One-Pot Synthesis, Stimuli Responsiveness, and White-Light

Article pubs.acs.org/Macromolecules

One-Pot Synthesis, Stimuli Responsiveness, and White-Light Emissions of Sequence-Defined ABC Triblock Copolymers Containing Polythiophene, Polyallene, and Poly(phenyl isocyanide) Blocks Zhi-Peng Yu, Na Liu, Li Yang, Zhi-Qiang Jiang, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, and Anhui Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei, Anhui Province 230009, China S Supporting Information *

ABSTRACT: A family of sequence-defined ABC triblock copolymers composed of poly(3-hexylthiophene) (P3HT), poly(hexadecyloxylallene) (PHA), and poly(phenyl isocyanide) (PPI) blocks were facilely synthesized in one pot using Ni(II) complex as a single catalyst. Although the monomers of the three blocks are different than each other and were polymerized under distinct polymerization mechanisms, the one-pot sequential block copolymerization was revealed to proceed in a living/ controlled chain-growth manner. The sequence of the blocks can be facilely tuned through the variation on the order of monomer feed additions. Thus, a series of triblock copolymers with defined sequences, controlled molecular weights (Mns), narrow molecular weight distributions (Mw/Mns), and tunable compositions can be readily prepared just by varying the initial feed ratios of monomers to the catalyst. Taking advantage of this synthetic method, amphiphilic triblock copolymers containing hydrophobic P3HT, hydrophilic poly(triethylene glycol allene), and hydrophilic PPI bearing triethylene glycol monomethyl ether chains were synthesized. Interestingly, such amphiphilic triblock copolymers exhibited tunable light emissions with response to various environmental stimuli such as solvent, pH, and temperature. Remarkably, white-light emission can be readily achieved in solution, gel, and also solid state.

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mers.11−14 However, most of the reported sequence-defined multiblock copolymers prepared by those approaches were realized through the living/controlled polymerizations of congeneric monomers under the same polymerization mechanism.15−18 Thus, development of a novel synthetic method for facile preparation of monodisperse, sequence-defined hybrid multiblock copolymers composed of different kinds of blocks via mechanismatically distinct, sequential living polymerizations of completely different kind of monomers is of great interest. It is well-known that poly(3-hexylthiophene) (P3HT) and its derivatives are one of the most widely studied conjugated polymers and have been widely studied owing to their great potential applications in organic electronic devices, such as organic photovoltaics (OPV),19−22 organic field effect transistors (OFETs),23−26 and organic light-emitting diodes (OLEDs).27,28 Generally, P3HT is prepared via a quasi-living Kumada catalyst-transfer polymerization (KCTP) using Ni(II) complex as catalyst.29−37 Taking advantage of this method,

INTRODUCTION Compared to the synthetic polymers, biomacromolecules such as peptides, DNA, and proteins exhibit exceptional structural, chemical, and biological properties. 1 It is the specific distribution/placement of monomer units of the polymers which endows the remarkable properties of these biopolymers. One of the major challenges of polymer chemistry is to follow such sequence regulation in polymer synthesis. In the past few years, various approaches have been reported on preparing sequence-controlled and sequence-defined macromolecules.2−8 The synthesis of artificial peptides via sequential condensation steps of different amino acids onto a solid support is one of the most successful attempts to synthesize polymers with predetermined and highly controlled sequence distributions.9,10 However, the less-than-quantitative reaction yields and timeconsuming deprotection/purification process frequently resulted in low yield and ill-defined afforded polymers. These difficulties may occur in other iterative synthetic techniques used for sequence-controlled polymers. Living/controlled polymerization techniques such as living anionic and controlled radical polymerizations have been widely employed for synthesis of sequence-controlled and defined block copoly© XXXX American Chemical Society

Received: December 5, 2016 Revised: April 7, 2017

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DOI: 10.1021/acs.macromol.6b02558 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. One-Pot Synthesis of ABC Triblock Copolymers Containing P3HT, PHA, and PPI Blocks

chains were readily synthesized. Such triblock copolymers showed responsiveness to multiple external stimuli including solvents, temperature, and acid. Interestingly, white-light emission can be easily achieved not only in solution but also in gel and in solid state.

P3HT has been fabricated into various diblock copolymers such as P3HT-b-poly(phenyl isocyanide)38−40 and P3HT-b-polyallene41−44 copolymers; however, the monodisperse P3HTcontaining ABC triblock copolymers have rarely been reported.45,46 Poly(phenyl isocyanide) is another kind of conjugated polymer which has attracted considerable research attention in recent years owing to not only their interesting rigid-rod helical conformation but also wide applications in enantiomeric separation, chiral recognition, and asymmetric catalysis.47−49 Helical polyisocyanides have been combined with π-conjugated polymers to form block copolymers in order to regulate their self-assembly morphology and the optoelectronic properties.50−52 For example, white-light emission can be readily achieved from the poly(3-triethylene glycol thiophene)b-poly(phenyl isocyanide) copolymer in a given solvent, while it was quenched in the solid state.50 Among the light-emitting materials, organic white-light-emitting polymers are considerably attractive candidates for applications in flexible full-color OLEDs and in backlights for liquid-crystalline displays because of their great potential in flexibility and easy solution processing.53−56 General methods for preparing such materials involve blending polymers with luminescent dyes that simultaneously emit over the whole visible range and making copolymers containing moieties emitting at different wavelengths.57−62 Although white-light-emitting P3HT block copolymers have been achieved in solution, its quenching in the solid state may limit the practical applications. Therefore, development of novel P3HT-containing triblock copolymers that exhibit white-light emission in both solution and solid state is of particular importance. In this contribution, we report on one-pot synthesis of monodisperse, sequence-defined ABC triblock copolymers containing π-conjugated P3HT, poly(hexadecyloxylallene) (PHA), and poly(phenyl isocyanide) (PPI) blocks via three distinct polymerization mechanisms using a Ni(II) complex as single catalyst. The sequence of the blocks can be facilely tuned through variation on the order of monomer feed additions. The one-pot triblock copolymerization was revealed to proceed in living/controlled chain-growth manners, affording sequencedefined, hybrid ABC triblock copolymers in high yield with controlled molecular weights (Mns), narrow molecular weight distributions (Mw/Mns), and tunable compositions. Taking advantage of this synthetic strategy, amphiphilic triblock copolymers containing hydrophobic P3HT, hydrophilic poly(triethylene glycol allene), and hydrophilic poly(phenyl isocyanide)s bearing triethylene glycol monomethyl ether

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RESULTS AND DISCUSSION Synthesis of Sequence-Defined ABC Triblock Copolymers. The ABC triblock copolymers composed of P3HT, PHA, and PPI blocks were attempted to be synthesized in one pot via three sequential living polymerizations of the corresponding monomers using Ni(II) complex as a single catalyst. As shown in Scheme 1, Ni(II)-terminated P3HT was first prepared through the polymerization of 2-bromo-3-hexyl5-chloromagnesiothiophene (1) with Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphanyl)propane) as catalyst in THF at room temperature ([1]0 = 0.20 M, [1]0/[Ni]0 = 30), following the KCTP mechanism.29−37 When the polymerization was completed as indicated by size exclusion chromatography (SEC) analysis, the Mn and Mw/Mn of the afforded Ni(II)terminated poly-130 (the subscript indicates the initial feed ratio of monomer to catalyst, the same below) were estimated to be 4.8 kg/mol and 1.21, respectively, with equivalent to polystyrene standards. The remaining polymer solution was treated with hexadecyloxyallene (2) in THF at room temperature ([2]0 = 0.20 M, [2]0/[Ni]0 = 40), which was chosen for its expected good solubility. When SEC analysis indicated the block copolymerization was completed, the resulting P3HT-b-PHA block copolymer poly(130-b-240) showed a Mn of 15.8 kg/mol, higher than that of the P3HT precursor, while the distribution was narrow (Mw/Mn = 1.19). After a small aliquot was taken out and quenched by methanol for further analysis, the remaining polymer solution of the Ni(II)-terminated P3HT-b-PHA was treated with tert-butyl 4isocyanobenzoate (3) in THF under a dry nitrogen atmosphere ([3]0 = 0.20 M, [3]0/[Ni]0 = 30). After the reaction solution was stirred for 2 h, the polymerization solution was quenched by methanol, and the precipitated solid was isolated by filtration. The Mn and Mw/Mn of the resulting P3HT-b-PHAb-PPI triblock copolymer poly(130-b-240-b-330) were estimated to be 21.6 kg/mol and 1.21, respectively. The Mn of the isolated poly(130-b-240-b-330) was higher than that of the corresponding poly(130-b-240) diblock copolymer and the poly-130 homopolymer. The recorded SEC curves of poly-130, poly(130-b-240), and poly(130-b-240-b-330) are displayed in Figure 1. All the polymers exhibited symmetric and single modal elution peaks. B


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Figure 1. Size exclusion chromatograms of the one-pot synthetic poly130 homopolymer, poly(130-b-240) diblock copolymer, and poly(130-b240-b-330) triblock copolymer. SEC conditions: eluent = THF, temperature = 40 °C.

As expected, the elution peak shifted to shorter retention time region after each chain extension reaction, suggesting the sequential block copolymerization of three monomers did take place. The formation of the hybrid ABC triblock copolymer was further verified by 1H NMR, UV−vis, and FT-IR spectroscopies (Figure 2; Figures S1 and S2 of the Supporting Information). The 1H NMR spectra of the one-pot synthetic P3HT homopolymer, P3HT-b-PHA diblock copolymer, and P3HTb-PHA-b-PPI triblock copolymer measured in CDCl3 at 25 °C are displayed in Figure 2. For comparison, 1H NMR spectra of poly-220 and poly-320 homopolymers prepared according to the reported literature are also shown in Figure 2.41−44,63 As expected, both the di- and triblock copolymers exhibited characteristic signals attributed to the corresponding blocks. For example, poly(130-b-240) showed a sharp resonance at 6.98 ppm assignable to the aryl protons of P3HT segment, and the resonance at 5.68−6.02 ppm is assignable to the double-bond protons of the PHA segment. Similarly, poly(130-b-240-b-330) exhibited the signals from not only the P3HT and PHA segments but also the PPI block at 6.52−7.80 and 5.27−6.27 ppm coming from the protons of benzene rings (Figure 2e). The block ratios of the di- and triblock copolymers deduced from 1H NMR almost agreed with the SEC analyses and approximately equal to the initial molar ratio of the monomers used in the sequential block copolymerization, indicating the one-pot block copolymerization may proceed in living/ controlled chain-growth manners. It should be noted that the new feed of monomer was added to the reaction solution when the Mn of the produced polymer ceased to increase; that is, the former monomer was consumed almost completely. However, due to the limited resolution of SEC, a small amount of unreacted former monomer can reside in the polymerization solution, which may cause some slight deviations on the recorded SEC Mns and block ratios deduced by 1H NMR with the expected one. To get more details, the one-pot block copolymerization was followed by 1H NMR and SEC analyses to estimate conversions of the three monomers and the values of Mn and Mw/Mn of the generated polymer at different polymerization stages ([1]0/ [2]0/[3]0/[Ni]0 = 20/20/20/1). Plots of the conversions of monomers 1, 2, and 3 with the polymerization time are displayed in Figure 3a. It was found that the polymerization of monomer 1 was relatively fast, which was almost completely

Figure 2. 1H NMR (600 MHz) spectra of poly-130 (a), poly-220 (b), poly-320 (c), poly(130-b-240) diblock copolymer (d), and poly(130-b240-b-330) triblock copolymer (e) measured in CDCl3 at 25 °C.

consumed within 1 h, and then the Mn ceased to increase. Upon addition of monomer 2 to the polymerization solution, the chain extension reaction occurred and the Mn increased again as indicated by SEC analysis. The block copolymerization 2 with Ni(II)-terminated P3HT was slower than that of monomer 1. It takes about 6 h to accomplish the chainextension reaction. In the third polymerization stage, monomer 3 was consumed within 2 h, affording the expected poly(120-b220-b-320) hybrid triblock copolymer. Kinetic studies revealed that the three sequential polymerizations obey the first-order rate law (Figure 3b). On the basis of the kinetic plots, the polymerization rate constants for monomers 1, 2, and 3 were estimated to be 8.33 × 10−4, 7.22 × 10−5, and 5.27 × 10−4 s−1, respectively. The block copolymerization of alkyl allenic monomers using the Ni(II)-terminated P3HT as a macromolecular initiator is known to proceed in a living/controlled chain-growth manner as we reported previously,41−44 while the behavior of the C


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Figure 3. (a) Plots of conversions of monomers 1 (black), 2 (blue), and 3 (red) with the polymerization time initiated by the Ni(II) complex in THF at room temperature (the conversion of total three monomers was designed as 100%). (b) First-order kinetics for the sequential one-pot block copolymerization for monomers 1 (black), 2 (blue), and 3 (red) using the Ni(II) complex as a single catalyst in THF at room temperature.

Figure 4. (a) Time-dependent size exclusion chromatograms of Ni(II)-terminated poly(120-b-220)-initiated block copolymerization of 3 in THF at 25 °C. (b) Plots of the Mn and Mw/Mn values as a function of the conversion of monomer 3 initiated by the Ni(II)-terminated poly(120-b-220) diblock copolymers in THF at 25 °C ([3]0/[Ni]0 = 20, [3]0 = 0.20 M).

Scheme 2. Synthesis of Triblock Copolymer with Different Sequences

Mw/Mn < 1.30. These results confirmed that the one-pot triblock copolymerization of phenyl isocyanide with the in situ generated Ni(II)-terminated P3HT-b-PHA as a macroinitiator did proceed in a living/controlled chain-growth manner. Because of the living nature of the block copolymerization of the three monomers, a variety of hybrid triblock copolymer, poly(1m-b-2n-b-3o)s with defined sequence, controlled Mns, narrow Mw/Mns, and tunable compositions were facilely prepared via the established one-pot triblock copolymerization strategy just through the variation on the initial feed ratios of monomers to the catalyst (Table S1 and Figure S3).

triblock copolymerization of phenyl isocyanide with Ni(II)terminated P3HT-b-PHA block copolymer has never been investigated. Thus, the controllability of the one-pot block copolymerization of monomer 3 with the Ni(II)-terminated P3HT-b-PHA block copolymer poly(120-b-220) was then investigated. As shown in Figure 4, the recorded SEC traces of all the P3HT-b-PHA-b-PPI triblock copolymers poly(120-b220-b-3m)s isolated at different polymerization stages exhibited symmetric and single model elution peaks. The Mns of the triblock copolymers increased linearly and in proportion to the conversion of monomer 3. Moreover, all the isolated triblock copolymers showed narrow molecular weight distributions with D


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Figure 5. (a) Size exclusion chromatograms of the one-pot synthetic poly-120 homopolymer, poly(120-b-315) diblock copolymer and the resulting poly(120-b-315-b-210) triblock copolymer. (b) Size exclusion chromatograms of the one-pot synthetic poly-230 homopolymer, poly(230-b-115) diblock copolymer, and the resulting poly(230-b-115-b-3100) triblock copolymer. SEC conditions: eluent = THF, temperature = 40 °C.

Scheme 3. One-Pot Synthesis of Amphiphilic Poly(1m-b-4n-b-5o) Triblock Copolymer

Ni(II)-terminated PHA homopolymer, poly-230, has a Mn of 8.3 kg/mol and Mw/Mn of 1.14 as determined by SEC analysis (Figure 5b). The polymerization solution was then chain extended with the thiophene monomer 1 under the KCTP mechanism ([1]0 = 0.20 M, [1]0/[Ni]0 = 15)29−37,41−44 to afford the Ni(II)-terminated PHA-b-P3HT diblock copolymer poly(230-b-115). SEC analyses of poly-230 and poly(230-b-115) indicated the block copolymerization did take place because the elution peak of the resulting diblock copolymer was shifted to the higher molecular weight region (Figure 5b). The Mn and Mw/Mn of the of the poly(230-b-115) were estimated to be 10.5 kg/mol and 1.29, respectively. The Mn was larger than that of poly-230 precursor, while the Mw/Mn remained narrow. The in situ generated Ni(II)-terminated PHA-b-P3HT block copolymer poly(230-b-115) was further treated with monomer 3 in THF at room temperature ([3]0 = 0.20 M, [3]0/[Ni]0 = 100). SEC analysis of the resulting PHA-b-P3HT-b-PPI triblock copolymer poly(230-b-115-b-3100) suggested the one-pot triblock copolymerization occurred because the elution peak was shifted to the shortest retention-time region as comparing to those of poly-230 and poly(230-b-115) precursors (Figure 5b). The Mn and Mw/Mn of the isolated poly(230-b-115-b-3100) were determined to be 31.4 kg/mol and 1.23, respectively. The Mn is much higher than that of the corresponding poly-230 homopolymer and poly(230-b-115) block copolymer, while the Mw/Mn of poly(230-b-115-b-3100) was narrow (Mw/Mn = 1.23). 1 H NMR, FT-IR, and UV−vis absorption spectra of the isolated poly(230-b-115-b-3100) also support the formation of expected triblock copolymer because the signals assignable to the three segments can be clearly observed (Figures S8−S11). These results confirmed that the one-pot triblock copolymerization of the three kinds of monomers via distinct polymerization mechanisms were also succeeded even it was performed under different polymerization sequence and that the sequence of the

To further verify the controllability on the sequence of aforementioned triblock copolymers, the syntheses of ACB triblock copolymers of poly(1m-b-3n-b-2o) and BAC triblock copolymers of poly(2m-b-1n-b-3o) containing P3HT, PHA, and PPI blocks with different sequence were then attempted through the variation on the order of monomer feed additions. As shown in Scheme 2, the in situ generated Ni(II)-terminated P3HT homopolymer poly-120 (Mn = 3.5 kg/mol, Mw/Mn = 1.21) was first treated with phenyl isocyanide monomer 3 instead of monomer 2 ([3]0 = 0.20 M, [3]0/[Ni]0 = 15), following the experimental procedure described above. The afforded Ni(II)-terminated P3HT-b-PPI diblock copolymer poly(120-b-315) (SEC: Mn = 6.4 kg/mol, Mw/Mn = 1.19) was then treated with the monomer 2 in THF at room temperature ([2]0 = 0.20 M, [2]0/[Ni]0 = 10). As shown in Figure 5a, the elution peak of the resulting P3HT-b-PPI-b-PHA triblock copolymer poly(120-b-315-b-210) was shifted to shortest retention-time region as compared to those of poly-120 and poly(120-b-315) precursors. The Mn was increased to 9.4 kg/ mol, larger than that of poly-120 and poly(120-b-315) precursors, while the Mw/Mn was still narrow (Mw/Mn = 1.22). These results suggested that the sequence of PHA and PPI segment of the one-pot synthetic triblock copolymer can be regulated by variation on the order of the monomer feed additions. The structure of the ACB triblock copolymer was further confirmed by 1H NMR, UV−vis, and FT-IR analyses (Figures S4−S7). The BAC triblock copolymer of poly(2m-b-1n-b-3o) composed of PHA, P3HT, and PPI segments was attempted to prepare by using PHA as the first block. As outlined in Scheme 2, the allenic monomer 2 was polymerized using a π-allyl Ni(II) complex as catalyst in THF at room temperature under the insertion polymerization mechanism of the terminal double bond ([2]0 = 0.20 M, [2]0/[Ni]0 = 30), following the procedure reported by our group previously.41−44 The afforded E


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Figure 6. (a) Normalized absorption spectra of poly(120-b-420-b-550) in CH2Cl2, methanol, and water at 25 °C and the expanded absorption spectra at the region of 350−650 nm. (b) Normalized emission spectra of poly(120-b-420-b-550) in CH2Cl2, methanol, and water at 25 °C (λexc = 365 nm, c = 0.20 g/L) and photographs of poly(120-b-420-b-550) in different solvents under UV light at 365 nm. AFM height images of the self-assembled structures formed from poly(120-b-420-b-550) in methanol (c) and water (d).

long wavelength emission (520−710 nm) can be attributed to π-conjugated P3HT, while the shorter wavelength one (390− 500 nm) was probably come from the emission of poly-420 and poly-550 segments which was supported by the emission spectra of poly-4m and poly-5m homopolymers (Figure S18). The quantum yield (QY) in CH2Cl2 was estimated to be 36%. In methanol, the emission at the long wavelength region of P3HT segment was completely quenched and only showed emission at shorter wavelength region as compared to that in CH2Cl2; thus. it showed a blue light emission (QY = 7.7%). This result may be ascribed to the intermolecular interactions of πconjugated P3HT segments of the triblock copolymer.64,65 The emission behavior of the poly(120-b-420-b-550) in water was different than those in methanol and CH2Cl2. The emission at the short wavelength region was completely quenched; only the long wavelength emission (570−720 nm) was observed (Figure 6b). The QY in water was estimated to be 4.2%. These results were probably due to the strong solvophobic interaction and intermolecular π−π interaction of π-conjugated P3HT segments of this amphiphilic triblock copolymer in different solvents.50,64,65 The details of the self-assembled supramolecular structures of poly(120-b-420-b-550) in these solvents were further investigated by dynamic light scattering (DLS) analysis. The average dynamic diameter in CH2Cl2 was estimated to be ca. 5.0 nm, suggesting the triblock copolymer was molecularly dissolved (Figure S19). When the solvent was transferred from CH2Cl2 to methanol and water, the diameter were increased to ca. 164 and 342 nm, respectively, suggesting the aggregation of poly(120-b-420-b-550) in these solvents. The morphologies of the self-assembled structures were further evidenced by atomic force microscope (AFM) observations. It was found that the triblock copolymer was self-assembled into well-defined spherical nanoparticles with good homogeneity

multiblock copolymers can be facilely regulated just by varying the order of the monomer feed additions (Table S1 and Figure S12). Responsiveness of the Amphiphilic Triblock Copolymer. By using the established one-pot synthetic method described above, amphiphilic ABC triblock copolymers poly(120-b-420-b-550) and poly(140-b-440-b-520) were facilely synthesized according to Scheme 3. The structures of the block copolymers were verified by SEC, 1H NMR, UV−vis, and FTIR analyses (Table S1 and Figure S13−S17). Because of the amphiphilic character, these triblock copolymers exhibited good solubility in most common organic solvents and in water as well. Interestingly, the amphiphilic triblock copolymers showed tunable absorption and emission properties depending on solvents used. For example, the absorption spectrum of poly(120-b-420-b-550) in CH2Cl2 showed intense absorptions around 250 and a weak hump around 400 nm (Figure 6a). When the solvent transferred from CH2Cl2 to methanol, the absorption maximum at 250 nm was decreased, accompanied by new weak absorptions at 560 and 605 nm, indicating the formation of π−π interactions associated with semicrystalline aggregation in methanol.64,65 Similarly, the bathochromically shifted absorption was also observed in water while the absorption was weaker. Furthermore, the emission of the poly(120-b-420-b-550) was also dependent on the solvents used. In nonselective CH2Cl2 solution, poly(120-b-420-b-550) showed an orange light emission under UV illumination at 365 nm, which can be ascribed to the π-conjugated P3HT chromophore (Figure 6b). In the selective methanol solution, poly(120-b-420b-550) exhibited a blue light emission, while it showed a reddish emission in water under UV light at 365 nm. According to the emission spectra, the triblock copolymer showed intense emission from 400 to 720 nm in CH2Cl2 (Figure 6b). The F


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Figure 7. (a) Photographs of poly(140-b-440-b-520) in CH2Cl2 upon cooling and heating taken under room light and UV light at 365 nm (c = 0.20 g/ L). (b) Normalized absorption (solid lines) and emission (dashed lines) spectra of poly(140-b-440-b-520) in CH2Cl2 measured at different temperatures (λexc = 365 nm, c = 0.20 g/L). (c) DLS curves for poly(140-b-440-b-520) measured in CH2Cl2 at 25 and −5 °C.

Figure 8. (a) Photographs of poly(120-b-420-b-550) in CH2Cl2 upon addition of TFA and TEA taken under UV light at 365 nm. The absorption (b) and emission (c) spectra of poly(120-b-420-b-550) in CH2Cl2 with the presence of TFA and TEA at 25 °C (c = 0.20 g/L, λexc = 365 nm).

°C, which gradually changed to red emission when cooling down to −15 °C (Figure 7a). As shown in Figure 7b, poly(140b-440-b-520) exhibited strong emission at 575 nm in CH2Cl2 at 25 °C, while it red-shifted to 581 nm when the solution was cooled down to −15 °C, accompanied by a new peak at 628 nm. The QY of the emission was determined to be 31% at 25 °C and decreased to 11% at −15 °C. This phenomenon was mainly because of the low-temperature promoted self-assembly of amphiphilic triblock copolymer. Note that the emission spectrum of poly(140-b-440-b-520) is much different than that of poly(120-b-420-b-550) measured in the same CH2Cl2, which can be ascribed to the different compositions of the two polymers. Compared to the poly(120-b-420-b-550) copolymer, poly(140-b440-b-520) contained more P3HT and less poly-5m segments; thus, it showed strong emission at the long wavelength region of π-conjugated P3HT block and weak emission at the shorter wavelength region. The aggregation of poly(140-b-440-b-520) at low temperature was further evidenced by the DLS analysis. The average diameter of poly(140-b-440-b-520) in CH2Cl2 was increased with the temperature decrease and eventually increased to 120 nm at −5 °C (Figure 7c). When the solution was rewarmed to 25 °C, both the emission and the diameter of poly(140-b-440-b-520) can be recovered, which further confirmed the reversibility of the aggregation.

(Figure 6c,d). The average diameter was estimated to be 150 nm in methanol and 320 nm in water, which were consistent to those obtained from DLS analyses. Probably the amphiphilic triblock copolymer self-assembled into well-defined micellar structures with the hydrophobic P3HT at the interior and exposing the hydrophilic segments at the exterior in the selective solvents due to the π−π interactions of P3HT segments and the immiscibility of the amphiphilic blocks. The emission properties of the one-pot synthetic amphiphilic triblock copolymers can be further tuned by temperature. For example, the yellow color solution of poly(140-b-440-b-520) in CH2Cl2 at 25 °C (c = 0.20 g/L) was gradually changed to orange when it was cooled down to −15 °C (Figure 7a). As shown in Figure 7b, it showed an intense absorption around 440 nm at 25 °C, corresponding to the P3HT segment. New absorptions at 570 and 604 nm was appeared when the solution was cooled, which is the characteristic of the aggregation and crystallization of the conjugated P3HT segment.41−44 The intensity of the absorption at 604 nm increased linearly with the decrease of the temperature (Figure S20). Note that the absorption spectra of the block copolymer can be completely recovered when the solution was rewarmed to 25 °C. Accordingly, the emission of poly(140-b-440-b-520) in CH2Cl2 was also dependent on the temperature. It showed an orange light emission upon illumination by UV light at 365 nm at 25 G


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Figure 9. (a) Photographs of the gel formed form poly(120-b-420-b-550) with NPLPA upon heating and cooling under room light and UV light at 365 nm. (b) Absorption (solid lines) and emission (dashed lines) spectra of the gel, melted gel, and xerogel prepared from poly(120-b-420-b-550) and NPLPA in CH2Cl2 (λexc = 365 nm). Insets: photographs of the white-light emission of the dried gel in film and in solid state under UV light at 365 nm.

changes were observed as that for poly(120-b-420-b-550) (Figure S23). This result suggests the TFA-induced white-light emission of the triblock copolymer is not only because of the interaction of the proton with the P3HT segment;40,43,44 the hydrophilic poly-420 and poly-550 blocks also play important roles, while the actual reason is not very clear at the current stage. White-Light Emission. Generally, conjugated block copolymers show intense emission in solution, while it was usually quenched in solid state, which may limit its practical applications. To investigate the emission properties of poly(120b-420-b-550) in the solid state, the white-light emission solution of poly(120-b-420-b-550) in CH2Cl2 with the presence with TFA was casted onto a glass substrate; however, the emission was completely quenched. To obtain more useful white-lightemitting materials, the acid-induced white-light emission of the triblock copolymer by TFA was attempted to use other organic acids. N-Pivaloyl-L-phenylalanine (NPLPA) was choose for its gelation ability which may maintain the microstructure of the block copolymer. NPLPA was added to the CH2Cl2 solution of poly(120-b-420-b-550) at 25 °C. The transparent yellowish solution was gelated immediately upon the addition of NPLPA. Interestingly, the resulting gel showed white-light emission under the illumination of UV light at 365 nm (Figure 9a). The absorbance and emission spectra of the gel are shown in Figure 9b, which exhibited emission from 400 to 700 nm with two major emission peaks located at 440 and 574 nm. The white-light emission gel also showed thermoresponsiveness; it transferred to a clear solution upon heating. Nevertheless, the white-light emission was maintained even it was melted to a solution, which was further confirmed by the emission spectrum. Compared to the emission of the gel, the emission at 440 nm was maintained in the solution while the emission at 574 nm was decreased. However, it still showed strong emission from 400 to 700 nm upon illumination by UV light at 365 nm. Interestingly, compared to the generally quenched emission of conjugated polymers in solid state, the xerogel of poly(120-b-420-b-550) with NPLPA also showed strong whitelight emission upon irradiation by UV light at 365 nm. The emission spectrum of the dried gel is shown in Figure 9b. Compared to the organogel, the emission at short wavelength region of the xerogel was shifted to 411 nm, while the emission at the long wavelength region (574 nm) was just slightly decreased. Note that the NPLPA showed no emission in solution and in solid state under the same experimental conditions. Because of the easy process of the solution and

The amphiphilic triblock copolymers also showed pH responsiveness. The orange light emission of poly(120-b-420-b550) in CH2Cl2 at 25 °C (c = 0.20 g/L) was gradually changed to blue upon the addition of trifluoroacetic acid (TFA, 0−0.2 M) under the illumination of UV light at 365 nm (Figure 8a). Remarkably, white-light emission was observed when the concentration of the added TFA reached 0.15 M. Neutralizing the acidic solution by triethylamine (TEA), the emission color can be recovered. To get great detail, the absorption spectra of the solution upon addition of TFA and TEA were recorded. As shown in Figure 8b, the absorption at 400 nm of the poly(120-b420-b-550) triblock copolymer was decreased upon the addition of TFA, accompanied by the appearance of a new absorption at 598 nm. The new peak at 598 nm was mainly induced by the protonation of P3HT (Figure S21).40,43,44 Neutralizing the acidic solution by TEA, the absorption of the triblock copolymer can be gradually recovered, suggesting the pHresponsiveness of the triblock copolymer is reversible. The emission spectra of poly(120-b-420-b-550) in CH2Cl2 with the presence of TFA and TEA are shown in Figure 8c. It was found that the emission of the triblock copolymer was gradually decreased upon the addition of TFA and showed a linear correlation to the concentration of TFA until it was saturated (Figure S22). However, the emission at the long wavelength region (574 nm) decreased more quickly than that at the short wavelength region (406, 431, and 457 nm). Before the addition of TFA, the ratio of emission intensity at low-energy band (574 nm) to high-energy band (431 nm) (I574/I431) is 1.66. It decreased to 0.40 when the concentration of TFA reached 0.20 M. Upon neutralization of the acidic solution by TEA, the emission can be recovered. Note that there is a little difference between the neutralized solution to the initial one on the emission spectra, which was probably because of the salt effect that formed during the acidification and neutralization. To get better insight into the acid-induced white-light emission of the triblock copolymer, three homopolymers of poly-120,38,41 (SEC: Mn = 3.5 kg/mol, Mw/Mn = 1.29), poly42043,44 (SEC: Mn = 4.1 kg/mol, Mw/Mn = 1.15), and poly-52040 (SEC: Mn = 5.9 kg/mol, Mw/Mn = 1.17) were prepared and blended in CH2Cl2 at 25 °C with the ratio of 2:2:5 (c = 0.20 g/ L), which is close to the block ratio of poly(120-b-420-b-550). The emission properties of the mixed solution were investigated with the addition of TFA. However, unlike the acid-induced emission changes of poly(120-b-420-b-550) triblock copolymer, the emission of the blended solution was quenched upon the addition of TFA; no similar acid-induced emission H


Article

Macromolecules

Figure 10. (a) Chromaticity coordinates of the gel, solution, and xerogel prepared from poly(120-b-420-b-550) and NPLPA in CH2Cl2 at 25 °C. White-light LED made by coating a thin layer of poly(120-b-420-b-550) and NPLPA on the surface of a UV lamp (365−370 nm emission, commercially available) with power off (b) and power on (c).

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xerogel prepared from poly(120-b-420-b-550) and NPLPA, the white-light-emitting film and powder can be facilely fabricated (insets of Figure 9b). Furthermore, the chromaticity coordinates (CIE) for the white-light emissions of the gel (0.32, 0.27), melted gel (0.30, 0.25), and xerogel (0.28, 0.24) were estimated (Figure 10), which were close to the pure white color light (0.31, 0.33). The solution processable property of poly(120-b-420-b-550) triblock copolymer makes it possible to be coated on a substrate. As shown in Figures 10b and 10c, an UV lamp (emission at 365−370 nm) coated with a film casted from the solution of the melted gel showed white-light emission as expected.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02558.

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Experimental procedure and additional spectroscopic data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.-Q.W.). ORCID

CONCLUSION

Zong-Quan Wu: 0000-0001-6657-9316

In summary, we developed a novel synthetic strategy for facile synthesis of sequence-defined triblock copolymers composed of conjugated polythiophene, polyallene, and polyisocyanide blocks. Although the three monomers were polymerized via distinct mechanisms, the one-pot sequential block copolymerization was revealed to proceed in living/controlled chaingrowth manners. The block sequence can be easily tuned through the variation on the order of monomer feed additions. Considering the modifications on the sequence and the structure of monomers, a variety of multiblock copolymers with defined sequence, controlled Mn, narrow Mw/Mn, and tunable composition can be facilely prepared. Amphiphilic triblock copolymer containing hydrophobic P3HT, hydrophilic polyallene, and poly(phenyl isocyanide) was readily prepared by using this method. Interestingly, such triblock copolymer exhibited tunable emissions with responsive to multiple stimuli. The white-light emission in solution, gel, and solid state can be easily achieved by using this triblock copolymers with the aid of an acid. We believe the present study provides not only a strategy for facile synthesis of sequence-defined conjugated multiblock copolymers but also a novel semiconducting materials with great potential.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by National Natural Science Foundation of China (21622402, 51673057, and 21574036). Z.W. thanks the 1000plan for Young Talents Program of China and Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201624) for financial support.

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One-Pot Synthesis, Stimuli Responsiveness, and White-Light

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