' in the spectrum. We also see the appearance of a multiplets around 3.2 and 3.4 ppm corresponding to protons H4, H5 and H6 in PGPTMS polymer spectrum after ring -opening polymerization, which is in good agreement with literature data [5,7,28]. The signal at 3.48 ppm corresponds to the protons in the alkoxide group (Si-O-CH3) in the two spectra confirms that only the ring opening polymerization is set up keeping the alkoxide function. The appearance of a peak at 5.4 ppm indicates the hydroxy end group O-H of polymer product due to the opening of the epoxide rings. The chemical shifts of the protons of the polymer synthesized are in agreement with those of the literature [4][5][6][7][8]28]. The results obtained by 1 H-NMR analysis show clearly the ring-opening polymerization of monomer, which are confirmed by the FTIR analysis.

29 Si-MAS/NMR analysis
The 29 Si-MAS/NMR ( 29 Si-Magic angle spinning/nuclear magnetic resonance) spectrum of polymer PGPTMS shown in Figure 6 allows to determine the alkoxysilane functions present in the hybrid polymer. It clearly shows the occurrence of hydrolysis by the presence of two major peaks, indicating the formation of two and three new siloxane bonds, respectively, from GPTMS [22,46]. The peaks at -58.30 and -66.66 ppm were observed for the corresponding absorption of monohydroxy-substituted silica Figure 7: RSi(OSi)2(OH) (T2) and nonhydroxysubtituted silica : RSi(OSi)3 (T3). These groups might be produced by the epoxy group reacting with water in air or adsorbed by Mag-H+ dur- ing the PGPTMS reprocessing procedure. The same results are obtained during the synthesis of epoxy-silica hybrid resins [47,48]. Therefore, the 29 Si-NMR results confirmed the conclusions drawn from the FT-IR analysis.

Thermogravimetry (TGA) analysis
In order to follow the thermal stability of the obtained product, the thermogravimetric analysis is used. The thermogravimetric curve Figure 7 represents the relative mass loss as a function of the heating temperature of PGPTMS compared with the GPTMS, hence depicting that the PGPTMS is thermally stable [28] with a degradation start temperature greater than 300 °C. The degradation rate of polymer increased with increasing temperature and reached its maximum at 370.5 °C, then is gradually decreased.
It also shows that the curve is consisted of a single distinct part associated with a single loss of mass indicating that the obtained product consist of a single type of polymer and not a mixture. Also shows that this polymer does not keep any trace of solvent or starting materials (monomer) knowing that the boiling point of the monomer used (3-glycidoxypropyltrimethoxysilane) is 120 °C and that the evaporation temperature of the solvent of the reaction (dichloromethane) is 40 °C. These two tem-peratures differ from those indicated on the curve confirming the formation of new product which is more stable than the monomers.  sult is in accordance with previous literature [28]. The low Tg indicates the good resistance of the polymer at low temperature.

Kinetic Study of the Rng-opening Polymerization of GPTMS Catalyzed by Mag-H +
The change in the reaction conditions of the PGPTMS synthesis reaction makes it possible to carry out a kinetic study on the ring-opening of the epoxide and the evolution of the PGPTMS yield. In order to find the optimum conditions of the polymerization of GPTMS, we studied the effect of different parameters and their influence on the yield of the obtained polymers, such as: the effect of catalyst amount, temperature and reaction time. Preliminary experiments were carried out with varied temperature, catalyst amount and reaction time ( Figures 11, 12 and 13).

Effect of Mag-H + amount on the yield of PGPTMS
The polymerization is carried out at 20 °C for 8 h in bulk and in solution (5 mL of CH2Cl2), using various amounts of Mag-H + (5; 10; 15; 20 and 25 wt%). For the obtained polymers in bulk, the results of the FTIR analysis in the Figure 9 show that the opening of the epoxy ring starts from the addition of 15% by weight of the catalyst Figure 9 (b) and continues with the increase of the amount of Mag-H + . This is indicated by the decrease of the intensity of the characteristic absorption peak of the epoxy group observed at around 910 cm -1 in the spectrum of monomer Figure 9 (a) and it's disappeared completely after polymerization of GPTMS Figure 9 (e), and the intensity of C-O-C bond becomes more intense by the increasing of the catalyst amount due to the formation of polymer chain. Also it is observed the widening  Figure 9 (e) and (f) which is more formed by the increasing of the ring opening . As is shown in the Figure 10, the yield of polymer increased with the amount of Mag-H + for both polymerization, in solution and in bulk. It reaches its maximum at 20% by weight of Mag-H + for polymers prepared in bulk, beyond this value the yield decreases. The obtained results show the effect of Mag-H + as a cationic catalyst, the increase of amount of catalyst increase the active sites responsible for the initiation and acceleration of the polymerization reaction. This is indicated by the higher viscosity of the environment in bulk polymerization of GPTMS giving best yield compared to those obtained for GPTMS polymerized in solu-tion. Similar results are obtained by other authors [32][33][34]37]. As is shown in the Figure 10, the yield of polymer increased with the amount of Mag-H + for both polymerization, in solution and in bulk. It reaches its maximum at 20% by weight of Mag-H + for polymers prepared in bulk, beyond this value the yield decreases. The obtained results show the effect of Mag-H + as a cationic catalyst, the increase of active sites available in catalyst responsible for the initiation and acceleration of the polymerization reaction until the saturation of these sites. This is indicated by the higher viscosity of the environment in bulk polymerization of GPTMS giving best yield compared to those of GPTMS polymerized in solution. Similar re-    sults are obtained by other authors [32][33][34]37]. Figure 11 shows the effect of temperature on the yield of the ring-opening polymerization of GPTMS. This study is carried out in bulk at different temperatures: 0; 20; 40; 60; 80; 100 and 120 °C for 8 h with 20% by wt of Mag-H + . The yield of the obtained product evolves relatively with the increase of the temperature and reaches maximum 40% at 60 °C; at this temperature (optimal point) a transparent polymer with better performance was formed. The yield decreases at 80 °C and It is also shown that at high temperature, the appearance of the polymer is changed and the color becomes light brown. For both temperatures 100 and 120 °C, the product obtained is an insoluble composite (clay / polymer). It is noted that the temperature has a great influence on the yield and the product appearance.

Effect of the time on the yield of PGPTMS
As could be seen in Figure 12, after three hours of polymerization in bulk, the conversion rises up dramatically and increases in presence of 20 wt% of Mag-H + at both temperatures 20 and 60 °C. Its be explained by The heterogeneous reaction which requires an adsorption process before monomer reaction with the acidic active site located in the Maghnite and being initiated, after a period of continuous stirring, the monomer started to contact with the acidic sites and polymerization took place. The Figure 12 shows that at the end of polymerization time 8 h, the best yields reaches (40% at 60 °C and 30% at 20 °C). It possible to conclude that the ring-opening of the epoxide is initiated from 8h of stirring, using 20 wt% of Mag-H + . After this time, the yield increases until reaching 68% after 7 days of stirring at room temperature (20 °C). In contrast to high temperature (60 °C), we recorded a remarkable decrease in performance beyond 24 hours (28%). Figure 13 shows that the polymerization is first order with respect to monomer. The polymerization of GPTMS initiated by Mag-H + can be described by the following formula: (1) where kp is the polymerization rate constant. At 20 °C, value of kp is 1.53×10 -6 s -1 .

Conclusion
PGPTMS were synthesized successfully under mild conditions, using Mag-H+ as an ecologic acidic catalyst for ring-opening polymerization of GPTMS, in bulk and in solution. The polymerization proceeds via a cationic mechanism due to the presence of intercalated protons in the lamellar structure of Maghnite. However, bulk polymerization remains a preferred technique for PGPTMS synthesis to be consistent with the principles of green chemistry. The optimum reaction conditions were defined by studying the effect of different parameters and their influence on the yield of the obtained polymers. The higher viscosity of the environment in bulk polymerization PGPTMS giving best yields compared to those of GPTMS polymerized in solution. The temperature and the polymerization time have a great influence on the yield and the product appearance. The structure of the obtained polymer has been perfectly confirmed by FTIR, 1 H NMR and 29 Si-NMR. The thermal properties have been given by TGA and DSC analysis. The good properties catalytic of the green catalyst (Mag-H+) and the simplicity of the method for the synthesis of PGPTMS without requiring very low or very high temperature nor high pressures, are less expensive and environmentally friendly. These make it attractive method for the green synthesis of PGPTMS and their derivates.