Low Dielectric Fluorinated Polynorbornene with Good Thermostability

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Low Dielectric Fluorinated Polynorbornene with Good Thermostability and Transparency Derived from a Bio-based Allylphenol (eugenol) Linxuan Fang, Junfeng Zhou, Yangqing Tao, Yuanqiang Wang, XingRong Chen, Xiaoyao Chen, Jiaren Hou, Jing Sun, and Qiang Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05527 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Sustainable Chemistry & Engineering

Low Dielectric Fluorinated Polynorbornene with Good Thermostability and Transparency Derived from a Biobased Allylphenol (eugenol) Linxuan Fang, Junfeng Zhou, Yangqing Tao, Yuanqiang Wang, Xingrong Chen, Xiaoyao Chen, Jiaren Hou, Jing Sun,* and Qiang Fang* 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 FOOTNOTE Corresponding Author: *To whom correspondence should be addressed. Tel & Fax: +86 21 5492 5337. E-mail: [email protected].

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ABSTRACT: A new bio-based functional norbornene was synthesized by the DielsAlder reaction between cyclopentadiene and the fluorinated eugenol. Based on the norbornene monomer, a new polynorbornene was prepared with a high molecular weight (Mn = 78,000 Da) by the standard ring opening metathesis polymerization (ROMP). This polymer displayed good film-forming ability and can be postpolymerized to form a cross-linked network at high temperature (> 150 °C) via the [2+2] cycloaddition reaction of trifluorovinyl ether (-OCF2=CF2) groups. The cross-linked polymer exhibited good transmittance (T % > 90% at 550 nm), high thermostability with a 5% weight loss temperature of 412 °C. Moreover, no obvious glass transition temperature (Tg) was observed.in the DMA measurement when the temperature was elevated near the decomposition temperature of the cross-linked polymer (about 350 °C). The cured resin displayed average dielectric constant (Dk) of below 2.65 with average dissipation factor (Df) of 4.3 × 10-3 for the frequencies varying from 40 Hz to 25 MHz. The cross-linked polymer film also possessed good hydrophobicity with a contact angle of 104° (water on the surface of the film) and a surface energy of 26.8 mJ m−2. These data indicate that the combination of norbornene and the fluorinated eugenol can greatly improve properties of polynorbornene, which could have potential application in microelectronic industry. Thus, this contribution may provide a new way for the usage of the biomass. KEYWORDS: Eugenol, Fluoropolymer, Polynorbornene, Dielectric constant, Thermostability, Hydrophobicity.


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INTRODUCTION In the past decades, considerable research has been focused on converting bio-based chemicals to polymer materials as an alternative to fossil oil raw materials, because of their renewability and remarkable abundance.1-5 Among the bio-based chemical compounds, eugenol, a kind of renewable plant oils, has attracted much interest. Eugenol is available in large quantity from clove or cinnamon plants,6 which has been used as the starting materials to prepare epoxy resins, cyanate esters, benzoxazine and polycarbonate.7-11 Considering it is easily available with low cost, more approaches to create new polymers with eugenol have been pursued to expand its application. It is noted that eugenol has reactive hydroxyl and allyl groups, which can be used to synthesize specific monomer to make full usage of these functional groups. For example, the allyl group can be used to construct norbornene monomer with dicyclopentadiene via Diels-Alder reaction to prepare polynorbornenes (PNBs). PNBs have recently aroused an increasing research attention, because of their enormous potential in theranostic materials, electronic materials, memory polymers and anion-exchange materials.12-20 Numerous polynorbornenes (PNBs) were prepared from the ring-opening metathesis polymerization (ROMP) due to the ease of their synthesis, highly functional groups tolerance and forming the polymers with high molecular weight and good process-ability.21-25 However, the ROMP-type PNBs possessing high thermo-stability were rarely reported. It is because that the structures of ROMP-type PNBs contain the flexible chains and excess unsaturated bonds, as a result of which most PNBs display low Tgs (< 200 °C).24, 26-28 Thus, developing the new ROMP-type PNBs with high thermo-stability is of significance. In this work, we prepared a novel ROMP-type PNB with high thermo-stability. good thermostability. In our case, a new norbornene monomer bearing thermo-curable trifluorovinyl ether (TFVE) units was elaborately designed. The TFVE-functionalized polymer can be easily converted to a cross-linked network having perfluorocyclobutyl ether (PFCB) units. Usually, PFCB groups may endow the polymers with high thermostability, transparency and good dielectric properties.29-32 In our case, eugenol



was selected as a feedstock to synthesize the new norbornene monomer because of its bifunctional nature, of which the allyl group can construct norbornene unit and hydroxyl group is a suitable block for the connection of TFVE unit. Moreover, dicyclopentadiene is a low-cost compound and is often considered as a byproduct from petrochemical industry (C5 fraction).33 Thus, exploring the new usage of dicyclopentadiene is very desirable. In this contribution, we combine a byproduct from petrochemical industry and a bio-based eugenol to develop the new sustainable materials. This work provides a new norbornene monomer with a biomass content of 52 %, prepared by a Diels-Alder reaction between cyclopentadiene and eugenol with fluorinated group (see Scheme 1). Starting from the monomer, a TFVE-containing polynorbornene was prepared with high molecular weight by the standard ROMP procedure. The obtained polymer displayed good solubility and good film-forming ability. Casting a solution of the polymer can give a flexible and highly transparent freestanding film. Treating this polymer film at high temperature offered a cross-linked network. Interestingly, even being processed at high temperature for hours, the film still remained highly transparent (T % > 90% at 550 nm). In addition, the glass transition temperature (Tg) of the cross-linked polymer cannot be obviously observed below 350 °C in DMA measurement. Moreover, thanks to the existence of fluoro-containing groups, the cross-linked polynorbornene displayed low dielectric constant of below 2.65, and good hydrophobicity with a low surface energy (26.8 mJ m−2) and a high contact angle of 104°. Thus, this work not only provides a new strategy for application of eugenol, but also prepares a new bio-based high-performance polymer. Herein, we report the details.

EXPERIMENTAL SECTION Materials. Dicyclopentadiene and hexadecane were purchased from TCI. The analytical grade solvents and Grubbs II catalyst were purchased from Adamas, China. 4-Allyl-1-(2-bromo-1, 1, 2, 2-tetrafluoroethoxy)-2methoxy benzene (1) was synthesized according to a previously reported procedure.34 Measurements. 1H NMR, 13C NMR and 19F NMR spectra were carried out on a Bruker


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400 spectrometer with CDCl3 as a solvent. Molecular weight was determined on a Waters Breeze2a 200 GPC instrument with polystyrene as the standard and tetrahydrofuran (THF) as the eluent at the column temperature of 35 oC. High performance liquid chromatography (HPLC) was performed on a Dionex Ultimate 3000 RS using a mixture of THF and H2O (8:2, v/v) as the eluent with a flow rate of 1.6 mL min-1. FT-IR spectra were measured on a Nicolet spectrometer at room temperature with KBr pellets. High resolution mass spectra (HRMS) were measured by an Agilent Technologies 5973N. The thickness of free-standing film was recorded on numerically display micrometer gauge. Elemental analysis (EA) was performed on an Elementar vario EL III system. UV-vis-NIR spectra were carried out on a Varian CARY 5000 at room temperature. Differential scanning calorimetry (DSC) was run on a TA Instrument DSC Q200 under nitrogen atmosphere with a heating rate of 10 °C min-1. Thermogravimetric analysis (TGA) was performed on a NETZSCH TG 209 apparatus under nitrogen and air flow with a heating rate of 10 °C min-1. Surface toughness of polymer films on silicon wafers were characterized by atom force microscopy (AFM) on a Shimadz SPM-9500J3. Dk and Df of the cured polymer sheets were analyzed at room temperature by a 4294A Precision Impedance Analyzer (Agilent). Contact angle of water or hexadecane on the surface of a cured polymer film was performed on a JC2000C tester. Based on the contact angle data, the surface energy was estimated. Dynamic mechanical analysis (DMA) was performed on a DMA Q800 instrument in air with a heating rate of 5 °C min-1. Thermomechanical analysis (TMA) was analyzed on a TMA DIL 402 Expedis instrument in nitrogen flow with a heating rate of 5 °C min-1. The bonding strength of a polymer film on the surface of a silicon wafer at room temperature was studied on UNHT/NST by nanoscratch tests. Synthesis of compound 2. Dicyclopentadiene (6.21 g, 47 mmol) and 1 (50.00 g, 146 mmol) were mixed with hydroquinone (0.02 g，0.018 mmol) in a 100 mL of sealed tube. The mixture was heated at 185 °C with stirring for 24 h and then was cooled to room temperature. The yellow reacting mixture was distilled under reduced pressure to afford 2 (b.p. 120 ~ 125 °C at 2 mmHg) as a clear and colorless liquid with a yield of 42 %. The unreacted compound 1 (b.p. 90 ~ 100 °C at 2 mmHg) was recycled. HPLC



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results displayed that 2 contained an endo and an exo isomer (82.46 and 15.88 %). 1H NMR (CDCl3, 400 MHz), δ (ppm): 7.12 (d, J = 8.1 Hz, 1H), 6.72 ~ 6.80 (m, 2H), 6.02 ~ 6.23 (m, 2H), 3.83 (s, 3H), 2.54 ~ 2.85 (m, 2H), 2.3 ~ 2.40 (m, 3H), 1.89 ~ 0.61 (m, 4H). 19F NMR (CDCl3, 376 MHz), δ (ppm): -67.44 (m, 2F), -85.89 (m, 2F). 13C NMR (CDCl3, 376MHz), δ (ppm): 152.17, 142.61, 142.36, 137.66, 136.80, 136.78, 135.67, 135.58, 132.44, 123.44, 119.13 ~ 100.43 (m, 2C), 113.65, 113.57, 56.16, 49.70, 45.91, 45.51, 45.17, 42.79, 42.49, 42.28, 40.84, 40.68, 40.31. HRMS-DATA (m/z): Calcd for C17H18O2BrF4 [M +H] + 409.0421; Found, 409.0419. Synthesis of NBE-TFVE. To a mixture of activated zinc (1.47 g, 22.4 mmol) in acetonitrile (10 mL) was added a solution of 2 (6.10 g 15.0 mmol) in acetonitrile (40 mL) during a period of over 0.5 h. The resulting mixture was heated at 95 °C for 24 h with stirring. After cooled to room temperature, the mixture was filtered and washed with hexane. The obtained organic layer was concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with hexane as the eluent. NBE-TFVE was prepared as a colorless oil in a yield of 78 %. 1H NMR (CDCl3, 400 MHz), δ (ppm): 6.98 (d, J = 8.3 Hz, 1H), 6.70 ~ 6.79 (m, 2H), 6.01 ~ 6.22 (m, 2H), 3.89 (s, 3H), 2.53 ~ 2.84 (m, 2H), 2.29 ~ 2.39 (m, 3H), 1.85 ~ 0.62 (m, 4H). 19F NMR (CDCl3, 376 MHz), δ (ppm): -120.91 ~ -120.44 (ddd, 1F), -127.15 ~ -126.56 (m, 1F), 134.05 ~ -133.58 (m, 1F). 13C NMR (CDCl3, 100 MHz), δ (ppm): 149.40, 146.81 (td), 142.10, 142.06, 140.35, 140.10, 137.64, 136.80, 134.19 (dt), 132.45, 120.96, 120.81, 116.45, 113.72, 113.64, 56.21, 49.69, 45.86, 45.50, 45.15, 42.79, 42.33, 42.27, 40.78, 40.66, 40.42, 33.02, 32.23. HRMS-DATA (m/z): Calcd for C17H18O2F3 [M +H]

+

311.1253; Found, 311.1249. Synthesis of PNBE-TFVE. Grubbs II catalyst (78.00 mg, 0.0923 mmol) was quickly added to a solution of NBE-TFVE (2.86 g, 9.23 mmol) under nitrogen flow in THF (55 mL). The reaction mixture was vigorously stirred for 4 h at the room temperature, and the obtained polymer was precipitated from methanol, filtered, and dried at 60 °C under vacuum overnight. PNBE-TFVE (2.76 g) was obtained as a light grey fibrous solid with a yield of 98.6%. GPC (polystyrene) results: Mn = 78,000 Da, PDI = 1.81. 1H

NMR (CDCl3, 400 MHz), δ (ppm): 6.92 (br s, 1H), 6.67 (br s, 1H), 5.20 ~ 5.44 (br,


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2H), 3.78 (br, 3H), 1.09 ~ 2.92 (m, 9H). 19F NMR (CDCl3, 376 MHz), δ (ppm): -120.93 ~ -120.41 (m, 1F), -127.16 ~ -126.61 (m, 1F), -134.06 ~ -133.91 (m, 1F). 13C NMR (CDCl3, 100MHz), δ (ppm): 149.82, 149.37, 149.20, 147.06, 146.44, 144.36, 143.73, 142.09, 140.28, 130.72~135.86, 120.96, 120.82, 116.42, 113.71, 56.20, 45.51, 37.04 ~ 40.36. Anal. Calcd for (C17H18O2F3)n:C: 65.80, H: 5.52, F: 18.37. Found: C: 65.61, H: 5.64, F: 18.62. Fabrication of PNBE-TFVE and PNBE-PFCB Sheets. A solution of PNBE-TFVE in CH2Cl2 (0.97 mol L−1) was placed in a flat-bottomed glass tube, and the solvent was naturally evaporated to obtain PNBE-TFVE sheet. The glass tube was slowly heated to 170 °C, and then kept under nitrogen at 170 °C for 2 h, 180 °C for 2 h, 200 °C for 3 h, 230 °C for 2 h, and 250 °C for 2 h, respectively. After gradually cooled to room temperature,

a

fully

cured

PNBE-PFCB

sheet

was

obtained.

Fabrication of PNBE-TFVE and PNBE-PFCB Films on Silicon Wafers. A smooth PNBE-TFVE film was obtained by spin-coating a solution of PNBE-TFVE in mesitylene (0.16 mol L−1) on to a silicon wafer. The wafer was placed on a hot-plate, which was preheated at 230 °C. After being maintained at the temperature for 1 min, the wafer was moved to an oven and heated under nitrogen atmosphere at 230 °C for 2 h, and 250 °C for 1 h, respectively. A fully cured PNBE-PFCB film was obtained. Fabrication of Free-standing Films. Casting a solution of PNBE-TFVE in CHCl2CHCl2 (0.65 mol L−1) into the surface of a glass sheet gave a film after natural evaporation of the solvent. A free-standing film with an average thickness of 16 m was obtained through keeping the glass sheet in alcohol for 10 min. For the preparation of PNBE-PFCB film, a PNBE-TFVE film on a glass sheet was treated under nitrogen atmosphere at 170 °C for 1 h, 230 °C for 2 h, and 250 °C for 2h, respectively. After removal of the glass sheet, a fully cured PNBE-PFCB free-standing film was obtained. A free-standing film can be also obtained by casting a solution of PNBE-TFVE in the halogen-free solvents, such as mesitylene and cyclohexanone. Because the fabrication of a film only needs a small quantity of the solvents, thus the environmental problem may be ignored.



RESULTS AND DISCUSSION Synthesis and Characterization Starting from eugenol, a new fluorinated norbornene with overall yield of 33% was synthesized via a three-step reaction (see Scheme 1). This monomer possessed biomass content of 53%. The atom efficiency of an individual step and an overall step was 100 %, and 66%, respectively. Intermediate 2 was synthesized by a solvent-free procedure via the Diels-Alder reaction between compound 1 and dicyclopentadiene. Pure compound 2 was obtained with a somewhat low yield due to the reversibility of Diels-Alder reaction, whereas the unreacted compound 1 can be easily recycled after distillation under reduced pressure. Based on compound 2, monomer NBE-TFVE was prepared in a good yield through an elimination reaction in the presence of zinc. This monomer was polymerized with a nearly quantitative yield through the standard ring-opening metathesis reaction (see Scheme 1). (see Scheme 1). In order to obtain a polymer with suitable molecular weight, the polymerization conditions had been optimized and the results are summarized in Supporting Information (Table S1). Because the polymerization required a critical concentration of monomer,35, 36 thus the polymer did not form at a low concentration of monomer. However, an insoluble gel was observed when the monomer concentration was too high. This phenomenon may be due to the crosslinking of the pendent alkene groups. To prepare a polymer with both high yield and good solubility, an optimal condition was determined with a monomer concentration of 0.16 mol L-1 in THF and a molar ratio of 1:1% between monomer and Grubbs II catalyst (Table S1, Entry 4). Thereby, PNBE-TFVE with a molecular weight of 78,000 was obtained according to the optimal condition.


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BrCF2CF2Br OCH3 OH Eugenol

185 oC

NaH, DMSO

OCH3 OCF2CF2Br

Zn, CH3CN

reflux

Dicyclopentadiene OCH3 OCF2CF2Br 2

1

n

Grubbs II OCH3 O

F

F

F

NBE-TFVE

THF r.t., 4 h

OCH3 O

F

F

F

PNBE-TFVE

Mn = 78,000 PDI = 1.81

Scheme 1. Procedure for the synthesis of PNBE-TFVE.

1H

NMR, 13C NMR, 19F NMR and elemental analysis were performed to confirm the

chemical structure of PNBE-TFVE. As can be seen from Figure 1, proton signals of 5.20 ~ 5.44 ppm are attributed to internal alkene. Peak signals of 1.09 ~ 2.92 ppm are belong to cyclopentane of polymer backbone and methylene group adjacent to cyclopentane moieties. As depicted in Figure 2, signals of cyclic alkene (137.64, 136.80, 132.45 ppm) of NBE-TFVE disappear after ROMP, and signals of internal alkene of PNBE-TFVE appear at 130.72 ~ 135.86 ppm.24 Meanwhile, characteristic carbon signals associate with eugenol unit are clearly saw in Figure 2. The typical characteristic peaks of TFVE group of PNBE-TFVE also were clearly shown in

19F

Figure 1). Hence, the chemical structure of PNBE-TFVE was confirmed.


NMR (see


Figure 1. 1H NMR (CDCl3, 400 MHz) and 19F NMR (CDCl3, 376 MHz) spectra of PNBE-TFVE.

Figure 2. 13C NMR (CDCl3, 100 MHz) spectra of NBE-TFVE and PNBE-TFVE. The insert (red) is DEPT 135o (CDCl3, 100 MHz) spectrum of NBE-TFVE. Curing behavior of PNBE-TFVE. Curing behavior of PNBE-TFVE was researched by DSC. As exhibited in Figure 3, PNBE-TFVE shows an onset curing temperature at


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150 °C and a maximum exothermic peak at 232 °C. No obvious exothermic peaks are observed at the second scan, suggesting the complete conversion of TFVE into PFCB units.31 Thermo-crosslinking reaction was also characterized by FT-IR spectroscopy. As can be seen from Figure 4 and Figure S7, the characteristic peak of TFVE at 1832 cm−1 can be observed in both of the FT-IR spectra of NBE-TFVE and PNBE-TFVE, indicating no change of TFVE during ROMP process. Nevertheless, the absorption peak at 1832 cm−1 disappears and a new obvious absorption peak attributed to PFCB at 962 cm−1 appears, suggesting that PNBE-TFVE has fully converted to PNBE-PFCB. The peak at 1158 cm-1 is attributed to C-F bond stretching vibration of TFVE group, which changes after forming PFCB group. The adsorption peak at 1605 cm−1 in both PNBE-TFVE and PNBE-PFCB is obviously observed, suggesting that no change of internal alkene occurs after crosslinking. In order to explore whether crosslinking occurs between the backbone alkene groups, a comparable polymer (P2) was synthesized, and the detailed synthetic procedure is shown in Supporting Information. As can be seen from Figure S8, no obvious exothermic peaks are observed in the DSC traces of P2, indicating no the crosslinking between the backbone alkene groups. Furthermore, 1H NMR spectra of P2 before and after heating at high temperature also indicate (see Figure S9) that the chemical structure of P2 does not change, suggesting that thermo-treatment does not result in the crosslinking of the double bonds in the main chain of P2. A schematic description for the conversion of PNBE-TFVE into PNBE-PFCB is depicted

in

Scheme


2.


Figure 3. DSC traces of PNBE–TFVE with a heating rate of 10 oC min-1 in N2.

Figure 4. FT-IR spectra of PNBE–TFVE and PNBE–PFCB.


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Scheme 2. A schematic description for the preparation of PNBE-PFCB via a [2 + 2] cycloaddition of PNBE–TFVE. Thermostability of the Crosslinked Polymer. Thermostability of the crosslinked polymer (PNBE-PFCB) was evaluated by TGA and the results were depicted in Figure 5. PNBE- PFCB exhibits a 5 % weight loss temperature of 412 °C and a char yield of 29 % at 1000 °C in nitrogen (the data in air are included in Figure S10). The 5% weight loss temperature of PNBE-PFCB is higher than those of the common thermosetting resins such as polycyanurates ( 90%) from 550 to 1100 nm. This result based on a PNBE-PFCB film is comparable to the reported transparent PNBs.45-47 The transmittance quickly decreases to near 0% in the UV region (

Low Dielectric Fluorinated Polynorbornene with Good Thermostability

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