High refractive index hyperbranched polymers with different naphthalene contents prepared through thiol-yne click reaction using di-substituted asymmetric bulky alkynes

Article abstract of DOI:10.1016/j.polymer.2014.07.030

In the present study, different aromatic moieties, naphthalene and benzene, were introduced into alkynes to constitute 1,3,5-tris(naphthylethynyl)benzene as B₃ monomer. This monomer could be reacted with different ratios of hexane-1,6-dithiol (

Full text of DOI:10.1016/j.polymer.2014.07.030

Polymer 55 (2014) 5600e5607
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
High refractive index hyperbranched polymers with different
naphthalene contents prepared through thiol-yne click reaction using
di-substituted asymmetric bulky alkynes
,
b
a
,
, b, *
Qiang Wei a , Robert Potzsch b, Hartmut Komber a, Doris Pospiech a, Brigitte Voit a
€
a Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany
Chair of Organic Chemistry of Polymers and Center for Advancing Electronics Dresden (cfaed), Technische Universitat Dresden, 01062 Dresden, Germany
b
€
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, different aromatic moieties, naphthalene and benzene, were introduced into al-
kynes to constitute 1,3,5-tris(naphthylethynyl)benzene as B3 monomer. This monomer could be reacted
with different ratios of hexane-1,6-dithiol (A2 monomer) via mono-selective Thiol-Yne reaction (TYR) to
form hyperbranched polymers PI. TYR yielded in both, model reactions and polymerizations, mono-
adducts with high regioselectivity and proved the absence of any bis-adducts. Polymers of very high
molecular weights of up to 625,000 g/mol could be achieved without any sign of gel formation. All the
polymers exhibit high transparency in the visible wavelength range and also show high refractive indices
of up to 1.745 at 589.7 nm due to the high incorporation of naphthalene and phenyl groups. Besides, all
polymers prove excellent solubility and processability together with excellent thermal properties.
© 2014 Elsevier Ltd. All rights reserved.
Received 2 June 2014
Received in revised form
14 July 2014
Accepted 18 July 2014
Available online 30 July 2014
Keywords:
Thiol-yne
Naphthalene
High refractive index
1. Introduction
waveguides [28], organic lasers [29] and OLED wrapping plastic
[30,31]. The TYR allowed us previously to synthesize a variety of
The thiol-yne reaction (TYR), often referred to as a type of “click
reactions” [1e3], is the reaction between alkyne groups and thiol
groups and can in principle lead to alkenyl sulfides (mono-adducts)
or 1,2-dithioethers (bis-adducts). The interest into the TYR was
mainly driven after the successful application of thiol-ene chem-
istry [4e7]. The TYR can be catalyzed by photo [8e10], thermo
[11e13], amine [14], and transitional metal catalysts [15,16]. In the
last few years, the TYR has been employed in many areas [3,17,18],
such as the construction of organic compounds [19], monomers
[20] and polymers [21], but also in the modification of polymers
[22,23], biomaterials [24,25], and surfaces [26], since the TYR is
characterized by advantages such as mild reaction conditions, high
atomic economy, functional group tolerance and excellent conver-
sion [3,17].
different linear and branched poly(vinylsulfide) (PVS) materials
with HRI [27]. As for the aimed optical applications, the radical-
mediated TYR has many advantages over other metaleion or
amine catalyzed methodologies towards PVS [14,32], since it is
conveniently done under mild reaction conditions, is metal-free,
uses easily available starting materials, and needs no specific
purification procedures. Although mono-adduct formation is
often accompanied by the formation of bis-adducts in radical-
mediated thiol-yne reactions [33e36], which can pose a chal-
lenge to the precise control over the polymeric structures, it was
found that phenylacetylene-based molecules adopted to react
with thiols resulted selectively in vinyl sulfide units through
monoaddition [37].
In our previous work [11,38] we have designed 1,3,5-
tris(phenylethynyl)benzene to construct hyperbranched polymers
by easy radical-mediated TYR, which showed both, high regio-
selectivity [37] and high refractive index (1.70 at 589 nm). In this
paper, in order to obtain polymers with even higher RI values, we
synthesized a new B3 monomer incorporating naphthalene groups
instead of phenyl groups (Scheme 1), since according to the Lor-
entzeLorenz equation [39], the naphthalene units exhibit higher
molar refraction than phenylene [40].
On the other hand, nowadays, increasing attention has been
paid to high refractive index (HRI) polymers [27], with broad
applications in diverse modern technologies, such as polymer
* Corresponding author. Leibniz-Institut für Polymerforschung Dresden e.V.,
Hohe Straße 6, 01069 Dresden, Germany.
E-mail address: voit@ipfdd.de (B. Voit).
http://dx.doi.org/10.1016/j.polymer.2014.07.030
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

Q. Wei et al. / Polymer 55 (2014) 5600e5607
5601
Scheme 1. Polymerization of B3 (alkyne 1) and A2 (thiol 2) to PIaec.
2. Experimental
dithiol, hexanethiol and 1-iodonaphthalene were also supplied by
SigmaeAldrich while 1,3,5-triethynylbenzene was purchased from
TCI.
2.1. Materials and instrumentations
1H (500.13 MHz) and 13C (125.75 MHz) NMR spectra were
recorded on an Avance III 500 NMR spectrometer (Bruker Biospin,
Germany) using CD2Cl2 as a solvent, lock, and internal standard
Anhydrous
tetrahydrofuran
(THF),
triethylamine
(TEA), and toluene sealed over molecular sieves were purch
ased from SigmaeAldrich. Triphenylphosphine, copper(I) iodide,
bis(triphenylphosphine)palladium(II)
(d
(1H) ¼ 5.31 ppm;
d
(
13C) ¼ 53.7 ppm). The signal assignments
dichloride,
hexane-1,6-
were determined from the combination of 1D and 2D NMR

5602
Q. Wei et al. / Polymer 55 (2014) 5600e5607
spectra. Quantitative 13C NMR spectra were obtained using inverse
gated decoupling,
/2 13C pulses and a pulse delay of 12 s. The
(125 MHz, CD2Cl2):
d
¼ 133.6 and 133.55 (C9 and C10), 132.0 (C14),
p
130.7 (C2), 129.2 (C4), 128.8 (C15), 128.8 (C16), 128.7 (C5), 127.2 (C7),
126.8 (C6), 126.4 (C8), 125.7 (C3), 123.7 (C13), 121.2 (C1), 94.7 (C12),
87.9 ppm (C11).
Fourier transform infrared spectra were recorded on a Perkin
Elmer 16 PC FT-IR spectrophotometer (samples prepared in KBr).
Thermal stability of the polymers and thermal transitions were
evaluated on TA Q5000 and Q1000 analyzer, respectively, under
nitrogen at a heating rate of 10 K minꢀ1. Gel permeation chro-
matography (GPC) was performed with a Waters Alliance HPLC
System with 2695 Separation Module and a combination of MALS
and RI detector (Wyatt DAWN HELEOS-II) with a laser operating at
663.1 nm and Wyatt Optilab rEX (RI) was used for determination
of absolute Mw values. UVeVis spectra were measured using of an
SPECORD 210 Plus spectrometer (Analytik Jena). Film thicknesses
were measured by ellipsometry (J.A. Woolam Company model
M2000DI VASE). Ellipsometry was also used to measure the
refractive index of the material. Ellipsometry measurements were
made at three angles (55, 65, 75ꢁ) and over a wavelength range of
400e1700 nm using a model consisting of a single-layer Cauchy
film on Si and the software Complete EASE has been utilized to
process the data. AFM measurements were performed on a
Dimension 3100 Nanoscopic IV from Digital Instruments Inc. in the
tapping mode. Light images were taken on Zeiss LSM 780 NLO
microscope.
2.3. Model reactions
Model Reaction a (alkyne:thiol ¼ 1:1). Into a baked 5 mL flask
with a stir bar, alkyne 3 (205 mg, 0.9 mmol), hexanethiol 4 (106 mg,
0.9 mmol, 135 mL), AIBN (7 mg, 0.045 mmol) and 1 mL anhydrous
toluene were added. After bubbling with nitrogen for about 10 min,
the flask was placed into 90 ꢁC oil bath for 2 h. The crude was
isolated by solvent evaporation in vacuum. A yellowish oil (301 mg)
was obtained (Scheme 2). The 1H and 13C, 2D NMR spectra are
shown in Figs. S1 and S2 respectively.
Model reaction b (alkyne:thiol ¼ 1:5). Similar to model reaction a,
3 (205 mg, 0.9 mmol), 4 (530 mg, 4.5 mmol, 675 mL) and AIBN
(37 mg, 0.225 mmol) in 1 mL anhydrous toluene were reacted at
90 ꢁC for 12 h resulting finally in 304 mg yellowish oil.
Model reaction c (alkyne:thiol ¼ 1:10). Similar to model reaction
a, 3 (205 mg, 0.9 mmol), 4 (1060 mg, 9 mmol, 1350 mL) and AIBN
(74 mg, 0.45 mmol) in 1 mL anhydrous toluene were reacted at
90 ꢁC for 20 h resulting finally in 304 mg yellowish oil.
2.2. Monomer synthesis
2.4. Polymer preparation
1,3,5-Tris(naphthalen-1-ylethynyl)benzene (1). PdCl2(PPh3)2
(537 mg, 0.84 mmol), CuI (80 mg, 0.42 mmol), PPh3 (110 mg,
0.42 mmol), 1,3,5-triethynylbenzene (1.05 g, 7 mmol) and 1-
iodonaphthalene (5.869 g, 23.1 mmol, 3.4 mL) were dissolved in
THF (60 mL) and triethylamine (20 mL) under nitrogen. The
mixture was stirred at 60 ꢁC for 12 h. After this, the mixture was
filtered and evaporated and then 50 mL water were added into the
mixture. The aqueous solution was then extracted with 50 mL of
dichloromethane for three times. The organic phases were com-
bined and dried over anhydrous magnesium sulfate overnight. Af-
ter the filtration and the solvent evaporation, the crude product
was purified using a silica gel column with hexane/ethyl acetate
(EA) mixture (10:1 by volume) as an eluent. Compound 1 (3.254 g)
was obtained as white needles in yield of 88% (Scheme S1). 1H NMR
The polymerization reactions were carried out under nitrogen
using standard Schlenk technique. PIb was set as a typical proce-
dure given below. Into a baked 10 mL Schlenk tube with a stopcock
in the side arm trialkyne 1 (147 mg, 0.30 mmol), hexane-1,6-
dithiol
2 (67 mg, 0.45 mmol, 70 mL) and AIBN (37 mg,
0.225 mmol) were added. The tube was evacuated and refilled
with nitrogen for three times. Thereafter, anhydrous toluene
(1.0 mL) was injected into the system via a syringe. After stirring at
90 ꢁC for 2 h, the mixture was diluted with 5 mL of chloroform and
precipitated by dropwise addition into 200 mL stirred hexane. The
precipitate was collected by filtration, and dried under vacuum at
45 ꢁC overnight.
Characteristics of PIa (molar ratio of 2 (A2):1 (B3) ¼ 1:1): yellow
solid in 70% yield. Mw 11,000 g/mol (LS); Ð ¼ 3.6 (RI, PS calibration).
1H NMR and 13C NMR spectra are shown in Figs. 1 and 2. IR (thin
(500 MHz, CD2Cl2):
H4 and H5), 7.84 (d, 3H, H2), 7.67 (t, 3H, H7), 7.58 (t, 3H, H6),
7.52 ppm (t, 3H, H3); 13C NMR (125 MHz, CD2Cl2):
d
¼ 8.50 (d, 3H, H8), 7.95 (s, 3H, H14), 7.92 (d, 6H,
film):
n
¼ 3068, 2910, 2833, 1583, 1502, 1460, 1415, 1393, 1255, 1131,
d
¼ 134.4 (C14),
1075, 1018, 859, 763, 735 cmꢀ1
.
133.6 (C9), 133.5 (C10), 131.1 (C2), 129.6 (C4), 128.7 (C5), 127.4 (C7),
126.9 (C6), 126.3 (C8), 125.6 (C3), 124.7 (C13), 120.6 (C1), 93.0 (C12),
Characteristics of PIb (molar ratio of 2 (A2):1 (B3) ¼ 3:2): yellow
solid in 87% yield. Mw 625,000 g/mol (LS); Ð ¼ 34.2 (RI, PS cali-
bration). 1H NMR and 13C NMR spectra are shown in Figs. 1 and 2. IR
89.2 ppm (C11). IR:
n
¼ 3047, 2200, 1590, 1502, 1415, 1403, 1340,
1011, 799, 774, 676 cmꢀ1
.
(thin film):
n
¼ 3068, 2910, 2833, 1583, 1502, 1460, 1415, 1393, 1255,
1-(Phenylethynyl)naphthalene (3). The synthesis of compound 3
was carried out by a Sonogashira-coupling reaction similar to
monomer 1. To be exact, PdCl2(PPh3)2 (280 mg, 0.4 mmol), CuI
(34 mg, 0.2 mmol), PPh3 (52.4 mg, 0.2 mmol) ethynylbenzene
(1.225 g, 12 mmol, 1.32 mL) and 1-iodonaphthalene (2.54 g,
10 mmol, 1.46 mL) were added into THF (30 mL) and triethylamine
(10 mL) in a 250 mL two-necked round-bottom flask purged with
nitrogen. After the mixture was stirred at 60 ꢁC for 12 h, 50 mL
water were added into the mixture. The aqueous solution was then
extracted with 50 mL of dichloromethane three times. The organic
phases were combined, and then dried over anhydrous magnesium
sulfate overnight. After the filtration and the solvent evaporation,
the crude product was purified by a silica gel column using hexane/
EA mixture (10:1 by volume) as an eluent. A yellowish powder of 3
(3.145 g) was obtained in yield of 95.2% (Scheme S2). 1H NMR
1131, 1075, 1018, 859, 763, 735 cmꢀ1
.
Characteristics of PIc (molar ratio of 2 (A2):1 (B3) ¼ 2:1): yellow
solid in 59% yield. Mw 34,600 g/mol (LS); Ð ¼ 5.1 (RI, PS calibration).
1H NMR and 13C NMR spectra are shown in Figs. 1 and 2. IR (thin
film):
n
¼ 3068, 2910, 2833, 2569, 1583, 1502, 1460, 1415, 1393,
1255, 1131, 1075, 1018, 859, 763, 735 cmꢀ1
.
2.5. Polymer films and solutions
Thin film samples were prepared by dissolving the PIs in
anhydrous 1,2-dichloroethane at 4 wt% concentration, filtering the
solution through 0.45 mm PTFE-membrane syringe filters, and spin-
coating it onto piranha-cleaned silicon wafers (Silicon Quest Int'l) at
3000 rpm for 30 s. The UV solutions were prepared by dissolving
1 mg of PI into 2 mL anhydrous 1,2-dichloroethane, filtering it
(500 MHz, CD2Cl2):
d
¼ 8.46 (d, 1H, H8), 7.90 (d, 1H, H5), 7.88 (d, 1H,
through 0.45 mm PTFE-membrane syringe filters and later diluting
0.25 mL of the prepared solution with another 2 mL anhydrous 1,2-
dichloroethane.
H4), 7.78 (d, 1H, H2), 7.67 (d, 2H, H14), 7.62 (t, 1H, H7), 7.56 (t,1H, H6),
7.49 (dd, 1H, H3), 7.45e7.39 ppm (3H, H15 and H16); 13C NMR

Q. Wei et al. / Polymer 55 (2014) 5600e5607
5603
Scheme 2. Model reaction between alkyne 3 and thiol 4.
Fig. 1. 1H NMR spectra of PIaePIc (aec) obtained from varying A2/B3 ratios measured
Fig. 2. 13C NMR spectra of PIaePIc (aec) obtained from varying A2/B3 ratios measured
in CD2Cl2.
in CD2Cl2.

5604
Q. Wei et al. / Polymer 55 (2014) 5600e5607
3. Results and discussion
conditions reached only a Mw of 41,600 g/mol, whereas a Mw of
625,000 g/mol was achieved for PIb (the data of the respective
polymer sample from Ref. [37] is added for comparison to Table 1).
This indicates a very high reactivity of the naphthalene-based B3
monomer prepared in this study towards thiol addition. It has to be
further noted that for the stoichiometric functionality ratio (thiol to
alkyne ¼ 1:1), as expected, the highest molecular weight was
achieved, accompanied with a very broad dispersity (Ð ¼ 34) as
typical for a polymerization system approaching the gel point (GPC
curve for PIb see Fig.S3), however, without any sign of macroscopic
gelation being observed for any sample under the used conditions.
Furthermore, thanks to their amorphous branched structures [43],
all the synthesized hb polymers are completely soluble in common
organic solvents, such as THF, chloroform, toluene and
dichloromethane.
3.1. Polymer synthesis
Hyperbranched polymers offer many advantages over linear
polymers including in general high solubility and formation of low-
viscous solutions which makes them highly processable. The
presence of a large number of functional groups on the other hand
makes it possible to crosslink them after deposition from solution.
In the field of HRI materials, the propensity of hb polymers to form
isotropic layers avoids the phenomenon of birefringence [27],
which in contrast is commonly observed in linear polymers, e.g.
polyimides, due to pep-stacking of the aromatic groups [41]. Out of
the different approaches towards hyperbranched polymers, like
using complex ABx monomer, we used the A2 þ B3 method, since
the monomers are more easily available and in addition, the
properties of the resulting hb polymers can be controlled by
different feed ratios of the monomers, while still allowing the
preparation of high molecular weight products under suitable re-
action conditions [11,42]. With this regard, trialkyne 1,3,5-
tris(naphthylethynyl)benzene was prepared as B3 monomer in
high yield by Sonogashira coupling starting from 1,3,5-
triethynylbenzene and 1-iodonaphthalene. The commercially
available hexanedithiol was chosen in this study as A2 monomer. In
order to compare the effects of different groups on the RI behavior
and other performances, polymers PIaePIc were synthesized by
three different molar ratios of B3 and A2 monomers (1:1, 2:3, and
1:2, respectively).
As in our previous study, the polymerization reactions were run
at 90 ꢁC for 2 h in toluene using 2,20-azobis(2-methylpropionitrile)
(AIBN) as a radical initiator [11]. Moreover, the concentration
alkyne B3 monomer was varied between 0.1 and 0.4 M in order to
obtain the optimum reaction concentration (Table S1). Thereby, it
was observed that a concentration of 0.3 M allowed obtaining the
polymer with the highest molecular weight Mw and very good yield
whereas a concentration of 0.4 M already led to the formation of
gels.
The optimized concentration of B3 monomer of 0.3 M was then
set as constant and A2 was reacted with B3 in three different ratios
(Scheme 1). All the polymerizations proceeded efficiently and
provided PI with both, high molecular weights and very good yields
after 2 h. The characteristics of the synthesized polymers are shown
in Table 1. It has to be noted that, surprisingly, the molecular
weights of the polymers obtained from naphthalene-based B3
monomer obtained under similar conditions exceeded significantly
the values obtained for structurally similar hyperbranched poly-
mers obtained from phenyl-based B3 monomer. Thus, a polymer
prepared from A2 and 1,3,5-tris(phenylethynyl)benzene in a 3:2
molar ratio (group ratio [C^C]:[SH] ¼ 1:1) reacted under the same
3.2. Structural characterization
The hyperbranched polymers PIaePIc have been studied by 1H
and quantitative 13C NMR spectroscopy (Figs. 1 and 2). Both spectra
allow calculating the ratio of A2 and B3 monomer in the polymer
from the total intensity of aliphatic and aromatic protons and car-
bons, respectively. The following values for the B3 (1):A2 (2) ratio
were obtained: PIa 1.22 (1H), 1.14 (13C), 1 (feed); PIb 1.67 (1H), 1.64
(
13C), 1.5 (feed); PIc 2.12 (1H), 2 (13C), 2 (feed). All samples are
characterized by a slightly lower content of the trialkyne monomer
1 in the polymer than expected from the monomer feed. Thiol
termination can be observed both for PIb and for PIc by 13C NMR
signals at ~34 ppm (CH2CH2SH) and ~24.5 ppm (CH2SH) whereas
for PIa the excess of alkyne functionalities results in unreacted
ethynyl groups as proved by 13C NMR signals at ~94 ppm and
~89 ppm (Fig. 2).
The broadened and overlapping NMR signals do not provide a
detailed insight in the polymer structure. Generally, the addition of
a thiol radical to the non-symmetrically substituted ethynyl bond of
1 can result in four vinyl sulfide isomers as shown for the reaction
of 3 with hexanethiol (4) in Scheme 2. The ratio of these isomers
after reacting 3 with 4 under similar conditions as applied for
polymerization (model reaction a) should allow a crude estimation
of the substitution pattern in the polymers. The 1H and 13C NMR
spectra of this reaction mixture are rather complex (Fig. S1) but
characteristic signals of the four isomers 5-E, 5-Z, 6-E, and 6-Z
could be identified by applying 1D and 2D NMR techniques (for
details see S1 and S2) and the isomers' ratio is given in Scheme 2.
The thiol radical adds to the sterically less hindered ethynyl carbon
adjacent to the phenyl group with more than 80% probability. The
5-E/5-Z ratio (42:58) corresponds to the value determined for a
similar reaction of 4 with symmetric diphenylacetylene [11].
Despite this model reaction does not account for the effects of
additional substitution on the 3- and 5-positions of 1, it is very
probable that also in the hyperbranched polymer the thiol radical
addition to the ethynyl group occurs preferentially at the phenyl
site. The steric hindrance of thiol radical addition at the bulky
naphthyl site could also be the reason that even at fivefold excess of
4 and elongated reaction time (model reaction b) no formation of
the bis-adduct was observed by characteristic methine proton
signals in the 4.0e4.5 ppm region. Only applying very harsh con-
ditions (tenfold thiol excess, 20 h reaction time; model reaction c),
two methine group signal regions with very low intensity could be
Table 1
Synthesis of hyperbranched polymers PIs from different ratios of alkyne 1 and thiol
2 at 90 ꢁC for 2 h in toluene.a
Entry
B3:A2 molar [C^C]:[SH] Mw (g/mol)b Ðc
Yield Solubilityd
(%)
ratio
P1a
P1b
P1c
1:1
2:3
1:2
3:2
3:3
3:4
3:3
11,000
625,000
34,600
41,600
3.6 70
34 88
Good
Good
Good
Good
5.1 59
4.3 90
Phenyl-based 2:3
Ref.e
observed at d
(1H) ~5.25 and ~4.45 ppm in the 1H NMR spectrum.
a
Concentration: [C^C] ¼ 0.9 M, AIBN:SH ¼ 5%.
The corresponding methine carbon signals at 56.9 and 56.7 ppm
and at 49.9 and 48.7 ppm, respectively, are in agreement with two
diastereomers of the non-symmetric bis-adduct [10]. The signals
are broadened by exchange processes indicating also steric
hindrance.
b
c
Determined by GPC in chloroform (LS detection).
Ð ¼ Mw/Mn, determined by GPC in chloroform (RI detection).
d
e
The solubility of polymers was tested in THF, chloroform, and dichloromethane.
Data from a structurally similar hyperbranched polymer (hb-P5) from Ref. [37]
based on hexane-1,6-dithiol and 1,3,5-tris(phenylethynyl)benzene.

Q. Wei et al. / Polymer 55 (2014) 5600e5607
5605
The IR spectra of PIb and its monomers are displayed in Fig. 3.
The stretching vibration bands of 2206 and 2254 cmꢀ1 can be
attributed to C^C of 1 and eSH of 2, respectively. Both bands are
missing in the spectrum of PIb, indicating that the triple bond of
monomer 1 and thiol group of monomer 2 exhibited high degrees
of conversion during the thiol-yne polymerization. On the other
hand, the band at 1600 cmꢀ1 turned to be much wider and blunder
after polymerization, suggesting that bands of the formed C]C
groups overlap this absorbance. The IR spectra of polymers PIa, PIb
and PIc were also compared (Fig. S4) and analyzed with regard to
variations in structural units. For analysis the aromatic proton
absorbance at 3052 cmꢀ1 was set as internal standard since it was
based on equal input of monomer 1 in the three polymerizations
and this absorbance did not change significantly upon polymeri-
zation. PIb exhibited the strongest absorbance between 1600 and
2000 cmꢀ1 as a result of its highest concentration of C]C resulting
from the 1:1 feed functionality ratio (molar ratio B3:A2 ¼ 2:3), while
PIc showed the strongest absorbance in the hexyl part from excess
thiol. Furthermore, there was a weak absorbance of thiol group left
at 2254 cmꢀ1 in PIc, due to its excess in this polymerization.
However, no evident triple bond's stretching vibration bands were
observed in PIa in spite of its excess as proved by NMR spectros-
copy. The reason may be that the absorbance of the triple bonds in
monomer 1 is already weak due to the di-substitution, and thus, the
even lower triple bond content in PIa cannot be detected.
was reduced. In general, PIa contains the highest concentration of
rigid structures (residues of monomer 1) exhibiting the highest Td5%
residual weight and Tg, while PIc has the highest amount of soft
units (hexyl group) and hence shows the lowest Tg temperature of
75 ꢁC.
3.4. Optical properties
On account of the unsaturated and aromatic structures in PIs,
the jump of valence electrons from
p to p
* is in the near ultraviolet
area. From the UV absorbances, PIb and PIc share almost the same
absorption peak at 324 nm which stands for the vinyl units; how-
ever, there is a shoulder in the peak at about 344 nm in PIa which
indicates the presence of unreacted triple bonds. In PIb, there
might be also some triple bonds left, but because it has the highest
molecular weight, the amount of unreacted triple bond seems to be
too small to be detected. In general, the polymers exhibit low ab-
sorption in the UV region. In the visual light area of 440e760 nm,
their solutions exhibited no detectable absorption, while their films
showed a small absorption detectable due to increased optical
density (Fig. 4).
In order to endow the PIs with high RI values, large contents of
sulfur atoms and aromatic rings (especially naphthalene) were
incorporated into the polymeric structures. Still, due to the hyper-
branched structure and high molecular weights, the PIs have good
solubility and film-forming properties. Homogeneous PI films of
about 90 nm thickness (Table 2) of low roughness, spin-coated on
wafers, were obtained (Fig. 5 for example). Within 450e1600 nm,
PI films have high refractive index values (Fig. 6) as determined by
wavelength-dependent refractive index measurements by ellips-
ometry. At 589.3 nm, the RI values of PIa, PIb and PIc are
decreasing slightly with the decreasing B3 ratio (1.7454, 1.7245 and
1.7132) (Table 2), due to the lowered content of naphthalene units.
Due to a high content of moieties with high molar refractions the
obtained RI values of PIs are close to the highest reported RI value
for polymers [27]. The Abbe numbers of PIs were also calculated
and the values turned out to be lower than 20 (14.6e17) as for most
polymers. When one compares the results obtained on our
naphthalene-based products with the benzene-based hyper-
branched materials of the previous work [37], significant higher RI
values were obtained, but keeping similar or even improved Abbe
numbers. For comparison reason we included the optical data for
the structurally similar benzene-based polymer in Table 2. Inter-
estingly, it could further be shown that using the same monomer
combination, the RI can be further increased by optimizing the
3.3. Thermal properties
The hyperbranched polymers PIaePIc lost 5% of their weight at
high temperatures of 382, 352 and 343 ꢁC, respectively (Fig. S5a)
and thus are in general thermally very stable. Moreover, glass
transition temperatures of PIa, PIb and PIc are 99, 96 and 75 ꢁC
respectively (Fig. S5b). The differences in thermal behavior among
the three polymers can be explained by the different contents of
structural components in the polymers. The structures of hyper-
branched polymers can be further divided into two parts: the hexyl
group as soft and thermally less stable part and the aromatic
structure as rigid, thermally stable part. From PIa to PIc, the content
of monomer 1 in the final polymer became smaller, thereby the
amount of rigid aromatic rings contained in the related polymer
Fig. 4. UVevis absorbance spectra of PIs, measured in 1,2-dichloroethane (DCE) so-
lution (solid line) and as films dip-coated on glass (dashed line).
Fig. 3. IR spectra of (a) A2, (b) B3 and (c) their polymer PIb.

5606
Q. Wei et al. / Polymer 55 (2014) 5600e5607
Table 2
Refractive index and Abbe number of PI.
Item
Thickness Raa
(nm)
RIb
RIb
RIb
Abbe
(nm) 486.1 nm 589.7 nm 656.3 nm numberc
PIa
PIb
PIc
92
95
89
0.2
0.2
0.3
0.2
1.7818
1.7560
1.7435
1.7470
1.7454
1.7245
1.7132
1.6824
1.7309
1.7117
1.7015
1.6630
14.6
16.4
17.0
14.3
Benzene-based 60e80
Ref.d
a
Roughness average obtained from AFM measurements.
Refractive index is represented by Fraunhofer D, F and C lines.
Abbe number is calculated through the formula below nD ¼ ðnD ꢀ 1Þ=ðnF ꢀ nCÞ;
b
c
where nD, nF and nC are the refractive indices of the material at the wavelengths of
the Fraunhofer D-, F- and C-spectral lines (589.3 nm, 486.1 nm and 656.3 nm
respectively).
d
Data from a structural similar hyperbranched polymer (hb-P6) from Ref. [37]
based on hexane-1,6-dithiol and 1,3,5-tris(phenylethynyl)benzene.
content of the aromatic alkyne B3 monomer. Thus, the hyper-
branched polymers PIs have the improved optical properties as
hoped for and thus a significant potential in optical applications.
Fig. 6. Refractive index spectra of PIs films spin-coated on silica wafers, measured by
ellipsometry.
4. Conclusion
monomer and a commercially available A2 monomer (hexane-1,6-
dithiol) of three different molar ratios was polymerized success-
fully in toluene at 90 ꢁC for 2 h using AIBN as a radical source to
afford soluble hyperbranched polymers PIaePIc with high molec-
ular weights (Mw up to 625,000 g/mol). Composition data
In the present study, we synthesized a new asymmetrically di-
substituted alkyne B3 monomer incorporating naphthalene
groups. Due to a high reactivity in the thiol-yne addition, the B3
Fig. 5. Light microscopic and AFM height images of PIb film on silicon wafer. a) and b) Light microscopic images in different magnification, c) and d) AFM height images, area
10 m.
m ꢂ 10
m
m

Q. Wei et al. / Polymer 55 (2014) 5600e5607
5607
calculated from 1H and 13C NMR spectra indicate a slightly higher
dithiol content than in the feed. From product analysis of model
reactions we conclude preferred thiol radical addition on the
ethynyl carbon at the phenyl core and absence of bis-adduct for-
mation by steric effects of the bulky naphthalene moiety. The
polymers are thermally stable (Td5% > 340 ꢁC) and exhibit reason-
able high glass transition temperatures (between 75 and 99 ꢁC) to
allow film application in optical devices. The polymers showed high
transparency in the visible range in solution as well as in 100 nm
thick films. Most importantly, they exhibit for polymers very high
refractive index values because of the high contents of benzene,
naphthalene and sulfur groups. Compared to previous work it could
be shown that the substitution of benzene units by naphthalene
moieties and optimizing the content of B3 units in the product
enhances significantly the RI values without compromising or even
enhancing the Abbe number. Thus, this study provides excellent
design rules towards high refractive index polymers for optoelec-
tronic applications.
[6] Lowe AB. Polym Chem 2010;1:17.
[7] Zou L, Zhu W, Chen Y, Xi F. Polymer 2013;54:481e4.
[8] Konkolewicz D, Gray-Weale A, Perrier S. J Am Chem Soc 2009;131:18075.
[9] Han J, Zhao B, Tang A, Gao Y, Gao C. Polym Chem 2012;3:1918.
[10] Chan JW, Zhou H, Hoyle CE, Lowe AB. Chem Mater 2009;21:1579e85.
€
[11] Potzsch R, Komber H, Stahl BC, Hawker CJ, Voit BI. Macromol Rapid Commun
2013;34:1772e8.
[12] Türünç O, Meier MAR. J Polym Sci Part a Polym Chem 2012;50:1689e95.
[13] Yao B, Mei J, Li J, Wang J, Wu H, Sun JZ, et al. Macromolecules 2014;47:
1325e33.
[14] Jim CKW, Qin A, Lam JWY, Mahtab F, Yu Y, Tang BZ. Adv Funct Mater 2010;20:
1319e28.
[15] Liu JZ, Lam JWY, Jim CKW, Ng JCY, Shi JB, Su HM, et al. Macromolecules
2011;44:68e79.
[16] Kuniyasu H, Ogawa A, Sato K, Ryu I, Kambe N, Sonoda N. J Am Chem Soc
1992;114:5902e3.
[17] Hoogenboom R. Angew Chem Int Ed Engl 2010;49:3415e7.
[18] Massi A, Nanni D. Org Biomol Chem 2012;10:3791e807.
[19] Li L, Zahner D, Su Y, Gruen C, Davidson G, Levkin PA. Biomaterials 2012;33:
8160e6.
ꢀ
ꢁ
ꢀ
[20] Gonzalez-Paz RJ, Lligadas G, Ronda JC, Galia M, Cadiz V. Polym Chem 2012;3:
2471.
[21] Yao B, Sun J, Qin A, Tang BZ. Chin Sci Bull 2013;58:2711e8.
[22] Wang J, Mei J, Zhao EG, Song ZG, Qin AJ, Sun JZ, et al. Macromolecules
2012;45:7692e703.
[23] Brummelhuis Nt, Schlaad H. Polym Chem 2011;2:1180.
[24] Kumar J, Bousquet A, Stenzel MH. Macromol Rapid Commun 2011;32:
1620e6.
[25] Ray JG, Ly JT, Savin DA. Polym Chem 2011;2:1536.
[26] Rahane SB, Hensarling RM, Sparks BJ, Stafford CM, Patton DL. J Mater Chem
2012;22:932.
[27] Liu J-g, Ueda M. J Mater Chem 2009;19:8907.
[28] Ma H, Jen AKY, Dalton LR. Adv Mater 2002;14:1339e65.
[29] Stroisch M, Woggon T, Teiwes-Morin C, Klinkhammer S, Forberich K,
Gombert A, et al. Opt Express 2010;18:5890e5.
[30] Ziebarth JM, Saafir AK, Fan S, McGehee MD. Adv Funct Mater 2004;14:451e6.
[31] Wang ZB, Helander MG, Qiu J, Puzzo DP, Greiner MT, Hudson ZM, et al. Nat
Photonics 2011;5:753e7.
Acknowledgments
€
Studienstiftung des deutschen Volkes and Forderverein des
Leibniz-Instituts für Polymerforschung are acknowledged for
financial support. P. Treppe, L. Haußler, Dr. M. Malanin and R.
Schulze (IPF Dresden) are acknowledged for conducting GPC, TGA
and DSC, FT-IR, and refractive index measurements. Dr. Yaoming
Zhang is also acknowledged for recording the light microscopic and
AFM images. This work was also partially supported by cfaed (DFG
Excellence Cluster).
€
[32] Ichinose Y, Wakamatsu K, Nozaki K, Birbaum J-L, Oshima K, Utimoto K. Chem
Lett 1987;16:1647e50.
Appendix A. Supplementary data
[33] Nuyken O, Siebzehnrübl F. Polym Bull 1988;19:371e5.
[34] Nuyken O, Maier G, Burger K. Macromol Chem Phys 1988;189:2245e53.
[35] Kobayashi E, Ohashi T, Furukawa J. Macromol Chem Phys 1986;187:2525e33.
[36] Kobayashi E, Yoshino T, Aoshima S, Furukawa J. J Polym Sci Part Polym Chem
1995;33:2403e14.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2014.07.030.
[37] Oswald AA, Griesbaum K, Hudson BE, Bregman JM. J Am Chem Soc 1964;86:
2877e84.
[38] Potzsch R, Stahl BC, Komber H, Hawker CJ, Voit BI. Polym Chem 2014;5:2911.
References
€
[39] Dechant JI, Brandrup J, Immergut EH (editors). 1990.
[40] Lange NASJG. Lange's handbook of chemistry. New York: McGraw-Hill; 2005.
[41] Lu J, Zhou N, Pan X, Zhu J, Zhu X. J Polym Sci Part a Polym Chem 2014;52:
504e10.
[1] Kolb HC, Finn MG, Sharpless KB. Angew Chem Int Ed 2001;40:2004e21.
[2] Iha RK, Wooley KL, Nystrom AM, Burke DJ, Kade MJ, Hawker CJ. Chem Rev
€
2009;109:5620e86.
[3] Lowe AB, Hoyle CE, Bowman CN. J Mater Chem 2010;20:4745.
[4] Hoyle CE, Bowman CN. Angew Chem Int Ed Engl 2010;49:1540e73.
[5] Cramer NB, Bowman CN. Thiol-X chemistries in polymer and materials sci-
ence. The Royal Society of Chemistry; 2013. p. 1e27.
[42] Voit BI, Lederer A. Chem Rev 2009;109:5924e73.
ꢀ
[43] Wooley KL, Frechet JMJ, Hawker CJ. Polymer 1994;35:4489e95.