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Bio-based Anethole-Functionalized Poly(phenylene oxides) (PPOs): New Low Dielectric Materials with High Tg and Good Dimensional Stability Yuanqiang Wang, Yangqing Tao, Junfeng Zhou, Jing Sun, and Qiang Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01596 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Bio-based Anethole-Functionalized Poly(phenylene oxides) (PPOs): New Low Dielectric Materials with High Tg and Good Dimensional Stability Yuanqiang Wang, Yangqing Tao, Junfeng Zhou, 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, PR China.

E-mail: [email protected]; [email protected] Keywords: Plant oil, anethole, poly(phenylene oxides) (PPOs), thermosetting polymers, CTE, Tg ABSTRACT: A series of modified poly(phenylene oxides) (PPOs) with low dielectric constant, high Tg and good dimensional stability were developed. These polymers were synthesized by treating brominated-PPOs with a bio-based anethole and polymerized at a high temperature in the presence of a peroxide. When the molar ratios of anethole in the PPO were 0.2, 0.35 and 0.5, respectively, the cross-linked PPOs displayed low dielectric constants of (less than 2.74) and high Tg (more than 220 C). In particular, the PPO with 0.5 molar ratio of anethole exhibited a coefficient of thermal expansion (CTE, 23.4 ppm/C), that is comparable to copper foil (ca. 18 ppm/C). This result indicated that the modified PPOs are suitable for the production of copper cladded laminates in electronic industry. Moreover, compared to a commercial PPO having a Tg 1 ACS Paragon Plus Environment

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of 153 C and CTE of 155.8 ppm/C, the modified PPOs with anethole units possessed a higher thermostability. Considering that PPOs are widely used in electronic industry, this contribution offers a new approach to prepare high performance materials using a bio-based feedstock.

INTRODUCTION Poly(phenylene oxides) (PPOs) are widely used as matrix resins for the production of printed circuit boards (PCBs) in electronic industry because they can be easily synthesized and display attractive properties, including low water absorption, good resistance against acid and alkali, and low dielectric constant (D < 2.6) as well.1-11 However, their weak resistance to organic solvents, low glass transition temperature (Tg) and high coefficient of thermal expansion (CTE) hinder their applications. In order to improve the properties of PPOs, many efforts have been made. The general approach is to introduce phenyl groups or crosslinkable groups into the backbones of the polymers for elevating the Tg of polymers,2, 12-18 but the introduction of phenyl group leads to high crystallization degrees of the polymers, resulting in their poor processability.15 In addition, although attaching crosslinkable groups to the PPOs can enhance their thermostability, the utilization of strong alkalis such as n-BuLi or t-BuLi is very inconvenient due to their high sensitivity to moisture16. Moreover, for the production of copper cladded laminates, the relatively high CTE of commercial PPOs cannot match with that of Cu foil (ca 18 ppm/C 19), easily resulting in failure of the bonding between 2 ACS Paragon Plus Environment

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the copper foil and the substrate. However, up to now, the research of decreasing the CTE of the PPOs is rarely reported.5,6 It still lacks an effective method to obtain the PPOs with good dimensional stability. Hence, it is significant to develop a facile and effective method to reinforce the properties of the PPOs, especially to improve their dimensional stability. Recently, because of the rapid consumption of fossil oil, more and more attention has been attracted to develop new approaches to take full advantage of renewable biomass resources.20-23 Among various biomass resources, anethole, a renewable plant oil with an aromatic skeleton and a reactive vinyl group, has been demonstrated to be able to enhance the thermostability of the polymers.24-27 The polymers based on this plant oil also display low D.28-30 Inspired by the good properties of the polymers derived from anethole, we have designed and synthesized new PPOs by introducing anethole to the backbone of the commercial PPOs (see Scheme 1). Three types of PPOs were obtained with the 0.2, 0.35 and 0.5 molar ratios of anethole (molar ratio of anethole is defined relative to that of phenylene oxide units), respectively. When treated the PPOs at a high temperature in the presence of a peroxide (dicumyl peroxide, DCP), they converted to the cross-linked polymer networks, which displayed high Tg, low CTE and low k. These results indicate that the combination of anethole and PPOs can prepare modified polymers with low D, high Tg and good dimensional stability, which can meet the requirements of high-performance resins for the production of PCBs and laminates in electronic industry.

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Scheme 1. Procedure for the synthesis of the anethole-containing PPOs.

EXPERIMENTAL SECTION Materials. Anethole was purchased from Nanjing Chemlin Chemical Industry Co., Ltd, China. A commercial poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with Mn of 16 kDa and Ð of 1.21 was purchased from Polysciences, Inc. Azoisobutyronitrile (AIBN), Nbromosuccinimide (NBS) and iodomethane (CH3I) were purchased from TCI Co. AIBN was purified by recrystallization from methanol before use. All other chemicals were used as received. (E)-4-Propenyl phenol was synthesized in a yield of 96% according to a route previously reported.29 Measurements. 1H NMR was measured on a Bruker AV400 instrument. Differential scanning calorimetric (DSC) analysis was recorded on a TA Instrument DSC Q200 at a heating rate of 10 C min-1 under nitrogen atmosphere. Dielectric constant and 4 ACS Paragon Plus Environment

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dissipation factor of polymer cylinders with a diameter of 20 mm and a height of 1mm were investigated at room temperature on a 4294A Precision Impedance Analyer (Agilent) by using the standard capacitance method in a range from 0.15 MHz to 30 MHz.31 Thermo-gravimetric analysis (TGA) was determined on a TG 209F1 apparatus with a heating rate of 10 C min-1 under N2. Thermomechanical analysis (TMA) was performed with a heating rate of 3 C min-1 in Nitrogen on a TMA Q400 V7.1 Build 89 instrument. Molecular weights were measured by using a Waters Breeze2a 200 GPC instrument with polystyrene as standards and tetrahydrofuran (THF) as the eluent. Fourier transform infrared spectra (FT-IR) were run on a Nicolet spectrometer with reflection. Synthesis of PPO-CH3. To a solution of PPO (11.48 g, 0.72 mmol) in THF (70 mL), CH3I (0.22 g, 1.53 mmol) and K2CO3 (0.24 g, 1.74 mmol) was added. The mixture was stirred at room temperature until the reaction completed (monitored by 1H NMR). The reacting mixture was filtered, concentrated and poured into methanol (100 mL) to afford a pale-yellow precipitate, which was filtered, washed with methanol (20 mL  3) and dried in vacuum. PPO-CH3 was obtained as a pale yellow solid in a yield of 98%. GPC (polystyrene standard) results: Mn = 16 kDa, and polydispersity (Ð) = 1.26. 1H NMR (400 MHz, CDCl3, ppm):  6.41 (-Ph, 2H), 3.75 (-OCH3, 0.01H). 2.03 (-PhCH3, 3H). Synthesis of P(BrPO-co-PO)x-co-(1-x). Three different P(BrPO-co-PO)x-co-(1-x) polymers (x = 0.2, 0.35 and 0.5) were prepared in an analogous procedure. Taking P(BrPO-coPO)0.2-co-0.8 as an example, the reaction was carried out according to the following 5 ACS Paragon Plus Environment

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procedure. To a stirring solution of PPO-CH3 (3.00 g, 24 mmol ) in carbon tetrachloride (20 mL) was added AIBN (0.12 g, 0.74 mmol) and NBS (1.32 g, 7.43 mmol). The mixture was stirred at 60 C for 8 h, then cooled to room temparature and diluted with water (100 mL) under vigorous stirring. The organic phase was separated and poured into methanol (100 mL). The formed precipitate was filtered and dried in vacuum to give a pale-yellow solid in a yield of 91%. GPC (polystyrene standard) results: Mn = 38 kDa and polydispersity (Ð) = 1.88. 1H NMR (400 MHz, CDCl3, ppm):  6.47 ~ 6.71 (Ph, 5.02H), 4.34 (-PhCH2Br, 1H), 2.09 (-PhCH3, 13.82H).The degree of bromination of PPO was estimated by 1H NMR spectum. Synthesis of P(APO-co-PO)x-co-(1-x). Taking P(APO-co-PO)0.2-co-0.8 as an example, the reaction was run according to the following procedure: to a stirring solution of P(BrPOco-PO)0.2-co-0.8 (3.00 g) in cholorobenzene (50 mL) was added a mixture of (E)-4propenyl phenol (1.60 g, 12 mmol), K2CO3 (1.66 g, 12 mmol) and DMSO (3 mL). The reacting mixture was heated to 80 C and kept at the temperature for about 20 h until the signal of -PhCH2Br disppeared in the 1H NMR spectrum. After being cooled to room temparature, the mixture was filtered, and the filtrate was washed with water (100 mL  3), dried over anhydrous Na2SO4, and concentrated until the volume of the solution was near to 5 mL. The residue was poured into methanol (100 mL) to afford a yellow precipitate, which was purified by using flash short column chromatography with CHCl3 as an eluent. P(APO-co-PO)0.2-co-0.8 was thus prepared in a yield of 90%. GPC (polystyrene standard) results: Mn = 43 kDa, and polydispersity (Ð) = 2.99. 1H NMR (400 MHz, CDCl3, ppm):  6.47-7.15 (-Ph, 7.41H), 6.26 (-CH=CHCH3, 0.50H), 6 ACS Paragon Plus Environment

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6.01 (-CH=CHCH3, 0.50H), 4.94 (-CH2OPh, 1H), 2.09 (-PhCH3, 14.40H), 1.80 (CH=CHCH3, 1.5H). Preparation of thermo-cross-linked products for the measurement of dielectic constant and thermo-mechanical behaviors. Casting a solution of P(APO-co-PO)xco-(1-x)

(x = 0.2, 0.35, 0.5) and DCP (5-wt%) in toluene on the surface of a clean and

non-pretreating glass sheet forms a smooth and transparent film. After the solvent was naturally evaporated, the glass sheet having the polymer film was placed into a vacuum oven and kept it in vacuum at room temperature for 12 h. Next, the glass sheet was moved to a quartz tube furnace, which was raised to 300 C at a heating rate of 5 C min-1 and kept at the temperature for 30 min. The film (Px) was thus fully thermo-crosslinked, which can be easily taken off from the glass sheet. RESULTS AND DISCUSSION Synthesis and characterization of anethole-containing PPOs. In order to synthesize the modified PPOs with anethole, a commercial PPO with a number average molecular weight (Mn) of 16 kDa was used as the starting material. Initially, the uncapped PPO was directly brominated and followed by the etherization. Unfortunately, the resulting polymer cannot form a smooth film. This may be attributed to the formation of some crosslinking structures between benzyl bromide and the -OH end group on another PPO chain, resulting in the poor solubility of corresponding polymers. Consequently, the -OH groups were capped with CH3I, and the formed PPOCH3 was then brominated to give a series of brominated-containing PPOs, P(BrPO-coPO)x-co-(1-x) (x = 0.2, 0.35, 0.5). These polymers were further treated with (E)-4-propenyl 7 ACS Paragon Plus Environment

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phenol (the demethylation product of anethole) to obtain the anethole-containing PPOs, P(APO-co-PO)x-co-(1-x) (x = 0.2, 0.35, and 0.5) in a high yield. The chemical structures of brominated-containing PPOs and anethole-containing PPOs, as well as the degree of bromination were characterized by 1H NMR spectra. The detailed data are listed in experimental section and Figure S1. As shown in Figure S1, a peak at 3.75 ppm is attributed to the -OCH3 group of methyl capped PPO. Meanwhile, the signals of -PhCH2Br and -PhCH3 appear at 4.43 and 2.09 ppm, respectively. For anethole-containing PPOs, the signals of -CH2OPh and -CH=CHCH3 groups are observed at 4.94 and 1.80 ppm, respectively. The bromination degree of PPOs are determined according to the ratios of the integrated area between -PhCH2Br and PhCH3 groups, and the detailed calculation is shown in supporting information. No signal of -PhCH2Br is observed in the 1H NMR spectra of anethole-containing PPOs, indicating that the bromine atoms at -PhCH2Br groups have been completely substituted by anethole moieties. The molecular weights of the modified PPOs were characterized by GPC. As shown in Figure 1 and Table S1, the GPC curve of the end-capped PPO is similar to that of the starting PPO with Mn of 16 kDa. After being brominated and substituted by anethole units, the molecular weights of the polymers increased and some double shoulder peaks appeared on the GPC curves. This may be ascribed to the maldistribution of the bromination degree of polymer chains, the bulk volume of side chains has an effect on the dispersity of the molecular weights of the polymers. However, this phenomenon did not influence the properties of the polymers including of the film-forming ability. Thus, 8 ACS Paragon Plus Environment

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the obtained polymers can form smooth and transparent films.

Figure 1. GPC curves of PPO, PPO-CH3, P(BrPO-co-PO)x-co-(1-x), P(APO-co-PO)x-co(1-x)

(x = 0.2, 0.35 and 0.5).

Solubility and film-forming ability. Anethole-containing PPOs were soluble in common organic solvents, such as THF, toluene and chloroform. All of them exhibited good film-forming ability. For instance, casting a solution of P(APO-co-PO)0.5-co-0.5 in toluene gave a smooth, transparent and free-standing film (see Figure 2). After being treated at a high temparature in the presence of an initiator (DCP), the anethole-containing PPOs became color-depening. As shown in Figure S2, the UV-Vis absorption spectrum of a P(APO-co-PO)0.5-co-0.5 film was red-shifted after cured, which can be asribed to the formation of conjugated structures in the polymers after the thermal oxidation32,33. The cross-linked polymers were insoluble in the common organic solvents, indicating solvent-resistance of PPOs improved as a result of the cross-linked networks formed. 9 ACS Paragon Plus Environment

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Figure 2. The images of a P(APO-co-PO)0.5-co-0.5 film before (left) and after (right) cured. Thermo-crosslinking reaction. The thermo-crosslinking reaction of the anethole-containing PPOs was characterized by DSC (see Figure 3), and the cross-linked degrees of the polymers were monitored by FT-IR spectroscopy (see Figure 4). As shown in Figure 3, the commercial PPO used in this work only shows a melting point of 244 C. For the anethole-containing PPOs with 5 wt% of dicumyl peroxide (DCP), they start to form cross-linked structures when the temperature is near 150 C. When the temperature is elevated to 270 C, a wide exothermic peak appears, indicating that the cross-linking reaction is further carried out. In addition, the DSC traces of the anethole-containing PPOs without DCP were also run (see Figure S3). As shown in Figure S3, only a weak exothermic peak is observed up to 270 C, indicating that the propenyl groups hardly reacted at this temperature without DCP. On the basis of the DSC results, the cured films were prepared, and the detailed method was descripted in the experimental section. The cured samples exhibited no obvious exothermic peak under DSC traces, reflecting the thermal crosslinking reactions of P(APO-co-PO)x-co-(1-x) (x = 0.2, 0.35, and 0.5) were completed. The crosslinking degree of anethole-containing PPOs was monitored by FTIR spectroscopy. Because all of the modified PPOs displayed a similar curing behavior, thus taking P(APO-co-PO)0.5-co-0.5 as an example to depict the IR spectrum (Figure 4). As showed 10 ACS Paragon Plus Environment

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in Figure 4, the characteristic peak at 960 cm-1 attributed to the out-of-plane vibration of the propenyl group at anethole, basically disappears after curing, indicating that the thermo-crosslinking reaction of anethole units in PPOs has completed.

Figure 3. DSC traces of PPO, P(APO-co-PO)x-co-(1-x) (x = 0.2, 0.35, 0.5) blended with 5 wt% of DCP, and cured polymers Px (x = 0.2, 0.35, 0.5) at a heating rate of 5 C min-1 in N2.

Figure 4. FT-IR spectra of P(APO-co-PO)0.5-co-0.5 before (red line) and after (blue line) cured.

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Figure 5. TGA curves of PPO, P0.2, P0.35 and P0.5 in N2 at a heating rate of 10 C min-1. Thermostability and dimensional stability. The thermostability of cured anethole-containing PPOs was investigated by TGA. As depicted in Figure 5, cured anethole-containing PPOs (Px, x= 0.2, 0.35, 0.5) exhibit 5wt% loss temperature (Td5) of 415 C, 406 C and 400 C, respectively, which are lower than that of the original PPO (Td5 = 437 C). It can be accounted for incorporation of weak benzyl ether bond. However, due to the existence of the cross-linked structures, the weight residues of cured anethole-containing PPOs (Px, x= 0.2, 0.35, 0.5) are about 37%, 44% and 52% at 1000 C (in N2), which are higher than that of the original PPO (about 29%). It is noted that high-performance PPOs should have high Tg and low CTE. Tg and CTE of the polymers were characterized by TMA. As shown in Table 1 and Figure 6, a commercial PPO used in this work has a Tg of 153 C and displays a CTE of 155.8 ppm/C at the range of temperatures from 40 to 133 C. When temperature is elevated to 153 C, the CTE of the PPO dramatically enlarged, indicating that the dimensional stability of the PPO decreased at high temperature. However, the cured anethole12 ACS Paragon Plus Environment

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containing PPOs (Px, x = 0.2 and 0.35) exhibit higher Tgs (224 C and 243 C) and lower CTEs (110.9 ppm/C and 87.0 ppm/C). Remarkably, P0.5 shows a Tg of 271C and a CTE of 23.4 ppm/C. Such a low CTE is comparable to copper foil (ca 18 ppm/C19). These data indicate that the dimensional stability of PPO has been greatly improved after incorporating a biorenewable anethole. Table 1. Properties of PPO and cured anethole-containing PPO, P x (x = 0.2, 0.35 and 0.5). Item

PPO

P0.2

P0.35

P0.5

D (at 30 MHz)

2.47

2.38

2.60

2.71

Df (at 30MHz)

0.001

0.002

0.001

0.005

Tg a

153 C

224 C

243 C

271 C

CTE b

155.8 ppm /C 110.9 ppm /C 87.0 ppm /C 23.4 ppm /C

Dielectric

Thermal

Solvent Resistance Chloroform

Soluble

Insoluble

Insoluble

Insoluble

Acid

Insoluble

Insoluble

Insoluble

Insoluble

Alkali

Insoluble

Insoluble

Insoluble

Insoluble

a:determined

by TMA; b: determined in the range of temperature from 40 C to Tg-20

C.

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Figure 6. (Left) Relative length changs of P x (red, x = 0.2; green, x= 0.3; blue, x= 0.5) versus temperature. (Right) The changes of Tg and CTE of Px with the increasing of the contents of anethole in PPOs.

Dielectric properties. In order to investigate the effect of introducing anethole units into the PPO on the dielectric properties of the polymer, D and Df of Px (x = 0.2, 0.35 and 0.5) were determined by the standard capacitance method,31 and the results are depicted in the Table 1 and Figure 7. Px (x = 0.2, 0.35 and 0.5) exhibit average D of 2.39, 2.62 and 2.74 with Df of 0.002, 0.001, 0.005 for frequencies ranging from 0.15 to 30 MHz, respectively, which are comparable to the commercial PPO (D =2.5 and Df =0.001). These results suggested that the introduction of anethole units into the PPO does not affect dielectric properties of the polymer.

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Figure 7. Dielectric constants (solid line) and dissipations (dash line) of PPO (black), P0.2 (red), P0.35 (green) and P0.5 (blue) films.

Conclusion In conclusion, the thermal and dimensional stability of a commercial PPO has been significantly improved by introducing a biorenewable anethole into the backbone of the polymer. The anethole-containing PPOs can be facilely thermo-cross-linked in the presence of a peroxide at a high temparature. The cross-linked polymers displayed better thermal and dimensional stability compared to the commercial PPO, while the good dielectric performance was retained. Especially, a cured PPO having a 0.5 molar ratio of anethole exhibits a CTE of 23.4 ppm/C, which is comparable to copper foil. The modified PPO with high Tg, good dimension stability and dielectric properties indicates that it can meet the requirements of matrix resins for the fabrication of printed circuit board in electronic industry.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Polymer characterizations, and additional tables and figures. AUTHOR INFORMATION ORCID Yuanqiang Wang: 0000-0001-7754-2148 Yangqing Tao: 0000-0002-2953-2938 Junfeng Zhou: 0000-0002-8201-6799 Jing Sun: 0000-0002-1714-0283 Qiang Fang: 0000-0002-3549-5600 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT (Financial support from the Ministry of Science and Technology of China (2015CB931900), the Natural Science Foundation of China (NSFC, No. 21574146, 21504103 and 21774142), the Science and Technology Commission of Shanghai Municipality (16JC1403800), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB 20020000) is gratefully acknowledged Reference (1). Zhang, Z.; Wang, D. H.; Litt, M. H.; Tan, L.-S.; Zhu, L. High-Temperature and High-Energy-Density Dipolar Glass Polymers Based on Sulfonylated Poly(2,6dimethyl-1,4-phenylene oxide). Angew. Chem. Int. Ed. 2017, 57, 1528-1531, DOI 16 ACS Paragon Plus Environment

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10.1002/anie.201710474. (2). Habaue, S.; Iwai, S.; Kubo, H.; Nagura, K.; Watanabe, T.; Muraki, Y.; Tsutsui, Y. synthesis of poly(2,6-dimethyl-1,4-phenylene oxide) derivatives containing hydroxyl and amino groups by oxidative coupling copolymerization. React. Funct. Polym. 2014, 83, 49-53, DOI 10.1016/j.reactfunctpolym.2014.07.009. (3). Chen, C.-W.; Lin, I. H.; Lin, C.-C.; Lin, J.-L.; Horie, M. Synthesis of poly(2,6dimethyl-1,4-phenylene oxide) derivatives in water using water-soluble copper complex catalyst with natural ligands. Polymer 2013, 54, 5684-5690, DOI 10.1016/j.polymer.2013.08.023. (4). Lin, C. H.; Tsai, Y. J.; Shih, Y. S.; Chang, H. C. Catalyst-free synthesis of phosphinated poly(2,6-dimethyl-1,4-phenylene oxide) with high-Tg and low-dielectric characteristic.

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The table of contents

New thermosetting poly(2,6-dimethylphenylene oxide)s (PPOs) were developed by functionalization with biomass anethole, which greatly improved Tg and dimensional stability of the polymers without degrading their dielectric properties.

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