A new selenium-based RAFT agent for surface-initiated RAFT polymerization of 4-vinylpyridine

Article abstract of DOI:10.1016/j.polymer.2013.07.060

A new selenium-based reversible addition-fragmentation chain transfer (RAFT) agent, 4-cyanopentanoic acid diselenobenzoate (RAFT-Se), was synthesized and utilized in the surface-initiated RAFT polymerization of 4-vinylpyridine (4VP) on silicon substrate. The results indicate that the RAFT-Se can control the surface-initiated RAFT polymerization, as evidenced by the number-average molecular weight that increase linearly with monomer conversion, molecular weights that agreed well with the predicted values, and the relatively low polydispersity indexes. The surface-initiated RAFT polymerization with the RAFT-Se was the same polymerization mechanism as its analog, 4-cyanopentanoic acid dithiobenzoate (RAFT-S). The grafting density of the poly(4-vinylpyridine) brushes prepared in the presence of RAFT-Se (σ_RAFT-Se) and RAFT-S (σ_RAFT-S) was estimated to be about 0.51 and 0.66 chains/nm², respectively. In addition, the end of polymer chains on silicon substrate contains selenium element which may be useful in biosensor applications.

Full text of DOI:10.1016/j.polymer.2013.07.060

Polymer 54 (2013) 5345e5350
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
A new selenium-based RAFT agent for surface-initiated RAFT
polymerization of 4-vinylpyridine
,
Serkan Demirci a, Selin Kinali-Demirci b, Tuncer Caykara b
*
a Department of Chemistry, Faculty of Arts and Sciences, Amasya University, 05100 Amasya, Turkey
b Department of Chemistry, Faculty of Science, Gazi University, 06500 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A new selenium-based reversible addition-fragmentation chain transfer (RAFT) agent, 4-cyanopentanoic
acid diselenobenzoate (RAFT-Se), was synthesized and utilized in the surface-initiated RAFT polymeri-
zation of 4-vinylpyridine (4VP) on silicon substrate. The results indicate that the RAFT-Se can control the
surface-initiated RAFT polymerization, as evidenced by the number-average molecular weight that in-
crease linearly with monomer conversion, molecular weights that agreed well with the predicted values,
and the relatively low polydispersity indexes. The surface-initiated RAFT polymerization with the RAFT-
Se was the same polymerization mechanism as its analog, 4-cyanopentanoic acid dithiobenzoate (RAFT-
S). The grafting density of the poly(4-vinylpyridine) brushes prepared in the presence of RAFT-Se (sRAFT-
Se) and RAFT-S (sRAFT-S) was estimated to be about 0.51 and 0.66 chains/nm2, respectively. In addition, the
end of polymer chains on silicon substrate contains selenium element which may be useful in biosensor
applications.
Received 9 May 2013
Received in revised form
21 July 2013
Accepted 23 July 2013
Available online 31 July 2013
Keywords:
Selenium
Polymer brushes
RAFT polymerization
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
RAFT has also contributed to generate polymer brushes via a
grafting to approach [15e20]. Surface-initiated RAFT polymeriza-
Modification of the surface properties of silicon substrates is
central to many areas of research, ranging from biomaterials to
advanced microelectronics [1,2]. A stable, densely packed polymer
chains covalently bonded to the silicon substrate can be used, for
example, as recognition layers in biosensors [3], and as passivation
layers in microelectromechanical systems [4]. Conventionally, free
radical polymerization is used to graft vinyl polymers from solid
substrate surface such as gold, glass, silicon etc. accepting the
disadvantage that the molecular weight and molecular weight
distribution of the grafted chains are not controlled. With the
advent of controlled free radical polymerization methods, such as
nitroxide-mediated polymerization (NMP) [5e7], atom transfer
radical polymerization (ATRP) [8e10], single electron transfer
living radical polymerization (SET-LRP) [11,12] and reversible
addition-fragmentation chain transfer (RAFT) [12e14] polymeri-
zation, it is now possible to synthesize polymers with pre-
determined molecular weight and low polydispersity for a great
variety of vinyl monomers. RAFT is of particular interest as a very
wide range of vinyl monomers can be polymerized in a controlled
manner under moderate reaction conditions (low temperature and
tolerance to oxygen) via this method.
tion from the initiator-containing self-assembled monolayers is
attractive, since a high density of initiators can be immobilized on
the solid substrate and the initiation mechanism is more well-
defined. Covalent attachment of initiator monolayers directly sili-
con surfaces via SieC bonds, in the absence of the native oxide
layer, has also attracted a great deal of attention, because of the
thermal and chemical stability of the resulting SieC bonds [1,21,22].
On the other hand, RAFT polymerization involves a reversible
addition-fragmentation cycle, in which transfer of the dithioester
groups between the activated and dormant polymer chains main-
tains the controlled character of the polymerization process. Because
of the analog of selenium (Se) with sulfur (S) in electronic structure,
selenium-based RAFT agents have also been explored in RAFT poly-
merization [23e25]. Recently, Zhu et al. [25] synthesized selenium-
based RAFT agents and used them as the RAFT agents for vinyl ace-
tate polymerization. They [24]alsoreported theRAFT polymerization
of styrene using the similar phosphinodiselenoic esters under UV
irradiation. Zhang et al. [26] have synthesized a new hyperbranched
polyselenide with multicatalytic sites at the branching units which
acts as a new glutathione peroxidase mimic. Considering that sele-
nium is an indispensable element in human body, the introduction of
selenium into polymer chains have been used as bio-related mate-
rials in biological applications [27e30]. However, although selenium
is an essential trace element, it can be dangerous forlife if the amount
* Corresponding author. Tel.: þ90 3122021124; fax: þ90 2122279.
E-mail address: caykara@gazi.edu.tr (T. Caykara).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.07.060

5346
S. Demirci et al. / Polymer 54 (2013) 5345e5350
exceeds the tolerable limits. Lee et al. [23] successfully synthesized
phosphinodiselenoic esters and used them as RAFT agents in styrene
polymerization. Thus, introduction the selenium into polymer chains
by controlled way would favor such researches. Although the sele-
nium containing RAFT agents were used in both homogeneous and
heterogeneous media, their use for the synthesis of polymer brushes
have not been reported to date.
In this study, we describe for the first time the synthesis of a
new selenium containing RAFT agent, 4-cyanopentanoic acid dis-
elenobenzoate (RAFT-Se) for comparison to mediate the surface-
initiated RAFT polymerization of the pH-responsive monomer 4-
vinylpyridine (4VP). The electrostatic changes arising from the
protonation/deprotonation of the pyridine groups of poly(4VP) are
responsible for tuning the surface charge [31e34] that provides an
attractive strategy for immobilization of DNA, enzymes, and poly-
anionic drugs via electrostatic interactions. According to the
mechanism of surface-initiated RAFT polymerization, the chain
ends of the polymer brushes are ideally terminated by the
dithioester or diselenoester groups. It is expected that the intro-
duction of selenium-contained RAFT agents into the 4VP poly-
merization would extend the available database of RAFT agents that
may be used to successfully perform surface-initiated RAFT poly-
merization of 4VP and develop a potential bio-related polymer
brush with controllable molecular weight and structure.
added to 500 mL round-bottomed flask equipped with magnetic
stir bar. This mixture was stirred for 1 h prior to the dropwise
addition of benzyl chloride (0.25 mol), at room temperature under
an argon atmosphere. The reaction mixture was heated in an oil
bath at 67 ꢁC for 12 h. After this time, the reaction mixture was
cooled to 7 ꢁC using an ice bath. The precipitated salt was removed
by filtration and the solvent removed in vacuum. To the residue was
added deionized water (250.0 mL), and solution was filtered to
finally yield a solution of sodium diselenobenzoate.
Potassium ferricyanide(III) (0.1 mol) was dissolved in deionized
water (500.0 mL). Sodium diselenobenzoate solution (250.0 mL)
was transferred to a 1 L conical flask equipped with a magnetic stir
bar. Potassium ferricyanide solution was added dropwise to the
sodium diselenobenzoate via an addition funnel over a period of 1 h
under vigorous stirring. The precipitate was filtered and washed
with deionized water. The solid was dried in vacuo at room tem-
perature overnight.
250 mL round-bottomed flask was added distilled ethyl acetate
(100.0 mL). To the flask was added dry ACVA (0.1 mol) and di(s-
elenobenzoyl) disulfide (0.05 mol). The reaction solution was
heated at reflux for 24 h. The ethyl acetate was removed in vacuum.
The target compound was recrystallized from benzene (Scheme 1).
RAFT-Se: Yield 32%, 1H NMR (CDCl3,
d
, ppm): 10.1 (br, 1H, OH), 7.8e
7.3 (m, 5H, ArH), 2.7e2.3 (m, 4H, eCH2CH2e), 2.2 (s, 3H, eCH3). FT-
IR (ATR-FTIR,
, cmꢂ1): 3300e3100 (s, br, COOeH), 2242 (v, C^N),
y
2. Experimental
1710 (vs, C]O), 853 (s, C]Se). MALDI-TOF: calcd. for C13H13NO2Se2
373.1678, found 374.0983 (þHþ).
2.1. Materials
4-cyanopentanoic acid dithiobenzoate (RAFT-S) was synthe-
sized according to a previously published protocol [36] and detailed
above except sulfur was used in place of selenium (Scheme 1).
4-Vinylpyridine (4VP, 95%, Aldrich), 4,40-azobis(4-cyanovaleric
acid) (ACVA, ꢀ98%, Aldrich), benzyl chloride (99%, Aldrich), sele-
nium (ꢀ99.5%, Aldrich), sulfur (ꢀ99.5%, SigmaeAldrich), potassium
ferricyanide(III) (99%, SigmaeAldrich), pyridine (99.8%, Aldrich),
diethyl ether (ꢀ99%, SigmaeAldrich), dichloromethane (98.5%,
SigmaeAldrich), acetone (99.8%, SigmaeAldrich) and ethanol
(99.8%, SigmaeAldrich), were purchased commercially. Dime-
thylformamide (DMF, ꢀ98%, Aldrich) was distilled over CuSO4 before
use. Silicon (111) wafers (n-type) obtained from Shin-etsu, Han-
doutai, and cleaned using “piranha” solution (30/70 30% aqueous
hydrogen peroxide solution/sulfuric acid) at 90 ꢁC for 2 h. It should
be noted that piranha solution is extremely reactive and as such
should be handled with great care. The synthesis and immobiliza-
tion of initiator, 4,40-azobis-4-cyanopentanoyl chloride (ACVC), on
silicon substrate have been reported previously [35]. Briefly, the
cleaned silicon wafers were etched for 1 min in a 2% hydrofluoric
acid solution, quickly rinsed in degassed deionized water and dried
in a stream of nitrogen. t-Butyloxycarbonyl (t-BOC)-protected
RAFT-S: Yield 48%, 1H NMR (CDCl3, , ppm): 1H NMR (CDCl3,
d d,
ppm): 10.2 (br, 1H, OH), 7.9e7.4 (m, 5H, ArH), 2.8e2.3 (m, 4H, e
CH2CH2e), 1.9 (s, 3H, eCH3). FT-IR (ATR-FTIR,
y
, cmꢂ1): 3300e3100
(s, br, COOeH), 2234 (v, C^N), 1706 (vs, C]O), 1040 (s, C]S).
2.3. Surface-initiated RAFT polymerization procedure
Surface-initiated RAFT polymerization of 4VP from Si-ACVC was
performed using two kinds of RAFTagent (RAFT-Se and RAFT-S) and
free initiator (ACVA). The molar ratio was [4VP]:[RAFT Agent]:[free
ACVA] 250:1:0.2. A dry glass reactor, which was designed to hold
six azo initiator-immobilized silicon wafers oriented normal to the
base of the reactor, was charged with RAFT agent (2.4 mmol), 4VP
(64.7 mL, 0.6 mol), DMF (35 mL), and ACVA (0.13 mg, 0.48 mmol).
After three freeze-pump thaw cycles, the reaction mixture was
immersed in a thermostated oil bath at 70 ꢁC, and from time to
time, small samples (w3 mL) were removed with a syringe. After
24 h, reaction was quenched by precipitation in methanol. The
polymer was filtered and purified by a Soxhlet extraction with
methanol to remove poly(4-vinylpyridine) [poly(4VP)] homopoly-
mer. The molecular weights and molecular weight distributions of
the polymers were measured by size exclusion chromatography
(SEC). For ellipsometric measurements, the substrates were also
removed from the reactor at different times and washed with the
buffer solution and ethanol in an ultrasonic bath. The substrates
were dried with N2, and the ellipsometric thicknesses of the dry
polymer films were measured at five different spots on each sample
allylamine (20
mL) was introduced onto the freshly prepared SieH
surface. After irradiation with a UV light (
l
¼ 254 nm), the modified
surfaces were ultrasonically washed with 25% trifluoroacetic acid
(TFA) in dichloromethane followed by a 3 min rinse in 10% NH4OH to
remove the t-BOC protecting group and dried in a stream of nitro-
gen. Next, pyridine and four pieces of amine-functionalized silicon
substrate were added to a 1 M solution of ACVC in anhydrous
dichloromethane. After the reaction, the silicon substrates with the
surface immobilized azo initiator (Si-Azo) were recovered from the
reaction mixture and repeatedly washed with dichloromethane,
acetone, ethanol, and dried under a stream of nitrogen.
2.2. Synthesis of RAFT-Se and RAFT-S
RAFT-Se was synthesized by a modification of the previously
published protocol [36], except selenium was used in place of sul-
fur. Sodium methoxide (30% solution in methanol, 0.5 mol), anhy-
drous methanol (100.0 mL), and elemental selenium (0.5 mol) were
Scheme 1. Synthesis of RAFT-Se and RAFT-S.

S. Demirci et al. / Polymer 54 (2013) 5345e5350
5347
and averaged. The conversion of the polymerization was deter-
mined gravimetrically.
a flood gun charge neutralizer system equipped with a mono-
chromated Al K- X-ray source (h
neutralizing equipment was used to compensate sample charging,
and the binding scale was referenced to the aliphatic component of
C1s spectra at 285.0 eV. The water contact angle measurements
were conducted at room temperature using a goniometer (DSA 100,
a
y
¼ 1486.6 eV). Charging
2.4. Characterization techniques
The nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker-Spectrospin Avance DPX 400 Ultra-Shield NMR spec-
trometer with chemical shifts in ppm (for CDCl3, tetramethylsilane
was used an as internal standard). The size exclusion chromatog-
raphy (SEC) analysis of poly(4VP) was performed with an Waters
system consisting of a binary pump, an autosampler, a temperature
controlled column oven, and differential refractometer. The
analytical separation was performed on Waters Styragel HR4 and
HR6 columns (7.8 ꢃ 300 mm) at 27 ꢁC with a flow rate of 1 mL/min.
Monodisperse polystyrene standards (Shodex) were used to cali-
brate the molecular weights. The ellipsometric measurements were
conducted in ambient conditions using an ellipsometry (DRE, EL
Krüss) equipped with a microliter syringe. Deionized water (5
mL,
18 M cm resistivity) was used as the wetting liquid. The
U
morphology of the surfaces was recorded on an atomic force mi-
croscope (AFM, Park Systems XE70 SPM Controller LSF-100 HS). A
triangular shaped Si3N4 cantilever with integrated tips (Olympus
Co. Ltd.) was used to acquire the images in the non-contact mode.
The normal spring constant of the cantilever was 0.02 N/m. The
force between the tip and the sample was 0.87 nN.
3. Results and discussion
X20C) equipped with a HeeNe laser (
l
¼ 632.8 nm) at a constant
It has been reported that in the preparation of polymer brushes,
a uniform a dense layer of immobilized initiators is indispensable
[35]. In this study, the azo initiator-immobilized on the silicon
substrate was used to initiate the growth of the polymer brushes in
the presence of chain transfer agent for RAFT polymerization. The
azo initiator ACVC was immobilized on the hydrogen-terminated
silicon (Si 111) (SieH) surface by the three-step process. First, UV-
induced coupling of t-BOC protected allylamine with SieH sur-
face, and then conversion of the t-BOC protected allylamine into the
free amine groups by TFA produce the amine-terminated SieNH2
surface. Subsequent the amide reaction of the SieNH2 with the
ACVC initiator yields the Si-Azo surface. The presence of a Cl 2p
incident angle of 75ꢁ. The average dry thickness of polymer brushes
on silicon substrate was determined by fitting the data with a
three-layer model [37]. The grafting density (s
, chains/nm2),
average distance between grafting points, D (nm) and radius of
gyration (Rg, nm) of the polymer chains were calculated from the
dry polymer thickness, h (nm), the number-average molar weight
and Mn (g/mol) (The molecular weight of the grafted polymer
chains is assumed to be similar to that of the free polymer in so-
lution [38,39]) values using Luzinov equation [40]. The X-ray
photoelectron spectra were recorded by using X-ray photoelectron
spectrometer (XPS) (Thermo Scientific). XPS was used by means of
Fig. 1. Results of surface-initiated RAFT polymerization of 4VP in DMF at 70 ꢁC, [4VP]:[RAFT-Se or RAFT-S]:[free ACVA] ¼ 250:1:0.2 polymerization kinetics (a), molecular weights
and polydispersity indices of poly(4VP) with increasing monomer conversion (b), SEC chromatograms of the free poly(4VP) synthesized in the presence of RAFT-Se (c) and RAFT-S
(d) at different monomer conversions.

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S. Demirci et al. / Polymer 54 (2013) 5345e5350
peak at approximately 200 eV for the Si-Azo surface indicates that
the azo initiator has been successfully immobilized on the silicon
surface (Fig. 1). After immobilization of ACVC, the thickness of the
monolayer on the Si-Azo surface was about 2.1 ꢄ 0.6 nm.
In this study, two kinds of RAFT agent, RAFT-Se and RAFT-S with
same R and Z groups (Scheme 1), were synthesized and used as
chain transfer agents in surface-initiated RAFT polymerization of
4VP, respectively. Surface-initiated RAFT polymerization of 4VP was
carried out in the presence of a free initiator ACVA, a RAFT agent
(RAFT-Se or RAFT-S) and the initiator-immobilized substrate to
produce free poly(4VP) and the corresponding polymer brushes on
the substrates simultaneously (Scheme 2). The kinetics and living
features of the polymerization explored and compared. The poly-
merization kinetics was given in Fig. 2a. A linear increase in the
semilogarithmic kinetic plots was observed for all the cases, and
there was an induction period of about 2 h (vide infra). The main
reason of the induction period maybe partly due to the lower
decomposition rate of intermediate radical [41] or the impurity of
chain transfer agent [42]. The apparent propagation rate constants
(kappp) in the presence of RAFT-Se and RAFT-S were 0.068 and
0.083 hꢂ1, respectively. Excellent control of the molecular weights,
which increased linearly with conversions, and low polydispersity
values around 1.43 were achieved for two agents (Fig. 2b). The SEC
traces of the free polymers shown in Fig. 2ced are unimodal and
clear shift of the traces to smaller elution time (higher molecular
weight) with the polymerization time. These results indicated that
both of the RAFT-Se and RAFT-S are effective RAFT agents in con-
trolling the polymerization of 4VP.
Fig. 2. Survey scan XPS spectra of Si-Azo (a), poly(4VP) brushes synthesized in the
presence of RAFT-Se (b) and RAFT-S (c), respectively.
The relationship between the thickness (h, nm) of the poly(4VP)
brushes and Mn of the corresponding free polymers formed in the
presence of RAFT-Se and RAFT-S is shown in Fig. 3. A direct pro-
portion relationship was observed between the Mn of the free
polymer (we assumed that the free polymer and the brush with the
same Mn are formed in the same reaction batch [38,39]) and brush
thickness (h ¼ Mn= NA ꢃ 10ꢂ21 where
s
r
s
(chains/nm2) is the
grafting density,
r
(1.15 g/cm3) is the density of polymer and NA
(6.02 ꢃ 1023 molꢂ1) is Avogadro’s number [40]). From the slope of
the linear line (Fig. 3), the grafting density of the polymer brushes
prepared in the presence of RAFT-Se (sRAFT-Se) and RAFT-S (sRAFT-S
)
was estimated to be 0.51 and 0.66 chains/nm2, respectively. In this
estimation, average distance between grafting points, DRAFT-Se and
DRAFT-S (D ¼ (4/ps
)
1/2) [40], was found to be 1.58 and 1.38 nm,
respectively. The radius of gyration (Rg, nm) of poly(4VP) was
estimated using the equation: Rg ¼ bðDPn=6Þ1=2 where DPn is the
degree of polymerization and b, is the segment length (assumed to
be 0.797 nm for poly(4VP) chains) [43]. An indication of having
polymeric brushes prepared in the presence of RAFT-Se and RAFT-S
can be when the interchain distances are less than the radius of
gyration of the corresponding free polymer chains [44]. We have
achieved high grafting density for poly(4VP) prepared in the
presence of both RAFT-Se and RAFT-S, and the interchain grafting
Fig. 3. Evolution of the thickness (h, nm) of the poly(4VP) brushes synthesized in the
presence of RAFT-Se and RAFT-S as a function of the number-average molecular weight
ðMnÞ.
Scheme 2. Schematic diagram illustrating the process of the surface-initiated RAFT
polymerization of 4VP from the Si-Azo surface.

S. Demirci et al. / Polymer 54 (2013) 5345e5350
5349
Fig. 4. 2D AFM images (5 ꢃ 5
m mL water droplets (top) on the poly(4VP) brushes synthesized in the presence of RAFT-Se (a) and
m2) and photographs in ambient conditions of 4
RAFT-S (b). The cross sections corresponding to the red line shown in the AFM images are given below each image. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
distances are less than the radius of gyration of these free poly(4VP)
chains in solution, indicating that the chains are indeed in a
stretched brush-like conformations. The formation of poly(4VP)
brushes was also confirmed by XPS measurements, as shown in
Fig. 2bec. Two peaks at 400.1, and 285.0 eV were observed, cor-
responding to nitrogen and carbon, respectively. The atomic ratios
of N/C in the poly(4VP) brushes prepared in the presence of RAFT-
Se and RAFT-S were estimated to be 0.20/0.80 and 0.17/0.83,
respectively, which were relatively close to the theoretical values
calculated from atomic composition of poly(4VP) (0.14 N and
0.86 C). Apart from these peaks, there appears three signals at
about 229.0, 162.0 and 54.5 eV assignable to Se 3s, Se 3p and Se 3d,
respectively, in the survey scan spectrum of the poly(4VP) brushes
prepared in the presence of RAFT-Se (Fig. 4a). However, in the
survey scan spectrum of the brushes prepared in the presence of
RAFT-S, two signals at about 163.5 and 226.2 eV assignable to S 2p
and S 2s, respectively, were observed (Fig. 2c). Meanwhile, the
disappearance of photoelectron lines of Si indicates that the pol-
y(4VP) films on the surface of the silicon substrate is thicker than
the XPS probing depth (w10 nm) [45,46]. In the topographic AFM
images of the poly(4VP) brushes prepared in the presence of RAFT-
Se and RAFT-S using the non-contact mode, the root-mean-square
(rms) values of the surface roughness were found to be 4.18 and
polymerized on the silicon surface via the surface-initiated RAFT
polymerization technique, using RAFT-Se or RAFT-S as the chain
transfer agent and surface immobilized azo species as the initiator.
Kinetic studies revealed a linear increase in the thickness of the
surface-graft polymerized film with reaction time, indicating that
the chain growth from the surface was a controlled process with a
“living” character. The grafting density of the poly(4VP) brushes
prepared in the presence of RAFT-Se (sRAFT-Se) and RAFT-S (sRAFT-S
)
was estimated to be about 0.51 and 0.66 chains/nm2, respectively.
AFM images revealed that surface-initiated polymerization via the
RAFT process had preceded uniformly on silicon surfaces. To the
best of our knowledge, this maybe the first example of using a
selenium-based RAFT agent in the surface-initiated RAFT poly-
merization of 4VP. Hence, the present study has illustrated that
new surface functionalities and molecular architecture arising well-
defined graft chains with selenium-based end group can be incor-
porated onto the silicon substrates via surface-initiated RAFT
polymerization.
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4. Conclusions
A new selenium-based RAFT agent (RAFT-Se) was synthesized
accompany with its sulfur analog (RAFT-S). 4VP was graft

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