A new approach for the polymerization of tetraphenyltetramethylcyclotetrasiloxane by an environmentally friendly catalyst called Maghnite-H⁺

2.1 Materials

D₄^Ph,Me (99%) was used as purchased without further purification. Methanol was purified by vacuum distillation. All products were obtained from the aldrich chemical annex of Telemcen in Algeria. Maghnite was obtained from the Algerian company of bentonite (BENTAL), without any pretreatment.

2.2 Preparation of Maghnite-H⁺

A mass of 30 g of raw Maghnite was combined with 120 ml of distilled water at room temperature; the suspension was left under stirring. After 30 min, 100 ml of a solution of sulfuric acid (0.23 m) was added and the stirring was continued for 48 h. After filtration and subsequent washing, the activated Maghnite was dried in an oven for 24 h at a temperature of 105°C. Finally, Maghnite-H⁺ was crushed, sieved and stored away from air and moisture.

2.3 Polymerization procedure

Some 0.08 g of Maghnite-H⁺ was heated before use under vacuum with mechanical stirring for 30 min. The polymerization was carried in bulk. The dried amount of Maghnite-H⁺ was added to a flask containing 4 g (3.54 ml) of D₄^Ph,Me; the flask was immersed in an oil bath and brought to a temperature of 60°C under reflux while being stirred. After 150 min, the reaction was stopped by deactivating the Maghnite-H⁺ by adding cold water to the reaction mixture. The Maghnite-H⁺ was recovered by filtration, and the filtrate was precipitated in methanol (non-solvent). The insoluble product was dried at 80°C in vacuum for 3–4 h and weighed as polymer. Excess water was retrieved by evaporation at 105°C; the amount of water necessary to stop the reaction would then be the difference between the initial amount and the recovered amount. It was assumed that the residual material was the remaining monomers and the oligomers formed during the reaction. Regarding the kinetic study, the same procedure described above was repeated by changing the temperature, time and the percentage of the catalyst.

2.4 Characterization methods

2.4.3 Nuclear magnetic resonance:

¹H and ¹³C nuclear magnetic resonance (NMR) spectrums were recorded under ambient temperature on a Bruker Avance 300 NMR spectrometer, using tetramethylsilane as the internal standard and deuterated chloroform as the solvent.

2.4.4 Differential scanning calorimetry:

The different thermal characteristics such as glass transition temperature (Tg) of the synthesized polymer were measured by differential scanning calorimetry (DSC) from the corresponding thermal changes in the DSC thermogram using a Setaram 92 DSC apparatus.

2.4.5 Molecular weight measurements:

Gel permeation chromatography measurements of the samples were performed using a WISP Model 712, Waters Associates chromatograph. Tetrahydrofuran was used as a solvent and the apparatus was calibrated in an initial approximation with polymethyl methacrylate of known molecular weight.

3.1 XRD

Figure 1 shows the characterization by XRD analysis of the raw Maghnite and Maghnite treated with sulfuric acid. It is obvious that the treatment led to the removal of minerals such as calcite and mica. This is confirmed by the decline of the intensity of their peaks compared to the strong peak corresponding to montmorillonite (green area); this elimination is clearer for the quartz, as shown by the reduction of the two peaks at 2θ=21.93° and 26.71° (blue areas). Moreover, the acid treatment caused a shift of the peak of montmorillonite to small values of 2θ from 8.41° to 5.73°, corresponding to an increase of the interlayer distance of montmorillonite (d₀₀₁) from 10.50 Å to 15.41 Å; this can be explained by the substitution of interlamellar cations of Maghnite by the acid protons which have a larger atomic diameter.

Figure 1:

X-ray diffraction (XRD) patterns of the Maghnite before treatment (raw-Maghnite) and after treatment (Maghnite-H⁺).

3.2 IR spectroscopy

Figure 2 provides the IR spectra of the monomer and the obtained products for 90 min, 120 min, 150 min, 180 min and 210 min at 60°C. The broad peak seen at 3425 cm⁻¹ for the obtained products is attributed to the OH stretching of the Si-OH end groups in the polyphenylmethylsiloxane (PPMS) chains. The appearance of this peak is due to the linkage between the released proton of Maghnite-H⁺ and the oxygen atom at the end after the D₄^Ph,Me ring opening (Scheme 2A); the decrease in its intensity with time of the reaction is clearly noticeable, which may be explained by the increase of polymerization degree leading to a smaller number of OH chain ends. The small peak which appears at about 3072 cm⁻¹ is attributed to the C-H bond of the phenyl ring. The two bands at 2953 cm⁻¹ and 2928 cm⁻¹ are, respectively, due to C-H asymmetric/symmetric stretching of CH₃. The two bands seen, respectively, at 2888 cm⁻¹ and 2846 cm⁻¹ are assigned to the C-H asymmetric/symmetric stretching of CH₂. The small series of bumps at 1949 cm⁻¹, 1865 cm⁻¹, 1812 cm⁻¹ and 1748 cm⁻¹ are caused by overtones (harmonics) of the phenyl ring vibrational modes having stretching frequencies in the fingerprint region. The bonds which occur in pairs, one at 1610 cm⁻¹ and one at 1492 cm⁻¹, are due to the C=C ring stretching absorptions. The signal at 1281 cm⁻¹ is assigned to the CH₃ symmetric deformation of Si-CH₃. Peaks appearing at 1016 cm⁻¹, 1090 cm⁻¹ and 486 cm⁻¹ are, respectively, attributed to the stretching vibrations and deformation vibrations of the linear Si-O-Si structures. The signal at 825 cm⁻¹ is due to the Si-C stretching vibrations. The two bands at 743 cm⁻¹ and 690 cm⁻¹ are assigned to the stretching vibrations of C-H in the phenyl ring. The IR spectrum of the obtained PPMS using Maghnite-H⁺ as a catalyst revealed no differences from those obtained by other researchers [30], [31].

Figure 2:

Infrared (IR) spectra of polyphenylmethylsiloxane (PPMS) obtained by the polymerization of tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me) at a temperature of 60°C for different times.

Scheme 2:

Polymerisation of tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me) by Maghnite-H⁺; (A) linear polymer and (B) Crosslinked polymer.

3.3 Proton NMR

In order to identify more precisely the structure of the polymer obtained by the polymerization of D₄^Ph,Me using the Maghnite-H⁺ as catalyst, the product was analyzed before and after reaction by NMR analysis by comparing the two spectra; that of the monomer and that of polymer obtained at 60°C for 150 min. The results are shown in Figure 3A and B, which show the different chemical shifts. In both spectra, the dominant peak observed at about 1.20 ppm is attributed to the methyl groups. It is more intense in the spectrum of the polymer meaning the large number of methyl groups in the polymeric chain. The neighboring peaks at 6.83 ppm, 6.97 ppm and 7.13 ppm are assigned to the protons of the phenyl ring. The small peak appearing at 4.80 ppm is assigned to the OH groups at the ends of polymer chains during the reaction. Similar results were obtained by Fei et al. [32].

Figure 3:

¹H nuclear magnetic resonance (NMR) spectra of (A) tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me) and (B) polymer obtained at a temperature of 60°C for 150 min.

3.4 Carbon NMR (¹³C NMR)

It was therefore necessary to analyze the products obtained by ¹³C NMR to provide a complement to the previous study. The results are shown in Figure 4A–C showing the ¹³C NMR spectra of the monomer, and the polymer obtained after 150 min and 180 min, respectively. The peak located at approximately 0.27 ppm for the monomer and 0.78 ppm for the polymers corresponds to the carbon of CH₃. The peaks at 137.78 ppm, 130.44 ppm, 128.85 ppm and 125.20 ppm for the monomer, and at 137.81 ppm, 129.92 ppm, 127.30 ppm and 124.13 ppm for the two polymers are assigned to the phenyl ring carbons, as shown in Figure 4. Moreover, there is a creation of a down peak at 33.85 ppm on the distortionless enhancement of polarization transfer using a 45° decoupler pulse spectrum of the polymer obtained after 180 min, that is attributed to the carbon of CH₂, indicating the formation of ethylene bridges between linear polymer chains (Scheme 2B). These results show that beyond 180 min of reaction time, the polymer chains can be crosslinked to form organopolysiloxane elastomers.

Figure 4:

¹³C nuclear magnetic resonance (NMR) spectra of (A) tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me), (B) polymer obtained after 150 min and (C) polymer obtained after 180 min at a temperature of 60°C.

3.5 DSC

DSC was used as a thermal analysis, to identify and confirm at the same time the structure and the purity of the obtained product. Figure 5 shows the DSC thermogram of the polymer obtained after 150 min of reaction time. The thermogram shows a small thermal deformation at about −41.35°C, which corresponds to the glass temperature of the polymer (Tg=−41.35°C); this Tg value is comparable to that of linear PPMS. The results obtained by DSC largely support the results obtained by IR and NMR for the linear structure of the polymer obtained after 150 min; they also simulate, to a large extent, the results obtained in previous research [33], [34].

Figure 5:

Differential scanning calorimetry (DSC) thermogram of polyphenylmethylsiloxane (PPMS) obtained after 150 min.

3.6 Effect of temperature

In an effort to understand and control more the polymerization reaction of D₄^Ph,Me catalyzed by Maghnite-H⁺, we examined the effect of the temperature of the medium on the reaction that takes place there. Table 1 gives measured values of the monomer conversion and number of average molecular mass of the polymers obtained in a temperature range of 30–90°C. The increase in temperature leads to a significant increase in conversion reaching 85% at 60°C; beyond this temperature, this increase becomes negligible until the conversion stabilizes at its maximum at about 89%. By contrast, the variation in average molecular mass shows two different behaviors; a gradual increase from 30°C to 60°C, followed by a reduction after just exceeding its highest value at about 60°C. We assume that this is the ceiling temperature; the decrease in average molecular mass can be explained by the fragmentation of the chains and suggests the thermal decomposition of PPMS after the breaking of Si-O bonds when approaching the boiling point. The thermal degradation phenomenon reflects a wide divergence between the molecular mass values, resulting in the increase of the polydispersity index, which is found in Table 1.

3.7 Effect of Maghnite-H⁺/monomer weight ratio

In order to study the catalytic action of Maghnite-H⁺ as a heterogeneous catalyst in the polymerization reaction of D₄^Ph,Me, we performed the reaction with a catalyst content ranging from 0.5% to 4% by weight, so that for each catalyst content, the reaction time varied from 30 min to 210 min. The results of the influence of the Maghnite-H⁺ content on the monomer conversion and on the number average molecular mass are shown in Figures 6 and 7, respectively. In all tests, the reaction was carried out in bulk and at a temperature of 60°C. It is clearly noticeable that for all the different Maghnite-H⁺ contents, the reaction time had an effect proportionally positive on the monomer conversion before 180 min. After this duration, the effect of reaction time became negative (Figure 6). The reduction in the monomer conversion for large periods may be explained by the occurrence of depolymerization phenomenon of polymer chains caused by the active sites of the Maghnite-H⁺ still remaining in the reaction medium. This result indicates that Maghnite-H⁺ can play the opposite role after sufficiently large periods of time. Similar results were previously obtained by several authors [7], [8]. By contrast, the average molecular mass increased with increasing reaction time; the maximum value for the different Maghnite-H⁺ contents was about 150 min, then it stabilized almost up to 180 min, so that it began to decrease. The reduction over time of the peak corresponding to OH groups, which exist only at the end of the polymer chains of PPMS synthesized by Maghnite-H⁺, shown by the IR analysis (Figure 2), indicates that the chains became longer, resulting in large molecular mass. The temporary stabilization between 150 min and 180 min of the average molecular mass, and at the same time the increase in the monomer conversion, were due to the crosslinking phenomenon, leading to branched structures, because of the formation of ethylene bridges between the linear chains; this explanation is clearly supported by what was obtained by ¹³C NMR analysis (Figure 4C). The decrease in the average molecular mass after 180 min can be explained by backbiting degradation in the growing polymer chains, which generates oligomers and cyclic polysiloxanes of varying sizes, thereby increasing the polydispersity index (Table 2).

Figure 6:

Effect of Maghnite-H⁺/tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me) weight ratio on the conversion of monomer.

Figure 7:

Effect of Maghnite-H⁺/tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me) weight ratio on the average molecular mass.

3.8 Kinetics and mechanism of the polymerization

In order to study the chemical kinetics of the polymerization reaction of D₄^Ph,Me catalyzed by Maghnite-H⁺, we followed the evolution of the concentration of monomer over time; we were interested just for t≤180 min, where there was not the depolymerization phenomenon. The results clearly indicate that the reaction is first-order with respect to the monomer (Figure 8). Scheme 3 shows the probable reaction mechanism.

Figure 8:

Representation of Ln ([D₄^Ph,Me]₀/[D₄^Ph,Me]_t), where D₄^Ph,Me is tetraphenyltetramethylcyclotetrasiloxane, as a function of time (Maghnite-H⁺/D₄^Ph,Me weight ratio=2%, T=60°C).

Scheme 3:

Mechanism of tetraphenyltetramethylcyclotetrasiloxane (D₄^Ph,Me) polymerization catalyzed by Maghnite-H⁺.