Branched polystyrene with high reflex index synthesized from selenium-mediated polymerization

Article abstract of DOI:10.1002/pola.27023

In this article, the polymerization behavior has been investigated utilizing phenyl(4-vinylbenzyl)selane (PVBS) as an inimer under ultraviolet irradiation. Corresponding copolymers and homopolymers were synthesized by copolymerization PVBS with styrene an

Full text of DOI:10.1002/pola.27023

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Branched Polystyrene with High Reflex Index Synthesized
from Selenium-Mediated Polymerization
Jinjie Lu, Nianchen Zhou, Xiangqiang Pan, Jian Zhu, Xiulin Zhu
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and
Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
People’s Republic of China
Correspondence to: J. Zhu (E-mail: chemzhujian@suda.edu.cn) or X. Zhu (E-mail: xlzhu@suda.edu.cn)
Received 25 July 2013; accepted 5 November 2013; published online 25 November 2013
DOI: 10.1002/pola.27023
ABSTRACT: In this article, the polymerization behavior has been
investigated utilizing phenyl(4-vinylbenzyl)selane (PVBS) as an
inimer under ultraviolet irradiation. Corresponding copolymers
and homopolymers were synthesized by copolymerization
PVBS with styrene and homopolymerization itself. The branch-
ing factors (g0) of these branched polymers were characterized
along with the polymerization conditions. Moreover, the results
showed that the obtained polymers’ refractive index (RI) can
be enhanced by the introducing of selenium element. The
results also indicated that the RI of obtained polymers could
be adjusted extendedly by changing selenium content in them.
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KEYWORDS: branched; high refractive index; hyperbranched;
iniferter; living radical polymerization (LRP); optics; refractive
index; selenium mediated
mer15,16 or the reaction between selenium agent with poly-
mer.17–19 However, in these approaches, the amount of
incorporated selenium in final polymer was difficult to
adjust, which would evidently limit the variable RI range.
Recently, our group has reported the controlled radical poly-
merization, or reversible-deactivation radical polymerization
defined by IUPAC, using novel selenium compounds as the
mediator.20–24 In such way, the amount of the selenium con-
centration in polymer can be adjusted by molecular weight.
However, in these cases the selenium was attached at the
end of polymer chain, which made it difficult to incorporate
more than two selenium-containing groups into one polymer
chain. Thus, increasing the amount of polymer chain ends
would be the efficient way to increasing the concentration of
selenium among the polymer chain with controlled manner.
As it well known that hyperbranched polymer contained
highly branched structure with high concentration of chain
ends. Apart from this, hyperbranched polymers have been of
particular interest in polymer science for many years.25–27
Because the unique globular and highly branched structure
can increase their compactness compared to linear ones. And
then low viscosities, many functional end-groups and low
viscosities28 are also their proper features, which makes
them widely used as targeted drug delivery,29 viscosity
modifiers,30 supports for catalysts,31 and other materials.32,33
Yuki34,35 has briefly reported the synthesis of hyperbranched
INTRODUCTION Polymeric materials with high refractive
index (n) and high Abbe’s number (m) have attracted much
interest due to their potential applications in many areas, such
as coatings for light-emitting diodes (LEDs),1,2 microlens com-
ponents for charge coupled devices (CCD)3 or volume holo-
graphic recording materials,4 and so on.5,6 According to the
Lorentz–Lorenz equation, the introduction of substituents
with high molar refractions and low-molar volume groups
have become the common approaches for most researchers to
increase the refractive index of polymers.7 Traditional meth-
ods usually be adopted were (1) incorporating organic groups
or consisting of highly conjugated structures, such as aromatic
groups;8 (2) incorporation of halogen atoms, sulfur atoms and
silicon atoms with high molar refractions into the backbone or
side chain of polymers;9–11 (3) synthesizing polymers with
enwrapped inorganic nanoparticles.12 Table 1 showed the
molar refractions of common atoms or groups. As shown in
the right column of Table 113 the molar refraction (RM) of alkyl
selenides was higher than that of sulfides, which implied that
insertion of selenium atoms in polymer materials maybe a
more efficient way to improve the refractive index than incor-
poration of sulfur atoms. And as far as we know, few relevant
literatures have been reported.14
The selenium can be incorporated into polymer chain
through the polymerization of selenium containing mono-
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TABLE 1 Molar Refractions (RM) of Atoms or Organic Groupsa
Group
RM
Group
RM
H
1.100
2.418
1.733
2.398
2.211
1.525
Phenyl (C6H5)
25.463
43.000
13.900
7.970
C
Naphthyl (C10H7)
C@C
CBC
C@O
OAH
I
C@S
ASH
7.690
-SeR (R5alkyl)
11.170
a
Data were cited from ref. 13.
polystyrene using selenium containing compounds. However,
the detailed polymerization behavior and properties of the
polymer were still worth of investigation.
Here, through the route shown in Scheme 1, we have investi-
gated the synthesis of hyperbranched polystyrene utilizing
phenyl(4-vinylbenzyl)selenide (PVBS)36 as an inimer under
ultraviolet (UV) irradiation. Copolymers and homopolymers
have been synthesized by its copolymerization with styrene
and homopolymerization itself via self-condensing vinyl poly-
merization (SCVP).37–39 The results showed that the poly-
mers showed higher RI than polystyrene. Moreover, the RI of
the polymer can be adjusted by the concentration of sele-
nium in polymer.
EXPERIMENTAL
SCHEME 1 Synthesis of selenium-containing hyperbranched
polystyrene with PVBS as inimer. [Color figure can be viewed
in the online issue, which is available at wileyonline
library.com.]
Materials
Diphenyl diselenide (98%) and tetrabutylammonium hexa-
fluorophosphate were purchased from J & K Scientific). All
other reagents (analytical grade) used in this study were pur-
chased from Shanghai Chemical Reagents (China). Styrene was
washed with an aqueous solution of sodium hydroxide (5 wt
%) three times and then with deionized water until neutraliza-
tion. After being dried with anhydrous magnesium sulfate, sty-
rene was distilled under reduced pressure and kept in
refrigerator under 0 ꢀC. Tetrabutylammonium hexafluorophos-
phate was recrystallized after dissolving in ethyl acetate. 4-
Vinylbenzyl chloride was purified by neutral alumina column.
7.16–7.45 (m, C6H4 and C6H5, 9H). 77Se NMR (CDCl3, d,
ppm) d 384.5 (s, PhSe-, 1Se). EA: calculated for C15H14Se: C,
65.94%; H, 5.16%. Found: C, 65.38%; H, 5.09%.
Photopolymerization of PVBS with Styrene
A typical polymerization procedure was as follows: St (0.453
g, 4.3 mmol) and PVBS (0.119 g, 0.43 mmol) were added to
a dried ampoule with 0.6 mL anhydrous toluene. The content
was deoxygenated by bubbling argon for 15 min, and then
the ampoule was flame sealed. Afterward, the ampoule was
placed in the irradiation of ultraviolet lamp with 150-W
high-pressure mercury lamp in the distance of 20 cm at
30 ꢀC. After the desired reaction time, the ampoule was
opened and the products were dissolved in THF, precipitated
into cold methanol. The polymer was separated via filtration
and was dried under vacuum until constant weight at room
temperature. The conversion of monomer was determined
by gravimetrical method.
Synthesis of Phenyl(4-vinylbenzyl)selane
Diphenyldiselenide (1.25 g, 4.0 mmol) was dissolved in 10
mL of anhydrous THF, and then 10 mL of an aqueous solu-
tion of 0.30 g (8.0 mmol) of sodium borohydride was added.
The reaction was finished in a few minutes, and a colorless
sodium phenylselenide (PhSeNa) solution was obtained. The
obtained PhSeNa solution was immediately injected into 20
mL of a THF solution of 1.22 g (8.0 mmol) of 4-vinylbenzyl
chloride, and the reaction was performed at 50 ꢀC for 12 h
under argon flow. Then, the product was extracted with
diethyl ether and was purified by column chromatography
with n-hexaneas the eluent. The pure product showed as
white powder was obtained with yield of 71%.
Homopolymerization of PVBS
A typical polymerization procedure was as follows: PVBS (1
g, 3.65 mmol) with anhydrous toluene (1.6 mL) was added
to a dried ampoule. The content was deoxygenated by bub-
bling argon for 15 min, and then the ampoule was flame
1H NMR (CDCl3, d, ppm) d 4.09 (s, CH2, 2H), 5.20 (d, trans-
CH2A, 1H), 5.70 (d, cis-CH2@, 1H), 6.66 (dd, @CH, 1H), and
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FIGURE 1 ln([M]0/[M]) versus time for the polymerization of
styrene in the presence of PVBS in bulk under UV irradiation
with the molar ratio of [St]0:[PVBS]05 50/30/10:1, temperature5
30 ꢀC. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 2 Relationships between Mn, Mw/Mn and conversion
in the polymerization of St at various concentrations of PVBS.
[St]0:[PVBS]05 50/30/10:1, temperature5 30 ꢀC. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
sealed. Afterward, the ampoule was placed in the irradiation
of ultraviolet lamp. Postprocessing method is the same as
above.
increased with the increasing of the inimer concentration.
As showed in Scheme 1, the PVBS decomposed to one car-
bon centered radical and one selenium centered radical
under the irradiation of UV. The carbon centered radical
initiated the polymerization of styrene and selenium cen-
tered radical served as the dorment radical to recombinat-
ing with propagation radical through iniferter way.34,35
Thus, increasing the inimer concentration would result in
the increasing of initiation radical, which enhanced the
polymerization rate. Figure 2 indicated the relationship
between number-average molecular weight (Mn) and the
molecular weight distribution (Mw/Mn) to the monomer
conversion with different feeding ratios. The molecular
weight increased almost linearly with the monomer conver-
sion in all cases of polymerizations. The molecular weights
were determined by GPC with RI detector due to the low
polymer molecular weight (<8000 g/mol), which cannot be
measured precisely by LS detector. The obtained polymers
showed broad molecular weight distribution (Mw/Mn > 1.8),
which was normally found in hyperbranched polymers. The
molecular weight distribution of the resulting polymers was
ever broadening as the polymerization proceeds because of
the chain branching reaction, which has been predicated40
and reported41 previously. Meanwhile, the Mw/Mn value
increased with the monomer conversion, which was consist-
ent with iniferter polymerization characteristic. The GPC
traces of these polymers were showed in Figure 3. It
showed significant shoulders, tailings, or even multiple dis-
tributions appeared in GPC traces, suggesting the possible
partial loss of CTA functionality and different branching
generations. Yuki et al. have demonstrated that diphenyl
diselenide (DPDSe) formed in the polymerization pro-
cess.34,35 As the result, the broad Mw/Mn and multicompo-
nent may also partially caused by the “side” polymerization
initiated by the generated DPDSe, which lead to little linear
polymers mixed in the hyperbranched polymers.
Characterization
The number-average molecular weight (Mn) and molecular
weight distribution (Mw/Mn) of the resulting polymers were
determined using an Agilent PL-50gel permeation chromato-
graph (GPC) equipped with triple detectors (refractive-index
detector, two angles light scattering detector and viscosity
detector), using PL Mixed gel C (5-lm beads size) columns
with molecular weights ranging from 200 to 2 3 106 g/mol.
THF was used as the eluent at a flow rate of 1.0 mL/min
and 40 ꢀC. GPC samples were injected using a PL-AS RT
autosampler and calibrated with PS standards purchased
1
from PL. The H NMR spectrum of the polymer was recorded
on an INOVA 400-MHz nuclear magnetic resonance instru-
ment using CDCl3 as the solvent and tetramethylsilane (TMS)
as an internal standard. The 77SeNMR spectrum of the poly-
mer was recorded on a Varian 400 MHz instrument using
CDCl3 as the solvent (CH3SeCH3 as an external standard).
Thermogravimetric analysis (TGA) was carried out on a
2960 SDT TA instruments with a heating rate of 10 ꢀC/min
from the room temperature to 800 ꢀC under the nitrogen
atmosphere. Atomic absorption spectroscopy of polymer was
measured on Varian 220DuO instrument. The refractive
index of the polymer was determined on J. A. Woollam
Alpha-SE ellipsometer.
RESULTS AND DISCUSSION
Photopolymerization Behavior of St in the Presence of
PVBS
Here, first, the polymerization kinetic of styrene was inves-
tigated using PVBS as inimer in different feeding ratio of
monomer to inimer. The result was showed in Figure 1. As
shown in Figure 1, the polymerization rate of styrene was
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FIGURE 4 77Se NMR spectra of PVBS and PPVBS (Mn 5 13,400,
Mw/Mn 5 15.18) in CDCl3. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
after polymerization, indicating that the chemical environ-
ment of selenium has changed after polymerization, which
was consisted with the polymerization reaction showed in
Scheme 1. Furthermore, only single peak were detected in
each spectra, which illustrated that the synthesized PVBS has
high purity and the selenium components in hyperbranched
PPVBS have similar structures.
The Branching factor (g0) and Selenium Content of
Hyperbranched Polymers
The branching factor (g0) was used to investigate the degree of
branching of the hyperbranched polymers on a side-note. Usu-
ally g0 defined by Zimm and Stockmayer with equation:
g0 5 [g]b/[g]l, where [g]b and [g]l are intrinsic viscosities of
branched and linear polymers with same molecular weight,
respectively. Table 2 showed the results of photocopolymeriza-
tion of St with PVBS in different feeding ratios along with the
values of g0. The g0 values increased from 0.17 to 0.74 when the
St-to-PVBS feeding ratio increased from 10 to 50, which indi-
cated that branching factor becomes smaller with an increase
of the PVBS concentration. Thus, the homopolymer of PVBS
showed the lowest g0 value of 0.13 as shown in Table 2. These
results indicated that the homopolymer has more branching
structure than that of copolymers. Meanwhile, the selenium
content in polymers has been measured via atom absorption
spectroscopy (AAS). The value of selenium content was also
listed in Table 2. The values decreased from 8.1 to 3.0% when
the St-to-PVBS feeding ratio increased from 10 to 50. And the
AAS determined selenium content values were closed to the
theoretical ones (assumed that all the PVBS has participated in
the reaction). In the case of homopolymerization, the highest
selenium content was 28.9%, which was approximate to the
value in PVBS. The branching factors for copolymers obtained
by SCVP of St and PVBS at constant comonomer ratios were
also investigated. The results were listed in Table 3. It showed
that the branching structure was increased with the increasing
of conversion, which was consisted with the SCVP process.42,43
FIGURE 3 Evolution of GPC traces for the copolymerization of
St with PVBS in toluene under UV irradiation. [St]0 5 7.2 mol/L.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
The structure of obtained polymer was investigated utilizing
77Se NMR spectrum. Due to the low abundance of 77Se, the
homopolymer of PVBS, poly(phenyl(4-vinylbenzyl)selane)
(PPVBS), was synthesized by the UV-irradiated polymeriza-
tion (Scheme 1). The results were showed in Figure 4. The
observation of 77Se resonance in both of the spectra before
and after the polymerization indicated that selenium can be
successfully incorporated into polymer through polymeriza-
tion. The resonance of 77Se shifted from 384.5 to 276.3 ppm
The Thermostabilities of Hyperbranched Polymers
Figure 5 presented the thermogravimetric analysis (TGA)
results for hyperbranched polymers incorporated with
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TABLE 2 Branching Factor for Copolymers Obtained by SCVP of St and PVBS at Different Feeding Ratios
Feeding Ratioa
Conv.%b
Mn
Mn
Mw/Mn
g0
Se%f
Seth%
c
d
d
e
g
No.
P-1
P-2
P-3
P-4*
50
30
10
0
51.7
58.8
73.3
60.2
4300
4500
7000
7100
5150
4.03
2.12
6.49
15.18
0.74
0.54
0.17
0.13
3.0
5.6
8.1
28.9
2.8
4.0
8.3
29.2
6370
12900
13400
a
d
Feeding ratio: [St]0:[Inimer]0. Polymerization conditions: kUV 5365 nm,
Determined by GPC with triple detectors(RI, LS, and Viscosity).
e
in toluene at 30 ꢀC. [St]054.35 mol/L,* [Inimer]052.3 mol/L.
b
g0 5 [g]b/[g]l, where, [g]b is intrinsic viscosity of branched polymers,
The conversion of monomer was determined by
a
gravimetrical
[g]l is intrinsic viscosity of linear polymers.
f
method.
c
Determined by atomic absorption spectrophotometer.
g
Determined by GPC with RI detectors.
Calculated by formula: 80mPVBS/[274 3 Con.% 3 (mPVBS 1 mSt)].
TABLE 3 Branching Factor for Copolymers Obtained by SCVP of St and PVBS at Constant Comonomer Ratios
b
c
No.
Feeding Ratioa
t (h)
Conv.%
Mn
Mn
Mw
Mw/Mn
g0
P-5
P-6
P-7
P-8
10
10
10
10
24
40
50
65
42.0
61.5
68.0
70.0
2,200
3,300
4,000
4,800
4,360
5,600
8,160
10,000
7,540
1.73
2.83
3.42
3.85
0.58
0.51
0.45
0.33
15,800
27,900
38,600
a
b
Feeding ratio: [St]0:[Inimer]0. Polymerization conditions: kUV 5 365 nm,
in toluene at 30 ꢀC. [St]0 5 7.26 mol/L.
Determined by GPC with RI detectors.
c
Determined by GPC with triple detectors (RI, LS, and Viscosity).
kJ/mol).44 Additionally, at 450 C, all polymers were decom-
posed completely. The decline of weight above 450 ꢀC was
mainly due to the evaporation of selenium. Then, the last
platform of every curve also indicated the difference of the
selenium content in these polymers. The amount measured
in TGA was in good accordance with the measured values by
AAS.
ꢀ
different contents of selenium atoms. In generally, polysty-
rene showed decomposition temperature of 300–400 ꢀC. As
showed in Figure 5, the copolymer P-1, P-2, and P-3 showed
similar decomposition behavior in the range of 350–360 ꢀC.
The increasing of selenium concentration resulted in the
decreasing of thermostability. The homopolymer of PVSB (P-
4) with selenium content of 28.9% started decomposing at
275 ꢀC. The reason may attribute to the fact that CASe
bonds were existed abundantly and the bond energy of C-Se
bond (243 KJ/mol) was lower than that of CAC bond (347
The Optical Properties of Hyperbranched Polymers
Figure 6 showed UV–vis absorption spectra of the studied
polymers (P-1, P-2, P-3, and P-4) in CH2Cl2. All the samples
showed no absorption in the visible and infrared region
(>350 nm). And absorption around 240–325 nm becomes
larger in the order of P1 < P2 < P3 << P4, which probably
contributed from the stronger absorption capacity of -SePh
groups.
Optical transparency of the polymer films was a crucial fac-
tor for their use in optical applications. So, in this article, we
coated thin sample films on quartz wafer and evaluated the
transmittance with a UV–vis spectrometer. Figure 7 shows
the UV–vis absorption spectra of selenium-containing hyper-
branched polystyrene films with thickness of 1.4 lm approx-
imately. As shown in the figure, the optical transmittance
spectra of P-1, P-2, P-3, and P-4 were similar to each other.
Simultaneously, all the polymer films exhibit relatively low
cutoff wavelengths (determined at 50% transmittance)
ranged between 283 and 318 nm and excellent transparency
with transmittance much higher than 90% after the wave-
length over 350 nm, which indicate that the incorporation of
selenium atoms have little effect on the transmissivity of the
FIGURE 5 TGA curves of hyperbranched polymers under con-
stant nitrogen flow of 20 mL/min and heating rate of 10 ꢀC/
min. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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TABLE 4 Optical Properties of the Selenium-Containing Films
Entry
Se%
n486
n589
n656
mD
P-1
P-3
P-4
3.0
1.629
1.651
1.713
1.617
1.638
1.697
1.612
1.632
1.691
36.3
33.6
31.7
8.1
28.9
which suggest that the introduction of a selenium (-Se-) ele-
ment with high atom refraction and low molecular disper-
sion was effective to develop high-n and high-m polymers.
And with the increasing of selenium content in polymers
from 3.0 to 28.9%, their RI value at 583.9 nm increased
from 1.617 to 1.697. Figure 8(b) revealed the wavelength
dispersion (300–800 nm) of calculated refractive indices (nk)
of these three films with a fitted curve using the Cauchy’s
formula. The calculations well reproduce the experimental
refractive indices of polymer films, although the calculated
nk of polymers were slightly overestimated. This because the
existence of hyperbranched structure induces additional free
volume.45,46 So, based on the description above, we can
FIGURE 6 UV2vis absorption spectra of polymers dissolved in
CH2Cl2. Polymer concentration: 0.25 mg/mL. [Color figure can
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
polymers. These characters were favorable for the polymer
been used as optical materials.
In order to investigating the optical properties of the
obtained polymer, the refractive indices and Abbe’s number
of these polymer films were investigated. Table 4 shows the
refractive indices of three hyperbranched polymers with dif-
ferent selenium content at 486, 589, and 656 nm. Simultane-
ously, as shown in Table 4, all the films have a relative high
Abbe’s number of 36.3, 33.6, and 31.7, respectively, which
indicates that these materials were endowed with low-light
dispersion. The dates of refractive indices were measured
with an ellipsometer by spin-coating samples on silicon
wafer. The wavelength dispersion (300–800 nm) of the
refractive indices (nk) was plotted in Figure 8(a). All the
polymers incorporated with selenium element were of high
RI values compared to ordinary polystyrene (n 5 1.58),
FIGURE 7 Transmittance UV–vis spectra of the synthesized
hyperbranched polymers with film thickness ꢁ1.3 lm. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 8 Wavelength dispersion of (a) experimental and (b) cal-
culated refractive indices of three polymer films. The dispersions
are fitted by the Cauchy’s formula. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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ratio to get the materials with the different refractive index,
which probably was an effective method to expand the appli-
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CONCLUSIONS
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In summary, the polymerizable iniferter PVBS was designed
and synthesized. Hyperbranched PS was synthesized via the
combination of SCVP and iniferter polymerization techniques
using PVBS as the mediator. The branched nature of the result-
ing polymers was confirmed by 77Se NMR spectroscopy and
gel permeation chromatograph (GPC). The incorporation of
selenium can improve the refractive index of optical materials
effectively. And the RI value can be adjusted by simply chang-
ing the selenium content in polymers. Simultaneously, these
serial selenium-containing materials also have good transpar-
ency and relative high Abbe’s numbers. So, in consider of the
multifaceted functionalities mentioned above and high sele-
nium content analyzed, the synthesized hyperbranched poly-
mers may have potential application in optical area.
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The financial support from the National Nature Science Foun-
dation of China (No. 21074082, 21374067, and 21302132), the
Priority Academic Program Development (PAPD) of Jiangsu
Higher Education Institutions, and the Program of Innovative
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