Luminescent polyethylene with side-chain triarylboranes: Synthesis and fluoride sensing properties

Article abstract of DOI:10.1016/j.polymer.2011.02.010

The direct copolymerization of ethylene with an electron-deficient borane monomer (1) using a single-site catalytic system, Me₂Si(η ⁵-C₅Me₄)(η¹-N- ^tBu)TiCl₂ (CGC)/methylaluminoxa

Full text of DOI:10.1016/j.polymer.2011.02.010

Polymer 52 (2011) 1510e1514
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Polymer
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Luminescent polyethylene with side-chain triarylboranes: Synthesis and fluoride
sensing properties
,
b,
*
Myung Hwan Park a, Taewon Kim b, Jung Oh Huh a, Youngkyu Do a , Min Hyung Lee
**
a Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
b Department of Chemistry and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The direct copolymerization of ethylene with an electron-deficient borane monomer (1) using a single-
site catalytic system, Me2Si(
5-C5Me4)( 1-N-tBu)TiCl2 (CGC)/methylaluminoxane (MAO) efficiently leads
Received 24 December 2010
Received in revised form
1 February 2011
Accepted 6 February 2011
Available online 12 February 2011
h
h
to the novel class of luminescent polyethylene with pendent triarylborane groups. The catalytic system
provides high-molecular-weight copolymers with controlled incorporation of 1. Characterization by
multinuclear NMR spectroscopy reveals that the copolymers constitute a well-defined polymer structure
with base-free tricoordinate boron centers. While UVevis absorption and PL spectra of copolymers
display almost identical absorption and emission features with those of 1, the copolymer (8.8 mol% of 1)
shows the enhanced emission quenching efficiency with 70% amplification effect upon addition of
fluoride ion in solution. Particularly, the thin film of the copolymer exhibits the high fluorescence
Keywords:
Metallocene catalysts
Polyethylene
quenching response in the presence of a very low concentration of fluoride ion (4.5 mM) in THF.
Triarylborane
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
incorporated into the polymer backbone by such catalytic systems
via a highly controlled manner [31e35]. In particular, Chung et al.
have previously shown that the Lewis acidic trialkylborane
monomers are compatible with the cationic active species of group
4 metal complexes and the cocatalysts like methylaluminoxane
(MAO) [36e38]. It may be thus expected that the tailored polymer
architecture with a controlled amount of borane moiety gives us
more detailed insight into the polymer properties such as signal
amplification effect [39] in anion sensing that might occur in this
type of polymer [14]. In addition, the excellent physical/mechanical
properties derived from a polyolefin backbone structure could
further lead to a great improvement in processibility, stability, and
film-forming ability required for the practical applications.
Electron-deficient organoboron polymers have received great
attention as promising materials in the various applications such as
organic light emitting devices [1e6], nonlinear optics [7,8] n-type
conductors [9e11], and anion sensors [12e16]. While the majority of
research efforts have been made on the conjugated main-chain
polymers [2,15,17e24], the side-chain organoboron polymers have
recentlyattracted focus due to the advantages in controlling polymer
architecture in terms of molecular weight, functionality and degree
of functionalization that may lead to intriguing properties [25].
Regarding to this, Jäkle et al. elegantly demonstrated that highly
Lewis acidic triaylborane moieties can be facilely introduced into the
polymer backbone via side-chain modifications [3,5,14,26e30].
However, a direct polymerization using borane monomers has rarely
been investigated although it may constitute a simple and straigh-
forward route toward various side-chain boron polymers [6,8].
For this purpose, the use of single-site catalytic systems based
on group 4 metal complexes could be considered as a viable
approach for the preparation of polyolefins with pendent triar-
To this end, we set out to explore the direct copolymerization of
ethylene with
electron-deficient borane group using [Me2Si(
u
-triarylboryl-
a
-olefin comonomer bearing an
h
5-C5Me4)( 1-N-tBu)
h
TiCl2] (CGC)/MAO catalytic system to develop a novel class of
polyethylene-based luminescent boron polymers. Herein, the
synthesis, characterization, and fluoride sensing properties of the
polymers are described.
ylborane groups as it is well known that a variety of
a-olefins
including sterically hindered functional olefins can directly be
2. Experimental
2.1. Materials
* Corresponding author. Tel.: þ82 52 259 2335; fax: þ82 52 259 2348.
** Corresponding author. Tel.: þ82 42 350 2829; fax: þ82 42 350 2810.
E-mail addresses: ykdo@kaist.ac.kr (Y. Do), lmh74@ulsan.ac.kr (M.H. Lee).
Anhydrous grade solvents (Aldrich) were dried by passing
through an activated alumina column and stored over activated
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.02.010

M.H. Park et al. / Polymer 52 (2011) 1510e1514
1511
molecular sieves (5 Å). Commercial reagents were used without
any further purification after purchasing from Aldrich (Mes2BF,
n-BuLi (2.5 M solution in n-hexanes), n-Bu4NF$3H2O (TBAF),
1-octene, chlorobenzene, o-dichlorobenzene (ODCB)) and TCI
(1,1,2,2,-tetrachloroethane, TCE). 1-Bromo-4-(oct-7-en-1-yl)ben-
10% to avoid a drift in monomer composition in the feed. The reac-
tions were quenched by the injection of ca. 2 mL of EtOH. The
resultant mixture was then poured into the large volume of acidified
i-PrOH (2 vol.%, 500 mL) and stirred for 1 h. The precipitated polymer
was subsequently collected by filtration, and washed with EtOH
(3 ꢂ 50 mL). The resulting polymers were finally dried in a vacuum
oven at 70 ꢁC to constant weight.
zene [40] and Me2Si(h h
5-C5Me4)( 1-N-tBu)TiCl2 (CGC) [41] were
synthesized by the published procedures. Polymerization-grade
ethylene monomer from Honam Petrochemical Co. was used after
purification by passing through LabclearÔ and OxiclearÔ filters.
1-Octene was dried by passing through an activated alumina
column. Methylaluminoxane (MAO) was used as a solid MAO
obtained by evaporation of the solvent from a toluene solution of
PMAO (Chemtura, 30T). CDCl3 and 1,1,2,2,-tetrachloroethane-d2
(C2D2Cl4) from Cambridge Isotope Laboratories were used after
drying over activated molecular sieves (5 Å).
2.5. Polymer analysis
1H and 11B NMR spectra of the polymers were recorded on
Bruker Spectrospin 400 spectrometer (400.13 MHz for 1H,
128.38 MHz for 11B) at 100 ꢁC. 13C NMR spectra were obtained from
Bruker AMAX 500 (13C; 125.77 MHz) spectrometer at 120 ꢁC with
90ꢁ pulse angle, 2 s acquisition time, and 8 s relaxation delay. The
samples were dissolved in C2D2Cl4 (for 1H, ca. 5 mg and for 13C and
11B, ca. 50 mg in 0.5 mL) in 5 mm tubes. The comonomer contents in
the polymers were determined from 1H NMR spectra. Chemical
shifts are given in ppm, and are referenced against external Me4Si
(1H, 13C) and BF3$Et2O (11B). 13C NMR peak assignments of the
polymers in the aliphatic region were analogously made according
to the reported literatures [42,43]. The molecular weight (Mw and
Mn) and polydispersity index (PDI, Mw/Mn) of the polymers were
analyzed by high temperature gel-permeation chromatography
(GPC) on Polymer Laboratories PL 220 at 140 ꢁC in 1,2,4-tri-
chlorobenzene and calibrated using narrow polystyrene standards
as a reference. The melting transition temperatures (Tm) of the
polymers were measured by differential scanning calorimetry (DSC,
TA Instrument Q100) at a heating rate of 10 ꢁC/min. Any thermal
history in the polymers was eliminated by the first heating the
samples to 180 ꢁC at 20 ꢁC/min, cooling to ꢀ70 ꢁC at 10 ꢁC/min, and
then recording the second DSC scan from ꢀ70 ꢁC to 180 ꢁC. Ther-
mogravimetric analyses (TGA) were performed under N2 atmo-
sphere using a TA Instrument Q500 at a heating rate of 20 ꢁC/min
from 50 ꢁC to 800 ꢁC.
2.2. Measurements
NMR spectra of monomer were recorded on a Bruker Avance
400 spectrometer (400.13 MHz for 1H, 100.62 MHz for 13C,
128.38 MHz for 11B) at ambient temperature. Chemical shifts are
given in ppm, and are referenced against external Me4Si (1H, 13C)
and BF3$Et2O (11B). HR EI-MS measurement (JEOL JMS700) was
carried out at Korea Basic Science Institute (Daegu). UVevis and PL
spectra were recorded on a Jasco V-530 and a Spex Fluorog-3
Luminescence spectrophotometer, respectively, in TCE solvent with
a 1 cm quartz cuvette.
2.3. Synthesis of monomer (1)
1-Bromo-4-(oct-7-en-1-yl)benzene (4.00 g, 15.0 mmol) in
30 mL diethyl ether was treated with 1 equiv of n-BuLi (6.0 mL) at
ꢀ78 ꢁC. The reaction mixture was allowed to warm to room
temperature and stirred for 2 h. To the mixture was slowly added
1 equiv of Mes2BF (4.46 g based on 90% purity) in diethyl ether
(30 mL) at 0 ꢁC. After stirring overnight at room temperature, the
solvent was removed under reduced pressure and the residue was
extracted with n-hexane (2 ꢂ 40 mL). Following filtration and
evaporation of the solvent, the oily residue was purified by flash
column chromatography on silica (eluent: n-hexane/CH2Cl2 ¼ 4/1,
v/v), affording 1-dimesitylboryl-4-(oct-7-en-1-yl)benzene (1) as
2.6. UVevis absorption and photoluminescence measurements
Due to high toxicity of TCE solvent, great caution should be
exercised when handling and preparing solutions. UVevis absorp-
tion and PL measurements were performed in TCE solvent with
a 1 cm quartz cuvette. The TCE solvent was used after purification by
distillation. The polymers used for the measurements were purified
by precipitation from a hot ODCB solution into EtOH. The samples
were first dissolved in hot TCE and then diluted to the desired
concentrations based on the borane groups at ambient temperature.
Fluorescence titrations were performed by addition of incremental
amounts of fluoride anions (TBAF, 5.0 ꢂ 10ꢀ3 M) into the solution of
polymer and 1 (w10ꢀ6 M) in TCE, respectively. The fluorescence
intensity was monitored at lem ¼ 382 nm for polymer and 384 nm
for 1. The intensity data acquired were fitted to the SterneVolmer
equation ([I]0/[I] ¼ KSV[F] þ 1) to estimate the KSV. The thin film of
polymer was obtained by spin-coating a chlorobenzene solution
(0.2 wt%) onto the quartz slide at 2000 rpm for 30 s.
colorless oil (5.96 g, 91%). 1H NMR (CDCl3):
d 1.33 (m, 6H, 3,4,5-CH2),
1.59 (m, 2H, 2-CH2), 1.99 (s, 12H, MeseCH3), 2.03 (m, 2H, 6-CH2),
2.29 (s, 6H, MeseCH3), 2.62 (t, J ¼ 7.8 Hz, 2H, 1-CH2), 4.92 (dd,
J ¼ 10.2/1.6 Hz, 1H, CH2]CH), 4.97 (dd, J ¼ 17.2/1.6 Hz, 1H, CH2]
CH), 5.79 (ddt, J ¼ 17.2/10.2/6.7 Hz, 1H, CH2]CH), 6.80 (s, 4H,
MeseCH), 7.13 (d, J ¼ 7.8 Hz, 2H, PheCH), 7.41 (d, J ¼ 7.8 Hz, 2H,
PheCH). 13C NMR (CDCl3):
d 21.17 (MeseCH3), 23.40 (MeseCH3),
28.80, 28.91, 29.15 (3,4,5-CH2), 31.05 (2-CH2), 33.71 (6-CH2), 36.22
(1-CH2), 114.21 (CH2]CH), 128.05 (PheC3), 128.10 (MeseCH),
136.77 (PheC2), 138.29 (MeseCparaMe), 138.97 (CH2]CH), 140.71
(MeseCorthoMe), 141.85 (BeCMes), 142.99 (BeCPh), 147.42 (PheC4).
11B NMR (CDCl3):
436.3301; found, 436.3304.
d
þ73.3. HR EI-MS: m/z calcd for C32H41B,
2.4. Polymerization
3. Results and discussion
Into the 250 mL-glass reactor charged with a pre-weighed MAO
([Al]/[Ti] ¼ 2000) was transferred a toluene solution of a prescribed
amount of comonomer (49.5 mL), and the temperaturewas adjusted
to 75 ꢁC using an external bath. Ethylene monomer was then satu-
rated at 1 bar with vigorous stirring for at least 10 min after
degassing several times. Polymerizationwas started by the injection
3.1. Synthesis and characterization
The newly designed
u-borane substituted a-olefin, 1-dimesi-
tylboryl-4-(oct-7-en-1-yl)benzene (1) was employed as como-
nomer.1 was prepared in high yield (91%) by the reaction of Mes2BF
with the lithium salt derived from 1-bromo-4-(oct-7-en-1-yl)
benzene [40]. The copolymerization reactions of ethylene and 1
were achieved at different comonomer concentrations using
of a toluene solution of catalyst (0.5 mL, 1.0
mmol of Ti). The poly-
merization time was varied to keep comonomer conversion below

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M.H. Park et al. / Polymer 52 (2011) 1510e1514
1
into the polyethylene backbone gradually increases with
increasing comonomer concentration in the feed. Although much
inferior to that in the 1-octene copolymerization (EBO3 vs. EO3),
the level of comonomer incorporation upon varying the como-
nomer concentration in the feed is reasonably high, thus indicating
that the CGC catalyst allows the facile incorporation of 1 into the
polyethylene backbone despite the steric bulkiness of triarylborane
moiety at the 8-position of 1, as similarly found in other copoly-
merization using sterically hindered monomers [44]. The copoly-
merization parameters estimated from the FinemaneRoss plot
(r1 z 0.0685 for 1; rE z 12.4 for ethylene; r1ꢂrE z 0.85) further
reveal very low reactivity ratio of 1 as well as a nearly random
distribution of 1 in the polymer chain as shown from 13C NMR
spectrum. The melting transition temperatures (Tm) of EBO1
(99.1 ꢁC) and EBO2 (85.8 ꢁC) correlate well with the increasing
content of 1 while EBO3 and EBO4 bearing higher contents do not
show any apparent melting transition. It is noteworthy that the
copolymers show high thermal stability with the Td5 values over
300 ꢁC although the weight loss of all polymers appears to begin at
around 220 ꢁC. Particularly, the EBO3 and EBO4 possess the high
Td5 values (>400 ꢁC) comparable to that of poly(ethylene-co-1-
octene) [32]. This stability is likely to be delivered by not only the
polyethylene backbone, but also the steric protection of the boron
center by the two mesityl groups [14]. Despite the limited solubility
in common organic solvents, all the EBO copolymers are soluble in
the chlorinated solvents such as 1,1,2,2,-tetrachloroethane (TCE)
and o-dichlorobenzene (ODCB) at elevated temperature. The
copolymers are stable toward air and moisture either in solution or
in solid state.
Scheme 1. Synthesis of EBO copolymers with CGC/MAO catalyst.
a CGC/MAO catalytic system at 75 ꢁC (Scheme 1). As summarized in
Table 1, the copolymers (EBO1 w EBO4) are readily produced with
decreasing order of activity [(kg polymer)/((mol of cat.) h bar)] as
the comonomer concentration in the feed increases. The much
lower activity than that of 1-octene copolymerization (EBO3 vs.
EO3) under the identical conditions indicates that the catalytic
activity is largely affected by the steric bulkiness of the triarylboron
group in 1. The molecular weight of copolymers decreases with
increasing comonomer concentration in the feed and the narrow
PDI values indicate the involvement of single catalytic active
species.
The identity of the copolymers has been fully characterized by
multinuclear NMR spectroscopy. For example, the 1H, 13C, and 11B
NMR spectra of EBO3 are shown in Fig. 1. The 1H NMR spectrum
(bottom) indicates that the resonances at
corresponding to the protons at the 1 and 2 positions of the side-
chain, respectively, are well-resolved fromthe peaks ( 1.2e1.5 ppm)
from the polyethylene backbone. Three singlet resonances at
d 2.71 ppm and d 1.72 ppm
d
3.2. Fluoride sensing properties
d
6.84 ppm,
7.46 ppm and
d 2.35 ppm, and d 2.06 ppm as well as two doublets at
d
d
7.19 ppm (3J ¼ 7.2 Hz) are also distinctly observed
To examine the optical properties of the copolymers, UVevis
and PL experiments have been carried out using the solutions of
10ꢀ5 w 10ꢀ6 M concentrations based on the borane unit in TCE
solvent. For example, UVevis and PL spectra of EBO3 are compared
with those of monomer 1 in Fig. 2. The absorption spectrum of
for the protons in the triarylborane moieties. Inspection of the
aliphatic region of the 13C NMR spectrum (Fig. 1, middle) reveals
apparent resonances ascribable to the isolated comonomer units
although the very weak signals associated with sequential como-
nomer incorporation are concomitantly observed possibly due to
the moderate content of 1 (8.8 mol%) in EBO3. The 11B NMR signal
EBO3 shows a major absorption band at 313 nm (log
3
¼ 4.19)
assignable to the dominant epp(B) transition in the borane
p
detected at around
d 74 ppm (Fig. 1, top) further confirms the
moiety [14,45e50], which is almost identical with that of the
monomeric borane 1 (labs ¼ 311 nm). The invariant absorption
features indicate that the ground state is not affected by the exis-
tence of neighboring borane moieties. The PL spectrum also
exhibits a very similar emission feature with that of 1 showing the
fluorescence maximum wavelength at around 382 nm. There is no
indication of the excimer formation that has often been encoun-
tered in other side-chain luminescent polymers depending on the
content and type of luminophore [31,32,51]. In fact, all the EBO
copolymers display identical emission features in solution (Fig. 3).
presence of a base-free trigonal planar boron center being in parallel
with that observed in 1 (73 ppm). These spectroscopic features are
also consistently observed for all the EBO copolymers (Supporting
Information), implying that the triarylborane monomer
1 is
compatible with the CGC/MAO catalytic system.
The estimation of the comonomer contents from the 1H NMR
spectra which show essentially identical resonances except for
different intensities upon the change of comonomer concentration
in the feed for all the copolymers indicates that the incorporation of
Table 1
Polymerization results of ethylene and 1 with CGC/MAOa.
Entry
[1] (M)
tp (sec)
Yield (g)
Activityb (ꢂ10ꢀ3
)
10ꢀ3Mw (g/mol)
PDIc
Cont. of 1d (mol%)
11B (ppm)
c
e
Tm (ꢁC)
f
Td5 (ꢁC)
EBO1
EBO2
EBO3
EBO4
EO3g
0.027
0.054
0.108
0.162
0.108
90
240
120
180
44
0.174
0.395
0.121
0.100
0.155
7.0
5.9
3.6
2.0
12.7
227
156
101
38.3
129
2.57
2.31
1.50
1.54
2.06
2.8
5.0
8.8
12.2
21.7
99.1
85.8
n.oh
n.o
320
305
414
438
73.9
74.5
74.2
74.1
a
Conditions: CGC ¼ Me2Si(
h
5-C5Me4)(
h
1-N-tBu)TiCl2; MAO ¼ methylaluminoxane; PE ¼ 1 bar; Tp ¼ 75 ꢁC; [Ti] ¼ 1.0
mol; [Al]/[Ti] ¼ 2000; Solvent ¼ 50 mL of toluene.
m
b
c
Activity given in units of (kg polymer)/((mol of cat.) h bar).
Determined by GPC.
d
e
f
Determined by 1H NMR.
Determined by DSC.
Determined by TGA at 5% decomposition.
1-Octene comonomer.
Not observed.
g
h

M.H. Park et al. / Polymer 52 (2011) 1510e1514
1513
Fig. 1. 1H (bottom), an aliphatic region of 13C (middle), and 11B NMR (top) spectra of EBO3 (*from C2D2Cl4).
triarylborane moiety in EBO3 (Fig. 4) [12,52], i.e. the observed
fluorescence quenching of EBO3 could be related to static binding
of fluoride to the Lewis acidic boron center rather than collisional
quenching. The SterneVolmer constant (KSV) which quantifies the
quenching efficiency of receptor was estimated from the titrations
to be ca. 6.9 ꢂ 104
M
ꢀ1, indicating an efficient fluorescence
quenching. Comparison with the emission quenching constant for
the monomer 1 (KSV ¼ 4.0 ꢂ 104
M
ꢀ1) indicates an approximately
70% amplification effect for EBO3 (Fig. S7). Even though not
significant, such enhancement in the emission quenching might be
attributed to the possible formation of lower-energy aggregate
states upon fluoride complexation as suggested by Jäkle et al. [14].
They reported that the 8-fold large enhancement in the emission
quenching is observed for the polystyrene homopolymer contain-
ing triarylborane moieties [14]. In EBO3, it seems that the small
amount of borane groups (8.8 mol%) which affords well-separation
between the adjacent borane moieties is mainly responsible for the
slight enhancement in quenching efficiency.
Finally, by taking advantage of the good film-forming capability
and the low solubility in common organic solvents, the fluores-
cence response of thin film of EBO3 toward fluoride ion was tested
in THF. Prior to fluoride addition, it was first confirmed that the
spin-coated thin film of EBO3 exhibits stable blue emission in
THF (Fig. 5). Different from the PL spectrum obtained in solution,
the appearance of a lower-energy emission band with weak
Fig. 2. UVevis absorption (left) and PL (right) spectra of 1 and EBO3 in TCE.
This result might be ascribable to both the sterically bulky nature of
the borane moiety which should be unfavorable to the formation of
aggregates or excimers and the relatively low content of borane
moiety.
To explore the anion sensing properties of the polymers, the
fluorescence quenching experiments using fluoride anion have
been further carried out in a TCE solution of EBO3. Upon addition of
fluoride ion, the emission intensity at 382 nm gradually decrea-
ses as a result of the binding of fluoride to the boron atom of
Fig. 4. Spectral changes in the fluorescence of a solution of EBO3 (7.0 ꢂ 10ꢀ6 M, TCE)
upon addition of n-Bu4NF (0e2.5 ꢂ 10ꢀ5 M). The inset shows the experimental (-) and
the SterneVolmer plot (ꢀ) of the intensity change at 382 nm as a function of [Fꢀ]. The
Fig. 3. PL spectra of copolymers in TCE.
quenching constant is estimated to be KSV ¼ 6.9 ꢂ 104 Mꢀ1
.

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M.H. Park et al. / Polymer 52 (2011) 1510e1514
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We have demonstarted for the first time that the direct copoly-
merization of ethylene with an electron-deficient borane monomer
(1) using a single-site catalytic system (CGC/MAO) can lead to the
novel class of luminescent polyethylene with pendent triarylborane
groups. The catalytic system provided high-molecular-weight
copolymers with high activity. Multinuclear NMR spectroscopic
methods revealed that the copolymers constitute a well-defined
polymer structure. While UVevis absorption and PL spectra of
copolymers display almost identical absorption and emission
features with those of borane monomer, it was found that the
copolymer shows better emission quenching efficiency upon addi-
tion of fluoride ion than monomer in solution. In particular, the thin
film of copolymer exhibited high fluorescence quenching response
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This work was supported by the 2009 Research Fund of
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Supplementary material associated with this article can be
found online version at doi:10.1016/j.polymer.2011.02.010.