Fluoro-containing Polysiloxane Thermoset with Good Thermostability

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Fluoro-Containing Polysiloxane Thermoset with Good Thermostability and Acid-Resistance Based on the Renewable Multi-functional Vanillin Yangqing Tao, Junfeng Zhou, Linxuan Fang, Yuanqiang Wang, Xiaoyao Chen, XingRong Chen, Jiaren Hou, Jing Sun, and Qiang Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00370 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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ACS Sustainable Chemistry & Engineering

Fluoro-Containing

Polysiloxane

Thermoset

with

Good

Thermostability and Acid-Resistance Based on the Renewable Multifunctional Vanillin AUTHOR NAMES Yangqing Tao, Junfeng Zhou, Linxuan Fang, Yuanqiang Wang, Xiaoyao Chen, Xingrong Chen, Jiaren Hou, Jing Sun* and Qiang Fang* AUTHOR ADRESS Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China. Corresponding Author: *To whom correspondence should be addressed. Tel & Fax: +86 21 5492 5337. E-mail: [email protected]. KEY WORDS Vanillin, polysiloxane, trifluorovinyl ether (TFVE), Thermostability, Acid-resistance. ABSTRACT Vanillin, an only available aromatic biomass in industrial scale derived from lignin, has been transformed to a fluoro-containing polysiloxane thermoset for the first time based on its multi-functional nature. Conventionally, the methoxy groups in vanillin are always ignored in the preparation of polymers, whereas in this work the groups are efficiently utilized to polymerize with a commercially available disiloxane through the

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Piers-Rubinsztajn reaction in the presence of B(C6F5)3 (0.1%) at room temperature. Furthermore, the incorporation of thermally polymerizable trifluorovinyl ethers (TFVE)s into the polysiloxane main chain imparts good thermostability and acidresistance to it. In addition, the cross-linked fluoro-containing polysiloxane also exhibits low dielectric constant and good transmittance. Considering the widespread existence of methoxy groups in natural feedstocks and the good performance of the new fluoro-polysiloxane thermoset, the strategy we developed may provide a new method for the application of biomass in the near future. INTRODUCTION Sustainable development is undoubtedly one of the most significant issues that scientists are facing today, especially considering the skyrocketing global demand of energies and raw materials, as well as the adverse consequences of environmental pollution.1-3 In the past decades, quantities of studies have proved that utilizing renewable biomass as chemical feedstocks, rather than fossil sources, might be the most effective solution to achieve sustainability. In recent decades, many achievements have been made in constructing materials based on renewable biomass,4-7 such as plant oil79

and carbonhydrates.10-12 However, the use of food crops for the production of

materials sometimes is questioned especially considering the existing famine in someplace of the world, so the critical point is developing finely value-added raw materials based on marginal or abandoned waste. Notably, lignin is the second most abundant natural polymer in plants and accounting for approximately 30 % of organic carbon in the biosphere. It has been estimated that


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the current annual production of lignin isolated from pulping is about 60 million tons, whereas only 2% of which is used commercially while the rest was burned as a lowvalue fuel.13 The low utilization of lignin may be caused by its poor solubility in common solvents and complex structure, which make it fairly difficult for transformation to commercial materials through chemical methods.14 Thanks to the development of bio-refining techniques, many fine smaller compounds with fascinating structures such as vanillin, cresol, guaiacol, ferulic and coumaric acids can be obtained by depolymerization of lignin.15,16 Among all these phenol derivatives, vanillin is the only biomass-derived phenol that can be manufactured in an industrial scale, which make it an attractive and ideal sustainable feedstock for polymer materials.17 With regard to the chemical structure of vanillin, there are three functional groups connected to the benzene skeleton, which are hydroxyl, aldehyde group and methoxy group, respectively. It is noteworthy that these functional groups presenting in vanillin can be a great advantage over traditional phenols to allow simplified, atom-economic synthetic procedures and impart unique properties to materials. In most cases, vanillin is usually transformed to di-hydroxyl or multi-hydroxyl compounds through either redox reaction or friedel-crafts reaction of the aldehyde group.18 Upon getting these polyphenols, various vanillin-based polymers have been fabricated including epoxy resins,19-23 polyesters24,25 and polyimines.26-29 However, to the best of our knowledge, as for the functionalization or polymerization of methoxy group in vanillin, rare research has been reported. It is well-documented that the introduction of the post-polymerizable trifluorovinyl



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ethers (TFVEs) could apparently improve the thermal stability and hydrophobicity of materials after a post-curing heat-treatment without any additives.30,31 Consequently, a vanillin–based di-TFVE functionalized monomer was facilely synthesized by using the phenolic hydroxyl groups. Usually, the di-TFVE functionalized monomers are able to polymerize but only to obtain linear oligomers, which is far from desirable high performance materials due to its poor thermal stability and chemical resistance.32 With the motivation to prepare materials with satisfying properties, fortunately, we found that the remaining methoxys of the di-TFVE functionalized monomer derived from vanillin are able to undergo the mild Piers-Rubinsztajn reaction with disiloxane catalyzed by tiny amounts of B(C6F5)3 (0.1%) without destroying the TFVEs to get polysiloxane. In this case, the TFVEs are act as hooks to the main polysiloxane chains, which can be cross-linked through a thermal [2+2] cyclization to form perfluorocyclobutene (PFCB) between the polysiloxane chains. On the one hand, we envisioned that the presence of crosslinking in polysiloxane may improve the thermal stability, on the other hand, the introduction of hydrophobic PFCB units could increase the acid resistance of polysiloxane, which is one of the critical limitations for their broad application. The synthetic details and the properties of the fluoro-polysiloxane based on vanillin are introduced here. EXPERIMENTAL SECTION Materials. Vanillin was purchased from Bide Pharmatech. Ltd., Titanium tetrachloride (TiCl4), tetrafluorodibromoethane (BrCF2CF2Br), magnesium and zinc granules were provided

by

Alfa

chemistry.

1,1,3,3-Tetramethyl-


disiloxane

and

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Tris(pentafluorophenyl)borane (B(C6F5)3) were purchased from Sigma-Aldrich. Palladium-carbon (Pd/C), glacial acetic acid (HOAc) and cesium carbonate (Cs2CO3) were bought from Sinopharm Chemical Reagent Co., Ltd. Solvents such as tetrahydrofuran (THF), acetonitrile (CH3CN) and petroleum ether were bought from Adamas Reagent, Ltd. All above mentioned chemicals were used without further purification. The solvents CH2Cl2 and toluene used for polymerization were redistilled before being used. Characterization. 1H NMR,

13C

NMR,

19F

NMR and

29Si

NMR spectra were

characterized by a JEOL ECZ400 NMR spectrometer at room temperature using TMS as the internal standard. Fourier transform infrared spectra (FT-IR) were recorded on a Nicolet spectrometer from 400 to 4000 cm-1 with KBr pellets. Elemental analysis was performed on an Elementar vario MicroCube machine. High resolution mass spectra (HRMS) were recorded on an Agilent Technologies 5973N instrument. Differential scanning calorimetry (DSC) curves were collected with a Q200DSC (TA, US) at a heating rate of 10 or 20 oC/min over a temperature range of 40 to 350 oC under N2 flow. Thermogravimetric analysis (TGA) was carried out on a NETZSCH TG 209 apparatus at a heating rate of 10 °C/min from room temperature to 1000°C under a N2 atmosphere. Dynamic mechanical analysis (DMA) was performed on the Mettler Toledo DMA/SDTA861e instruments with a heating rate of 3 °C min−1 in air. Gel permeation chromatography (GPC) curves were collected from a Waters Breeze2a 200 GPC instrument with polystyrene as standards and tetrahydrofuran (THF) as the eluent. The tensile tests were conducted on a universal testing machine (UTM, INSTRON 5583) at



room temperature. The ultraviolet-visible near-infrared (UV−vis−NIR) spectra was used to characterize the transparency of cured films on Varian CARY 5000 at room temperature with a scan rate of 600 nm min−1. Synthesis of V-BP. A mixture of magnesium granules (7.99 g, 329 mmol) and 300 mL THF were chilled to -78 oC, then TiCl4 (49.87 g, 263 mmol, 29 mL) was added dropwise, the obtained bright yellow mixtures were stirred vigorously at room temperature for 1 hour to form a black solution. To this solution, a THF (100 mL) solution of vanillin (20.00 g, 131 mmol) was added dropwise through a constant pressure drop funnel. The mixture was then heated to reflux at 80 oC for 3h. The solvent THF was removed by reduced pressure distillation and recycled. The residual solid was treated with HCl (240 mL, 2M) solution to produce the aimed V-BP as the precipitate in a dark solution. Through filtration and washing with water and cold ethanol, the vanillin derived bisphenol V-BP was obtained as a pale brown solid in a yield of 57%. 1H NMR (400 MHz, DMSO-d6): δ 9.06 (s, 2H), 7.14 (s, 2H), 6.95 (s, 2H), 6.92 (s, 2H), 6.75 (d, J = 8.1 Hz, 2H), 3.82 (s, 6H). 13C NMR (101 MHz, DMSO-d6): δ 148.3 (2C), 146.6 (2C), 129.7 (2C), 126.3 (2C), 120.0 (2C), 116.1 (2C), 109.9 (2C), 56.0 (2C). Synthesis of RV-BP. To a solution of V-BP (10 g, 37 mmol) in THF (100 ml), palladium-carbon catalyst (10%, 1.17 g, 1.1 mmol) and glacial acetic acid (2 ml) were added. The mixture was stirred at room temperature for 24 h under hydrogen atmosphere (1 atm), and then filtered through diatomaceous earth to remove catalyst. The resulting solution was concentrated and precipitated in hexane (300 mL), after dried in vacuum, the reduced product RV-BP was obtained as a white solid in a


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quantitative yield. 1H NMR (400 MHz, DMSO-d6): δ 8.61 (s, 2H), 6.72 (s, 2H), 6.64 (d, J = 7.9 Hz, 2H), 6.59 – 6.51 (m, 2H), 3.70 (s, 6H), 2.69 (s, 4H).

13C

NMR (101

MHz, DMSO-d6): δ 152.5 (2C), 149.6 (2C), 137.8 (2C), 125.6 (2C), 120.4 (2C), 117.8 (2C), 60.7 (2C), 42.3 (2C). Synthesis of M-RV-TFVE. The TFVE-functionalized monomer was synthesized according to a previously reported method.33 M-RV-TFVE was obtained as white crystals with an overall yield of 60%. 1H NMR (400 MHz, CDCl3): δ 6.97 (t, J = 16.0 Hz, 2H), 6.76 – 6.69 (m, 2H), 6.67 (s, 2H), 3.82 (s, 6H), 2.87 (s, 4H). 13C NMR (101 MHz, CDCl3): δ 149.2 (2C), 146.6 (2C), 142.1 (2C), 139.13 (2C), 134.2 (2C), 120.5 (2C), 116.4 (2C), 113.4 (2C), 55.9 (2C), 37.5 (2C). 19F NMR (376 MHz, CDCl3): δ 120.75 (dd, J = 99.0, 57.3 Hz), -126.98 (dd, J = 110.0, 98.6 Hz), -134.04 (dd, J = 109.2, 58.4 Hz). HRMS-ESI (m/z): Calcd. for C20H17F6O4 [M+H]+ 435.1026. Found 435.1028. Anal. Calcd. for C20H16F6O4: C, 55.31; H, 3.71; F,26.25; Found: C, 55.33; H, 3.84; F, 25.80. Synthesis of PSi-RV-TFVE. To a 25 mL Schlenk flask with a balloon, the M-RVTFVE monomer (1.0 g, 2.3 mmol) and B(C6F5)3 (1.18 mg, 2.3×10-3 mmol) were added. Then the flask was degassed and filled with argon for three times. 2 mL CH2Cl2 was added to the flask and heated by a hair dryer to form a homogeneous solution, to the flask 1,1,3,3-Tetramethyl-disiloxane (0.46 g, 3.4 mmol) was carefully added through an injection syringe. The mixture was stirred at room temperature for 1 h, and then filtered through silica gel to remove catalyst. The resulting solution was concentrated in vacuum to remove the solvent and the excess disiloxane, subsequently the liner



polysiloxane PSi-RV-TFVE was obtained as a clear and colorless viscous liquid in a quantitative yield. 1H NMR (400 MHz, CDCl3): δ 6.92 (d, J = 8.3 Hz, 2H), 6.74 (d, J = 1.8 Hz, 2H), 6.64 (dd, J = 8.3, 1.5 Hz, 2H), 2.71 (s, 4H), 0.20 (s, 12H). 19F NMR (376 MHz, CDCl3): δ -120.95 (dd, J = 100.2, 58.5 Hz), -126.58 ~ -127.81 (m), -133.11 ~ 133.92 (m). 29Si NMR (79 MHz, CDCl3): δ -10.25. Molecular weight (GPC in THF) Mn = 1.20 ×104 g mol-1; PDI = 1.43. Preparation of PSi-RV-TFCB sheets and free-standing films. In order to prepare cured PSi-RV-TFVE sheets, the viscous liquid of PSi-RV-TFVE was heated at 60 oC to make it convenient to be placed into a flat-bottom glass tube. To prepare the freestanding films, a CH2Cl2 solution of PSi-RV-TFVE (0.5 g/mL) was dropped onto the surfaces of flat glass sheets, after which they were kept at room temperature for 3 h to remove the solvent. Then the tube and glass sheets were transferred to a quartz tube furnace and treated with a heating process consisting of 200 oC for 2 h and 230 oC for 6 h under a N2 atmosphere. After being cooled to room temperature, cross-linked transparent sheets of PSi-RV-TFCB and free-standing films were obtained. Stability in acid solution. The cured bulk samples (cylinder with a diameter of 10 mm and a height of 10 mm) of PSi-RV- TFCB were pre-dried in vacuum to constant weights at 80 oC. After that, the sheets were immersed in HCl solution (1 mol/L) at room temperature for several days. The weight change can be calculated via the formula WC (%) = (M – M0)/M0, where M and M0 are the weights after soaking in acid solution and initial weights respectively. RESULTS AND DISCUSSION


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Preparation of polysiloxane based on vanillin. At the very beginning, vanillin was designed to transform to its TFVE-functionalized monomer to polymerize with hydrosilanes, considering that both the aldehyde and methoxy groups are able to undergo the Piers-Rubinsztajn reaction. However, there were two serious challenges that standing in our way, for one thing, the preparation of TFVE-functionalized vanillin was fairly difficult because of the existence of strong electron withdrawing aldehyde group,34 for another, the polymerization process was too hard to control since that aldehyde groups can be further reduced by hydrosilanes to methyl groups,35 which not only significantly influenced the preparation of high molecular weight polysiloxane but also made the polymer much more complex. As a result, the aldehyde groups in vanillin were evaded through McMurry coupling reaction to obtain its bisphenol V-BP, and the double bond was further reduced with Pd/C under H2 to gain its reduced bisphenol RVBP to get rid of the unfavourable influence of unsaturated vinyl on the polymer performance (Scheme 1).36 By using a method we previously reported, the bisphenol was transformed to the TFVE-functionalized monomers successfully, whose structure had been confirmed by NMR and HRMS analysis. Upon getting the TFVE-functionalized monomer M-RV-TFVE, we first tried to get thermoset material from the monomer by direct thermal polymerization (Scheme 1, Control group 1), however, the cured samples suffered from some problems. First, there were many undesired bubbles in the cured sample pieces, in order to figure out where these bubbles come from, we first excluded the possibility of solvent residue by drying the monomer under vacuum for 24 hours, and direct TGA analysis of the monomer



showed that about 25 % of the weight loss appeared in the early curing process (210 oC,

Figure S1), we speculated that these bubbles may be due to the sublimation of the

monomer M-RV-TFVE, and the sublimation experiment (where white crystal can be obtained when heating at 150 oC) of the monomer in vacuum did confirmed this. Second, the cured samples were soluble in THF, GPC indicated that only macromolecules with a Mn of 1.60 ×104 were obtained (Figure S2). And what’s more, the material exhibited low glass transition temperature (Tg =68 oC, Figure S3), which was also unsatisfied for its application.


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O H

OH

O TiCl4, Mg THF

O

HO

OH

Pd/C, H2

O

V-BP

O

THF

HO

RV-BP

O

O

Vanillin O 1. BrCF2CF2Br, Cs2CO3 2. Zn, CH3CN F2C

F C

F2C

F C

C F

H O H Si Si

CF2

O O O

O

C F

B(C6F5)3, CH2Cl2

M-RV-TFVE

CF2

F F

O o

> 150 C

O O

Si

O

Si n

F F

O F O F

m

O O

PSi-RV-TFVE

Si

O

Si n

PSi-RV-PFCB Control group 1: O

F2C

F C

C F

CF2

O

O F

> 150 oC

O O

O

M-RV-TFVE

Fn

O

P-RV-PFCB

Mn = 18,000, PDI = 1.8 O

M-RA

F

O

Control group 2:

O

F F

F

O

H O H Si Si B(C6F5)3 CH2Cl2

O

Si

O

Si n

PSi-RA Mn = 54,000, PDI = 1.9

Scheme 1. Synthetic processes of bio-based fluoro-polysiloxane PSi-RV-PFCB and the control polymers. To overcome these drawbacks, we thus reflected on solutions to deal with the methoxy groups originated from vanillin. And fortunately, we found that the two methoxy groups are able to react with disiloxanes in the presence of tiny amounts of B(C6F5)3 to produce polysiloxanes.37-39 It was worthy to mention that there are various catalysts can be used to catalyse the reaction between hydrosilanes and bifunctional methoxy monomers,40



and the reasons why we chose the Piers-Rubinsztajn reaction as the polymerization reaction were as follows: the mild reaction condition protects the TFVEs from being destroyed,32 in addition, the catalyst B(C6F5)3 is easily available and removed compared with other metal catalysts such as manganese, palladium and rhodium.41 In order to find the suitable conditions to acquire polysiloxane with high molecular weight, at first, the commonly used solvent toluene was applied, nevertheless only oligomers with a Mn of 1800 were prepared. Later we tried another solvent, CH2Cl2 to conduct the polymerization, and the reaction was found to become effective with a Mn of 4.68 × 104 (see Figure S4). Moreover, the influence of catalyst loading and the polymerization time on reaction were also studied. The results showed that neither decreasing the catalyst loading nor prolonging the reaction time can improve the Mn of PSi-RV-TFVE. As a result, the optimal polymerization condition was determined as the reagents reacted in CH2Cl2 for an hour with 0.1% loading of catalyst. In addition, to get better insight into the influence of cross-linkable TFVEs to the polysiloxane, a similar liner polysiloxane PSi-RA based on anisaldehyde was prepared and compared (Scheme 1, Control group 2). The details of the synthesis of PSi-RA were described in supporting information (Scheme S1).


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Figure 1. NMR Spectra of M-RV-TFVE and PSi-RV-TFVE. The chemical structures of the as-prepared polysiloxane were characterized by 1H NMR, 19F

NMR and 29Si NMR. Typically, for PSi-RV-TFVE, as shown in Figure 1, the peak

at 3.81 ppm of methoxy groups disappeared when compared with the monomer M-RVTFVE, and new peak of methyls connected to silicon at 0.20 ppm of the polymer occurred, moreover, the doublet peak of aromatic hydrogen in M-RV-TFVE was increased from 6.67 ppm to 6.74 ppm due to the change of chemical environment around it. In addition, the appearance of silicon signals at -10.25 ppm also confirmed the structure of PSi-RV-TFVE. No extra peaks can be observed in 19F NMR spectra (Figure S6) demonstrated that the TFVEs remained unaffected in the polymerization process, which guaranteed the consequent cross-linking of liner PSi-RV-TFVE to form



the cured thermoset PSi-RV-PFCB. Curing behaviour of PSi-RV-TFVE. TFVE-based monomers or oligomers can be cured without any catalyst and any small volatile molecules by simply heating to form the PFCB-based thermosets.30-33 Differential scanning calorimetric (DSC) curves and Fourier transform infrared (FT-IR) spectra were used to evaluate the thermal crosslinking process of the liner PSi-RV-TFVE. As shown in Figure 2, the DSC curves were analogous to those of previously reported PFVE-based materials. At the first scan, the polysiloxane PSi-RV-TFVE showed an onset temperature of curing at 150 oC and a maximum exothermic peak at 240 oC, suggesting a wide window of processing temperature. When scanning for the second time, the cured PSi-RV-PFCB showed a glass transition temperature of 135 oC (the red line in Figure 2). Similarly, the dynamic mechanical analysis (DMA, see Figure S7) revealed that the thermoset has an identical Tg (133 oC), which was much higher than those of common liner polysiloxanes.40 The better thermal stability was resulted from the forming of PFCB cross-links between the polymer chains.


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Figure 2. DSC curves of PSi-RV-TFVE. As a result, the thermal curing of PSi-RV-TFVE was conducted by simply heating the polysiloxane at temperature above 150 oC for several hours, after which transparent films and sheets can be obtained. FT-IR spectra (Figure 3) were used to further analyse the thermal cross-linking behaviour of PSi-RV-TFVE. The comparison between the monomer M-RV-TFVE and polysiloxane PSi-RV-TFVE showed that the stretching vibrations of methyl at 2950 cm-1 disappeared after polymerization, as well as the peaks of C-O-C stretching vibrations of ether at 1250 cm-1 and 1030 cm-1, the presence of new peaks attributed to Si-O-Si at 980 cm-1 and 1080 cm-1 also confirmed the structure of PSi-RV-TFVE. Notably, the characteristic peak of TFVE groups at 1830 cm-1 maintained unaffected in the polysiloxane,30-33 suggesting the mildness of the polymerize reaction. After curing, the occurrence of characteristic peak at 960 cm-1



assigned to the PFCB groups, along with the vanishing of signals of TFVE at 1830 cm-1, indicating that the TFVEs have been fully transformed to TFCBs.

Figure 3. FT-IR spectra of M-RV-TFVE, PSi-RV-TFVE and PSi-RV-PFCB. The curing process also changed the solubility and stability of polysiloxane in various solvent. The linear PSi-RV-TFVE, a viscous liquid with good solubility in common organic solvents such as CH2Cl2, THF, and ethyl acetate, was transformed to an insoluble and infusible solid after curing at high temperature. While the control group PSi-RA was vulnerable to ambient condition, which turned to white brittle fragments only after being in atmosphere for 24 h. The results showed that the polysiloxane we developed not only exhibited excellent processing ability by both bulk method and solvent method, but also displayed excellent environmental tolerance. Properties of the thermoset PSi-RV-PFCB. Thermostability. The thermostability of a thermoset has always been acknowledged as one of the key factors for application


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since it determines the upper limit of operating temperature. As depicted in Figure 4, the PSi-RV-PFCB exhibited a 5 wt% loss temperature (Td5) at 385 oC and a final char yield of 35%, which were much better than those of the controlled liner polysiloxane PSi-RA (Figure S5, Td5 = 342 oC, char = 8%). The good heat resistance of PSi-RVPFCB can be attributed to the existence of PFCB cross-links between polysiloxane chains.

Figure 4. TGA curves of PSi-RV-PFCB in N2 atmosphere. Acid resistance. A long-term and urgent challenge for polysiloxanes that limit their widespread application is their poor stability in water environment, especially in an acid solution, where polysiloxanes would quickly degrade within few hours at room temperature.42 In our strategy, the introduction of PFCB, not only improved the thermal stability, but also overcome the sensibility of polysiloxanes to moisture due to the large amounts of hydrophobic C-F bonds. The average water contact angle of PSi-RV-PFCB



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films was as large as 103 oC (see Image S1), which indicated its good hydrophobicity. Moreover, the bulk sample of PSi-RV-PFCB was immersed in 1 M HCl for a week to investigate the stability in acid condition. No change in appearance can be observed after soaking in HCl solution for a week, and only a tiny weight increment of 0.08% was found (see Figure S8). While for the control polysiloxane PSi-RA, it was dissolved in 1 M HCl aqueous solution to form a homogeneous solution after stirring for 24 hours. The results of GPC analysis of the extract indicated that the Mn was less than 1200 along with a larger retention time (~25 min), which illustrated its degradation in acid solution. The results above demonstrated the excellent stability of PSi-RV-PFCB in acid media. Transparency. Polysiloxanes, usually liquids, are famous for its good transparency and colorless nature which enable their applications as heating media, coating and cosmetics.43 To our delight, the thermoset PSi-RV-PFCB in our work retained the good transparency of polysiloxanes. The UV-vis-NIR spectra of three PSi-RV-PFCB films with different thickness revealed their excellent optical transparency (Figure 5). The films exhibited good transmittance in the range of visible area (400 nm – 800 nm), and typically, the transmittance of the films were all above 89 % at wavelengths that larger than 450 nm. In addition, the photo of free-standing PSi-RV-TFVE film in Figure 5 also displayed its good visual transparency. The results showed that the bio-based PSiRV-TFVE

thermoset

may

be

used

as

transparent


coating

materials.

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Figure 5. UV-vis-NIR spectra of free-standing PSi-RV-PFCB films with different thickness. Dielectric properties. Quantities of research have found that the PFCB-based materials usually have satisfying dielectric properties because of the introduction of the least polarizable groups such as C-F, C-O and C-C bonds into the polymer skeleton.31 And similarly, the cross-linked PSi-RV-PFCB was endowed with good dielectric ability with an average dielectric constant (k) lower than 2.68 at a range of 0.1 to 30 MHz (Figure S9), which was better than the widely used SiO2-based (k = 3.9)44 or other organosiloxane dielectric materials (k = 2.7 – 3.0).45 Mechanical properties. DMA and tensile tests were performed to evaluate the mechanical properties of the bio-based polysiloxane. A storage modulus of nearly 400 MPa was observed for the PSi-RV-PFCB (Figure S6), which was much better than that of cross-linked PDMS (1 – 10 MPa).46,47 On the other hand, tensile experiments showed



that the bio-based thermoset exhibited an elastic modulus of 790 MPa with an elongation at break of 1.7 % (Figure S10), indicating its good mechanical robustness. CONCLUSION Considering the severe shortage of fossil sources and the environmental factors, the skillful use of natural compounds as raw materials is quite meaningful and valuable, especially those possessing interesting chemical structures. In this paper, an aromatic biomass, vanillin, has been transformed to a high performance fluoro-containing polysiloxane based on its multi-functional nature. First the aldehyde groups of vanillin were transformed through McMurry reaction to avoid undesired side effects, then diTFVE-functionalized monomer were obtained by reacting the hydroxyl groups with BrCF2CF2Br. Notably, regarding the methoxy groups, which are always ignored in preparing high performance materials, are first reported to polymerize with hydrosilanes through the mild Piers-Rubinsztajn reaction. With the help of post-polymerization, a new bio-based polysiloxane thermoset was prepared. The introduction of the curable TFVEs into the polysiloxane chains not only enhances the thermal stability by forming PFCB cross-links, but also endows the cured thermoset with unexpected resistance in water and acids despite of the degradability of polysiloxanes. Furthermore, the cross-linked bio-based polysiloxane also shows good dielectric and mechanical properties, which make it possible for further application in electronic packaging industry as a high-performance thermoset. Given that the widely presence of methoxy groups in natural feedstocks, the method we developed may open a door to explore the application of the multi-functional biomass and develop new bio-


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based materials.

ASSOCIATED CONTENT

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications. TGA and DSC curves of monomer M-RV-TFVE, detailed and additional characterization of polymers PSi-RA, PSi-RV-TFVE and PSi-RV-PFCB. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Qiang Fang: 0000-0002-3549-5600 Jing Sun: 0000-0002-1714-0283 Yangqing Tao: 0000-0002-2953-1938 Junfeng Zhou: 0000-0002-8201-6799 Linxuan Fang: 0000-0003-4658-2336 Yuanqiang Wang: 0000-0001-7754-2148 Xiaoyao Chen: 0000-0002-0545-6395

NOTES



The authors declare no competing financial interest. ACKNOWLEDGMENT This work was implemented under the financial support from the Ministry of Science and Technology of China (2017YFB0404701 and 2015CB931900), National Natural Science Foundation of China (No. 21574146, 21774140 and 21774142), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB 20020000). REFERENCES 1. Schneiderman, D. K.; Hillmyer, M. A. 50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers. Macromolecules 2017, 50, 3733-3749, DOI: 10.1021/acs.macromol.7b00293. 2. Katzer, J. R. Sustainable Research, Development, and Demonstration (RD&D). Ind. Eng. Chem. Res. 2010, 49, 10154–10158, DOI: 10.1021/ie1005965 3. Beller, m.; Centi, G.; Sun, L. Chemistry Future: Priorities and Opportunities from the Sustainability Perspective. ChemSusChem 2017, 10, 6–13, DOI: 10.1002/cssc.v10.1. 4. Gandini, A.; Lacerda, T. M. From Monomers to Polymers from Renewable Resources: Recent Advances. Prog. Polym. Sci. 2015, 48, 1-39, DOI: 10.1016/j.progpolymsci.2014.11.002. 5. Zhu, Y.; Romain, C.; Williams, C. K. Sustainable Polymers from Renewable Resources. Nature 2016, 540, 354-362, DOI: 10.1038/nature21001. 6. Kumar, S.; Samal, S. K.; Mohanty, S.; Nayak, S. K. Recent Development of Biobased Epoxy Resins: A Review. Polym. Plast. Technol. Eng. 2016, 57, 133– 155. DOI:10.1080/03602559.2016.1253742. 7. Lligadas, G.; Ronda, J. C.; Galia M.; Cadiz, V. Plant Oils as Platform Chemicals for Polyurethane Synthesis: Current State-of-the-Art. Biomacromolecules 2010, 11, 2825–2835. DOI:10.1021/bm100839x. 8. Mashouf Roudsari, G.; Mohanty, A. K.; Misra, M. Green Approaches to Engineer Tough Biobased Epoxies: A Review. ACS Sustainable Chem. Eng. 2017, 5, 95289541, DOI: 10.1021/acssuschemeng.7b01422. 9. Gandini, A.; Lacerda, T. M.; Carvalho, A. J.; Trovatti, E. Progress of Polymers from Renewable Resources: Furans, Vegetable Oils, and Polysaccharides. Chem. Rev. 2016, 116, 1637-69, DOI: 10.1021/acs.chemrev.5b00264. 10. Sheldon, R. A. The Road to Biorenewables: Carbohydrates to Commodity Chemicals. ACS Sustainable Chem. Eng. 2018, 6, 4464-4480, DOI: 10.1021/acssuschemeng.8b00376.


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For Table of Contents Use Only

Synopsis The easily available biomass vanillin derived from renewable lignin was transformed to a new fluoro-containing polysiloxane based on its multi-functional nature for the first time, the as-prepared fluoro-polysiloxane thermoset exhibited good processability, thermal stability and acid resistance.


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254x190mm (96 x 96 DPI)


Fluoro-containing Polysiloxane Thermoset with Good Thermostability

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