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PolymerChemistry

PAPER

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MOE Key Laboratory of Macromolecular Syn

of Polymer Science and Engineering, Zhejian

P. R. China. E-mail: [email protected]

† Electronic supplementary information (EFTIR spectra and 1H NMR spectra for seTEM images of DMB-Py and DMB-TPE. Se

Cite this: Polym. Chem., 2013, 4, 4450

Received 29th April 2013Accepted 29th May 2013

DOI: 10.1039/c3py00546a

www.rsc.org/polymers

4450 | Polym. Chem., 2013, 4, 4450–

Dendritic molecular brushes: synthesis via sequentialRAFT polymerization and cage effect for fluorophores†

Sipei Li and Chao Gao*

Multifunctional dendritic molecular brushes (DMBs) with pendent poly(glycidyl methacrylate) (PGMA)

chains were synthesized via a sequential reversible addition–fragmentation chain transfer (RAFT)

polymerization approach. By controlling the kinetics of the polymerization, DMBs with a high number-

averaged molecular weight (Mn) of �1.4 � 106 g mol�1 and narrow polydispersity index (PDI) of 1.50

were obtained. DMBs were reacted with sodium azide, giving rise to giant macromolecules with a high

density of hydroxyl and azide bifunctional groups. Pyrene moieties (with a conventional chromophore)

were efficiently attached to the DMBs by esterification, affording solution-emissive blue-light DMBs.

Ultrahigh excimer emission at 475 nm was achieved for these fluorescent DMBs at a very low

concentration of pyrene (�10�7 mol L�1). Alternatively, tetraphenylethylene (TPE, with an aggregation-

induced emission (AIE) chromophore) units were attached to DBMs with a conversion of 100% by

copper-catalyzed alkyne–azide click chemistry. The solid-state-emissive blue-light TPE-functionalized

DMBs exhibited typical AIE (rather than aggregation enhanced emission, AEE) effect which was rarely

observed for dendritic scaffolds and its quantum yield at aggregated state is two times higher than that

of fluorophore monomer. Such topology-induced unusual emission phenomena were attributed to the

cage effect of DMBs.

Introduction

With the developments of well controlled polymerization tech-niques1 and precise conjugation chemistry,2,3 chemists havemade great efforts to shape macromolecules into differentarchitectures, including circles,4 stars,5 tadpoles,6 brushes7 andso on. Among them, the molecular brush is a majorcategory consisting of 1D brush,8,9 2D brush10 and 3D brush(Fig. 1a–c).3,11,12 In spite of the well developed dendritic poly-mers,7 dendritic molecular brushes (DMBs), a kind of 4Dbrushes with side-chains grown along a segmented dendriticpolymer scaffold, were rarely reported, which is likely to be dueto the lack of multifunctional sparsely branched backbones.12

To solve this dilemma, our group recently explored DMBs withshort hydrophobic, hydrophilic or amphiphilic side-chainsthrough a “graing to” approach based on multifunctionalsegmented hyperbranched polymer (SHP) scaffolds.3,12–15

Compared with the SHPs and those conventional hyper-branched polymers (CHPs),12 DMBs possess higher density offull-scaffold functionality, just like trees with thick leaves along

thesis and Functionalization, Department

g University, 38 Zheda Road, Hangzhou,

SI) available: Experimental conditions,quential functionalization of DMB-Py,e DOI: 10.1039/c3py00546a

4460

the whole branches (Fig. 1). However, the unique topology-induced properties of DMBs have not been disclosed yet.

On the other hand, in the area of photochemistry, two kindsof chromophores are becoming the focus, i.e. pyrene and tet-raphenylethylene (TPE).16,17 The potential of pyrene to readilyform excimers makes it attractive for applications in solution-based uorescent sensors,16 and the unique aggregation-induced emission (AIE) attribute of TPEmakes it very promisingfor application in solid-state devices.18 To date, most of thepyrene-based polymers exhibited no or negligible excimeremission in highly diluted solutions,19 andmost of the dendriticpolymers with AIE moieties only exhibited the aggregation-enhanced emission (AEE) phenomenon, rather than the typicalAIE effect as shown in the linear polymer analogues.20 Hence, tofacilely fabricate pyrene-based polymer materials with highexcimer emission and to realize AIE performance in dendriticpolymer systems are still challenges.

To answer the aforementioned questions, here we synthe-sized DMBs based on multifunctional SHP backbones3,12 via a“graing from” approach that allows convenient control overthe length of side chains. The facile availability of the complexDMBs paves the way for further functionalization and exploitingtheir properties. Subsequent post-modication of DMBs withpyrene and TPE moieties gave birth to compact dendriticmacromolecules showing ultrahigh excimer-emission atextremely low concentrations and classic AIE effect, respec-tively, due to the signicant cage effect of DMBs. The high

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3py00546a

http://pubs.rsc.org/en/journals/journal/PY

http://pubs.rsc.org/en/journals/journal/PY?issueid=PY004016

Fig. 1 (a) 1D molecular brush. (b) 2D molecular brush. (c) 3D molecular brush. (d) Densely branched trees with leaves mostly around the periphery. (e) Sparselybranched trees with leaves along the whole branches. (f) Densely branched trees with thick leaves along the whole branches. (g) Conventional hyperbranched polymers(CHPs) with functional groups mostly around the periphery. (h) Segmented hyperbranched polymers (SHPs) with functional groups along the whole scaffold. (i) 4Dfunctional dendritic molecular brushes (DMBs) with higher-density functional groups along the whole scaffold.

Paper Polymer Chemistry

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solubility, ease of solution processing, and unique cage effect ofDMBs would foster the advancements in design and fabricationof novel organic electronic devices. As a prototype, we made acomplex solid pattern that emits strong blue light using theTPE-attached DMBs as printable ink.

Experimental sectionMaterials

Mono-ethynyl TPE (METPE), (s)-1-doceyl-(s)-(a,a0-dimethyl-a0 0-acidic acid) trithiocarbonate (DDMAT) and 2-((2-(((dodecylthio)-carbonothioyl)thio)-2-methylpropanoyl)oxy)ethyl acrylate (ACDT)were synthesized according to previous literature.3,21 Glycidylmethacrylate (GMA, 97%), sodium azide (99.5%), propargylalcohol (99%), CuBr (98%) and 2,20-azobisisobutyronitrile (AIBN)were purchased from Sigma-Aldrich Corporation. 4-Dimethyla-minopyridine (DMAP, 99%), N-(3-dimethylaminopropyl)-N0-


ethylcarbodiimide hydrochloride (EDCI, 98.5%), and 1-pyr-enebutyric acid were purchased from Aladdin Chemical Co.China. 1,1,4,7,7-Pentamethyldiethylenetriamine (PMDETA,98%), propargyl bromide and N,N-dimethylpropargyamine werepurchased fromAlfa Aesar Corporation. Chloroform-d (99.8 atom%D, stabilized with silver foil), dimethyl sulfoxide-d6 (99.9 atom%D) were purchased from J&K chemical. Dimethylformamide(DMF), 1,4-dioxane, chloroform, dichloromethane, ammoniumchloride and other organic solvents were purchased from Sino-pharm Chemical Reagent Co. Ltd. GMA was passed through acolumn of basic alumina before use and other materials wereused as received.

Instrumentation

Gel permeation chromatography (GPC) was carried out on aPerkin Elmer HP 1100, using DMF/LiBr (0.02 mol L�1) as the

Polym. Chem., 2013, 4, 4450–4460 | 4451


Polymer Chemistry Paper

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eluent at a ow rate of 1 mL min�1, RI-WAT 150 CVt + as thedetector and linear poly(methyl methacrylate) as the calibrationstandard at 60 �C for detection of apparent molecular weights.Light scattering data were recorded through gel permeationchromatography with multiple angle laser scattering detection(GPC-MALLS) system using LiBr/DMF (0.05mol L�1) as eluent ata ow rate of 1 mL min�1 and linear poly(methyl methacrylate)as the calibration standard at 60 �C for characterization ofabsolute molecular weights. The detection system consisted of adn/dc RI detector (Dawn DSP Laser Photometer, Wyatt Tech-nology) and multiangle laser light scattering detector (OPILABDSP Interferometric Refractometer, Wyatt Technology). 1HNMR (400 MHz) spectroscopy measurements were carried outon a VarianMercury plus 400 NMR spectrometer using CDCl3 orDMSO-d6 as solvent. Fourier transform infrared (FTIR) spectrawere recorded on a PE Paragon 1000 spectrometer (lm or KBrdisk). UV-visible spectra were obtained by using a Varian Cary300 Bio UV-visible spectrophotometer. Fluorescence spectrawere measured with a RF-5301PC uorophotometer (ShimadzuCorp.). Atomic force microscopy (AFM) was performed undertapping mode on a NanoScope IIIa SPM from Digital Instru-ments Inc. The AFM samples were obtained via a dip-coatingmethod on a freshly peeled mica wafer. Transmission electronmicroscopy (TEM) studies were performed on a JEOL JEM2010electron microscope at 200 kV. Dynamic light scattering (DLS)was measured on a Brookhaven 90 Plus particle size analyzer.The samples for dynamics study were obtained under nitrogengas purge and subjected to characterization directly without anypurication.

Synthesis of alkynyl chain transfer agent (alk-CTA)

1 g DDMAT (1 equiv.), 0.5 mL propargyl alcohol (3.26 equiv.),0.783 g EDCI (1.5 equiv.) and 0.5 g DMAP (1.5 equiv.) weredissolved in 10 mL dichloromethane in a 25 mL round bottomask deoxygenated with nitrogen purge. The solution was stir-red at room temperature for 24 h and then subsequently washedwith 1.2 MHCl and deionized water. Aer dried with anhydrousmagnesium sulfate and ltered, the solution was evaporatedusing a vacuum evaporator and kept in a vacuum oven at roomtemperature for 24 h. Viscous yellow product was obtained andsubjected to 1H NMR analysis. 1H NMR (400 MHz, CDCl3): 4.70(s, CH2C^CH), 3.28 (t, C11H23CH2SC(]S)SC(CH3)2), 2.48 (t,CH2C^CH), 1.72 (s, C11H23CH2SC(]S)SC(CH3)2), 1.26 (m,CH3C10H20CH2SC(]S)SC(CH3)2), 0.89 (t, CH3C10H20CH2SC(]S)SC(CH3)2).

Synthesis of segmented hyperbranched poly(chain transferagent) (SHP-CTA)

Segmented hyperbranched poly(glycidyl methacrylate) (SHP-1)was synthesized via reversible addition–fragmentation chaintransfer self-condensing vinyl polymerization (RAFT-SCVP) ofGMA and ACDT in a feed ratio of 30 : 1. The composition of thetwo components in the obtained SHP-1 was close to the feedratio and each segment contains about 30 GMA units. Subse-quently, SHP-1 was reacted with NaN3 in DMF at 50 �C for 24 h,yielding SHP backbone (SHP-2) with both hydroxy and azido

4452 | Polym. Chem., 2013, 4, 4450–4460

groups at each repeating unit.12 SHP-CTA was synthesized byreacting SHP-2 with alk-CTA. Typically, 530 mg SHP-2 (1 equiv.),2.31 g alk-CTA (2 equiv.), and 0.55 mL PMDETA (1 equiv.) werecharged into a 10 mL Schlenk ask with a mixture of 9 mL DMFand 9 mL chloroform. Aer deoxygenation, 383 mg CuBr(1 equiv.) were instantly added to the solution under nitrogenpurge. The solution was kept at room temperature for 24 h andthen poured into 200 mL methanol for purication. The driedproduct of SHP-CTA was characterized by FTIR and 1H NMRanalyses. 1H NMR (400 MHz, CDCl3): 7.90 (methine proton onthe triazole ring), 5.20 (triazole-CH2OOCC(CH2)2SC(]S)SCH2(CH2)10CH3), 4.80–3.91 (C(]O)OCH2CH(OH)CH2-tri-azole), 3.26 (triazole-CH2OOCC(CH3)2SC(]S)SCH2(CH2)10CH3),1.76 (triazole-CH2OOCC(CH3)2SC(]S)SCH2(CH2)10CH3), 1.27(triazole-CH2OOCC(CH3)2SC(]S)SCH2(CH2)10CH3), 0.89 (tri-azole-CH2OOCC(CH3)2SC(]S)S–CH2(CH2)10CH3). (For

1H NMRinformation for SHP-1 and SHP-2, see ref. 12).

Synthesis of DMB with poly(glycidyl methacrylate) side-chains(DMB-1)

In a typical procedure, 25 mg SHP-CTA (1 equiv.), 2.977 g GMA(500 equiv.) and 1.375 mg AIBN were charged into a 10 mLround bottom ask with 4.2 mL dioxane. Aer the solution wasbubbled with nitrogen gas in an ice bath for deoxygenation, theask was put into a 75 �C thermostatic oil bath. Aer 1.25 h, thesolution was quenched by liquid nitrogen. Aer thawing,the diluted solution was then precipitated into 10-fold excess ofmethanol. The precipitates were collected and dried in vacuo atroom temperature for 1 h. 1H NMR (400 MHz, CDCl3): 4.30(COOCH2CHO, COOCH2CH2OCO), 3.73 (COOCH2CHO), 3.20(COOCH2CHO, S]CSCH2(CH2)10CH3), 2.80 and 2.66 (–CH2O–in epoxide ring), 2.2–1.4 (side chain proton), 1.24 (S]CSCH2(CH2)10CH3), 0.98 (methyl protons on side chain back-bone), 0.8 (S]CSCH2(CH2)10CH3).

Synthesis of DMB with poly(2-hydroxy-3-azidopropylmethacrylate) side-chains (DMB-2)

In a typical procedure, 264 mg DMB-1 (1 equiv.), 121 mg sodiumazide (3 equiv.) and 100 mg ammonium chloride (3 equiv.) weredissolved in 12 mL dry DMF in a 25 mL round bottom askequipped with a stirring bar. Aer deoxygenation, the ask wasimmersed in a 50 �C oil bath and stirred for 24 h. The solutionwas then precipitated into 10-fold excess of deionized water.The precipitates were then desiccated via a vacuum freeze-drier.1H NMR (400 MHz, DMSO-d6): 5.44 (hydroxyl proton), 3.87(COOCH2CH(OH)CH2N3), 3.35 (COOCH2CH(OH)CH2N3), 2.20–1.40 (side chain proton), 1.23 (S]CSCH2(CH2)10CH3), 0.92(methyl protons on side chain backbone), 0.76 (S]CSCH2(CH2)10CH3).

Synthesis of pyrene-functionalized DMB (DMB-Py)

In a typical procedure, 50 mg DMB-2 (1 equiv.), 225 mg 1-pyr-enbutyric acid (3 equiv.), 140 mg EDCI (3 equiv.) and 105 mgDMAP (3 equiv.) were dissolved in 1.5 mL dry DMF in a 10 mLround bottom ask sealed with a rubber septum. Aer purgingwith nitrogen in an ice bath for 15 min, the solution was




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immersed in a 60 �C thermostatic oil bath and stirred for 24 h.The solution was poured into 10-fold excess of deionized water.The precipitates were dried in a vacuum freeze-drier andredissolved in 1.5 mL chloroform. The solution was precipitatedinto 30 mL ethyl acetate and dried. The excitation and emissionslit widths for PL spectrum were both 5 nm. 1H NMR (400 MHz,CDCl3): 8.08–7.45 (aromatic protons of pyrene ring), 5.04(CHCH2N3), 4.31–3.66 (COOCH2CH2OCO, COOCH2CHCH2N3),3.36–2.79 (pyrene-CH2CH2CH2COOCH, COOCH2CHCH2N3,SCH2(CH2)9CH3), 2.26 (pyrene-CH2CH2CH2COOCH), 1.93 (pyr-ene-CH2CH2CH2COOCH), 1.27 (S]CSCH2(CH2)10CH3), 1.04(methyl protons on side chain backbone), 0.87 (S]CSCH2(CH2)10CH3).

Synthesis of TPE-functionalized DMB (DMB-TPE)

In a typical procedure, 52.6 mg DMB-2 (1 equiv.), 97 mg METPE(2 equiv.) and 30 mL PMDETA (0.5 equiv.) were dissolved in 0.5mL DMF in a 10 mL Schlenk ask and purged with nitrogen gasfor 15 min. Aerwards, 11.5 mg CuBr was added under nitrogenprotection. The reaction was kept at room temperature andstirred for 36 h. The solution was then precipitated in 14 mLdiethyl ether. The precipitates were then redissolved in 6 mLchloroform and passed through a column of basic alumina toremove residual copper. The excitation and emission slit widthsfor PL spectrum were both 3 nm. 1H NMR (400 MHz, DMSO-d6):8.39 (proton on triazole ring), 6.92 (aromatic protons on TPEring), 5.52 (hydroxyl proton), 4.70–3.59 (C(]O)OCH2CH(OH)-CH2-triazole), 2.21–1.69 (side chain proton)1.41–0.86 (S]CSCH2(CH2)10CH3, S]CSCH2(CH2)10CH3, methyl protons onside chain backbone).

Scheme 1 Synthesis route to fluorescent dendritic molecular brushes.

Results and discussionSynthesis of DMBs via “graing from” approach

Three methodologies called “graing to”, “graing through”and “graing from” have been developed to synthesize polymerbrushes.7 The “graing to” strategy has been employed tosynthesize DMBs in our previous work,3,12 which allows theconvenient immobilization of short side chains onto the SHPbackbone but is of limited use for growing long multifunctionalpendent chains. Accordingly, we designed an alternative routeto synthesize DMBs with long multifunctional chains via“graing from” approach (Scheme 1). Firstly, the SHP-1 wasprepared by RAFT-SCVP of GMA and ACDT with a feed ratio of30 : 1. Subsequent ring-opening of epoxide groups of SHP-1 inthe presence of NaN3 gave SHP-2 with both hydroxy and azidogroups at each repeating unit. Aerwards, CTA units wereintroduced onto SHP backbone by azide–alkyne click chemistryefficiently. The resulting SHP-CTA with about 30 RAFT agentson each segment was used to “initiate” the RAFT polymerizationof clickable side chains of PGMA, affording DMBs with a highdensity of epoxy groups.22 The molecular parameters of SHP-1,and SHP-2 are listed in Table 1. Notably, sequential RAFTpolymerization process has been employed to synthesize multi-arm star polymers (or star-like brushes, Fig. 1c) with a SHP corewhich only has CTA groups at terminal units. In this work, CTA


groups are attached on each repeating unit of the whole SHPscaffold, and thus the resulting macromolecules are coined asdendritic molecular brushes which contain much higherdensity of side gras than the conventional star-like brushes.

Fig. 2 shows the 1H NMR and FTIR spectra of SHP-CTA. Inthe 1H NMR spectrum (Fig. 2a), peak “f” represents the triazolering resulted from the click cycloaddition of azide and alkyne,and peak “b”, “g” and “h” represent the Z groups of the attachedCTA. The disappearance of the adsorption peak at �2100 cm�1

from the FTIR spectrum of SHP-CTA indicates the �100%consumption of azido groups on SHP-2 (Fig. 2b).

Based on the sparsely branched dendritic macro-CTA(SHP-CTA), we prepared series of DMBs with different lengths ofside arms through RAFT polymerization of GMA by adjustingthe reaction conditions such as the feed ratio of GMA to CTA,the concentration of GMA, and polymerization time (Table 2).

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Table 1 Molecular parameters and sizes of synthesized dendritic polymers

Entry Mna (g mol�1) Mw

a (g mol�1) Mza (g mol�1) PDIa Mn

b (g mol�1) Sizec (nm)

SHP-1 52 400 52 300 74 000 1.234 18 400 135.0SHP-2 64 900 119 200 262 900 1.835 26 700 155.4DMB-1 346 000 521 200 988 400 1.507 113 000 164.4DMB-Py 964 900 1 264 000 2 595 000 1.310 192 700 182.2DMB-TPE 1 164 000 2 148 000 4 167 000 1.845 223 400 176.7

a Absolute number-averaged molecular weight (Mn), weight-averaged molecular weight (Mw), z-averaged molecular weight (Mz) and polydispersityindex (PDI) determined by GPC-MALLS. b Apparent number-averagedmolecular weight (Mn) determined by conventional GPC. c Determined by DLSat concentration of 1 � 10�5 mg mL�1 at 25 �C.

Fig. 2 (a) 1H NMR spectrum of SHP-CTA in CDCl3. (b) FTIR spectra of SHP-CTAand SHP-2.


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Codes 1–4 show that higher monomer and CTA concentrationgive rise to quicker polymerization but poorer polydispersity.Codes 5 and 6 show that higher monomer/CTA ratio guaranteesbetter control over the polydispersity. The conditions with feedratio of 500 : 1 and GMA concentration of 0.5 M were selectedfor further kinetics study. The results are shown in Table 3 andFig. 3. The curve of “ln([M0]/Mt) vs. time” showed ideal linearityat the early stage of the polymerization, revealing the “living/controlled” nature of RAFT polymerization during this stage

Table 2 Selected conditions and results for RAFT polymerization of DMB-1

Code Feed ratio Time [GMA] (mol L�1) Mna (g mol�1) M

1 500 : 1 40 min 1 133 7002 500 : 1 90 min 1 382 3003 500 : 1 40 min 2 245 0004 500 : 1 70 min 2 394 2005 2000 : 1 3 h 0.5 934 200 16 2000 : 1 6 h 0.5 1 457 000 2

a Absolute number-averaged molecular weight (Mn), weight-averaged moleindex (PDI) determined by GPC-MALLS. b Apparent number-averaged mo

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(the rst two hours) (Fig. 3a). The increasing trend of theabsolute Mn detected by GPC-MALLS was much sharper thanthat of the apparent Mn detected by conventional GPC tech-nique (Fig. 3b). Given that conventional GPC technique is basedon detecting molecular volume,23 the difference between theapparent Mn and the absolute Mn means that although theDMB's molecular weight increased dramatically, its molecularsize did not increase obviously. During the later period ofpolymerization, a shoulder peak gradually appeared (Fig. 3c),suggesting the happening of intermolecular coupling caused bythe limited motion of the high density free radicals in thehyperbranched macro-CTA systems (aer the rst two hours).This is also conrmed by the ln([M0]/Mt) vs. time curve that itgradually deviated from the tted linear curve (Fig. 3a). There-fore, the key to obtaining high molecular weight DMBs withnarrow polydispersity is to decrease the monomer concentra-tion, increase the monomer/CTA ratio and to stop the poly-merization at low conversion. This is because lower monomerconcentration leads to lower free-radical concentration andhigher monomer/CTA rate makes the high-density free radicalsaround the DMB backbones surrounded by more uninitiatedmonomer, reducing the inter-radical contact rate. According tothe aforementioned kinetics analyses, the DMB sample withboth high Mn (�350 000 g mol�1), relatively low PDI (�1.5) andabout 33 repeating units on the side chain (Code 5, Table 3) waschosen for subsequent functionalization.

DLS measurements also proved that the sizes of SHP-1 andDMB-1 were close (135.0 nm and 164.4 nm, respectively) (Table 1).The relatively large hydrodynamic radius of SHP-1 indicates thesparsely branched structure of SHP-1 which accounts for its highfunctionality.12 The similar molecular size of the DMBs to their

wa (g mol)�1 Mz

a (g mol�1) PDIa Conversion Mnb (g mol�1)

170 900 301 100 1.279 — 36 000651 600 1 461 000 1.704 10.7% 141 200372 700 662 000 1.521 2.9% 83 460813 900 2 377 000 2.065 13.0% 126 600240 000 1 892 000 1.328 7.4% 264 200194 000 4 140 000 1.506 41.2% 381 000

cular weight (Mw), z-averaged molecular weight (Mz) and polydispersitylecular weight (Mn) determined by conventional GPC.



Table 3 Kinetics information for RAFT polymerization of DMB-1 at feed ratio of 500 : 1

Code Time Conversion Mna (g mol�1) Mw

a (g mol�1) Mza (g mol�1) PDIa Mn

b (g mol�1) mc

1 10 min — 115 900 207 800 477 900 1.793 27 000 —2 20 min — 188 100 233 100 325 800 1.239 43 000 —3 30 min 1.0% 244 300 345 800 592 000 1.415 79 900 54 45 min 3.8% 317 300 450 400 779 700 1.419 126 200 195 1 h 6.5% 346 000 521 200 988 400 1.507 113 000 336 1.5 h 12.3% 386 100 744 400 196 900 1.928 129 700 627 2 h 16.0% 482 700 1 113 000 3 638 000 2.306 149 600 808 3 h 27.5% 673 200 2 186 000 8 158 000 3.248 165 000 1389 4 h 42.8% 955 200 2 976 000 10 660 000 3.115 173 600 214

a Absolute number-averaged molecular weight (Mn), weight-averaged molecular weight (Mw), z-averaged molecular weight (Mz) and polydispersityindex (PDI) determined by GPC-MALLS. b Apparent number-averaged molecular weight (Mn) determined by conventional GPC. c Number of GMArepeating unit on each of the gra side-chain.


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SHP precursors indicates that they have a much more compactstructure than the precursor. TEM observations showed that thedry-state size of DMB-1 was �100 nm (Fig. 4a) which was alsoconrmed by the AFM data (Fig. 4b and c). Moreover, the molec-ular height is about 3 nm similar to the data of the linear coun-terpart of DMBs-the cylindrical molecular brushes (1.8–4.2 nm inheight), indicating the mono-layer dispersion of the macromole-cules.9 Notably, given the highly compact structural nature ofDMB-1, a small amount of physical coupling and self-assemblymight take place even at extremely low sampling concentrations. Itis smaller than the DMB size in the solution-state, indicating thatthe DMB backbone is relatively so and scaffold shrinkage takesplace in the dry state. AFM measurements revealed that dry-stateDMB-1 had a lateral size of �100 nm and a height of �3 nm(Fig. 4b–d), conrming the shrinkage of so DMB backbone.

Fig. 3 (a) Dependence of ln([M]0/[Mt]) on time. (b) Dependence of Mn on GMA coDotted lines are linear fitting curves.


The following azidation on oxirane units of DMB-1 yieldedDMB-2, which is the rst case of polymer brushes with hetero-bifunctional side-chains. The structure of DMB-2 was charac-terized by 1H NMR and FTIR analyses. The peaks at 4.3 and 3.8ppm representing the methene protons near the oxirane ringand the peaks at 3.2, 2.8 and 2.6 ppm assigned to the protons onthe oxirane ring were not detected any more in the 1H NMRspectrum of DMB-2 (Fig. 5a), demonstrating a very highconversion (�100%) for the ring-opening reaction despite theextremely high compactness of DMBs. A new peak was found at5.4 ppm belonging to the hydroxy groups of DMB-2. The strongabsorption peak at �2100 cm�1 in the FTIR spectrum of DMB-2declared the presence of azido groups. This hetero-bifunctionalDMB promises the controlled immobilization of different typesof uorophores.

nversion. (c) GPC peak evolution of DMB-1 during RAFT polymerization of GMA.

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Fig. 4 (a) Representative TEM image of DMB-1. The scale bar corresponds to 1 mm (large picture) and 200 nm (inset picture) respectively. Sample concentration: 1 �10�5 mg mL�1. (b) AFM image of DMB-1. The scale bar corresponds to 500 nm. Samples concentration: 1 � 10�5 mg mL�1. (c) Section analysis of diameter of typicalparticle in (b) (�98 nm). (d) Section analysis of height of typical particle in (b) (�3 nm).


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Cage effect: excimer-enhanced DMB-Py

With the characters of easy excimer formation and structuralsensitivity,16 pyrene was widely used in material science andanalysis chemistry. In particular, the switch between excimeremission andmonomer emissionmakes them hot in the area ofratiometric uorescent sensors.24 However, the materials with ahigh IE/IM ratio (excimer emission intensity/monomer emissionintensity) at a low concentration were rarely developed, whichgreatly limits the application of pyrenyl compounds in high-resolution sensors. The crux of this challenge is to improve theintermolecular contact rate of excited pyrene moieties and theground-state pyrenemoieties.25 Based on this principle, DMB-Pywith a highly compact structure was designed and synthesizedby esterication between DMB-2 and 1-pyrenebutyric acid(Scheme 1). In the corresponding 1H NMR spectrum (Fig. 5a),the overlapped peak around 8.0 ppm represents the aromaticsignal of the attached 1-butyricpyrene acid, and the integrationratio of peak at 5.1 ppm to peak at 3.2 ppm equals 1 : 4, which

Fig. 5 1H NMR spectra (a) and FTIR spectra (b) of DMB-1, DMB-2, DMB-Py and DM

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indicates the complete conversion of hydroxy groups and thatabout 33 pyrene units were attached onto each of the sidechains. The remaining azido groups showed a strong absorp-tion peak at �2100 cm�1 in the FTIR spectrum of DMB-Py(Fig. 5b). DMB-Py showed a molecular size of 182.2 nm insolution-state (DLS data, Table 1) and �150 nm at dry-state(TEM result, Fig. S1a†), which are close to the correspondingvalues of its precursor of DMB-1. Such a cage-like compactstructure lled with pyrene moieties makes their higher contactrate possible. We then measured the uorescent spectra ofDMB-Py at various concentrations (Fig. 6a). Very strong excimeremission at 475 nm and ultrahigh IE/IM value were found atextremely low concentrations (�10�7 M). Such concentrationsare compared with our recently reported pyrene-based SHPswith very strong excimer emission.12 Signicantly, both theexcimer emission and IE/IM are higher than those of the highlyemissive pyrene-based SHPs at the comparable pyreneconcentrations (Fig. 6b).12 For instance, the IE/IM value of

B-TPE.




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DMB-Py reached 7.76 at pyrene concentration of 0.181 mM,�32% higher than that of pyrene-based SHP at a roughly two-fold higher concentration of 0.349 mM. Moreover, comparedwith other compact macromolecular pyrene-conjugated struc-tures, like dendrimers (IE/IM value of 2.19 at pyrene concentra-tion of 1.5 � 10�6 M) and hollow polymer capsules (IE/IM valueof 1.29 at polymer concentration of 1 mg mL�1), DMB-Py alsoshowed higher IE/IM at much lower concentration.26 These factsprove that the highly compact structure of DMB has a strongamplifying inuence on the emission of conventional chromo-phores such as pyrene, which is coined as the “cage effect” inthis article. In addition, the remaining clickable azido sitesalong the scaffold promises broader applications of DMB-Py. Ina preliminary attempt, the azido sites were completelyconverted into tertiary amine sites and then cationic alkynylsites via a sequential click chemistry approach (Scheme S1 andFig. S2–S4†).

Cage effect: AIE-featured DMB-TPE

The serious problem of uorescence quenching in aggregation-state, against which scientists have been ghting for decades,27

limits the application of conventional chromophores in solid-state uorescence. Alternatively, Tang and co-workers recentlydiscovered the novel effect of aggregation-induced emission(AIE) for some conjugated molecules,18 which essentiallyfurthers the advancement of organic electronic devices.28 As hasbeen reported, when the AIE moieties were introduced intodendritic systems, they usually exhibited aggregation-enhancedemission (AEE) rather than the typical AIE effect mainly becausethe rigidness of dendritic scaffolds caused the restriction inrotation (RIR) effect of the AIE units in solution-state.20 Mostrecently, Tang and Qin et al. reported a kind of spring-likehyperbranched TPE macromolecule which exhibited typical AIEfeatures.29 This inspired us to attach a TPE moiety to ourshrinkable DMBs. Through simple click coupling betweenMETPE and DMB-2, we synthesized DMB-TPE (Scheme 1).

Fig. 6 (a) PL spectra of DMB-Py excited at 344 nm. (b) Squares: dependence of IE/pyrene concentration; triangles: monomer intensity on pyrene concentration. Red c


Fig. 5a shows the 1H NMR spectrum of DMB-TPE, and theoverlapped peaks around 7.2 ppm represent the attached TPEmoieties. No absorption peak was detected at �2100 cm�1 inthe FTIR spectrum of DMB-TPE, indicating the 100% conver-sion of the click chemistry reaction (Fig. 5b) and that about 33TPE units were attached onto each of the side chains. Fig. 7shows the PL spectra and quantum yields (QYs) of DMB-TPEupon gradual addition of water to the THF solution of DMB-TPE. The PL intensity at 475 nm only slightly increased to 72when the water faction ( fw) arose to 60 vol%, and thendramatically increased to 185 at fw of 70 vol%, and jumped to975 at fw of 90 vol% (Fig. 7a), which is apparently a typical AIEphenomenon. The corresponding QYs displayed the sameregular pattern as a function of fw (Fig. 7b). When fw is below 70vol%, the curve is almost parallel to the abscissa, while as fwincreased to 90 vol%, the QY rapidly increased from 5.69% to36.6%, conrming the AIE effect of DMB-TPE. In the photo-graph of DMB-TPE with different fws excited at 365 nm (Fig. 7b,inset), it is obvious that visible blue light appeared at fw of 70 vol%and the light quickly became stronger at fws of 80 and 90 vol%.This typical AIE effect is attributed to the unique structure ofDMB, just like a “spring-like cage” that can more tightly holdloads when shrunken but won't hamper the rotation of the TPEgroup in the solution state. This cage effect makes the rotationof TPE moieties more conned in the shrinking DMBs uponaddition of water, resulting in a sharp transition of PL intensityat the critical shrinking point ( fw � 70 vol% in our case), real-izing the AIE effect in this high molecular weight dendriticsystems. Moreover, compared with the AIE-effect-featuredMETPE monomer, the QY of the DMB-TPE is about two timeshigher (Fig. 7b), indicating that this shrunken cage also exertedan amplifying effect for the AIE uorophore at aggregationstate. This solid-state uorescent dendritic material guaranteeswide applications in functional ink and coating. As a prototypefor application, we made a complex pattern (Zhejiang Universityschool badge) by Chinese-brush painting with the ink of DMB-TPE, which emits strong blue light under UV radiation (Fig. 8).

IM on pyrene concentration; circles: dependence of excimer emission intensity onurves are data for DMB-Py and black curves are data for pyrene-modified SHP.12

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Fig. 7 (a) PL spectra of DBPY in THF–water mixtures with water fraction from 0 to 90% at concentration of 10 mg mL�1. lex ¼ 330 nm. (b) Blue line: quantum yields ofDBPYat varying water fraction, with quinine sulfate in 0.1 N H2SO4 (FF ¼ 54.6%) as reference; red line: quantum yields of METPE at varying water fraction. Inset pictureis the photograph of DBPY with varying THF–water fraction at lex of 365 nm.


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Conclusions

In this work, we presented a sequential RAFT polymerization“graing from” approach to synthesize multifunctional long-chain dendritic molecular brushes (DMBs). The sparselybranched segmented hyperbranched backbone was rstlysynthesized via a SCVP-RAFT approach, which allows thecomplete immobilization of CTA on the whole scaffold. Subse-quent RAFT polymerization on the macro-CTA precursor gavebirth to the clickable DMBs with a high molecular weight of�1.4 � 106 g mol�1 and low PDI of 1.50. Further azidation onthe epoxy ring yielded DMBs with azide/hydroxy hetero-bifunctionality. When conjugated with conventional chromo-phores (pyrene) and AIE chromophores (tetraphenylethylene,TPE) respectively, the compact and exible scaffold of the DMBsshowed unique “cage effect” for both uorophores. Thecompactness makes pyrene-based DMBs have high intra-molecular contact rate and thus exhibit unprecedented strongexcimer emission at highly diluted concentration. The exibilitymakes TPE-based DMBs have a spring-like scaffold and there-fore exhibit typical AIE effect which is precious and rarely found

Fig. 8 (a) AIE painting of the Zhejiang University school badge in daylight. (b)The school badge exposed to 365 nm UV light.

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in dendritic systems. Such DMBs were used as printable ink tomake a complex pattern that can emit strong blue light at solidstate when excited under UV-irradiation. These blue-light uo-rescent macromolecules with special structures probably havepromising applications in high-resolution solution sensors andsolid-state organic electronic devices. This is the rst reportregarding the strong inuence of dendritic brush architectureon the optical behavior of different kinds of chromophores,breaking new ground for design, synthesis, and applications ofmultifunctional dendritic molecular brushes.

Acknowledgements

This work was nancially supported by the National NaturalScience Foundation of China (no. 20974093 and no. 51173162),Qianjiang Talent Foundation of Zhejiang Province (no.2010R10021), Fundamental Research Funds for the CentralUniversities (no. 2013XZZX003), Research Fund for the DoctoralProgram of Higher Education of China (no. 20100101110049),and Zhejiang Provincial Natural Science Foundation of China(no. R4110175).

Notes and references

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7 C. Gao and D. Yan, Prog. Polym. Sci., 2004, 29, 183; D. Yan,C. Gao and H. Frey, Hyperbranched Polymers: Synthesis,Properties and Applications, John Wiley & Sons, Hoboken,NJ, 2011; K. Inoue, Prog. Polym. Sci., 2000, 25, 453;M. Kakimoto and M. Jikei, Prog. Polym. Sci., 2001, 26, 1233;C. R. Yates and W. Hayes, Eur. Polym. J., 2004, 40, 1257.

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10 B. Zhao and W. J. Brittain, Prog. Polym. Sci., 2000, 25, 677;M. A. Cohen Stuart, W. T. S. Huck, J. Genzer, M. Muller,C. Ober, M. Stamm, G. G. Sukhorukov, I. Szleifer,V. V. Tsukruk, M. Urban, F. Winnik, S. Zauscher,I. Luzinov and S. Minko, Nat. Mater., 2010, 9, 101; X. Huand C. Gao, Sci. Rep., 2012, 2, 767; L. Kan, Z. Xu andC. Gao, Macromolecules, 2011, 44, 444.

11 C. Zhang, Y. Zhou, Q. Liu, S. Li, S. Perrier and Y. Zhao,Macromolecules, 2011, 44, 2034; Y. Han and C. Gao, Sci.China: Chem., 2010, 53, 2461; Z. Dong and Z. Ye,Macromolecules, 2012, 45, 5020.

12 S. Li, J. Han and C. Gao, Polym. Chem., 2013, 4, 1774.13 References regarding SHPs synthesized using the SCVP

approach: A. P. Vogt and B. S. Sumerlin, Macromolecules,2008, 41, 7368; H. Mori, A. H. E. Muller andP. F. W. Simon in Hyperbranched Polymers: Synthesis,Properties and Applications, ed. D. Yan, C. Gao and H. Frey,John Wiley & Sons, Hoboken, NJ, ch. 5, 2011; A. Pfaff andA. H. E. Muller, Macromolecules, 2011, 44, 1266;S. Muthukrishnan, D. P. Erhard, H. Mori andA. H. E. Muller, Macromolecules, 2006, 39, 2743; C. Cheng,K. L. Wooley and E. J. Khoshdel, J. Polym. Sci., Part A:Polym. Chem., 2005, 43, 4754; C. Gao and X. Zheng, SoMatter, 2009, 5, 4788; M. Zhang, H. Liu, W. Shao, K. Miaoand Y. Zhao, Macromolecules, 2013, 46, 1325.


14 References regarding SHPs synthesized using theHyperMacs approach: L. R. Hutchings, J. M. Dodds andS. J. Roberts-Bleming, Macromolecules, 2005, 38, 5970;L. R. Hutchings, So Matter, 2008, 4, 2150;L. R. Hutchings, J. M. Dodds and S. J. Roberts-Bleming,Macromol. Symp., 2006, 240, 56; L. R. Hutchings,J. M. Dodds, D. Rees, S. M. Kimani, J. J. Wu and E. Smith,Macromolecules, 2009, 42, 8675; J. M. Dodds andL. R. Hutchings, Macromol. Symp., 2010, 291–292, 26;L. Z. Kong, M. Sun, H. M. Qiao and C. Y. Pan, J. Polym.Sci., Part A: Polym. Chem., 2010, 48, 454; S. Hilf, F. Wurmand F. M. Kilbinger, J. Polym. Sci., Part A: Polym. Chem.,2009, 47, 6932; D. Konkolewicz, C. K. Poon, A. Gray-Wealsand S. Perrier, Chem. Commun., 2011, 47, 239; J. M. Dodds,E. de Luca, L. R. Hutchings and N. Clarke, J. Polym. Sci.,Part B: Polym. Phys., 2007, 45, 2762; N. Clarke, E. de Luca,J. M. Dodds, S. M. Kimani and L. R. Hutchings, Eur. Polym.J., 2008, 44, 665; D. Konkolewicz, M. J. Monteiro andS. Perrier, Macromolecules, 2011, 44, 7067.

15 References regarding SHPs synthesized using the A2 + B3

macromonomer approach: A. R. Fornof, T. E. Glass andT. E. Long, Macromol. Chem. Phys., 2006, 207, 1197;S. Unal, I. Yilgor, J. P. Sheth, G. L. Wilkes and T. E. Long,Macromolecules, 2004, 37, 7081; S. Unal and T. E. Long,Macromolecules, 2006, 39, 2788; M. G. McKee, T. Park,S. Unal, I. Yilgor and T. E. Long, Polymer, 2005, 46, 2011;S. Unal, G. Ozturk, K. Sisson and T. E. Long, J. Polym. Sci.,Part A: Polym. Chem., 2008, 46, 6285; A. J. Duncan,J. M. Layman, M. P. Cashion, D. J. Leo and T. E. Long,Polym. Int., 2010, 59, 25.

16 T. M. Figueira-Duarte and K. Mullen, Chem. Rev., 2011, 111,7260.

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24 R. Yang, W. Chan, A. W. M. Lee, P. Xia, H. Zhang and K. Li, J.Am. Chem. Soc., 2003, 125, 2884; S. V. Wegner, A. Okesli,

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