Solvent free oxidative coupling polymerization of 3-hexylthiophene (3HT) in the presence of FeCl₃ particles

Abstract

A solvent free oxidative coupling reaction of 3-hexylthiophene (3HT) within a nanocavity is reported. It is found that 3HT can be encapsulated in nanocavities larger than 1 nm, which corresponds to the size of the molecule. In this case, the side reaction at the 4-position in 3HT is regulated.

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Poly(3-hexylthiophene) (P3HT) has been widely used in p-type semiconductor devices including organic solar cells and transistors.^1,2 Due to its simplicity compared to other strategies, oxidative coupling polymerization of 3-hexylthiophene (3HT) in the presence of FeCl₃ particles is a widely used approach to prepare P3HT for academic studies.^3–5 However, this method requires the use of organic solvents, especially CHCl₃ or CH₂Cl₂ containing halogens, which pose an environmental challenge and limit the relevance of the technique to larger scale production. Solvent free oxidative coupling polymerization has been reported for thiophene derivatives such as 3,4-propylenedioxylthiophene,⁶ but has not been previously reported for 3HT. The difference may be attributed to the reactive positions in the thiophene derivatives, which are at 2 and 5, whereas in the case of 3HT, they are at 2, 4, and 5. The difference in reactive position may lead to a defect at the 4 position for P3HT when prepared by solvent free oxidative coupling. There remains a significant need for novel oxidative coupling polymerization strategies to prepare P3HT in solvent free conditions.

One potential approach to modify reactivity is to carry out the polymerization reaction in a nanocavity.^7–10 For example, divinylbenzene forms a cross-linked structure when polymerization by radical reaction in the bulk state. In contrast, when the reaction is carried out in a nanocavity with similar diameter as divinylbenzene, a linear polymer was obtained.¹¹ In this work, we have explored the use of a nanocavity for the oxidative coupling polymerization of 3HT with a target of producing high molecular weight P3HT via a solvent free approach.

Fig. 1 shows the (a) ¹H nuclear magnetic resonance (¹H NMR) and (b) Fourier-transform infrared spectroscopy (FT-IR) spectra of P3HT prepared by oxidative coupling polymerization in CHCl₃ and solvent free (bulk) condition. The stoichiometric ratios of 3HT and FeCl₃ were 1 : 4. In the bulk condition, insoluble components are formed and only the soluble part is available for characterization. The signals at 0.87 and 7 ppm in ¹H NMR spectra correspond to the methyl group and 4-position of 3HT, respectively.¹² For the polymerization carried out in CHCl₃, the ratio of integral intensity of the signals was 3 to 1, which indicates that side reactions involving chlorination at the 4-position or 2, 4-coupling reaction were regulated. In the bulk condition, the ratio of integral intensities was 3 to 0.4, which indicates that a side reaction occurred. The FT-IR spectra of obtained P3HT are shown in Fig. 1b. The bands at 3090–3030 cm⁻¹ are assigned to the stretching vibration mode of thiophene at the 4 position (C_β–H),^12,13 while the bands at 3000–2800 cm⁻¹ are assigned to CH₃ and CH₂ stretching vibrations of hexyl group.¹³ The intensities of the FT-IR spectra were normalized based on the anti-symmetric stretching vibration of CH₃ at 2955 cm⁻¹. The integral intensity of the C_β–H stretching vibration is much more pronounced for the P3HT polymerized in CHCl₃ than in the bulk condition, which supports that a side reaction takes place at the 4-position in bulk condition. Moreover, the number-average molecular weight (M_n) of P3HT prepared in CHCl₃ was higher (M_n = 35 600, polydispersity index (PDI) = 2.1) than that in the bulk (M_n = 3000, PDI = 4.5). The results support that the regulation of side reaction at 4-position in 3HT is necessary to achieve solvent free oxidative coupling polymerization for 3HT.

Fig. 1 (a) ¹H NMR and (b) FT-IR spectra for P3HT prepared in bulk and in CHCl₃ solution.

To investigate the role of encapsulation on the polymerization reaction, 3HT was placed in zeolite (molecular sieve 13×) with pore size of about 1 nm. The molecular size of 3HT was estimated to be about 1 nm when the hexyl group is fully stretched. Fig. 2 shows thermogravimetric analysis (TGA) for 3HT, zeolite with and without the 3HT, and purified P3HT which is prepared by oxidative coupling polymerization in zeolite. The significant increase in weight loss between zeolite with and without the 3HT confirms that the 3HT is encapsulated in the zeolite nanocavity. Based on the TGA measurements, the mass of 3HT in zeolite was determined. Based on the content of 3HT, 3HT in zeolite and FeCl₃ particles were mixed with stoichiometric ratio of 1 : 4.

Fig. 2 TGA curves of P3HT, zeolite, zeolite including 3HT, and 3HT.

Oxidative coupling polymerization was performed by mixing the zeolite with 3HT and FeCl₃ in an inert gas environment. The reaction was performed at room temperature for 24 h. The zeolite was subsequently washed by THF for 24 h, then filtrated and evaporated. The primary structure of the residue was evaluated using ¹H NMR and size exclusion chromatography (SEC) measurements. The ratio of integral intensity at 0.87 and 7 ppm was found to be 3 to 1 (Fig. ESI 4a†), which indicates that the nanocavity regulates the side reaction of 3HT at the 4-position. The signals in the ¹H NMR at 2.55 and 2.8 ppm correspond to head-to-head (H–H) and head-to-tail (H–T) configurations, respectively.¹² The populations of HT–HT, TT–HT, HT–HH, and TT–HH segments were evaluated based on the signals at 6.98 ppm, 7.00 ppm, 7.02 ppm, and 7.05 ppm, respectively.¹² The results are summarized in Table 1. The findings indicate that although P3HT was successfully produced using solvent-free oxidative coupling polymerization by encapsulating 3HT in a zeolite nanocavity, the stereoregularity is not controlled compared with the non-encapsulated reaction in CHCl₃. The influence of cavity size on the polymerization reaction was evaluated by the same protocol using a halloysite nanotube, which is a cylindrical nanoparticle with diameter of about 15 nm and length of several micrometers.¹⁴ The primary structure evaluated using ¹H NMR spectrum showed the ratio of integral intensity at 0.87 and 7 ppm to be 3 to 0.7 (Fig. ESI 4b†). SEC measurements revealed that the M_n of P3HT prepared in the halloysite nanotube is higher than that obtained in the bulk (Table 1). In this reaction condition, almost all parts of P3HT are soluble in CHCl₃. Taking these results into account, it is clear that a side reaction at 4-position of 3HT still occurs, but is limited with respect to the bulk condition. These results demonstrate that size of the nanocavity strongly affects the nature of the side reaction.

Table 1 Summary of molecular characterization for P3HT prepared under different conditions

Samples	Dyads^a (%)		Triads^a (%)				Mn^b (g mol^−1)	PDI^b
	HT	HH	HT–HT	TT–HT	HT–HH	TT–HH
a Determined by ^1H NMR spectrum.b Determined by SEC measurements using PS as standards.
P3HT in bulk	48	52	35	27	19	19	3000	4.5
P3HT in CHCl3	66	34	53	17	18	12	35 600	2.1
P3HT in zeolite	60	40	41	19	21	19	22 700	2.9
P3HT in halloysite	52	48	45	15	22	18	7000	4.0
P3HT in MOF	66	34	48	16	20	16	29 500	3.2

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One limitation of the polymerization reaction using zeolite is that a significant amount of organic solvent is still necessary to isolate the obtained P3HT from the nanocavity. It would be advantageous to use a less stable structure to regulate the reaction to facilitate recovery. A metal organic framework (MOF) consisting of [Cu₂(bdc)₂–(ted)]_n (bdc = 1,4-benezenedicarboxylate, ted = triethylenediamine) was selected. This MOF has pore size of 0.75 × 0.75 nm² and can be dissolved in an ethylenediaminetetraacetic acid (EDTA) aqueous solution.^10,15 It is anticipated that the oxidative coupling reaction in MOF can reduce the solvent requirements during the recovery step. To investigate the stability of the MOF in the presence of the oxidant, the MOF and FeCl₃ were mixed at room temperature for 24 h. The molecular aggregation structure was subsequently investigated using wide-angle X-ray diffraction (WAXD) measurements (Fig. 3a) and X-ray absorption fine structure (XAFS) measurements (Fig. 3b). After 24 h mixing, no significant change could be observed from either WAXD or XAFS spectrum. The findings indicate that the MOF is stable in the presence of the oxidant and that the average size of the nanocavity does not change over time.

Fig. 3 (a) WAXD line profile for MOF, MOF including 3HT and MOF with FeCl₃. (b) XAFS spectra for MOF and MOF with FeCl₃.

The ability of the MOF to encapsulate 3HT was evaluated using WAXD and TGA measurements (Fig. 3a and ESI 5†). For WAXD, the diffraction intensity depends on the electron density of the materials. The integral diffraction intensity of the MOF was much higher than that of the MOF containing 3HT, which supports that the 3HT is encapsulated in the MOF nanocavities. The MOF is made up of square pores with the length of 0.75 nm, which is shorter than the estimated size of 3HT, ∼1 nm. However, the diagonal of the pore is 1.06 nm, which may accommodate the monomer in a specific configuration. The TGA results are consistent with the expected stoichiometric ratio for the system, which confirms that the 3HT is effectively encapsulated by MOF. The polymerization reaction was carried out by mixing 3HT in the MOF with FeCl₃ at stoichiometric ratio of 1 : 4 at room temperature. After 24 h, the P3HT was isolated from the MOF using EDTA aqueous solution. The primary structure of the obtained polymer was evaluated using ¹H NMR and SEC measurements. The polymer shows high HT and HT–HT ratios, which demonstrate that the side reaction at the 4-position is effectively regulated using MOF (Table 1). The findings demonstrate that solvent free oxidative coupling polymerization was achieved using MOF. A summary of the results showing the influence of cavity size on polymerization is shown in Fig. 4. Although the zeolite and MOF possess different 3D network structure, linear P3HTs with high M_n were obtained in both cases. These results suggest that molecular motion of 3HT monomer and active chain ends in the zeolite and MOF are confined in 2 dimensions, resulting in the formation of linear P3HT.

Fig. 4 Schematic illustration of 3HT oxidative coupling polymerization in nanocavity.

In the case of the oxidative coupling polymerization in CHCl₃, it is necessary to add the solid oxidant FeCl₃ at a 4 times higher concentration than 3HT to yield P3HT. It has been suggested that HCl generated in the reaction system reacts with FeCl₃ to form FeCl₄⁻, which serves to regulate the extent of reaction in the system.⁴ On the other hand, we reported that the outermost surface of FeCl₃ is surrounded by an insoluble fraction of polymer.¹⁶ It should be possible to determine if a similar regulation takes place for the encapsulated system by decreasing the concentration of FeCl₃ relative to 3HT below 1 : 1. To evaluate the regulation behavior, FeCl₃ was mixed with 3HT encapsulated in MOF at mixture ratios of 3HT : FeCl₃ = 1 : 1, 2 : 1, 4 : 1 and 10 : 1. The reaction was performed at room temperature for 24 h. The primary structure of the obtained polymer is summarized in Table 2. Interestingly, using the MOF, P3HT could be obtained even if the stoichiometric amount of FeCl₃ is lower than that of 3HT. This results suggest that the presence of the nanocavity not only enables the solvent free oxidative coupling reaction to take place, but also permits the amount of oxidant to be reduced. The ability to reduce the concentration of oxidant is attributed to a decrease in the amount of insoluble polymer built up around the FeCl₃ particles.

Table 2 Molecular characteristics of P3HT obtained using different ratios of FeCl₃ to 3HT

[FeCl3]/[3HT]	HT^a (%)	Mn^b (g mol^−1)	PDI^b
a Determined by ^1H NMR.b Determined by SEC measurements.
1.0	67	21 000	2.3
0.5	66	21 000	2.6
0.25	67	20 300	2.5
0.1	66	22 400	2.7

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Conclusions

We have successfully achieved solvent free oxidative coupling polymerization reaction for 3HT using different types of nanocavities. The findings demonstrate that the pore size of the nanocavity strongly affects the nature of the side reaction at the 4-position. In the case of nanocavity with similar dimension as the 3HT, the side reaction is regulated and favours the formation of a linear polymer. For larger nanocavities, the side reaction is not regulated and polymerization occurs similar to the bulk, resulting in a branched polymer. The amount of FeCl₃ can be effectively reduced using oxidative coupling reaction with nanocavity because the cavity regulates the anchoring of P3HT molecules at the surface of FeCl₃.

Acknowledgements

This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We also acknowledge support from the World Premier International Research Center Initiative (WPI) MEXT, Japan and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. Part of this work was supported by the Impulsing Paradigm Change through Disruptive Technologies (ImPACT) Program. The XAFS experiments were performed at the Kyushu University Beamline (BL06/SAGA-LS) with proposal numbers 2014IIIKN009, 2015IK008, 2015IIK006, 2015IIIK003, 2016IK009. The FT-IR measurements were performed at the BL43IR beamline of SPring-8 under proposals 2014A1234, 2014B1288, 2015A1588. We also would like to thank for Keiji Horio of MicrotracBEL Corp. helpful feedback.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23178k

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