Synthesis of Polyquinolines via One-Pot Polymerization of Alkyne

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

Synthesis of Polyquinolines via One-Pot Polymerization of Alkyne, Aldehyde, and Aniline under Metal-Free Catalysis and Their Properties Weiqiang Fu,† Lichao Dong,† Jianbing Shi,*,† Bin Tong,† Zhengxu Cai,† Junge Zhi,‡ and Yuping Dong*,† †

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, and ‡Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *

ABSTRACT: A novel synthetic route to polyquinolines with 6substituted quinoline as the structural unit was developed based on the polymerization of alkyne−aldehyde monomers and aniline derivatives under the catalysis of Lewis acid B(C6F5)3. The polymerization was conducted in dichloroethane at 100 °C for 36 h under air atmosphere, affording polyquinolines with molecular weights up to 13 100 and good solubility in most organic solvents. The substituents in aniline exhibited significant effects on the molecular weight, yield, and solubility of the produced polyquinolines. The structures of prepared polymers were characterized and confirmed by GPC, NMR, and FT-IR. The thermogravimetry (TGA) and differential scanning calorimetry (DSC) analysis suggests that the polyquinolines are highly thermal stable. Further photoluminescence behaviors of the prepared polyquinolines were investigated. Based on the characterization results and small molecule reaction mechanism, the polymerization pathway of the polyquinolines was proposed. Our work has provided a novel simple strategy for the preparation of multifunctional polyquinolines with unique architectures by one-pot synthesis under metal-free catalysis.

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INTRODUCTION The polymerization of carbon−carbon triple bonds, such as azide−alkyne, thiol−yne, and amino−yne click polymerizations, has attracted considerable attention. The functional polymerization products usually possess excellent solubility and stability and have been demonstrated with great application potentials in a variety of fields, such as organic light-emitting diodes (OLEDs),1 chemosensors,2,3 bioprobes,4−8 organic photovoltaics,9−11 organic thin-film transistors (OTFTs),12−18 photocatalytic,19 electrochromic materials,20 and so on. The polymerization of carbon−carbon triple bonds is usually realized via the organic reaction of alkynes. For practical application, the organic reaction should meet with the following criteria: mild reaction condition, high catalytic efficiency, and easily isolated products. For example, Arndtsen et al. synthesized a pyrrolebased π-conjugated polymer by the metal-free catalyzed multicomponent reaction of imines, acid chlorides, and alkynes.21 In the past few decades, polyquinolines have been extensively studied due to their excellent optical, electrical, and mechanical properties.22−31 For example, Jenekhe et al. found that the color of n-type polyquinolines could be tuned visibly from blue to red, which could be potentially used for the preparation of efficient and bright light-emitting devices.32 However, extremely limited polymerization routes including Suzuki (or © XXXX American Chemical Society

Sonogashira) couplings, oxidative polymerizations, and Friedländer synthesis are available for the preparation of polyquinolines.33 Several years ago, Gorodetsky’s group reported the synthesis of polyquinolines with a bifunctional monomer via AB-type and AA/BB-type aza-Diels−Alder polymerization reactions.34,35 2-Substituted polyquinolines with completely conjugated main chains were obtained by two-step metal-free catalyzed reactions. The Schiff bases were obtained and purified, which then reacted with alkyne monomers to produce the corresponding structural units of quinolines. The alkyl chains, as the side groups, were found to be significant in improving the solubility of these polymers with rigid main chains. In 2016, Tang’s group discovered that the polyannulations of internal diynes and O-acyloxime derivatives could produce functional poly(isoquinoline)s under the catalysis of [Cp*RhCl2]2 using Cu(OAc)2·H2O as the oxidant.36 The thin films of the synthesized poly(isoquinoline)s exhibited high refractive indices in a wide wavelength region. It has been well recognized that 2,4-diarylquinolines can be synthesized via the aldehyde−aniline−alkyne reaction that forms motif upon cyclization/oxidation.37 Two types of Received: November 28, 2017 Revised: March 15, 2018

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DOI: 10.1021/acs.macromol.7b02494 Macromolecules XXXX, XXX, XXX−XXX

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metal-free one-pot polymerization of alkyne−aldehyde monomers and aniline derivatives is simple and can be conducted with commercially available aniline derivatives for the synthesis of a variety of multifunctional polyquinolines with great application potentials in different fields.22−31

catalysts have been generally utilized for the cyclization/ oxidation, which follows different catalytic mechanisms. The transition metal or lanthanide catalysts can form metal acetylide nucleophiles with alkynes during the catalytic cycle, while nonmetal catalysts, Brønsted and certain Lewis acids, activate the imine for the nucleophilic attack by alkyne in a variant of the Povarov reaction.38 Most of the syntheses of substituted quinolines usually require metal catalysts, such as Cu(OTf)2, FeCl3, Cu(OTf)2, YCl3, Pd, and so on.39−44 In 2017, however, Fasano et al. successfully initiated the aldehyde−aniline−alkyne reaction with nonmetal catalyst Lewis acid, B(C6F5)3, for the synthesis of 2,4-substituted quinolines under ambient conditions.45 Inspired by Fasano’s work, we first wanted to develop a polymerization strategy for the synthesis of quinoline-based polymers via the one-pot three-component tandem polymerization of 4,4′-diethynylbiphenyl, terephthalaldehyde, and 4-nbutylaniline but failed with poorly soluble products because of the unordered attack of alkynes at the iminium cation and cross-linking during the polymerization. Nevertheless, in the present work, aldehyde and alkyne groups were then combined into one molecule to form an alkyne−aldehyde bifunctional compound that could be readily polymerized with excessive aniline under the catalysis of B(C6F5)3, as shown in Scheme 1.

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RESULTS AND DISCUSSION Polymerization. Seven alkyne−aldehyde monomers were synthesized in good yields via the procedures reported in the literature46−48 and characterized by NMR and HR-MS. The detailed characterization can be found in Figures S1−S21 of the Supporting Information. Considering the accessibility of monomers and the solubility of corresponding polymers, alkyne−aldehyde 1a and aniline 2d were chosen to explore the polymerization conditions. The effects of B(C6F5)3 concentration on the polymerization efficiency were first determined (Table 1). No polymer was Table 1. Effects of Catalyst Concentration on the Polymerizationa entries

B(C6F5)3 (mol %)

yield (%)

Mwb

Đb

0 5 10 20 30

− 10.5 44.6 64.6 70.4

− 5300 13100 10000 9400

− 1.56 1.63 1.56 1.40

1 2 3 4 5

Scheme 1. Synthetic Route to Polyquinolines Catalyzed by B(C6F5)3

c

Conducted in dichloroethane (DCE) at 100 °C for 32 h in the presence of B(C6F5)3, [1a] = 0.33 M and [1a]:[2d] = 1:2. b Determined by GPC in THF using a linear polystyrene as calibration standard; Mw = weight-average molecular weight; Đ = Mw/Mn, where Mn = number-average molecular weight. cNo polymerization. a

formed in the absence of B(C6F5)3 catalyst (Table 1, entry 1). The reaction with 0.05 equiv of B(C6F5)3 to monomer 1a afforded a polymer with low Mw of 5300 in a low yield of 10.5% (Table 1, entry 2). Increasing the amount of B(C6F5)3 to 0.1 equiv produced a polymer with the desired Mw of 13 100 in the moderate isolated yield of 44.6% (Table 1, entry 3). Further increasing the catalyst amount led to decline Mw of the polymer product. Therefore, 10 mol % B(C6F5)3 was used as the catalyst in the following experiments. The effects of monomer concentration on the polymerization were then investigated. As shown in Table 2, the highest Mw (13 100) was obtained at the monomer concentration of 0.33 mol/L (Table 2, entry 2). Decreasing or increasing the concentration of 1a resulted in slightly lower yields and/or Mws (Table 2, entries 1 and 3), suggesting that the optimal Table 2. Effects of Monomer Concentration on the Polymerizationa

On the basis of the FT-IR, 1H NMR, and 13C NMR characterization results, it was found that the produced polyquinolines contained 2,4-substituted quinoline structural units and 6-substituted functional side groups. In addition, the main chain structure could be fully conjugated or nonconjugated as R1 was varied, which enriched the variety of polyquinolines and their properties. Most of obtained polyquinolines showed excellent solubility. Furthermore, the effects of substituents on monomers 1 and 2 on the polymerization were investigated and discussed. In all, this

entries

[1a] (mol/L)

[2d] (mol/L)

yield (%)

Mwb

Đb

1 2 3 4 5 6 7

0.16 0.33 0.66 0.33 0.33 0.33 0.33

0.33 0.66 1.32 0.16 0.33 0.99 1.32

35.2 44.6 56.8 9.9 24.6 38.3 −c

5300 13100 9500 3900 3300 21000 −

1.32 1.63 1.28 1.56 1.50 2.69 −

Carried out in DCE at 100 °C for 32 h in the presence of 10 mol/L B(C6F5)3. bDetermined by GPC in THF using a linear polystyrene as calibration standard. cNot available.

a

B


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Table 5. Effects of Time Course on the Polymerizationa

concentration range of monomer 1a was very narrow. It can be explained that the collision probability is lower at low monomer concentrations and thus results in lower yields. The viscosity of polymerization system increases with the increases of monomer concentration, which makes the addition of monomer to the polymeric segments difficult and thus leads to the low Mw of the produced polymer. The polymerizations with excess amounts of monomer 2d produced polymers of higher molecular weights in higher yields (Table 2, entries 2, 4−6). However, the products were poorly soluble and could not be isolated for characterization (Table 2, entry 7). Based on these results, the monomer concentration was optimized to be [1a] = 0.33 mol/L and [1a]:[2d] = 1:2. The reaction solvent also exhibited significant effects on the polymerization (Table 3). Four different solvents including Table 3. Effects of Reaction Solvent on the Polymerization

entries

time (h)

yield (%)

Mwb

Đb

1 2 3

24 36 48

34.8 44.6 49.8

6400 13100 10200

1.45 1.63 1.92

a Carried out in DCE at 100 °C in the presence of 10 mol % B(C6F5)3, [1a]:[2d] = 1:2. bDetermined by GPC in THF using a linear polystyrene calibration standard.

Finally, the effects of the electron affinity of aniline monomer on the polymerization were studied. Monomer 1a was used as the alkyne−aldehyde monomer, and the aniline monomer was varied among 2a−j. As shown in Table 6, the variation of Table 6. Polymerizations of 1a and Different Aniline Monomersa

a

entries

solvent

yield (%)

Mwb

Đb

entries

1 2 3 4

DCE DMF DMSO toluene

44.6 24.9 13.2 35.6

13100 3800 4500 3100

1.63 1.08 1.80 1.07

1 2 3 4 5 6 7 8 9 10

a Carried out at 100 °C for 32 h in the presence of 10 mol % B(C6F5)3, [1a]:[2d] = 1:2. bDetermined by GPC in THF using a linear polystyrene as calibration standard.

DCE (haloalkane solvent), N,N-dimethylformamide (DMF, aprotic solvent), dimethyl sulfoxide (DMSO, aprotic solvent), and toluene (aromatic hydrocarbon solvent) were tested. The highest yield (44.6%), as well as the highest Mw (13 100), was achieved in DCEa suitable solvent for the synthesis of quinolines via aldehyde−aniline−alkyne reaction. 45,49,50 Although the polymerization also occurred in DMF, DMSO, and toluene, both Mw and yields of the polymer products were significantly lower. Therefore, DCE was used as the polymerization solvent in the following experiments. The effects of temperature on the polymerization are summarized in Table 4. The yield of polymer product increased

temp (°C)

yield (%)

Mwb

Đb

1 2 3 4

60 80 100 120

23.5 35.9 44.6 58.8

3900 5900 13100 8200

1.56 1.34 1.63 1.54

yield (%)

Mwb

Đb

solubilityc

+ + + + + + + + + +

77.9 50.3 43.1 46.6 56.7 88.5 77.5 12.4 8.6 15.3

13700 6500 8500 13100 4600 6600 1900 3600 2300 3900

2.07 1.58 1.49 1.63 1.48 1.61 1.18 1.16 1.28 1.18

△ ○ ○ ○ ○ △ △ △ △ ○

1a 1a 1a 1a 1a 1a 1a 1a 1a 1a

2a 2b 2c 2d 2e 2f 2g 2h 2i 2j

Carried out in DCE at 100 °C for 32 h in the presence of 10 mol % B(C6F5)3, [monomer 1]:[monomer 2] = 1:2. bDetermined by GPC in THF using a linear polystyrene as calibration standard. c○ = completely soluble in THF, DMF, and CHCl3 and △ = partially soluble in THF, DMF, and CHCl3. a

substituent significantly affected both polymerization efficiency and solubility of polymerization product. The electron-neutral and electron-rich aniline monomers were able to react smoothly with 1a, producing polymers with higher Mws in moderate yields and good solubility (Table 6, entries 1−6). However, it is worth noting that the polymers prepared with the monomers containing no flexible alkyl groups, such as 2a and 2f, exhibited poor solubility (Table 6, entries 1 and 6). Although the polymerization of 2g containing an amide group resulted in a good yield, the Mw of corresponding polymer was very low (Table 6, entry 7), probably because the product was partially cross-linked by hydrogen bonds during drying, and only low-Mw molecules were allowed to pass the filter membrane for GPC analysis. The polymerization efficiencies of aniline monomers with electron-withdrawing substituents were very low under the same conditions, and the yields of the corresponding polymers were below 20% (Table 6, entries 8− 10). It can be explained that based on the mechanism of aldehyde−aniline−alkyne reaction, the electron-withdrawing substituents inhibited B(C6F5)3 to activate the imine to the nucleophilic attack by alkyne, and thus reduced the catalyst activity, leading to lower yields and Mws of the polymer product. Under the optimized reaction conditions, the scope of alkyne−aldehyde monomer was investigated using monomers 1a−g (Table 7). Monomer 1b is more rigid than 1a, and thus its polymerization produced a product (P1b2d) with a lower Mw than that of P1a2d. The alkyne−aldehyde monomers with electron-donating groups, such as ether, linked to phenyl-

Table 4. Effects of Temperature on the Polymerizationa entries

monomer

a Carried out in DCE for 32 h in the presence of 10 mol % B(C6F5)3, [1a]:[2d] = 1:2. bDetermined by GPC in THF using a linear polystyrene as calibration standard.

with the increase of temperature from 60 to 120 °C. However, the highest Mw was obtained at 100 °C (Table 4, entry 4). Therefore, the polymerization temperature was optimized as 100 °C. The effects of time course on the polymerization were evaluated as shown in Table 5. Both yield and Mw of the polymer product gradually increased as the polymerization time increased from 24 to 36 h. However, further prolonging the reaction time to 48 h resulted in a lower Mw of 10 200, suggesting that the polymer chain length growth became difficult after a certain time. Based on these observations, the reaction time was optimized as 36 h. C


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Macromolecules Table 7. Polymerizations of Different Alkyne−Aldehyde Monomers 1 and 2da entries c

1 2 3 4 5 6 7

monomers

yield (%)

Mwb

Đb

solubilityc

1a + 2d 1b + 2d 1c + 2d 1d + 2d 1e + 2d 1f + 2d 1g + 2d

46.6 79.9 86.5 89.8 57.9 −d −

13100 8800 4700 3400 5200 − −

1.63 1.31 1.38 1.13 1.49 − −

○ ○ △ △ ○ -

Carried out in DCE at 100 °C for 32 h in the presence of 10 mol % B(C6F5)3, [1]:[2d] = 1:2. bDetermined by GPC in THF using a linear polystyrene as calibration standard. c○ = completely soluble in THF, DMF, and CHCl3 and △ = partially soluble in THF, DMF, and CHCl3. dNo polymerization. a

Figure 1. FT-IR spectra of 1a (A), 2d (B), MC (C), and P1a2d (D).

acetylene are favorable for the polymerization and thus can produce polymers in higher yields under same conditions (Table 7, entries 3 and 4). However, the solubility of the corresponding products, P 1c2d and P 1d2d, are much lower due to the high activities of the triple bonds and faster polymerization. It is interesting that the polymerization of 1e with the electron-donating ether group linked to its benzaldehyde was able to produce highly soluble polymer P1e2d in a moderate yield (Table 7, entry 5). The alkyne− aldehyde monomers containing electron-withdrawing groups, such as ester group, made the polymerization very difficult to proceed (Table 7, entry 6). In addition, monomer 1g with benzaldehyde linked to alkyl alkyne was not polymerized under the optimal reaction conditions (Table 7, entry 7). On the basis of these results, it can be concluded that the activity of the triple bond in monomer is the key factor for a successful polymerization. Structural Characterization. To assist the structural characterization of synthesized polymers, a model compound MC was prepared for tracking the changes in polymerization caused by different substituents. MC was synthesized under the optimal conditions for the polymerization using benzaldehyde 3, phenylacetylene 4, and 4-n-butylaniline 2d at the molar ratio of 1:3:3 (Scheme 2). P1a2d was selected as a representative of polymerization products for the characterization. As shown in Figure 1A for FT-IR spectra, the monomer exhibited stretching vibration bands of CC and ≡C−H at 2100 and 3230 cm−1, respectively. The C−H stretching vibration in aldehyde group was observed at 2740 and 2838 cm−1. The peak at 1684 cm−1 was attributed to the CO stretching vibration in the aldehyde group. All these peaks disappeared and a cluster of peaks at 1450−1650 cm−1 appeared in the FT-IR spectra of both MC and P1a2d, partially due to their aromatic quinoline core (Figure 1D).51−53 The stretching vibration peaks of −NH2 in 2d at 3441 and 3355 cm−1 also disappeared, and the stretching

vibration of C−H in the n-butyl group of 2d exhibited as a broad band at 2853−2959 cm−1 in FT-IR spectra of MC and P1a2d. These results indicate that the reactants reacted with each other, following the synthetic route proposed above. The compounds 1a, 2d, MC, and P1a2d were further characterized with NMR spectroscopy. As shown in Figure 2

Figure 2. 1H NMR spectra of 1a in CDCl3 (A), 2d in CDCl3 (B), MC in CD2Cl2 (C), and P1a2d in d8-THF (D). The solvent peaks are marked with asterisks.

for their 1H NMR spectra, the peaks at δ 3.29 ppm and δ 9.98 ppm that were assigned to the resonances of Ha and Hb in 1a, respectively, almost disappeared after the polymerization. The peak at δ 3.57 ppm that was ascribed to the resonance of Hc in 2d also disappeared, and several new peaks at δ 8.15−8.50 ppm appeared in the NMR spectra of MC and P1a2d due to the resonances of Hd and He. The 13C NMR spectra shown in Figure 3 are consistent with the results of IR and 1H NMR

Scheme 2. Synthetic Route To Model Compound MC

D


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thermal stability of the polyquinolines with different morphologies endows them application potentials as carbon materials. Photophysical Properties. Figure 5 shows the UV absorption spectra of MC and synthesized polyquinolines in

Figure 3. 13C NMR spectra of 1a in CDCl3 (A), 2d in CDCl3 (B), MC in CD2Cl2 (C), and P1a2d in d8-THF (D). The solvent peaks are marked with asterisks.

analyses. The chemical shifts at 81.14, 82.64, and 191.35 ppm assigned to Ca, Cb, and Cc in 1a and that at 144.15 ppm assigned to Cd in 2d completely disappeared, and two new peaks appeared at 147.76 and 148.59 ppm in spectra of MC and P1a2d due to the resonances of Ce and Cf. These results confirmed the formation of quinoline ring in the polymers. Thermal Stability. The thermal stability of P1a2a, P1a2d, P1b2d, and P1d2d was evaluated by thermogravimetric analysis (TGA). As shown in Figure 4, all of the polymers lost only 5%

Figure 5. Normalized UV−vis absorption spectra of MC, P1a2d, P1a2e, P1b2d, and P1c2d in THF (10 μM).

THF. The spectrum of MC is featured with an absorption onset at 357 nm. The absorption onset shifted to 458 nm in P1a2e due to the higher electron donating ability of its methoxy group that could improve the electronic conjugation of the polymer.55−57 The absorption onsets of other polyquinolines also red-shifted to 432−467 nm, compared with that of MC, due to the formation of macromolecules that extended the πconjugated lengths. The photoluminescence (PL) spectra of MC and polyquinolines in THF are shown in Figure 6. Upon photoexcitation, MC exhibited an emission peak centered at 386 nm, while those of P1a2d, P1a2e, P1b2d, and P1c2d red-shifted to 483, 461, 502,

Figure 4. Thermograms of P1a2a, P1a2d, P1b2d, and P1d2d measured under N2 at the heating rate of 10 °C/min.

weight as heated at 212−255 °C under a nitrogen atmosphere due to the degradation of the unstable 6-substituent group, suggesting that they were highly thermal-stable.54 The mass ratio of quinoline fragment in the polymer was calculated to be ∼50%. The weight losses of P1a2a, P1a2d, and P1b2d at 800 °C were less than 50% due to their stable aromatic conjugated structures. Other polyquinolines including P1a2b, P1a2f, P1c2d, and P1e2d also showed good thermal stability (Figure S34). No glass transition or crystal transition of the polyquinolines P1a2b, P1a2d, and P1e2d was found with the DSC analysis under a nitrogen atmosphere (Figures S35−S37) due to the relatively rigid main chains that made the segmental relaxation and crystallization very difficult. Therefore, the good

Figure 6. Normalized PL spectra of MC, P1a2d, P1a2e, P1b2d, and P1c2d in THF (10 μM). Excitation wavelength: 300 nm for MC, 390 nm for P1a2d, 320 nm for P1a2e, 400 nm for P1b2d, and 380 nm for P1c2d. E


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Figure 7. (A) PL spectra of MC in THF/H2O mixtures (10 μM). Excitation wavelength: 300 nm. (B) Variation of maximum PL intensity of MC with the water fraction in THF/H2O. (C) Transmittance spectra of MC in THF/H2O mixtures (10 μM).

Figure 8. (A) PL spectra of P1a2d in THF/H2O mixtures with different water fractions ( f w) (10 μM). Inset: zoom-in spectra at water fractions of 70%, 80%, and 90%. (B) Relative PL intensities of P1a2d, P1a2e, P1b2d, and P1c2d in pure THF and the THF/H2O mixture containing 90% water (10 μM). Excitation wavelength: 390 nm for P1a2d, 320 nm for P1a2e, 400 nm for P1b2d, and 380 nm for P1c2d.

and 464 nm, respectively, because of their longer conjugated system and higher rigidity and coplanarity.

The aggregation-induced emission behaviors of the prepared polyquinolines in the mixture of THF and H2O were then F


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Macromolecules Scheme 3. Proposed Polymerization Pathway of Alkyne−Aldehyde and Aniline Catalyzed by B(C6F5)3

close vicinity, which enhanced their π−π stacking interactions and resulted in PL annihilation upon gradually increased water content. Polymerization Process. On the basis of the discussion above and the formation mechanism of small molecule quinolines reported in the literature,45 we proposed a polymerization pathway for preparation of polyquinolines as Scheme 3. First, H2O−B(C6F5)3 is reversibly deprotonated by the aniline or quinoline, a weak Brønsted base. Extremely high or low amounts of aniline are unconducive to the polymerization (Table 2, entries 2, 4−7). The protons of H2O− B(C6F5)3 are seized by the iminium cation (I) produced from the Schiff base reaction at high temperatures, forming [HOB− (C6F5)3]−, a suitable weakly coordinating anion that can provide sufficient electrophilicity for the nucleophilic attack by alkyne. The vinyl cation intermediate II is difficult to be produced or is produced but unstable to prevent chain propagation with the R1 containing an electron-withdrawing group or an aliphatic chain (Table 7, entries 6 and 7). The species II are then subjected to the proactive nucleophilic attack by dihydroquinolinium species III that is generated from the ortho-carbon of anilines, releasing a proton to form species IV. The oxidation of dehydroquinoline furnishes the final quinolines with an equivalent of imine acting as the hydrogen acceptor. Unlike those in other small molecule reactions for quinoline preparation, the terminal alkyne with the ability to carry out nucleophilic attack is still present in species V. The pathway is then repeated from species V following the same catalytic process, which eventually forms polyquinolines. Theoretically, the molecular weight can be continuously increased as long as residual terminal alkynes are available.

investigated. MC exhibited a weak fluorescence emission in pure THF, and the emission intensity gradually increased with the increase of H2O fraction and peaked with 3-fold increase at the H2O fraction of 70% (Figure 7). To confirm the formation of MC aggregates, the transmission of MC solution was monitored with a UV−vis spectrophotometer as H2O gradually added. As shown in Figure 7C, the transmission peak gradually shifted from 350 to 550 nm with the increase of water content, and transmittance was dramatically decreased as the water fraction increased to 70%, consistent with the results of fluorescence analysis. Dynamic light scattering (DLS) analysis further confirmed the formation of nanosized particles of the polymers at different water fractions (Figure S38). Evidently, it can be concluded that the emission of MC can be spectacularly boosted by the aggregation, and MC is an aggregationenhanced emission (AEE) active compound.58 However, the PL of P1a2d was significantly quenched up to 90% as H2O added to its THF solution (Figure 8A). The other three polyquinolines, P1a2e, P1b2d, and P1c2d, also showed similar properties (Figure 8B) due to the aggregation-caused quenching (ACQ) as demonstrated by DLS analysis (Figure S39). It can be explained that the THF solution became more polar as H2O added, which promoted the interactions between quinolines containing aromatic structures and thus caused the fluorescence quenching.59 A shoulder peak appeared at 560 nm as the water fraction increased to 70% (inset in Figure 8A) and became clear with increased polymer concentration as shown in Figure S40, suggesting that the excimer formation occurred due to the close proximity of main chains after the aggregation.60,61 The volumetric shrinkage of the polymer chains of P1a2d in the poor solvent might push the aromatic rings together in G


Macromolecules However, the molecular weight of polyquinoline products remained constant in a certain reaction time, even though the reaction time was prolonged to 48 h with the residual terminal alkyne present in the system as demonstrated by the 1H NMR analysis result of P1a2d (Figure 2D, entries 2 and 3 in Table 5), possibly due to the high viscosity of the reaction system at the late stage of polymerization and high rigidity of the produced polyquinolines that inhibited the diffusion of the terminal alkyne.

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ASSOCIATED CONTENT

The polymerization of different monomers and characterization data can be found in the Supporting Information. The polymerization of Table 1, entry 3 is described in the following as an example. Into a 10 mL Schlenk tube equipped with a magnetic stir bar were added 4ethynylbenzaldehyde (130 mg, 1.0 mmol), 4-n-butylaniline (320 uL, 2.0 mmol), B(C6F5)3 (51.2 mg, 0.1 mmol), and 3 mL of DCE. The reaction mixture was heated in an oil bath at 100 °C for 36 h under constant stirring under air atmosphere, cooled to room temperature, and poured into 200 mL of methanol under vigorous stirring. The precipitate was filtered, washed with methanol, and vacuum-dried to the constant weight at room temperature to afford the polymer product. The polymerizations of other monomers followed the same procedure under varied conditions.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02494.

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ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS In the present work, a one-pot metal-free polymerization of alkyne−aldehyde and aniline derivatives catalyzed by B(C6F5)3 was developed for the synthesis of polyquinolines. The aniline derivatives bearing electron-donating substituents were able to improve the solubility of polyquinolines, yet in moderate yields. The polymerization was found to be highly tolerant to oxygen and moisture and thus could be conducted even under air atmosphere, which significantly simplified the reaction system. The one-pot reaction makes the polymerization strategy amenable to scale-up, and the functional groups in aniline derivatives endow multifunction of the produced polyquinolines. The potential applications of these polyquinolines are underway in our lab. EXPERIMENTAL SECTION

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This work was financially supported by the National Natural Scientific Foundation of China (Grants 21490574, 51673024, and 21474009) and the National Basic Research Program of China (973 Program; Grant 2013CB834704).

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Experimental procedures and figures showing NMR spectra and GPC chromatograms of representative polymer products (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.S.). *E-mail: [email protected] (Y.D.). ORCID

Jianbing Shi: 0000-0002-4847-1472 Zhengxu Cai: 0000-0003-0239-9601 Yuping Dong: 0000-0001-7437-0678 Notes

The authors declare no competing financial interest. H


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Synthesis of Polyquinolines via One-Pot Polymerization of Alkyne

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