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Ind. Eng. Chem. Res. 2006, 45, 3406-3411
Synthesis and Properties of Fluorinated Ester-Functionalized Polythiophenes in Supercritical Carbon Dioxide Hullathy Subban Ganapathy, Ha Soo Hwang, and Kwon Taek Lim* DiVision of Image and Information Engineering, Pukyong National UniVersity, Pusan 608-739, Korea
A series of fluorinated ester-functionalized conjugated polythiophenes and their random copolymers with 3-octylthiophene (OT) were synthesized by oxidative polymerization with FeCl3 in supercritical carbon dioxide (scCO2). The polymerizations were compared with those prepared from the conventional solvent, chloroform, in terms of yield, molecular weight, and polydispersibility. The effects of side-chain organization of polythiophenes with different types of fluoroalkyl esters on their solubility in scCO2 were also investigated. While homo-polymers of poly[(2-(3-thienyl)acetyl 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanate)] (PSFTE) and poly[2-(3-thienyl)methylheptafluorobutyrate] (PFTE3) show solubility in scCO2, polymers prepared from fluorinated esters of 3-thienylmethanol were found to be insoluble in both scCO2 and common organic solvents. The copolymers, P(OT-SFTE), with different molar compositions of comonomers prepared in CO2 were also soluble in scCO2. 1. Introduction Supercritical carbon dioxide (scCO2) and conjugated polymers have attracted considerable attention in recent years because of their excellent applications in different areas. While conjugated polymers are an attractive class of materials because of their potential applications as electronic materials and devices,1 scCO2, essentially a “green” solvent and environmentally benign, is rapidly becoming a viable alternative solvent for various chemical processes including fine chemicals, polymers, and material synthesis.2 However, practical use of scCO2 has been limited in the field of polymer synthesis because of the poor solubility of most polymers except amorphous fluoro-polymers and silicones.3 Apart from CO2 solubility, the incorporation of fluorine atoms into polymers has been extensively studied because of the resultant low surface energies, remarkable chemical and oxidative resistance, hydrophobicity, rigidity, thermal stability, and self-organization of perfluoro alkyl chains.4 The combination of the unique characteristics of fluorine and the electronic characteristics of conjugated polymers (CPs) may lead to the development of new materials which are of interest to the scientific and industrial communities. Among numerous CPs studied, polythiophene (PTh) derivatives have been the focus of extensive research because of the unique combination of electronic properties including electrochemical stability, electrochromism, and structural versatility.1 Roncali reported the synthesis of PTh-possessing perfluorinated alkyl side chains containing 50 wt. % fluorine that had conductivities similar to those of poly(3-alkylthiophene) analogues.5 Besides side-chain organization of simple poly(3-alkylthiophene)s, a number of semifluoro- and perfluoroalkyl-substituted polythiophenes and their copolymers with alkyl-substituted PThs have been prepared with the aim of controlling molecular architecture and improving environmental stability.6 Alternating units of polythiophenes bearing hydrocarbon and fluorocarbon side chains gave rise to amphiphilic polymers that self-assembled into a lamellar structure. This suggests that fluoroalkyl-substituted PThs may prove valuable in the development of liquid crystal based on PThs.7 A copolymer of 3-(methoxyethoxyethoxymethyl)th* Corresponding author. Fax: +82-51-625-2229. E-mail: ktlim@ pknu.ac.kr.
iophene and 3-(perfluoroalkyl)thiophene was found to be fluorescent and displayed minimum absorption and emission spectra overlap, which can be used for lasing technology.8 Prior to this report, a number of groups had reported the conducting polymers synthesized in scCO2 using a chemical oxidant, typically iron(III) triflate. Alkyl thiophenes,9 polypyrrole,10 and their conducting composite with polystyrene9 were prepared using scCO2 as a solvent. However, none of these polymers are soluble in scCO2. Recently, Li et al. have shown that poly(perfluorooctylthiophene) is soluble in scCO2.11 However, the amount of polymer soluble in CO2 or the corresponding cloud-point profile to study the CO2-polymer phase behavior was not reported. Given the fluorophilicity of scCO212 and the strong interaction between scCO2 and carbonyl groups,13 we have recently shown that, by modifying the polythiophene backbone with semi- and perfluoro-alkyl ester groups, highlyCO2-soluble polymers at low pressure and temperature could be obtained.14 CO2-soluble conjugated polymers offer the potential advantage to prepare novel conducting polymer particles by the rapid expansion of supercritical CO2 solution (RESS) process,15 which is extensively being studied for using CO2 for particle engineering in pharmaceutical and polymeric materials.16 In this paper, we have synthesized a series of fluorinated esterfunctionalized polythiophenes and their copolymers with 3-octylthiophene using scCO2 as a solvent and ferric chloride as an oxidant. The comparison of the polymerization results with the conventional organic solvent, chloroform, was carried out. Furthermore, a detailed investigation of the solubility of the homo-copolymers and random copolymers in CO2 is also presented. 2. Experimental Section 2.1. Materials. 2-(3-Thienyl)ethanol, 3-thienylmethanol, heptaflouorobutyryl chloride, pentadecafluorooctanoyl chloride, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol, 3-octylthiophene, and ferric chloride (Aldrich) were used as received. Thiophene3-acetic acid (TAA) (Aldrich) was recrystallized in 1:1 mixture of ethyl ether and hexane mixed solvent. Thionyl chloride (Junsei) was purified by distillation. Pyridine and dichloromethane (Junsei) were distilled from calcium hydride prior to use.
10.1021/ie050718t CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3407
Figure 1. Schematic diagram of the experimental apparatus.
2.2. Characterization. 1H NMR spectra were recorded using a JNM-ECP 400 (JEOL) spectrometer, with residual chloroform as the internal reference (δH ) 7.26 ppm). 13C NMR (100 MHz) spectra were recorded in CDCl3 on the same spectrometers with the central peak of chloroform as the internal reference (δc ) 77.0 ppm). IR spectra were measured on Bomem B-100 infrared spectrometer. High-resolution mass spectroscopy (HR-MS) measurements were taken with a JEOL-JMS 700 spectrometer. Size-exclusion chromatography (SEC) was carried out with an HP1100 apparatus equipped with a set of four columns (105104-103-102 Å: polymer standards service) with THF as the eluent. Polystyrene samples were used as standards to construct the calibration curve. 2.3. Experimental Setup for Polymerization in CO2. The experimental apparatus used in the polymer synthesis and solubility studies in supercritical carbon dioxide is shown in Figure 1. It consists of a high-pressure stainless-steel reactor equipped with a sapphire quartz window and a high-pressure syringe pump (ISCO model 500D series) for pressurizing the carbon dioxide. Heating was provided by a water bath, and the temperature was measured with a thermocouple (Doric Trendicator 400A). The water bath was controlled with a temperature controller (Jisico model J-IVW8, Korea), and a Teflon-coated magnetic stir bar was used to mix the cell contents. 2.4. Polymer-scCO2 Phase-Behavior Measurements. The apparatus and techniques used to obtain polymer-CO2 phasebehavior data is described elsewhere.17 The main component of the experimental apparatus is a high-pressure, variable-volume cell (∼30 cm3 working volume). The cell is first loaded with a measured amount of polymer. To remove entrapped air, the cell is degassed very slowly at pressures 99.9%) for 10 min. The cell was then pressurized at 20 MPa and 40 °C for 2 h. After polymerization, the cell contents were cooled to room temperature and depressurized slowly. The oxidized polymer was poured into methanol, and the precipitated dark brownish polymer was collected as dry powder. Random copolymers were prepared in a similar way with the required molar composition of 3-octylthiophene to fluorinated monomers. 3. Results and Discussion 3.1. Characterization of Polymers. Comparisons of yields, molecular weights, and solubilities of polymers prepared in conventional organic solvent, chloroform, and supercritical carbon dioxide are shown in Table 1. While PSFTE and PFTE3 were soluble in common organic solvents and scCO2, PFTE1 and PFTE2 were insoluble in any known solvent. It is well-
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Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
Scheme 1. Synthesis of Semifluorinated Ester-Substituted Polythiophene (PSFTE) and Its Random Copolymers with 3-Octylthiophene
Scheme 2. Synthesis of Various Perfluorinated Ester-Substituted Polythiophenes and Their Copolymers.
Table 1. Comparative Study of the Synthesis of Fluorinated Ester-Functionalized Polythiophenes in ScCO2 and CHCl3 a
entry polymer solvent 1 2 3 4 6 6 7 8 9 10
PSFTE PSFTE PFTE1 PFTE1 PFTE2 PFTE2 PFTE3 PFTE3 PFTE4 PFTE4
CHCl3 scCO2 CHCl3 scCO2 CHCl3 scCO2 CHCl3 scCO2 CHCl3 scCO2
solubility yieldb Mwc (%) (g mol-1) PDI scCO2 CHCl3 THF 60 55 80 70 63 63 85 69 90 80
12 000 11 500
1.15 1.20
66 000 52 000 5 000d 4 000d
2.39 3.11 1.20 1.30
O O X X X X O O D D
O O X X X X O O X X
O O X X X X O O D D
a O, soluble; X, insoluble; ∆, partially soluble. b Polymerization conditions: 2 h at 40 °C with 1:4 monomer-to-oxidant ratio. c Determined by GPC. d THF-soluble portion.
known that the delocalized electronic structures of π-conjugated polymers tend to yield relatively stiff chains with little flexibility and with relatively strong interchain attractive interactions, which are the primary reasons for their insoluble nature.20 Thus, in the quest for a soluble and processable conducting polythiophene, alkylthiophenes were polymerized by introducing flexible alkyl chains on the thiophene ring.1 Though chemically synthesized poly(3-methylthiophene) was insoluble,1 environmentally stable and soluble poly(3-alkylthiophenes) were pre-
pared with alkyl groups longer than butyl chains, which can readily be solution-processed into films.1 On our investigation of fluorinated ester-derivatized polythiophenes, we observed different behavior in solubility for polymers prepared from different precursors. It is noteworthy that, while the polymers (PFTE1 and PFTE2) which have a methylene (sCH2s) spacer between the thiophene ring and the oxygen atom of the carbonyl ester (sOsCdO) were insoluble, those that have an ethylene (sCH2sCH2s) spacer (PFTE3) were readily soluble in common organic solvents. Though, the reason for the difference is not clear, there have been reports on the insolubility of polythiophene derivatives which had one methylene spacer, while the same polymer with an ethylene spacer was soluble.21 However, PFTE4, which has an ethylene spacer, was insoluble in CHCl3 and was found to be partially soluble in THF and CFCl3. This might be due to the longer perfluoroalkyl groups versus thos on PFTE3. Our effort to get soluble polymers by making copolymers with soluble precursors such as 3-octylthiophene failed for FTE1 and FTE2. The random copolymers, P(OT-FTE1) and P(OT-PFTE2) were insoluble in any organic solvent. On the other hand, copolymers of P(OT-SFTE) and P(OT-PFTE3) were soluble in CHCl3 and THF. We observed that the conventional oxidant, iron(III) chloride, effectively oxidizes the monomers despite its very low solubility in scCO2. The yield and molecular weight of homo-copolymers and random copolymers obtained in scCO2 and chloroform were almost similar with a small negligible difference. However, conversion was slightly less for the polymerization in scCO2, which could be attributed to the lower solubility of FeCl3 in CO2. Moreover, while the polymers were synthesized in room temperature for organic solvent, the temperature was slightly increased to 35-40 °C for the polymerization in scCO2 to allow the CO2 to attain supercritical condition. Table 1 shows the molecular weight of homopolymers, determined by GPC with THF as the eluent. The weight-average molecular weights (Mw) of PSFTE and PFTE3 prepared by scCO2 were ca. 11 500 and 52 000 g mol-1 with polydispersibilities of 1.19 and 3.11, respectively. These values were almost similar for polymers synthesized in CHCl3. The THF-soluble fraction of PFTE4 has a Mw of ca. 4000 g mol-1. We have carried out the synthesis of a series of random copolymers of P(OT-SFTE) in varying compositions of 3-octylthiophene with fluorinated monomer, SFTE, in CO2. Yield, molecular weight, and the degree of polymerization results are shown in Table 2. The random copolymers of P(OT-SFTE) with a molar composition of 1:1 (OT/SFTE) was readily soluble in
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3409 Table 2. Comparative Study of the Synthesis of Random Copolymers of Various Fluorinated Ester-Functionalized Thiophenes with 3-Octylthiophene in ScCO2 and CHCl3
e
polymer
solvent
yieldb
P(OT-SFTE) P(OT-SFTE) P(OT-SFTE) P(OT-SFTE) P(OT-PFTE1) P(OT-PFTE1) P(OT-PFTE2) P(OT-PFTE3) P(OT-PFTE3)
CHCl3 scCO2 scCO2 scCO2 CHCl3 scCO2 CHCl3 CHCl3 scCO2
90 70 80 83 85 72 80 73 64
(%)
Mwc
(g
mol-1)
PDI
d
DPw (OT/SFTE)
mol. comp.e ([OT]/[SFTE])
34 000 29 000 28 600 28 000
2.1 2.8 2.5 2.1
86/34 73/29 60/34 47/38
1:1 1:1 1:1.4 1:2
74 000 61 000
3.12 3.67
151/136 122/112
1:1.5 1:1.5
solubilitya scCO2 CHCl3 O O O O X X X X X
O O O O X X X O O
a O, soluble; X, insoluble. b Polymerization time: 2 h with 1:4 monomer-to-oxidant ratio. c Determined by GPC. d Degree of polymerization for comonomers. Determined by NMR.
Figure 2.
1H
NMR spectra (400 MHz, CDCl3) of PSFTE.
Figure 4.
Figure 3.
1H
NMR spectra (400 MHz, CDCl3) of PFTE3.
scCO2. The actual molar composition of the copolymers was determined by 1H NMR and found to be similar to the feed ratio of the comonomers. While the Mw of P(OT-SFTE) prepared in CO2 with different compositions was ca. 29 000 g mol-1, P(OT-FTE) was found to have a Mw of ca. 61 000 g mol-1. 1H NMR spectroscopy was used further to characterize the microstructure of fluoroalkyl-ester-substituted polythiophenes prepared in scCO2. 1H NMR spectra of PSFTE, PFTE, and the 1:1 random copolymer of P(OT-SFTE) were presented in Figures 2, 3, and 4, respectively. As was expected for oxidative polymerization, 1H NMR spectra indicate that the polymers prepared in CO2 using FeCl3 have a regiorandom linkage. Regioregularity (ratio of HT to HH (or TT)) was related to the presence of splitting of the signal for the methylene group directly attached to the thiophene ring. The head-to-tail (HT) content was evaluated by calculating the intensities of the two peaks of the R-methylene protons in the range from 2.4 to 3.5 ppm. HT contents for PSFTE, PFTE, and the copolymer P(OTSFTE) were approximately 67%, 55%, and 65%, respectively. It may be noted that the other characteristics of conducting
1H
NMR spectra (400 MHz, CDCl3) of P(OT-SFTE).
polymers such as conjugation length, UV-vis absorption, and conductivity were not affected by the high-pressure supercritical carbon dioxide.9,14 3.2. Solubility of Polymers in scCO2. In general, the level of polymer solubility in supercritical CO2 is sensitive to polarity and backbone flexibility.22,23 In this study, we have investigated the role of the ester side chain on the solubility of derivatized polythiophenes as a function of the alkoxy tail length and its fluorine content. The polarity of the ester group was kept relatively constant in the examples studied here. It was established that CO2 does not dissolve nonpolar polyolefins. Hence, the fluorination of the alkoxy tail was anticipated to impact on the solubility of polythiophenes in scCO2 on the basis of the expected polarity and flexibility of functionalized polythiophenes. The solubility of polymers in scCO2 was studied by the cloudpoint method. A known quantity of the sample was taken into a variable-volume stainless-steel cell equipped with a sapphire quartz window and a magnetic stir bar. Results were observed in temperature range of 35-60 °C and at pressures up to 35 Mpa. We observed that the fluorinated ester-functionalized polymers which showed solubility in common organic solvents were also soluble in CO2, except the random copolymer P(OTPFTE3). While PFTE was readily soluble in CO2, polymers with similar structural units except with only one methylene spacer group, PFTE1 and PFTE2, were insoluble in CO2 even at higher pressures. Though these polymers possess high fluorine content, the insolubility could be due to their stiff chains with little flexibility and with relatively strong interchain attractive interactions. Thus, the interaction between the CO2-ester complex and the CO2-fluoroalkyl chains does not likely overcome the rigid nature of PFTE1 and PFTE2.
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Figure 5. Cloud-point profile of PSFTE and P(OT-SFTE): O, PSFTE 6.65 wt % in CO2; b, P(OT-SFTE) 1:2 random copolymer 0.052 wt % in CO2; 1, P(OT-SFTE) 1:2 random copolymer 0.052 wt % in CO2.
The cloud-point profile of PSFTE and its copolymers with 3-octylthiophene (POT-SFTE) is shown in Figure 5. The optically transparent one-phase region is above each curve in the plot. The results indicate that the PSFTE is highly soluble in scCO2 at low temperatures and pressures, as it takes
