Synthesis of Functional Poly(propargyl imine)s by MulticomponentPolymerizations of Bromoarenes, Isonitriles, and AlkynesHanchu 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 andReconstruction, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute ofMolecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and Technology, ClearWater 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 Materialsand Devices, South China University of Technology, Guangzhou 510640, China

*S Supporting Information

ABSTRACT: Here we reported a versatile and multi-component polymerization (MCP) approach that enabled thesynthesis of functional poly(propargyl imine)s with well-defined structures and high molecular weight (Mw up to38 200) in excellent yields (up to 93%) from readily accessiblemonomers of dibromoarenes, isonitriles, and diynes. This MCPhad the advantages of simple operation, wide substrate scope,and mild reaction conditions. The resulting polymers possessedgood solubility and showed high thermal stability and refractiveindices. The tetraphenylethene-containing polymer displayed aphenomenon of aggregation-induced emission and couldrespond to various acidic vapors.

The development of new catalysts and polymerizationroutes continues to attract great attention in polymer

chemistry. A new efficient and selective polymerization methodenables scientists to generate well-defined, functional materialsto address many scientific problems not only in chemistry butalso in other fields.1 In the last decades, one- or two-componentpolymerization routes were widely used in polymer synthesis.However, most of them are not suitable for constructingpolymers with complicated structures and multifunctionalitiesdue to the narrow diversity of the resulting backbone structure.2

Recently, multicomponent reactions (MCRs) have beenintroduced to the field of polymer synthesis and appearpromising for generating polymers with well-defined struc-tures.3 A series of popular MCRs, such as Passerini reaction,4

Mannich reaction,5 Ugi reaction,6 Hantzsch reaction,7 andBiginelli reaction,8 have been exploited to access various novelmaterials with unique properties due to their high efficiency togenerate complex molecules.9 However, the biggest challenge ishow to find a suitable multicomponent reaction due to thedifficulties such as the tedious synthesis of monomers and poorsolubility of the resulting polymers.10

Alkyne-based multicomponent polymerizations (MCPs)have recently attracted much attention because of the richchemistry of alkynes.11 Polymerizations of alkyne monomers

generally give rise to polymers with diverse structures andfunctional properties. For example, while MCPs of alkynes,sulfonyl azides, and amines or alcohols generated poly(N-sulfonylamidines) or poly(N-sulfonylimidates),12 those ofa lkynes , a ldehydes , and amines produced poly-(propargylamine).13 On the other hand, multicomponenttandem polymerizations of alkynes, acyl chlorides, andmercaptoacetates/hydrazines generated conjugated polymerssuch as polythiophenes and polypyrazoles.14 Imine-containingpolymers have attracted great attention because of theirpotential applications as information storage, chemosensor,and catalyst. However, their syntheses have been limited toone-component polymerization methods.15 Therefore, MCPswill be promising synthetic techniques for producing variousimine-containing polymer structures. In 2013, Ji reported anefficient Pd-catalyzed three-component reaction of arylbromides, isonitriles, and alkynes to generate a library ofalkynones in moderate to excellent yields (Scheme 1). Thisreaction is believed to occur via Pd-catalyzed oxidative additionof aryl bromide followed by a successive isonitrile insertion

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

Letter

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© 2017 American Chemical Society 1352 DOI: 10.1021/acsmacrolett.7b00872ACS Macro Lett. 2017, 6, 1352−1356

process and coupled with alkyne.16 It can be conducted withease under mild reaction conditions and wide monomer scopeand shows higher selectivity than traditional Sonogashiracoupling.17 Despite these advantages, it has never beendeveloped into useful tools for the synthesis of functionalpolymers.Encouraged by the recent development of MCPs, here we

reported a versatile synthesis of a library of poly(propargylimine)s via Pd-catalyzed MCPs of various dibromides,isocyanides, and diynes. The resulting polymers were generatedin excellent yields with well-defined structures and highmolecular weights. They possessed good solubility in commonorganic solvents, high thermal stability, good film-formingability, and high refractive indices. The tetraphenylethene(TPE)-containing polymer displayed a phenomenon ofaggregation-induced emission (AIE) and showed strong lightemission in the solid state.18 This made them promisingmaterials for generating fluorescent photopatterns by UVphotolithography and could sensitively respond to variousacidic vapors.Utilizing the reported conditions by Ji et al. for the synthesis

of low molecular weight alkynyl imines and alkynones, a modelpolymerization of 4,4′-dibromobiphenyl 1a, t-butyl isocyanide2a, and 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene 3a wascarried out in dimethyl sulfoxide (DMSO) in the presence ofpalladium(II) acetate [Pd(OAc)2], bis[(2-diphenylphosphino)-phenyl]ether (DPEPhos), and cesium carbonate (Cs2CO3) at100 °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) couldbe used to enhance the efficiency of the polymerization (Table1, entry 2). On the other hand, as suggested by the result inTable 1, entry 3, the stoichiometric balance between 1a and 3awas very important because both were involved in the polymerchain propagation. Such a result also indicates that there is noHay−Glaser side reaction, which otherwise will break thestoichiometric balance and lead to dramatic decrease ofmolecular weight. Under these reaction conditions, the catalyticactivity of other palladium catalysts was screened. As shown inTable S1, Pd(PPh3)4, Pd(PPh3)2Cl2, PdCl2, and PdBr2 gavepoor 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 withlow 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 ashoulder peak around 26 min in the DMSO/THF cosolventsystem, thus THF is the optimized solvent for this MCP. By

simple precipitation of the reaction mixture into methanol toremove the catalyst and unreacted monomers and oligomers,the polymers were isolated as yellow powders. The GPC tracesof every entry were shown in Figure S1.To assist the structural characterization of the obtained

polymers, a model compound 4 was synthesized according tothe synthetic route shown in Scheme S1 and characterized byIR, NMR, and mass spectroscopies (Figure S2). The IR andNMR spectra of monomers 1a, 2a, and 3a, the modelcompound 4, and the corresponding polymer P1a/2a/3awere compared and analyzed. While the CN stretchingvibration of 2a was observed at 1660 cm−1 (Figure S3B), theC−H and CC stretching vibrations of 3a were observed at3279 and 2108 cm−1 (Figure S3C). All these peaks disappearedin both the spectra of 4 and P1a/2a/3a (Figures S3D andS3E). Instead, two new peaks associated with CN and CCstretching vibrations emerged at 1584 and 2199 cm−1,respectively. Meanwhile, the IR spectrum of P1a/2a/3a largelyresembled that of model compound 4. All these resultssuggested 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). Thisfurther confirmed that the terminal triple bond had beencompletely consumed by the polymerization. The methylprotons (c) of 2a resonated at δ 1.43, which shifted to δ 1.54after the reaction. Meanwhile, the phenyl peaks (a) of 1aexperienced a large shift from δ 7.54 to δ 8.12 due to thestronger electron-withdrawing effect of the imine group thanthe 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 consistentwith the theoretical value calculated from the monomers(2:2:9) and indicated an efficient and thorough polymerizationmethod. The results from 13C NMR analysis further confirmedthe proposed structure of P1a/2a/3a. As shown in Figure S4,the 13C NMR spectrum of P1a/2a/3a was similar to 4, showingcharacteristic CN, CC, and t-Bu resonance peaks at δ146.6, 99.1, 84.5, and 57.2. It is noteworthy that signals relatedto Sonogashira and Hay−Glaser coupling product were notobserved in structural characterizations, suggesting higher

Scheme 1. Three-Component Reactions to Afford AlkynylImines

Table 1. Optimization of the Model MCPa

entry [1a]/[2a]/[3a] solventyield(%) Mw

b Mw/Mnb DPn

c

1 1.0/2.0/1.0 DMSO 70 26300 3.2 122 1.0/2.4/1.0 DMSO 71 33000 3.0 163 1.2/2.4/1.0 DMSO 56 9000 5.3 24 1.0/2.4/1.0 toluene 86 15700 3.1 75 1.0/2.4/1.0 DMF 72 24700 2.3 156 1.0/2.4/1.0 dioxane 69 23100 4.0 87 1.0/2.4/1.0 THF 85 38200 2.2 258 1.0/2.4/1.0 DMSO/

THFd81 26800 2.5 15

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

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selectivity of the present MCP than the traditional Sonogashiraand 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 takenout at different times during the polymerization for 1H NMRand GPC analyses. As shown in Figure 2A and Figure S5, all the

monomers were almost consumed within 6 h to formoligomers. Afterward, polymers with increased molecularweights formed with prolonging the polymerization time(Figure 2B and Figure S6), confirming a step-growthpolymerization mechanism.19 The polymerization went toalmost completion in 18 h, and the molecular weight of thepolymer was slightly increased when extending the reactiontime 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 aromaticgroups and diynes 3b carrying different aryl rings all proceeded

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 molecularweight (Mw = 18 800) polymer was also isolated in a high yield(81%) when 2b was used as monomer (Table 2, entry 5), whileno polymer was obtained using aromatic isocyanide 2c−e(Table 2, entries 6−8). What’s more, insoluble product wasobserved when activated alkyne 3c was used (Table 2, entry10), probably due to higher reactivity of such electron-deficientalkyne species. All the obtained polymers P1a−d/2a−b/3a−bwere soluble in common organic solvents, such as THF,chloroform, DCM, and DMSO. Their structures were fullycharacterized by IR, 1H NMR, and 13C NMR (Figures S7−S9).Their thermal properties were evaluated by thermogravimetricanalysis (TGA) and differential scanning calorimetry (DSC)analysis. All the polymers enjoyed high thermal stability, losing5% of their weight at temperature ranging from 230 to 300 °C(Figure S10). DSC analysis showed that only P1c/2a/3aexhibited a glass transition temperature at around 170 °C, whileno signals were detected in other polymers even when heatedto 300 °C, presumably due to their rigid structures (FigureS11). Films of P1/2/3 with good quality could be readilyfabricated by solution spin-coating and exhibited high refractiveindices of 1.6637−1.6219 (Figure S12), thanks to thenumerous aromatic rings and heteroatoms present in theirstructures.

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.

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

Table 2. Polymerization Results of Different Monomersa

entry monomers yield (%) Mwb Mw/Mn

b DPnc

1 1a/2a/3a 85 38200 2.23 252 1b/2a/3a 93 13200 1.59 123 1c/2a/3a 86 30500 1.99 234 1d/2a/3a 82 14300 1.76 105 1a/2b/3a 81 18800 1.85 136 1a/2c/3a trace / / /7 1a/2d/3a trace / / /8 1a/2e/3a trace / / /9 1d/2a/3b 78 20800 1.95 1610 1a/2a/3c insoluble / / /

aPolymerization 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 basisof a polystyrene calibration. cDPn = Mn/M0. M0 is the molecularweight of the repeating unit.

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The absorption spectra of dilute THF solutions (10 μM) ofP1/2/3 are shown in Figure S13. All the polymers absorbed atsimilar wavelength and exhibited a maximum at around 320 or350 nm because they possessed a similar chromophore in theirbackbone. The photoluminescence (PL) of P1a/2a/3a wasfurther investigated in THF/water mixtures with different waterfractions ( fw). As shown in Figure 3A, the PL spectrum of the

polymer in THF was basically a flat line parallel to the abscissa,suggesting that the polymer was nonemissive when molecularlydissolved. Addition of water, a poor solvent of P1a/2a/3a, intoits THF solution had gradually strengthened its emission. Thehighest fluorescence intensity was achieved at fw of 80%, whichwas 29-fold higher than that of in pure THF solution (Figure3B). Such PL behavior suggested that P1a/2a/3a was AIEactive, which was inherited from monomer 3a containing TPEchromophore. The model compound 4 also show typical AIEcharacteristics, while significant emission enhancement can onlybe observed after 70% water was added (Figure S14). Thegraduate fluorescence enhancement of P1a/2a/3a in smallwater fraction is due to its poorer solubility in water, and thepolymer chain entanglement also helps to restrict the molecularmotion of TPE. The PL intensity of P1a/2a/3a slightlydecreased at fw > 90%, probably due to the decrease in theeffective dye concentration by forming large aggregates in thepresence 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 usedfor fabricating fluorescent photopattern by UV photolithog-raphy (Figure S15) but also function as acid vapor-responsive

fluorescent materials. As shown in Figure 3C, the P1a/2a/3apowder placed on filter paper was strongly emissive under UVirradiation. However, after fuming with HCl vapor for 30 s, theemission was quenched. This was further proved by the PLanalysis shown in Figure 3D. Fuming with other volatile acidssuch as formic acid, acetic acid, and trifluoroacetic acid can alsoquench the emission of P1a/2a/3a to some extent, which wasconsistent with the volatility of these acids. No florescencechange was observed for nonvolatile H2SO4 “fuming” (FigureS16). Such an emission “turn-off” phenomenon was closelyassociated with two possible mechanism: (1) protonation of theimine unit of P1a/2a/3a, which triggered the photoinducedelectron transfer process, and (2) hydrolysis of propargyl iminetoward propagylic ketone, which was nonemissive. Theprotonation quenching effect was confirmed by the partialfluorescence recovery after NH3 fuming (Figure S17).Complete hydrolyzed poly(propargyl ketone) P4 was obtainedby strong acid post transformation of P1a/2a/3a (Scheme S2)and was fully characterized (Figures S18 and S19). P4 showsalmost no emission due to the quenching effect of the ketonegroup and no fluorescence recovery upon NH3 fuming (FigureS20). These results proved that both protonation andhydrolysis occurred during the acid fuming. The combinationof the AIE property and the reactive diversity of imine willrender P1/2/3 as potential versatile materials in many differentfields.In this work, we developed a new multicomponent

polymerization route for the synthesis of poly(propargylimine)s from dibromoarenes, isonitriles, and diynes. ThisMCP showed higher selectivity than Sonogashira reaction,affording high molecular weight and well-defined imine-containing polymers in high yields. All the poly(propargylimine)s showed good solubility and high thermal stability. Theincorporation of AIE luminogen and imine functionality intothe polymer backbone rendered the resulting polymers withAIE activity and capability to fabricate well-resolved fluorescentphotopattern and acid vapor-responsive fluorescent materials.Further research will be conducted to exploit this multi-component polymerization to access new classes of functionalpolymers.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsmacro-lett.7b00872.

Tables, experimental methods, and additional exper-imental data (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: chjacky@ust.hk.*E-mail: tangbenz@ust.hk.

ORCIDZijie Qiu: 0000-0003-0728-1178Ting Han: 0000-0003-1521-6333Ryan T. K. Kwok: 0000-0002-6866-3877Ben Zhong Tang: 0000-0002-0293-964XAuthor Contributions#H.H. and Z.Q. contributed equally to the work.

Figure 3. (A) PL spectra of P1a/2a/3a in THF/water mixtures withdifferent water fractions ( fw). 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 mixturewith 80% water taken under 365 nm UV irradiation. (C) Fluorescentphotographs of powder of P1a/2a/3a on filter paper taken under UVirradiation before and after fuming with HCl vapor. (D) PL spectra ofP1a/2a/3a powder before and after fuming with HCl vapor. Excitationwavelength: 370 nm.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was partially supported by the National BasicResearch Program of China (973 Program; 2013CB834701),the National Science Foundation of China (21490570 and21490574), the Research Grants Council of Hong Kong(6308116, 6303815, 6305014, C2014−1567 and A-HKUST605/16), the Nissan Chemical Industries, Ltd., theInnovation and Technology Commission (ITCRD/17-9), andthe University Grants Committee of Hong Kong (AoE/P-03/08). B.Z.T. thanks the support of the Guangdong InnovativeResearch Team Program (201101C0105067115), the Shenz-hen Peacock Plan, and the Science and Technology Plan ofShenzhen (JCY20160229205601482).

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