Core Cross-Linked Multiarm Star Polymers with Aggregation-Induced Emission and Temperature Responsive Fluorescence Characteristics

Article abstract of DOI:10.1021/acs.macromol.7b00506

Aggregation-induced emission (AIE) active core cross-linked multiarm star polymers, carrying polystyrene (PS), polyethylene (PE), or polyethylene-b-polycaprolactone (PE-b-PCL) arms, have been synthesized through an arm-first strategy, by atom transfer radical copolymerization (ATRP) of a double styrene-functionalized tetraphenylethene (TPE-2St) used as a cross-linker with linear arm precursors possessing terminal ATRP initiating moieties. Polyethylene macroinitiator (PE-Br) was prepared via the polyhomologation of dimethylsulfoxonium methylide with triethylborane followed by oxidation/hydrolysis and esterification of the produced PE-OH with 2-bromoisobutyryl bromide; polyethylene-block-poly(?-caprolactone) diblock macroinitiator was derived by combining polyhomologation with ring-opening polymerization (ROP). All synthesized star polymers showed AIE-behavior either in solution or in bulk. At high concentration in good solvents (e.g., THF, or toluene) they exhibited low photoluminescence (PL) intensity due to the inner filter effect. In sharp contrast to the small molecule TPE-2St, the star polymers were highly emissive in dilute THF solutions. This can be attributed to the cross-linked structure of poly(TPE-2St) core which restricts the intramolecular rotation and thus induces emission. In addition, the PL intensity of PE star polymers in THF(solvent)/n-hexane(nonsolvent) mixtures, due to their nearly spherical shape, increased when the temperature decreased from 55 to 5 °C with a linear response in the range 40-5 °C.

Full text of DOI:10.1021/acs.macromol.7b00506

Article
pubs.acs.org/Macromolecules
Core Cross-Linked Multiarm Star Polymers with Aggregation-
Induced Emission and Temperature Responsive Fluorescence
Characteristics
Zhen Zhang,† Panayiotis Bilalis,† Hefeng Zhang,†,§ Yves Gnanou,‡ and Nikos Hadjichristidis*,†
‡
†Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory and Physical Sciences and
Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
S
* Supporting Information
ABSTRACT: Aggregation-induced emission (AIE) active core
cross-linked multiarm star polymers, carrying polystyrene (PS),
polyethylene (PE), or polyethylene-b-polycaprolactone (PE-b-
PCL) arms, have been synthesized through an “arm-first”
strategy, by atom transfer radical copolymerization (ATRP) of a
double styrene-functionalized tetraphenylethene (TPE-2St)
used as a cross-linker with linear arm precursors possessing
terminal ATRP initiating moieties. Polyethylene macroinitiator
(PE−Br) was prepared via the polyhomologation of dimethyl-
sulfoxonium methylide with triethylborane followed by
oxidation/hydrolysis and esterification of the produced PE−
OH with 2-bromoisobutyryl bromide; polyethylene-block-poly-
(ε-caprolactone) diblock macroinitiator was derived by combin-
ing polyhomologation with ring-opening polymerization (ROP). All synthesized star polymers showed AIE-behavior either in
solution or in bulk. At high concentration in good solvents (e.g., THF, or toluene) they exhibited low photoluminescence (PL)
intensity due to the inner filter effect. In sharp contrast to the small molecule TPE-2St, the star polymers were highly emissive in
dilute THF solutions. This can be attributed to the cross-linked structure of poly(TPE-2St) core which restricts the
intramolecular rotation and thus induces emission. In addition, the PL intensity of PE star polymers in THF(solvent)/n-
hexane(nonsolvent) mixtures, due to their nearly spherical shape, increased when the temperature decreased from 55 to 5 °C
with a linear response in the range 40−5 °C.
unique structures or chemical compositions, AIE properties can
be combined with polymer properties to obtain emission in
response to external stimuli such as heat,13,14 pH,15 light,16
chemicals,17 etc. For example, Tang and co-workers synthesized
a hyperbranched poly(silylenephenylene) which acts as as a
highly sensitive chemosensor for detection of explosives with a
super amplification effect due to the three-dimensional (3D)
structure.18,19 Taniguchi et al. synthesized TPE cross-linked
poly(dimethylsiloxane) (PDMS) elastomers through the hydro-
silylation reaction of tetravinyl-modified TPE derivative and H-
terminated PDMS.20 Such AIE elastomers exhibited stimuli-
sensitive behavior in organic solvents with the extent of swelling
influenced by the nature of the solvent used. Studies have not
only focused on hyperbanched and cross-linked polymers but
also linear, star, dendritic, microporous, and supermolecular
polymers have been investigated and their remarkable proper-
ties reported.21 However, in most cases, the reported AIE
polymers are nonemissive or weakly fluorescent when dissolved
in their good solvents, a typical AIE feature.18 The applications
INTRODUCTION
■
Luminescent materials have attracted strong interest in recent
decades due to their significant applications in electronic and
bioimaging areas.1−4 The conventional luminophores experi-
ence high emission in dilute solution, but undergo partial or
complete light emission quenching when aggregated.5 This
undesirable aggregation-caused quenching (ACQ) effect
hinders the practical use of luminescent materials in solid or
aggregated state. In contrast, propeller-like molecules, e.g.
tetraphenylethene (TPE), are nonluminescent in dilute
solution but emissive in aggregated state. This exactly opposite
phenomenon of ACQ was discovered and documented as
aggregation-induced emission (AIE) by Tang in 2001.6,7 The
restriction of intramolecular rotation (RIR) in the aggregate
state leads to the radiative decay of excitations, which has been
identified as the main cause for the AIE effect.8 In contrast,
active rotations of peripheral phenyl rings effectively annihilate
the exited states in dilute solution. Until now, many new AIE
molecules have been designed/synthesized and applied in
chemosensing, bioimaging, optoelectronics, stimuli-responsive
systems, and so on.9−11
Received: March 8, 2017
Revised: May 15, 2017
To achieve facile fabrication of thin films and devices, AIE-
active polymers have been largely explored.12 Owing to the
© XXXX American Chemical Society
A
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and Styragel HR4) with THF eluent at 35 °C, flow rate =1.0 mL/min
and differential refractive index (RI) detector. For PE-based linear and
star polymers, Mn,GPC and Đ were measured on a Viscoteck high
temperature gel permeation chromatography (HT-GPC) module 350
instrument with two PLgel 10 μm MIXED-B columns and 1,2,4-
trichlorobenzene (TCB) as the eluent at a flow rate of 0.8 mL/min at
150 °C. The absolute molecular weight (Mw,GPC‑LS) of star polymers
and ĐGPC‑LS were determinted by triple-detection GPC or HT-GPC
(refractometry, light scattering at λ = 670 nm and viscometry). Both
GPCs were calibrated with PS standard (Mw = 115 × 103 g/mol, Đ =
1.05). DSC measurements were performed using a Mettler Toledo
DSC1/TC100 system under inert atmosphere (nitrogen). The sample
was heated from room temperature to 150 °C, cooled to −10 °C and
finally heated again to 150 °C with a heating/cooling rate of 10 °C/
min. The second heating curve was used to determine the glass
transition temperature (Tg) and melting temperature (Tm). Trans-
mittance measurements were performed on a Thermo Evolution 600
UV−vis spectrophotometer equipped in quartz cuvettes of 10 mm
path length at a wavelength of 500 nm at room temperature.
Photoluminescence spectra were recorded on a Thermo Lumina
fluorescence spectrometer equipped an external water circulator for
the thermostated cell holder. Dynamic light scattering (DLS)
measurements were made with a Brookhaven BI-200SM multiangle
goniometer with a TurboCorr correlator. The light source was a 30
mW He−Ne laser emitting vertically polarized light of 632.8 nm
wavelength.
Synthesis of Cross-Linker 1,2-Diphenyl-1,2-bis(4-((4-
vinylbenzyl)oxy)phenyl)ethane (TPE-2St). Zn powder (11.6 g,
0.18 mol) and 4-hydroxybenzophenone (8.0 g, 0.040 mol) were placed
into a 250 mL two-necked flask equipped with a condenser. The flask
was evacuated under vacuum and flashed with argon three times,
followed by addition of anhydrous THF (200 mL). Then the mixture
was cooled to −78 °C and TiCl4 (10.0 mL, 0.18 mol) was added
dropwise. The mixture was slowly warmed to room temperature,
stirred for 30 min, and then refluxed overnight. After cooling down to
room temperature, the reaction was quenched by 10% aqueous K2CO3
solution, and after vigorous stirring for 5 min, the dispersed insoluble
material was removed by vacuum filtration using a Celite pad. The
organic phase was separated and the aqueous layer was extracted three
times with diethyl ether (60 mL × 3). The combined organic fractions
were washed with saturated NaCl solution and dried over MgSO4. The
solvent was removed to give the crude 4,4′-(1,2-diphenylethene-1,2-
diyl)diphenol (TPE-2OH).46
of AIE luminogens are thus limited in aggregate state. To
further promote their potential applications, developing AIE
systems or synthesizing AIE polymers with AIE effect not only
in aggregate state but also in good solvent is a challenging topic.
Although star polymers, carrying AIE moieties either in the
core or in the arms have already reported,22−24 stars with a
cross-linked AIE core have never been synthesized/investigated
until now. Furthermore, the ATRP “arm-first” strategy,
developed by Matyjaszewski, Sawamoto, Chen, Qiao, et al.
for a variety of styrenic and (meth)acrylic star polymers,25−31
was applied only in one case of PE stars. The ATRP of
divinylbenzene was initiated by 2-bromoisobutyryl end-capped
and branched PE-macroinitiators, the latter being synthesized
by Pd-catalyzed polymerization of ethylene.32−34
In contrast to the polymerization of ethylene by coordination
metal complexes, polyhomologation, an organoborane-initiated
polymerization of dimethylsulfoxonium methylide,34 has been
proven as an efficient tool to synthesize well-defined and
perfectly linear hydroxyl-terminated polymethylene (PE ana-
logue), as well as other PE-based architectures in combination
with other polymerization techniques.35−40 Recently, we
developed a “core-first” strategy by using a functionalized B-
thexylboracycle initiator for the synthesis of 4-arm star PE and
in combination with ATRP for the synthesis of PE-based 3-
miktoarm stars.41−43
Herein, we wish to report the synthesis via “arm-first”
strategy of core cross-linked multiarm stars made of polystyrene
(PS), polyethylene (PE) or polyethylene-b-poly(ε-caprolac-
tone) (PE-b-PCL) arms, and a cross-linked TPE-2St core. The
cross-linking reaction was carried out by ATRP, using TPE-2St
as cross-linker and the above-mentioned Br-terminated
precursors as macroinitiators. These macroinitiators were
themselves synthesized by ATRP, polyhomologation and
ROP. As the rigid cross-linked structure of P(TPE-2St) core
significantly inhibited the intramolecular motions of TPE’s
phenyls, the synthesized star polymers were highly emissive not
only in the aggregate state but also in the dilute solutions.
Because of the highly compact nanoscale three-dimensional
globular structure, the star polymers exhibited temperature
responsive fluorescence.
Without purification, the crude TPE-2OH was dissolved in
acetonitrile (150 mL). Then K2CO3 (0.20 mol, 28 g) and 4-
vinylbenzyl chloride (0.060 mol, 9.3 mL) were added under argon, and
the mixture refluxed for 5 h. After filtration and removal of solvent, the
residue was chromatographed over silica gel (dichloromethane/
petroleum ether: 1/5) to give main product E-TPE-2St as white
solid (8.3 g, 70% yield in two steps). E-TPE-2St: 1H NMR (500 MHz,
CDCl3) δ 4.96 (s, 4H), 5.27 (2H, d, J = 10.0 Hz), 5.78 (2H, d, J = 20.0
Hz), 6.71−6.77 (6H, m), 6.93−6.95 (3H, m), 7.04−7.15 (10H, m),
7.36−7.37 (3H, m), 7.42−7.44 (4H, m). 13C NMR (100 MHz,
CDCl3) δ 69.6, 113.9, 114.0, 114.1, 126.2, 126.3, 127.5, 127.8, 131.4,
132.5, 136.4, 136.5, 136.7, 137.3, 139.7, 144.1, 157.2. HRMS (ESI):
calcd for C44H37O2 [M + H]+, m/z 597.2794; found, m/z 597.2766.
Synthesis of Linear Polyethylene Macroinitiator PE−Br. PE−
OH was prepared according to Shea’s work.36 A 500 mL flask, charged
with dimethylsulfoxonium methylide solution (165 mL, 0.6 M in
toluene), was heated to 60 °C with stirring under argon. Then
triethylborane solution (0.4 mL, 1.0 M in THF) was rapidly injected
via syringe. After 10 min, a drop of the mixture was taken out and
added to water containing phenolphalein. The neutral solution
indicated the complete consumption of the ylide. Then TAO·2H2O
(0.80 g) was added, and the reaction stirred at 80 °C for 5 h. The
mixture was concentrated and poured into cold methanol (300 mL).
Filtration of the precipitate, washing with methanol, and drying under
vacuum afforded a white solid (1.3 g, Mn,NMR = 1460, Mn,theor = 1260,
EXPERIMENTAL SECTION
■
Materials. 4-Hydroxybenzophenone (98%), zinc powder, titanium
tetrachloride (TiCl4, ≥ 98%), 4-vinylbenzyl chloride (90%), potassium
carbonate (anhydrous, ≥ 99.0%), copper(I) bromide (CuBr,
99.999%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA,
99%), triethylborane solution (1.0 M in THF), trimethylamine N-
oxide dihydrate (TAO·2H2O) (≥99%), ethyl α-bromoisobutyrate
(98%), phosphazene base P2-tBu solution (∼2 M in THF), 2-
bromoisobutyryl bromide (BIBB, 98%), and pyridine (99.8%) were
purchased from Aldrich and used as received. Styrene (St, ≥ 99%), and
ε-caprolactone (CL, 97%) were distilled over calcium hydride (CaH2)
under reduced pressure. Tetrahydrofuran (THF) and toluene were
refluxed over sodium/benzophenone and distilled under a nitrogen
atmosphere just before use. Acetonitrile (CH3CN) was used as
received. Dimethylsulfoxonium methylide was prepared according to
the Corey’s method followed by switching the solvent from THF to
toluene.44 PS−Br was prepared according to the typical ATRP
procedure initiated by ethyl α-bromoisobutyrate with CuBr/PMDETA
as catalyst.45
Measurements. The 1H and 13C NMR spectra were recorded with
a Bruker AVANCE III-400, 500, or 600 spectrometer. The molecular
weight (Mn,GPC) and molecular weight distribution (Đ) of linear and
star polystyrenes were measured by gel permeation chromatography
(GPC) (Viscoteck 305 instrument with two columns of Styragel HR2
1
Đ = 1.15). H NMR (600 MHz, CDCl2CDCl2, 90 °C): δ 0.91−0.96
B
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Scheme 1. Reaction Steps for the Synthesis of Cross-Linker TPE-2St
Scheme 2. Synthetic Routes for Macroinitiators (a) PS−Br, (b) PE−Br, and (c) TBDMSO-PE-b-PCL−Br and Core Cross-
Linked Multiarm Star Polymers (d) (PS)n-P(TPE-2St) or (PE)n-P(TPE-2St) and (e) (TBDMSO-PE-b-PCL)n-P(TPE-2St)
(9H, m), 1.34 (204H, brs), 1.59−1.63 (2H, m), 3.66 (2H, t, J = 6.0
Hz).
set at 100 °C to start the polymerization under stirring. After 25 h, the
polymerization was stopped by cooling in a liquid nitrogen bath. The
cloudy solution was diluted with toluene (∼5 mL), heated to clear and
poured into cold methanol (300 mL) with stirring. The solid was
filtered, washed with methanol, dried under vacuum, and characterized
PE−OH (1.0 g, 0.68 mmol of OH) and toluene (40 mL) were
placed into a 100 mL Schlenk flask and the mixture was kept under
vigorous stirring at 100 °C under Ar. Pyridine (0.49 mL, 6.0 mmol)
and 2-bromoisobutyryl bromide (0.90 mL, 7.2 mmol) were added
dropwise. After being stirred for 12 h, the reaction mixture was cooled
to room temperature and poured into 300 mL of acidic methanol
(containing 30 mL of 1 M aqueous HCl). The solid was filtered,
washed successively with methanol (2 × 20 mL), 1 M aqueous HCl (2
× 10 mL), and methanol (2 × 20 mL), and dried at 50 °C for 3 h in
vacuum to give an off-white solid (0.80 g, Mn,NMR = 1610, Đ = 1.16).
1H NMR (600 MHz, CDCl2CDCl2, 90 °C): δ 0.90−0.95 (9H, m),
by 1H NMR and HT-GPC (0.60 g, Mn,GPC‑LS = 875.5 × 103, ĐGPC‑LS
=
2.38).
The synthetic procedures of (PS)n-P(TPE-2St) and P(TBDMSO-
PE-b-PCL)n-P(TPE-2St) were similar to that of (PE)136-P(TPE-2St).
RESULTS AND DISCUSSION
■
Cross-Linker: Synthesis and Photoluminescence. The
AIE cross-linker TPE-2St was synthesized according to the
steps shown in Scheme 1.
1.33 (204H, brs), 1.70−1.75 (2H, m), 1.97 (6H, s), 4.21 (2H, t, J = 6.0
Hz).
The first step was the McMurry coupling reaction of 4-
hydroxybenzophenone to afford dihydroxyl functionalized
tetraphenylethylene (TPE-2OH). Crude TPE-2OH can be
used for the next step without purification. The second step was
the Williamson ether reaction between the hydroxyl groups of
TPE-2OH and 4-vinylbenzyl chloride which afforded tetraphe-
nylethylene having two styrene groups (TPE-2St) (70% final
yield in the two steps). To avoid the contamination of TPE-2St
with unreacted 4-vinylbenzyl chloride, it was carefully purified
by silica gel chromatography.
The photoluminescence (PL) of TPE-2St in THF and THF/
H2O mixtures was investigated (Figure S1, Supporting
Information). For TPE molecule, the restriction of intra-
molecular rotation (RIR) of the phenyl rotors in the aggregates
has been demonstrated experimentally and theoretically as the
main source for the AIE effect.47−51 Thus, only a slight
fluorescent signal at 474 nm could be detected in dilute THF
solution or in aqueous mixtures with fw 0−70% water content.
Synthesis of Linear Diblock Macroinitiator TBDMSO-PE-b-
PCL−Br. As described in our previous work, TBDMSO-PE−OH
(TBDMSO-: tert-butyldimethylsilyoxyl) was prepared from allyl
alcohol via three steps including protection of allyl alcohol,
hydroboration and polyhomologation reactions.39 Then ring-opening
polymerization of ε-caprolactone with TBDMSO-PE−OH as macro-
initiator and phosphazene base P2-tBu as catalyst, TBDMSO-PE-b-
PCL−OH was obtained.43 TBDMSO-PE-b-PCL−OH was converted
to TBDMSO-PE-b-PCL−Br by reacting with 2-bromo-2-methylpro-
pionyl bromide in the presence of pyridine in toluene at 100 °C as
more details described above.
Synthesis of Core Cross-Linked Multiarm Star Polymers. As
an example the synthesis of (PE)136-P(TPE-2St) star is given. CuBr
(14 mg, 0.10 mmol), TPE-2St (0.60 g, 1.0 mmol), PE−Br (0.16 g,
0.10 mmol, Mn,NMR = 1610), and toluene (6.0 mL) were placed into a
100 mL Schlenk flask. The mixture was subjected to three freeze−
pump−thaw cycles and then PMDETA (42 μL, 0.20 mmol) was
added and the mixture was subjected to another one freeze−pump−
thaw cycle. The solution was immediately immersed into an oil bath
C
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Table 1. Molecular Characterization Results of Linear Macroinitiators (MIs)
a
a
b
b
entry
MI
Mn,NMR (kg/mol)
DP
54
Mn,GPC (kg/mol)
Đ
1
2
3
4
PS−Br
PE−Br-1
PE−Br-2
5.7
2.5
1.6
6.6
6.8
6.4
1.2
7.2
1.09
1.11
1.16
1.23
166
103
c
d
TBDMSO-PE-b-PCL−Br
108, 45
a
b
Mn,NMR and degree of polymerization (DP) were calculated from 1H NMR spectra. Mn,GPC, Đ = Mw/Mn, determined by GPC (for PS−Br: THF, 35
°C, PS standards; for PE−Br and TBDMSO-PE-b-PCL−Br, trichlorobenzene, 150 °C, PS standards). TBDMSO-: tert-butyldimethylsilyoxyl. DPPE
c
d
= 108; DPPCL = 45
a
Table 2. ATRP Reaction Conditions and Molecular Characteristics of Synthesized Core Cross-Linked Star Polymers
[MI]
(mM)
time
(h)
Mn,RI
(kg/mol)
Mw,GPC‑LS
c
b
b
c
c
d
entry
MI
[TPE-2St]:[MI]
ĐRI
(kg/mol)
ĐGPC‑LS Rh (nm)
Narm
1
2
3
4
5
6
7
8
PS−Br
PS−Br
PS−Br
PE−Br-1
PE−Br-2
PE−Br-2
TBDMSO-PE-b-PCL−Br
TBDMSO-PE-b-PCL−Br
27.5
20.0
12.5
12.5
16.7
16.7
12.5
12.5
22
22
64
65
25
25
22
23
10:1
10:1
5:1
gel
40.9
37.0
36.8
26.4
25.1
93.9
1.77
1.37
1.21
2.36
2.92
1.97
e
661.7
2.20
1.84
e
11.5
8.4
e
54.9
14.3
e
128.8
10:1
10:1
13:1
13:1
10:1
e
875.5
2008
e
2.38
2.01
e
11.1
14.6
e
136
262
e
e
e
e
e
e
a
b
Polymerization conditions: toluene, 90 °C, [CuBr]:[PMDETA]:[MI] = 1:2:1. Mn,RI and ĐRI, determined by GPC (entries 2 and 3) or HT-GPC
(entries 4−7) RI detector. Mw,GPC‑LS, ĐGPC‑LS and Rh (hydrodynamic radius) determined by triple-detection GPC (entries 2 and 3) or HT-GPC
(entries 5 and 6). Average number of arms: Narm = (MW,star × armwt %)/Mn,NMR,arm. Not measured due to the poor solubility.
c
d
e
When a large amount of water was added into the THF
solution the emission intensity was boosted to ∼75-fold (fw =
80%), and to ∼300-fold (fw = 90%) of that of pure THF
solution, caused by the aggregation and the resulting restriction
in intramolecular rotation.
triple-detection GPC) and average number of arms (Narm) 54.9
after 22 h reaction (Table 2, entry 2; for GPC traces and NMR
spectra, see Figure S2 and S3). The weight fraction of PS arms
(armwt %) was determined by 1H NMR. The average number of
arms of the star polymer was calculated using the following
equation: Narm = (MW,star × armwt %)/Mn,NMR,arm. By decreasing
further the macroinitiator concentration to 12.5 mM and with a
cross-linker to macroinitiator ratio of 5:1, longer time (65 h)
was needed to form a soluble star polymer which exhibited a
molecular weight of 128.8 kg/mol and a Narm of 14.3 (Table 2,
entry 3; for GPC traces, see Figure S4).
Macroinitiators. Different kinds of linear macroinitiators
were synthesized via living and controlled/living polymerization
techniques. PS macroinitiator (PS−OH) was prepared through
ATRP of styrene initiated by ethyl α-bromoisobutyrate
(Scheme 2a). Polyhomologation of dimethylsulfoxonium
methylide initiated by triethylborane led to hydroxyl terminated
PE, which was converted to PE−Br ATRP macroinitiator by
esterification using 2-bromoisobutyryl bromide (Scheme 2b).
TBDMSO-PE−OH was prepared via polyhomologation
reaction initiated by tri[3-(tert-butyldimethylsilyloxyl)propyl]-
borane,39 followed by ring-opening polymerization of ε-
caprolactone from TBDMSO-PE−OH as macroinitiator and
phosphazene base P2-tBu as catalyst. TBDMSO-PE-b-PCL−
OH was further transformed to ATRP macroinitiator
TBDMSO-PE-b-PCL−Br by reaction with 2-bromoisobutyryl
bromide (Scheme 2c). The 1H NMR and GPC/HT-GPC
characterization results, summarized in Table 1, show the well-
defined nature of all synthesized macroinitiators.
Upon using the same initial concentration of 12.5 mM for
PE−Br macroinitiator, it was found that most of the PE−Br
remained unreacted and subsequent separation of the star
polymer from the linear PE was difficult (Table 2, entry 4 and
Figure S5). However, upon increasing the MI concentration
from 12.5 to 16.7 mM, PE star polymers with different
molecular weights (875.5 kg/mol and 2008 kg/mol) could be
successfully synthesized, under different TPE-2St to MI ratios
(10:1 and 13:1, respectively) (Table 2, entry 5 and 6). From
the GPC traces it is evident that most of the PE−Br was
consumed and the star polymer was monomodal (Figure 1). As
1
shown in the H NMR spectra (Figure 2), both signals due to
Core Cross-Linked Star Polymers. The general con-
ditions allowing the synthesis of core cross-linked multiarm star
polymers (PS)n-P(TPE-2St), (PE)n-P(TPE-2St), and
(TBDMSO-PE-b-PCL)n-P(TPE-2St) are shown in Scheme 2d
and 2e.
Initially, PS−Br was chosen as model macroinitiator to
investigate the reactivity of TPE-2St in the “arm-first”
methodology. The concentration of PS−Br was found to
have a significant impact on the extent of star formation.
Gelation occurred after 22h for a macroinitiator concentration
of 27.5 mM and a cross-linker to macroinitiator ratio of 10:1
(Table 2, entry 1). Upon decreasing the PS−Br concentration
from 27.5 to 20.0 mM, a soluble star polymer was obtained
with a molecular weight of 661.7 kg/mol (determined with
PE arms and P(TPE-2St) core could be identified. Two other
star polymers were synthesized from linear diblock MI
TBDMSO-PE-b-PCL−Br, but their absolute molecular weights
could not be determined by triple detection GPC due to the
weak RI signal and poor solubility (Table 2, entry 7 and 8;
Figure S6). As shown in Figure 3, star-block copolymers could
also be successfully synthesized, as indicated by the signals
corresponding to poly(TPE-2St), PE and PCL in the 1H NMR
spectra.
DSC Characterization. The DSC traces for the linear PE,
PE-b-PCL macroinitiators and for the corresponding star
polymers are shown in Figure 4. The linear PE with Mn,NMR
1600 exhibited a melting temperature (Tm) of 109 °C. For
(PE)136-P(TPE-2St) star polymer, the melting endotherm due
D
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chains, respectively. For the first PE-b-PCL star polymer
(sample of Table 2, entry 7), a broad melting endotherm in a
temperature range of 74 to 104 °C with a peak at 93 °C was
observed along with a weak glass transition at 123 °C. For
another sample (sample of Table 2, entry 8), only a broad peak
range from 75 to 106 °C with Tm equal to 98 °C was found.
Indicated by the disappearance of the melting endotherm in
both samples, steric crowding probably prevented the move-
ments of chains which in turn prevented the PCL segments
from crystallizing. On the other hand, the PE arms of (PE-b-
PCL)s are in the periphery and not as sterically hindered as the
arms of PCL, but their flexibility and regularity were reduced,
which resulted in a lower Tm. Considering the highly restricted
mobility of cross-linked core, the observed high Tg could be
attributed to the P(TPE-2St)n core. In the case of (PS)14.3
-
P(TPE-2St) (Tg: 106 °C) and (PS)54.9-P(TPE-2St) (Tg: 110
°C), the glass transition of PS segments shifted to higher
temperatures in comparison to that of linear PS−Br (Tg: 96
°C) due to the increase of arm number and resulting reduced
mobility (Figure S7).
Figure 1. HT-GPC (TCB at 150 °C) traces of linear PE−Br-2 and
(PE)136-P(TPE-2St), (PE)262-P(TPE-2St) stars. (The negative peak of
PE−Br-2 is due to the negative DRI of PE.)
Aggregation-Induced Emission. The emission character
of the solutions of the synthesized star polymers was then
investigated. As shown in Figure 5, parts a and b, one of the
sample investigated [(PE)136-P(TPE-2St)] was weakly emissive
with an almost flat peak in the emission spectra at the
concentration of 0.001 g/L in THF. When the concentration
to PE segments at 109 °C almost disappeared. With nearly
double the number of arms, the melting endotherm vanished
for (PE)262-P(TPE-2St). Because of the increasing steric
crowding in the PE shell, caused by an increased number of
arms, the segmental mobility of the PE chains is reduced.32 In
the case of TBDMSO-PE-b-PCL−Br, two melting points at 55
and 109 °C were found corresponding to the PCL and PE
increased from 0.001 to 0.1 g/L, the emission intensity at λem
=
484 nm gradually increased by ∼53.1-fold compared to the
initial intensity (0.001 g/L). Unexpectedly, the emission
Figure 2. 1H NMR (600 MHz) spectra of (a) TPE-2St in chloroform-d at 25 °C and (b) PE−Br and (PE)n-P(TPE-2St) in 1,1,2,2-
tetrachloroethane-d2 at 90 °C.
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1
Figure 3. H NMR (600 MHz) spectra of TBDMSO-PE-b-PCL−OH, TBDMSO-PE-b-PCL−Br, and (TBDMSO-PE-b-PCL)n-P(TPE-2St) in
toluene-d8 at 90 °C.
concentration, the emission character of the (PE)136-P(TPE-
2St) toluene solution was investigated, since the star polymer
had a good solubility in toluene (Figure S8). A similar tendency
in toluene was observed as in THF. The same phenomenon
was also found for another sample (PS)54.9-P(TPE-2St)
exhibiting a good solubility in THF (Figure S9). DLS results
revealed only a slight shift of the mean diameter once the
concentration increased (Figure S9). Thus, the aggregation is
not the reason for the emission quenching. Moreover, it was
observed that only the top layer of the high concentration
solution exhibited a strong emission under UV as illustrated in
the photos of Figure 5c, S8, and S9, and other parts of the
solution had slight emission since the excitation light was
incapable of reaching these sites. Hence, the most possible
reason is the inner filter effect because of the high
concentrations of absorbing molecules.53 The intensity of the
excitation light is not constant through the polymer solution
Figure 4. DSC thermograms of synthesized PE-based macroinitiators
and star polymers.
and only a small percentage of the excitation light reaches the
TPEs that are visible for the detection system.54
To avoid the detrimental inner filter effect, dilute rather than
high concentration solutions should be employed to study the
photophysical properties of the star polymer solutions. Thus,
the AIE behaviors of (PE)136-P(TPE-2St) were investigated in
dilute THF (solvent)/n-hexane(nonsolvent) mixture with a
concentration of 0.1 g/L. As shown in parts a and b of Figure 6,
addition of n-hexane into the THF solution induced the
molecules to aggregate and gradually enhanced its PL
intensities. The highest emission intensity was observed in 90
vol % mixtures and its intensity enhanced about 5.6-fold
compared with that of its THF solution. When the volume
content of n-hexane ranged from 0 to 70%, the transmittance of
the solution stayed the same as 100% which is an indication of
intensity dropped to 14% of the highest intensity (0.1 g/L)
upon increasing the concentration further from 0.1 to 1 g/L.
Emission intensities remained in a range between 14 and 20%
of the highest intensity for concentrations ranging from 1 to 5
g/L. Moreover, the transmittance of the solution dropped from
100% to ∼80% when the concentration increased from 0.1 to 1
or 2 g/L. Less than 30% transmittance was observed in the
solutions with concentrations ≥3 g/L which can be attributed
to the poor solubility of PE arms in THF.
As reported by Tang, poly(naphthopyran)s with TPE units
anchored on the backbone underwent “ACQ” in the aggregated
state.52 To clarify if it was indeed “ACQ” or not at high
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Figure 5. (a) Photoluminescence (PL) spectra of (PE)136-P(TPE-2St) in THF at different concentrations as indicated measured at room
temperature. (b) Plot of normalized PL intensity and transmittance versus concentration. (c) Photographs of (PE)136-P(TPE-2St) in THF at 0.01,
0.1, and 1 g/L concentrations with UV irradiation (365 nm) from the top.
Figure 6. (a) Photoluminescence (PL) spectra of (PE)136-P(TPE-2St) in THF/n-hexane mixtures with different n-hexane fractions at a
concentration of 0.1 g/L. (b) Plot of I/I0 and transmittance versus n-hexane fraction in the THF/n-hexane mixture; I is PL intensity in THF/n-
hexane mixtures with different n-hexane fractions, I0 is PL intensity in pure THF solution (inset: photographs of THF/n-hexane mixtures of polymer
with 0 and 90 vol % n-hexane fractions taken under room light). (c) Photographs of THF/n-hexane mixtures of (PE)136-P(TPE-2St), and THF/H2O
mixtures of TPE-2St with 0, 50, and 90 vol % n-hexane or H2O fractions at a concentration of 0.1 g/L taken under 365 nm UV illumination. (d)
Images of solid powder of (PE)136-P(TPE-2St) taken under room light and under 365 nm UV irradiation.
good solubility of PE star in the THF/n-hexane mixture.
Transmittance 83% was observed in 80 vol % of n-hexane, and
79% transmittance in 90 vol % of n-hexane, along with the
increase of PL intensity as the PE star aggregated.
It is noteworthy that a strong emission was observed from
the (PE)136-P(TPE-2St) in pure THF or in THF/n-hexane
mixtures with low fraction of the nonsolvent n-hexane. In sharp
contrast, small molecular luminogen TPE-2St displayed high
emission only with a large amount of water (nonsolvent) (≥80
vol %) was added to the THF solutions (Scheme 1S). As
exemplified by the images in Figure 6c, the differences in
emission were obvious between the star polymer and cross-
linker TPE-2St in THF solutions with 0 and 50 vol %
nonsolvent (n-hexane or water) fractions. The reason for the
nonemission of TPE-2St in dilute THF solutions is the active
intramolecular rotations of the TPE components. In the case of
PE stars, the core cross-linked structure restricts the intra-
molecular rotation which forcefully blocked the nonradiative
path and activated radiative decay, resulting in the emission
enhancement of the star polymer in dilute solutions. As for the
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Figure 7. (a) PL spectra of (PE)136-P(TPE-2St) in 50/50 vol % THF/n-hexane mixture at a concentration of 0.1 g/L with temperature decreasing
from 55 to 5 °C by a step size of 2.5 °C. (b) Plot of normalized PL intensity versus temperature, I (5 °C) = 1.0.
aggregate state that we can see from Figure 6d, the solid
powder of (PE)136-P(TPE-2St) was white color in appearance
under normal room lighting within the lab, but emitted a strong
greenish-blue light under UV irradiation at 365 nm.
For other star polymers (PS)n-P(TPE-2St) and (TBDMSO-
PE-b-PCL)n-P(TPE-2St), they showed the emission in dilute
THF solutions and addition of water to their THF solution also
resulted in a rapid increase of PL intensities (Figure S10 and
S11). The rubbery structure played a key role for their
luminescent properties especially in dilute solutions, although
these polymers possess different kinds of arms.
strategy by ATRP of the corresponding macroinitiators with
AIE TPE-2St, used as a cross-linker. Polyhomologation was
utilized for the synthesis of PE-based macroinitiators. All
synthesized star polymers exhibited AIE-characters either in
bulk or in solution. The emission of these polymers in dilute
solutions is caused by the restriction of the intramolecular
rotation induced by the core cross-linked structure. Because of
their nearly spherical shape, the star polymers exhibited
temperature responsive emission in THF/n-hexane mixtures,
which increasing linearly with decreasing temperature from 40
to 5 °C. We hope the presented results will open new horizons
toward a variety of novel luminescent core cross-linked star
polymers. Further research on pH-responsive core cross-linked
miktoarm stars is underway.
Temperature Response. Next, the photoluminescence of
(PE)136-P(TPE-2St) star polymer in 50/50 vol % of THF/n-
hexane mixture was investigated under varying temperatures
(Figure 7). Interestingly, PL intensity increased when
decreasing the temperature of the mixture from 47.5 to 5 °C,
but remained almost unchanged from 55 to 47.5 °C. Notably,
within the responsive window of 40−5 °C, apparent linear
relationship of PL intensity versus temperature was observed.
The same linear decrease also occurred with another sample
(PE)262-P(TPE-2St) star polymer in THF/n-hexane with 50 or
70 vol % n-hexane fraction (Figure S12). A similar
phenomenon was reported by Zhao et al. for poly(N-
isopropylacrylamide) (PNIPAM)-based core/shell hydrogel
nanoparticles, and their red-emission rare-earth complex and
blue-emission quaternary ammonium tetraphenylethylene
derivative (d-TPE), which also exhibited a linear temperature
response in the range from 10 to 80 °C.55 They ascribed such a
behavior to the shrinkage of the shell with a temperature
decrease, which led to the inhibition of intramolecular rotations
and vibrations of d-TPE molecules. The inhibition of
intramolecular motions enhanced the PL emission. In our
case, due to the nearly spherical shape,56 TPE molecules may
have experienced a shrinkage of arms and be immobilized at
low temperature (5−45 °C) similar to the core/shell hydrogel
nanoparticles. At a high temperature (47.5−55 °C), the star
polymer may have relaxed and intramolecular motions of TPE
favored at the same level resulting in the almost same PL
emission.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b00506.
Figure S1−S12 showing NMR, GPC, DSC, and
photoluminescence (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*(N.H.) Telephone: + 966-(0)12-8080789. E-mail: nikolaos.
hadjichristidis@kaust.edu.sa.
ORCID
Panayiotis Bilalis: 0000-0002-5809-9643
Yves Gnanou: 0000-0001-6253-7856
Nikos Hadjichristidis: 0000-0003-1442-1714
Present Address
§College of Science, Shantou University, Shantou, Guangdong,
515063 P. R. China.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CONCLUSION
■
■
In summary, core cross-linked multiarm star polymers with PS,
PE, or PE-b-PCL arms were synthesized via the “arm-first”
Research reported in this publication was supported by King
Abdullah University of Science and Technology (KAUST).
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