Introduction

Poly(3-hexylthiophene) (P3HT) is a conjugated polymer with high hole mobility and applied to organic and perovskite solar cells and organic thin-film transistors [1,2,3,4]. A wide range of studies have been reported for fine-tuning the optoelectronic and physical properties of P3HT through molecular engineering of its microstructure, i.e., the regioregularity, side chains, end groups, composition, and architectures. In particular, the introduction of a versatile synthetic handle to the P3HT end-group is important for the preparation of functional P3HT and its block copolymers. Hence, P3HT with reactive end groups such as alkynes and pentafluorobenzene has been developed, especially for click coupling reactions [5,6,7]. In addition, alcohol end-functionalized P3HT plays a central role in the synthesis of conjugated rod-coil block copolymers in conjunction with controlled polymerization reactions, including ring-opening, atom-transfer radicals, reversible addition-fragmentation chain-transfer, and cationic polymerizations, and a wide range of functional conjugated materials have been obtained with high precision [8,9,10,11,12].

The development of living polymerization methods has paved the way to the production of well-defined end-functional polymers [13,14,15,16]. In particular, catalyst-transfer polycondensation (CTP) is a controlled chain-growth polymerization method used to produce conjugated polymers with controlled molecular weight, low dispersity, and well-defined end groups [17]. In a seminal work, Kumada CTP (KCTP) revolutionized the controlled synthesis of conjugated polymers, including P3HT [18]. Given the power of this method, a wide range of well-defined end-functional P3HTs have been prepared with precisely controlled molecular weights and narrow dispersity [12]. In particular, the well-established protocol for the synthesis of P3HT-OH via KCTP involves the preparation of allyl end-functional P3HT and its postmodification by hydroboration-oxidation (Scheme 1a). However, this protocol requires multistep postmodifications that can reduce the end-group fidelity of P3HT, as well as involves time-consuming purification steps. Another method involves the use of functional initiators for KCTP, which might ensure high end-group fidelity of P3HT (Scheme 1b) [19]. However, because of the use of the Grignard reagent in KCTPs, the alcohol end-group should be protected during polymerization, eventually necessitating additional deprotection and purification steps (Scheme 1b). To address this issue, Negishi CTP (NCTP) with an alcohol-functional zincate complex used as a terminator has been developed and can directly produce alcohol end-functional P3HT without the need for a protection technique (Scheme 1c) [20, 21]. Nevertheless, this method requires an excessive number of terminators to guarantee high chain-end fidelity [21].

Scheme 1
scheme 1

Preparation of P3HT-OH by (a) KCTP and postmodifications, (b) KCTP with a protected functional initiator, (c) NCTP with an alcohol-functionalized terminator, and (d) SCTP with an alcohol-functionalized initiator

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To improve accessibility to P3HT-OH and its block copolymers, a new direct synthetic pathway for P3HT-OH needs to be developed. The Suzuki-Miyaura reaction features high coupling efficiency and good functional group tolerance for polar functionalities [22]. Moreover, recent advances in Suzuki-Miyaura CTP (SCTP) have paved the way for the synthesis of P3HT with excellent control and high productivity [23, 24]. Thus, the introduction of an alcohol-functional initiator to this SCTP can allow the preparation of well-defined P3HT-OH without compromising polymerization control (Scheme 1d). Additionally, the process occurs in one pot even without a protecting group or the use of an excessive amount of terminators, which is beneficial to the improved synthetic accessibility of P3HT-OH (Scheme 1d).

Herein, we report the one-pot synthesis of P3HT-OH from an unprotected alcohol initiator via SCTP. The effects of the catalyst amount and reaction time were systematically investigated to obtain high-quality P3HT-OH. Under optimized reaction conditions, the preparation of low to high molecular weight P3HT-OH was attempted. Finally, the utility of P3HT-OH for block copolymerization was shown by the preparation of P3HT-block-polystyrene.

Results and discussion

Since the Suzuki-Miyaura reaction has high functional group tolerance, we attempted SCTP using 4-iodobenzyl alcohol without a protecting group. The RuPhos Pd G3 precatalyst was used to efficiently generate RuPhos Pd(0), enabling facile external initiation with 4-iodobenzyl alcohol to produce Ar-Pd(RuPhos)-I [23, 25]. Additionally, extra RuPhos ligand was added during polymerization to stabilize the active Pd chain-end and promote the catalyst-transfer reaction [25, 26]. A N-methyliminodiacetic acid (MIDA)-protected monomer (M) was chosen to minimize side reactions such as protodeboronation and boronate homocoupling [26]. Initially, polymerization was carried out using 4.0 equivalent of the RuPhos Pd G3 precatalyst relative to that of the alcohol initiator to ensure complete consumption of the initiator (Table 1, entry 1). Size-exclusion chromatography (SEC) analysis of the final product confirmed the formation of a polymer with an Mn of 2.0 kDa and Ð of 1.31 (Table 1, entry 1). A closer investigation of the end-group of the final polymer by 1H NMR revealed two major end-group signals at 4.72 and 9.99 ppm corresponding to the methylene group of benzyl alcohol and aldehyde end groups, respectively (Fig. 1a). The formation of aldehyde was probably due to the Pd-mediated oxidation of benzyl alcohol, which significantly reduced the alcohol end-group fidelity to 62% (Table 1, entry 1). To address this issue, we systematically lowered the [Pd]/[I] ratio to suppress oxidation and simultaneously improve polymerization control. As the [Pd]/[I] ratio was decreased from 4.0 to 1.5 and then to 1.0, the end-group selectivity of alcohol over aldehyde gradually increased from 62% to 74% and then to 97% (Table 1, entries 1−3 and Fig. 1a). Moreover, the polymerization control was greatly improved, affording P3HT-OH with an Mn of 5.2 kDa and Ð of 1.09 (Table 1, entry 3). Since there was a minimal amount of free Pd(0) catalyst at a [Pd]/[I] ratio of 1.0, the homocoupling of monomers producing Br-aryl-Pd(II)-Br might also have been suppressed, thus enhancing both polymerization control and end-group fidelity [26]. No further improvement was observed when a [Pd]/[I] ratio of 0.7 was adopted (Table 1, entry 4). Therefore, in terms of atom economy and polymerization control, the optimal [Pd]/[I] ratio was set to 1.0. Additionally, matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass analysis of the product in entry 3 revealed a major series of signals corresponding to the mass of the benzyl alcohol/H end-group, further corroborating the formation of the well-defined P3HT-OH (Fig. 1b, Fig. S2).

Table 1 Optimization of the synthesis of P3HT-OH by SCTP
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Fig. 1
figure 1

Structural analysis of P3HT-OH. (a) 1H NMR spectra addressing the end groups of P3HT depending on the [Pd]/[I] ratio. (b) MALDI-TOF mass spectrum of P3HT-OH in Table 1, entry 3

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After determining the optimal [Pd]/[I] ratio, we investigated the effect of reaction time on the polymerization control and fidelity of the alcohol end-group. Initially, we prepared three independent batches of P3HT-OH with different reaction times (Table 1, entries 3, 5, and 6). Within 24 h, the polymerization was well controlled (Ð = 1.09) with high alcohol end-group fidelity (97%) (Table 1, entry 3). However, when the reaction was prolonged from 24 h to 72 h and then to 120 h, the SEC trace broadened, and the intensity of the higher molecular weight shoulder signal gradually increased (Fig. S1). This phenomenon can probably be attributed to disproportionation of the active Pd end-group, causing P3HT dimer formation [27, 28]. Nevertheless, the prolonged reaction did not affect the high alcohol end-group fidelity (Table 1, entries 3, 5, and 6). With this observation, we carried out kinetic monitoring of the reaction to determine the relationship between monomer conversion and the formation of the shoulder signal in the SEC trace. Interestingly, a unimodal SEC trace was observed until 98% conversion of the monomer, suggesting that disproportionation might occur when the monomer is completely consumed (Fig. 2). Considering these data, we identified the following two conditions to obtain high-purity P3HT-OH: (1) the [Pd]/[I] ratio must be ≤1.0, and (2) the polymerization should be quenched before complete consumption of the monomer.

Fig. 2
figure 2

Monitoring of SEC traces depending on the reaction time and monomer conversion when [M]/[I] = 25

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The distinct advantage of controlled polymerization over conventional step-growth polymerization is its ability to control molecular weight with high precision. Thus, using optimal conditions, we next examined the control of the molecular weight of P3HT-OH while maintaining high alcohol end-group fidelity. Notably, as the targeted degree of polymerization increased, the ratio of [RuPhos]/[RuPhos Pd] increased from 1.5 to 10 to effectively stabilize the active Pd chain-end, thereby achieving well-controlled polymerization [25, 26]. Upon varying the [M]/[I] ratio from 15 to 75, the SEC traces of the final polymer gradually shifted to the higher molecular weight region up to 16.5 kDa, with overall excellent control (Ð ≤ 1.10) and high yield (80−95%) (Fig. 3). Moreover, 1H NMR analysis of the product revealed excellent alcohol end-group selectivity over aldehydes (≥95%) for low to high molecular weight P3HT (Table 1, entries 3, 7, and 8; Fig. S3).

Fig. 3
figure 3

Controlled synthesis of P3HT-OH with low to high molecular weights. (a) SEC traces and (b) Mn, Ð, and the isolated yield of P3HT-OH depending on the [M]/[I] ratio from 15 to 75

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P3HT-containing block copolymers show unique self-assembly behavior and can therefore form various nanostructures that are useful for applications in solar cells, thin-film transistors, batteries, and sensors [29,30,31,32,33,34,35]. In particular, block copolymers from P3HT and functional coil-type polymers can enhance the processability, optoelectronic properties, and self-assembly behavior of the P3HT homopolymer [36, 37]. Thus, as a showcase example, we carried out the straightforward synthesis of P3HT-block-PS from P3HT-OH (Fig. 4). P3HT-OH with Mn = 5.0 kDa and Ð = 1.07 was prepared by the one-pot method described herein and was readily converted to an atom-transfer radical polymerization (ATRP) macroinitiator with high efficiency (Fig. 4 and S3). Subsequently, the ATRP of styrene using a P3HT-Br macroinitiator was performed to produce the desired block copolymers (Fig. 4). 1H NMR showed the characteristic signals corresponding to P3HT and PS blocks, and the SEC analysis revealed a complete shift of the SEC trace of P3HT to the higher molecular weight region while maintaining its unimodal trace shape (Mn = 9.3 kDa and Ð = 1.12), indicating the formation of a well-defined block copolymer (Fig. 4 and S3). This successful block copolymerization confirmed the high end-group fidelity of P3HT-OH, demonstrating the effectiveness of the one-pot SCTP protocol for synthesizing P3HT-OH.

Fig. 4
figure 4

Synthesis of P3HT-block-PS using P3HT-OH. Synthetic scheme (left) and the SEC traces (right)

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Conclusions

In summary, alcohol end-functionalized P3HT was successfully prepared by one-pot SCTP without protection techniques. A systematic investigation revealed that high chain-end fidelity of P3HT can be achieved only when the [Pd]/[I] ratio is ≤1 because at this ratio alcohol oxidation is suppressed. In addition, the polymerization should be quenched before full conversion of the monomer to minimize the dimerization of P3HT chains. Under optimized reaction conditions, low to high molecular weight P3HT-OH can be prepared with high precision and productivity. Moreover, the high end-group fidelity and utility of P3HT-OH were highlighted by the synthesis of P3HT-block-PS.

Experimental section

Materials

Unless otherwise noted, all reagents were purchased from commercial sources and used without further purification. Tetrahydrofuran (THF) was purified by passing through the activated alumina column. Radical inhibitors were removed from styrene before use by running styrene through a basic aluminum oxide plug. All solvents and styrene were degassed by argon bubbling for 10 min before polymerization.

General analytical information

1H NMR spectra were recorded with a JEOL Resonance ECZ600R (600 MHz) spectrometer. Chemical shifts are reported in delta (δ) units on the basis of parts per million (ppm) downfield from TMS. Size-exclusion chromatography (SEC) was performed at 35 °C using Shodex LF-804 columns eluted with tetrahydrofuran in a Waters 1515 pump, a Waters 2414 refractive index detector, and a Waters 2489 UV detector. Number average molecular weights (Mn) and weight average molecular weights (Mw) were calculated relative to linear polystyrene standards. Dispersity (Đ) values are reported as the quotient of Mw/Mn. MALDI-TOF mass analysis was performed with an Ultraflex III TOF/TOF spectrometer (Bruker Daltonics) using dithranol as the matrix.

Preparation of 5-bromo-4-n-hexylthien-2-yl-MIDA-boronate (M)

Monomers (M) were prepared according to a previously described procedure [25].

Preparation of P3HT-OH via SCTP

To a round-bottom flask equipped with a magnetic stir bar, the RuPhos Pd G3 precatalyst (0.033 g, 0.040 mmol), 4-iodobenzyl alcohol (0.0094 g, 0.040 mmol), RuPhos (0.028 g, 0.060 mmol), and K3PO4 (1.3 g, 6.0 mmol) were added. The flask was evacuated and backfilled with argon three times. Then, THF (10 mL) and H2O (3.6 mL) degassed with argon were added to the flask. The mixture was heated and stirred at 50 °C for 1 h to prepare the externally initiated catalyst. M (1 mmol) in degassed THF (85 mL) was then added to the flask, and the mixture was stirred at 45 °C for 24 h before quenching with 6 N HCl (25 mL). The crude reaction mixture was diluted with CHCl3 and washed with brine. The organic layer was concentrated under reduced pressure and purified by precipitation into MeOH. The precipitate was collected by filtration, washed with MeOH, and dried under vacuum to afford P3HT-OH as a violet solid. The molecular weight (Mn = 5.2 kg/mol) and dispersity (Ð = 1.09) of the polymer were obtained by SEC, with elution by THF (using a polystyrene standard). The reaction conditions with various [M]/[I] ratios are summarized below.

[M]/[I]

initiator (mmol)

cat. (mmol)

M (mmol)

RuPhos (mmol)

K3PO4 (mmol)

THF (mL)

H2O (mL)

yield (%)

15

0.067

0.067

1.0

0.1

6

10 + 85

3.6

95

25

0.04

0.04

1.0

0.06

6

10 + 85

3.6

84

75

0.0053

0.0053

0.4

0.053

2.4

4 + 34

1.4

80

Preparation of P3HT-Br macroinitiator

P3HT-Br macroinitiator was prepared according to a previously described procedure [25]. P3HT-OH (0.12 g, 0.028 mmol), anhydrous THF (2 mL), triethylamine (0.14 mL, 1.0 mmol), and 2-bromoisobutyryl bromide (0.069 mL, 0.56 mmol) were added to a 20-mL vial equipped with a magnetic stir bar. The mixture was degassed by purging with argon for 10 min and then heated overnight at 50 °C. The crude reaction mixture was diluted in chloroform and washed with brine three times. The organic layer was concentrated and precipitated into MeOH to afford P3HT-Br as a violet solid (yield = 80%). The molecular weight (Mn = 5.0 kg/mol) and dispersity (Ð = 1.07) of the polymer were obtained by SEC, with elution by THF (using a polystyrene standard).

Preparation of P3HT-block-PS

P3HT-block-PS was prepared according to a previously described procedure [25]. P3HT-Br macroinitiator (0.078 g, 0.018 mmol), CuBr (0.0025 g, 0.018 mmol), and PMDETA (0.0037 mL, 0.018 mmol) were added to a 4-mL vial equipped with a magnetic stir bar. The vial was evacuated and backfilled with argon three times. Toluene (0.6 mL) and styrene (0.6 mL) degassed by purging with argon for 10 min in a separate vial were added to the vial containing the mixture. Subsequently, the vial was sealed with Teflon tape and Parafilm and stirred at 90 °C for 4 h. The crude reaction mixture was diluted with CHCl3 and washed with brine. The organic layer was concentrated under reduced pressure and purified by precipitation into MeOH. The precipitate was collected by filtration, washed with MeOH, and dried under vacuum. The molecular weight (Mn = 9.3 kg/mol) and dispersity (Ð = 1.12) of the polymer were obtained by SEC, with elution by THF (using a polystyrene standard).