“On Water” Effect for Green Click Reaction: Spontaneous Polymerization of Activated Alkyne with “Inert” Aromatic Amine in Aqueous Media

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES1 Nov 2022“On Water” Effect for Green Click Reaction: Spontaneous Polymerization of Activated Alkyne with “Inert” Aromatic Amine in Aqueous Media Xinyue Liu†, Chen Zhang†, Lianrui Hu, Jinghan Wang, Kai Li, Hai-Tao Feng, Jacky W. Y. Lam, Benzhao He and Ben Zhong Tang Xinyue Liu† Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 HKUST-Shenzhen Research Institute, Shenzhen 518057 †X. Liu and C. Zhang contributed equally to this work.Google Scholar More articles by this author , Chen Zhang† Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 HKUST-Shenzhen Research Institute, Shenzhen 518057 Department of Biomedical Engineering, SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055 †X. Liu and C. Zhang contributed equally to this work.Google Scholar More articles by this author , Lianrui Hu Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 HKUST-Shenzhen Research Institute, Shenzhen 518057 Google Scholar More articles by this author , Jinghan Wang Department of Biomedical Engineering, SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055 Google Scholar More articles by this author , Kai Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Biomedical Engineering, SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055 Google Scholar More articles by this author , Hai-Tao Feng AIE Research Center, Shaanxi Key Laboratory of Phytochemistry, College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013 Google Scholar More articles by this author , Jacky W. Y. Lam *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 HKUST-Shenzhen Research Institute, Shenzhen 518057 Google Scholar More articles by this author , Benzhao He *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Center for Advanced Materials Research, Beijing Normal University at Zhuhai, Zhuhai 519085 Google Scholar More articles by this author and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 HKUST-Shenzhen Research Institute, Shenzhen 518057 School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen 518172 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202392 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic reactions in water have attracted great attention due to their advantages such as unique reaction performance and environmental friendliness. Organic reactions as well as polymerizations in aqueous media have been extensively investigated, and so far, there has been a massive amount of reporting about polymerizations in water. However, reports about click polymerization in water have been rare. Herein, click polymerization of activated alkyne and aromatic amine in aqueous media is developed. The “on water” effect facilitates polymerization in aqueous media better than in organic solvents, and its mechanism is deciphered through experimental data and theoretical calculations. Water participates in the reaction and reduces the energy barrier to some extent. Besides, polymerization makes it possible for aromatic amine with low reactivity to be linked. By using this strategy, polymers with high molecular weights can be obtained in high yields (up to 95.4%). They show good thermal stability and high refractivity. They can be photodegraded. The polymers with tetraphenylethylene moieties show aggregation-induced emission and can be used as materials for generating photopatterns and visualizing agents for specific staining of lysosome in living cells. Download figure Download PowerPoint Introduction In nature or living systems, a variety of chemical reactions occur in water media or with the aid of water.1 Thus, nature has utilized water to its utmost. Nonetheless, artificial synthesis adopts another route that often requires no water and no oxygen. One of the reasons is that water has been regarded as a bad solvent in organic chemistry for a long time because of the poor solubility of most chemical compounds in water. In contrast, the use of traditional organic solvents has made it possible to synthesize various compounds via different types of chemical reactions but at the same time has brought diverse problems. For example, organic solvents are harmful to human health.2 The corrosion by organic solvents of containers or devices can greatly increase the cost of their use and lead to some security problems. Additionally, the posttreatment of organic solvents contributes to huge environmental problems like the release of greenhouse gases and severe pollution of ecosystems. Such problems and risks pose great challenges. However, the essence of nature reminds researchers of another possibility that mimics nature. Learning from nature and coming back to nature is what we can explore. Thus, developing reactions in water is urgently required with huge challenges but also enormous opportunities. Some researchers, attracted by the advantages of using water, have been dedicated to exploring water-related reactions.3–13 In 1980, Rideout and Breslow14 developed the hydrophobic acceleration of the Diels–Alder reaction. Since then, research on reactions in water has gained considerable attention. Topics include the exploration of different reactions in water and explanation of the mechanism by which water benefits reactions. In 2005, Sharpless et al.15 coined the “on water” effect, and the mechanistic investigation has progressed to a systematic stage. Although many explorations have been made in this field, research into reactions in water proceeds at a slow pace and is still in its infancy. The difficulty lies in many aspects varying from the limited reaction types to the complex mechanisms behind the reactions. Additionally, most reactions in water are heterogeneous reactions. In nature, heterogeneous reactions are widely found and utilized to produce important substances. For example, amino acids are water-soluble while some proteins are insoluble in water. We can see that nature has already developed most of the heterogeneous reactions. The subtle and precise regulation of these reactions has been a huge mystery and continues to intrigue researchers with its immense potentials. The development of reactions that “love” water can be traced back to the exploration of properties of water. There are several typical properties of water, including high polarity and proton-donating ability. When the reactants are highly polarized, water can facilitate the charge separation and stabilization of these species. Thus, for any reaction that involves intermediates, transition species, or even products stabilized with the aid of water, it is likely to be developed into a reaction in water. We are intrigued by developing water-based reactions, especially polymerizations in water. In fact, polymerizations in water have attracted great interest from researchers. Many trials have been successful, including radical polymerization, oxidative polymerization, and supramolecular polymerization in water.16–23 In 1997, Tang et al.24 studied the polymerization of phenylacetylene by organorhodium complexes and found that water is superior to other traditional organic solvents for polymerization in terms of reaction rate, polymer yield, and molecular weight. However, click polymerization in water has rarely been reported. The click reaction has been a hot research topic in recent years and has found wide applications in synthetic chemistry and the biological field.25–29 In particular, the alkyne-azide click reaction is widely used in biology and shows great biorthogonality30,31 but requires the prefunctionalization of the biomolecules in the living body. Additionally, the click reaction of alkyne and azide often proceeds under the catalysis of copper. Although a catalyst-free reaction can be achieved if strain-promoted cyclooctyne is used, such an alkyne molecule is costly. On the other hand, amine is widely found in living systems and can serve as an ideal agent for bioconjugation without the need of any prefunctionalization. Consequently, the exploration of click reactions of alkynes and amines has gained considerable attention. Previously, we have extended the click reaction to the polymerizations of activated alkynes and amines.32–34 An electron-withdrawing ester group was attached to the alkyne to activate the whole molecule, and in this case, the click polymerizations of activated alkynes and aliphatic amines proceeded without any catalysts.35 However, the polymerization was still performed in organic solvents, and the scope of amine was limited to aliphatic amines. Aromatic amines cannot be used due to their weak nucleophilicity, and they make the click reaction using aromatic amines a great challenge. Thus, the click reaction of alkyne and aromatic amine as a synthetic tool is urgently needed. Because of the low nucleophilicity of aromatic amines, we tried to further activate alkyne based on previous attempts with two ester groups attached on both ends of the triple bond. The monomer with many electron-deficient groups is largely polarized, and the polymerization using this type of monomer may show good performance in water. Further, the reaction of dimethyl acetylenedicarboxylate (DMAD) with aniline was reported by Ziyaei-Halimehjani and Saidi in 2008.36 Inspired by the success of Saidi’s work, we would like to develop a click polymerization based on activated alkyne that can react with inert aromatic amine and test if this polymerization can proceed in water and be accelerated by water. It turns out that green click polymerizations of activated alkynes and aromatic amines have been achieved in aqueous media by us (Scheme 1). Empirical experiments and theoretical calculations have been conducted to gain insights into the “on water” effect of this polymerization. The properties and applications of the obtained polymers have been investigated thoroughly as well. Scheme 1 | Synthesis of polyenamines via click polymerizations in aqueous media. Download figure Download PowerPoint Experimental Section Materials and instruments Activated alkyne 1 was synthesized according to previous reported procedure.37 Aromatic amines 2a–2f were purchased from Dieckmann (Hong Kong, China), Meryer (Shanghai, China), J&K Scientific (Guangdong, China), Aldrich (Hong Kong, China) and used after further purification through precipitation and column chromatography. 2g was synthesized according to previously reported procedure.38 Relative weight-average (Mw) molecular weights and polydispersity indices (Ð = Mw/Mn) of the polymers were estimated on a Waters gel permeation chromatography (GPC) system equipped with a Waters 486 wavelength-tunable UV–vis detector. Tetrahydrofuran (THF) was used as eluent at a flow rate of 1.0 mL/min. A set of monodispersed linear polystyrenes covering the Mw range of 103–107 g/mol were utilized as standards for molecular weight calibration. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vertex 70 FT-IR spectrometer, Bruker (Hong Kong, China) using a KBr disk. 1H and 13C NMR spectra were obtained on a Bruker AR 400 NMR spectrometer, Bruker (Hong Kong, China) using CDCl3 as solvent. The thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements were conducted on a Perkin Elmer TGA 7 analyzer, Perkin Elmer (Hong Kong, China) and a TA Instruments DSC Q1000, TA Instruments (Hong Kong, China), respectively, at a heating rate of 10 °C/min under nitrogen. UV–vis spectra were measured on SHIMADZU UV-2600i UV–vis spectrophotometer, Shimadzu (Shanghai, China). Photoluminescence spectra were measured on Edinburgh Instruments spectrofluorometer FS5, Edingburgh Instruments (Hong Kong, China). Thin films for refractive index measurements and photopatterning were fabricated by spin-coating the 1,2-dichloroethane solutions of the polymers (∼20 mg mL−1) on silicon wafers at 600 rpm for 6 s and 1000 rpm for 1 min and then drying them in a vacuum oven at room temperature. The fluorescent photopatterns were generated by irradiating the polymer thin films through a photomask in air at room temperature. The photomask with a grid pattern was coated with copper in the square areas whereas the grid lines were transparent glass substrate. The photoirradiation process was conducted using UV light from an Oriel Mercury Arc Lamp at a distance of 8 cm, and the applied power of the Mercury Arc Lamp (200–400 nm) was 180 W. The fluorescent photopatterns were taken on a fluorescence optical microscope (Nikon Eclipse 80i, Nikcon, Hong Kong, China) under a UV light source. Refractive index (RI) values were determined on a Woollam ellipsometer, J.A. Woollam (Shanghai, China) with a model of Alpha-SE with a wavelength tunability from 380 to 900 nm. Polymer synthesis Without additional notes, all the polymerization reactions were conducted at room temperature in air. The synthetic procedure of P 1/2a (Table 3, entry 1) is given below as an example. To a 10 mL Schlenk tube were added activated alkyne 1 (0.1 mmol), diamine 2a (0.12 mmol), and 1 mL of THF/H2O mixture (1/9, v/v). The reaction mixture was stirred at room temperature in air for 4 h. The crude product was obtained by extraction with dichloromethane or ethyl acetate. The organic layer was added dropwise in 100 mL hexane, and the precipitate was collected and dried under vacuum to a constant weight. The structural characterization data for all polymers can be found in the Supporting Information. Computational methods All structures were optimized at the B3LYP39,40 level of theory in the gas phase with the Def2SVP41 basis set for all atoms. Empirical dispersion correction has been considered by using Grimme’s density functional theory (DFT) empirical dispersion correction with the Becke–Jonson (D3BJ) damping function.42,43 Optimized minima and transition states (TSs) were verified at the same level of theory by harmonic vibrational analysis to have no and one proper imaginary frequency, respectively. Single-point calculations with a larger basis set Def2TZVP were then based on these optimized structures to refine the calculated energy. The electronic potential (ESP) population mapped electron density surface was drawing with an isovalue of 0.004 at the same level. The solvent effect was modeled in these single-point calculations by employing the solvation model based on density SMD continuum solvation model,44 taking hexane or water as the solvent for each reaction. The reported free energies in this work were based on the electronic energy of single-point calculations, including the Gibbs free energy thermal correction obtained from vibrational analysis at the corresponding experimental reaction temperature (25 °C), as well as the DFT-D3(BJ) empirical dispersion correction.42,43 All DFT geometry optimizations and single-point calculations were performed with the Gaussian 16 program.45 Further details about Cartesian coordinates for the optimized structures can be found in the Supporting Information. Biological experiments HeLa and 293T cells were purchased from American Type Culture Collection (ATCC, Hong Kong, China). Phosphate buffered saline (PBS, 1×), Dulbecco’s modified eagle medium (DMEM), penicillin-streptomycin solution, trypsin-ethylenediaminetetraacetic acid disodium salt (EDTA) (0.5% trypsin and 5.3 mM EDTA tetrasodium), and fetal bovine serum (FBS) were bought from Gibco (Billings, Montana, USA). A Cell Counting Kit-8 (CCK-8) was obtained from Beyotime Biotechnology (Jiangsu, China). Lysotracker Red (LTR) was obtained from Invitrogen (Waltham, Massachusetts, USA). The confocal imaging was conducted on a Carl Zeiss LSM 980 machine (Berlin, Germany). The cytotoxicity assessment was conducted using CCK-8 assay. HeLa and 293T cells were seeded in a medium containing 90% DMEM and 10% FBS and then cultured at 37 °C in a 5% CO2 incubator. The cells were seeded in 96-well plates for 24 h. Then HeLa and 293T cells were treated with 2, 5, 10, 20, and 50 μM of P 1/2g for 18 h in the dark, respectively. After another 24 h, the cells were incubated with CCK-8 assay for 4 h. After shaking for 20 s, the OD450 values were measured with a Tecan Infinite M200 monochromator-based multifunction microplate reader for calculating the relative cell viabilities. For bioimaging, after incubation with 10 μM of P 1/2g at 37 °C for 1 h, HeLa and 293T cells were stained with 50 nM LTR for an additional 0.5 h, respectively. The cells were rinsed with PBS three times and fixed with 4% paraformaldehyde (PFA). After another three rinsings with PBS, the cells were observed by confocal microscopy. The excitation wavelength of P 1/2g was 405 nm while LTR were excited at 543 nm. The emission signals were collected at 480 ± 30 nm for P 1/2g and 620 ± 20 nm for LTR. Results and Discussion Effect of “water” on the polymerization All the polymerizations were conducted without any catalyst at room temperature. Monomers 1 and 2g were prepared according to the previously published procedure.37,38 Aromatic amines were used as monomers and are commercially available. Our goal is to explore the function of water. We used 1 and 2a as monomers to systematically investigate the “on water” effect. First, we tried to add water to see whether the polymerization could be improved. It turns out that by increasing the volume fraction of water, the obtained polymers show higher molecular weights (Table 1). The change of the yield with a higher water fraction is not obvious since the system is heterogeneous. With the increase of the water fraction, polymers with higher molecular weight can be obtained even though the molecular weight of the polymers obtained in pure water is slightly lower than that of polymers in an aqueous mixture (AM) with 90% water. That is partly due to the poor solubility of the monomers in water. Thus, they cannot have good contact with each other to convert into polymers and result in lower molecular weight. Nevertheless, the polymerization result is satisfactory. Table 1 | Effect of Water on the Click Polymerizationa Entry fw (vol %) Yield (%) Mwb Ðb 1 0 52.5 3800 1.27 2 10 84.0 4400 1.24 3 20 79.1 5100 1.15 4 30 91.9 6700 1.45 5 40 65.6 7100 1.50 6 50 72.6 7700 1.58 7 60 79.8 7900 1.57 8 70 82.2 8100 1.59 9 80 91.7 9400 1.73 10 90 95.4 14,800 2.21 11 100 70.2 13,800 2.06 aCarried out in air at room temperature for 4 h in THF/water mixture with different volume fractions of water (fw) in a catalyst-free manner. [1] = 0.1 M. [1]:[2] = 1:1.2. bDetermined by GPC in THF on the basis of a linear polystyrene calibration. Abbreviation: Ð = polydispersity index = Mw/Mn. Water has two dominant properties. One is its high polarity, and the other is its outstanding proton-donating ability. We thus tried other solvents to test whether these properties function in promoting polymerization. We chose both aprotic and protic solvents for the investigation. As shown in Table 2, all the polymers were obtained in good yields ranging from 78.4%–95.7% in different solvents, but their molecular weights were dramatically different. For example, the polymerization in MeOH can yield polymers with molecular weight of 12,100 and yield of 89.4%. Although polymerization in 2-PrOH showed the great yield of 95.7%, the polymers obtained had low molecular weights. Also, the polymerization in aprotic solvents gave out polymers with lower molecular weight. Table 2 | Solvent Effect on the Click Polymerizationa Entry Solvent Dielectric Constant Yield (%) Mwb Ðb Protic Solvent 1 MeOH 32.6 89.4 12,100 1.86 2 EtOH 24.3 86.8 7600 1.56 3 2-PrOH 18.0 95.7 4900 1.30 4 Butanol 17.3 78.4 5000 1.47 Aprotic Solvent 5 ACN 37.5 85.9 3300 1.35 6 Acetone 20.5 82.3 3700 1.44 aCarried out in air at room temperature for 4 h in different solvents in a catalyst-free manner. [ 1] = 0.1 M. [ 1]:[ 2] = 1:1.2. bDetermined by GPC in THF on the basis of a linear polystyrene calibration. Abbreviation: MeOH = methanol, EtOH = ethanol, 2-PrOH = 2-propanol, ACN = acetonitrile, Ð = polydispersity index = Mw/Mn. Table 3 | Polymerization of 1 with Different Comonomer 2 in an AM and Toluene (Tol)a Entry Polymer Yield (%) Mwb Ðb AM Tol AM Tol AM Tol 1 P 1/2a 95.4 90.2 14,800 4200 2.21 1.30 2 P 1/2b 92.6 83.7 19,500 8000 3.09 2.88 3 P 1/2c 89.0 27.5 10,400 500 3.09 1.06 4 P 1/2d 97.8 66.4 10,300 22300 2.13 2.76 5 P 1/2e 98.3 54.9 10,500 9300 2.50 2.76 6 P 1/2f 94.8 32.5 17,000 600 2.60 1.07 7 P 1/2g 72.9 1.6 3000 1700 1.13 1.77 aCarried out in air at room temperature for 4 h in an AM of THF/water mixture with 90% water fraction in a catalyst-free manner. [ 1] = 0.1 M. [ 1]:[ 2] = 1:1.2. bDetermined by GPC in THF on the basis of a linear polystyrene calibration. Abbreviation: Ð = polydispersity index = Mw/Mn. Thus, it seems that solvents with high polarity and excellent proton-donating ability can help optimize polymerization. Comparing aprotic and protic solvents with similar polarity, the polymerization in protic solvents shows much better performance. In water, both properties are unparalleled, especially when compared with other solvents. Thus, their superior performance in water is reasonable. In addition, we have systematically studied the effect of pH on polymerization. Mixtures of PBS buffer at different pHs and THF with 90% PBS buffer were prepared as solvents. The polymerizations were conducted under the same conditions except for the pH of solvents. As shown in the Supporting Information Table S1, we found that pH has little effect on the yields. The difference in yield is small. However, taking molecular weight into account, the neutral condition is not beneficial to the polymerization. Instead, acidic or basic conditions can help promote the performance of the polymerization. That may be attributed to the mechanism of nucleophilic addition under acidic or alkaline conditions. However, in both acidic and basic environments, the polymers show high polydispersity, and their molecular weight demonstrates a broad distribution. Later, we systematically investigated the effects of reaction time, monomer ratio, and monomer concentration on polymerization, and the results are shown in the Supporting Information Tables S2–S4. Under optimized conditions, polymers derived from different monomers were obtained, and the polymers synthesized in pure toluene were used for comparison. As shown in Table 3, polymers were produced in higher yields in AM than in toluene. For example, P 1/2c was obtained in 27.5% yield in toluene, and that value is much lower than in the AM. Especially, only a trace amount of P 1/2g was obtained in toluene, confirming that the monomers undergo sluggish polymerization in toluene. Structural characterization To confirm the structures of the obtained polyenamines, we synthesized a model compound 5 using DMAD 3 and aniline 4 as reactants for comparison according to Supporting Information Scheme S1. Later, NMR and FT-IR analyses of the monomers, model compound, and the polymers were carried out. The 1H NMR spectra of monomer 1, 2a, model compound and corresponding polymer are shown in Figure 1a–d. The 1H NMR spectrum of 5 exhibits a new peak associated with the resonance of the vinyl proton “f” at δ 5.39 (Figure 1c). It also shows a new peak at δ 9.67 which corresponds to the resonance of the secondary amino proton. Correspondingly, polymer P 1/2a also shows these two peaks at δ 5.39 and δ 9.67 (Figure 1d), verifying that the alkyne reacts with the aromatic amine to form a vinyl unit and secondary amino moiety. The 13C NMR spectra shown in Figure 2a–d further confirm that 1 and 2a are consumed to form polymers. The peak at δ 74.6 which corresponds to the triple bond carbon resonance disappears, and new resonance peaks for two vinyl carbon atoms emerge at δ 93.70 and δ 148.14, respectively. Besides, the FT-IR spectra further verify the formation of polyenamines. For example, the N–H stretching vibration of 2a occurs at 3416 cm−1, which is not found in the FT-IR spectra of 5 and P 1/2a ( Supporting Information Figure S1). Correspondingly, a new peak for C=C stretching vibration appears at 1668 cm−1, indicative of the success of the polymerization. Besides, to further investigate the selectivity of the reaction, a compound 10 was synthesized according to the synthetic route shown in Supporting Information Scheme S2. In the 13C NMR spectrum of 10 shown in Supporting Information Figure S2, there are four peaks at δ 94.03, 93.96, 93.67, and 93.55 associated with the double-bond carbon resonance, which means that the reaction is not regioselective. The amino group can attack both ends of the triple bond due to the similar electro-affinity of the carbon atoms of the internal ethynyl groups. As for stereoselectivity, in a previous report by Saidi, they are stereoselective with all cis structures.36 Figure 1 | 1H NMR spectra of monomer (a) 1, (b) 2a, (c) model compound 5, and (d) corresponding polymer P1/2a in CDCl3. Download figure Download PowerPoint The FT-IR spectra and 1H and 13C NMR spectra of other polymers are given in Supporting Information Figures S3–S17, respectively, and all suggest that polymers are formed with molecular structures as depicted in Scheme 1. Figure 2 | 13C NMR spectra of monomer (a) 1, (b) 2a, (c) model compound 5, and (d) the corresponding polymer P1/2a in CDCl3. Download figure Download PowerPoint “On water” mechanism exploration Obviously, water plays an important role in this polymerization. Thus, what is the mechanism behind the phenomenon? In fact, there have been some investigations on the mechanism,46–49 and they have concluded that the acceleration of the reaction by water is due to the hydrophobic effect, cohesive energy, hydrogen bonding, and other factors. Some researchers have proposed other mechanisms such as the Marcos-Jung model and the Kobayashi model.48,50 Based on the experimental data found in this polymerization, we have conducted the theoretical calculation on the reaction pathway. As shown in Figure 3a,b, DMAD 3 and aniline go through nucleophilic attack and form intermediate 6 which is charge-separated.