Grafting of polymers from electrodeposited macro-RAFT initiators on conducting surfaces

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.05.013

A novel route for grafting polymers via electropolymerization of a macro-reversible addition fragmentation chain-transfer (RAFT) agent onto a conducting surface is reported. The electro-deposition of the chain transfer agent (CTA) on the conducting surface was done using cyclic voltammetry (CV). The statistically exposed dithiobenzoate moieties served as a CTA for the polymerization of styrene on a matrix of an electrodeposited conjugated polymer. The polystyrene (PS)-modified substrate was then used as the macro-CTA for the synthesis of the second block of poly-tert-butyl acrylate (PTBA) on the surface. The polymerization from the surface was characterized by surface analytical methods including AFM, XPS, and contact angle measurements.

Full text of DOI:10.1016/j.reactfunctpolym.2011.05.013

Reactive & Functional Polymers 71 (2011) 938–942
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Grafting of polymers from electrodeposited macro-RAFT initiators on conducting
surfaces
Carlos D. Grande c, Maria Celeste Tria a, Guoqian Jiang a, Ramakrishna Ponnapati a, Yushin Park a,
Fabio Zuluaga c, Rigoberto Advincula a,b,
⇑
a Department of Chemistry, University of Houston, TX 77204-5003, USA
b Department of Chemical and Biomolecular Engineering, University of Houston, TX 77204-5003, USA
c Departamento de Quimica, Universidad del Valle, AA 25360, Cali, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel route for grafting polymers via electropolymerization of a macro-reversible addition fragmenta-
tion chain-transfer (RAFT) agent onto a conducting surface is reported. The electro-deposition of the
chain transfer agent (CTA) on the conducting surface was done using cyclic voltammetry (CV). The sta-
tistically exposed dithiobenzoate moieties served as a CTA for the polymerization of styrene on a matrix
of an electrodeposited conjugated polymer. The polystyrene (PS)-modified substrate was then used as the
macro-CTA for the synthesis of the second block of poly-tert-butyl acrylate (PTBA) on the surface. The
polymerization from the surface was characterized by surface analytical methods including AFM, XPS,
and contact angle measurements.
Received 14 March 2011
Received in revised form 22 May 2011
Accepted 30 May 2011
Keywords:
Grafting
Electropolymerization
Macro-CTA
Polymer brushes
Thiophene
Ó 2011 Elsevier Ltd. All rights reserved.
RAFT
1. Introduction
as silica-nanocomposites, silica nanoparticles with combination of
click-chemistry and RAFT polymerization, silicon wafers, carbon
The fabrication of nanostructured ultrathin films is of consider-
able interest for creating functional coatings for optical devices,
biomedical implants, sensors, and electronic semiconductors.
Methods of producing these ultrathin films ranges from simple
evaporative to directed-assembly techniques including spin-
coating, Langmuir–Blodgett films [1], layer-by-layer [2], and drop-
let evaporation to name a few. However, these polymer films can
be easily desorbed or delaminated under the right solvent and
temperature conditions. To circumvent this problem, surface-initi-
ated polymerization (SIP) has been used to immobilize polymers at
interfaces [3]. In this method, polymer brushes are usually grown
from initiators or chain transfer agents (CTAs) covalently tethered
on surfaces to create a more robust film of high grafting density.
This is typically done by self-assembled monolayers (SAM) on
the surface either through initiator thiols, for gold surfaces, or si-
lanes, for hydroxyl-terminated surfaces. For reversible addition
fragmentation chain transfer (RAFT) polymerization, the CTA was
first immobilized and subsequent polymerization using the ‘‘graft-
ing from’’ approach was demonstrated from different surfaces such
nanotubes, clay, CdS nanoparticles, cellulose and membranes [4].
Although this strategy addresses the issue of film stability on these
surfaces, specific initiators and CTAs may have limited application
towards electrode interfaces and the electrochemical environment.
This is the case especially with thiol derivatives which can be
reductively desorbed at specific potentials. Moreover, the issue of
macroinitiator functionality in electrochemically active interfaces
has not been widely explored. To our knowledge, only one report
has demonstrated RAFT grafting from indium–tin oxide (ITO) re-
cently using titanium-diol coordination. In their case, the RAFT
agent was self-assembled on the surface by a catechol-functional
RAFT agent and not directly electrodeposited as in our case [5].
Herein, we report a novel method of introducing a CTA on con-
ducting and electrode substrates via direct electrografting (Scheme
1). Conveniently, only one type of CTA was needed to graft poly-
mers on both gold and indium tin oxide (ITO) electrode surfaces,
i.e. no need for a separate thiol or silane derivative synthesis. The
presence of the electro-active moiety, thiophene, in the CTA,
enabled its direct surface immobilization through electropolymer-
ization. The growth of the polymer brush from the electro-immo-
bilized CTA was then demonstrated by using the surface initiated
RAFT polymerization (SI-RAFT). The RAFT technique should extend
the versatility of the system towards a wide range of monomers
⇑
Corresponding author at: Department of Chemistry, University of Houston, TX
77204-5003, USA. Fax: +1 713 743 1755.
E-mail address: radvincula@uh.edu (R. Advincula).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.05.013

C.D. Grande et al. / Reactive & Functional Polymers 71 (2011) 938–942
939
instrument with a triple detector array (RALS, IV, RI, or UV)
equipped with 2 GMHHR-M and 1 GMHHR-L mixed bed ViscoGel
columns (eluent: THF; flow rate: 1 mL minꢀ1). CV was performed
on an Amel 2049 potentiostat and power lab/4SP system with a
three-electrode cell. In all the measurements, the counter electrode
was a platinum wire, and ITO or gold was used as a working elec-
trode. Atomic force microscopy (AFM) imaging was performed un-
der ambient conditions with a PicoSPM II (PicoPlus, Molecular
Imaging) in the intermittent contact mode. Ellipsometric measure-
ments were carried out using the Multiskop, Optrel GbR with a
632.8 nm helium–neon laser at 60o angle of incidence. Refractive
indices were fixed at 1.50 for all respective surface modifications
except for polystyrene (PS) and poly(t-butylacrylate) (PTBA), which
were 1.55 and 1.46, respectively. The Au-glass electrodes were
cleaned with a plasma ion cleaner (Plasmod, March). X-ray photo-
electron spectroscopy (XPS) was carried out in a Physical Electron-
ics 5700 instrument with photoelectrons generated by the non-
monochromatic Al Ka irradiation (1486.6 eV). Photoelectrons were
collected at a take-off angle of 45o using a hemispherical analyzer
operated in the fixed retard ratio mode with an energy resolution
setting of 11.75 eV. The binding energy scale was calibrated prior
to analysis using the Cu 2p3/2 and Ag 3d5/2 lines. Charge neutraliza-
tion was ensured through co-bombardment of the irradiated area
with an electron beam and the use of the non-monochromated
Scheme 1. General scheme of the preparation of polymer brushes from the electro-
immobilized CTA.
Al Ka source. This placed the adventitious C 1s peak at a binding
energy of 284.6 eV.
[6]. Moreover, by using anodic electropolymerization to introduce
the CTA on the surface, a g-conjugated or conducting polymer film
can be deposited simultaneously, which presents a broad range of
potential applications in sensors, electro-optical materials, and
semi conductor devices [7]. Thus, the combination of the SI-RAFT
polymerization of different functional polymers with conducting
or g-conjugated polymers may serve as a platform for a novel class
of ultra-thin polymer films with multiple stimuli mechanisms
ranging from solvent-selective to electrochemical response.
2.3. Synthesis of the electrograftable CTA (Scheme 2)
2.3.1. Synthesis of 4-Cyano-4-((thiobenzoyl)sulfanyl)pentanoic acid
(1) [1].
4,4’-Azobis(4-cyanovaleric acid) (4.19 g, 0.0149 mol) and
bis(thiobenzoyl)disulfide (3.07 g, 0.01 mol) were dissolved in ethyl
acetate (200 mL) in a 500 mL round-bottom flask equipped with a
condenser. The mixture was degassed by bubbling through with N2
and heated to reflux for 20 h under N2. The reaction was allowed to
cool to room temperature, and the solvent was removed in vac-
uum. The crude product was purified by column chromatography
(silica gel) with ethyl acetate: hexanes 2:3 as the eluent. After
the removal of solvent, the red fraction gave 4-cyano-4-((thi-
obenzoyl)sulfanyl)-pentanoic acid as a red oil. The product solidi-
fied upon sitting at ꢀ20 °C. 1H NMR (CDCl3) d (ppm): 1.93 (s, 3H,
CH3); 2.38–2.80 (m, 4H, CH2CH2); 7.42 (m, 2H, m-ArH); 7.56 (t,
1H, J = 8 Hz p-ArH); 7.91 (d, 2H, J = 7.3 Hz, o-ArH). 13C NMR (CDCl3)
d (ppm): 24.1, 29.5, 32.9, 45.6, 118.4, 126.7, 128.6, 133.1, 144.4,
177.3, 222.1.
2. Experimental part
2.1. Materials
Reagent chemicals and monomers were purchased from Aldrich
and were used without further purification unless otherwise indi-
cated. Tetrahydrofuran (THF) used in the syntheses was distilled
from sodium benzophenone. Styrene and tert-butylacrylate (TBA)
were passed through a column of activated basic alumina to re-
move the inhibitor. 2,2-azobis(isobutyronitrile) (AIBN) which was
recrystallized twice from ethanol.
2.3.2. Synthesis of 2-(thiophen-3-yl)ethyl 4-cyano-4-
2.2. Characterization
(phenylcarbonothioylthio) pentanoate, CTA (2)
In a 100 mL round-bottom flask equipped with a stir bar and an
addition funnel, a solution of 4-cyano dithiobenzoate pentanoic
acid (0.675 g, 2.42 mmol), 2-(thiophen-3-yl)ethanol (0.36 g,
2.90 mmol), and 4-(dimethylamino)pyridine (DMAP) (30 mg,
NMR spectra were recorded on a General Electric QE-300 spec-
trometer at 300 MHz for 1H. UV–Vis was recorded on an Agilent
8453 spectrometer. GPC was carried out on a Viscotek 270
Scheme 2. Synthesis of the electro-graftable chain transfer agent (CTA) (2).

940
C.D. Grande et al. / Reactive & Functional Polymers 71 (2011) 938–942
0.245 mmol) in 30 mL of dry CH2Cl2 was cooled to 0 °C under N2. N,
N0-Dicyclohexylcarbodiimide (DCC) (0.598 g, 2.89 mmol) was dis-
solved in 5 mL of CH2Cl2 and added dropwise to the reaction flask
under stirring. After complete addition of DCC, the reaction was
stirred for 5 min at 0 °C and then allowed to warm to room temper-
ature overnight. Then, the solid was removed by filtration, and the
filtrate was washed with diluted aqueous sodium bicarbonate
(20 mL) and water (2 ꢁ 20 mL) and finally dried over anhydrous
MgSO4. The solution was filtered and the solvent removed to yield
the crude product mixture as red oil, which was further purified by
column chromatography on silica gel using 2:1 hexane/ethyl ace-
tate as eluent. The final product was obtained as viscous red oil
(0.342 g, 36.3% yield). 1H NMR (CDCl3) d (ppm): 1.91 (s, 3H, CH3);
2.34–2.73 (m, 4H, CH2CH2); 2.98, (t, 2H, CH2, J = 8 Hz) 4.32 (t, 2H,
-OCH2, J = 7.6 Hz); 6.97 (d, 1H, J = 4 Hz, thiophene); 7.03 (s, 1H, thi-
ophene); 7.28 (d, 1H, thiophene); 7.40 (t, 2H, J = 8.6 Hz, m-ArH);
7.57 (t, 1H, p-ArH, J = 8 Hz); 7.91 (d, o-ArH, 2H, J = 9 Hz). 13C NMR
d (ppm) (CDCl3): 24.0, 29.4, 29.8, 33.3, 45.6, 64.8, 118.4, 121.6,
125.7, 126.4, 126.6, 128.5, 133.0, 137.6, 144.5, 171.4, 222.2. Ele-
mental analysis calculated for C19H19NO2S3: C, 58.5; H, 4.9; N,
3.6; O, 8.2; S, 24.6. Found: C, 58.2; H, 4.9; N, 3.7; S, 24.2.
of MeOH. This procedure was repeated until no monomer signals
were observed by 1NMR. The resulting polymer was dried under
vacuum at room temperature until no weight loss was observed
and then analyzed by gel permeation chromatography (GPC). For
the copolymer synthesis, the procedure used was the same as that
for polystyrene synthesis, but the monomer used was TBA with the
ITO or gold modified polystyrene surfaces and the free polymer
was used to estimate the molecular weight and the monomer
conversion.
3. Results and discussion
As shown in Scheme 1, the fabrication of the polymer brush
starts with the electro-deposition of the CTA copolymerized with
carbazole on the conducting surface using cyclic voltammetry
(CV). Copolymerization with the carbazole is a direct demonstra-
tion of the potential for electropolymerizability with other con-
ducting polymers as previously reported by our group [8]. The
statistically exposed dithiobenzoate moieties on the surface then
serves as the CTA for polymerization of styrene as the first block.
The polystyrene (PS)-modified substrate was then used as the
macro-CTA for the synthesis of the second block of poly-tert-butyl
acrylate (PTBA) on the surface. Again, the copolymerization (di-
block) demonstrates the feasibility for SIP living polymerization
methods in this system [4]. The surfaces were then characterized
by atomic force microscopy (AFM), X-ray photoelectron spectros-
copy (XPS), ellipsometry, water contact angle, and UV–Vis
spectroscopy.
During the electropolymerization process, it can be observed
that copolymerization of the thiophene bearing CTA with carbazole
lowered the onset of oxidation (0.6 V) (Fig. 1a) compared to that of
the CTA alone (1.0 V) (Fig. 1b). This is preferred to prevent possible
decomposition of the polymer film at higher oxidation potentials
[8]. This technique is an advantage because it avoids the use of a
thiophene dimer or trimer, to lower the overall oxidation potential
of electropolymerization of polythiophenes, which requires multi-
ple steps of synthesis [9]. It can be seen from Fig. 1 that these films
have high cyclic stability with the oxidation and reduction waves
consistent with a thiophene and carbazole co-electropolymeriza-
tion. The electropolymerized film on the ITO was characterized fur-
ther using the UV–Vis spectroscopy, confirming the presence of a
2.4. Electrodeposition of CTA on conducting surfaces
The electrodeposition of the CTA RAFT agent was done using the
cyclic voltammetry (CV) technique. In a three-electrode cell, 0.1 M
tetrabutylammonium hexafluorophosphate (TBAP) as supporting
electrolyte, 0.25 mM of the CTA RAFT agent and 0.75 mM of 9H-
carbazole or 1:3 M ratio in CH2Cl2 were mixed and stirred sweep-
ing the voltage at a scan rate of 50 mV/s during 20 cycles from 0 to
1.3 V using ITO or gold surfaces as working electrodes and also as
the substrates, the reference electrode was Ag/AgCl and the coun-
ter electrode was platinum.
2.5. Surface-initiated RAFT polymerization
In
a
typical run, 5.0 mL of styrene (4.53 g, 43.49 mmol),
mol) of the CTA, 1.42 mg (8.69 mol) of AIBN
47.94 mg (86.98
l
l
initiator and 3.0 mL of THF were added to a 50 mL Schlenk Flask
and three freeze-pump cycles were carry out to remove any dis-
solved gas. After that, the contents were stirred gently and purged
with nitrogen for one hour. To a second Schlenk tube backfilled
with nitrogen the ITO or gold modified with the CTA surfaces were
added and the content of the first schlenk tube was added via can-
nula. The tube was sealed with septum and placed in a preheated
oil bath at 60 °C and the polymerization was carried out within 8 h.
Untethered polymer was removed from the substrate via Soxhlet
extraction overnight at 60 °C in THF. Free polymer from the poly-
merization solutions was precipitated using an excess of 10 times
p-conjugated polymer at the interface (Fig. 2). The peaks observed
at 400 and 760 nm corresponded to the absorption of polaronic
and bipolaronic species of the combined polycarbazoles and poly-
thiophenes species [8,10]. The immobilization of the copolymer
film was further verified by XPS. The XPS scan of the electrodepos-
ited film (Fig. 3a) reveals the presence of the expected C, O, N, and S
signals derived from the statistical distribution of carbazole and
CTA functional group [10]. The high resolution scan of the S 2p
Fig. 1. Cyclic voltammograms of (a) the CTA thiophene and carbazole (Inset) The potential range for the oxidation and reduction peak and (b) the Tp-CTA alone after 20 cycles
at scan rate of 50 mV/s during the electropolymerization on ITO.

C.D. Grande et al. / Reactive & Functional Polymers 71 (2011) 938–942
941
Table 1
Experimental conditions and molecular weight properties of solution polymerizations
determined by GPC.
0.08
0.06
0.04
0.02
0.00
-0.02
a
Polymer
[M]o:[CTA]o:[I]o
Mn,theory
Mn,GPC
%Convb
PDI
PS
PTBA
500:1:0.1
500:1:0.1
9410
12503
9.50 ꢁ 103
14.0
15.3
1.05
1.09
1.27 ꢁ 103
a
Mn,theory = [M]/[CTA] ꢁ conv ꢁ MWM + MWCTA
.
b
Conversion determined by RI response for styrene and NMR for TBA.
400 nm
the PS-b-PTBA (Fig. 3b and c) corroborated with the expected the-
oretical distribution of the peaks where greater C 1s peak intensity
is observed for the PS brush and an increase in O 1s intensity rel-
ative to the C 1s peak was seen after incorporation of the PTBA
block. The AFM topographies of the films showed transition in
morphological features from the electropolymerized macroiniti-
ator layer (granular) (rms roughness = 0.35 nm) to the formation
of the homopolymer (globular) (rms roughness = 0.92 nm) and di-
block brushes (ridged features) (rms roughness = 3.87 nm) (Fig. 4).
The thickness was determined using ellipsometry and also in-
creased additively from the electro-immobilized carbazole-CTA
film (5.3 nm); PS brushes (17.8 nm); diblock copolymer of the
PS-b-PTBA (30.7 nm).
To investigate the versatility on another electrode, the whole
process was also conducted on a gold surface. The thickness and
contact angle of the films on each step were monitored to examine
the deposition of the films introduced on the surface. Table 2 sum-
marizes the thicknesses and contact angles of the films on the sur-
face after each step. The increasing thickness of the film after each
surface modification step proved the stepwise incorporation of the
materials deposited beginning with electrodeposition up to the
formation of the diblock copolymer. The static water contact angle
of the electro-grafted CTA changed from 75° to 97° after the
760 nm
800
300
400
500
600
700
Wavelength (nm)
Fig. 2. UV–Vis spectrum of the co-electropolymerized CTA (2) and carbazole on ITO
surface (0.25 mM CTA 1 and 0.75 mM of carbazole monomer in CH2Cl2).
peak confirmed the assignment of thiophene and the dithiobenzo-
ate moieties of the CTA.
The CTA-modified ITO surface was then subjected to SI-RAFT
polymerization of styrene and then TBA as a second block. Free
CTA was added in the monomer solution to ensure controlled
polymerization kinetics and for molecular weight estimation of
the polymer brushes (Table 1). Narrow PDI values were obtained
(<1.2), signifying a well-controlled polymerization. XPS results of
the PS brush and the diblock copolymer of PS-b-PTBA likewise
confirmed the successful growth of the polymer brushes. Compar-
ison of the XPS wide scans of the C 1s and O 1s peaks of the PS and
Fig. 3. (a) XPS wide scan of the co-electropolymerized CTA (S2) and carbazole. (Inset) Narrow scan of the S 2p peak from the electropolymerized film, and XPS narrow scan of
the (b) C 1s and (c) O 1s of the PS and PS-b-PTBA brushes.
Fig. 4. AFM topographies of the (a) electro-copolymerized CBz-TP-CTA, (b) PSty homopolymer after first RAFT polymerization, and (c) PSty-b-PTBA diblock copolymer brush
after the second RAFT polymerization on ITO substrate. (Inset) The 3-D representation of each topography. A scale bar is equivalent to 1 lm.

942
C.D. Grande et al. / Reactive & Functional Polymers 71 (2011) 938–942
Table 2
E-1551. C.D. Grande also acknowledge support from the Instituto
Colombiano de Investigaciones Cientificas, COLCIENCIAS and the Cen-
tro de Excelencia en Nuevos Materiales CENM for economic support.
Technical Support from Viscotek, Malvern Instruments and Optrel
is also acknowledged.
Thickness and contact angle measurements for each stage of grafting.
Surface
Thickness (nm)
Contact angle (degrees)
CBz-Tp-CTA
PS brush
PS-b-PTBA brush
2.2 0.2
11.3 1.0
26.1 1.3
75 1.03
97 0.56
63 0.97
Appendix A. Supplementary data
growth of the more hydrophobic PS brush and then to 63° after the
PTBA block, which is more hydrophilic. The films formed showed
stability on the surface as proven by its adhesion even after over-
night Soxhlet extractions. This stability originates from the insolu-
bility of the highly cross-linked network of the macroinitiator on
the surface as demonstrated in our previous report on electrode-
posited macroprecursors [11].
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.reactfunctpolym.2011.05.013.
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This work was supported by the National Science Foundation
CBET-0854979, DMR-10-06776, and Robert A Welch Foundation,