Single Component Polymerization of Diisocyanoacetates toward

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

Single Component Polymerization of Diisocyanoacetates toward Polyimidazoles Tianyu Cheng,† Yizhao Chen,† Anjun Qin,*,† and Ben Zhong Tang*,†,‡ †

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State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640, China ‡ Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Centre for Tissue Restoration and Reconstruction, Institute for Advanced Study, and Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: The polymerization of triple-bond building blocks will generate functional polymers owing to their unsaturated backbones, which facilitate the electron delocation within the main chains. Currently, the research focus in this area is the alkynebased polymerization, and the carbon−nitrogen triple-bond based polymerization to produce stable nitrogen-containing polymers is rarely reported. In this paper, a new polymerization of diisocyanoacetate was successfully established. The silver acetatecatalyzed polymerization of diisocyanoacetate in the acetonitrile or DMF readily produced soluble polyimidazoles with high molecular weights (Mw up to 32500) in excellent yields (up to 94%) at room temperature after 2 h. Moreover, this polymerization performed in a single component fashion, which shows remarkable advantages over the traditional two- or multicomponent ones. In addition, the resultant polyimidazoles could be postfunctionalized to yield ionized polyelectrolytes and could feature the aggregation-induced emission (AIE) characteristics by incorporation of an AIE-active tetraphenylethene moiety in its polymer chains. This single component polymerization will become a powerful tool for the preparation of polyimidazoles and be potentially applicable in materials and biological fields.

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bond building blocks,15,16 and remarkable progress in the polymerizations especially based on carbon−carbon triple bonds, i.e., alkyne monomers (CC), has been made. For example, the Cu(I)- and Ru(II)-catalyzed and metal-free azide−alkyne click polymerizations,17−23 transition-metal-catalyzed, photoinitiated, organobase-catalyzed, and spontaneous thiol−yne click polymerizations,24−26 Cu(I)-catalyzed and spontaneous amino−yne click polymerizations,27−29 and organobase-catalyzed hydroxyl−yne click polymerizations30,31 as well as alkyne-based multicomponent polymerizations32,33 have been successfully developed and used for the preparation of functional polymers with versatile properties and diverse applications.16 As for the triple-bond building blocks, besides the alkynes, there are other kinds of carbon−nitrogen ones, e.g., nitrile (−CN) or isonitrile (−N+C−) monomers. The polymerizations based on carbon−nitrogen triple bonds will readily generate nitrogen-containing polymers with unique properties. However, these polymerizations are rarely reported besides the

INTRODUCTION The development of powerful and efficient polymerization reactions is of vital importance to polymer science, through which advanced polymer materials with versatile properties could be facilely generated. In the past decades, polymer scientists have been enthusiastically devoted themselves to exploring new polymerization reactions based on double-bond building blocks, especially alkenes.1−6 As we know, when the double bonds of the monomeric species are polymerized, the polymers with saturated single bonds in their main chains are produced, which generally make them electronically inactive. This feature limits their real applications in the optoelectronic field, etc. To overcome this limitation and to prepare more functional materials, polymer scientists are committing to the research of polymerizations based on triple-bond building blocks.7−9 Such polymerizations can produce polymers with repeating units knitted by electronically unsaturated double or triple bonds or (hetero)aromatic rings. As a result, polyacetylene10 and other conjugated polymers11−13 have been prepared and applied in diverse areas.14 Attracted by the rich reaction types and versatile properties of resultant polymers, more and more researchers have been involved in developing new polymerizations based on triple© XXXX American Chemical Society

Received: June 3, 2018 Revised: July 7, 2018

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DOI: 10.1021/acs.macromol.8b01179 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules polymerization of isonitriles that yields functional poly(phenyl isocyanide)34−37 and multicomponent polymerization based on isonitriles.38−41 According to textbook knowledge, the cyano group is much more stable than the isocyano one. Therefore, the reactivity of the latter is much higher than that of the former. Indeed, many elegant organic reactions based on isocyano groups have been reported,42−45 which offers us great opportunities to develop isonitrile-based polymerizations because of the fact that most of the polymerizations are established based on the most efficient and powerful organic reactions.46 Among the reported organic reactions based on isonitriles, we found that the isocyanoacetates could readily undergo the cyclodimerization in the presence of the catalyst of silver acetate under mild reaction conditions (the reaction mechanism is provided as Scheme S1 in the Supporting Information), and imidazole derivatives were facilely produced in excellent yields (Scheme 1).47 Interestingly, the resultant imidazole derivatives are easy

Scheme 3. Polymerization of M1−M5 toward Polyimidazoles of PI−PV

established polymerization will not only greatly simplify the experimental operations but also facilitate the preparation of functional polyimidazoles for high-tech applications.

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RESULTS AND DISCUSSION Cyclodimerization of Isocyanoacetates. To develop the cyclodimerization of isocyanoacetates into an efficient single component polymerization for the preparation of functional polyimidazoles, monomers with multiple functional groups were rationally designed and facilely prepared. The monomers of M1−M5 were efficiently prepared according to route shown in Scheme 2. After confirming their structures by the spectroscopical methods, we systematically optimized the polymerization conditions using M2 as monomer in the presence of the catalyst of silver acetate under nitrogen. First, we investigated the effect of solvents on the polymerization. The results are summarized in Table 1. The polymerization performed in

Scheme 1. Silver Acetate-Catalyzed Cyclodimerization of Isocyanoacetate

to be postmodified to imidazole salts.48 Moreover, the imidazole derivatives are widely used in diverse areas as ionic liquids,49,50 ligand for organometal catalyst,51 antimicrobial materials,52 and so on. In addition, it is worth noting that the diisocyanoacetate monomers could be readily synthesized through two simple steps as shown in Scheme 2. Surprisingly, as far as we know, diisocyanoacetates have not been used as monomers, and there is no report on the diisocyanoacetatebased polymerization.

Table 1. Effect of Solvent on the Polymerization of M2a

Scheme 2. Synthetic Routes to Monomers of M1−M5

entry

solvent

yield (%)

Mwb

Đb

1 2 3 4 5

DCM THF chloroform DMF acetonitrile

82 39 87 69 96

45500 37400 25200 52400 46700

1.85 1.94 1.68 1.83 1.89

a

Carried out under nitrogen at room temperature for 4 h. [M]0 = 0.5 M. bEstimated by gel permeation chromatography (GPC) in DMF containing 0.05 M LiBr on the basis of a PMMA calibration; Mw = weight-average molecular weight; Đ = polydispersity index.

the solvents, such as dichloromethane (DCM), chloroform, N,N-dimethylformamide (DMF), and acetonitrile, could furnish products with satisfactory weight-average molecular weights (Mw) in high yields. However, when the solvent was changed to tetrahydrofuran (THF), the yield of the product decreased dramatically. Considering the yield and Mw of the product, we chose acetonitrile as the solvent for further investigation. Second, we studied the effect of temperature on the polymerization. As can be seen from Table 2, the polymerization became much faster with temperature enhancing from 0 to 60 °C. Interestingly, even we lowered the temperature to 0 °C, this polymerization could still occur and produce the product with the Mw of 5100. When we carried out this polymerization at 60 °C, we found that polymerization of M2 was so fast that the entire system turned into a lump of gel in as short as 0.5 h probably due to the Mw of the product is too large to be soluble in acetonitrile. Delightfully, a soluble product with Mw of 48800 could be obtained in 93% yield at

Attracted by this elegant reaction, in this paper, we developed it into a new polymerization after systematically investigating the reaction conditions (Scheme 3). This polymerization could be carried out under mild reaction conditions, and polyimidazoles with weight-average molecular weights (Mw up to 32500) could be readily produced in excellent yields (up to 94%). More importantly, this polymerization is performed only with one single monomer of diisocyanoacetate, which avoids the problem of stoichiometry of the monomers in the two- or three-component polymerizations.32,40 Furthermore, the resultant polyimidazoles are important starting materials for the preparation of polyimidazoliums that are diversely applied in many fields including electrochemical device, solar cells, electrolytes or binders for batteries, and other smart materials.53−56 Thus, our B


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With the optimized reaction conditions in hand, we performed the polymerization using other diisocyanoacetate monomers to confirm its universality. The results showed that all the polymerizations propagated smoothly, and the polymers of PI−PV with high Mw values (up to 50300) (Figure S1) were produced in excellent yields (up to 96%) (Table 5), manifesting the universality of this powerful and efficient polymerization.

Table 2. Effect of Temperature on the Polymerization of M2a entry

T (°C)

yield (%)

Sb

Mwc

Đc

1 2 3 4 5

0 r.t. 30 40 60

74 93 93 94 86

√ √ √ √ △

5100 48800 64500 66200 57500

1.15 1.85 2.01 2.23 1.91

a The polymerizations in entries 1−4 were carried out in acetonitrile under nitrogen for 4 h, r.t. = room temperature. The reaction entry 5 was carried out in acetonitrile under nitrogen for 0.5 h; [M]0 = 0.5 M. b Solubility (S) tested in DMF and DMSO; √ = completely soluble, Δ = partially soluble. cEstimated by gel permeation chromatography (GPC) in DMF containing 0.05 M LiBr on the basis of a PMMA calibration; Mw = weight-average molecular weight; Đ = polydispersity index.

Table 5. Click Polymerizations of Different Diisocyanoacetate Monomers

room temperature. Thus, we preferred room temperature for further experiments. Third, we followed the time course of the polymerization. As shown in Table 3, both of the yields and the Mw of the

t (h)

yield (%)

Mwb

Đb

1 2 3 4 5 6 7

0.5 1 2 4 6 12 24

40 93 94 94 95 99 99

6600 22500 32500 40700 43900 45000 73900

1.20 1.99 1.85 1.93 1.99 2.14 2.23

monomer

polymer

yield (%)

Mwb

Đb

1 2 3 4 5

M1 M2 M3 M4 M5

PI PII PIII PIV PV

85 96 81 82 82

24400 50300 65900 31700 33500

1.73 2.06 2.28 1.80 2.08

a

The experiments in entries 1−4 were carried out in acetonitrile under nitrogen at room temperature for 4 h. The experiment in entry 5 was carried out in DMF under nitrogen at room temperature for 4 h. bEstimated by gel permeation chromatography (GPC) in DMF containing 0.05 M LiBr on the basis of a PMMA calibration; Mw = weight-average molecular weight; Đ = polydispersity index.

Table 3. Time Course of the Polymerization of M2a entry

entrya

The resultant polymers are soluble in highly polar organic solvents, such as DMF and DMSO, and thermally stable. As evaluated by thermogravimetric analysis (TGA) (Figure S2), the temperatures of 5% weight loss of the PI−PV are between 250 and 280 °C. Structural Characterization. As aforementioned, the resultant polymers are all soluble in highly polar organic solvents, which enables us to characterize their structures spectroscopically. Satisfactory analysis data corresponding to their expected molecular structures (see the Experimental Section) were obtained. As the FT-IR spectral profiles of PI− PV are similar, the FT-IR spectra of PII and its monomer M2 are discussed here as an example (Figure 1). The stretching vibration of isocyano groups is observed at 2247 cm−1. This

a

Carried out in acetonitrile under nitrogen at room temperature. [M]0 = 0.5 M. bEstimated by gel permeation chromatography (GPC) in DMF containing 0.05 M LiBr on the basis of a PMMA calibration; Mw = weight-average molecular weight; Đ = polydispersity index.

products showed an uptrend with the extension of reaction time. These two values of the resultant polymers increased inconspicuously over time after 4 h. Although higher yield and Mw of the polymer could be obtained in 24 h than that in 4 h, we adopted 4 h as the optimal reaction time when considering the efficiency of the polymerization. Finally, we investigated the effect of monomer concentration on the polymerization. The experimental results showed that the best Mw (52700) and yield (99%) of polymer could be obtained when the monomer concentration is 0.5 M (Table 4). Doubled or halved the monomer concentration resulted in the polymers with lower Mw and yields. Thus, this monomer concentration was used in the future polymerization. Table 4. Effect of Monomer Concentration on the Polymerization of M2a entry

[M]0 (M)

yield (%)

Mwb

Đb

1 2 3

0.25 0.5 1

50 99 96

51700 52700 50700

1.52 2.06 1.95

a

Carried out in acetonitrile under nitrogen at room temperature for 4 h. bEstimated by gel permeation chromatography (GPC) in DMF containing 0.05 M LiBr on the basis of a PMMA calibration; Mw = weight-average molecular weight; Đ = polydispersity index.

Figure 1. FT-IR spectra of (A) M2 and (B) its polymer of PII. C


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Figure 2. Kinetic study of the polymerization of M2 by reactIR. (A) Time-dependent peak intensity at 1740 and 1753 cm−1. (B) Threedimensional FT-IR profiles of the peaks at ∼1740 and ∼1753 cm−1 for the polymerization.

characteristic peak, however, disappeared in the spectrum of PII, indicating that the isocyano group of M2 has been consumed by the polymerization reaction. Moreover, the other obvious difference between the FT-IR spectra of PII and M2 is the peaks at about 1700 cm−1 which belong to the stretching vibration of carbonyl groups. In the spectrum of M2, only one peak was observed at near 1700 cm−1, indicating that only one kind of carbonyl group existed. However, in the spectrum of PII, two peaks were observed. These results suggest that there are two kinds of carbonyl groups in the polymer structure, and the polymerization indeed occurred. All the FT-IR spectra of different polymers show the same phenomenon (Figures S3− S6). Moreover, we also used the reactIR to monitor the IR absorption peak change of the carbonyl group during the polymerization (Figure 2). The peak at 1753 cm−1, which belongs to the absorption of carbonyl group in the monomer, decreases dramatically at the beginning of the polymerization, indicative of high efficiency of this polymerization. Meanwhile, the new peak at 1740 cm−1 that belongs to the carbonyl group adjacent to the newly formed imidazole ring in the polymer chain increases quickly. Those characteristic peaks become unchanged after 4 h, suggestive of the completion of reaction. These results further confirm our experimentally optimized reaction time. 1 H NMR spectroscopy could offer more detailed information about the polymer structures. To assist the structural characterization of the polymers, the model compound 2 was prepared by the efficient cyclodimerization of isocyanoacetate using silver acetate as catalyst under nitrogen at room temperature (Scheme 1). The 1H NMR spectra of M2, model compound 2, and PII are shown in Figure 3 as an example. The new peaks emerged at δ 7.65 and 7.97 in the spectra of 2, and PII represent the aromatic protons in the imidazole rings. On the other hand, the resonance peak of methylene protons between the carbonyl group and the isocyanide group at δ 4.76 in the spectra of the monomer shifted to downfield at δ 5.04. These results suggest that the isocyano groups have been converted into imidazole moieties by the polymerization. Similar results were observed for PI− PV (Figure 7). The 13C NMR spectra of M2, model compound 2, and PII further substantiate the conclusion drawn from their 1H NMR

Figure 3. 1H NMR spectra of (A) M2, (B) its polymer PII, and (C) model compound 2 in DMSO-d6. The solvent and water peaks are marked with asterisks.

spectral analysis (Figure 4). The resonance peak of the carbon atom of the isocyano group disappeared in the spectra of both 2 and PII, suggesting that it has been completely converted by the reaction. Meanwhile, new peaks emerging between δ 120 and 160 were readily assignable to the aromatic carbon atoms of the imidazole moiety. In addition, two carbon resonant peaks of the carbonyl group could be observed in the 13C NMR spectrum of PII because it contains two types of carbonyl groups in the repeating unit. Similar results were observed for PI−PV (Figures S11−S14). Interestingly, according to polymer structures shown in Scheme 3 and Figure 5, there are two repeating units in the polymer chains. However, no isomeric unit peaks were observed in the 1H and 13 C NMR spectra. When having detailed investigation of the reaction mechanism, only one structure could be found. D


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postfunctionalized PII (Figure 6). This result together with the data of 13C NMR spectra (Figure S15) suggest that the

Figure 6. 1H NMR spectra of (A) PII and (B) postfunctionalized PII in DMSO-d6. The solvent and water peaks are marked with asterisks.

postfunctionalization reaction is successful. It is worth noting that most imidazole-based polyelectrolyte are directly synthesized from imidazole-containing monomers,53−56 and very few polymerizations that in situ generate imidazole rings in the polymer skeleton were reported.57 Thus, our developed polymerization of diisocyanoacetate offers a new facile way to prepare imidazole-based polyelectrolyte, which might find diverse application in materials and biological fields. Aggregation-Induced Emission (AIE). Our established polymerization of diisocyanoacetate is tolerant to functional groups. Moreover, these groups could also be facilely incorporated into the monomers via efficient substitution reaction. These advantages enable the polymers to possess versatile properties, such as aggregation-induced emission (AIE). The AIE was conceptually termed by our group in 2001 and refers to a unique phenomenon that the nonemissive or weakly emissive molecules become strongly luminescent upon aggregation or in the solid state.58−61 Among the reported AIE-active luminogens (AIEgens), tetraphenylethene (TPE) is a widely studied one owing to its remarkable advantages, such as facile preparation, intense emission, and easy functionalization. By taking these advantages, we also prepared a TPEcontaining polyimidazole of PVI. To facilitate the property investigation of PVI in solution state, we synthesized it with lower Mw (15600) than PI−PV by the polymerization of TPEcontaining diisocyanoacetate (M6) in a yield of 88% (Scheme S3). After fully characterizing its structure by FT-IR, and 1H NMR, and 13C NMR spectra (Figures S16−S18), we investigated its photophysical property. The UV−vis spectrum of PVI in DMF shows a maximal absorption peak at 310 nm (Figure S19), which was used as excitation wavelength to measure it photoluminescence (PL) spectrum. Next, we investigated the AIE activity of PVI by addition of poor solvent of water into its DMF solution (Figure 7). When excited at 310 nm, the PL spectrum of the diluted DMF solution of PVI gave almost a flat line parallel to the abscissa, manifesting that this polymer is weakly emissive when molecularly dissolved. With addition of water, its

Figure 4. 13C NMR spectra of (A) M2 and (B) its polymer of PII and (C) model compound 2 in DMSO-d6. The solvent peaks are marked with asterisks.

Figure 5. Formation of “one structure, two repeating units” in a polymer.

However, in the polymer skeleton, the two linking types led to two different repeating units but with the same chemical conditions. Thus, this “one structure, two repeating units” phenomenon of this polymerization offers us a unique innovation to polymer chemistry. Postmodification. Thanks to the fact that the formed imidazole rings could be easily ionized through reacting with haloalkanes, we used this strategy to postmodify our resultant polyimidazoles. As showed in Scheme S2, the imidazole-based polyelectrolyte can be easily obtained by the reaction of imidiazole and benzyl bromide under mild reaction conditions. The 1H NMR spectra show that the proton resonant peaks of e and f in imidiazole rings of PII completely disappeared, and new peaks of e′ and f′ were observed in the downfield in E


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Figure 7. (A) PL spectra of PVI in DMF/water mixtures with different water fractions. Polymer concentration: 10 μM. Excitation wavelength: 310 nm. (B) Plot of relative PL intensity versus water fraction in DMF/water mixtures, where I = peak intensity and I0 = peak intensity in pure DMF. Inset: fluorescent images of PVI in DMF and DMF/water mixture with water fraction of 50% taken under a hand-held UV lamp.

emission intensity was enhanced dramatically. The highest PL intensity is reached in the DMF/water mixture with water fraction of 50%, which is 129 times higher than that its pure DMF solution (Figure 7B), indicating that PVI is also AIEactive.The absolute fluorescence quantum yield (ΦF) of PVI was measured to be as high as 31.8% in DMF/water mixture with water fraction of 50%. It is worth noting that this value is higher than those of most of AIE-active polymers synthesized by our group previously62 probably because the polymer main chains greatly restrict the intramolecular rotation of TPE units in the aggregate state. The high ΦF value of PVI makes it promising in biological application.59

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.J.Q.). *E-mail [email protected] (B.Z.T.). ORCID

Anjun Qin: 0000-0001-7158-1808 Ben Zhong Tang: 0000-0002-0293-964X Notes

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The authors declare no competing financial interest.

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CONCLUSIONS In this paper, we realized the polymerization of isonitrile monomers and successfully established an efficient and powerful polymerization of isocyanoacetates. This polymerization could be carried out under very mild reaction conditions and readily furnished polyimidazoles of PI−PVI with high Mw values in high yields at room temperature after 4 h. More importantly, this polymerization is a single-component reaction, which shows many advantages over the two- or multicomponent ones, such as simple polymerization procedure, easy access to monomers, and no influence from monomer ratio. In addition, the in situ formed imidiazole rings could be further postfunctionalized to generate polyelectrolytes. Moreover, thanks to its excellent function group tolerance, TPE-containing polyimidiazole was facilely prepared by polymerization of TPE-containing monomer, and the resultant PVI showed the unique AIE feature. Thus, this efficient and powerful polymerization could be potentially used in materials and biological fields.

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FT-IR, 1H NMR, and 13C NMR spectra of PI−PVI; UV spectrum of PVI in DMF solution (PDF)

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21788102, 21525417, and 21490571); The National Program for Support of Top-Notch Young Professionals; the Natural Science Foundation of Guangdong Province (2016A030312002); the Fundamental Research Funds for the Central Universities (2015ZY013); and the Innovation and Technology Commission of Hong Kong (ITC-CNERC14S01).

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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.8b01179. Experimental section; detailed synthetic routes to M6, PVI and postfunctionalized PII; proposed mechanism; the GPC traces of PI−PVI; TGA curves of PI−PVI; F


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Single Component Polymerization of Diisocyanoacetates toward

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