Synthesis and characterization of polyvinylmethylsiloxanes by cationic polymerization using a solid green catalyst

2.1 Materials

Pentavinylpentamethylcyclopentasiloxane (V5D5, 99%) was used as purchased from Aldrich chemical (Algeria), without further purification. Methanol was purified by vacuum distillation. All other products have been used as received. Maghnite was obtained from Algerian company of bentonite (BENTAL), without any pretreatment. All products were obtained from the Aldrich chemical annex in Algeria (Sigma-Aldrich is a known chemical supplier on a global scale).

2.2 Preparation of Maghnite-H⁺

A mass of 30 g of raw maghnite is combined with 120 ml of distilled water at room temperature, the suspension is left under stirring. After 30 min, 100 ml of a solution of sulfuric acid (0.23 m) is added, the stirring is continued for 48 h. After filtration and subsequent washing, the activated maghnite is 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

A total of 0.1 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 5 g of V5D5, the flask was immersed in an oil bath and brought to a temperature of 60°C under reflux at while being stirred. After 6 h, 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 6–8 h and weighed as polymer. Excess water was retrieved by evaporation at 105°C, the amount of water necessary to stop the reaction would be then 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

3.1 X-ray diffraction

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:

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

3.2 Infrared spectroscopy (IR)

Figure 2 provides the IR spectra of the monomer and the obtained products for 2, 4, 6, 8 and 10 h at 60°C. The broad peak seen at 3510 cm⁻¹ for the obtained products is attributed to the OH stretching of the Si-OH end groups in the PVMS 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 V5D5 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 smaller number of OH chain ends. The small peak appears at about 3062 cm⁻¹ is attributed to the C-H bond of the vinyl group (CH=C). The band corresponds to the stretch C=C is seen at 1639 cm⁻¹. The decrease in the intensity of this two peaks corresponds to C-H and C=C of the vinyl group for PDVS obtained after long periods of time is perhaps explained by the rupture of the double bond between carbon atoms to establish interchain linkages with CH₃ leading to the formation of crosslinked polymers (Scheme 2B). The two bands at 2907 and 2861 cm⁻¹ are, respectively, due to C-H asymmetric/symmetric stretching of CH₃. The two peaks seen, respectively, at 2788 and 2734 cm⁻¹ are assigned to the C-H asymmetric/symmetric stretching of CH₂. The signal at 1272 cm⁻¹ is assigned to the CH₃ symmetric deformation of Si-CH₃. Peaks appearing at 1029, 1099 and 469 cm⁻¹ are, respectively, attributed to the stretching vibrations and deformation vibrations of the linear Si-O-Si structures. The signal at 816 cm⁻¹ is due to the Si-C stretching vibrations. The IR spectrum of obtained PVMS using Maghnite-H⁺ as the catalyst revealed no differences from those obtained by other researchers (28), (29).

Figure 2:

IR spectra of PVMS obtained by the polymerization of V5D5 at a temperature of 60°C for different times.

Scheme 2:

Polymerization of V5D5 by Maghnite-H⁺.

3.3 Proton nuclear magnetic resonance (¹H NMR)

In order to identify more precisely the structure of the polymer obtained by the polymerization of V5D5 using the Maghnite-H⁺ as catalyst, the product was analyzed before and after reaction by NMR analysis by comparing the obtained spectra: that of the monomer and those of the polymers obtained after 6 h and 8 h at 60°C, respectively. The results are shown in Figure 3A–C, which shows the different chemical shifts. In the three spectra, the dominant peak observed at about 0.12 ppm, is attributed to the methyl groups, it is more intense in the spectra of polymers meaning the large number of methyl groups in the polymeric chain. The two peaks at 5.90 and 6.12 ppm are, respectively, assigned to CH₂ and CH of the vinyl group. The small peak appearing at 4.86 ppm in the polymer spectra is assigned to the OH groups at the ends of polymer chains. Similar results were obtained by Yang et al. (30). The peak appeared at 1.30 ppm, exclusively for the polymer obtained after 8 h is attributed to the CH₂ of the alkane group. These groups can be derived from the breaking of the vinylic double bond by establishing inter-chain propylene bridges.

Figure 3:

¹H NMR spectra of (A) V5D5, (B) polymer obtained after 6 h and (C) polymer obtained after 8 h at a temperature of 60°C.

3.4 Carbon nuclear magnetic resonance (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, the polymer obtained after 6 h and after 8 h, respectively. The peak located at approximately −0.35 ppm for the monomer and −0.65 ppm for the polymers corresponds to the carbon of CH₃. The other two neighboring peaks that appear at 130.02 and 140.95 ppm for the monomer and at 130.39 and 141.01 ppm for the polymers are assigned to CH₂ and CH of the vinyl group, respectively. Moreover, there is a creation of two down peaks at 20.10 and 32.87 ppm on the DEPT-135 spectrum of the polymer obtained after 8 h (Figure 4C), which are attributed to the carbon of CH₂ of the alkane group, indicating the formation of propylene bridges between linear polymer chains. These results confirm that beyond 6 h of reaction time, the polymer chains can be crosslinked to form organopolysiloxane elastomers.

Figure 4:

¹³C NMR spectra of (A) V5D5, (B) polymer obtained after 6 h and (C) polymer obtained after 8 h at a temperature of 60°C.

3.5 Differential scanning calorimetry (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 6 h of reaction time. The thermogram shows a small thermal deformation at about −136.43°C, which corresponds to the glass temperature of the polymer (Tg=−136.43°C), this Tg value is comparable to that of linear PVMS. The result obtained by DSC largely supports the results obtained by IR and NMR for the structure of the polymer obtained after 6 h, it also simulates to a large extent the results obtained in previous research (31), (32).

Figure 5:

DSC thermogram of obtained PVMS obtained after 6 h.

3.6 Effect of temperature

In an effort to understand and control more the polymerization reaction of V5D5 catalyzed by Maghnite-H⁺, we have 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 average molecular mass of the polymers obtained in a temperature range of 10–110°C. The increase in temperature leads to a significant increase in conversion reaching 89% at 60°C, beyond this temperature, this increase becomes negligible until the conversion stabilizes at its maximum at about 93%. On the other hand, the variation in average molecular mass shows two different behaviors, a gradual increase from 10 to 60°C, followed by a reduction after just exceed its highest value at about 60°C, we assume that it is the ceiling temperature, this decrease in average molecular mass can be explained by the fragmentation of the chains suggests the thermal decomposition of PVMS 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 V5D5, 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 0.5 to 10 h. 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 has an effect proportionally positive on the monomer conversion before 8 h. After this duration, the effect of reaction time has become negative (Figure 6). The reduction in the monomer conversion for large periods may be explained by the occurence of depolymerization phenomenon of polymer chains caused by the active cites of the Maghnite-H⁺ still remaining in the reaction medium. This result indicates that the Maghnite-H⁺ can play the opposite role after periods of time sufficiently large. Similar results were previously obtained by several authors (33), (34). On the other hand, the average molecular mass increases with increasing reaction time, the maximum value for the different Maghnite-H⁺ contents is about 6 h, then it is almost stabilizes for 2 h, so that it begins to decrease. The reduction over time, of the peak corresponding to OH groups exist only at the end of the polymer chains of PVMS synthesized by Maghnite-H⁺ showing by the IRI analysis (Figure 2), indicates that the chains became longer, resulting in large molecular mass. The temporary stabilization between 6 and 8 h of the average molecular mass and at the same time the increase in the monomer conversion, due to the crosslinking phenomenon leading to branched structures because of the formation of propylene bridges between the linear chains, this explanation is clearly supported by what has been obtained by ¹³C NMR analysis (Figure 4C). The decrease in the average molecular mass after 8 h 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⁺/V5D5 weight ratio on the conversion of monomer.

Figure 7:

Effect of Maghnite-H⁺/V5D5 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 V5D5 catalyzed by Maghnite-H⁺, we followed the evolution of the concentration of monomer over time, we were interested just for t≤8 h, where there was not the depolymerization phenomenon. The results clearly indicate that the reaction is first-order with respect to monomer (Figure 8). Scheme 3 shows the probable reaction mechanism.

Figure 8:

Representation of Ln ([V5D5]₀/[V5D5]_t) as a function of time (Maghnite-H⁺/V5D5 weight ratio=2%, T=60°C).

Scheme 3:

Mechanism of D5V5 polymerization catalyzed by Maghnite-H⁺.