A Facile Synthetic Route to Multifunctional Poly(3-hexylthiophene)-b

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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A Facile Synthetic Route to Multifunctional Poly(3hexylthiophene)‑b‑poly(phenyl isocyanide) Copolymers: From Aggregation-Induced Emission to Controlled Helicity Lei Xu, Xun-Hui Xu, Na Liu,* Hui Zou,* and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, and Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei University of Technology, Hefei, Anhui Province 230009, China Macromolecules Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/18/18. For personal use only.

S Supporting Information *

ABSTRACT: New synthetic methods are of great significance in enabling access to valuable compounds and materials. Although conjugated poly(3-hexylthiophene) (P3HT) has been widely investigated, P3HT copolymers containing functional groups (such as amino, imino, hydroxyl, etc.) are rarely reported because of inherent synthetic difficulties. Here P3HT copolymers bearing diverse functional pendants, which cannot be obtained by the direct block copolymerization of the corresponding monomers, were designed and facilely synthesized through the postpolymerization modification method. First, P3HT-b-poly(phenyl isocyanide) (P3HT-b-PPI) with pentafluorophenyl (PFP) ester was prepared in one-pot using Ni(dppp)Cl2 as the catalyst. Then, through the postpolymerization modification of the copolymer with alcohols and amines, P3HT-b-PPIs copolymers bearing defined functional groups were facilely obtained. The introduction of functional pendants endowed P3HT-b-PPIs copolymers with diverse functionalities. When pyrene pendants were introduced into PPI block, the obtained copolymer presented both aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) properties when methanol was added into its THF solution. As for the copolymer containing chiral pendants, an excess of right-handed helix of the PPI backbone was induced after annealing in toluene at 100 °C for 7 days, and left-handed helical bundles were formed by the self-assembly of the annealed copolymer in THF. Meanwhile, the copolymer containing PEG pendants was amphiphilic and could self-assembled into worm-like or spherical structures in different solutions. In addition, cross-linked P3HT-b-PPI was obtained after postpolymerization modification of P3HT-b-PPI copolymer with glycol.

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INTRODUCTION Conjugated polymers have attracted increasing attention because of their potential applications in electronic1−5 and photonic materials,6−11 light-emitting devices,12 biomedicine,13,14 etc. Among the class of conjugated polymers, poly(3hexylthiophene) (P3HT) has been widely investigated owing to its high environmental stability,15 good electrical properties,16,17 and excellent synthetic versatility.18 Generally, P3HT is prepared by Grignard metathesis polymerization (GRIM), which is also known as Kumada catalyst-transfer polycondensation (KCTP), using the Ni(II) complex as catalyst through the quasi-living catalyst-transfer polycondensation mechanism.19,20 This polymerization proceeds in a living, chain-growth manner to produce P3HT with narrow dispersities, high regioregularities, and controllable molecular weights. Moreover, it has been © XXXX American Chemical Society

found that the Ni(II) complex can perform other polymerization reactions. Thus, a variety of P3HT-containing block copolymers have been designed and synthesized.21−26 For example, P3HT diblock copolymers, such as P3HT-b-poly(6,7-dimethylquinoxaline-2,3-diyl) 2 7 and P3HT-b-poly(hexadecyloxyallene),28 as well as P3HT triblock copolymers, such as P3HT-b-poly(hexadecyloxyallene)-b-P3HT 29 and poly(hexadecyloxyallene)-b-P3HT-b-poly(hexadecyloxyallene),30 have been designed and prepared by our group using Ni(II)terminated P3HT as macroinitiator. Meanwhile, we have developed well-defined P3HT-b-polyisocyanide block copolyReceived: July 10, 2018 Revised: September 7, 2018

A

DOI: 10.1021/acs.macromol.8b01478 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. (A) Synthesis of P3HT-b-PPI Copolymer (Poly(1m-b-2n)); (B) Facile Synthesis of Functional P3HT-b-PPI Copolymers via the Postpolymerization Modification Method

origin of homochirality in nature. Therefore, development of novel P3HT-containing block copolymers that can self-assemble into well-defined supramolecular helical structures is of great interest and significance. Poly(phenyl isocyanide) (PPI) is a kind of typical helical polymer which has been the focus of intense efforts in the past few decades due to its unique helical structure and wide potential applications.44−48 What is more, we found that PPIs with defined functional pendants could be prepared by a postpolymerization modification method.49 In this method, PPI with pentafluorophenyl (PFP) ester is synthesized first, and then through the postpolymerization modification with amines or alcohols, PPIs with desired functional pendants linked by amide or ester bonds respectively are obtained.50−58 Therefore, if we use PFP-containing PPI to incorporate with P3HT, the formed copolymers can be further modified by the reaction with amines or alcohols, leading to the obtainment of P3HT copolymers bearing functional groups. What is more, due to the helical conformation of PPIs, the obtained P3HT-b-PPI copolymers

mers through copolymerization of 3-hexylthiophene and isocyanide monomers in one pot.31 However, because Ni(II) complex is sensitive to some functional groups, such as amino, imino, hydroxyl, etc., P3HT copolymers with these groups cannot be prepared by the direct polymerization. The quick and easy preparation of P3HT copolymers with functional groups is still a challenge. In addition, although a lot of research has been conducted on P3HT-containing copolymers, most of the studies just investigate the copolymers self-assembling into supramolecular structures, such as spherical particles, nanofibers, and nanosheets.32−38 P3HT polymers that can form supramolecular helical structures have also been explored,39,40 but such reports are relatively rare. It is well-known that helix is one of the essential structural elements in nature, and one-handed helical structure can be easily found in biological systems at the macromolecular and supramolecular levels, such as the righthanded α-helix of proteins and the double helix of DNA.41−43 It is a long-standing concern whether helicity correlates with the B

DOI: 10.1021/acs.macromol.8b01478 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Size exclusion chromatograms of poly-120, poly(120-b-240), and poly(120-b-240-b-260). (b) Plots of Mn and Mw/Mn values of poly(120-b2m)s with the initial feed ratio of monomer 2 to Ni(II) initiator.

thiophene (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 = 20), following the KCTP mechanism. The polymerization of monomer 1 was traced by size exclusion chromatography (SEC) analysis of the aliquots taken out at appropriate time intervals. When the polymerization of 1 was completed as indicated by SEC, that is, the Mn of the afforded Ni(II)-terminated poly-120 (here and elsewhere the subscript indicates the initial feed ratio of monomer to initiator) ceased to increase, an aliquot was taken out and quenched by methanol for further analysis. The Mn and Mw/Mn of the obtained poly-120 measured by SEC analysis were estimated to be 4.1 kDa and 1.32, respectively, as equivalent to the polystyrene standard. MALDI-TOF analysis showed that the P3HT precursor was with a mixture of H/H and H/Br end groups (Figure S1, Supporting Information). Then PFP-ester bearing a phenyl isocyanide monomer (2) was added into the remaining Ni(II)-terminated poly-120 solution, and the temperature of the system was increased to 45 °C to extend the second block ([2]0 = 0.20 M, [2]0/[Ni]0 = 40). After reaction for 2 h, the polymerization solution was precipitated into a large amount of methanol. The block copolymer poly(120-b-240) was collected by centrifugation in 75% yield over the two steps. The success of the one-pot block copolymerization was first confirmed by the SEC analysis of the macroinitiator precursor poly-120 and the resulting block copolymer poly(120-b-240). As shown in Figure 1a, the poly-120 and poly(120-b-240) both exhibited symmetric and single modal elution peaks, and the SEC trace of the poly(120-b-240) located at a shorter retentiontime region compared with that of the poly-120 precursor. A tailing could be observed in the low molecular region for SEC traces. This was probably due to the interaction of the polymer with the GPC columns during the SEC measurements. The Mn of the poly(120-b-240) was estimated to be 15.6 kDa (Mw/Mn = 1.20), larger than that of the poly-120 (Mn = 4.1 kDa, Mw/Mn = 1.32). It should be noticed that the poly-120 and resulting poly(120-b-240) were both with narrow molecular weight distributions, indicating that the one-pot block copolymerization might proceed in a living/controlled chain-growth manner. To verify this, a series of the block copolymerizations were performed using Ni(II)-terminated poly-120 (Mn = 4.1 kDa, Mw/ Mn = 1.32) as the common macroinitiator with different initial feed ratios of monomer 2 to the macroinitiator. All the block copolymers were isolated in high yields and exhibited symmetric and single modal elution peaks on SEC traces (Figure S2). The plots of the Mn and Mw/Mn values of the obtained block copolymers to the initial feed ratios of monomer 2 to the

may not only present interesting self-assembly and optical properties but also form supramolecular helical structures. Herein, P3HT-b-PPIs copolymers bearing diverse functional pendants, which cannot be prepared through the direct block copolymerization of the corresponding monomers, were designed and facilely synthesized by the postpolymerization modification method as shown in Scheme 1. The P3HT-b-PPI copolymer bearing PFP ester was first prepared in one-pot by using Ni(dppp)Cl2 as the single catalyst (Scheme 1A). After postpolymerization modification of the copolymer with alcohols and amines, P3HT-b-PPI copolymers with a variety of desired functional pendants, such as pyrene groups, chiral groups, poly(ethylene glycol) (PEG) groups, and cross-link agents, were obtained (Scheme 1B). The introduction of these functional pendants endowed P3HT-b-PPIs copolymers with diverse functionalities. When pyrene pendants were introduced into PPI block, the copolymer presented both aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) properties upon adding methanol into its THF solution. Meanwhile, when chiral pendants were introduced into PPI block, an excess of right-handed helix of the PPI main chain was induced after annealing in toluene at 100 °C for 7 days, and lefthanded helical bundles were formed by the self-assembly of the annealed copolymer in THF. In addition, after PEG pendants were introduced into the copolymer, the P3HT-b-PPI copolymer bearing PEG pendants was amphiphilic, and it selfassembled into worm-like supramolecular helical structures in THF/MeOH = 7/3 (v:v) mixtures and spherical micelles in pure MeOH solutions. What is more, after postpolymerization modification of P3HT-b-PPI with glycol, cross-linked copolymer would be obtained. The results demonstrate that the postpolymerization modification of PFP ester containing P3HT-b-PPI copolymer with alcohols and amines is a facile synthetic method to prepare P3HT-b-PPIs copolymers that cannot be synthesized via the direct block copolymerization of the corresponding monomers, and the obtained copolymers were with not only diverse functional pendants but also novel optical and self-assembly properties.

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RESULTS AND DISCUSSION Synthesis of Poly(1m-b-2n) Copolymers. The poly(1m-b2n) copolymers were synthesized in one-pot via two sequential living polymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene (1) and PFP ester-functionalized phenyl isocyanide (2) using the Ni(II) complex as the catalyst as shown in Scheme 1A. First, Ni(II)-terminated P3HT was prepared through the polymerization of 2-bromo-3-hexyl-5-chloromagnesioC

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Figure 2. (a) Plots of the Mn and Mw/Mn values as a function of the conversion of monomer 2 initiated by Ni(II)-terminated poly-120 in THF at 45 °C ([2]0 = 0.10 M, [2]0/[Ni(II)]0 = 100). (b) First-order kinetics plots for polymerization of 2 initiated by Ni(II)-terminated poly-120.

macroinitiator, Ni(II)-terminated poly-120 are summarized in Figure 1b. It can be observed that the Mn of the block copolymers was increased linearly and in proportion to the initial feed ratios of monomer to initiator. Meanwhile, all the block copolymers were with narrow molecular weight distributions. These results suggested that the block copolymerization of 2 initiated by the Ni(II)-terminated poly-120 proceeded in a living/controlled chain-growth manner, and the active PFP-ester pendants had no negative effect on the living copolymerization. To further obtain the details, the polymerization of 2 with Ni(II)-terminated poly-120 as the catalyst was traced by 1H NMR and SEC measurements of the aliquots taken out from the polymerization solution at appropriate time intervals. It can be seen that the polymerization was very fast, and more than 90% of 2 was consumed within 10 min (Figure S3). The SEC traces of the copolymers isolated at different polymerization stages are shown in Figure S4, and they were all with single modal elution peaks and shifted to the higher-molecular-weight region with the increase of monomer conversion. The correlations of Mn and Mw/Mn values of the isolated copolymers vs the conversion of 2 are shown in Figure 2a. The Mn increased linearly with the conversion of monomer 2, indicating no chain transfer reactions occurred. Kinetic studies revealed that the polymerization obeyed the first-order reaction rule (Figure 2b), suggesting the absence of termination reactions. The reaction rate constant was estimated to be 3.63 × 10−3 s−1. The chemical structure of poly(120-b-240) was further verified by 1H and 19F NMR, FT-IR, and UV−vis measurements. The 1 H NMR spectra of poly-120 and poly(120-b-240) measured in CDCl3 at 25 °C are shown in Figures 3a and 3b, respectively. It can be observed that the poly(120-b-240) exhibited the signals from not only the P3HT block (sharp resonance at 6.97 and 2.81 ppm) but also the PPI block (broader resonance at 7.62 and 5.84 ppm coming from the phenyl protons). The 19F NMR spectrum of poly(120-b-240) is displayed in Figure 4, and three broad peaks located at upfield were observed, suggesting the successful copolymerization of PPI block. The FT-IR spectrum of poly(120-b-240) showed a characteristic vibration of CN of the polyisocyanide main chain at 1605 cm−1 and C−F vibration of the PFP pendant at 1048 and 1013 cm−1 (Figure 5b), which further confirmed the successful preparation of poly(120-b-240). To further confirm the living nature of the Ni(II) complex catalyzed polymerization of the PPI monomer, polymerization of monomer 2 with Ni(II)-terminated poly(120-b-240) (Mn = 15.6 kDa, Mw/Mn = 1.20) as the macroinitiator was performed in THF at 45 °C ([2]0 = 0.20 M, [2]0/[Ni]0 = 40). The SEC trace

Figure 3. 1H NMR (600 MHz) spectra of poly-120 (a), poly(120-b-240) (b), and poly(120-b-2a40) (c) measured in CDCl3 at 25 °C.

Figure 4. 19F NMR (564 MHz) spectra of poly(120-b-240) (a), poly(120-b-2a40) (b), poly(120-b-2b40) (c), and poly(120-b-2c40) (d) measured in CDCl3 at 25 °C.

of poly(120-b-240-b-260) is shown in Figure 1a. It can be seen that the obtained copolymer was still with a symmetric and single modal elution peak, and its SEC trace shifted to the shortest retention time region as compared with its precursor poly(120-b2 40). The M n of the obtained poly(120-b-240 -b-2 60) as determined by SEC was 38.6 kDa, which was larger than that of the poly(120-b-240), while the Mw/Mn kept narrow (Mw/Mn = 1.22). The result demonstrated that the Ni(II)-terminated D

DOI: 10.1021/acs.macromol.8b01478 Macromolecules XXXX, XXX, XXX−XXX

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The thermal stability of the block copolymer poly(120-b-240) and its corresponding homopolymers was studied by thermogravimetric analysis (TGA) under N2 with 10 °C/min heating rate (Figure S6a). For poly(120-b-240), an initial 55% weight loss was observed from 300 to 430 °C, followed by an 17% weight loss from 430 to 604 °C. As a comparison, the TGA of poly-120 and poly-240 was tested. It was found that the weight loss was 67% for poly-120 from 403 to 556 °C and 70% for poly240 from 295 to 790 °C. These results indicated that both the poly-120 segment and poly-240 segment existed in the diblock copolymer. The thermal properties of poly(120-b-240) were analyzed by differential scanning calorimetry (DSC). The DSC curves of the copolymer as well as the two homopolymers of poly-120 and poly-240 are shown in Figure S6b. It was found that poly(120-b-240) exhibited two glass transition temperatures (Tg) and two melting temperatures (Tm), corresponding to the poly120 and poly-240 blocks. This result supported the phase segregation of the poly(120-b-240) in the solid state. The crystalline property of poly-120 and poly(120-b-240) was characterized by X-ray differaction (XRD) measurement. As shown in Figure S7a, the poly-120 presented characteristic diffraction peaks at 2θ of 5.61°, 10.59°, and 15.82°, which were ascribed to the lamellar structure of P3HT with a lattice constant of 1.69 nm for the (h00) plane.59,60 As for poly(120-b-240) copolymer, the diffraction peaks associated with poly-120 were clearly observed (Figure S7b), indicating that the poly-240 block did not completely affect the crystallization of poly-120 in the copolymer. Postpolymerization Modification of Poly(120-b-240) with Alcohols and Amines. After the successful synthesis of poly(120-b-240), P3HT-b-PPIs copolymers with diverse defined functional pendants were obtained by the postpolymerization modification method as shown in Scheme 1b. When poly(120-b240) was treated with decanol (2a, 1.5 equiv with respect to the repeating unit 2 in poly(120-b-240)) in the presence of 4(dimethylamino)pyridine (DMAP) as catalyst in THF at 55 °C, poly(120-b-2a40) was obtained after reaction for 12 h. The resultant poly(120-b-2a40) was isolated via filtration and analyzed by SEC. The addition of excess 2a was to accelerate the reaction rates. If stoichiometric quantity of 2a was added, the postpolymerization modification could also proceed very well, and the PFP pendants were fully modified, too. In addition, if a substoichiometric quantity of 2a was added, just partial postpolymerization modification occurred, and some of the PFP pendants were maintained. Thus, the polymer composition was dependent on the stoichiometric quantity used in the postpolymerization reaction. As shown in Figure S8, the elution peak was single model and symmetric, and the Mn was 15.2 kDa, smaller than that of poly(120-b-240), with a narrow Mw/Mn of 1.16. The 1H NMR spectrum of poly(120-b-2a40) is shown in Figure 3c. Compared with the 1H NMR spectrum of poly(120-b240) in Figure 3b, the resonances of phenyl protons in poly(120b-2a40) were shifted to upfield (Figure 3c). This was because the electron-deficient PFP groups were replaced with decanol groups. In the meantime, the peaks of phenyl protons in poly(120-b-2a40) (d at 7.31 and c at 5.74 ppm) were sharper than those in poly(120-b-240), which was probably due to the less steric hindrance of the decanol ester. What is more, a new broad resonance at 4.06−3.79 ppm coming from the protons of −OCH2 groups in poly(120-b-2a40) was clearly observed, and the ratio of the integration of the proton signals at 7.31 ppm (peak d in Figure 3c) to that at 5. 74 ppm (peak c in Figure 3c) and 4.06−3.79 ppm (peak e in Figure 3c) was estimated to be

Figure 5. FT-IR spectra of poly-120 (a), poly(120-b-240) (b), and poly(120-b-2a40) (c) measured at 25 °C using KBr pellets.

poly(120-b-240) was living, and it could still be used to polymerize phenyl isocyanide monomers. We also tried to perform the polymerization of the monomer 2 first and then continued with polymerization of the monomer 1. The Ni(II)-terminated poly-240 was obtained successfully with the Mn of 14.5 kDa (Figure S5). However, when the monomer 1 was added to extend the second block, the SEC trace moved to a longer retention-time region after reaction for 2 h (Figure S5). The failure of the polymerization of the monomer 1 using Ni(II)-terminated poly-240 as catalyst was probably because the chloromagnesiothiophene monomer 1 was reacted with PFP ester pendants. UV−vis absorption spectra of poly-120 and poly(120-b-240) were conducted as shown in Figure 6. The absorption peak of

Figure 6. UV−vis absorption spectra of poly-120 and poly(120-b-240) in THF (solid line, c = 0.02 mg/mL) and in thin film (dashed line).

poly-120 was at 440 nm, while it was at 414 nm for poly(120-b240). It can be seen that the absorption band of P3HT block in poly(120-b-240) was blue-shifted compared with that in poly120.The reason should be due to the influence of PPI block. What is more, the optical property of the polymers in thin film state was also explored. As shown in Figure 6, the absorption spectra of thin films casted from poly(120-b-240) were significantly red-shifted about 90 nm relative to its solution absorption spectra, indicating intermolecular π−π stacking of the P3HT segment in thin films. The λend values of poly-120 and poly(120-b-240) were 701 and 722 nm, respectively, which corresponded to the narrow optical band gap of about 1.77 and 1.72 eV. These results suggested the oriented self-assembly of the P3HT moieties. E

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Compared with the UV−vis spectrum of poly(120-b-240) in THF solution (Figure 6), new peaks at 240, 265, 277, 329, and 344 nm, which were the characteristic absorption bands of pyrene, appeared in Figure 7a. Meanwhile, it can be seen that the absorption peak of P3HT blocks in poly(120-b-240) (Figure 6) and poly(120-b-2e40) (Figure 7a) were both broad, suggesting 3HT moieties existed in the intense π-stacking form in the poly(120-b-240) and poly(120-b-2e40) copolymers. It is known that pyrene is a typical fluorescent group, and P3HT shows fluorescence, too. Thus, poly(120-b-2e40) was expected to present interesting fluorescent properties. The fluorescence emission spectra of poly(120-b-2e40) in mixed THF and methanol with different ratios are shown in Figure 7b. Two major emissions at ∼394 and ∼575 nm, corresponding to PPI and P3HT segments, respectively, could be clearly observed in pure THF. When the fraction of methanol increased to 20%, the emission intensity at ∼394 nm increased significantly, while the emission intensity at ∼575 nm decreased significantly. Moreover, with the further addition of methanol, the emission intensity at ∼394 and ∼575 nm continued to increase and decrease, respectively. The reason for this interesting change of the emission spectra may be that methanol was the poor solvent for both PPI and P3HT segments. When methanol was added into the THF solution of poly(120-b-2e40), the copolymers aggregated with each other. Then the intramolecular rotation of pyrene moieties on PPI blocks was restricted, and AIE of PPI segments happened, leading to the increase of emission intensity at ∼394 nm.61,62 As for the P3HT block, because of the π−π stacks, ACQ of P3HT segments occurred, resulting in the decrease of emission intensity at ∼575 nm. Therefore, the copolymer poly(120-b-2e40) presented both AIE and ACQ properties. Synthesis and Self-Assembly of Optically Active Poly(120-b-2f40). The postpolymerization modification of poly(120-b-240) bearing PFP ester pendants can also be applied to alkamine, which should have a higher reactivity than alcohols. This method is meaningful because it can be used to prepare polymers that cannot be synthesized by direct polymerization. For example, the copolymer poly(120-b-2f40) bearing alcoholic hydroxyl groups could not be synthesized from the direct polymerization because the Ni(II) complex was sensitive to hydroxyl groups. However, poly(120-b-2f40) was easy obtained by postmodification of poly(120-b-240) with amine 2f with the presence of DMAP as the catalyst. The reaction was followed by measuring the 19F NMR spectrum of the generated copolymer. When no signal could be detected by 19F NMR, i.e., all the PFPester pendants were transferred to amides, the reaction solution was precipitated into a large amount of methanol, and then the

2:2:2, which agreed well with the proposed structure of poly(120b-2a40). The chemical structure of poly(1 20-b-2a40) was also confirmed by 19F NMR and FT-IR measurements. As shown in Figure 4b, no resonance could be detected on the 19F NMR spectrum of poly(120-b-2a40). Meanwhile, the FT-IR spectra showed that the C−F vibrations at 1048 and 1013 cm−1 in poly(120-b-240) (Figure 5b) disappeared in poly(120-b-2a40) (Figure 5c). The 19F NMR and FT-IR results indicated that all the PFP ester groups in poly(120-b-240) were replaced with the decanol ester pendants through the postpolymerization modification reaction. Collectively, these studies confirmed the success of postpolymerization modification of poly(120-b240) with alkyl alcohol. Taking advantage of the postpolymerization modification method, a series of well-defined P3HT-b-PPIs copolymers with diverse defined functional pendants, controlled molecular weight, and narrow polydispersity index were facilely synthesized in high yields through the modification of poly(120-b-240) with various alcohols and amines (Table 1, runs 1−8). The Table 1. Results for the Synthesis of P3HT-b-PPI Copolymers with Different Pendants run

polymer

Mnb (Da)

Mw/Mnb

yieldc (%)

1a 2a 3a 4a 5a 6a 7a 8a

poly(120-b-2a40) poly(120-b-2b40) poly(120-b-2c40) poly(120-b-2d40) poly(120-b-2e40) poly(120-b-2f40) poly(120-b-2g40) poly(120-b-2h40)

1.52 × 104 1.82 × 104 0.91 × 104 0.78 × 104 1.73 × 104 1.39 × 104 2.41 × 104 1.81 × 104

1.16 1.18 1.20 1.15 1.22 1.21 1.25 1.25

87 82 81 87 90 86 70 83

a

These polymers were prepared according to Scheme 1b. bMn and Mw/Mn values were determined by SEC with equivalent to polystyrene standards. cThe isolated yields.

structures of these obtained copolymers were investigated by SEC, 1H NMR, 19F NMR, and FT-IR spectroscopies (Figures S8−S27), and the results confirmed the successful preparation of P3HT-b-PPIs copolymers with diverse functional pendants. Optical Properties of Poly(120-b-2e40). The copolymer poly(120-b-2e40) was synthesized by the postpolymerization modification of poly(120-b-240) with 1-pyrenemethanol. The successful preparation of poly(120-b-2e40) was confirmed by SEC, 1H NMR, 19F NMR, and FT-IR analyses (Figures S8 and S16−S18). Meanwhile, the UV−vis absorption spectrum of poly(120-b-2e40) was also investigated as shown in Figure 7a.

Figure 7. (a) UV−vis spectra of poly(120-b-2e40) in THF (c = 0.05 mg/mL). (b) Emission spectra of poly(120-b-2e40) in the mixtures of THF and methanol with different volume ratios (c = 0.1 mg/mL, λexc = 344 nm). F

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Figure 8. (a) CD and UV−vis spectra of poly(120-b-2f40) before and after anneal in THF (c = 0.15 mg/mL). (b) TEM image of the annealed poly(120b-2f40) in THF (c = 0.20 mg/mL). (c) A plausible model for the annealed poly(120-b-2f40) self-assembly into supramolecular helical structures.

Figure 9. (a) CD and UV spectra of poly(120-b-2f40) after anneal in mixed THF and methanol with different volume ratios (c = 0.15 mg/mL). (b) Emission spectra of poly(120-b-2f40) in mixed THF and methanol with different volume ratios (0.15 mg/mL, λexc = 365 nm).

segment, and the immiscibility of the P3HT with the PPI segments, exposing the PPI segments at the exterior. A plausible assembly model is outlined in Figure 8c. To gain the self-assembly structure, a poor solvent, methanol, was added to the THF solution of the annealed poly(120-b-2f40) at room temperature. As shown in Figure 9a, the UV−vis absorption spectra of the annealed copolymer revealed that new absorptions at 517, 550, and 600 nm were observed when the volume ratio of THF/MeOH changed from 10/0 to 8/2, 6/4, 5/ 5, and 4/6. These new UV−vis absorptions were due to the intramolecular π−π interaction of the conjugated P3HT block. Meanwhile, it can be seen that the CD spectrum of the annealed poly(120-b-2f40) did not change upon the addition of methanol, suggesting no Cotton effects of the P3HT block were generated. Meanwhile, the fluorescence emission spectra of poly(120-b2f40) in mixed THF and methanol with different ratios were investigated as shown in Figure 9b. It can be seen that poly(120b-2f40) exhibited strong emission of P3HT block at ∼570 nm in pure THF, while the intensity decreased obviously and redshifted slightly when the volume ratio of THF/MeOH changed to 8/2 and 6/4. This was because methanol was a poor solvent

resultant copolymer poly(120-b-2f40) was obtained. The chemical structure of poly(120-b-2f40) was explored by SEC, 1 H NMR, 19F NMR, and FT-IR measurements (Figures S8 and S19−S21). The CD and UV−vis spectra of poly(120-b-2f40) are displayed in Figure 8a. The Cotton effect at 250 nm, which was due to the chiral pendants on PPI block, could be observed before and after annealing. After being annealed in toluene at 100 °C for 7 days, poly(120-b-2f40) was observed with an additional positive Cotton effect at 364 nm (Δε364 = +9.75), which corresponded to the absorption region of the PPI backbone. Thus, it can be reasonably assigned to the excess of right-handed helix of the PPI backbone induced by the chiral pendants; however, the possibility of the scattering of the chiral aggregates of the polymer cannot be completely excluded. Although the annealed poly(120-b-2f40) contained an excess of right-handed helix of the PPI segment, it self-assembled into left-handed helical bundles as revealed by transmission electron microscopy (TEM) (Figure 8b). The mechanism of the self-assembly of annealed poly(120b-2f40) to form such left-handed helical bundles might be ascribed to the π−π interactions of the conjugated P3HT G

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structures were formed as shown in Figure 10b, while welldefined spherical nanoparticles were clearly observed when the menthol fraction increased to 100% (Figure 10c). This was because methanol was the good solvent for PEG chains and poor solvent for P3HT block. Thus, the copolymer poly(120-b-2g40) was amphiphilic and could self-assemble in THF/MeOH mixtures. When the fraction of methanol changed, the hydrophilic/hydrophobic ratio of the system changed as well. Therefore, different self-assembly structures were observed. Cross-Linked Copolymer Poly(120-b-2h40). The crosslinked copolymer poly(120-b-2h40) was obtained through postpolymerizaton modification of poly(120-b-240) with glycol (2h). The successful preparation of poly(120-b-2h40) was confirmed by 1H NMR, 19F NMR, FT-IR, and SEC measurements (Figures S8 and S25−S27). The morphology of poly(120b-2h40) was investigated via AFM observations. The thin film for AFM observation was prepared by spin-casting the THF solution of poly(120-b-2h40) onto silicon wafers. AFM images showed that the cross-linked copolymer was with spherical morphologies, and the average surface roughness (rms) measured was 0.49 nm (Figure 11).

for poly(120-b-2f40). P3HT blocks aggregated with each other upon the addition of methanol, leading to the fluorescence quenching. Thus, the fluorescence intensity decreased and redshift happened. Self-Assembly of Amphiphilic Polymer Brush Poly(120b-2g40). After postpolymerizaton modification of poly(120-b240) with PEG monomethyl ether (2g), the amphiphilic polymer brush poly(120-b-2g40) was obtained (Scheme 1b). The chemical structure of poly(120-b-2g40) was verified by 1H NMR, 19F NMR, FT-IR, and SEC analyses (Figures S8 and S22−S24). The obtained poly(120-b-2g40) was with good solubility in most common organic solvents, such as THF, DCM, and toluene. In the meantime, benefiting from the excellent hydrophilicity of PEG pendants, poly(120-b-2g40) was amphiphilic and might exhibit interesting self-assembly behaviors in water, methanol, or other selective solvents. The UV−vis absorption spectra of poly(120-b-2g40) in THF and methanol mixtures with different ratios are shown in Figure 10a. It can be seen that the block copolymer presented a major

Figure 11. Height (a) and phase (b) AFM images of thin film casted from the THF solution of poly(120-b-2h40) onto Si wafers, (c = 0.05 mg/mL, rms = 0.49 nm).

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CONCLUSIONS In summary, functional block copolymers containing conjugated P3HT and helical PPI with diverse pendants were facilely prepared by the postpolymerization modification method. The copolymer poly(120-b-240) was first synthesized in one-pot using Ni(dppp)Cl2 as the catalyst, and through postpolymerization modification of poly(120-b-240) with functional alcohols and amines, P3HT-b-PPI copolymers with various defined functional pendants, e.g., pyrene groups, chiral groups, PEG groups, and cross-link agents, were obtained. The obtained copolymers presented diverse functionalities. As for poly(120-b-2e40) containing pyrene pendant, it presented an interesting fluorescent property. The AIE and ACQ properties could be observed at the same time upon adding methanol in the THF solutions of poly(120-b-2e40). When chiral pendants were introduced into PPI block, an excess of right-handed helix of the PPI backbone in poly(120-b-2f40) was induced after annealing, and the annealed poly(120-b-2f40) self-assembled into left-handed helical bundless in THF. After postpolymerization modification of P3HT-b-PPI copolymer with PEG monomethyl ether groups, amphiphilic copolymer poly(120-b2g40) with PEG pendants was obtained, which could selfassemble into worm-like structure in THF/MeOH = 7/3 (v:v) mixtures and spherical micelles in pure MeOH solutions. What

Figure 10. (a) UV−vis spectra of the amphiphilic polymer brush poly(120-b-2g40) in THF/MeOH mixtures with different volume ratios (c = 0.05 mg/mL). AFM image of the poly(120-b-2g40) in THF/MeOH mixtures with volume ratio of 7/3 (b) and in pure MeOH (c) (c = 0.05 mg/mL).

absorption band located at 445 nm, corresponding to P3HT segments. When methanol, a selective solvent for hydrophilic PEG pendants, was added into THF solution, the absorption maximum at 445 nm of poly(120-b-2g40) blue-shifted to 407 nm. Meanwhile, two new absorptions at 550 and 603 nm appeared, which might have resulted from the formation of intermolecular π−π interactions associated with semicrystalline aggregation of the conjugated P3HT segments, indicating that new selfassembly structures were generated. The morphology of the new self-assembly structures was investigated using atomic force microscopy (AFM). The thin film for AFM observation was prepared by spin-casting the solution of poly(120-b-2g40) in THF/MeOH mixtures with different ratios onto silicon wafers. When the volume ratio of THF:MeOH was 7:3, worm-like supramolecular helical H

DOI: 10.1021/acs.macromol.8b01478 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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is more, when P3HT-b-PPI copolymer was postpolymerization modified with glycol, cross-linked copolymer poly(120-b-2h40) was obtained, and it was with spherical morphologies in THF. We believe this study provides not only a method for synthesis of P3HT-b-PPI copolymers bearing functional groups that cannot be synthesized via the direct block copolymerization but also a clue for designing functional P3HT copolymers with novel fluorescent property, controlled self-assembly morphology, and helicity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01478.

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Experimental procedure and characteristic data (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Hui Zou: 0000-0002-6716-9746 Zong-Quan Wu: 0000-0001-6657-9316 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Scientific Foundation of China (21371043, 51303044, 21574036, 51673058, and 21622402). Z.-Q. Wu thanks the Thousand Young Talents Program of China for Financial Support.

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DOI: 10.1021/acs.macromol.8b01478 Macromolecules XXXX, XXX, XXX−XXX