All reagents were purchased from Sigma-Aldrich Corp. (>99%) and used as received unless otherwise stated. Tetrahydrofuran (THF, 99.5%, Wako Chemicals Co., Ltd.) was refluxed over sodium benzophenone under nitrogen for 2 h, then distilled just before use. THF for the solvent of living anionic polymerization was refluxed over Na wire, then distilled from lithium aluminum hydride. It was finally distilled from sodium naphthalenide on a high vacuum line. 1,1-Diphenylethylene (DPE) was distilled from calcium hydride and 1,1-diphenylhexylthium under reduced pressure. α,ω-DPE difunctional P3HT (DPE-P3HT-DPE) [16] and TVS [59] were synthesized and purified according to the previous papers.

Living anionic polymerization of TVS was performed by using a standard high vacuum technique. In a typical experiment, sec-BuLi (2.0 mmol) was added to a THF solution (5.0 mL) of DPE (0.100 mmol) at −78 °C, and the solution was warmed to room temperature and kept for 1 h. After the solution was again cooled to −78 °C, a THF solution of TVS (0.50 g, 3.0 mmol) was added with vigorous stirring. After 8 h, the solution was poured into a mixture of methanol/water (90 mL/10 mL) to precipitate the polymer. The polymer was purified by reprecipitation using THF and methanol/water, followed by freeze-drying from its absolute benzene solution to afford PTVS as a white solid (0.48 g, 92%).

PTVS-b-P3HT-b-PTVS was synthesized by using a standard high vacuum technique. To a THF solution (50.0 mL) of DPE-P3HT-DPE (M_n = 12,500, PDI = 1.10, r.r. = 97%, 0.100 g, 0.008 mmol) sec-BuLi (0.320 mmol) was added at −78 °C, and the solution was warmed to room temperature and kept for 1 h. After the solution was cooled to −78 °C, a THF solution of TVS (0.628 mmol) was added with vigorous stirring. After 24 h, the solution was poured into a mixture of methanol/water (90 mL/10 mL) to precipitate the polymer. The polymer was purified by fractional precipitation using ethyl acetate, followed by freeze-drying from its absolute benzene solution to afford PTVS-b-P3HT-b-PTVS as a purple solid (0.120 g, 61%).

PTVS-b-P3HT-b-PTVS (1.0 μm thickness) was obtained by spin coating from a chloroform solution (10 mg/mL) onto a glass substrate. The film was dried under vacuum and then heated at 170 °C for 2 h. After cooling down to room temperature and purging with nitrogen, a dark purple PA-b-P3HT-b-PA film was obtained.

Molecular weights (MWs) and polydispersity indices (PDIs) were measured with a HPLC system equipped with a pump, refractometer (RI), absorbance detector (UV, λ = 254 nm), online RALLS detector (λ = 670 nm), sample processor, and polystyrene gel columns connected in the following series: 650, 200, and 75 Å. THF was used as a carrier solvent at a flow rate of 1.0 mL/min at room temperature. ¹H NMR was recorded on a Bruker DPX (300 MHz) in chloroform-d calibrated to tetramethylsilane as an internal standard (δH 0.00). TGA was performed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a heating rate of 10 °C/min. FT-IR spectra were measured by a Horiba FT-120 Fourier transform spectrophotometer. UV-vis absorption spectra of a polymer solution in chloroform and the thin film were recorded on a Jasco FP-750 spectrometer over a wavelength range of 250–800 nm. The films were obtained by spin coating from a toluene solution, followed by drying under vacuum. AFM phase images were taken with a SII-NT SPA 400 operating in a tapping mode. Electric conductivity was measured by the typical four point probe technique using a cast film from a toluene solution (5 mg/mL) onto a glass substrate.

As mentioned in the Introduction, Hogen-Esch et al. demonstrated that the polymerization of phenyl vinyl sulfoxide proceeded in a living manner to afford polymers with controlled MWs and relatively low PDIs in the range of 1.2 to 1.4 [58]. They also succeeded in a quantitative transformation reaction into PA simply by thermal treatment. By this success, poly(phenyl vinyl sulfoxide) obtained by living anionic polymerization becomes an advantageous PA precursor, soluble in organic solvents and well-controlled in chain length. Recently, Hirao reported that the use of a DPE anion as the initiator in the presence of a 10 ∼ 20 fold excess of lithium halides (LiCl or LiBr) is effective to further reduce the PDI value (PDI = 1.07–1.2) of a similar polymer, PTVS, which could also be thermally converted to PA in 100% yield [59,60]. However, the addition of a large amount of salts in polymerization media requires a thorough drying process of the salts. In a macroinitiator system, this becomes quite a serious problem, because even a small amount of contamination from salts affords a mixture of a deactivated homopolymer with the expected BCP. Therefore, a new living system has to be developed, especially suited for the macroinitiator system.

To prepare the α,ω-difunctional P3HT macroinitiator, if a precursory DPE difunctional P3HT (DPE-P3HT-DPE) is reacted with a large excess of sec-butyllithium (sec-BuLi) toward the DPE moieties, the expected macroinitiator can be obtained by an addition reaction of sec-BuLi with the terminal DPE moieties, leaving an excess of sec-BuLi. However, sec-BuLi can selectively decompose with a THF solvent into ethylene gas and lithium enolate by elevating the temperature to 25 °C (Scheme 3) [17]. It should be noted that stabler initiating DPE anions survive under this condition and the generated ethylene and lithium enolate are inert in the living polymerization [17]. We expected that the in situ-generated lithium enolate would function to lower the PDI of PTVS instead of the lithium halides.

A model homopolymerization of TVS was carried out in THF initiated by 3-methyl-1,1-diphenylpentyllithium in the presence of lithium enolate, derived from DPE and a large excess of sec-BuLi (Scheme 4). The results are summarized in Table 1. The molecular weight ranges from 5,200 to 27,000 with low PDI (1.10–1.22), as expected. The representative SEC profile is shown in Figure 1. In a blank experiment, the PDI increases to around 1.5 without lithium enolate. Consequently, the in situ-generated salt, lithium enolate, is found to be effective to obtain PTVS with controlled MWs and low PDIs.

To synthesize PTVS-b-P3HT-b-PTVS, precursory DPE difunctional P3HT (DPE-P3HT-DPE, M_n = 12,500, PDI = 1.10, r.r. = 97%) was reacted with a 20-fold excess of sec-butyllithium (sec-BuLi) toward the DPE moieties in THF at −78 °C for 10 min. The excess sec-BuLi decomposed with THF by warming to room temperature and keeping for 1 h, resulting in in situ generation of excess lithium enolate. The system was then cooled back to −78 °C. TVS was next added at −78 °C to start the living anionic polymerization, followed by quenching with degassed methanol (Scheme 2). As shown in the SEC chromatograms (Figure 2), the peak top for the precursory DPE-P3HT-DPE obviously shifts to a higher molecular weight region after the polymerization. Unfortunately, there is a minor shoulder corresponding to unreacted DPE-P3HT-DPE, probably because of incomplete initiation from the partially aggregated macroinitiator in THF at −78 °C , similar to the case in the synthesis of PMMA-b-P3HT-b-PMMA [17]. Eventually, it was successfully eliminated from the mixture by fractional precipitation using ethyl acetate. The isolated PTVS-b-P3HT-b-PTVS possesses a unimodal sharp peak (Figure 2). The M_n value of PTVS-b-P3HT-b-PTVS, determined by SEC-RALLS (24,500), is in good agreement with the calculated one (22,300). The PDI value determined by SEC is acceptable (1.33).

Moreover, ¹H NMR spectroscopy of PTVS-b-P3HT-b-PTVS confirms the absence of a signal for the vinyl protons of the DPE moieties at 5.4 ppm, as well as a reasonable composition of two segments being almost consistent with that targeted (Figure 3(B)). Thus, the expected PTVS-b-P3HT-b-PTVS (6,000-12,500-6,000) could be successfully synthesized. It should be noted that the DPE chemistry employed here is quite elegant and efficient for designing a wide range of well-defined BCPs, because the 1,1-diphenylalkyl anion is able to undergo living anionic polymerization of many other kinds of monomers, acrylates, acryl amides, vinylpyridines, epoxides, propylene sulfide, and so on.

The transformation from PTVS-b-P3HT-b-PTVS into PA-b-P3HT-b-PA was performed simply by thermal treatment at 170 °C for 2 h under vacuum (Scheme 2). Figure 4(A) shows the TGA curve of PTVS-b-P3HT-b-PTVS. The first weight-loss from 120 to 250 °C may be ascribed to the elimination of 4-tolylsulfenic acid from the PTVS segments, as confirmed by the weight-loss behavior of a PTVS homopolymer (Figure 4(B)). The weight loss percentage at temperatures in the range of 120 to 250 °C was nearly consistent with that assumed by complete elimination. The second weight-loss was seen from 350 to 470 °C, corresponding to the degradation of the PA-b-P3HT-b-PA segments.

The FT-IR spectrum of PTVS-b-P3HT-b-PTVS (Figure 5(A)) showed a strong absorption band at 1,045 cm⁻¹, assignable to S=O stretching, which completely disappeared after thermal treatment. Instead, an absorption band at 1,005 cm⁻¹, attributed to trans-vinylene out-of-plane deformation from PA main chains, newly appeared (Figure 5(B)). In addition, the polymer became insoluble in common organic solvents, while precursory PTVS-b-P3HT-b-PTVS was well soluble in toluene, chloroform, and THF.

These analytical results clearly confirmed the complete conversion from PTVS to PA chains by quantitatively eliminating 4-tolylsulfenic acid in PTVS-b-P3HT-b-PTVS.

The optical property of PTVS-b-P3HT-b-PTVS was studied by UV-vis spectroscopy. The solution-state UV-vis spectrum of PTVS-b-P3HT-b-PTVS in chloroform showed maximum absorption (λ_max) for the π–π* transition of the P3HT inner segment at around 442 nm (Figure 6 (yellow line)). In the film state, the λ_max value of the polymer was bathochromically shifted to around 565 nm compared with that in the solution state (pink line). In addition, the polymer showed a shoulder at around 610 nm related to vibronic absorption, similar to the pristine P3HT film (purple line). This observation indicates a high degree of ordering in the polymer films even if PTVS is covalently linked to P3HT. After thermal transformation, the absorbance was somewhat red-shifted to obtain an off-set value at a longer wavelength (∼700 nm, green line)). This indicates the successful conversion from PTVS to PA segments in the PTVS-b-P3HT-b-PTVS film. Interestingly, even after generating a highly rigid-rod PA segment, the shoulder at around 610 nm remains intact, indicative of highly ordered P3HT packing structures. Therefore, the thermal transformation reaction seems not to destroy the self-assembled structures, derived from precursory PTVS-b-P3HT-b-PTVS in the film state.

Electrical conductivity of PA-b-P3HT-b-PA was then measured by using a typical four-point probe technique. After iodine doping, relatively high conductivity of 0.126 S/cm was obtained, while overflow resistance was obtained for an undoped sample of PA-b-P3HT-b-PA. In order to improve the conductivity to more than 10 S/cm, a film-forming condition to obtain nanofibril or nanowire structures may be required [61].

To gain insight into the morphology, a thin film of PTVS-b-P3HT-b-PTVS was prepared by drop casting from a toluene solution onto a mica substrate at room temperature. The film thickness was estimated to be 100 nm by Dektak³. Figure 7(A) and Figure 7(B) show tapping-mode AFM height and phase images of the PTVS-b-P3HT-b-PTVS thin film surface, respectively, after solvent annealing with toluene vapor at room temperature for 3 h. Although a periodic microphase-separated morphology is not observed, continuous nanofibril structures are observed at 10–20 nm sizes in whole areas, without macrophase separation or large aggregation. Figure 7(C) and Figure 7(D) show tapping-mode AFM height and phase images of the PA-b-P3HT-b-PA thin film obtained by a thermal transformation reaction of PTVS-b-P3HT-b-PTVS on a mica substrate. As can be seen, there is almost no difference in morphology before and after the transformation, which might be coincident with the observation by UV-vis spectroscopy showing a high degree of ordering in P3HT packing even after the thermal transformation. In contrast, the roughness determined by height images significantly changed after the transformation from 6.5 to 17.6 nm. This result suggests that the PA-b-P3HT-b-PA film possesses discrete pits [22] in the matrix on a nano-scale, probably generated by eliminating ca. 85 vol% of a large leaving group, 4-tolylsulfenic acid, from PTVS domains, which are anchored by rigid and crystalline P3HT domains in the film state. Such semiconducting nanoporous films are quite interesting materials, especially for solar cell device applications [62], after back-fill of n-type semiconductors into nanoporous matrices, possibly achieving large p/n interface areas with controlled morphological domain widths which correspond to exciton diffusion lengths.