Surface-initiated ring-opening metathesis polymerization (SI-ROMP): Synthesis and electropolymerization of terthiophene-functionalized olefin peripheral dendrons

Article abstract of DOI:10.1021/ma101746e

We report on the synthesis and electrodeposition of peripheral olefin dendrons with electropolymerizable focal point terthiophene units. These films were utilized for surface initiated ring-opening metathesis polymerization (SI-ROMP) of norbornene to form

Full text of DOI:10.1021/ma101746e

10262 Macromolecules 2010, 43, 10262–10274
DOI: 10.1021/ma101746e
Surface-Initiated Ring-Opening Metathesis Polymerization (SI-ROMP):
Synthesis and Electropolymerization of Terthiophene-Functionalized
Olefin Peripheral Dendrons
Guoqian Jiang, Ramakrishna Ponnapati, Roderick Pernites, Mary Jane Felipe, and
Rigoberto Advincula*
Department of Chemistry and Department of Chemical and Biomolecular Engineering, University of Houston,
Houston, Texas 77204-5003, United States
Received July 31, 2010; Revised Manuscript Received October 30, 2010
ABSTRACT: We report on the synthesis and electrodeposition of peripheral olefin dendrons with electro-
polymerizable focal point terthiophene units. These films were utilized for surface initiated ring-opening
metathesis polymerization (SI-ROMP) of norbornene to form grafted polynorbornene brushes. The dendrons
were first electrodeposited on an electrode surface, forming a highly dense and uniform polythiophene-type
film, and their electrochemical behavior was investigated. Subsequently, after activation with a transition
metal metathesis catalyst, polynorbornene brushes were grown from the electrodeposited films which were
found to be highly dependent on the density of exposed olefin functional groups and dendron generation. The
change in film morphology was examined by atomic force microscopy (AFM). X-ray photoelectron spectros-
copy (XPS) was used to prove polynorbornene brush film growth and composition on top of the electro-
polymerized layer. Brush growth kinetics experiments were also carried out to understand the correlation
between the structure of the dendrons and polynorbornene brush growth mechanism. The method presented in
this paper provides a facile route to prepare robust, uniform, and controllable polymer brushes grafted from an
underlying π-conjugated polymer layer.
1. Introduction
Choi and co-workers have reported the formation of diblock copoly-
mer brushes via SI-ROMP and their morphological changes upon
exposure to different solvents.6
Polymer brushes via surface-initiated polymerization (SIP) on
flat surfaces and colloidal particles have received much interest
because of their synthetic inimitability.1 There are, in general, two
different approaches;“grafting to” and “grafting from”;to grow
polymer brushes. Compared to the “grafting to” method which
usually results in nonuniform films with low surface coverage,
slow grafting, and poor adhesion, the “grafting from” approach
or SIP allows monomers of interest to controllably grow from
surface-bound initiators. Therefore, the properties of brush films
generated by SIP can be modified by varying parameters related
to the type of initiation mechanism, grafting density, and control
on degree of polymerization.2 The surface initiators, inmost cases,
are covalently attached to surfaces via self-assembled monolayers
(SAMs). This provides a uniform surface coverage of the initiat-
ing functional groups with the possibility for obtaining block
copolymers or mixed polymer brush systems.
Surface-initiated ring-opening metathesis polymerization (SI-
ROMP) has emerged as a novel method to synthesize polymer
brushes using late transition metal catalysts.3 As a living/controlled
polymerization technique, SI-ROMP offers the capability of pre-
paring uniform polymer brushes and block copolymers by meta-
thesis methods. In addition, it has shown rapid reaction kinetics
under mild conditions. Jennings et al. recently investigated SI-ROMP
kinetics by varying the alkyl chain length on the side of norbornene
monomers.4 Different functional groups have been introduced
into norbornene monomers to form polymer brushes for a variety
of applications. For example, photochromic polymer brushes have
been reported by Locklin et al. based on photoinduced iso-
merization of spiropyran-functionalized polynorbornene films.5
In the immobilization of surface initiators onto flat electrically
conductive substrates, the formation of SAMs from silane or
thiol molecules provides two-dimensionally ordered surfaces for
polymer brush growth. Besides SAMs, different strategies have
been developed to prepare polymer layers on conductive sur-
faces.7-13 Electrochemical deposition of polyacrylate films pro-
vides a sublayer which includes active surface initiators. The poly-
mer brushes are then chemically attached to the conducting
electrode surface.8-10 Detrembleur et al. combined electrograft-
ing and ROMP to prepare polynorbornene brushes on conduc-
tive substrates.14 In their work, they first electrochemically grafted
poly(norbornenylmethylene acrylate) films which were activated
by Grubbs catalyst on the norbornenyl pendant groups. Then a
SI-ROMP procedure of norbornene monomer generated poly-
(norbornene-co-acrylate) brush films with several micrometers
in thickness. However, this strategy usually yields only thin
initiator-containing polymer films due to rapid passivation of the
cathode by the insulating polyacrylate layer and early termina-
tion of polymerization. Moreover, in the cathodic electropoly-
merization process, monomers containing protic functional groups
such as alcohol, amine, and carboxylic acid cannot be electro-
grafted due to their reduction at the same or at a less cathodic
potential than the (meth)acrylate. Relatively anodic polymeriza-
tion of monomers bearing surface initiating functionalities is
more facile and controllable over a wider range of solvents.
Herein, we report a new method to attach ROMP surface
initiators onto electrode surfaces by electrochemically depositing
a polythiophene film with different generations of dendritic
peripheral olefin functional groups (Scheme 1). From a statis-
tical point of view, there should be a controlled number of free
*To whom correspondence should be addressed. E-mail: radvincula@
uh.edu.
pubs.acs.org/Macromolecules
Published on Web 11/24/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 24, 2010 10263
Scheme 1. Electrochemical Deposition of Frechet-Type Peripheral Olefin Dendrons Leading to a Dendronized Polythiophenea
a This is followed by SI-ROMP of norbornene on the eletropolymerized films. Shown below the diagram is a possible polymer structure for a G1
thiophene dendron with three olefin groups.
series of generational Frechet-type peripheral olefin dendrons
with terthiophene at the focal point as shown in Figure 1. By
cyclic voltammetry (CV), the polymerization of the terthiophene-
containing dendron molecules leadsto the deposition of polythio-
phene films onto the electrodes. By this anodic electrochemical
polymerization method involving radical cations, polythiophene
films can be continuously deposited without significant passiva-
tion of the electrodes. Thus, it is possible to achieve high density
of free olefins as well as generate an electrically conducting poly-
mer sublayer. Generation 1 and 2 of these dendritic molecules
were synthesized with a range of 1-6 olefinic groups based on a
branched phenyl and benzyl ether functionality (Figure 1) in an
attempt to correlate their number with electrochemical behavior
and outcome in a SI-ROMP procedure. Polynorbornene as the
model brush film was then grown from the electropolymerized
films after activation by the first-generation Grubbs catalyst, as
shown in Scheme 1. Ellipsometric measurements were first carried
out to track the polymer film growth ex situ. Polymer growth
kinetics was then investigated by measuring the polymer brush
film thickness as a function of polymerization time. X-ray photo-
electron spectroscopy (XPS) was used to investigate film compo-
sition. Atomic force microscopy (AFM) was used to examine
morphology changes before and after SI-ROMP.
This method is seemingly more complicated, as compared to
the normal method which introduces surface initiators from SAMs.
However, SAMs are typically compatible only with specific metal
or nonmetal substrates. The restricted two-dimensional arrange-
ment also limits the conformational freedom and accessibility of
the functional groups on the surface. Our group has recently
reported the preparation of protein and cell-resistant poly(poly-
ethylene glycol methyl ether methacrylate) (PPEGMEMA) brushes
by surface-initiated reversible addition-fragmentation chain
transfer (SI-RAFT) polymerization. The process started with
the electrochemical deposition of an electroactive chain transfer
agent (CTA) including carbazole units on the Au surface, which
Figure 1. First and second generations of the Frechet-type olefin den-
drons with terthiophene at the focal point used in this study and the
serves as the surface initiating layer.15 In our present paper, we
varying number of olefinic peripheral groups. Here we define “G1/G2” as
the first/second generation, “M/D/T” as the mono/di/trisubstituted olefin
side chains, and “3T” as terthiophene. For example, G1M3T represents
the first generation (G1) dendron with the monosubstituted (M) olefin
functional group as the pendant and the terthiophene (3T) group as the
focal point.
aim to not only extend this idea to SI-ROMP but also illustrate
how the electroactive dendron structure and electropolymeriza-
tion conditions affect the subsequent polymer brush growth.
2. Experimental Section
2.1. Materials. Reagent chemicals were purchased from Sigma-
Aldrich, VWR, Alfa Aesar, or Acros Organics and used without
further purification unless otherwise indicated. Tetrahydrofuran
(THF) used in synthesis was distilled from sodium/benzophenone
olefin functional groups available for Grubbs’ catalyst activation.
Polymer brush density will therefore be highly dependent on the
population of activated olefins. In this regard, we synthesized a

10264 Macromolecules, Vol. 43, No. 24, 2010
Jiang et al.
Scheme 2. Synthesis of the Monosubstituted Terminal Olefin Dendrons
(G1M3T and G2M3T) with Electropolymerizable Terthiophenes at the
Focal Points
Scheme 3. Synthesis of the Disubstituted Terminal Olefin Dendrons
(G1D3T and G2D3T) with Electropolymerizable Terthiophenes at the
Focal Points
ketyl. Methylene chloride (CH2Cl2) used in synthesis was distilled
from CaH2. Grubbs first-generation catalyst was purchased from
Aldrich and used as received. Carbazole-containing alkylthiol and
alkylsilane were synthesized according to previous reports.16,17
2.2. Instrumentation. Nuclear magnetic resonance (NMR) spec-
tra were recorded on a General Electric QE-300 spectrometer at
300 MHz for 1H NMR. The electropolymerization of the dendron
molecules was carried out on a Princeton Applied Research Parstat
2263. Carbazole-SAM-covered indium tin oxide (ITO) or gold
substrates were used as working electrodes coupled with a Pt wire
as counter electrode and Ag/AgCl wire as reference electrode. The
gold-coated thin film (∼150 nm) substrates were prepared from
thermoevaporation on glass or silicon wafer slides under high vacuum
(<5 ꢀ 10-7 Torr). The ITO substrates were pretreated with the
RCA recipe (H2O/H2O2/NH3::15.1 g/26.6 g/8.57 g). Cyclic vol-
tammetry (CV) was utilized to prepare the polythiophene films
from a 5 mM solution of the dendrons in 0.1 M TBAPF6/CH3CN,
where TBAPF6 is tetrabutylammonium hexafluorophosphate. For
all the measurements, the electrode area exposed to the supporting
electrolyte solution is 0.785 cm2. The surface morphology of the
electropolymerized films and the polymer brush films were exam-
ined by tapping mode AFM [PicoPlus System, Molecular Imaging
(now Agilent Technologies), Tempe, AZ] under ambient condi-
tions (24 °C and ∼55% relative humidity). All image processing
was performed using the SPIP software (Scanning Probe Image
Processor, Image Metrology). XPS was carried out on a Physical
Electronics 5700 instrument with photoelectrons generated by the
nonmonochromatic Al KRirradiation (1486.6 eV). Photoelectrons
were collected at a takeoff angle of 45° using a hemispherical ana-
lyzer operated in the fixed retard ratio mode with an energy reso-
lution setting of 11.75 eV. The binding energy scale was calibrated
2.3. Synthesis of the Peripheral Olefin Dendrons. The details of
the synthesis protocols are outlined below, and the synthesis
routes are outlined in Schemes 2, 3, and 4.
2.3.1. General Synthesis Procedure for Etherification. To a sus-
pension of methyl benzoate with each of the different hydroxyl-
substituted phenyl groups (mono-, di-, and trisubstituted), potas-
sium carbonate, and 18-crown-6 ether in acetone was added
5-bromopentene dropwise under continuous stirring. The suspen-
sion was then refluxed for 48 h. Acetone was evaporated under
vacuum, and the residue was washed between water and CH2Cl2.
The organic layer was separated, and the aqueous layer was
extracted three times with CH2Cl2. Finally, the organic layer was
combined and dried over anhydrous Na2SO4. After removal of
the solvent, the crude product was purified by a silica gel column
chromatography using pure CH2Cl2 as the eluent.
2.3.2. General Synthesis Procedure for Reduction by LAH. To
a suspension of lithium aluminum hydride (LAH) in THF
precooled to 0 °C was added a THF solution of the esters drop-
wise through a funnel. Once the addition was complete, the solu-
tion was allowed to stir for another 5 min and then warmed to
room temperature and stirred for 24 h. The reaction was quenched
by the addition of water. The pH of the reaction mixture was
then adjusted with 2 M aqueous HCl to ∼7.0. The solvent was
evaporated under vacuum. The residue solution was extracted
with CH2Cl2, and each organic layer was washed with an equal
volume of water. The combined organic layers were dried over
Na2SO4, filtered, and evaporated.
3d5/2
prior to analysis using Cu2p and Ag
lines. Charge neutraliza-
2.3.3. General Synthesis Procedure for Mitsunobu Coupling.
The Mitsunobu etherification was carried out under sonication
in order to expedite the reaction according to a previous report.18
The mixture of alcohol, phenol, and triphenylphosphine (PPh3)
3/2
tion was ensured through cobombardment of the irradiated area
with an e-beam using nonmonochromated Al KR source and
placing the C1s peak at a binding energy of 284.6 (0.2) eV.

Article
Macromolecules, Vol. 43, No. 24, 2010 10265
Scheme 4. Synthesis of the Trisubstituted Terminal Olefin Dendrons (G1T3T and G2T3T) with Electropolymerizable Terthiophenes at the
Focal Points
in minimal THF was cooled to 5 °C in an ice-water bath and
sonicated for 10 min. Under sonication, a solution of diisopropyl
azodicarboxylate (DIAD) was added dropwise through a
syringe under the protection of nitrogen. The water temperature
was allowed to warm to room temperature, and precautions
were taken to maintain the temperature. (Caution: water in the
sonicator turns very hot if sonicated for a long time.) The reac-
tion was quenched by adding water, and the THF was removed
by vacuum evaporation. The aqueous layer was extracted with
CH2Cl2, and each organic layer was washed with dilute KOH
solution and an equal volume of water. The product was puri-
fied using 4:1 CH2Cl2/hexane as an eluent by silica gel column
chromatography.
2.3.4. General Synthesis Procedure of Hydrolysis. A solution
of methyl benzoate ester in THF was mixed with ethanol. Satu-
rated KOH aqueous solution was then added under vigorous
stirring. The mixture was refluxed at 80 °C under a nitrogen
atmosphere for 24 h. THF and EtOH were then removed under
vacuum evaporation. The residue was diluted with water and the
pH adjusted to 3 with a concentrated HCl solution. The solution
was washed with CH2Cl2 and water. The organic layers were
combined, dried over Na2SO4, filtered, and evaporated.
2.3.5. General Synthesis Procedure for Esterification. A solu-
tion of terthiophene alcohol (3TOH) was synthesized according
to an earlier report;19 terminal olefin dendron carboxylic acid
and p-(dimethylamino)pyridine (DMAP) in anhydrous CH2Cl2
were kept stirring at 0 °C in an ice-water bath under a nitrogen
atmosphere for 15 min. Dicyclohexylcarbodiimide (DCC) pre-
dissolved in anhydrous CH2Cl2 was then added dropwise to the
stirring mixture. After the addition, the reaction mixture was
allowed to stir at 0 °C for 5 min and then warmed to room tem-
perature overnight. Then the precipitated solids were removed
by gravity filtration; the filtrate was washed with a dilute solu-
tion of sodium bicarbonate (NaHCO3) and water and finally
dried over Na2SO4. The solution was filtered and the solvent was
removed under vacuum evaporation. The crude products were
further purified by silica gel column chromatography using a
mixture of 4:1 CH2Cl2/hexane as the eluent.
Synthesis of Methyl 4-(Pent-4-enyloxy)benzoate (G1ME). The
reaction mixture of 4.90 g (32.86 mmol) of 5-bromopentene,
5.40 g (35.58 mmol) of methyl 4-hydroxybenzoate, 32.0 g of
K2CO3, and 92 mg of 18-crown-6 in 400 mL of acetone afforded
6.0 g of oil product in 82.9% yield. 1H NMR (CDCl3): δ (ppm)
7.97 (2H, d, J=9.0 Hz), 6.89 (2H, d, J=9.0 Hz), 5.94 (1H, ddt,

10266 Macromolecules, Vol. 43, No. 24, 2010
Jiang et al.
J=19.8, 10.5, 6.6 Hz), 5.13 (2H, ddt, J=17.7, 17.1, 1.5 Hz), 4.00
(3H, t, J=6.6 Hz), 3.87 (3H, s), 2.24 (2H, dd, J=11.1, 5.1 Hz),
1.89 (2H, dt, J=14.4, 6.6 Hz).
product in 92.8% yield. 1H NMR (CDCl3): δ (ppm) 6.50 (2H, d,
J=2.1 Hz), 6.38 (2H, t, J=2.4 Hz), 5.85 (2H, ddt, J=17.1, 10.5,
6.6 Hz), 5.03 (4H, ddt, J=17.7, 17.1, 1.5 Hz), 4.61 (2H, s), 3.95
(2H, t, J=6.6 Hz), 2.23 (4H, dd, J=11.1, 5.1 Hz), 1.87 (4H, dt,
J=14.4, 6.6 Hz), 1.78 (1H, s).
Synthesis of 3,5-Bis(pent-4-enyloxy)benzoic Acid (G1DCOOH).
The reaction mixture of 2.25 g (7.39 mmol) of G1DE and 2.0 g of
KOH in a mixed solvent of 15 mL of THF, 10 mL of EtOH, and
2.0 mL of H2O afforded 1.96 g of oil product in 91.2% yield. 1H
NMR (CDCl3): δ (ppm) 7.23 (2H, d, J=2.4 Hz), 6.69 (2H, t, J=
2.4 Hz), 5.86 (2H, ddt, J=17.1, 10.5, 6.6 Hz), 5.04 (4H, m), 4.00
(4H, t, J=6.6 Hz), 2.25 (4H, dd, J=11.1, 5.1 Hz), 1.90 (4H, dt,
J=14.4, 6.6 Hz).
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethyl 3,5-
Bis(pent-4-enyloxy)benzoate (G1D3T). The reaction mixture of
1.86 g (6.41 mmol) of G1DCOOH, 1.82 g (6.22 mmol) of 3TOH,
1.32 g (6.41 mmol) of DCC, and 0.0783 g (0.64 mmol) of DMAP
in 25 mL of CH2Cl2 afforded 2.85 g of green or yellow oil
product in 81.5% yield. 1H NMR (CDCl3): δ (ppm) 7.35-7.00
(9H, m), 6.63 (1H, t, J=2.4 Hz), 5.85 (2H, ddt, J=17.1, 10.5,
6.6 Hz), 5.03 (4H, m), 4.55 (2H, t, J=6.9 Hz), 3.94 (4H, t, J=
6.6 Hz), 3.22 (2H, t, J=6.6 Hz), 2.22 (4H, dd, J=11.1, 5.1 Hz),
1.86 (4H, dt, J=14.4, 6.6 Hz).
Synthesis of Methyl 3,5-bis(3,5-bis(pent-4-enyloxy)benzyloxy)-
benzoate (G2DE). The reaction mixture of 5.26 g of G1DOH
(19.0 mmol), 1.76 g (10.5 mmol) of methyl 3,5-dihydroxybenzoate,
4.98 g (19.0 mmol) of PPh3, and 3.84 g (19.0 mmol) of DIAD
in 15 mL of THF afforded 4.86 g of oil product in 78.9% yield.
1H NMR (CDCl3): δ (ppm) 7.29 (2H, d, J=2.1 Hz), 6.79 (1H, t,
J=2.1 Hz), 6.56 (4H, d, J=2.1 Hz), 6.41 (2H, t, J=2.1 Hz), 5.85
(4H, ddt, J=17.1, 10.5, 6.6 Hz), 5.05 (8H, m), 4.99 (4H, s), 3.96
(8H, t, J=6.6 Hz), 3.91 (3H, s), 2.24 (8H, dd, J=11.1, 5.1 Hz),
1.88 (8H, dt, J=14.4, 6.6 Hz).
Synthesis of 4-(Pent-4-enyloxy)phenylmethanol (G1MOH). The
reaction mixture of 3.80 g (17.25 mmol) of G1ME and 0.98 g
(25.88 mmol) of LAH in 120 mL of THF afforded 3.13 g of oil
product in 94.4% yield. 1H NMR (CDCl3): δ (ppm) 7.34 (2H, d,
J=8.1 Hz), 6.96 (2H, d, J=8.4 Hz), 5.94 (1H, ddt, J=17.1, 10.5,
6.6 Hz), 5.13 (2H, ddt, J=17.7, 17.1, 1.5 Hz), 4.65 (2H, s), 4.05
(2H, t, J=6.6 Hz), 2.33 (2H, dd, J=11.1, 5.1 Hz), 2.24 (1H, s), 1.97
(2H, dt, J=14.4, 6.6 Hz).
Synthesis of 4-(Pent-4-enyloxy)benzoic Acid (G1MCOOH).
The reaction mixture of 1.00 g (4.54 mmol) of G1ME and 1.2 g
of KOH in a mixed solvent of 15 mL of THF, 10 mL of EtOH,
and 1.5 mL of H2O afforded 0.86 g of oil product in 91.9% yield.
1H NMR (CDCl3): δ (ppm) 8.06 (2H, d, J=9.0 Hz), 6.94 (2H, d,
J=9.0 Hz), 5.86 (1H, ddt, J=17.1, 10.5, 6.6 Hz), 5.04 (2H, ddt,
J=17.7, 17.1, 1.5 Hz), 4.04 (t, 2H, J=6.6 Hz), 2.26 (2H, dd,
J=11.1, 5.1 Hz), 1.92 (2H, dt, J=14.4, 6.6 Hz).
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethyl
4-(Pent-4-enyloxy)benzoate (G1M3T). The reaction mixture of
0.65 g (3.15 mmol) of G1MCOOH, 1.10 g (3.78 mmol) of 3TOH,
0.78 g (3.78 mmol) of DCC, and 0.0464 g (0.38 mmol) of DMAP
in 15 mL of CH2Cl2 afforded 1.14 g of dark green oil product in
75.1% yield. 1H NMR (CDCl3): δ (ppm) 7.95 (2H, d, J=8.7 Hz),
7.35-7.00 (7H, m), 6.89 (2H, d, J = 8.7 Hz), 5.85 (1H, ddt,
J=17.1, 10.5, 6.6 Hz), 5.04 (2H, ddt, J=17.7, 17.1, 1.5 Hz), 4.53
(2H, t, J = 6.6 Hz), 4.02 (2H, t, J = 6.6 Hz), 3.21 (2H, t,
J = 7.2 Hz), 2.25 (2H, dd, J = 11.1, 5.1 Hz), 1.90 (dt, 2H,
J=14.4, 6.6 Hz).
Synthesis of Methyl 3,5-Bis(4-(pent-4-enyloxy)benzyloxy)-
benzoate (G2ME). The reaction mixture of 4.20 g (21.8 mmol) of
G1MOH, 1.93 g (11.5 mmol) of methyl 3,5-dihydroxybenzoate,
5.72 g of PPh3 (21.8 mmol), and 4.41 g (21.8 mmol) of DIAD in
15 mL of THF afforded 4.25 g of oil product in 75.4% yield. 1H
NMR (CDCl3): δ (ppm) 7.40-6.47 (11H, m), 5.85 (2H, ddt, J=
17.1, 10.5, 6.6 Hz), 5.14-4.83 (8H, m), 4.10-3.87 (4H, m), 3.83
(3H, s), 2.24 (4H, dd, J=11.1, 5.1 Hz), 1.90 (4H, dt, J=14.4,
6.6 Hz).
Synthesis of 3,5-Bis(4-(pent-4-enyloxy)benzyloxy)benzoic Acid
(G2MCOOH). The reaction mixture of 1.33 g (2.57 mmol) of
G2ME and 0.8 g of KOH in a mixed solvent of 10 mL of THF,
8 mL of EtOH, and 0.8 mL of H2O afforded 1.16 g of oil product
in 90.0% yield. 1H NMR (CDCl3): δ (ppm) 7.42-6.48 (11H, m),
5.86 (2H, ddt, J = 17.1, 10.5, 6.6 Hz), 5.17-4.86 (8H, m),
4.12-3.88 (4H, m), 2.26 (4H, dd, J=11.1, 5.1 Hz), 1.92 (4H,
dt, J=14.4, 6.6 Hz).
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethyl 3,5-Bis-
(4-(pent-4-enyloxy)benzyloxy)benzoate (G2M3T). The reaction
mixtureof 0.74 g (1.47 mmol) of G2MCOOH, 0.516 g (1.76 mmol)
of 3TOH, 0.361 g (1.76 mmol) of DCC, and 0.0215 g (0.176 mmol)
of DMAP in 10 mL of CH2Cl2 afforded 0.83 g of dark green oil
product in 72.8% yield. 1H NMR (CDCl3): δ (ppm) 7.42-6.48
(18H, m), 5.86 (2H, ddt, J = 17.1, 10.5, 6.6 Hz), 5.17-4.86
(8H, m), 4.51 (2H, t, J=6.6 Hz), 4.12-3.88 (4H, m), 3.18 (2H, t,
J=7.2 Hz), 2.25 (4H, dd, J=11.1, 5.1 Hz), 1.91 (4H, dt, J=14.4,
6.6 Hz).
Synthesis of Methyl 3,5-Bis(pent-4-enyloxy)benzoate (G1DE).
The reaction mixture of 5.40 g (36.23 mmol) of 5-bromopentene,
3.35 g (19.93 mmol) of methyl 3,5-dihydroxybenzoate, 35.0 g of
K2CO3, and 108 mg of 18-crown-6 in 400 mL of acetone afforded
4.68 g of oil product in 84.9% yield. 1H NMR (CDCl3): δ (ppm)
7.16 (2H, d, J=2.4 Hz), 6.63 (2H, d, J=2.4 Hz), 5.85 (2H, ddt,
J=17.1, 10.5, 6.6 Hz), 5.03 (4H, ddt, J=17.7, 17.1, 1.5 Hz), 3.98
(4H, t, J=6.6 Hz), 3.89 (3H, s), 2.23 (4H, dd, J=11.1, 5.1 Hz),
1.88 (4H, dt, J=14.4, 6.6 Hz).
Synthesis of 3,5-Bis(3,5-bis(pent-4-enyloxy)benzyloxy)benzoic
Acid (G2DCOOH). The reaction mixture of 2.11 g (3.08 mmol)
of G2DE and 1.0 g of KOH in a mixed solvent of 15 mL of THF,
10 mL of EtOH, and 1.0 mL of H2O afforded 1.84 g of oil
product in 89.1% yield. 1H NMR (CDCl3): δ (ppm) 7.33 (2H, d,
J=2.4 Hz), 6.83 (2H, t, J=2.4 Hz), 6.56 (4H, d, J=2.1 Hz), 6.41
(2H, t, J=2.1 Hz), 5.85 (4H, ddt, J=17.1, 10.5, 6.6 Hz), 5.04 (8H,
m), 5.00 (4H, s), 3.96 (8H, t, J=6.6 Hz), 2.24 (8H, dd, J=11.1,
5.1 Hz), 1.88 (8H, dt, J=14.4, 6.6 Hz).
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethyl 3,5-Bis-
(3,5-bis(pent-4-enyloxy)benzyloxy)benzoate (G2D3T). The reac-
tion mixture of 1.37 g (2.04 mmol) of G2DCOOH, 0.716 g
(2.45 mmol) of 3TOH, 0.505 g (2.45 mmol) of DCC, and
0.0299 g (0.245 mmol) of DMAP in 15 mL of CH2Cl2 afforded
1.43 g of green or yellow solid product in 74.0% yield. 1H NMR
(CDCl3): δ (ppm) 7.34-6.95 (9H, m), 6.78 (1H, t, J=2.4 Hz),
6.54 (4H, d, J=2.1 Hz), 6.41 (2H, t, J=2.1 Hz), 5.85 (4H, ddt,
J=17.1, 10.5, 6.6 Hz), 5.04 (8H, m), 4.93 (4H, s), 4.56 (2H, t,
J = 6.9 Hz), 3.95 (8H, t, J = 6.6 Hz), 3.23 (2H, t, J = 6.9 Hz),
2.24 (8H, dd, J=11.1, 5.1 Hz), 1.87 (8H, dt, J=14.4, 6.6 Hz).
Synthesis of Methyl 3,4,5-Tris(pent-4-enyloxy)benzoate (G1TE).
The reaction mixture of 6.76 g (45.36 mmol) of 5-bromopentene,
2.68 g (14.56 mmol) of methyl 3,4,5-trihydroxybenzoate, 40.0 g of
K2CO3, and 125 mg of 18-crown-6 in 400 mL of acetone afforded
4.87 g of oil product in 86.0% yield. 1H NMR (CDCl3): δ (ppm)
7.27 (2H, s), 5.94-5.77 (3H, m), 5.02 (6H, m), 4.04 (6H, dt, J=
6.6, 6.0 Hz), 3.89 (3H, s), 2.34-2.21 (6H, m), 2.34-2.21 (6H, m).
Synthesis of (3,4,5-Tris(pent-4-enyloxy)phenyl)methanol
(G1TOH). The reaction mixture of 5.34 g (13.75 mmol) of
G1TE and 0.782 g (20.62 mmol) of LAH in 100 mL of THF
afforded 4.62 g of oil product in 93.2% yield. 1H NMR (CDCl3):
δ (ppm) 6.50 (2H, s), 5.94-5.77 (3H, m), 5.01 (6H, m), 4.50
(2H, s), 3.95 (6H, dt, J=6.6, 6.0 Hz), 2.62 (1H, s), 2.32-2.19
(6H, m), 1.94-1.78 (6H, m).
Synthesis of (3,5-Bis(pent-4-enyloxy)phenyl)methanol (G1DOH).
The reaction mixture of 6.23 g (24.47 mmol) of G1DE and 1.39 g
(30.70 mmol) of LAH in 150 mL of THF afforded 6.27 g of oil
Synthesis of 3,4,5-Tris(pent-4-enyloxy)benzoic Acid (G1TCOOH).
The reaction mixture of 2.11 g (3.08 mmol) of G1TE and 1.0 g of

Article
Macromolecules, Vol. 43, No. 24, 2010 10267
KOH in a mixed solvent of 15 mL of THF, 10 mL of EtOH, and
1.0 mL of H2O afforded 1.84 g of oil product in 89.1% yield. 1H
NMR (CDCl3): 8.98 (1H, b), 7.33 (2H, s), 5.94-5.78 (3H, m),
5.03 (6H, m), 4.06 (6H, dt, J=10.8, 6.3 Hz), 2.32-2.21 (6H, m),
1.94-1.80 (6H, m).
dendrimers which can be cross-linked at the peripheries
via ring-closing metathesis reaction has been reported by
Zimmerman et al.20-23 These dendrimers have found inter-
esting applications especially in host-guest chemistry. Shon
and Choi have recently synthesized gold nanoparticles capped
with peripheral olefin dendron thiols prepared by a conver-
gent approach.24 In our present work, the synthesis of an electro-
polymerizable dendron molecule was achieved by Steglich
esterification of 2,20:50,200-terthiophene ethanol (3TOH) with
the Frechet-type peripheral olefin dendrons at the carboxylic
acid foci in the presence of DCC and DMAP. In this manner,
the electrochemically active terthiophene moiety was
attached at the focal point of the dendron. The commercially
available methyl benzoate with mono-, di-, and trihydroxy
on the phenyl groups offers a facile pathway to vary the num-
ber of terminal olefins, as shown in Schemes 2, 3, and 4, respec-
tively. Mitsunobu coupling reaction was applied to bring the
first-generation dendrons onto the 3- and 5-positions of the
dihydroxymethyl benzoate, giving the second-generation
dendrons. This coupling reactions were either carried out by
conventional synthesis methods involving reflux or by the
sonication-assisted approach reported in our group.18
3.2. Electropolymerization of the Terminal Olefin Dendrons.
2,20:50,200-Terthiophene containing monomers and polymers
have shown very good electroactivity in device applications,
nanopatterning and electrochromic devices, etc.19,25-27
The electrochemical oxidation of the terthiophene unit
toward polymer synthesis is more facile compared to thiophene
and carbazole in that it has a lower oxidation potential.19
Herein we studied their electropolymerizability as dendro-
nized monomers.
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethyl 3,4,5-
Tris(pent-4-enyloxy)benzoate (G1T3T). The reaction mixture of
1.37 g (2.04 mmol) of G1TCOOH, 0.716 g (2.45 mmol) of 3TOH,
0.505 g (2.45 mmol) of DCC, and 0.0299 g (0.245 mmol) of DMAP
in 15 mL of CH2Cl2 afforded 1.43 g of green or yellow solid
product in 74.0% yield. 1H NMR (CDCl3): δ (ppm) 7.36-7.00
(9H, m), 5.98-5.80 (3H, m), 5.05 (6H, m), 4.59 (2H, t, J =
6.9 Hz), 4.08 (2H, t, J=6.3 Hz), 3.98 (4H, t, J=6.3 Hz), 3.25
(2H, t, J=6.6 Hz), 2.37-2.21 (6H, m), 1.97-1.83 (6H, m).
Synthesis of Methyl 3,5-Bis(3,4,5-tris(pent-4-enyloxy)benzyloxy)-
benzoate (G2TE). The reaction mixture of 5.31 g (14.73 mmol) of
G1TOH, 1.13 g (6.69 mmol) of methyl 3,5-dihydroxybenzoate,
3.86 g (14.73 mmol) of PPh3 and 2.98 g (14.73 mmol) of DIAD in
12 mL of THF afforded 3.94 g of oil product in 69.0% yield. 1H
NMR (CDCl3): δ (ppm) 7.30 (1H, d, J= 2.4 Hz), 6.82-6.36
(6H, m), 5.95-5.76 (6H, m), 5.10-4.96 (12H, m), 4.96 (4H, s),
3.99 (12H, t, J = 6.3 Hz), 3.91 (3H, s), 2.34-2.18 (12H, m),
1.97-1.79 (12H, m).
Synthesis of 3,5-Bis(3,4,5-tris(pent-4-enyloxy)benzyloxy)benzoic
Acid (G2TCOOH). The reaction mixture of 2.44 g (2.86 mmol)
of G2TE and 1.0 g of KOH in a mixed solvent of 15 mL of THF,
10 mL of EtOH, and 1.0 mL of H2O afforded 2.03 g of oil prod-
1
uct in 83.2% yield. H NMR (CDCl3): δ (ppm) 7.28 (1H, d,
J=2.4 Hz), 6.80-6.33 (6H, m), 5.94-5.75 (6H, m), 5.11-4.96
(12H, m), 4.96 (4H, s), 3.99 (12H, t, J = 6.3 Hz), 2.34-2.18
(12H, m), 1.97-1.79 (12H, m).
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethyl 3,5-Bis-
(3,4,5-tris(pent-4- enyloxy)benzyloxy)benzoate (G2T3T). The reac-
tion mixture of 1.18 g (1.41 mmol) of G2TCOOH, 0.494 g (1.69
mmol) of 3TOH, 0.349 g (1.69 mmol) of DCC, and 0.0208 g (0.17
mmol) of DMAP in 10 mL of CH2Cl2 afforded 1.12 g of dark green
oil product in 71.3% yield. 1H NMR (CDCl3): δ (ppm) 7.33-6.93
(7H, m), 6.81-6.36 (6H, m), 5.96-5.77 (6H, m), 5.11-4.95 (12H,
m), 4.89 (4H, s), 4.57 (2H, t, J=7.2 Hz), 3.99 (12H, t, J=6.3 Hz),
3.23 (2H, t, J=6.6 Hz), 2.35-2.20 (12H, m), 1.97-1.82 (12H, m).
2.4. Surface-Initiated Ring-Opening Metathesis Polymeriza-
tion (SI-ROMP) of Norbornene on the Electropolymerized Films.
Norbornene was chosen as the monomer for demonstrating SI-
ROMP on the electropolymerized films because it is commer-
cially available and is facile in the ring-opening process. All the
SI-ROMP was conducted in a glovebox under a N2 atmosphere.
The electropolymerized films were first immersed into a 0.35 M
of the first generation Grubbs catalyst solution in CH2Cl2 for 30
min to activate the surfaces. Then the films were washed consec-
utively three times with CH2Cl2 to completely remove any unbound
catalyst. After washing, the catalyst-activated slides were
immediately dipped into a 0.2 M norbornene monomer solution
in CH2Cl2 up to 60 min. In the kinetics studies, a slide at a certain
time interval was taken out from the monomer solution followed
by copious washing with CH2Cl2. A total of six slides for each
kinetics study were used to plot the polymer growth curve.
2.5. Ellipsometric Measurement of the Film Thickness before
and after SI-ROMP. After electropolymerization of the den-
dron and after the SI-ROMP, the samples were dried completely
and the film thickness was measured by ellipsometry (HeNe
laser, 632 nm). The incident angle was fixed at 70° on the gold
electrode samples. The thickness value was averaged by taking
six different spots on the polymer films. The refractive indices of
the electropolymerized films and the resulting polymer brush
films were assumed to be 1.45 and 1.50, respectively.
3.2.1. Electropolymerization of the First-Generation Term-
inal Olefin Dendrons. Electropolymerization was done first
with the first-generation dendrons on bare gold and ITO.
Although typical CV curves for electropolymerization of the
terthiophene molecules were obtained, the polymer films
were partially washed away by THF or CH2Cl2. This can be
attributed to the poor adhesion of the polythiophene films
which is only physically adsorbed on the electrode surfaces.
It is not unreasonable considering that dendritic side chains
enhances polythiophene solubility compared to alkyl side
chains. In order to improve polymer film adhesion, a 11-(9H-
carbazol-9-yl)undecane-1-thiol (CbzC11SH) or9-(11-(trimeth-
oxysilyl)undecyl)-9H-carbazole or (CbzC11-Silane) was used
to modify the gold or ITO electrode surfaces. The synthesis
of the thiol or silane molecule was reported in our earlier pub-
lications.16,17 5 mM of the thiol or silane was used to prepare
a monolayer film. More specifically, the carbazole-terminated
alkylsilane was dissolved in toluene, and the ITO electrodes
were then immersed in this solution at 60 °C for 12 h. The
carbazole-terminated alkylthiol solution in THF was used to
functionalize the gold electrodes for 2 h. The electrodes were
examined by AFM, before and after the procedure. As shown
in Figure S1 (Supporting Information), the silane-functio-
nalized ITO surfaces exhibited a more “fuzzy” morphology
with aggregates compared to the clear boundaries on bare
ITO surface. Ellipsometric measurements on the CbzC11SH-
modified gold electrode gave a film thickness ∼1.1 nm, indicat-
ing monolayer surface coverage. During the electropolymer-
ization process, the carbazole moieties in the SAMs should
be copolymerized (electrografted) with the terthiophene units,
leading to covalent linkages or chemical adhesion between
the SAM-modified electrode surface and the polythiophene
films. As expected, the electropolymerized films exhibited
much better resistance to solvent washing. Even after sonica-
tion in good solvents like THF and CH2Cl2, the film thick-
ness was maintained.
3. Results and Discussion
3.1. Synthesis of the Electropolymerizable Peripheral Olefin
Dendrons. Peripheral olefin dendrons as building blocks for

10268 Macromolecules, Vol. 43, No. 24, 2010
Jiang et al.
Figure 2. CV of the dendrons after 5 CV cycles (G1M3T, a; G1D3T, c; and G1T3T, e) and 10 CV cycles (G1M3T, b; G1D3T, d; and G1T3T, f). The
CV measurements were carried out in 5 mM of each monomer in 0.1 M of TBAPF6 solution in acetonitrile. Potential scan window: 0-1.2 V; scan rate:
50 mV/s; reference electrode: Ag/AgCl; counter electrode: Pt wire; working electrode: ITO.
The CV experiments were carried out in different solvent
systems including THF, CH2Cl2, and acetonitrile (ACN).
Electropolymerization in ACN gave the highest film thick-
ness and the best quality film morphology in terms of surface
roughness and homogeneity. To determine the effect of electro-
polymerized film thickness on polymer brush growth, we
applied 5 and 10 CV cycles and compared brush thickness
and polymer brush growth kinetics. The CV traces for 5 cycles
are presented in Figure 2. In the first cycles, the monomer
oxidation peak position for all three dendrons was found at
almost the same position at 1.15 V. Upon scanning in a reverse
direction, the reduction peak around 0.75 V showed a signifi-
cantly lower current. With the succeeding scans, two new oxi-
dation waves appeared between 0.6 and 1.0 V, corresponding
to the oxidation of the newly formed polythiophene from the
first scan. However, they were not observed from CV results
of a previous report on polynorbornene bearing terthio-
phene in the side chains using ACN as the solvent.28 Also,
only a single polymer oxidation peak was observed from the
CV curves using THF and CH2Cl2 as the solvent. These
differences may be related to polythiophene oxidation with
formation of two electronic transitions: polarons and bipo-
laron species in a stepwise manner. The polymerization oxi-
dation process most likely occurs at the same time scale with
the doping and dedoping of the already deposited polymer.
Previously, in situ electron spin resonance (ESR) and optical
measurements on polythiophenes have shown that spinless
species (bipolarons) are generated at the maximum oxidation
level of each step and spin-bearing species (polarons) pro-
duced at an intermediate oxidation in the first process only.29
Usually, low-temperature CV measurements are done to
observe the two peaks in which ion transport is very slow.
In this case, the use of the ACN solvent and the dendronized
monomer perhaps gives a better resolution of this two-step
doping process by a better control of thickness and a more
restricted internal morphology in terms of ion transport.
Further investigation into the redox properties and polaronic
structures of the polymers, e.g., by spectroelectrochemistry
or low-temperature measurements, may help understand this
phenomenon better.
The cathodic waves from 1.0 to 0.4 V correspond to the
reduction process from the oxidized and conductive polythio-
phenes to their neutral state (dedoping). Two partially separated
reduction peaks were similarly observed, with each correlat-
ing with the corresponding oxidation peaks. Repeated cycles
showed an even growth interval in both oxidation and reduc-
tion currents, which is in agreement with the cyclic deposi-
tion of the electroactive polymer films. This suggests that the
olefins in these dendrons are not involved in the electropoly-
merization.30,31 Moreover, vinyl groups are known to poly-
merize only under negative potential cathodic electropoly-
merization conditions through radical anion mechanisms.8-10,14

Article
Macromolecules, Vol. 43, No. 24, 2010 10269
Table 1. Anodic and Cathodic Peak Potentials and Currents on the
Final Scans from the CV of the First-Generation Dendronsa
Table 2. Anodic and Cathodic Peak Potentials and Currents on the
Final Scans from the CV of the Second-Generation Dendronsa
films
E1pa E2pa
i1pa
i2pa
E1pc E2pc
i1pc
i2pc
films
Epa
ipa
Epc
ipc
G1M3T_5 0.71 0.82 171.3 290.5
G1M3T_10 0.75 0.83 565.0 600.5
0.74
0.70
-351.0
-623.7
G2M3T_5
G2M3T_10
G2D3T_5
G2D3T_10
G2T3T_5
G2T3T_10
0.95
0.93
0.96
0.85
0.91
0.98
22.4
59.9
28.5
91.1
24.9
26.9
0.82
0.79
0.83
0.67
0.80
0.87
-53.9
-86.1
-33.8
-120.4
-50.8
-39.3
G1D3T_5
G1D3T_10 0.71 0.85 318.4 433.1 0.63 0.74 -321.4 -464.7
G1T3T_5 0.66 0.83 152.1 184.2 0.61 0.74 -134.8 -185.8
0.70 0.83 104.0 221.9 0.62 0.75 -149.2 -217.7
G1T3T_10 0.69 0.86 275.8 249.5 0.60 0.73 -224.2 -268.1
a The units for anodic and cathodic peak potentials are V, and the units
for anodic and cathodic peak currents are μA. Because of the serious
overlap of the reduction waves for G1M3T, only the peak at the higher
position was included in this table. The onset for all the CV curves is at
0.57 V. G1M3T_5: electrografted polythiophene films after 5 CV cycles from
G1M3T. 1 donates the higher oxidation/reduction peak, and 2 donates the
lower oxidation/reduction peak. Epa: anodic peak potential; ipa: anodic
peak current; Epc: cathodic peak potential; ipc: cathodic peak current.
a G2M3T_5: electrografted polythiophene films after 5 CV cycles from
G2M3T.
Table 3. Film Thicknesses Measured by Ellipsometry before and after
1 h SI-ROMP of Norbornenea
E-films
thickness (nm)
PNb films
thickness (nm)
E-G1M3T-5
E-G1D3T-5
E-G1T3T-5
E-G1M3T-10
E-G1D3T-10
E-G1T3T-10
E-G2M3T-5
E-G2D3T-5
E-G2T3T-5
E-G2M3T-10
E-G2D3T-10
E-G2T3T-10
27.2 ( 3.3
22.5 ( 2.5
18.7 ( 0.9
41.3 ( 5.5
35.7 ( 4.1
30.5 ( 2.6
5.2 ( 1.3
PNb-G1M3T-5
PNb-G1D3T-5
PNb-G1T3T-5
PNb-G1M3T-10
PNb-G1D3T-10
PNb-G1T3T-10
PNb-G2M3T-5
PNb-G2D3T-5
PNb-G2T3T-5
PNb-G2M3T-10
PNb-G2D3T-10
PNb-G2T3T-10
11.2 ( 2.2
14.6 ( 2.7
18.4 ( 3.1
10.8 ( 2.4
13.9 ( 3.0
17.2 ( 3.3
13.8 ( 0.9
19.2 ( 1.1
21.6 ( 2.9
14.0 ( 1.0
19.5 ( 1.3
21.8 ( 6.5
In all cases, the oxidation peaks moved to higher potentials
and the reduction peaks shifted toward lower potentials.
This gradually broadened peak separation is a typical feature
for semiconducting polymer electrodeposition.29 As more
polymers are electrochemically grafted onto the electrodes,
the electron transfer kinetics in turn becomes more hetero-
geneous. It should be noted that even though some poly-
thiophenes were copolymerized with the SAM carbazole
groups on the first cycle, a certain amount of polymers would
still redissolve into the supporting electrolyte solution with sub-
sequent cycles.16-18 The dissolved polymers doped with the
anions were visibly seen from the much darker orange color
of the electrolyte solution compared to the light green or yellow
starting monomer-electrolyte solution. As good solvents like
THF and CH2Cl2 were used, the electrolyte solutions after
electropolymerization were even deeper orange in color,
giving further evidence of more dissolved polymer species.
The CV experiments on the CbzC11SH-covered gold elec-
trodes showed similar results. Note that all thicknesses after
electropolyermization and SI-ROMP procedure were mea-
sured by ellipsometry on these gold electrodes.
Table 1 summarizes the CV results in terms of oxidative
and reductive peak potentials and currents from the final
scan. There is no distinct difference in both the onset and the
peaks of the anodic (oxidative) and cathodic (reductive)
waves for all the three dendrons. Only slightly lower anodic
peaks (∼50 mV) were observed with G1T3T. All these obser-
vations reveal that the olefin dendron side chains have negli-
gible effects on the electrochemical reactivity of the terthio-
phene moiety and the polythiophene film formation.8-10
Interestingly, both polymer oxidation and reduction peak
currents decreased with increasing number of olefin peripheral
groups on changing from G1M3T to G1D3T to G1T3T.
This is mainly attributed to a decreased tendency to deposit
more material per cycle on the electrode (perhaps a decrease
in molecular weight of the polythiophene as well). More
branching can hinder terthiophene reactivity and highly
branched side chains on the formed polymer increases poly-
mer solubility, thereby reducing its adhesion on the surface.
The lowest molecular weights and the smallest polymer aggre-
gates would then be expected from G1T3T. In contrast,
electropolymeriation of G1M3T would tend to give longer
polymer chains or bigger aggregates on the electrode surfaces
(as shown later by AFM).
4.7 ( 1.5
16.3 ( 4.9
7.7 ( 2.2
22.7 ( 3.2
18.5 ( 7.6
a E-G1M3T-5 represents the electropolymerized film from G1M3T
after 5 CV cycles under the listed conditions. PNb-G1M3T-5 represents
the polynorbornene brush film on top of the electropolymerized films
from G1M3T after 5 CV cycles.
3.2.2. Electropolymerization of the Second-Generation Pe-
ripheral Olefin Dendrons. Because of the limited solubility of
the second-generation dendrons in ACN, THF was used as
solvent. Except for the solvent used, all other conditions were
the same as those in the electropolymerization of the first-
generation dendrons. As shown in Figure S4 (Supporting
Information), the CV traces appeared remarkably different
from those of the first-generation dendrons in ACN. During
the first scan, the monomer oxidation peaks were not well-
defined. Nonetheless, in the retro scans, the CV waves in the
range from 0.70 to 0.83 V corresponding to the polymer reduc-
tions are well behaved. In the following scans, the polymer
oxidation and reduction were both significantly suppressed,
similar to the first-generation dendrons using THF as sol-
vent. As seen in Table 2, all the currents were about 1 order of
magnitude lower than those obtained from Figure 2. In addi-
tion, the anodic and cathodic peak potentials were relatively
higher. All these results strongly indicate that electropoly-
merization of these dendrons was not as favorable even in
good solvents compared to the first generation. Thus, thin-
ner films were obtained in this series of CV experiments (as
measured by ellipsometry, shown in Table 3). Regardless of
these observations, the films were still found to be stable
against CH2Cl2 washing and even sonication or extraction,
which is crucial for the SI-ROMP.
3.3. SI-ROMP of Norbornene on the Electropolymerized
Films. Prior to SI-ROMP, the electropolymerized films were
initially immersed in the first-generation Grubbs catalyst solu-
tions for 30 min, leading to the Ru-activated surfaces. Any
unbound ruthenium complexes were extensively washed by
CH2Cl2 at least three times. Then the surface-activated sam-
ple was immediately dipped into a 0.2 M norbornene solu-
tion in CH2Cl2. After a specific period of time, a slide was
taken out and washed with CH2Cl2. The polymerization was
allowed to proceed up to 60 min at room temperature. The
polynorbornene brush films were then investigated by AFM,
Moreover, the polymer oxidation and reduction waves
were reversible as evidenced by the comparable peak cur-
rents. This means that the electron transfer rate is faster than
the scan rate (50 mV/s) and that all oxidized and reduced
polythiophene species deposited should be stable without any
significant side reactions.

10270 Macromolecules, Vol. 43, No. 24, 2010
Jiang et al.
Figure 3. 2D tapping-mode AFM images of the electropolymerized films from the first-generation dendrons after 5 CV cycles (G1M3T, a; G1D3T, b;
and G1T3T, c) and the polynorbornene (PNB) brush films grown on their corresponding electropolymerized films after 60 min (PNb-G1M3T-5, d;
PNb-G1D3T-5, e; and PNb-G1T3T-5, f).
ellipsometry, and XPS to confirm the film surface morphology,
thickness, and the atomic composition (elemental analysis).
3.3.1. AFM Study of the Electropolymerized Films and Poly-
norbornene Brush Films. The film morphology after electro-
polymerization of the dendrons before ROMP and after growth
of polynorbornene brushes was examined by AFM. Figure 3
shows tapping-mode AFM images after electrografting of
the first-generation dendrons with 5 CV cycles and after 1 h
of SI-ROMP on their corresponding electropolymerized films.
It can be seen that the polythiophene films highly covered the
CbzC11-silane-modified ITO electrodes surface as compared
with Figure S1b (Supporting Information). Similar surface
morphology was also observed from the electropolymerized
films with first-generation dendrons at 10 CV cycles, as pre-
sented in Figure S2 (Supporting Information). The AFM
images (d, e, and f in Figure 3 and Figure S2) after SI-ROMP
of norbornene on all electropolymerized films showed different
morphologies based on the globule size shown in Table S1,
which includes both the size of the globular features and the
surface roughness from all the AFM images in Figure 3 and
Figure S2. Although it is difficult to perform any quantita-
tive analysis of the polymer brush density based on the AFM
images alone, they are quite useful to allow us to confirm the
successful growth of polymer brush and further compare the
morphological changes before and after SI-ROMP. After 5 or
10 CV cycles, the size of those globular aggregates decreases
from the electropolymerized films with G1M3T to G1T3T,
in accordance with the discussions in the electropolymeriza-
tion section.
The size of the globular and conical features from the poly-
norbornene films increases with dendron generation. More
isolated islands are observed on the polymer brush film from
the electropolymerized films with G1M3T. However, on the
brush film from the electropolymerized films with G1T3T,
the features become more interconnected. This also supports
the fact that polymer brush growth can be controlled by
changing the number of terminal olefin on the polythiophene
side chains. Furthermore, the size of the aggregates before
and after SI-ROMP, along with the surface roughness, is
similar to both 5 and 10 CV cycles (Figure S2, Supporting
Information). Therefore, once the electropolymerized films
are fully covered by the dendrons, statistically the number of
free olefins available from the polymer brush growth will
only depend on the structure of the dendrons.
The surface morphology of the electropolymerized films
from the second-generation dendrons in THF gave a much

Article
Macromolecules, Vol. 43, No. 24, 2010 10271
Figure 4. 2D tapping-mode AFM images of the electropolymerized films from the second-generation dendrons after 5 CV cycles (G2M3T, a;
G2D3T, b; and G2T3T, c) and the polynorbornene brush films grown on their corresponding electropolymerized films (d, e, and f) after 60 min. All the
images are 5 ꢀ 5 μm2.
different appearance, presumably due to solvent effects. In con-
trast to the densely packed globular features of polythiophene
aggregates, the aggregates on the electropolymerized films in
Figure 4 and Figure S3 are more isolated. The background of
the ITO sharp domain edges is still visible. After SI-ROMP,
the smoothness of polynorbornene films significantly im-
proves, except for a few aggregates on the electropolymer-
ized G2T3T films for both 5 and 10 CV cycles. The morphol-
ogy of the polynorborne brush films here are actually very
similar to those grown from surface-initiated films using
SAMs.4,32 In addition, there are some large pinholes or defects
from the polymer brushes in all cases. The pinholes can have
some unwanted effects for practical applications of the brush
films, like electrical shorting in FET device fabrication, but
they can usually be removed by thermal annealing.3b
3.3.2. Film Thicknesses of the Electropolymerized Films and
the Polynorbornene Brush Films. To evaluate the effect of
peripheral olefin number and the dendron generation on poly-
mer brush growth, we measured the polymer brush thick-
nesses by ellipsometry. Table 3 summarizes the thickness
values of the electropolymerized films and their correspond-
ing polymer brush films on gold substrates. From G1M3T to
G1T3T, in the case of both 5 and 10 CV cycles, an increase in
polynorbornene brush thickness was observed. This is in
contrast to a decrease in the electropolymerized film thick-
ness during electrodeposition. This clearly demonstrates that
the number of active olefins for ROMP on the electropoly-
merized films is statistically determined by the number of
olefinic substituents present, which, in turn, controls poly-
mer brush thickness as well as grafting density. Moreover,
the thickness of the corresponding polynorbornene brush
films increases disproportionally. By immersing the electro-
polymerized films from G1T3T into a 0.6 M of the Grubbs
catalyst solution, no distinct thickness difference was obtained.
This excludes the possibility that only a certain percentage of
the free olefin functionalities are activated in the 0.35 M cat-
alyst solution. One possible reason is that free olefins in the
electropolymerized films from G1T3T are located in closer
proximity. Therefore, there is a higher probability for cross-
metathesis upon catalyst activation, although the vinyl-
terminated functional groups are generally less susceptible
with this catalyst. This can essentially decrease the density of
the active sites for ROMP. To achieve a maximum polymer
brush grafting density and the highest film thickness, in some

10272 Macromolecules, Vol. 43, No. 24, 2010
Jiang et al.
Figure 5. High-resolution XPS spectra of the C1s, O1s, S2p, and In3d peaks on the polynorbornene-covered brush film grown from 5 CV cycle
5/2
electropolymerized G2T3T (PNb-G2T3T-5).
Table 4. Relative Density of XPS Signals on the Polynorbornene
Films Grown from the Electropolymerized Films with the Second-
Generation Dendrons after 5 CV Cycles
cases, inert molecules were intentionally mixed into the sur-
face initiator matrix to dilute the density of the active sites.33
Another possible reason for this disproportionate thickness
increase lies in the conformation of the side chains on the
electrodeposited polythiophenes. This may be improved by
thermal annealing.
5/2
films
C1s
O1s
S2p
In3d
PNb-G2M3T-5
PNb-G2D3T-5
PNb-G2T3T-5
95.82
97.30
97.39
3.01
2.15
2.57
0.07
0.08
0.04
1.10
0.47
0.00
No trend is found from the thickness of the electropoly-
merized films with the second-generation dendrons. Most of
the thicknesses, in this series, are less than those of the first-
generation dendrons electropolymerized in ACN. Nonethe-
less, polynorbornene brush thickness still increased from
G2M3T to G2T3T, indicating the density of the free olefins
still plays a key role in polymer brush growth even with
thinner electrodeposited materials. By comparing G1D3T
and G2M3T which both have two terminal olefin functional
groups in each molecule, the brush film thickness were found
identical, meaning the slightly larger G2M3T does not
obviously change the statistical distribution of the effective
olefinic sites. Again, a disproportionate relation between the
electropolymerized film thickness and the number of per-
ipheral olefins from the dendrons exists. Finally, the poly-
norbornene brush film thicknesses, in our study, are relatively
lower than those grown from SAMs-activated surfaces with
similar monomer concentration. This is perhaps due to a
lower density of transition-metal-activated sites for ROMP.34
Polymer brush synthesis on top of layer-by-layer (LBL) films
containing initiators has been developed by the Armes group
and our group very recently.35-37 Polyelectrolyte macroini-
tiators were synthesized and then built into a multilayer archi-
tecture, leading to an amplified polymer brush growth by
increasing porosity of the LBL films. On the other hand,
using other chemically adsorbed macroinitiators on solid sub-
strates provides another pathway to grow polymer brushes.38
As a comparison, varying the number of peripheral olefins in
our present work offers another useful method for polymer
brush synthesis to control the growth of polymer brush.
3.3.3. XPS Analysis of the Electropolymerized Films and the
Polynorbornene Brush Films. XPS analysis was performed at
a takeoff angle of 45° for elemental analysis and surface con-
tent ratio. In the XPS study, we picked the brush films grown
after 60 min from the electropolymerized films (5 CV cycles)
with G2M3T, G2D3T, and G2T3T. High-resolution scans of
the carbon (C), oxygen (O), sulfur (S), and indium (In) regions
were obtained and then fitted with Gaussian equations for peak
area analysis. Figure 5 shows a typical XPS spectra showing
the 4 atom peaks in the film PNb-G2T3T-5. XPS data provide
the qualitative and semiquantitative evidence of the presence
of C1s with a binding energy of 286 eV, O1s with a binding
energy of 533 eV, S2p with a binding energy of 164 eV, and
In3d with a binding energy of 445 eV. Table 4 shows the
5/2
relative concentration of the C1s, O1s, S2p, and In3d peaks
5/2
on these films.
Evidently, the In3d peak is not present in the film PNb-
5/2
G2T3T-5 which indicates the more highly dense and thicker
brush, as supported by AFM and ellipsometry. As a compar-
ison, the more porous and thinner film from PNb-G2M3T-5
shows the highest concentration of In3d . Furthermore, the
5/2
film PNb-G2T3T-5 provides the highest intensity of C1s and
the lowest intensity of S2p. Additionally, the relative concen-
tration of O1s in the film PNb-G2T3T-5 is higher than that in
the film PNb-G2D3T-5. It is not surprising that the theore-
tical percentage of oxygen atom in the dendron G2T3T is
higher than that in G2D3T.

Article
Macromolecules, Vol. 43, No. 24, 2010 10273
Figure 6. Ellipsometric film thicknesses of the polynorbornene brushes grown on the different electropolymerized films from the dendrons after 5 and
10 CV cycles.
3.3.4. Growth Kinetics of the Polynorbornene Brush Films.
It is very important to understand polymer brush growth
kinetics because it not only allows a better understanding of
the basic fundamentals in film growth but also helps in the
synthesis of polymer films with the desired thickness or
grafting density. Recently, Jennings et al. have studied the
kinetics of polynorbornene brush growth on the ROMP-
active monolayer films based on some assumptions.4 It was
found that with increasing pendant alkyl chain length on
norbornene monomer the growth kinetics of the polymer
brush films drastically decreased by 2 orders of magnitude.
Unfortunately, in our case, the growth kinetics in terms of
initiation, propagation, and termination rate constants can-
not be quantitatively analyzed, given that the density of the
active olefins varies with a few parameters and hard to quantify.
Thus, we attempted to qualitatively investigate the kinetics
of the polynorbornene brushes grown on the electropoly-
merized films ex situ. As depicted in Figure 6, the polymers
grow very rapidly in the first 20 min and then start to reach
saturation slowly in the next 40 min. This is similar to pre-
vious reports on the measurements of polynorbornene (or
their derivatives) growth kinetics.3a,4 Therefore, the strategy
we utilize in this work provides for another effective pathway
to grow polymer brush films in a short time. Compared to
solution (bulk) ROMP reactions which often complete within
minutes, the brush growth kinetics is much slower by
several orders of magnitude. And this effect is amplified by
introducing more bulky functional groups in the norbornene
pendants. In all the four cases listed in Figure 6, by increasing
the number of the substituted terminal olefins, the polymer
film thickness increases faster and approaches the constant
equilibrium value earlier. This provides a strong proof that
the density of the available alkenes for catalyst activation
and film growth can be controlled. Furthermore, growth
kinetics of the polynorbornene brush seemed to be very com-
parable regardless of the CV cycles, indicating that the density
of the active olefins on the electropolymerized films from the
same dendron molecules are not much different.
4. Conclusions
In this work, a series of peripheral olefin dendrons with electro-
polymerizable terthiophene functional group at the focal point
were successfully synthesized. Electrochemical polymerization of
these dendron molecules onto conducting substrates enabled ultra-
thin films with free olefins available for catalyst activation/initi-
ation. This opens up a new route to prepare polymer brushes which
can be achieved on any type and shape of conducting surfaces. By
varying dendron generation or the number of the olefinic sub-
stituent on the branching sides, the density of such olefins can be
changed. Therefore, the controlled synthesis of polymer brushes
in terms of chain length, grafting density, and growth kinetics can
be achieved. Polynorbornene was chosen as the model brush poly-
mer to demonstrate the utility of these dendrons as surface initiators.
AFM, XPS, and growth kinetics measurements confirmed the
effect of the dendron structures on the growth kinetics and film
thickness of the polymer brushes. Future work can focus on the
nature of the electrodeposited conjugated polymer and its electro-
optical properties. Other monomers designs can also be explored.
Acknowledgment. The authors acknowledge funding from
the NSF CBET-0854979, DMR-10-06776, CHE-10-41300,
Robert A. Welch Foundation E-1551, and the Texas NHARP
01846. Technical support from Agilent Technologies (Molecular
Imaging), Malvern Instruments (Viscotek), and Optrel GmbH
is also acknowledged.
Supporting Information Available: AFM images of the
electrodeposited polythiophene films and the grafted polynor-
bornene brushes including summary of feature dimensions; electro-
chemical CV data on second-generation dendrons. This material is
available free of charge via the Internet at http://pubs.acs.org.

10274 Macromolecules, Vol. 43, No. 24, 2010
Jiang et al.
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