Synthesis of Functional Poly(propargyl imine)s by Multicomponent

Nov 22, 2017 - Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices ...
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Synthesis of Functional Poly(propargyl imine)s by Multicomponent Polymerizations of Bromoarenes, Isonitriles, and Alkynes Hanchu Huang,†,‡,# Zijie Qiu,†,‡,# Ting Han,†,‡ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,*,†,‡,§ and Ben Zhong Tang*,†,‡,§ †

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡ Guangdong Provincial Key Laboratory of Brain Science, Diseases and Drug Development, HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan, Shenzhen 518057, China § Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Here we reported a versatile and multicomponent polymerization (MCP) approach that enabled the synthesis of functional poly(propargyl imine)s with welldefined structures and high molecular weight (Mw up to 38 200) in excellent yields (up to 93%) from readily accessible monomers of dibromoarenes, isonitriles, and diynes. This MCP had the advantages of simple operation, wide substrate scope, and mild reaction conditions. The resulting polymers possessed good solubility and showed high thermal stability and refractive indices. The tetraphenylethene-containing polymer displayed a phenomenon of aggregation-induced emission and could respond to various acidic vapors.

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generally give rise to polymers with diverse structures and functional properties. For example, while MCPs of alkynes, sulfonyl azides, and amines or alcohols generated poly(Nsulfonylamidines) or poly(N-sulfonylimidates),12 those of alkynes, aldehydes, and amines produced poly(propargylamine).13 On the other hand, multicomponent tandem polymerizations of alkynes, acyl chlorides, and mercaptoacetates/hydrazines generated conjugated polymers such as polythiophenes and polypyrazoles.14 Imine-containing polymers have attracted great attention because of their potential applications as information storage, chemosensor, and catalyst. However, their syntheses have been limited to one-component polymerization methods.15 Therefore, MCPs will be promising synthetic techniques for producing various imine-containing polymer structures. In 2013, Ji reported an efficient Pd-catalyzed three-component reaction of aryl bromides, isonitriles, and alkynes to generate a library of alkynones in moderate to excellent yields (Scheme 1). This reaction is believed to occur via Pd-catalyzed oxidative addition of aryl bromide followed by a successive isonitrile insertion

he development of new catalysts and polymerization routes continues to attract great attention in polymer chemistry. A new efficient and selective polymerization method enables scientists to generate well-defined, functional materials to address many scientific problems not only in chemistry but also in other fields.1 In the last decades, one- or two-component polymerization routes were widely used in polymer synthesis. However, most of them are not suitable for constructing polymers with complicated structures and multifunctionalities due to the narrow diversity of the resulting backbone structure.2 Recently, multicomponent reactions (MCRs) have been introduced to the field of polymer synthesis and appear promising for generating polymers with well-defined structures.3 A series of popular MCRs, such as Passerini reaction,4 Mannich reaction,5 Ugi reaction,6 Hantzsch reaction,7 and Biginelli reaction,8 have been exploited to access various novel materials with unique properties due to their high efficiency to generate complex molecules.9 However, the biggest challenge is how to find a suitable multicomponent reaction due to the difficulties such as the tedious synthesis of monomers and poor solubility of the resulting polymers.10 Alkyne-based multicomponent polymerizations (MCPs) have recently attracted much attention because of the rich chemistry of alkynes.11 Polymerizations of alkyne monomers © 2017 American Chemical Society

Received: November 6, 2017 Accepted: November 20, 2017 Published: November 22, 2017 1352

DOI: 10.1021/acsmacrolett.7b00872 ACS Macro Lett. 2017, 6, 1352−1356

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ACS Macro Letters Table 1. Optimization of the Model MCPa

Scheme 1. Three-Component Reactions to Afford Alkynyl Imines

entry [1a]/[2a]/[3a] 1 2 3 4 5 6 7 8

process and coupled with alkyne.16 It can be conducted with ease under mild reaction conditions and wide monomer scope and shows higher selectivity than traditional Sonogashira coupling.17 Despite these advantages, it has never been developed into useful tools for the synthesis of functional polymers. Encouraged by the recent development of MCPs, here we reported a versatile synthesis of a library of poly(propargyl imine)s via Pd-catalyzed MCPs of various dibromides, isocyanides, and diynes. The resulting polymers were generated in excellent yields with well-defined structures and high molecular weights. They possessed good solubility in common organic solvents, high thermal stability, good film-forming ability, and high refractive indices. The tetraphenylethene (TPE)-containing polymer displayed a phenomenon of aggregation-induced emission (AIE) and showed strong light emission in the solid state.18 This made them promising materials for generating fluorescent photopatterns by UV photolithography and could sensitively respond to various acidic vapors. Utilizing the reported conditions by Ji et al. for the synthesis of low molecular weight alkynyl imines and alkynones, a model polymerization of 4,4′-dibromobiphenyl 1a, t-butyl isocyanide 2a, and 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene 3a was carried out in dimethyl sulfoxide (DMSO) in the presence of palladium(II) acetate [Pd(OAc)2], bis[(2-diphenylphosphino)phenyl]ether (DPEPhos), and cesium carbonate (Cs2CO3) at 100 °C, which afforded a polymer with Mw of 26 300 in 70% yield after 18 h (Table 1, entry 1). Excess 2a (2.4 equiv) could be used to enhance the efficiency of the polymerization (Table 1, entry 2). On the other hand, as suggested by the result in Table 1, entry 3, the stoichiometric balance between 1a and 3a was very important because both were involved in the polymer chain propagation. Such a result also indicates that there is no Hay−Glaser side reaction, which otherwise will break the stoichiometric balance and lead to dramatic decrease of molecular weight. Under these reaction conditions, the catalytic activity of other palladium catalysts was screened. As shown in Table S1, Pd(PPh3)4, Pd(PPh3)2Cl2, PdCl2, and PdBr2 gave poor results, and Pd(OAc)2 still exhibited the highest activity. The solvent effect on the polymerization was then investigated (Table 1, entries 2 and 4−8). While polymerizations in toluene, dimethylformamide, and 1,4-dioxane generated polymers with low molecular weights, that conducted in tetrahydrofuran (THF) produced a high molecular weight (Mw = 38 200) polymer in a high yield of 85%. On the other hand, there is a shoulder peak around 26 min in the DMSO/THF cosolvent system, thus THF is the optimized solvent for this MCP. By

1.0/2.0/1.0 1.0/2.4/1.0 1.2/2.4/1.0 1.0/2.4/1.0 1.0/2.4/1.0 1.0/2.4/1.0 1.0/2.4/1.0 1.0/2.4/1.0

solvent DMSO DMSO DMSO toluene DMF dioxane THF DMSO/ THFd

yield (%)

Mwb

70 71 56 86 72 69 85 81

26300 33000 9000 15700 24700 23100 38200 26800

Mw/Mnb DPnc 3.2 3.0 5.3 3.1 2.3 4.0 2.2 2.5

12 16 2 7 15 8 25 15

a Polymerization at 100 °C under nitrogen for 18 h in the presence of Pd(OAc)2. [3a] = 0.1 M, [Pd] = 5 mol % [3a], [DPEPhos] = 10 mol % [3a], Cs2CO3 = 2.1 equiv. bDetermined by GPC in THF on the basis of a polystyrene calibration. cDPn = Mn/M0. M0 is the molecular weight of the repeating unit. dVolume ratio of DMSO:THF = 1:1.

simple precipitation of the reaction mixture into methanol to remove the catalyst and unreacted monomers and oligomers, the polymers were isolated as yellow powders. The GPC traces of every entry were shown in Figure S1. To assist the structural characterization of the obtained polymers, a model compound 4 was synthesized according to the synthetic route shown in Scheme S1 and characterized by IR, NMR, and mass spectroscopies (Figure S2). The IR and NMR spectra of monomers 1a, 2a, and 3a, the model compound 4, and the corresponding polymer P1a/2a/3a were compared and analyzed. While the CN stretching vibration of 2a was observed at 1660 cm−1 (Figure S3B), the  C−H and CC stretching vibrations of 3a were observed at 3279 and 2108 cm−1 (Figure S3C). All these peaks disappeared in both the spectra of 4 and P1a/2a/3a (Figures S3D and S3E). Instead, two new peaks associated with CN and CC stretching vibrations emerged at 1584 and 2199 cm−1, respectively. Meanwhile, the IR spectrum of P1a/2a/3a largely resembled that of model compound 4. All these results suggested the occurrence of the polymerization. On the other hand, the 1H NMR spectrum of P1a/2a/3a displayed no ethynyl proton (e) of 3a at δ 3.1 (Figure 1). This further confirmed that the terminal triple bond had been completely consumed by the polymerization. The methyl protons (c) of 2a resonated at δ 1.43, which shifted to δ 1.54 after the reaction. Meanwhile, the phenyl peaks (a) of 1a experienced a large shift from δ 7.54 to δ 8.12 due to the stronger electron-withdrawing effect of the imine group than the bromide atom. The integral ratio of protons at δ 8.12, 7.36, and 1.54 in Figure 1E was 2.0:2.1:9.4, which was consistent with the theoretical value calculated from the monomers (2:2:9) and indicated an efficient and thorough polymerization method. The results from 13C NMR analysis further confirmed the proposed structure of P1a/2a/3a. As shown in Figure S4, the 13C NMR spectrum of P1a/2a/3a was similar to 4, showing characteristic CN, CC, and t-Bu resonance peaks at δ 146.6, 99.1, 84.5, and 57.2. It is noteworthy that signals related to Sonogashira and Hay−Glaser coupling product were not observed in structural characterizations, suggesting higher 1353

DOI: 10.1021/acsmacrolett.7b00872 ACS Macro Lett. 2017, 6, 1352−1356

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ACS Macro Letters

Table 2. Polymerization Results of Different Monomersa

Figure 1. 1H NMR spectra of (A) 1a, (B) 2a, (C) 3a, (D) 4, and (E) P1a/2a/3a in CDCl3. The solvent peaks were marked with asterisks.

selectivity of the present MCP than the traditional Sonogashira and Hay−Glaser coupling. This unprecedented polymerization prompts us to track its mechanism. After addition of monomers, catalyst, and solvent, the mixture was stirred at 100 °C for 24 h. Samples were taken out at different times during the polymerization for 1H NMR and GPC analyses. As shown in Figure 2A and Figure S5, all the

entry

monomers

yield (%)

Mwb

Mw/Mnb

DPnc

1 2 3 4 5 6 7 8 9 10

1a/2a/3a 1b/2a/3a 1c/2a/3a 1d/2a/3a 1a/2b/3a 1a/2c/3a 1a/2d/3a 1a/2e/3a 1d/2a/3b 1a/2a/3c

85 93 86 82 81 trace trace trace 78 insoluble

38200 13200 30500 14300 18800 / / / 20800 /

2.23 1.59 1.99 1.76 1.85 / / / 1.95 /

25 12 23 10 13 / / / 16 /

Polymerization at 100 °C under nitrogen for 18 h. [1] = 0.1 M, [2] = 0.12 M, [3] = 0.1 M, [Pd] = 5 mol % [3a], [DPEPhos] = 10 mol % [3a], Cs2CO3 = 2.1 equiv. bDetermined by GPC in THF on the basis of a polystyrene calibration. cDPn = Mn/M0. M0 is the molecular weight of the repeating unit. a

smoothly and generated polymers in excellent yields (78−93%) with high molecular weights ranging from 13 200 to 30 500 (Table 2, entries 2−4 and entry 9). Notably, a high molecular weight (Mw = 18 800) polymer was also isolated in a high yield (81%) when 2b was used as monomer (Table 2, entry 5), while no polymer was obtained using aromatic isocyanide 2c−e (Table 2, entries 6−8). What’s more, insoluble product was observed when activated alkyne 3c was used (Table 2, entry 10), probably due to higher reactivity of such electron-deficient alkyne species. All the obtained polymers P1a−d/2a−b/3a−b were soluble in common organic solvents, such as THF, chloroform, DCM, and DMSO. Their structures were fully characterized by IR, 1H NMR, and 13C NMR (Figures S7−S9). Their thermal properties were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. All the polymers enjoyed high thermal stability, losing 5% of their weight at temperature ranging from 230 to 300 °C (Figure S10). DSC analysis showed that only P1c/2a/3a exhibited a glass transition temperature at around 170 °C, while no signals were detected in other polymers even when heated to 300 °C, presumably due to their rigid structures (Figure S11). Films of P1/2/3 with good quality could be readily fabricated by solution spin-coating and exhibited high refractive indices of 1.6637−1.6219 (Figure S12), thanks to the numerous aromatic rings and heteroatoms present in their structures.

Figure 2. (A) Plot of monomer conversion against polymerization time. (B) GPC curves at different reaction time.

monomers were almost consumed within 6 h to form oligomers. Afterward, polymers with increased molecular weights formed with prolonging the polymerization time (Figure 2B and Figure S6), confirming a step-growth polymerization mechanism.19 The polymerization went to almost completion in 18 h, and the molecular weight of the polymer was slightly increased when extending the reaction time to 24 h. With the optimized polymerization conditions, we further explored the monomer scope. As shown in Table 2, dibromoarenes 1b−d with different alkyl, alkyloxyl, or aromatic groups and diynes 3b carrying different aryl rings all proceeded 1354

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ACS Macro Letters

fluorescent materials. As shown in Figure 3C, the P1a/2a/3a powder placed on filter paper was strongly emissive under UV irradiation. However, after fuming with HCl vapor for 30 s, the emission was quenched. This was further proved by the PL analysis shown in Figure 3D. Fuming with other volatile acids such as formic acid, acetic acid, and trifluoroacetic acid can also quench the emission of P1a/2a/3a to some extent, which was consistent with the volatility of these acids. No florescence change was observed for nonvolatile H2SO4 “fuming” (Figure S16). Such an emission “turn-off” phenomenon was closely associated with two possible mechanism: (1) protonation of the imine unit of P1a/2a/3a, which triggered the photoinduced electron transfer process, and (2) hydrolysis of propargyl imine toward propagylic ketone, which was nonemissive. The protonation quenching effect was confirmed by the partial fluorescence recovery after NH3 fuming (Figure S17). Complete hydrolyzed poly(propargyl ketone) P4 was obtained by strong acid post transformation of P1a/2a/3a (Scheme S2) and was fully characterized (Figures S18 and S19). P4 shows almost no emission due to the quenching effect of the ketone group and no fluorescence recovery upon NH3 fuming (Figure S20). These results proved that both protonation and hydrolysis occurred during the acid fuming. The combination of the AIE property and the reactive diversity of imine will render P1/2/3 as potential versatile materials in many different fields. In this work, we developed a new multicomponent polymerization route for the synthesis of poly(propargyl imine)s from dibromoarenes, isonitriles, and diynes. This MCP showed higher selectivity than Sonogashira reaction, affording high molecular weight and well-defined iminecontaining polymers in high yields. All the poly(propargyl imine)s showed good solubility and high thermal stability. The incorporation of AIE luminogen and imine functionality into the polymer backbone rendered the resulting polymers with AIE activity and capability to fabricate well-resolved fluorescent photopattern and acid vapor-responsive fluorescent materials. Further research will be conducted to exploit this multicomponent polymerization to access new classes of functional polymers.

The absorption spectra of dilute THF solutions (10 μM) of P1/2/3 are shown in Figure S13. All the polymers absorbed at similar wavelength and exhibited a maximum at around 320 or 350 nm because they possessed a similar chromophore in their backbone. The photoluminescence (PL) of P1a/2a/3a was further investigated in THF/water mixtures with different water fractions ( f w). As shown in Figure 3A, the PL spectrum of the

Figure 3. (A) PL spectra of P1a/2a/3a in THF/water mixtures with different water fractions ( f w). Solution concentration: 10 μM; excitation wavelength: 370 nm. (B) Plot of relative emission intensity (I/I0) versus the composition of the THF/water mixture of P1a/2a/ 3a, where I0 = peak intensity in pure THF. Inset in panel B: photographs of P1a/2a/3a in pure THF and a THF/water mixture with 80% water taken under 365 nm UV irradiation. (C) Fluorescent photographs of powder of P1a/2a/3a on filter paper taken under UV irradiation before and after fuming with HCl vapor. (D) PL spectra of P1a/2a/3a powder before and after fuming with HCl vapor. Excitation wavelength: 370 nm.

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polymer in THF was basically a flat line parallel to the abscissa, suggesting that the polymer was nonemissive when molecularly dissolved. Addition of water, a poor solvent of P1a/2a/3a, into its THF solution had gradually strengthened its emission. The highest fluorescence intensity was achieved at f w of 80%, which was 29-fold higher than that of in pure THF solution (Figure 3B). Such PL behavior suggested that P1a/2a/3a was AIE active, which was inherited from monomer 3a containing TPE chromophore. The model compound 4 also show typical AIE characteristics, while significant emission enhancement can only be observed after 70% water was added (Figure S14). The graduate fluorescence enhancement of P1a/2a/3a in small water fraction is due to its poorer solubility in water, and the polymer chain entanglement also helps to restrict the molecular motion of TPE. The PL intensity of P1a/2a/3a slightly decreased at f w > 90%, probably due to the decrease in the effective dye concentration by forming large aggregates in the presence of a large amount of poor solvent. Due to the AIE feature and imine functionality, the aggregates or powders of P1a/2a/3a could not only be used for fabricating fluorescent photopattern by UV photolithography (Figure S15) but also function as acid vapor-responsive

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00872. Tables, experimental methods, and additional experimental data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zijie Qiu: 0000-0003-0728-1178 Ting Han: 0000-0003-1521-6333 Ryan T. K. Kwok: 0000-0002-6866-3877 Ben Zhong Tang: 0000-0002-0293-964X Author Contributions #

H.H. and Z.Q. contributed equally to the work.

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DOI: 10.1021/acsmacrolett.7b00872 ACS Macro Lett. 2017, 6, 1352−1356

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ACS Macro Letters Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Basic Research Program of China (973 Program; 2013CB834701), the National Science Foundation of China (21490570 and 21490574), the Research Grants Council of Hong Kong (6308116, 6303815, 6305014, C2014−1567 and AHKUST605/16), the Nissan Chemical Industries, Ltd., the Innovation and Technology Commission (ITCRD/17-9), and the University Grants Committee of Hong Kong (AoE/P-03/ 08). B.Z.T. thanks the support of the Guangdong Innovative Research Team Program (201101C0105067115), the Shenzhen Peacock Plan, and the Science and Technology Plan of Shenzhen (JCY20160229205601482).

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DOI: 10.1021/acsmacrolett.7b00872 ACS Macro Lett. 2017, 6, 1352−1356