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Synthesis and gas permeability of methylol-group-containing Poly(diphenylacetylene)s with high CO2 permeability and permselectivity
Polymer 190 (2020) 122230
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Synthesis and gas permeability of methylol-group-containing Poly (diphenylacetylene)s with high CO2 permeability and permselectivity Toshikazu Sakaguchi *, Aiko Takeda , Tamotsu Hashimoto Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, Bunkyo 3-9-1, Fukui, 910-8507, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Membrane Gas separation CO2 permeability poly(diphenylacetylene) Methylol group
Diphenylacetylene monomers bearing a methylol group protected by a t-butyldiphenylsilyl group [tBuPh2 – SiOCH2C6H4C– –CC6H4R; R ¼ SiMe3 (1a) and R ¼ SiMe2nC18H37 (1b)] were synthesized and then polymerized with TaCl5/n-Bu4Sn to afford the corresponding poly(diphenylacetylene)s 2a and 2b, respectively. The molecular weights of 2a and 2b were high (i.e., Mw ¼ 1,020,000 and 1,560,000, respectively), and free-standing mem branes were obtained by solution casting. The deprotection of membranes of 2a and 2b using n-Bu4NþF afforded poly(diphenylacetylene)s bearing methylol and silyl groups (3a and 3b). On the other hand, treatment of membranes of 2a and 2b with trifluoroacetic acid led to deprotection and desilylation, producing poly (diphenylacetylene)s bearing trifluoroacetate moieties (4a and 4b, respectively), and then treatment with methanol afforded poly(diphenylacetylene)s bearing only methylol groups (5a and 5b). The carbon dioxide permeability coefficients (PCO2) of membranes of 2a and 2b were 99 and 78 Barrer, respectively. Deprotection led to the increment of CO2 permeability without the decline of the CO2 permselectivity. Poly(diphenylacetylene) s bearing methylol and silyl groups (3a and 3b) exhibited high permeability (PCO2 ¼ 2000 and 120 Barrer, respectively), but not high permselectivity (PCO2/PN2 ¼ 17 and 16, respectively). Methylol-group-containing poly (diphenylacetylene) without silyl groups 5b exhibited the best CO2 permeation property, i.e., high CO2 permeability and high permselectivity (PCO2 ¼ 220 Barrer, PCO2/PN2 ¼ 32).
1. Introduction Gas separation has been increasingly applied for hydrogen recovery, nitrogen generation, oxygen-enriched air production, hydrocarbon pu rification, and carbon dioxide removal. In particular, the capture and storage of carbon dioxide has attracted considerable attention because global warming and climate change are presumably caused by the increment in the concentration of carbon dioxide in the atmosphere. Unlike separation techniques such as distillation and adsorption, mem brane separation has been investigated because of its cost-effectiveness, low energy consumption, facile operation, and small footprint [1–3]. Thus far, various polymers have been investigated for gas separation membranes, and poly(substituted acetylene)s [4–7] and polymers of intrinsic microporosity [8–10] have been considered as promising ma terials because they exhibit high gas permeability, which is mainly related to the extremely high free volume. However, such high free volume generally leads to the decline of permselectivity. Our group has synthesized various substituted acetylene polymers and investigated their gas permeability [5,6]. A majority of poly
(substituted acetylene)s exhibit high gas permeability and low permse lectivity, while poly(substituted acetylene)s bearing polar groups exhibit relatively high CO2 permselectivity [11–17]. Polar-group-containing polymers are known to exhibit a good affinity toward CO2 molecules; thus, the solubility of CO2 in polymer membranes increases [18]. For example, sulfonated poly(diphenylacetylene) exhibits high CO2 perm selectivity, with a separation factor (PCO2/PN2) of 54. However, the CO2 permeability coefficient (PCO2) is as low as 3.9 Barrer [13]. Poly(diphe nylacetylene) bearing phenolic hydroxyl groups exhibits good permse lectivity (PCO2/PN2 ¼ 48) and a relatively high CO2 permeability (PCO2 ¼ 110 Barrer) [19]. In this way, polar groups increase the CO2 permse lectivity but decrease the permeability because the interaction between polymer chains is strengthened by the high polarity. Previously, our group has synthesized poly(diphenylacetylene) bearing silanol groups, the polarity of which is not extremely high [20]. The polymer membrane exhibits a high PCO2 of 1200 Barrer, but its permselectivity is not high (PCO2/PN2 ¼ 15). In this regard, it is imperative to synthesize a polymer with appropriate polarity to achieve good CO2 separation performance. In this study, novel poly(diphenylacetylene)s bearing methylol
* Corresponding author. E-mail address: [email protected] (T. Sakaguchi). https://doi.org/10.1016/j.polymer.2020.122230 Received 26 November 2019; Received in revised form 21 January 2020; Accepted 25 January 2020 Available online 27 January 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.
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groups is developed. Poly(diphenylacetylene) derivatives are well known to be synthesized by metathesis polymerization using a TaCl5 catalyst [21,22]. This TaCl5 catalyst is deactivated by polar groups such as alcohol, which in turn does not permit the polymerization of polar-group-containing diphenylacetylene monomers. Hence, dipheny lacetylenes bearing methylol groups protected by silyl groups are syn thesized and subsequently polymerized by metathesis polymerization (Scheme 1). The deprotection of the obtained polymers afforded methylol-group-containing poly(diphenylacetylene)s. The membrane of poly(diphenylacetylene) bearing methylol groups exhibited higher CO2 permeability than that of poly(diphenylacetylene) bearing phenolic hydroxyl groups, as well as higher CO2 permselectivity than that of poly (diphenylacetylene) bearing silanol groups.
addition of p-trimethylsilylphenylacetylene (11.2 g, 64.2 mmol) in triethylamine (50 mL) at room temperature. The reaction mixture was stirred at reflux temperature for 24 h and then allowed to cool to room temperature. After the evaporation of triethylamine, the crude product was dissolved in ether (300 mL), and the insoluble salt was filtered off. The solution was washed three times using 1.0 M HClaq, and ether was evaporated. This product was purified by silica-gel column chromatog raphy (eluent: CHCl3) to afford 1-(p-trimethylsilyl)phenyl-2-(p-hydrox ymethyl)phenylacetylene (9.3 g, 62% yield). First, 1-(p-trimethylsilyl)phenyl-2-(p-hydroxymethyl)phenylacetylene (8.3 g, 29.6 mmol), imidazole (10.0 g, 147 mmol), and DMF (100 mL) were transferred to a 500 mL three-necked flask equipped with a dropping funnel, a three-way stopcock, and a magnetic stirring bar. Second, a so lution of t-butyldiphenylchlorosilane (12.2 g, 44.4 mmol) in DMF (50 mL) was added dropwise at 0 � C under nitrogen. After the addition, the reaction mixture was stirred for 10 h at room temperature. Next, ether (200 mL) was added to the flask, and the solution was washed three times with water. The ethereal solution was dried over anhydrous sodium sulfate and then concentrated in vacuo. The crude product was purified by silica-gel col umn chromatography (eluent: hexane/chloroform (9/1)) to afford the desired product 1a (10.1 g, 66%) as a white solid. 1H NMR (CDCl3, ppm): 7.76–7.62 (m, 4H, Ar), 7.59–7.28 (m, 14H, Ar), 4.78 (s, 2H, CH2O), 1.12 (s, 9H, C(CH3)3), 0.26 (s, 9H, Si(CH3)3). 13C NMR (CDCl3, ppm): 141.3, 140.9, 135.5, 133.3, 131.5, 130.6, 129.8, 127.8, 125.9, 123.6, 121.7, 89.9, 89.2, 65.2, 26.8, 19.3, 1.2. Anal. Calcd for C34H38OSi2: C, 78.7; H, 7.38; O, 3.08; Si, 10.83. Found: C, 79.8; H, 7.36.
2. Experimental 2.1. Materials The polymerization solvent, toluene, was purified by distillation over calcium hydride. The main catalyst, TaCl5 (Aldrich, 99.999%), was used without further purification. The cocatalyst, n-Bu4Sn (Wako, Japan), was purified by distillation over calcium hydride. p-Bromobenzyl alcohol, t-butyldiphenylchlorosilane, tetra-n-butylammonium fluoride (n-Bu4NþF ), trifluoroacetic acid, and typical organic solvents were purchased from Wako, Japan and used without further purification. pTrimethylsilylphenylacetylene and p-dimethyl-n-octadecylsilylphenyla cetylene were synthesized according a previously reported study [23]. Monomers were synthesized according to Scheme 2. The synthetic procedures and analytical data of monomers are as follows.
2.3. Synthesis of 1-(p-dimethyl-n-octadecylsilyl)phenyl-2-[p-(tbutyldimethylsiloxymethyl)] phenylacetylene (1b) Monomer 1b was prepared by the same method as that adopted for 1a using p-dimethyl-n-octadecylsilylphenylacetylene instead of p-tri methylsilylphenylacetylene. Overall yield: 49%, white solid. 1H NMR (CDCl3, ppm): 7.75–7.66 (m, 4H, Ar), 7.58–7.31 (m, 14H, Ar), 4.78 (s, 2H, CH2O), 1.46–1.16 (m, 32H, CH2(CH2)16CH3), 1.10 (s, 9H, C(CH3)3), 0.88 (t, 3H, CH2(CH2)16CH3), 0.74 (t, 2H, CH2(CH2)16CH3), 0.25 (s, 9H, Si(CH3)3). 13C NMR (CDCl3, ppm): 141.3, 140.2, 135.5, 133.4, 131.5, 130.6, 129.7, 127.8, 125.9, 123.6, 121.7, 89.9, 89.2, 65.2, 33.6, 31.9, 29.7, 29.6, 29.3, 26.8, 23.8, 22.7, 19.3, 15.6, 14.2, 3.1. Anal. Calcd for
2.2. Synthesis of 1-(p-trimethylsilyl)phenyl-2-[p-(tbutyldimethylsiloxymethyl)] phenylacetylene (1a) First, a 500 mL three-necked flask was equipped with a reflux condenser, a three-way stopcock, and a magnetic stirring bar, followed by flushing with nitrogen. Second, p-bromobenzyl alcohol (10.0 g, 53.5 mmol), PdCl2(Ph3P)2 (0.0964 g, 0.138 mmol), Ph3P (0.157 g, 0.598 mmol), and CuI(I) (0.149 g, 0.780 mmol) were added into the flask. Third, triethylamine (150 mL) was added into the flask, followed by the
Scheme 1. Synthesis of poly(diphenylacetylene) bearing methylol groups. 2
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Scheme 2. Synthesis of diphenylacetylene monomers.
C51H72OSi2: C, 80.9; H, 9.58; O, 2.11; Si, 7.42. Found: C, 80.1; H, 9.20.
2.6. Deprotection (synthesis of 3a and 3b)
2.4. Polymerization
Deprotection of 2a and 2b was carried out using n-Bu4NþF in THF at 65 � C for 24 h. A detailed procedure is as follows: Polymer 2a (0.32 g, 0.62 mmol repeating unit) was added into a flask and then flushed with nitrogen. Dry THF (10 mL) was added into the flask, and the solution was stirred until the polymer was completely dissolved. A 1 M THF so lution of n-Bu4NþF (6.2 mL, 10 equiv.) was then added to the solution and stirred at 65 � C for 24 h. The polymer was then isolated by pre cipitation into a large excess of water and dried under reduced pressure. The polymer was dissolved in THF again and purified by precipitation into hexane. The polymer was dried at room temperature for 24 h under reduced pressure.
Polymerization was carried out in a glass tube equipped with a threeway stopcock under dry nitrogen. Unless otherwise specified, the reac tion was carried out at 80 � C for 24 h under the following conditions: [Monomer] ¼ 0.20 M, [TaCl5] ¼ 40 mM, and [n-Bu4Sn] ¼ 400 mM. A detailed polymerization procedure is as follows: The monomer solution was prepared in a glass tube by mixing 1a (0.52 g) and dry toluene (2.5 mL). Another glass tube was charged with TaCl5 (36 mg) and 0.8 M of nBu4Sn in toluene (2.5 mL); the resulting catalyst solution was aged at 80 � C for 10 min. The monomer solution was then added to the catalyst solution. Polymerization was carried out at 80 � C for 24 h, which was quenched using a small amount of methanol. Toluene (25 mL) was added into the flask, and the insoluble product was separated using a glass filter. The toluene-soluble polymer was isolated by precipitation into a large excess of acetone, and its yield was determined gravimetrically.
2.7. Desilylation (synthesis of 4a and 4b) The desilylation of 2a and 2b was performed with trifluoroacetic acid using polymer membranes. Membranes of 2a and 2b were immersed in trifluoroacetic acid at room temperature for 24 h, followed by washing with hexane and immersing in hexane for 24 h. The obtained membranes were dried at room temperature for 24 h under reduced pressure.
2.5. Measurements The molecular weights and polydispersity ratios of polymers were estimated by gel permeation chromatography (THF was used as the eluent, polystyrene was used for calibration) at 40 � C on a Shimadzu LC10AD chromatograph equipped with three polystyrene gel columns (Shodex KF-8025 � 1 and A-80M � 2) and a Shimadzu RID-6A refractive index detector. IR spectra were recorded on a Nicolet Magna 560 spectrometer. Thermogravimetric analyses (TGA) were conducted with Rigaku TG-DTA 8078G1 in nitrogen at a 10 � C min 1 heating rate. NMR spectra were recorded on a Jeol ECX-500 spectrometer. Elemental an alyses of monomers were performed at A-Rabbit-Science Japan Co., Ltd. Gas permeability coefficients of polymer membranes were measured using a Rikaseiki K-315-N gas permeability apparatus at 25 � C at an upstream pressure of 1 atm. The permeability coefficient P expressed in unit of Barrer (1 Barrer ¼ 10 10 cm3(STP) cm 2 s 1 cmHg 1) was calculated from the slope of the steady-state line. The diffusion coeffi cient (D) value was determined by the time lag method using the following equation:
2.8. Conversion from trifluoroacetate to methylol (synthesis of 5a and 5b) The reaction was performed using membranes of 4a and 4b. Mem branes of 4a and 4b were immersed in methanol/triethylamine solution (2/1 vol ratio) at room temperature for 24 h, followed by washing with methanol and immersing in methanol for 24 h. The obtained membranes were dried at room temperature for 24 h under reduced pressure. 2.9. Membrane fabrication Membranes (30–60 μm thickness) of 2a and 2b were fabricated by casting their toluene solutions (0.4 wt% concentration) into Petri dishes. The dish was covered with a glass vessel to decrease the evaporation rate of solvent (3–4 days). After the formation of a membrane, the membrane was peeled off, and it was immersed in methanol for 24 h and dried to constant weight at room temperature. Membranes of 3a and 3b were fabricated by casting their THF so lutions (0.4 wt% concentration) into Petri dishes. The dish was covered with a glass vessel to decrease the evaporation rate of the solvent (2–3 days). After the formation of a membrane, the membrane was peeled off, and it was immersed in hexane for 24 h and dried to constant weight at room temperature.
D ¼ l2/6θ Here, l is the membrane thickness, and θ is the time lag, which is expressed by the intercept of the asymptotic line of the time–pressure curve to the time axis. The solubility coefficient (S) was calculated by using equation S ¼ P/D.
3
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2.10. Fractional free volume (FFV) of polymer membranes
(runs 1 and 4).
Membrane density was determined by hydrostatic weighing using a Mettler Toledo balance and a density determination kit. In this method, a liquid with a known density (ρ0) is required, and the membrane den sity (ρ) is expressed by the following equation:
3.2. Deprotection by n-Bu4NþF
ρ ¼ ρ0 � MA / (MA
The deprotection of 2a was carried out using n-Bu4NþF in THF at 65 � C for 24 h to produce 3a bearing hydroxy groups (Scheme 1). Fig. 1 shows the IR spectra of the polymers before and after the reaction. In the IR spectrum of 2a, an absorption peak observed at 1080 cm 1 corre sponded to the asymmetric stretching of Si–O–CH2 in the siloxy groups, while in the IR spectrum of 3a, an absorption peak at 1080 cm 1 was not observable. Furthermore, a broad absorption peak characteristic of the O–H stretching of the hydroxyl groups was observed at 3400 cm 1. Absorption peaks corresponding to trimethylsilyl groups were observed at 1260 and 1120 cm 1 in the IR spectrum of 2a. The intensities of these absorption peaks did not decrease after the reaction, suggesting that trimethylsilyl groups are not eliminated by the reaction. These results revealed the selective desilylation and generation of polymer 3a bearing hydroxyl and trimethylsilyl groups. The deprotection of 2b was also performed in THF at 65 � C (Fig. 2). Similar to the case of 3a, an ab sorption peak at 1080 cm 1 corresponding to Si–O–CH2 disappeared and an absorption peak observed at 3400 cm 1 corresponding to O–H were observed after the reaction. Furthermore, absorption peaks observed at 1260 and 1120 cm 1 did not clearly change, suggesting that the reaction using n-Bu4NþF affords polymer 3b bearing hydroxyl and dimethyl-noctadecylsilyl groups.
ML)
where MA is the weight of the membrane in air, and ML is the weight of the membrane in an auxiliary liquid. Aqueous sodium nitrate was used as the auxiliary liquid. FFV is calculated by the following equation [24]. FFV ¼ (vsp
v0)/vsp � (vsp
1.3 vw)/vsp
where vsp is the specific volume of the polymer, and v0 is the volume occupied by the polymer. The occupied volume is typically estimated as 1.3 times the van der Waals volume (vw), which is calculated by using the group contribution method [25]. 3. Results and discussion 3.1. Polymerization The polymerization of monomers (1a and 1b) was carried out using the TaCl5/n-Bu4Sn catalyst in toluene and cyclohexane at 80 � C. The molecular weight and yields of the polymers obtained by the polymer ization of diarylacetylene derivatives in cyclohexane have been reported to be greater than those obtained by the polymerization of the same derivatives in toluene [26,27]. Table 1 summarizes the results obtained from polymerization. The polymerization of 1a in toluene afforded a polymer in 33% yield, but it contained solvent-insoluble parts. The obtained polymer was dissolved in toluene, and the insoluble parts were filtered off. The yield of the toluene-soluble part was 21%, with a high Mw of 1,020,000 g/mol. The polymerization of 1a in cyclohexane afforded a polymer with a higher molecular weight in higher yield (87%). However, most parts were insoluble in all solvents, and the yield of the toluene-soluble part was only 5.6%. The insolubility will be caused by too high molecular weight because the IR spectrum of insol uble part is the same as that of soluble part. Monomer 1b bearing an n-octadecyl group polymerized in toluene afforded a polymer in a yield of 3.5%, without insoluble parts. Its Mw was 453,000 g/mol; this value is considerably less than that of polymer 2a. When cyclohexane was used as the polymerization solvent, the yield and molecular weight increased up to 68% and 1,560,000 g/mol, respectively. The yield of the toluene-soluble part was 45%; this value is considerably greater than that of 2a. In the metathesis polymerization of disubstituted acetylenes, the polymerizability of monomers is critically dependent on the steric hindrance of substituents [21,22]. Compared to 1a, monomer 1b bearing a long alkyl chain exhibited low polymerizability probably because of the steric effect. The solubility of the polymer improved due to long alkyl side chains. Membrane fabrication and polymer reaction were examined using the soluble polymers with relatively high yields
3.3. Desilylation by trifluoroacetic acid Desilylation by trifluoroacetic acid was performed using freestanding membranes because the obtained polymers were insoluble in all solvents, and the membranes could not be fabricated by solution casting. Membranes of 4a and 4b were obtained by the desilylation of membranes of 2a and 2b, respectively, using trifluoroacetic acid. Membranes of 5a and 5b were then obtained by the treatment of membranes of 4a and 4b, respectively, with methanol. Fig. 1 shows the IR spectra of 4a and 5a. In the spectrum of 4a, new absorption peaks – O, C(¼O)O, and C–F bonds were observed at 1,780, corresponding to C– 1,240, and 1140 cm 1, respectively. The absorption peak corresponding to the O–H bond around 3400 cm 1 was not observed. The absorption
Table 1 Polymerization of monomers 1a and 1b with TaCl5–n-Bu4Sna. Run
Monomer
Solvent
Yield, %b
Yield (soluble part), %c
Mw � 10 6 g/ mold
Mw/ Mn d
1 2 3 4
1a
toluene cyclohexane toluene cyclohexane
33 87 3.5 68
21 5.6 3.5 45
1.02 2.38 0.453 1.56
3.44 3.50 2.65 3.02
a b c d
1b
At 80 � C for 24 h; [M]0 ¼ 0.20 M, [TaCl5] ¼ 40 mM, [n-Bu4Sn] ¼ 400 mM. Acetone-insoluble product. Toluene-soluble and acetone-insoluble products. Measured by GPC.
Fig. 1. IR spectra of membranes of 2a, 3a, 4a, and 5a. 4
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Fig. 2. IR spectra of membranes of 2b, 3b, 4b, and 5b.
peak at 1080 cm 1 corresponding to the siloxy group and the peak at 1260 cm 1 corresponding to the trimethylsilyl group disappeared, indicating that both silyl groups are eliminated by trifluoroacetic acid. The strong absorption peak observed at 700 cm 1 in the spectrum of 4a, characteristic of monosubstituted benzene, was not observed in the spectra of 2a and 3a. This result supported the elimination of trime thylsilyl groups. Hence, the obtained polymer is thought to possess tri fluoroacetate groups (-OCOCF3) without trimethylsilyl groups (Scheme 1). The trifluoroacetate groups were easily decomposed by the treatment with methanol and hydroxyl groups formed in the polymer. In the spectrum of 5a, a broad absorption peak at around 3400 cm 1 corre sponded to the O–H bond, and absorption peaks at 1,780, 1,240, and 1140 cm 1, corresponding to trifluoroacetate groups were not apparent, indicating that polymer 5a has methylol groups and no silyl groups (Scheme 1). Polymer 2b bearing dimethyl-n-octadecylsilyl groups was desilylated by trifluoroacetic acid under the same conditions as those utilized for 2a. Fig. 2 shows the IR spectra of polymers. The spectral
variations were similar to the case of 2a–5a; i.e., the desilylation of both silyl groups occurred by the treatment with trifluoroacetic acid, affording polymer 4b bearing trifluoroacetate groups. Polymer 5b bearing methylol groups was then obtained by the treatment of 4b with methanol. 3.4. Solvent solubility Table 2 summarizes the solubility of the obtained polymers. Poly mers 2a and 2b obtained by polymerization contained insoluble parts, but this solubility test was conducted using toluene-soluble products. The polymers bearing t-butyldiphenylsiloxy groups (2a and 2b) dis solved in common organic solvents such as CCl4, CHCl3, and THF. The polymers bearing n-octadecylsilyl groups 2b exhibited relatively good solubility and dissolved even in hexane and diethyl ether. Polymer 3a, which was synthesized by the deprotection of 2a using n-Bu4NþF , was soluble in high-polarity solvents such as THF, acetone, and DMF, but it
Table 2 Solubility of polymers. 2a 2b 3a 3b 4a 4b 5a 5b
Hexane
CCl4
Toluene
Et2O
CHCl3
THF
Acetone
DMF
� þ – – – – – –
þ þ – � – – – –
þ þ – � – – – –
� þ � þ – – – –
þ þ � þ – – – –
þ þ þ þ – – – –
– – þ – – – – –
– – þ – – – – –
Symbols: þ, soluble; �, partly soluble; –, insoluble. 5
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was insoluble in low-polarity solvents such as CCl4 and toluene. The solubility change was due to the generation of hydroxyl groups. On the other hand, polymer 3b did not completely dissolve in low-polarity solvent such as toluene and high-polarity solvents such as acetone and DMF. The solubility of 3b can be attributed to the combination of hy drophilic hydroxyl groups and hydrophobic n-octadecyl groups. Desi lylation using trifluoroacetic acid afforded polymers with lower solubility. Polymers 4a and 4b were insoluble in all solvents. Polymers 5a and 5b also exhibited poor solubility and did not dissolve in any solvents, indicating that the silyl groups on the side chains are crucial to conferring solvent solubility to poly(diphenylacetylene) derivatives.
Table 3 Gas permeation properties and FFV of polymer membranes. Membrane
PO2a
PN2a
PCO2a
PO2/PN2
PCO2/PN2
Densityb
FFVc
2a 3a
20 330
5.6 120
99 2000
3.6 2.8
18 17
1.10 1.01
0.131 0.214
2b 3b 4b 5b
15 22 38 27
5.0 7.4 12 6.9
78 120 230 220
3.0 3.0 3.2 3.9
16 16 19 32
1.03 0.982 1.25 1.20
0.140 0.159 0.226 0.130
a
In the unit of Barrer [1 Barrer ¼ 1 � 10 10 cm3 (STP) cm 2 s 1 cmHg 1]. Determined by hydrostatic weighing. In the units of g cm 3. c FFV ¼ fractional free volume. Calculated using the group contribution method. b
3.5. Thermal property Thermogravimetric analyses (TGA) of 2a, 2b, 3a, and 3b were conducted in N2 (Fig. 3). The onset temperature of weight loss (T0) for 2a was 325 � C, showing its high thermal stability. The T0 values of 2b, 3a, and 3b were somewhat lower than that of 2a, but they indicated more than 270 � C. Poly(diphenylacetylene) without any substituents show very high thermal stability, and its T0 is 480 � C [28]. On the other hand, poly(diphenylacetylene)s possessing various silyl groups show the T0’s of 260–420 � C [29]. These results imply that the thermal degra dation of poly(diphenylacetylene)s occurs at the substituents connected to benzene rings at first. Anyway, the present polymers (2a, 2b, 3a, and 3b) have enough high thermal stability to apply for gas separation membranes. 3.6. Gas permeability The permeability of the membranes to oxygen, nitrogen, and carbon dioxide was examined at 25 � C (Table 3). Table 3 also summarizes the density and FFV of the membranes. Fig. 4 plots the PCO2 and separation factor (PCO2/PN2) data on a log–log scale. The PO2, PN2, and PCO2 of 2a were 20, 5.6, and 99 Barrer, respectively. These values are comparable to those reported previously for bulky-substituent-containing poly (diphenylacetylene)s [30–33]. For instance, the PO2 values for poly (diphenylacetylene)s bearing triisopropylsilyl and dimethylphenylsilyl groups are 20 and 26 Barrer, respectively [30,31]. The deprotection of 2a led to the drastic increase in the gas permeability. The PO2, PN2, and PCO2 of 3a were 330, 120, and 2000 Barrer, respectively. The FFV value of 3a was 0.214; this value is considerably greater than that of 2a. The phenyl groups of t-butyldiphenylsiloxy groups in the side chains could easily exhibit polymer chain packing due to their π–π interaction. Hence, the elimination of t-butyldiphenylsilyl groups leads to the increment of
Fig. 4. Relationship between PCO2 and PCO2/PN2 for the current polymers and the related poly(diphenylacetylene)s.
FFV. The PCO2/PN2 ratio of 3a was 17; this value is similar to that of 2a, indicating that the hydroxyl groups in 3a improve the CO2 permse lectivity. Generally, highly gas-permeable polymers exhibit low selec tivity. As shown in Fig. 4, the Robeson upper bound determined in 2008 represents the experimental limit of the separation performance of polymer membranes [34]. High-performance membranes for gas sepa ration were located beside the upper bound. Compared with that of 2a, the plot of 3a clearly lied near the upper bound (Fig. 4). Unfortunately, membranes of 4a and 5a were too brittle; hence, their gas permeation properties could not be investigated. Membranes of 2b and 3b exhibited a similar tendency to those of 2a and 3a; i.e., deprotection led to the enhancement of permeability without a decrease in permselectivity. However, the permeability of 3b was considerably less than that of 3a. Polymer 3b comprises n-octadecyl groups; these long alkyl groups occupy the free volume in a high-freevolume polymer such as poly(substituted acetylene)s. Previous studies have reported that the introduction of a long alkyl chain into poly (diphenylacetylene) leads to the decrease of the FFV [27,35]. Desilyla tion using trifluoroacetic acid leads to an increase in the gas perme ability. The PO2, PN2, and PCO2 of 4b were 38, 12, and 230 Barrer, respectively. Bulky t-butyldiphenylsilyl and dimethyl-n-octadecylsilyl groups were eliminated to increase the FFV of the membrane. The membrane of 5b exhibited low FFV and low gas permeability compared with those of 4b because of the presence of methylol groups in 5b. The
Fig. 3. TGA curves of 2a, 2b, 3a, and 3b (in N2, heating rate 10 � C min 1). 6
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PO2 and PN2 of 5b were similar to those of 3b bearing methylol and n-octadecyl groups. However, the PCO2 of 5b was roughly two times greater than that of 3b; thus, the permselectivity of 5b is two times that of 3b. The membrane of 5b exhibited good performance as a CO2 sep aration membrane (Fig. 3). Poly(diphenylacetylene) bearing phenolic hydroxy groups exhibited high CO2 permselectivity but low perme ability (PCO2 ¼ 110 Barrer, PCO2/PN2 ¼ 50) [19]. Poly(diphenylacety lene)s bearing aliphatic hydroxy groups used as CO2 separation materials in this study exhibited higher gas permeability. 3.7. CO2 diffusivity and solubility for polymer membranes The gas permeability of polymer membranes can be expressed in terms of diffusivity and solubility. Theoretically, the gas permeability coefficient (P) is expressed as a product of the gas diffusion coefficient (D) and the gas solubility coefficient (S) [P ¼ D � S]. To investigate the variation of gas permeability for the current polymers in detail, CO2 diffusion coefficients (DCO2) and CO2 solubility coefficients (SCO2) were calculated from the time lag of the permeation measurement. However, we could not obtain the diffusivity and solubility for oxygen and nitro gen because their time lags were extremely small for precise calculation. Fig. 5 shows the DCO2 and SCO2 data for the membranes. The large in crease in the CO2 permeability for 3a originated from the increase in diffusivity and solubility. This result can be explained by the large FFV and the generation of methylol groups. In the case of polymers 2b and 3b, we observed a similar trend although the changes were extremely small. Comparison of DCO2 and SCO2 between 3b and 5b showed that the values were quite different. Polymer 3b exhibited a large DCO2 value, corresponding to the flexible long chains of n-octadecyl groups. On the other hand, polymer 5b exhibited a large SCO2 value, which possibly related to the high affinity to CO2 molecules. Polymer 4b exhibited in termediate D and S values. Polymer 3b also possessed methylol groups, but the affinity to CO2 molecules probably low because the contribution of methylol groups in 3b was considerably less than that in 5b from the molecular weight of the repeating unit. Notably, the introduction of methylol groups into poly(diphenylacetylene)s is effective in improving the CO2 solubility.
Fig. 5. CO2 diffusion coefficients (D) and solubility coefficients (S) of poly mer membranes.
References [1] Z.-X. Low, P.M. Budd, N.B. McKeown, D.A. Patterson, Gas permeation properties, physical aging, its mitigation in high free volume glassy polymers, Chem. Rev. 118 (2018) 5871–5911. [2] J. Liu, X. Hou, H.B. Park, H. Lin, High-performance polymers for membrane CO2/ N2 separation, Chem. Eur J. 22 (2016) 1–12. [3] R.W. Baker, B.T. Low, Gas separation membrane materials: a perspective, Macromolecules 47 (2014) 6999–7013. [4] T. Sakaguchi, Y. Hu, T. Masuda, Substituted polyacetylenes, in: Y. Yampolskii, E. Finkelshtein (Eds.), Membrane Materials for Gas and Vapor Separation, Wiley, 2017, pp. 107–142. [5] T. Sakaguchi, T. Hashimoto, Synthesis of poly(diphenylacetylene)s bearing various polar groups and their gas permeability, Polym. J. 46 (2014) 391–398. [6] T. Sakaguchi, K. Yumoto, Y. Shida, M. Shiotsuki, F. Sanda, T. Masuda, Synthesis, properties, and gas permeability of novel poly(diaryacetylene) derivatives, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 5028–5038. [7] K. Nagai, T. Masuda, T. Nakagawa, B.D. Freeman, I. Pinnau, Poly[1(trimethylsilyl)-1-propyne] and related polymers: synthesis, properties and functions, Prog. Polym. Sci. 26 (2001) 721–798. [8] P.M. Budd, B.S. Ghanem, S. Makhseed, N.B. McKeown, K.J. Msayib, C. E. Tattershall, Polymers of intrinsic microporosity (PIMs): robust, solutionprocessable, organic nanoporous materials, Chem. Commun. (2004) 230–231. [9] P.M. Budd, E.S. Elabas, B.S. Ghanem, S. Makhseed, N.B. McKeown, K.J. Msayib, C. E. Tattershall, D. Wang, Solution-processed, organophilic membrane derived from a polymer of intrinsic microporosity, Adv. Mater. 16 (2004) 456–459. [10] N.B. McKeown, The synthesis of polymers of intrinsic microporosity, Sci. China Chem. 60 (2017) 1023–1032. [11] T. Sakaguchi, K. Kameoka, T. Hashimoto, Synthesis of PEG-functionalized poly (diphenylacetylene)s and their gas permeation properties, J. Appl. Polym. Sci. 113 (2009) 3504–3509. [12] T. Sakaguchi, K. Kameoka, T. Hashimoto, Synthesis of sulfonated poly (diphenylacetylene)s with high CO2 permselectivity, J. Polym. Sci., Part A: Polym. Chem. 47 (2009) 6463–6471. [13] T. Sakaguchi, Y. Shinoda, T. Hashimoto, A series of poly(diphenylacetylene)s bearing sulfonic acids: synthesis, characterization, and gas permeability, Polymer 53 (2012) 5268–5278. [14] T. Sakaguchi, Y. Shinoda, T. Hashimoto, Synthesis and gas permeability of nitrated and aminated poly(diphenylacetylene)s, Polymer 55 (2014) 6680–6685. [15] T. Sakaguchi, T. Tsuzaki, T. Masuda, T. Hashimoto, Synthesis, gas permeability, and metal-induced gelation of poly(disubstituted acetylene)s having p,mdimethoxyphenyl and p,m-dihydroxyphenyl groups, Polymer 55 (2014) 1977–1983. [16] T. Sakaguchi, H. Ito, T. Masuda, T. Hashimoto, Highly CO2-permeable and –permselective poly(diphenylacetylene)s having imidazolium salts: synthesis, characterization, gas permeation properties, and effects of counter anion, Polymer 54 (2013) 6709–6715. [17] T. Sakaguchi, S. Kaji, T. Hashimoto, Synthesis and CO2 permeation properties of sulfonium-substituted poly(diphenylacetylene)s, Polym. Bull. 75 (2018) 3011–3022.
4. Conclusions Methylol-group-containing poly(diphenylacetylene)s were synthe sized by the metathesis polymerization of diphenylacetylene bearing siloxy groups, followed by deprotection with n-Bu4NþF or trifluoro acetic acid. The membrane of poly(diphenylacetylene) bearing methylol and trimethylsilyl groups exhibited high CO2 permeability and relatively high CO2 permselectivity (PCO2 ¼ 2000 Barrer, PCO2/PN2 ¼ 17). The membrane of poly(diphenylacetylene) bearing methylol groups exhibi ted relatively high CO2 permeability and high CO2 permselectivity (PCO2 ¼ 220 Barrer, PCO2/PN2 ¼ 32). The CO2 permeability of the current polymers was greater than that of poly(diphenylacetylene)s bearing phenolic hydroxyl groups and sulfonic acid. Methylol groups were found to provide a good balance of CO2 permeability and permselectivity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Toshikazu Sakaguchi: Conceptualization, Methodology, Visualiza tion, Investigation, Validation, Writing - original draft. Aiko Takeda: Data curation, Software, Investigation. Tamotsu Hashimoto: Supervi sion, Writing - review & editing. 7
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Polymer 190 (2020) 122230 [27] T. Sakaguchi, M. Shiotsuki, T. Masuda, Synthesis and properties of Si-containing poly(diarylacetylene)s and their desilylated polymer membranes, Macromolecules 37 (2004) 4104–4108. [28] A. Niki, T. Masuda, T. Higashimura, Effects of organometallic cocatalysts on the polymerization of disubstituted acetylenes by TaCl5 and NbCl5, J. Polym. Sci., Part A: Polym. Chem. 25 (1987) 1553–1562. [29] T. Sakaguchi, K. Yumoto, M. Shiotsuki, F. Sanda, M. Yoshikawa, T. Masuda, Synthesis of poly(diphenylacetylene) membranes by desilylation of various precursor polymers and their properties, Macromolecules 38 (2005) 2704–2709. [30] M. Teraguchi, T. Masuda, Polymerization of diphenylacetylenes having very bulky silyl groups and polymer properties, J. Polym. Sci., Part A: Polym. Chem. 36 (1998) 2721–2725. [31] K. Tsuchihara, T. Masuda, T. Higashimura, Polymerization of Si-containing acetylenes. XIV. Polymerization of diphenylacetylenes with bulky silyl groups and polymer properties, J. Polym. Sci., Part A: Polym. Chem. 31 (1993) 547–552. [32] H. Kouzai, T. Masuda, T. Higashimura, Polymerization and polymer properties of diarylacetylenes, Polymer 35 (1994) 4920–4923. [33] T. Aoki, Y. Kobayashi, T. Kaneko, E. Oikawa, Y. Yamamura, Y. Fujita, M. Teraguchi, R. Nomura, T. Masuda, Synthesis and properties of polymers from disubstituted acetylenes with chiral pinanyl groups, Macromolecules 32 (1999) 79–85. [34] L.M. Robeson, The upper bound revised, J. Membr. Sci. 320 (2008) 390–400. [35] A. Takeda, T. Sakaguchi, T. Hashimoto, Synthesis of novel poly(diphenylacetylene) s with both trimethylsilyl and alkyl groups: the effect of desilylation on gas permeability, Polymer 50 (2009) 5031–5036.
[18] H. Lin, B.D. Freeman, Materials selection guidelines for membranes that remove CO2 from gas mixtures, J. Mol. Struct. 739 (2005) 57–74. [19] Y. Shida, T. Sakaguchi, M. Shiotsuki, F. Sanada, B.D. Freeman, T. Masuda, Synthesis and properties of poly(diphenylacetylene)s having hydroxyl groups, Macromolecules 38 (2005) 4096–4102. [20] T. Sakaguchi, A. Takeda, T. Hashimoto, Highly gas-permeable silanolfunctionalized poly(diphenylacetylene)s: synthesis, characterization, and gas permeation property, Macromolecules 44 (2011) 6810–6817. [21] F. Sanada, M. Shiotsuki, T. Masuda, Alkyne polymerization, in: K. Matyjaszewski, M. Moller (Eds.), Polymer Science: A Comprehensive, vol. 3, Elsevier BV, Amsterdam, 2012, pp. 875–954. [22] M. Shiotsuki, F. Sanada, T. Masuda, Polymerization of substituted acetylenes and features of formed polymers, Polym. Chem. 2 (2011) 1044–1058. [23] T. Aoki, H. Nakahara, Y. Hayakawa, M. Kokai, E. Oikawa, Trimethylsilyl-group containing polyphenylacetylenes for oxygen and ethanol permselective membranes, J. Polym. Sci., Part A: Polym. Chem. 32 (1994) 849–858. [24] A. Bondi, Physical Properties of Molecular Crystals, Liquids and Glasses, Wiley, New York, 1968. [25] D.W. VanKrevelen, Properties of Polymers: Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions, Elsevier, Amsterdam, 1990, p. 875. [26] T. Sakaguchi, G. Kwak, T. Masuda, Synthesis of poly(1-β-naphthyl-2phenylacetylene) membranes through desilylation and their properties, Polymer 43 (2002) 3937–3942.
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Synthesis and gas permeability of methylol-group-containing Poly (diphenylacetylene)s with high CO2 permeability and permselectivity Toshikazu Sakaguchi *, Aiko Takeda , Tamotsu Hashimoto Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, Bunkyo 3-9-1, Fukui, 910-8507, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Membrane Gas separation CO2 permeability poly(diphenylacetylene) Methylol group
Diphenylacetylene monomers bearing a methylol group protected by a t-butyldiphenylsilyl group [tBuPh2 – SiOCH2C6H4C– –CC6H4R; R ¼ SiMe3 (1a) and R ¼ SiMe2nC18H37 (1b)] were synthesized and then polymerized with TaCl5/n-Bu4Sn to afford the corresponding poly(diphenylacetylene)s 2a and 2b, respectively. The molecular weights of 2a and 2b were high (i.e., Mw ¼ 1,020,000 and 1,560,000, respectively), and free-standing mem branes were obtained by solution casting. The deprotection of membranes of 2a and 2b using n-Bu4NþF afforded poly(diphenylacetylene)s bearing methylol and silyl groups (3a and 3b). On the other hand, treatment of membranes of 2a and 2b with trifluoroacetic acid led to deprotection and desilylation, producing poly (diphenylacetylene)s bearing trifluoroacetate moieties (4a and 4b, respectively), and then treatment with methanol afforded poly(diphenylacetylene)s bearing only methylol groups (5a and 5b). The carbon dioxide permeability coefficients (PCO2) of membranes of 2a and 2b were 99 and 78 Barrer, respectively. Deprotection led to the increment of CO2 permeability without the decline of the CO2 permselectivity. Poly(diphenylacetylene) s bearing methylol and silyl groups (3a and 3b) exhibited high permeability (PCO2 ¼ 2000 and 120 Barrer, respectively), but not high permselectivity (PCO2/PN2 ¼ 17 and 16, respectively). Methylol-group-containing poly (diphenylacetylene) without silyl groups 5b exhibited the best CO2 permeation property, i.e., high CO2 permeability and high permselectivity (PCO2 ¼ 220 Barrer, PCO2/PN2 ¼ 32).
1. Introduction Gas separation has been increasingly applied for hydrogen recovery, nitrogen generation, oxygen-enriched air production, hydrocarbon pu rification, and carbon dioxide removal. In particular, the capture and storage of carbon dioxide has attracted considerable attention because global warming and climate change are presumably caused by the increment in the concentration of carbon dioxide in the atmosphere. Unlike separation techniques such as distillation and adsorption, mem brane separation has been investigated because of its cost-effectiveness, low energy consumption, facile operation, and small footprint [1–3]. Thus far, various polymers have been investigated for gas separation membranes, and poly(substituted acetylene)s [4–7] and polymers of intrinsic microporosity [8–10] have been considered as promising ma terials because they exhibit high gas permeability, which is mainly related to the extremely high free volume. However, such high free volume generally leads to the decline of permselectivity. Our group has synthesized various substituted acetylene polymers and investigated their gas permeability [5,6]. A majority of poly
(substituted acetylene)s exhibit high gas permeability and low permse lectivity, while poly(substituted acetylene)s bearing polar groups exhibit relatively high CO2 permselectivity [11–17]. Polar-group-containing polymers are known to exhibit a good affinity toward CO2 molecules; thus, the solubility of CO2 in polymer membranes increases [18]. For example, sulfonated poly(diphenylacetylene) exhibits high CO2 perm selectivity, with a separation factor (PCO2/PN2) of 54. However, the CO2 permeability coefficient (PCO2) is as low as 3.9 Barrer [13]. Poly(diphe nylacetylene) bearing phenolic hydroxyl groups exhibits good permse lectivity (PCO2/PN2 ¼ 48) and a relatively high CO2 permeability (PCO2 ¼ 110 Barrer) [19]. In this way, polar groups increase the CO2 permse lectivity but decrease the permeability because the interaction between polymer chains is strengthened by the high polarity. Previously, our group has synthesized poly(diphenylacetylene) bearing silanol groups, the polarity of which is not extremely high [20]. The polymer membrane exhibits a high PCO2 of 1200 Barrer, but its permselectivity is not high (PCO2/PN2 ¼ 15). In this regard, it is imperative to synthesize a polymer with appropriate polarity to achieve good CO2 separation performance. In this study, novel poly(diphenylacetylene)s bearing methylol
* Corresponding author. E-mail address: [email protected] (T. Sakaguchi). https://doi.org/10.1016/j.polymer.2020.122230 Received 26 November 2019; Received in revised form 21 January 2020; Accepted 25 January 2020 Available online 27 January 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.
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groups is developed. Poly(diphenylacetylene) derivatives are well known to be synthesized by metathesis polymerization using a TaCl5 catalyst [21,22]. This TaCl5 catalyst is deactivated by polar groups such as alcohol, which in turn does not permit the polymerization of polar-group-containing diphenylacetylene monomers. Hence, dipheny lacetylenes bearing methylol groups protected by silyl groups are syn thesized and subsequently polymerized by metathesis polymerization (Scheme 1). The deprotection of the obtained polymers afforded methylol-group-containing poly(diphenylacetylene)s. The membrane of poly(diphenylacetylene) bearing methylol groups exhibited higher CO2 permeability than that of poly(diphenylacetylene) bearing phenolic hydroxyl groups, as well as higher CO2 permselectivity than that of poly (diphenylacetylene) bearing silanol groups.
addition of p-trimethylsilylphenylacetylene (11.2 g, 64.2 mmol) in triethylamine (50 mL) at room temperature. The reaction mixture was stirred at reflux temperature for 24 h and then allowed to cool to room temperature. After the evaporation of triethylamine, the crude product was dissolved in ether (300 mL), and the insoluble salt was filtered off. The solution was washed three times using 1.0 M HClaq, and ether was evaporated. This product was purified by silica-gel column chromatog raphy (eluent: CHCl3) to afford 1-(p-trimethylsilyl)phenyl-2-(p-hydrox ymethyl)phenylacetylene (9.3 g, 62% yield). First, 1-(p-trimethylsilyl)phenyl-2-(p-hydroxymethyl)phenylacetylene (8.3 g, 29.6 mmol), imidazole (10.0 g, 147 mmol), and DMF (100 mL) were transferred to a 500 mL three-necked flask equipped with a dropping funnel, a three-way stopcock, and a magnetic stirring bar. Second, a so lution of t-butyldiphenylchlorosilane (12.2 g, 44.4 mmol) in DMF (50 mL) was added dropwise at 0 � C under nitrogen. After the addition, the reaction mixture was stirred for 10 h at room temperature. Next, ether (200 mL) was added to the flask, and the solution was washed three times with water. The ethereal solution was dried over anhydrous sodium sulfate and then concentrated in vacuo. The crude product was purified by silica-gel col umn chromatography (eluent: hexane/chloroform (9/1)) to afford the desired product 1a (10.1 g, 66%) as a white solid. 1H NMR (CDCl3, ppm): 7.76–7.62 (m, 4H, Ar), 7.59–7.28 (m, 14H, Ar), 4.78 (s, 2H, CH2O), 1.12 (s, 9H, C(CH3)3), 0.26 (s, 9H, Si(CH3)3). 13C NMR (CDCl3, ppm): 141.3, 140.9, 135.5, 133.3, 131.5, 130.6, 129.8, 127.8, 125.9, 123.6, 121.7, 89.9, 89.2, 65.2, 26.8, 19.3, 1.2. Anal. Calcd for C34H38OSi2: C, 78.7; H, 7.38; O, 3.08; Si, 10.83. Found: C, 79.8; H, 7.36.
2. Experimental 2.1. Materials The polymerization solvent, toluene, was purified by distillation over calcium hydride. The main catalyst, TaCl5 (Aldrich, 99.999%), was used without further purification. The cocatalyst, n-Bu4Sn (Wako, Japan), was purified by distillation over calcium hydride. p-Bromobenzyl alcohol, t-butyldiphenylchlorosilane, tetra-n-butylammonium fluoride (n-Bu4NþF ), trifluoroacetic acid, and typical organic solvents were purchased from Wako, Japan and used without further purification. pTrimethylsilylphenylacetylene and p-dimethyl-n-octadecylsilylphenyla cetylene were synthesized according a previously reported study [23]. Monomers were synthesized according to Scheme 2. The synthetic procedures and analytical data of monomers are as follows.
2.3. Synthesis of 1-(p-dimethyl-n-octadecylsilyl)phenyl-2-[p-(tbutyldimethylsiloxymethyl)] phenylacetylene (1b) Monomer 1b was prepared by the same method as that adopted for 1a using p-dimethyl-n-octadecylsilylphenylacetylene instead of p-tri methylsilylphenylacetylene. Overall yield: 49%, white solid. 1H NMR (CDCl3, ppm): 7.75–7.66 (m, 4H, Ar), 7.58–7.31 (m, 14H, Ar), 4.78 (s, 2H, CH2O), 1.46–1.16 (m, 32H, CH2(CH2)16CH3), 1.10 (s, 9H, C(CH3)3), 0.88 (t, 3H, CH2(CH2)16CH3), 0.74 (t, 2H, CH2(CH2)16CH3), 0.25 (s, 9H, Si(CH3)3). 13C NMR (CDCl3, ppm): 141.3, 140.2, 135.5, 133.4, 131.5, 130.6, 129.7, 127.8, 125.9, 123.6, 121.7, 89.9, 89.2, 65.2, 33.6, 31.9, 29.7, 29.6, 29.3, 26.8, 23.8, 22.7, 19.3, 15.6, 14.2, 3.1. Anal. Calcd for
2.2. Synthesis of 1-(p-trimethylsilyl)phenyl-2-[p-(tbutyldimethylsiloxymethyl)] phenylacetylene (1a) First, a 500 mL three-necked flask was equipped with a reflux condenser, a three-way stopcock, and a magnetic stirring bar, followed by flushing with nitrogen. Second, p-bromobenzyl alcohol (10.0 g, 53.5 mmol), PdCl2(Ph3P)2 (0.0964 g, 0.138 mmol), Ph3P (0.157 g, 0.598 mmol), and CuI(I) (0.149 g, 0.780 mmol) were added into the flask. Third, triethylamine (150 mL) was added into the flask, followed by the
Scheme 1. Synthesis of poly(diphenylacetylene) bearing methylol groups. 2
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Scheme 2. Synthesis of diphenylacetylene monomers.
C51H72OSi2: C, 80.9; H, 9.58; O, 2.11; Si, 7.42. Found: C, 80.1; H, 9.20.
2.6. Deprotection (synthesis of 3a and 3b)
2.4. Polymerization
Deprotection of 2a and 2b was carried out using n-Bu4NþF in THF at 65 � C for 24 h. A detailed procedure is as follows: Polymer 2a (0.32 g, 0.62 mmol repeating unit) was added into a flask and then flushed with nitrogen. Dry THF (10 mL) was added into the flask, and the solution was stirred until the polymer was completely dissolved. A 1 M THF so lution of n-Bu4NþF (6.2 mL, 10 equiv.) was then added to the solution and stirred at 65 � C for 24 h. The polymer was then isolated by pre cipitation into a large excess of water and dried under reduced pressure. The polymer was dissolved in THF again and purified by precipitation into hexane. The polymer was dried at room temperature for 24 h under reduced pressure.
Polymerization was carried out in a glass tube equipped with a threeway stopcock under dry nitrogen. Unless otherwise specified, the reac tion was carried out at 80 � C for 24 h under the following conditions: [Monomer] ¼ 0.20 M, [TaCl5] ¼ 40 mM, and [n-Bu4Sn] ¼ 400 mM. A detailed polymerization procedure is as follows: The monomer solution was prepared in a glass tube by mixing 1a (0.52 g) and dry toluene (2.5 mL). Another glass tube was charged with TaCl5 (36 mg) and 0.8 M of nBu4Sn in toluene (2.5 mL); the resulting catalyst solution was aged at 80 � C for 10 min. The monomer solution was then added to the catalyst solution. Polymerization was carried out at 80 � C for 24 h, which was quenched using a small amount of methanol. Toluene (25 mL) was added into the flask, and the insoluble product was separated using a glass filter. The toluene-soluble polymer was isolated by precipitation into a large excess of acetone, and its yield was determined gravimetrically.
2.7. Desilylation (synthesis of 4a and 4b) The desilylation of 2a and 2b was performed with trifluoroacetic acid using polymer membranes. Membranes of 2a and 2b were immersed in trifluoroacetic acid at room temperature for 24 h, followed by washing with hexane and immersing in hexane for 24 h. The obtained membranes were dried at room temperature for 24 h under reduced pressure.
2.5. Measurements The molecular weights and polydispersity ratios of polymers were estimated by gel permeation chromatography (THF was used as the eluent, polystyrene was used for calibration) at 40 � C on a Shimadzu LC10AD chromatograph equipped with three polystyrene gel columns (Shodex KF-8025 � 1 and A-80M � 2) and a Shimadzu RID-6A refractive index detector. IR spectra were recorded on a Nicolet Magna 560 spectrometer. Thermogravimetric analyses (TGA) were conducted with Rigaku TG-DTA 8078G1 in nitrogen at a 10 � C min 1 heating rate. NMR spectra were recorded on a Jeol ECX-500 spectrometer. Elemental an alyses of monomers were performed at A-Rabbit-Science Japan Co., Ltd. Gas permeability coefficients of polymer membranes were measured using a Rikaseiki K-315-N gas permeability apparatus at 25 � C at an upstream pressure of 1 atm. The permeability coefficient P expressed in unit of Barrer (1 Barrer ¼ 10 10 cm3(STP) cm 2 s 1 cmHg 1) was calculated from the slope of the steady-state line. The diffusion coeffi cient (D) value was determined by the time lag method using the following equation:
2.8. Conversion from trifluoroacetate to methylol (synthesis of 5a and 5b) The reaction was performed using membranes of 4a and 4b. Mem branes of 4a and 4b were immersed in methanol/triethylamine solution (2/1 vol ratio) at room temperature for 24 h, followed by washing with methanol and immersing in methanol for 24 h. The obtained membranes were dried at room temperature for 24 h under reduced pressure. 2.9. Membrane fabrication Membranes (30–60 μm thickness) of 2a and 2b were fabricated by casting their toluene solutions (0.4 wt% concentration) into Petri dishes. The dish was covered with a glass vessel to decrease the evaporation rate of solvent (3–4 days). After the formation of a membrane, the membrane was peeled off, and it was immersed in methanol for 24 h and dried to constant weight at room temperature. Membranes of 3a and 3b were fabricated by casting their THF so lutions (0.4 wt% concentration) into Petri dishes. The dish was covered with a glass vessel to decrease the evaporation rate of the solvent (2–3 days). After the formation of a membrane, the membrane was peeled off, and it was immersed in hexane for 24 h and dried to constant weight at room temperature.
D ¼ l2/6θ Here, l is the membrane thickness, and θ is the time lag, which is expressed by the intercept of the asymptotic line of the time–pressure curve to the time axis. The solubility coefficient (S) was calculated by using equation S ¼ P/D.
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2.10. Fractional free volume (FFV) of polymer membranes
(runs 1 and 4).
Membrane density was determined by hydrostatic weighing using a Mettler Toledo balance and a density determination kit. In this method, a liquid with a known density (ρ0) is required, and the membrane den sity (ρ) is expressed by the following equation:
3.2. Deprotection by n-Bu4NþF
ρ ¼ ρ0 � MA / (MA
The deprotection of 2a was carried out using n-Bu4NþF in THF at 65 � C for 24 h to produce 3a bearing hydroxy groups (Scheme 1). Fig. 1 shows the IR spectra of the polymers before and after the reaction. In the IR spectrum of 2a, an absorption peak observed at 1080 cm 1 corre sponded to the asymmetric stretching of Si–O–CH2 in the siloxy groups, while in the IR spectrum of 3a, an absorption peak at 1080 cm 1 was not observable. Furthermore, a broad absorption peak characteristic of the O–H stretching of the hydroxyl groups was observed at 3400 cm 1. Absorption peaks corresponding to trimethylsilyl groups were observed at 1260 and 1120 cm 1 in the IR spectrum of 2a. The intensities of these absorption peaks did not decrease after the reaction, suggesting that trimethylsilyl groups are not eliminated by the reaction. These results revealed the selective desilylation and generation of polymer 3a bearing hydroxyl and trimethylsilyl groups. The deprotection of 2b was also performed in THF at 65 � C (Fig. 2). Similar to the case of 3a, an ab sorption peak at 1080 cm 1 corresponding to Si–O–CH2 disappeared and an absorption peak observed at 3400 cm 1 corresponding to O–H were observed after the reaction. Furthermore, absorption peaks observed at 1260 and 1120 cm 1 did not clearly change, suggesting that the reaction using n-Bu4NþF affords polymer 3b bearing hydroxyl and dimethyl-noctadecylsilyl groups.
ML)
where MA is the weight of the membrane in air, and ML is the weight of the membrane in an auxiliary liquid. Aqueous sodium nitrate was used as the auxiliary liquid. FFV is calculated by the following equation [24]. FFV ¼ (vsp
v0)/vsp � (vsp
1.3 vw)/vsp
where vsp is the specific volume of the polymer, and v0 is the volume occupied by the polymer. The occupied volume is typically estimated as 1.3 times the van der Waals volume (vw), which is calculated by using the group contribution method [25]. 3. Results and discussion 3.1. Polymerization The polymerization of monomers (1a and 1b) was carried out using the TaCl5/n-Bu4Sn catalyst in toluene and cyclohexane at 80 � C. The molecular weight and yields of the polymers obtained by the polymer ization of diarylacetylene derivatives in cyclohexane have been reported to be greater than those obtained by the polymerization of the same derivatives in toluene [26,27]. Table 1 summarizes the results obtained from polymerization. The polymerization of 1a in toluene afforded a polymer in 33% yield, but it contained solvent-insoluble parts. The obtained polymer was dissolved in toluene, and the insoluble parts were filtered off. The yield of the toluene-soluble part was 21%, with a high Mw of 1,020,000 g/mol. The polymerization of 1a in cyclohexane afforded a polymer with a higher molecular weight in higher yield (87%). However, most parts were insoluble in all solvents, and the yield of the toluene-soluble part was only 5.6%. The insolubility will be caused by too high molecular weight because the IR spectrum of insol uble part is the same as that of soluble part. Monomer 1b bearing an n-octadecyl group polymerized in toluene afforded a polymer in a yield of 3.5%, without insoluble parts. Its Mw was 453,000 g/mol; this value is considerably less than that of polymer 2a. When cyclohexane was used as the polymerization solvent, the yield and molecular weight increased up to 68% and 1,560,000 g/mol, respectively. The yield of the toluene-soluble part was 45%; this value is considerably greater than that of 2a. In the metathesis polymerization of disubstituted acetylenes, the polymerizability of monomers is critically dependent on the steric hindrance of substituents [21,22]. Compared to 1a, monomer 1b bearing a long alkyl chain exhibited low polymerizability probably because of the steric effect. The solubility of the polymer improved due to long alkyl side chains. Membrane fabrication and polymer reaction were examined using the soluble polymers with relatively high yields
3.3. Desilylation by trifluoroacetic acid Desilylation by trifluoroacetic acid was performed using freestanding membranes because the obtained polymers were insoluble in all solvents, and the membranes could not be fabricated by solution casting. Membranes of 4a and 4b were obtained by the desilylation of membranes of 2a and 2b, respectively, using trifluoroacetic acid. Membranes of 5a and 5b were then obtained by the treatment of membranes of 4a and 4b, respectively, with methanol. Fig. 1 shows the IR spectra of 4a and 5a. In the spectrum of 4a, new absorption peaks – O, C(¼O)O, and C–F bonds were observed at 1,780, corresponding to C– 1,240, and 1140 cm 1, respectively. The absorption peak corresponding to the O–H bond around 3400 cm 1 was not observed. The absorption
Table 1 Polymerization of monomers 1a and 1b with TaCl5–n-Bu4Sna. Run
Monomer
Solvent
Yield, %b
Yield (soluble part), %c
Mw � 10 6 g/ mold
Mw/ Mn d
1 2 3 4
1a
toluene cyclohexane toluene cyclohexane
33 87 3.5 68
21 5.6 3.5 45
1.02 2.38 0.453 1.56
3.44 3.50 2.65 3.02
a b c d
1b
At 80 � C for 24 h; [M]0 ¼ 0.20 M, [TaCl5] ¼ 40 mM, [n-Bu4Sn] ¼ 400 mM. Acetone-insoluble product. Toluene-soluble and acetone-insoluble products. Measured by GPC.
Fig. 1. IR spectra of membranes of 2a, 3a, 4a, and 5a. 4
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Polymer 190 (2020) 122230
Fig. 2. IR spectra of membranes of 2b, 3b, 4b, and 5b.
peak at 1080 cm 1 corresponding to the siloxy group and the peak at 1260 cm 1 corresponding to the trimethylsilyl group disappeared, indicating that both silyl groups are eliminated by trifluoroacetic acid. The strong absorption peak observed at 700 cm 1 in the spectrum of 4a, characteristic of monosubstituted benzene, was not observed in the spectra of 2a and 3a. This result supported the elimination of trime thylsilyl groups. Hence, the obtained polymer is thought to possess tri fluoroacetate groups (-OCOCF3) without trimethylsilyl groups (Scheme 1). The trifluoroacetate groups were easily decomposed by the treatment with methanol and hydroxyl groups formed in the polymer. In the spectrum of 5a, a broad absorption peak at around 3400 cm 1 corre sponded to the O–H bond, and absorption peaks at 1,780, 1,240, and 1140 cm 1, corresponding to trifluoroacetate groups were not apparent, indicating that polymer 5a has methylol groups and no silyl groups (Scheme 1). Polymer 2b bearing dimethyl-n-octadecylsilyl groups was desilylated by trifluoroacetic acid under the same conditions as those utilized for 2a. Fig. 2 shows the IR spectra of polymers. The spectral
variations were similar to the case of 2a–5a; i.e., the desilylation of both silyl groups occurred by the treatment with trifluoroacetic acid, affording polymer 4b bearing trifluoroacetate groups. Polymer 5b bearing methylol groups was then obtained by the treatment of 4b with methanol. 3.4. Solvent solubility Table 2 summarizes the solubility of the obtained polymers. Poly mers 2a and 2b obtained by polymerization contained insoluble parts, but this solubility test was conducted using toluene-soluble products. The polymers bearing t-butyldiphenylsiloxy groups (2a and 2b) dis solved in common organic solvents such as CCl4, CHCl3, and THF. The polymers bearing n-octadecylsilyl groups 2b exhibited relatively good solubility and dissolved even in hexane and diethyl ether. Polymer 3a, which was synthesized by the deprotection of 2a using n-Bu4NþF , was soluble in high-polarity solvents such as THF, acetone, and DMF, but it
Table 2 Solubility of polymers. 2a 2b 3a 3b 4a 4b 5a 5b
Hexane
CCl4
Toluene
Et2O
CHCl3
THF
Acetone
DMF
� þ – – – – – –
þ þ – � – – – –
þ þ – � – – – –
� þ � þ – – – –
þ þ � þ – – – –
þ þ þ þ – – – –
– – þ – – – – –
– – þ – – – – –
Symbols: þ, soluble; �, partly soluble; –, insoluble. 5
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Polymer 190 (2020) 122230
was insoluble in low-polarity solvents such as CCl4 and toluene. The solubility change was due to the generation of hydroxyl groups. On the other hand, polymer 3b did not completely dissolve in low-polarity solvent such as toluene and high-polarity solvents such as acetone and DMF. The solubility of 3b can be attributed to the combination of hy drophilic hydroxyl groups and hydrophobic n-octadecyl groups. Desi lylation using trifluoroacetic acid afforded polymers with lower solubility. Polymers 4a and 4b were insoluble in all solvents. Polymers 5a and 5b also exhibited poor solubility and did not dissolve in any solvents, indicating that the silyl groups on the side chains are crucial to conferring solvent solubility to poly(diphenylacetylene) derivatives.
Table 3 Gas permeation properties and FFV of polymer membranes. Membrane
PO2a
PN2a
PCO2a
PO2/PN2
PCO2/PN2
Densityb
FFVc
2a 3a
20 330
5.6 120
99 2000
3.6 2.8
18 17
1.10 1.01
0.131 0.214
2b 3b 4b 5b
15 22 38 27
5.0 7.4 12 6.9
78 120 230 220
3.0 3.0 3.2 3.9
16 16 19 32
1.03 0.982 1.25 1.20
0.140 0.159 0.226 0.130
a
In the unit of Barrer [1 Barrer ¼ 1 � 10 10 cm3 (STP) cm 2 s 1 cmHg 1]. Determined by hydrostatic weighing. In the units of g cm 3. c FFV ¼ fractional free volume. Calculated using the group contribution method. b
3.5. Thermal property Thermogravimetric analyses (TGA) of 2a, 2b, 3a, and 3b were conducted in N2 (Fig. 3). The onset temperature of weight loss (T0) for 2a was 325 � C, showing its high thermal stability. The T0 values of 2b, 3a, and 3b were somewhat lower than that of 2a, but they indicated more than 270 � C. Poly(diphenylacetylene) without any substituents show very high thermal stability, and its T0 is 480 � C [28]. On the other hand, poly(diphenylacetylene)s possessing various silyl groups show the T0’s of 260–420 � C [29]. These results imply that the thermal degra dation of poly(diphenylacetylene)s occurs at the substituents connected to benzene rings at first. Anyway, the present polymers (2a, 2b, 3a, and 3b) have enough high thermal stability to apply for gas separation membranes. 3.6. Gas permeability The permeability of the membranes to oxygen, nitrogen, and carbon dioxide was examined at 25 � C (Table 3). Table 3 also summarizes the density and FFV of the membranes. Fig. 4 plots the PCO2 and separation factor (PCO2/PN2) data on a log–log scale. The PO2, PN2, and PCO2 of 2a were 20, 5.6, and 99 Barrer, respectively. These values are comparable to those reported previously for bulky-substituent-containing poly (diphenylacetylene)s [30–33]. For instance, the PO2 values for poly (diphenylacetylene)s bearing triisopropylsilyl and dimethylphenylsilyl groups are 20 and 26 Barrer, respectively [30,31]. The deprotection of 2a led to the drastic increase in the gas permeability. The PO2, PN2, and PCO2 of 3a were 330, 120, and 2000 Barrer, respectively. The FFV value of 3a was 0.214; this value is considerably greater than that of 2a. The phenyl groups of t-butyldiphenylsiloxy groups in the side chains could easily exhibit polymer chain packing due to their π–π interaction. Hence, the elimination of t-butyldiphenylsilyl groups leads to the increment of
Fig. 4. Relationship between PCO2 and PCO2/PN2 for the current polymers and the related poly(diphenylacetylene)s.
FFV. The PCO2/PN2 ratio of 3a was 17; this value is similar to that of 2a, indicating that the hydroxyl groups in 3a improve the CO2 permse lectivity. Generally, highly gas-permeable polymers exhibit low selec tivity. As shown in Fig. 4, the Robeson upper bound determined in 2008 represents the experimental limit of the separation performance of polymer membranes [34]. High-performance membranes for gas sepa ration were located beside the upper bound. Compared with that of 2a, the plot of 3a clearly lied near the upper bound (Fig. 4). Unfortunately, membranes of 4a and 5a were too brittle; hence, their gas permeation properties could not be investigated. Membranes of 2b and 3b exhibited a similar tendency to those of 2a and 3a; i.e., deprotection led to the enhancement of permeability without a decrease in permselectivity. However, the permeability of 3b was considerably less than that of 3a. Polymer 3b comprises n-octadecyl groups; these long alkyl groups occupy the free volume in a high-freevolume polymer such as poly(substituted acetylene)s. Previous studies have reported that the introduction of a long alkyl chain into poly (diphenylacetylene) leads to the decrease of the FFV [27,35]. Desilyla tion using trifluoroacetic acid leads to an increase in the gas perme ability. The PO2, PN2, and PCO2 of 4b were 38, 12, and 230 Barrer, respectively. Bulky t-butyldiphenylsilyl and dimethyl-n-octadecylsilyl groups were eliminated to increase the FFV of the membrane. The membrane of 5b exhibited low FFV and low gas permeability compared with those of 4b because of the presence of methylol groups in 5b. The
Fig. 3. TGA curves of 2a, 2b, 3a, and 3b (in N2, heating rate 10 � C min 1). 6
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Polymer 190 (2020) 122230
PO2 and PN2 of 5b were similar to those of 3b bearing methylol and n-octadecyl groups. However, the PCO2 of 5b was roughly two times greater than that of 3b; thus, the permselectivity of 5b is two times that of 3b. The membrane of 5b exhibited good performance as a CO2 sep aration membrane (Fig. 3). Poly(diphenylacetylene) bearing phenolic hydroxy groups exhibited high CO2 permselectivity but low perme ability (PCO2 ¼ 110 Barrer, PCO2/PN2 ¼ 50) [19]. Poly(diphenylacety lene)s bearing aliphatic hydroxy groups used as CO2 separation materials in this study exhibited higher gas permeability. 3.7. CO2 diffusivity and solubility for polymer membranes The gas permeability of polymer membranes can be expressed in terms of diffusivity and solubility. Theoretically, the gas permeability coefficient (P) is expressed as a product of the gas diffusion coefficient (D) and the gas solubility coefficient (S) [P ¼ D � S]. To investigate the variation of gas permeability for the current polymers in detail, CO2 diffusion coefficients (DCO2) and CO2 solubility coefficients (SCO2) were calculated from the time lag of the permeation measurement. However, we could not obtain the diffusivity and solubility for oxygen and nitro gen because their time lags were extremely small for precise calculation. Fig. 5 shows the DCO2 and SCO2 data for the membranes. The large in crease in the CO2 permeability for 3a originated from the increase in diffusivity and solubility. This result can be explained by the large FFV and the generation of methylol groups. In the case of polymers 2b and 3b, we observed a similar trend although the changes were extremely small. Comparison of DCO2 and SCO2 between 3b and 5b showed that the values were quite different. Polymer 3b exhibited a large DCO2 value, corresponding to the flexible long chains of n-octadecyl groups. On the other hand, polymer 5b exhibited a large SCO2 value, which possibly related to the high affinity to CO2 molecules. Polymer 4b exhibited in termediate D and S values. Polymer 3b also possessed methylol groups, but the affinity to CO2 molecules probably low because the contribution of methylol groups in 3b was considerably less than that in 5b from the molecular weight of the repeating unit. Notably, the introduction of methylol groups into poly(diphenylacetylene)s is effective in improving the CO2 solubility.
Fig. 5. CO2 diffusion coefficients (D) and solubility coefficients (S) of poly mer membranes.
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4. Conclusions Methylol-group-containing poly(diphenylacetylene)s were synthe sized by the metathesis polymerization of diphenylacetylene bearing siloxy groups, followed by deprotection with n-Bu4NþF or trifluoro acetic acid. The membrane of poly(diphenylacetylene) bearing methylol and trimethylsilyl groups exhibited high CO2 permeability and relatively high CO2 permselectivity (PCO2 ¼ 2000 Barrer, PCO2/PN2 ¼ 17). The membrane of poly(diphenylacetylene) bearing methylol groups exhibi ted relatively high CO2 permeability and high CO2 permselectivity (PCO2 ¼ 220 Barrer, PCO2/PN2 ¼ 32). The CO2 permeability of the current polymers was greater than that of poly(diphenylacetylene)s bearing phenolic hydroxyl groups and sulfonic acid. Methylol groups were found to provide a good balance of CO2 permeability and permselectivity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Toshikazu Sakaguchi: Conceptualization, Methodology, Visualiza tion, Investigation, Validation, Writing - original draft. Aiko Takeda: Data curation, Software, Investigation. Tamotsu Hashimoto: Supervi sion, Writing - review & editing. 7
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