Facile synthesis and characterization of highly branched polystyrene by self-condensing atom transfer radical polymerization

Article abstract of DOI:10.1080/15421406.2018.1450827

To simply prepare hyperbranched polystyrene with dendritic structure for optical and electronic applications, the copper-mediated self-condensing atom transfer radical polymerization of 4-(2-bromopropionyl)oxy styrene has been attempted. Given the unequal reactivity of two propagating species owing to different substituents which stabilize radicals, the influence of polymerization conditions on the structure of dendritic polystyrene were investigated. Higher polymerization temperature leaded to the lower functionality of Br in the branched polystyrene due to HBr elimination. As the catalyst to monomer ratio increased, the degree of branching of the prepared polystyrene was enhanced by increasing the reaction chance of the less stabilized propagating site.

Full text of DOI:10.1080/15421406.2018.1450827

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Facile synthesis and characterization of highly
branched polystyrene by self-condensing atom
transfer radical polymerization
Jun Hyup Lee
To cite this article: Jun Hyup Lee (2017) Facile synthesis and characterization of highly branched
polystyrene by self-condensing atom transfer radical polymerization, Molecular Crystals and Liquid
Crystals, 659:1, 46-51, DOI: 10.1080/15421406.2018.1450827
To link to this article: https://doi.org/10.1080/15421406.2018.1450827
Published online: 02 May 2018.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gmcl20

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Facile synthesis and characterization of highly branched
polystyrene by self-condensing atom transfer radical
polymerization
Jun Hyup Lee
Department of Chemical Engineering, Myongji University, Yongin, Korea
ABSTRACT
KEYWORDS
Atom transfer radical
polymerization;
hyperbranched polymer;
polystyrene; self-condensing
vinyl polymerization
To simply prepare hyperbranched polystyrene with dendritic struc-
ture for optical and electronic applications, the copper-mediated
self-condensing atom transfer radical polymerization of 4-(2-
bromopropionyl)oxy styrene has been attempted. Given the unequal
reactivity of two propagating species owing to different substituents
which stabilize radicals, the influence of polymerization conditions
on the structure of dendritic polystyrene were investigated. Higher
polymerization temperature leaded to the lower functionality of Br in
the branched polystyrene due to HBr elimination. As the catalyst to
monomer ratio increased, the degree of branching of the prepared
polystyrene was enhanced by increasing the reaction chance of the less
stabilized propagating site.
1. Introduction
Dendritic polymers are a new class of functional polymers where a large number of branches
and functional groups are in the spherical shape [1–4]. These materials are usually prepared
by condensation reaction of AB2 monomers with branching unit. There has been a growing
interest in dendritic polymers which, because of the three-dimensional unique structure and
extremely high functionality, exhibit superior optical, electrochemical, and biological proper-
ties than linear polymer materials [5, 6]. Among dendritic polymers, dendrimers with well-
defined spherical structure require multiple chemical reaction steps which must be followed
by time-consuming purification process. As a good alternative to dendrimers, hyperbranched
polymers can be synthesized on a large scale via one-pot process. While most of hyper-
branched polymers have been prepared by condensation polymerization, vinyl monomers
could be polymerized through the combined method of self-condensing vinyl polymeriza-
tion (SCVP) and atom transfer radical polymerization (ATRP) [7, 8]. To mimic the well-
defined dendritic structure of dendrimers, the majority of the hyperbranched vinyl polymers
are based on acrylic monomers having the equal reactivity of the two propagating species
for high degree of branching. However, hyperbranched polyacrylates have low mechanical
strength due to low glass transition temperature and limited optical and electronic properties
for using in some specific areas [9].
CONTACT Jun Hyup Lee
-, Korea.
junhyuplee@mju.ac.kr
Department of Chemical Engineering, Myongji University, Yongin
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
47
In this study, new type of highly branched polystyrene were synthesized by the self-
condensing vinyl polymerization of 4-(2-bromopropionyl)oxy styrene using atom transfer
radical polymerization catalyzed by the complex of CuBr/2,2-bipyridyl. Unlike the conven-
tional hyperbranched polyacrylates, the newly designed hyperbranched polystyrene has the
unequal reactivity of the two propagating species due to different substituents which sta-
bilize radicals, which may reduce the degree of branching compared to the conventional
hyperbranched polyacrylates with the equal reactivity of the propagating sites. To achieve
the improved degree of branching and high functionality, polymerization experiments were
carried out with various reaction temperature and catalyst to monomer ratio, and the effects
of polymerization conditions on the structure of branched polymers were investigated.
2. Experimental
2.1. Materials
4-Acetoxystyrene (Aldrich), potassium hydroxide (KOH, Junsei), hydrochloric acid (HCl,
Oriental chemical), pyridine (Junsei), and 2-bromopropionyl bromide (Aldrich) were used
without further purification. Copper(I) bromide (Cu(I)Br, Aldrich) and 2,2ꢀ- bipyridine (Bipy,
˚
Aldrich) were used as received. Chlorobenzene (Junsei) was dried over molecular sieves (4 A).
The silica gel with 70–230 mesh (Merck, Kieselgel 60) was used for column chromatography.
2.2. Synthesis of monomer
The synthesis of 4-(2-bromopropionyl)oxy styrene (BPOS) was based on the previous report
except that 4-hydroxystyrene was used in place of 2-hydroxyethyl acrylate [10]. To synthe-
size the 4-hydroxystyrene, solution consisting of 4-acetoxystyrene (170 mmol) and KOH
(255 mmol) in 260 mL methylene chloride was firstly prepared under nitrogen atmosphere.
Then, 26 mL methanol was added with vigorous stirring at room temperature. After 2 h,
300 mL water was added to the reaction mixture and the aqueous phase was separated. A
certain amount of 1N HCl was added for acidification, and extracted with 300 mL methy-
lene chloride. The organic phase was separated, and washed with saturated aqueous sodium
chloride solution. After methylene chloride was evaporated, 4-hydroxystyrene was obtained
as white solid. 1H NMR (300 MHz, CDCl3): δ 1.93 (d, J = 6.9 Hz, 3H), 4.58 (q, J = 6.9 Hz,
1H), 5.25 (d, J = 10.8 Hz, 1H), 5.71 (d, J = 17.6 Hz, 1H), 6.69 (dd, J = 10.8, 17.6 Hz, 1H), 7.08
(d, J = 8.6 Hz, 2H), 7.42 (d, J = 8.6 Hz, 2H).
2.3. Polymerization
For the solution polymerization, a two-necked flask equipped with a reflux condenser was
placed with Cu(I)Br (0.561 mmol) and Bipy (1.122 mmol), and then the BPOS (11.2 mmol)
was added via a syringe under vacuum. To remove any oxygen in the flask, the three cycle
of vacuum-nitrogen was conducted, and then 2 mL chlorobenzene was added via a syringe.
After 8 h at 100°C, the reaction mixture was cooled with ice, followed by removing the catalyst
with glass filter. The filtrate was evaporated, and then diluted with tetrahydrofuran. Finally,
the polymer solution was added to a large excess of methanol, resulting in the polymer pre-
cipitate. To examine the effect of polymerization conditions, the polymerization temperature
was varied with 60, 80, and 100°C, and the molar ratio of Cu(I)Br catalyst to monomer was
0.025, 0.050, and 0.075.

48
J. H. LEE
Scheme . Polymerization process for hyperbranched polystyrene.
2.4. Characterization
1H NMR spectroscopy was conducted on a JEOL JNM-LA 300 spectrometer (300 MHz)
with CDCl3 as the solvent. Glass transition temperatures (Tg) of the polymers were deter-
mined using a TA DSC 2010 at a heating rate of 10°C min−1. The relative molecular weight
and molecular weight distribution were obtained by gel permeation chromatography (GPC)
equipped with a Viscotek LR125 laser refractometer and three polystyrene gel columns.
Tetrahydrofuran was used as eluting solvent at a flow rate of 1.0 mL min−1, and the calibration
curve was determined by using polystyrene standards. The Wyatt mini-DAWN Tristar light
scattering (LS) detector and Optilab DSP refractive index (RI) detector were used to measure
absolute molecular weight of the polymers. After separation through GPC column, polymer
sample was detected by RI and LS detector at 45, 90, and 135° sequentially.
3. Results and discussion
The overall polymerization scheme for hyperbranched polystyrene (HPS) is shown in
Scheme 1. During copper-mediated ATRP, a dynamic equilibrium between active and dor-
mant species is present, and can be shifted by control of the concentration of monomer or
catalyst [11]. Figure 1 shows 1H NMR spectrum of the prepared HPS polymer. Broad signals
between 7.0 and 7.5 ppm corresponding to benzene rings in the polymer are observed with
the same positions as for the monomer. In addition, the peaks at around 4.5 and 1.9 ppm cor-
respond to the proton and methyl group geminal to the bromine atom at B∗ site, respectively.
The new peaks at about 5.1 and 1.3 ppm, which were not observed for the BPOS monomer,
are attributed to the proton geminal to the bromine atom at A∗ site and methyl group geminal
to the debrominated B∗ site, respectively. This result indicates that the B∗ sites can be con-
tinuously converted to new A∗ sites during the polymerization, resulting in the generation of
dendritic unit inside the polymer.
Table 1 summarizes the polymerization conditions and characteristics of the prepared
polymers. Most of hyperbranched polystyrenes exhibited very high conversion of 99% under

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
49

Figure . H NMR spectrum of the prepared HPS polymer.
the different reaction conditions. Moreover, the molecular weights of HPS polymers were rea-
sonably as high as those of conventional hyperbranched polyacrylates [9]. Glass transitions
of the prepared HPSs were observed above room temperature (40–60°C), which is ascribed
to the rigid polystyrene structure compared to hyperbranched aliphatic polyacrylate. The
increased Tg properties can provide improved mechanical strength and thermal stability of
hyperbranched polymers for specific applications.
In order to examine the effect of polymerization temperature on the structure of the pre-
pared HPS polymer, 1H NMR spectra were recorded for 60, 80, and 100°C at the fixed catalyst
ratio of 0.075, as shown in Fig. 2(a). As reaction temperature increased, the peaks at around
6.5 and 3.5 ppm were newly detected, and their signal intensities were increased. These peaks
may be due to dehydrobromination reaction, reported in a previous study by Matyjaszewski
[12], which generates carbon-carbon double bond in polymer backbone leading to decrease
in active site of Br. To quantitatively analyze the amount of dehydrobromination during the
polymerization, the content of Br in the prepared polymer can be determined from the 1H
NMR spectra by dividing the integration of the peaks at around 5.1 and 4.5 ppm by the inte-
gration of the peaks between 7.0 and 7.5 ppm. Since the number of B functional group is equal
to the number of repeating unit in hyperbranched AB2-type polymer, the amount of Br in the
HPS polymer is theoretically 100 for 100 repeating unit. As shown in Fig. 2(b), the Br content
in 100 repeat units of HPS was decreased from about 90 to 55 by raising reaction temperature
regardless of catalyst amount. Therefore, it is confirmed that the dehydrobromination occurs
Table . Polymerization conditions and characteristics of the HPS polymers.
b
b
Temp. (°C)
Cu(I)Br (equiv.)
Time (h)
Conv.a(%)
Mn
Mw
PDIb
Tg (°C)

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.



























.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.


aAs determined by H NMR (consumption of double bonds).
bAs determined by GPC calibrated with polystyrene.

50
J. H. LEE
Figure . (a) H NMR spectra and (b) Br content of HPS polymers prepared at , , and °C.
severely during polymerization at high temperature leading to low functionality in the pre-
pared HPS polymer, and low reaction temperature of 60°C is preferred for high functionality
of active site.
To investigate the effect of catalyst amount on the structure of hyperbranched polymer,1H
NMR spectra were analyzed for various molar ratios of Cu(I)Br to monomer involving 0.025,
0.050, and 0.075. Since degree of branching cannot be determined from the 1H NMR spectra
due to peak overlap of the debrominated B∗ site at around 1.3 ppm, the number difference
between A∗ and B∗ sites (LA − LB) was calculated from the peaks at about 5.1 and 4.5 ppm.
Figure . (a) The number difference between A∗ and B∗ sites (LA − LB), (b) the relative and absolute molec-
ular weights, and (c) GPC traces of the prepared HPS polymers according to the catalyst to monomer ratio.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
51
As show in Fig. 3(a), the positive value of (LA − LB) is observed for low catalyst amount
of 0.025, which is ascribed to the higher reactivity of A∗ site with more radical-stabilizing
benzene group compared to ester group at B∗ site. As the catalyst amount increased, the
(LA − LB) was decreased to negative value, indicating that high catalyst amount can promote
the reaction chance of the less stabilized B∗ site.
To confirm the degree of branching in the prepared HPS polymers, the absolute molecular
weight of the polymers was examined by light scattering measurement. Commonly, hyper-
branched polymers have lower molecular weight obtained by GPC than linear polymers due
to the smaller hydrodynamic volume stemming from densely packed branched structure
[1–4]. Figure 3(b) and 3(c) show the molecular weights and GPC traces of the prepared
HPS polymers according to the catalyst to monomer ratio at 60°C. As the catalyst amount
increased, the difference between absolute and relative molecular weights was increased, sug-
gesting that higher degree of branching can be obtained by raising the amount of Cu(I)Br
catalyst, and the catalyst to monomer ratio of 0.075 is favored for highly branched structure.
4. Conclusions
A new type of hyperbranched polystyrene was prepared by copper-mediated self-condensing
ATRP method. The prepared hyperbranched polystyrene showed high glass transition com-
pared to conventional hyperbranched polyacrylate. The low reaction temperature and high
catalyst amount produced the hyperbranched polymers with the improved degree of branch-
ing and high functionality. Further study for application in electro-optical sensors is in
progress.
Funding
This research was supported by Basic Science Research Program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03931997).
References
[1] Zheng, Y., Li, S., Weng, Z., & Gao, C. (2015). Chem. Soc. Rev., 44, 4091.
[2] Abbasi, E., Aval, S. F., Akbarzadeh, A., Milani, M., Nasrabadi, H. T., Joo, S. W., Hanifehpour, Y.,
Nejati-Koshki, K., & Pashaei-Asl, R. (2014). Nanoscale Res. Lett., 9, 247.
[3] Dong, R., Zhou, Y., & Zhu, X. (2014). Acc. Chem. Res., 47, 2006.
[4] Ding, S.-G., Yu, L., Wang, L.-H., Wang, L.-D., Yu, Z.-Q., & You, Y.-Z. (2016). J. Mater. Chem. B, 4,
6462.
[5] Kao, S.-Y., Lin, Y.-S., Hu, C.-W., Leung, M.-K., & Ho, K.-C. (2015). Sol. Energ. Mat. Sol. C., 143,
174.
[6] Yang, X., Shang, H., Ding, C., & Li, J. (2015). Polym. Chem., 6, 668.
[7] Aydogan, C., Yilmaz, G., & Yagci, Y. (2017). Macromolecules, 50, 9115.
[8] Popescu, D.-L., & Tsarevsky, N. V. (2012). Macromol. Rapid Commun., 33, 869.
[9] Yoo, S. H., Yoon, T. H., & Jho, J. Y. (2001). Macromol. Rapid Commun., 22, 1319.
[10] Yoo, S. H., Lee, J. H., Lee, J.-C., & Jho, J. Y. (2002). Macromolecules, 35, 1146.
[11] Matyjaszeweski, K., & Wang, J.-S. (1995). Macromolecules, 28, 7901.
[12] Matyjaszeweski, K., Davis, K., & Patten, T. E. (1997). Tetrahedron, 53, 15321.