Photocaged pendent thiol polymer brush surfaces for postpolymerization modifications via thiol-click chemistry

Article abstract of DOI:10.1002/pola.26468

In this work, a postpolymerization surface modification approach is reported that provides pendent thiol functionality along the polymer brush backbone using the photolabile protection chemistry of both o-nitrobenzyl and p-methoxyphenacyl thioethers. Poly

Full text of DOI:10.1002/pola.26468

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Photocaged Pendent Thiol Polymer Brush Surfaces for
Postpolymerization Modifications via Thiol-Click Chemistry
Ryan M. Hensarling, Emily A. Hoff, Arthur P. LeBlanc, Wei Guo,
Santosh B. Rahane, Derek L. Patton
School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, Mississippi 39406
Correspondence to: D. Patton (E-mail: derek.patton@usm.edu)
Received 12 September 2012; accepted 3 November 2012; published online 27 November 2012
DOI: 10.1002/pola.26468
ABSTRACT: In this work, a postpolymerization surface modifica-
tion approach is reported that provides pendent thiol function-
ality along the polymer brush backbone using the photolabile
protection chemistry of both o-nitrobenzyl and p-methoxyphe-
nacyl thioethers. Poly(2-hydroxyethyl methacrylate) (pHEMA)
brushes were synthesized via surface-initiated atom transfer
radical polymerization, after which the pHEMA hydroxyl
groups were esterified with 3-(2-nitrobenzylthio)propanoic acid
or 3-(2-(4-methoxyphenyl)-2-oxoethylthio)propanoic acid to
provide the photolabile protected pendent thiols. Addressing
the protecting groups with light not only affords spatial control
thiol-mediated transformations with isocyanates and malei-
mides providing a modular route to create functional polymer
surfaces. This concept was extended to block copolymer brush
architectures enabling the modification of the chemical func-
tionality of both the inner and outer blocks of the block copoly-
C
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mer surface. 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part
A: Polym. Chem. 2013, 51, 1079–1090
KEYWORDS: functionalization of polymers; polymer brush; post-
polymerization modification; surface-initiated polymerization;
surfaces; thin films; thiol-click
of reactive thiol functionality but enables
a plethora of
INTRODUCTION Engineering polymer surfaces with precise
control over polymer architecture, chemical functionality,
and spatial orientation of functional groups throughout the
based PPM strategies;11 however, concern over the presence
of residual metal impurities following copper-catalyzed click
reactions has driven the development of alternate, metal-free
surface modification strategies. Consequently, metal-free click
reactions—such as strain-promoted azide-alkyne cycloaddi-
tions,12 Diels–Alder cycloadditions,13–17 and thiol-based
reactions18—are rapidly becoming methods of choice for
postpolymerization surface modification strategies.
interface represents
a
grand challenge for polymer
chemistry—particularly as demand increases for surfaces
presenting complex chemistries and morphologies. A rapidly
growing strategy to address this challenge involves postpoly-
merization modification (PPM) of polymer surfaces.1 PPM of
surfaces is
a process based on the polymerization of
Our group, along with others, has demonstrated thiol-based
click reactions—including thiol-ene,19–22 thiol-yne,22–24 and
thiol-isocyanate25—as efficient and modular strategies
towards engineering multifunctional surfaces.26 Thiol-click
reactions are advantageous for PPM of surfaces in that they
proceed at room temperature with high efficiency and rapid
reaction rates, in the presence of oxygen and water, without
expensive and/or toxic catalysts, and exhibit high tolerance
toward a broad range of functional groups.27,28 In addition,
we have recently exploited the orthogonal nature of radical-
mediated thiol-yne reactions in sequential combination with
base-catalyzed thiol-isocyanate, thiol-epoxy, and thiol-bromo
reactions for the design of multifunctional polymer brush
surfaces with controlled surface compositions and wetting
properties.29 Furthermore, thiol-click reactions also have a
significant advantage in that a large number of functional
monomers with functional groups that are inert under the
polymerization and/or film formation conditions, but can
subsequently be quantitatively converted into a broad range
of other functional groups. Thus, PPM enables the versatile
and modular transformation of physiochemical properties of
surfaces, and has been demonstrated using modification
chemistries ranging from activated esters2,3 and ring open-
ing4–7 to more efficient and robust chemistries based on the
‘‘click’’ family of reactions.8,9 Click reactions—with the
copper assisted azide-alkyne cycloaddition (CuAAC) reaction
being the prominent example10—exhibit salient features
such as high yields, fast reaction kinetics, orthogonal reactiv-
ity, and are tolerant to a broad range of reaction conditions.
Successful utilization of CuAAC for PPM of surfaces has
provided the impetus for continued development of click-
Additional Supporting Information may be found in the online version of this article.
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thiols are commercially available eliminating the need for
multistep synthesis of postmodifiers often encountered in
other click-based strategies. In order to exploit the library of
commercially available thiols, most examples of thiol-click
PPM of surfaces have relied on the immobilization of alkenes,
alkynes, or isocyanates as thiol-reactive handles on the sur-
face. However, by using the reverse scenario whereby thiols
are immobilized as the reactive handle on the surface, one
could easily take advantage of the vast libraries of
commercially available maleimides, acrylates, isocyanates, and
so forth—all of which are attainable carrying a broad range
of pendent functionalities. Such an approach would vastly
broaden the thiol-click toolbox for postmodification of surfa-
ces. Aside from serving as a handle for PPM, well-defined
polymer surfaces with polyfunctional pendent thiols may also
be of interest for immobilization of metallic nanoparticles
and for heavy metal capture and remediation applications.
polymerization, but amendable via sequential light-triggered
deprotection and thiol-ene PPM process. Wosnick and
Shoichet53 covalently modified agarose hydrogels with a
6-bromo-7-hydroxycoumarin sulfide derivative, which upon
phototriggered deprotection and thiol-Michael reaction with
maleimide-functionalized biomolecules, enabled the develop-
ment of three-dimensional chemical patterns within the
hydrogel. Examples using surface-bound photolabile pro-
tected thiols as reactive handles for postmodification of
surfaces have been limited to self-assembled mono-
layers.41,42,46 For example, Chen et al.46 described photopat-
terning of biomolecules on planar surfaces upon photolysis
of an o-nitrobenzyl protected thiol monolayer. Upon exposure
of the thiol, biomolecules were immobilized via disulfide and
thiol-Michael reactions. Wavelength-selective PPGs have also
been used to expose thiols as reactive head groups on
monolayers by exploiting wavelength-selective photolysis of
various PPG derivatives enabling independently addressable
functional moieties.41,42 Aside from our own example
demonstrating the PPM of pendent thiols via thiol-Michael
on cysteine-containing polypeptide brushes,21 we are cur-
rently unaware of any literature reporting the synthesis of
well-defined polymer brush surfaces bearing pendent thiols
for modular PPM.
Unfortunately, the desirable characteristics of thiols—i.e.,
high reactivity and efficiency toward an array of functional
groups—also make them intolerable under radical polymer-
ization conditions. The large chain transfer constants of
thiols in vinyl polymerizations, self-association via disulfide
linkages, and numerous other side reactions eliminate any
possibility of incorporating unprotected thiols into macromo-
lecules by direct polymerization. While there are numerous
examples of polymer pendent polyfunctional thiols,30,31
researchers often resort to protection/deprotection schemes
that require harsh reaction conditions to yield the thiol (i.e.,
conversion of halogens to thioesters followed by extended
reflux under basic conditions). For engineering functional
surfaces based on tethered thiols, particularly those with
delicate underlying substrates, milder synthetic conditions
toward the thiol are desirable.
In the present work, we report a postpolymerization surface
modification approach that provides pendent thiol function-
ality along the polymer brush backbone using the photolabile
protection chemistry of both o-nitrobenzyl (o-NB) and
p-methoxyphenacyl (p-MP) thioethers. Addressing the pro-
tecting groups with light enables a plethora of thiol-mediated
transformations with isocyanates and maleimides providing
a versatile route to create complex, functional polymer
surfaces. The experiments described in this article were
performed using poly(hydroxyethyl methacrylate) (pHEMA)
brushes synthesized via surface-initiated atom transfer
radical polymerization (SI-ATRP), which were esterified with
3-(2-nitrobenzylthio)propanoic acid or 3-(2-(4-methoxy-
phenyl)-2-oxoethylthio)propanoic acid to provide the photo-
labile protected pendent thiols. SI-ATRP of HEMA was chosen
for this work as it serves as a model brush platform allowing
excellent control over film thickness, and it enables facile
synthesis of block copolymer brushes for investigation of
photolysis and thiol-click PPM processes on more advanced
brush architectures. A principal advantage of the postmodifi-
able brush platform is that it provides a larger number of
modifiable sites per unit area of substrate as compared to
conventional self-assembled monolayers (SAMs), while also
decoupling the polymer synthesis step from the immobiliza-
tion of sensitive functional groups on the surface thereby
avoiding expensive monomer synthesis and reducing poten-
tial side reactions.
‘‘Caged’’ compounds, or structures containing photolabile
protecting groups (PPGs), are well established in the areas
of organic32 and biochemistry as mild alternatives to
chemically-induced deprotections.33 PPGs can be removed by
exposure to ultraviolet (UV) light under neutral and reagent-
free conditions to yield a variety of reactive functional
groups including acids, alcohols, and amines. Light can be
used in a direct fashion to immediately trigger a desired
modification or indirectly to release a reactive moiety that
will then participate in a given activity or modification
process, such as PPM. In this way, PPGs have been used for
initiator,34 end group35 and side-chain polymer modifica-
tions,36–38 block copolymers,37,39,40 monolayers,41–44 two-
dimensional surface modifications,45–47 and polymer
brushes.48 PPG strategies have also been reported for the
efficient photolysis of protected thiols based on 2-nitroben-
zyl,49 phenacyl,50,51 benzoinyl,52 and coumarinyl49 protecting
groups; however, these examples are mostly related to bio-
chemistry applications with fewer examples describing pho-
todeprotection of thiols for polymer or surface modifications.
Recently, Barner-Kowollik et al. demonstrated o-nitrobenzyl
protected thiols as latent pendents on methacrylate-based
homopolymers38 and acrylamide-based copolymers38 render-
ing the thiol functionality inert during controlled radical
EXPERIMENTAL
Materials
All reagents and solvents were obtained at the highest purity
available from Aldrich Chemical Company or Fisher Scientific
and used without further purification unless otherwise speci-
fied. Single-side polished silicon wafers were purchased from
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University Wafers. Monomers, 2-hydroxyethyl methacrylate
(HEMA, 97% Aldrich), and 2-(dimethylamino)ethyl methacry-
late (DMAEMA, 99% Aldrich), were passed through a neutral
alumina column to remove the inhibitor. Reagents, 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU), dimethylphenylphosphine
(DMPP), and anhydrous triethylamine (TEA) for deprotection
and thiol-click reactions were also obtained from Aldrich
and used as received. Cyanophenyl maleimide was synthe-
sized according to reported literature procedures.54,55
phy (5:1 hexanes:ethyl acetate, Rf ¼ 0.7) to give 7.25 g
(91%) of the ester as a colorless oil.
1H NMR (CDCl3, d ppm): 1.28–1.72 (br m, 14H), 1.93 (s, 6H),
2.05 (q, 2H), 4.16 (t, 2H), 4.9–5.02 (m, 2H), 5.74–5.87 (m,
1H) 13C NMR (CDCl3, d ppm): 25.93, 28.50, 29.07, 29.23,
29.30, 29.52, 29.56, 30.95, 33.95, 56.16, 66.36, 114.25,
139.34, 171.89.
Synthesis of (11-(2-Bromo-2-methyl)propionyloxy)
undecyltrichlorosilane (ATRP Initiator-Tricholorsilane)
In a glovebox under a N2 atmosphere, 10-undecen-1-yl
2-bromo-2-methylpropionate (0.27 g, 0.84 mmol, 1 equiv),
trichlorosilane (0.57 g, 4.2 mmol, 5 equiv), about 3 mL of
anhydrous toluene, and five to six drops of Pt-divinyl tetra-
methyl disiloxane complex in vinyl silicone were allowed to
react overnight. Toluene and excess trichlorosilane were
removed under vacuum to yield (11-(2-bromo-2-methyl)
propionyloxy)undecyltrichlorosilane (0.37 g, 96.9%). Dry
toluene (3.1 mL) was added creating a stock 271 mM
solution. The catalyst and any solids were removed by a
syringe filter before use.
Characterization
A Varian Mercury Plus 300 MHz NMR spectrometer operat-
ing at a frequency of 300 MHz with VNMR 6.1 C software
was used for proton and carbon analysis. Wettability of the
unmodified and modified polymer brushes was measured
ꢀ
using a Rame-hart 200-00 Std.-Tilting B. goniometer. Static
(ysw) contact angles were measured using 10-lL water
droplets in combination with DROPimage Standard software.
Ellipsometric measurements were carried out using a Gaert-
ner Scientific Corporation LSE ellipsometer with a 632.8 nm
laser at 70ꢀ from the normal. Refractive index values of 3.86,
1.45, 1.43, and 1.5 for silicon, oxide layer, photoinitiator
monolayer, and all polymer layers, respectively, were used to
build the layer model and calculate layer thicknesses.56,57
The chemical nature of the polymer brush surfaces was
characterized by Fourier transform infrared spectroscopy
(FTIR) in grazing angle attenuated total reflectance mode
1H NMR (CDCl3, d ppm): 1.23–1.45 (br m, 16H), 1.54–1.75
(m, 4H), 1.93 (s, 6H), 4.16 (t, 2H) 13C NMR (CDCl3, d ppm):
22.16, 24.22, 25.69, 28.26, 29.07, 29.22, 29.38, 30.71, 55.88,
66.02.
Immobilization of ATRP Initiator-Trichlorosilane
on SiO2 Surfaces
(GATR-FTIR) using
(Nicolet 8700) equipped with
a
ThermoScientific FTIR instrument
a
VariGATRTM accessory
Silicon wafers were cut into appropriate sized pieces and
ultrasonically cleaned in DP2300 ultrahigh performance
general purpose cleaner and degreaser (Branson Ultrasonics)
for 5 min. The wafers were then wiped gently with lens
paper or a cotton-tipped applicator to remove silicon dust
from the wafer dicing process. After wiping, the wafers were
ultrasonicated for an additional 10 min, rinsed multiple
times with DI water, and ultrasonicated in deionized water
for 15 min. The wafers were then placed into a RCA-1
solution (five parts deionized H2O, one part 27% ammonium
hydroxide, and one part 30% hydrogen peroxide) for 15 min
at 70 ꢀC to remove any organic residues before initiator
immobilization. The wafers were rinsed thoroughly with DI
water, dried under a stream of N2, and transferred into an
acrylic glove box where they were placed into a toluene
solution of (11-(2-bromo-2-methyl)propionyloxy)undecyltri-
chlorosilane (4 mM) at room temperature for 16 h without
stirring. The wafers were removed from the solution, rinsed
extensively with toluene, dichloromethane, and dimethylfor-
mamide before drying under a stream of N2. The initiator-
functionalized silicon wafers were stored in toluene at ꢁ20
ꢀC until use.
(grazing angle 65ꢀ, germanium crystal; Harrick Scientific).
Spectra were collected with a resolution of 4 cmꢁ1 by accu-
mulating a minimum of 128 scans per sample. All spectra
were collected while purging the VariGATRTM attachment and
FTIR instrument with N2 gas along the infrared beam path
to minimize the peaks corresponding to atmospheric mois-
ture and CO2. Spectra were analyzed and processed using
Omnic software. Atomic force microscopy was performed
using a Bruker Icon in tapping mode. The samples were
scanned with T300R-25 probes (Bruker AFM Probes) with a
spring constant of 40 Nmꢁ1. Confocal microscopy was
performed on fluorescently patterned surfaces using a Zeiss
LSM 710 operating with two lasers (433 and 548 nm) corre-
lating to the absorption of fluorescein and rhodamine. The
fluorescent images were processed using ZEN software.
Synthesis of 10-Undecen-1-yl 2-Bromo-2-
methylpropionate (ATRP Initiator Precursor)
10-Undecen-1-yl 2-bromo-2-methylpropionate was synthe-
sized according to literature procedures.58 Pyridine (2.1 g,
26.5 mmol) was added to x-undecylenyl alcohol (4.27 g,
25.1 mmol) in 25 mL of anhydrous tetrahydrofuran (THF)
and subsequently cooled to 0 ꢀC followed by the dropwise
addition of 2-bromoisobutyryl bromide (6.1 g, 26.5 mmol).
The reaction was stirred overnight at r.t. followed by dilution
with hexanes (50 mL) and washing with 2N HCl (2ꢂ) and
deionized H2O (2ꢂ). The organic phase was dried with
MgSO4, filtered, and concentrated via rotary evaporation. The
colorless oily residue was purified using flash chromatogra-
Synthesis of 3-(2-Nitrobenzylthio) Propanoic Acid (o-NB)
3-Mercaptopropionic acid (2.95 g, 23.1 mmol) and anhy-
drous TEA (1.83 g, 18.1 mmol) were added to 2-nitrobenzyl
bromide (2.5 g, 11.6 mmol) in anhydrous acetone (150 mL)
under a N2 atmosphere and allowed to react overnight. The
salt by-product was filtered and the crude product was
isolated via rotary evaporation. The crude product was
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redissolved in ethyl acetate (100 mL), washed with 0.5 M
HCl (4ꢂ, 75 mL wash) and brine (1ꢂ, 75 mL), dried using
MgSO4. After filtration, the product was concentrated via
rotary evaporation. The product crystallized upon removing
excess solvent. The crystalline product was finally washed
with hexanes and dried under vacuum (1.9 g, 68.1%).
and N,N0-diisopropyl carbodiimide (DIPC) (57 mg, 0.45
mmol). DIPC in anhydrous DMF (1 mL) was added dropwise
over 5 min before placing the reaction on a shaker for 16 h.
The substrates were rinsed extensively with DMF, THF, and
toluene and dried under a stream of N2.
Photodeprotection of Brush Pendent o-NB- and
p-MP-Protected Thiols
1H NMR (CDCl3, d ppm): 2.59–2.64 (t, 2H), 2.70–2.74 (t, 2H),
4.10 (s, 2H), 7.26–7.59 (m, 3H), 7.96–7.99 (d, 1H), 11.12 (b
s, 1H) 13C NMR (CDCl3, d ppm): 26.64, 33.86, 34.47, 125.76,
128.76, 132.1, 133.37, 134.0, 149.01, 177.94.
Deprotection of the protected thiols was facilitated by irradi-
ating the substrates with UV light (365 nm, 70 mW cmꢁ2, 2
h) in N2 purged anhydrous dichloromethane with catalytic
amounts of DMPP (3.5 ꢂ 10ꢁ5 M). GATR-FTIR was used
to monitor the disappearance of the o-nitrobenzyl and
p-methoxyphenacyl groups.
Synthesis of 3-(2-(4-Methoxyphenyl)-2-oxoethylthio)
Propanoic Acid (p-MP)
3-Mercaptopropionic acid (2.32 g, 21.8 mmol) and anhy-
drous TEA (1.44 g, 14.2 mmol) were added to 2-bromo-40-
methoxyacetophenone (2.5 g, 10.9 mmol) in anhydrous
acetone (150 mL) under an N2 atmosphere and allowed to
react overnight. The salt by-product was filtered and crude
product was isolated via rotary evaporation. The crude prod-
uct was redissolved in ethyl acetate (100 mL), washed with
0.5 M HCl (4ꢂ, 75 mL wash) and brine (1ꢂ, 75 mL), dried
using MgSO4 followed by concentration of the product via
rotary evaporation. The product crystallized upon placing
into freezer at ꢁ20 ꢀC. The crystalline product was washed
with hexanes to remove any residue impurities and dried
under vacuum (2.6 g, 88.7%).
One-Pot Photodeprotection and Thiol-Click Modification
Protected substrates were irradiated with UV light under the
above conditions to facilitate photolysis of the o-NB or p-MP
moieties. The light source was turned off followed by the
addition of reagents to facilitate Michael-type thiol-ene
and base-catalyzed thiol-isocyanate surface modifications.
Modification of the thiol with various functionalities was
monitored by ellipsometry, GATR-FTIR and static water
contact angle. Details for each set of thiol-click reactions are
given below.
Thiol-Isocyanate Modification
A N2 purged solution consisting of 2-nitrophenyl isocyanate
(98.5 mg, 0.6 mmol, 0.1 M), 4-methoxybenzyl isocyanate
(85.7 mL, 0.6 mmol, 0.1 M), dodecyl isocyanate (144.6 mL, 0.6
mmol, 0.1 M), furfuryl isocyanate (64.3 mL, 0.6 mmol, 0.1 M),
or 1-adamantyl isocyanate (0.11 mg, 0.6 mmol, 0.1 M) in
anhydrous DCM (6 mL) was added to the reaction vessel
containing the reactive pendent thiol polymer brushes. For
rapid reaction kinetics, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) (0.3 mol % with respect to isocyanate) was used as
catalyst. The reaction was allowed to react overnight to
ensure completion; however, our group has previously
reported quantitative base-catalyzed thiol-isocyanate reac-
tions within minutes.25
1H NMR (CDCl3, d ppm): 2.70–2.73 (d, 2H), 2.81–2.87 (d,
2H), 3.79 (s, 2H), 3.88 (s, 2H), 6.92–6.97 (d, 2H), 7.93–7.97
(d, 2H), 11.12 (b s, 1H) 13C NMR (CDCl3, d ppm): 26.81,
34.15, 36.93, 55.65, 114.14, 128.18, 131.39, 164.14, 177.27,
193.43.
Synthesis of pHEMA Brush Surfaces by Surface-Initiated
Atom Transfer Radical Polymerization
SI-ATRP was carried out in vacuum purged test tubes
equipped with rubber septa. In one tube, HEMA and a
water/methanol mixture (1:4 v/v) were degassed by
bubbling through with N2 for 45 min. In a second tube, 2,20-
bipyridyl, and copper(I)bromide (40:1:0.5 mol % monomer/
ligand/Cu(I)Br) were degassed by three vacuum/N2 purge
cycles. The monomer solution was transferred by cannula to
the tube containing the ligand/Cu(I)Br, and the mixture was
stirred for 45 min or until a deep-red, homogeneous solution
was obtained. The monomer/catalyst complex was then
transferred by cannula into a degassed tube containing the
initiator-modified silicon substrate. The reaction proceeded
at room temperature. Reaction times were varied to obtain
the desired thickness of pHEMA brushes. pHEMA modified
substrates were rinsed extensively with water and methanol
following polymerization.
Thiol-Michael Modification
The reactive pendent thiol polymer brushes were submerged
into a N2 purged anhydrous dichloromethane (6 mL) solu-
tion containing cyanophenyl maleimide (0.1 M, 118.9 mg, 0.6
mmol) with 5 equiv. TEA (0.5 M, 418 mL, 3 mmol) with
respect to maleimide. The reaction was allowed to react
overnight to ensure completion, however, reactions times are
known to be much faster.27
Surface Patterning via Photodeprotection and
Orthogonal Thiol-Click Chemistries
Photomasks (copper grids, 200 mesh, hole width: 90 mm,
bar width: 37 mm) were placed directly on top of o-NB-pro-
tected thiol polymer brushes and secured in place with a
microscope cover glass slide. The glass slide ensured the
photomasks were in intimate contact with the surface as
well as limited the mobility of the photomasks upon the
addition of solvent. For photodeprotection, the substrates
were irradiated with UV light (365 nm, 70 mW cmꢁ2) for
Carbodiimide-Mediated Esterification of pHEMA Brush
Surface with o-NB- and p-MP-Protected Thiols
The pendent hydroxyl groups of the pHEMA brushes were
modified in anhydrous DMF (6 mL) with o-nitrobenzyl (3-(2-
nitrobenzylthio)propanoic acid) or phenacyl (3-(2-(4-methox-
yphenyl)-2-oxoethylthio)propanoic acid) derivatives (0.3
mmol) using 4-dimethylaminopyridine (7.3 mg, 0.06 mmol)
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SCHEME 1 (a) General approach for the synthesis of polymer brush surfaces with pendent photolabile protected thiols and (b)
subsequent photodeprotection and thiol-click modification.
1 h in anhydrous dimethylformamide (DMF) to facilitate the
photolysis of the o-NB moieties. The photomask was
removed and the sample was extensively washed in DMF,
THF and toluene followed by reaction of the newly generated
reactive pendent thiols with fluorescein isothiocyanate. Only
areas exposed to UV light generate free thiols that are avail-
able for reaction. A solution of fluorescein isothiocyanate
(9.9 mg, 12.7 mM) and DBU (0.127 mM, 100:1 mol/mol %
isothiocyanate:DBU) in anhydrous DMF (2 mL) was placed
into the reaction vessel containing the polymer brushes and
allowed to react for 1 h. After washing the substrate with
DMF, THF and toluene the areas within the polymer brush
still in a protected state were deprotected to form additional
free thiols available for reaction by irradiating the sample
with UV light (365 nm, 70 mW cmꢁ2) for 1 h in anhydrous
copolymer brushes were modified with o-nitrobenzyl thio-
ether units as previously described. For AFM studies, the
modified block copolymers were submerged into a 0.01 M
HCl aqueous solution for 30 min to protonate the pDMAEMA
domains within the block copolymer.
RESULTS AND DISCUSSION
Synthesis of Pendent Thiol Polymer Brushes
Scheme 1(a) shows the general approach for the synthesis of
photolabile protected pendent thiol polymer brushes. Silicon
substrates were first functionalized with a chlorosilane ATRP
initiator derivative, 11-(2-bromo-2-methyl) propionyloxyunde-
cenyltrichlorosilane.58 p(HEMA) brushes were then prepared
using SI-ATRP of HEMA in a water/methanol mixture, followed
by carbodiimide-mediated esterification with 3-(2-nitroben-
zylthio)propanoic acid (1) (o-NB) or 3-(2-(4-methoxyphenyl)-
2-oxoethylthio)propanoic acid (2) (p-MP) to provide the
polymer brushes with pendent photolabile protected thiols.
Conversion to the desired ester derivatives was confirmed by
GATR-FTIR (vide infra). Attempts to incorporate the o-NB
functional group in the brush by direct SI-ATRP of an o-NB
functional methacrylate monomer were successful; however,
this approach yielded miniscule film thickness (<10 nm) pre-
sumably due to the inhibitory effects of the ANO2 group.59
R
DMF followed by reaction with Texas RedV C2 maleimide. A
R
solution of Texas RedV C2 maleimide (0.1 mM) and TEA (0.5
mM, 5 equiv in respect to maleimide) in anhydrous DMF
(2 mL) was purged with N2 and subsequently added to the
reaction vessel containing the polymer brushes and allowed
to react for 1 h. The substrates were extensively washed
with DMF, THF, and toluene before confocal microscopy was
performed.
Synthesis, Photodeprotection, and Thiol-Click
Modification of Block Copolymer Brush Surfaces
The inhibition effect is likely amplified given the low concen-
tration of propagating chains relative to monomer concentra-
tion in surface-initiated polymerizations. In contrast, SI-ATRP
of a p-MP methacrylate monomer enabled a direct polymeriza-
tion approach providing an evident advantage over the o-NB
derivative, but for comparative purposes, the carbodiimide
esterification route was adopted for both systems.
pHEMA-b-pDMAEMA and pDMAEMA-b-pHEMA brush surfa-
ces were prepared in an analogous manner as previously
described for the homopolymer pHEMA brush surfaces using
2,2’-bipyridyl and copper(I)bromide (40:1:0.5 mol % mono-
mer/ligand/Cu(I)Br). pHEMA or pDMAEMA brushes served
as macroinitiator substrates for the chain extension reac-
tions. Reaction times were varied to obtain the desired thick-
ness of outer pDMAEMA or pHEMA blocks. The pendent
hydroxyl groups of the pHEMA block within the block
As shown in Scheme 1(b), the o-NB and p-MP moieties are
cleaved by irradiation with UV light at 365-nm yielding
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protection mechanism of p-methoxyphenacyl derivatives
(16). Givens et al.64 indicated the photodeprotection pro-
ceeded via a triplet-excited state and a spiroketone interme-
diate (not shown), which may account for byproduct (20) in
the case of p-MP-protected thiols, as (20) would result from
a nucleophilic ring opening of the spiroketone by a thiol.51
A
more recent study by An et al.65 suggested a concerted
triplet deprotection and solvolytic rearrangement with little
observation of a spiroketone intermediate to provide the
deprotected thiol moiety (17) and a series of byproducts
(18, 19).
SCHEME 2 Photodeprotection of o-nitrobenzyl (o-NB) pro-
Photoinduced deprotection of the caged thiols was investi-
gated under various conditions to facilitate complete removal
of the photolabile protecting groups while maximizing thiol
yield. Pauloehrl et al.38 previously showed that DMPP, when
added in catalytic amounts, prevented the formation of
disulfides during photodeprotection of o-NB thioethers elimi-
nating the need for a separate reduction step, thus we
adopted similar conditions. The photodeprotection reaction
was monitored with GATR-FTIR by observing the disappear-
ance of the asymmetric and symmetric NO2 stretching vibra-
tions inherent to the aromatic nitro derivative at 1527 and
1350 cmꢁ1 for the o-nitrobenzyl PPG, and the aromatic C¼¼C
stretching vibrations of the phenacyl derivative at 1600,
1576, and 1515 cmꢁ1 for the p-methoxyphenacyl PPG.66 The
formation of thiol upon deprotection could not be monitored
by GATR-FTIR due to the band for SAH stretching vibration
tected thioether.
pendent thiol brush precursors. The resulting pendent thiol
moieties along the polymer brush backbone then serve as
reactive handles for subsequent thiol-click reactions. In the
following sections, we describe the results of photodeprotec-
tion of both o-NB and p-MP PPGs and postmodification via
base-catalyzed thiol-isocyanate reactions and thiol-Michael
additions with maleimides. The reactions were carried out in
a one-pot reaction, meaning that the UV light was turned off
following deprotection, and base-catalyzed thiol-isocyanate
reactions or Michael-type thiol-ene additions were facilitated
in the same pot by syringing in solutions of either an isocya-
nate or a maleimide along with the respective catalysts.
being very weak in the region of 2540–2600 cmꢁ1 66
Figure
.
Photodeprotection of o-NB- and p-MP-Protected Thiol
Brushes and One-Pot Thiol-Click Modification
1 shows the GATR-FTIR spectra for the photolabile caged
o-NB- and p-MP-protected pendent thiol polymer brushes
upon just photodeprotection and also one-pot photodepro-
tection and sequential thiol-isocyanate click reactions. The o-
NB-protected thiol polymer brushes [Fig. 1(a)] were subse-
quently deprotected by exposing the surface to UV365nm light
irradiation (70 mW cmꢁ2) for 2 h in N2 purged anhydrous
DCM in the presence of a catalytic amount of DMPP. Success-
ful deprotection to the thiol was indicated by the complete
disappearance of the NO2 stretching vibrations in the GATR-
FTIR [Fig. 1(b)] and a decrease in polymer brush thickness
(ca. 9 6 1 nm). Similar results were obtained for the photo-
deprotection of the p-MP modified brush surfaces. As shown
in Figure 1(d), the aromatic C¼¼C stretching vibrations of the
phenacyl derivative at 1600, 1576, and 1515 cmꢁ1 present
in the protected form were no longer observed following
photodeprotection [Fig. 1(e)]. In both cases, the surface
becomes more hydrophilic on deprotection and conversion
from the aromatic PPG to the pendent thiol (i.e., water con-
tact angle decreases from 73ꢀ to 52ꢀ for o-NB surface and
from 74ꢀ to 49ꢀ for p-MP surface) (Supporting Information
Fig. S1).
Schemes 2 and 3 show the commonly accepted mechanisms
for photodeprotecton reactions of o-NB and p-MP derivatives
(shown as thiol derivatives for relevance to the current
work). A brief discussion of these schemes provides insight
for the ensuing photodeprotection studies to yield pendent
thiols on the brush platforms. Photodeprotection of o-nitro-
benzyl derivatives (9) is known to be initiated by the
abstraction of benzylic hydrogen from an excited nitro group
producing aci-nitro intermediates (10) and (11).33,60,61 Irre-
versible cyclization to the benzisoxazoline intermediate (12)
through the neutral nitronic acid (10) followed by ring-open-
ing yields (13) and ultimately the products 2-nitrosobenzal-
dehyde (14) and R-SH (15).33,61 A strong solvent and pH
dependence has also been shown for the photodeprotection
of o-nitrobenzyl derivatives.62 The reaction of the depro-
tected thiol (15) with the nitroso moiety of 2-nitrosobenzal-
dehyde (14) should be noted as a potential side reaction63
which would reduce the available thiol concentration within
the brush. Much less is known about the detailed photode-
To gain better insight and provide a route to quantify the
yield of thiol functionality on the surface following photode-
protection, the pendent thiols were ‘‘tagged’’ with isocyanates
bearing a structural resemblance to the cleaved PPG via the
base-catalyzed thiol-isocyanate reaction. Namely, thiols pro-
duced from the photodeprotection of o-NB and p-MP groups
were subsequently tagged with 2-nitrophenyl isocyanate (3)
SCHEME 3 Photodeprotection of p-methoxyphenacyl (p-MP)
protected thioether.
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reactive free thiols, respectively, on photodeprotection when
performed in the presence of a catalytic amount of DMPP as
a reducing agent. Additionally, an increase in brush thickness
was observed upon one-pot photodeprotection and thiol-
isocyanate click in both o-NB- and p-MP-protected samples,
where an increase from 21.4 6 0.2 nm (o-NB protected)
to 24.2 6 0.1 nm (clicked with 3) and from 19.1 6 0.8 nm
(p-MP protected) to 24.1 6 0.2 nm (clicked with 4) was
measured, respectively (Supporting Information Table S1).
The slight increase in brush thickness despite less than
quantitative availability of thiol can be attributed to the
replacement of the thioether-linked o-NB and p-MP pendent
groups with the larger molecular weight thiourethane-linked
pendent groups derived from 3 and 4. Despite the use of
DMPP, the formation of disulfides or other adventitious side
products resulting from the photodeprotection precludes the
possibility of achieving quantitative yields of reactive free
thiol on the brush surface. For comparison, the formation of
free thiols for both o-NB- and p-MP-protected polymer
brushes decreased to 63 6 1% and 75 6 2%, respectively,
when reducing agent was not used during photodeprotec-
tion. GATR-FTIR spectra of samples with and without DMPP
during the photodeprotection are available in the Supporting
Information (Fig. S1).
FIGURE 1 GATR-FTIR spectra of photolabile-caged o-NB and
p-MP-protected pendent thiol polymer brushes, subsequent
deprotection, and thiol-isocyanate click reactions: (a) photola-
bile o-NB-protected pendent thiol polymer brush, (b) depro-
tected pendent thiol polymer brush (3.5 ꢂ 10ꢁ5 M DMPP in
DCM), (c) one-pot photodeprotection and thiol-isocyanate reac-
Upon optimization of the photodeprotection and thiol-click
modifications of the brush surfaces, the scope of the thiol-
click reactions was broadened to include other functional-
ities. Photodeprotection of the o-NB- and p-MP-protected
brushes was facilitated as previously described in the pres-
ence of catalytic amounts of DMPP (3.5 ꢂ 10ꢁ5 M). Figure 2
and Supporting Information Figure S2 show the GATR-FTIR
of the photolabile o-NB- and p-MP-protected thiol polymer
brushes after modification via one-step photodeprotection
and thiol-click reactions. For brevity, the thiol-click reactions
for the o-NB and p-MP derivatives will be discussed collec-
tively as the results were similar in each case. The polymer
brushes before and after one-pot photodeprotection and
thiol-click surface modification were characterized by GATR-
FTIR, ellipsometry, and water contact angle measurements.
Figure 2(a–c) and Supporting Information Figure S2(a–c)
show the GATR-FTIR following thiol-isocyanate click of the
pendent thiols produced from o-NB and p-MP cleavage,
respectively, in the presence of DBU (0.3 mol % in respect to
isocyanate) with dodecyl isocyanate (5), furfuryl isocyanate
(6) and adamantyl isocyanate (7). In each case, FTIR
confirms a successful thiol-click modification due to the
appearance of peaks indicative of the functional isocyanates,
for instance, aliphatic CAH stretching vibrations (2954,
2924, and 2852 cmꢁ1) was observed for dodecyl isocyanate
[Fig. 2(a) and Supporting Information Fig. S2(a)] and for
adamantyl isocyanate [Fig. 2(c) and Supporting Information
Fig. S2(c)]. For modification with furfuryl isocyanate, CAH
and ¼¼CAH stretching vibrations occur between 3000 and
2800 cmꢁ1; however, spectral overlap of the polymer back-
bone in the ether region (CAOAC asymmetric stretch, 1270–
1060 cmꢁ1) makes explicit confirmation of a successful
thiol-isocyanate modification with furfuryl isocyanate by
GATR-FTIR difficult [Fig. 2(b) and Supporting Information
tion with 2-nitrophenyl isocyanate (3) (0.3 mol
% DBU in
respect to isocyanate), (d) p-MP-protected pendent thiol poly-
mer brush, (e) deprotected pendent thiol polymer brush (3.5 ꢂ
10ꢁ5 M DMPP in DCM), and (f) one-pot photodeprotection and
thiol-isocyanate reaction with 4-methoxybenzyl isocyanate (4)
(0.3 mol % DBU in respect to isocyanate).
and 4-methoxybenzyl isocyanate (4), respectively, in a one-
pot fashion. Thus, using GATR-FTIR, the ratio of the ANO2
peak (or the AOCH3 peak) before photodeprotection and
after thiol-isocyanate click will provide insight into the quan-
tity of thiols available for modification. Importantly, we have
previously shown that thiol-isocyanate postmodification of
brush surfaces proceeds rapidly to full conversion, so we are
confident this approach will tag any available thiols for
analysis.25 Figure 1 shows the GATR-FTIR spectra of o-NB
[Fig. 1(a)] and p-MP [Fig. 1(d)] protected pendent thiol poly-
mer brushes and subsequent one-pot photodeprotection and
thiol-isocyanate click reactions with 2-nitrophenyl isocyanate
[Fig. 1(c)] or 4-methoxybenzyl isocyanate [Fig. 1(f)]. Photo-
chemical conversion to free thiol was estimated by taking
the ratio of the integrated peak area of the NO2 symmetric
stretching vibration (1350 cmꢁ1) for the o-NB brushes
before photodeprotection and after thiol-isocyanate click,
and the integrated peak area of the aromatic phenacyl C¼¼C
stretching vibration (1515 cmꢁ1) before photodeprotection
and after thiol-isocyanate click for the p-MP brushes. The
carbonyl band (C¼¼O, 1730 cmꢁ1) was used as a reference
peak as the area of this peak remained constant during the
surface modifications, (i.e., the C¼¼O peak associated with
formation of a thiourethane linkage (ASACOANHA) on the
thiol-isocyanate click reaction appears at 1650 cmꢁ1).66
Accordingly, the photolabile o-NB- and p-MP-protected
pendent thiol polymer brushes yield 77 6 2% and 88 6 3%
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cein on the brush surface only in the light exposed areas
creating a well-defined pattern as shown by fluorescence
microscopy (Supporting Information Fig. S4). The remaining
o-NB protecting moieties were subsequently cleaved by UV
flood exposure liberating free thiols available for further
R
functionalization. Texas RedV C2 maleimide was then immo-
bilized onto the surface in the presence of TEA to generate
multifunctional micropatterns in a sequentially orthogonal
fashion. Figure 3 shows the fluorescence microscopy images
R
of the fluorescein/Texas RedV C2 micropatterns under illumi-
nation with two lasers (433 and 548 nm). Well-defined
edges [Fig. 3(b)] indicate a sharp interface between the two
domains illustrating spatially resolved patterns can be
achieved via cleavage of PPGs and PPM thiol-click processes.
Postpolymerization Modification of Block
Copolymer Brushes
With the retention of the bromine end group from SI-ATRP,
chain extension enables preparation of block copolymer
brushes containing photolabile thiol moieties available for
postmodification within the upper or lower block (Scheme
4). Relatively few reports have demonstrated the PPM of
block copolymer brush surfaces despite the potential of
using the block copolymer brush architecture to control
access and the microenvironment of pendent functional
groups.3,67 Block copolymer brushes with o-NB-protected
thiols in the inner block were synthesized using pHEMA
with a thickness of 14.6 6 0.4 nm as a macroinitiator for
polymerization of DMAEMA. The thickness of the DMAEMA
block was 22.1 6 0.3 nm after 40 min of polymerization at
room temperature. Block copolymers with o-NB-protected
thiols in the outer block were prepared analogously using
instead a pDMAEMA macroinitiator for chain extension with
HEMA. In both cases, the o-NB-protected thiol was added via
esterification of the pHEMA as described for the homopoly-
mer brush samples resulting in a net thickness increase of
about 15 nm.
FIGURE 2 GATR-FTIR spectra after one-pot photodeprotection
and thiol-click modifications (base-catalyzed thiol-isocyanate
and Michael-type thiol-ene) of o-NB-protected pendent thiol
polymer brushes with (a) dodecyl isocyanate, (b) furfuryl iso-
cyanate, (c) adamantyl isocyanate, and (d) cyanophenyl
maleimide.
Fig. S2(b)]. Lastly, Figure 2(d) and Supporting Information
Figure S2(d) show the GATR-FTIR spectra for polymer
brushes modified with cyanophenyl maleimide (8) via
Michael-type thiol-ene reactions in the presence of 5 equiv
of TEA. The incorporation of cyanophenyl maleimide in the
brush was evident by the appearance of a CBN stretching
vibration at 2227 cmꢁ1 and aromatic C¼¼C stretching vibra-
tions at 1610 and 1510 cmꢁ1. All one-pot reactions show
expected water contact angles (Supporting Information Fig.
S3) and increases in film thickness on modification
(Supporting Information Table S1).
Figure 4 shows the GATR-FTIR spectra of the block copoly-
mer surfaces with o-NB pendent thiol in the inner [Fig. 4(a)]
and outer [Fig. 4(c)] blocks. Both surfaces show the charac-
teristic peaks for the –NO2 asymmetric and symmetric
stretching vibrations (1527 and 1350 cmꢁ1) of the o-NB
block and for aliphatic amine NACAH stretching vibrations,
2820 and 2770 cmꢁ1, and CAN stretching vibration 1270
cmꢁ1 of the DMAEMA block. When the outer block consisted
of DMAEMA units, protonation in 0.01 M HCl (pH 2) resulted
in a weak NHþ stretching vibration at 2250 cmꢁ1. Upon
one-pot photodeprotection and thiol-isocyanate click with
dodecyl isocyanate, both samples show the disappearance of
the NO2 stretching vibrations and appearance of CAH
stretching vibrations at 2989, 2930, and 2860 cmꢁ1 and
(CH2)n 1470 cmꢁ1 indicative of dodecyl moieties within
inner [Fig. 4(b)] and outer [Fig. 4(d)] blocks. The corre-
sponding GATR-FTIR spectra for the homopolymer brushes
can be found in Supporting Information Figure S5.
Surface Patterning via Photodeprotection and
Orthogonal Thiol-Click Chemistries
As a stimulus for deprotection, light has the added benefit of
spatial and temporal control useful for surface patterning. To
demonstrate this control, we conducted sequential/area-
selective orthogonal surface reactions via base-catalyzed
thiol-isothiocyanate and thiol-Michael additions with fluores-
R
cein isothiocyanate and Texas RedV C2 maleimide, respec-
tively, using a simple photopatterning technique. Photomasks
(copper grids, hole width: 90 mm, bar width: 37 mm) were
placed in contact with the brush surface, immersed in anhy-
drous DMF and irradiated with UV light for 1 h (365 nm, 70
mW cmꢁ2) to facilitate the photolysis of the o-NB moieties
only in the light exposed areas. After exposure, substrates
were immersed in a solution of fluorescein isothiocyanate/
DBU for 1 h. The initial click reaction immobilized fluores-
Changes in the topography of the block copolymer surfaces
as a result of thiol-click modifications of the inner and outer
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FIGURE 3 Fluorescence microscopy images of polymer brushes patterned with fluorescein isothiocyanate (squares) and Texas
R
RedV C2 maleimide (bars) with 433 and 548 nm lasers, respectively, at magnifications of (a) 5ꢂ and (b) 20ꢂ.
blocks as well as solvent treatment were characterized by
AFM in tapping mode. The height images show the geometric
roughness of the surface while the phase images reveal the
distribution of different polymer domains present at the
brush interface. The DMAEMA block was protonated to
create a more hydrophilic domain compared to the more
hydrophobic pendent dodecyl-modified block. After protona-
tion with 0.1 M HCl, the surfaces were dried, rinsed with
toluene, and finally dried with a stream of N2. To better
understand the morphological changes of the block copoly-
mer surfaces, the height and phase images of each system
are shown in Figure 5. For comparison, the AFM images for
unmodified and modified homopolymer brushes and the
unprotonated block copolymers are given in the supporting
information (Supporting Information Figs. S6 and S7). Figure
5(a,c) show the topography and phase images for the block
copolymer surfaces with o-NB-protected pendent thiol in the
inner and outer blocks, respectively. These surfaces do not
FIGURE 4 GATR-FTIR of block copolymers (p(inner block)-b-
p(outer block)) after PPG modification and thiol-click reaction
with dodecyl isocyanate: (a) photolabile o-NB pendent thiol-
modified pHEMA-b-pDMAEMA polymer brush (protonated), (b)
pHEMA pendent thiol-b-pDMAEMA polymer brush clicked with
dodecyl isocyanate (protonated), (c) pDMAEMA-b-NB pendent
thiol-modified pHEMA polymer brush (protonated), and (d)
pDMAEMA-b-pHEMA pendent thiol polymer brush clicked with
dodecyl isocyanate (protonated).
SCHEME 4 General approach for block copolymer synthesis,
one-pot photodeprotection and thiol-click modification. The
analogous inverse block sequence (pendent thiol upper block)
was also synthesized, but is not shown.
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FIGURE 5 Tapping mode AFM images (height/phase) of block copolymers: (a) photolabile o-NB pendent thiol-modified pHEMA-b-
pDMAEMA polymer brush (protonated), (b) pHEMA pendent thiol-b-pDMAEMA polymer brush click with dodecyl isocyanate (pro-
tonated), (c) pDMAEMA-b-photolabile o-NB pendent thiol-modified pHEMA polymer brush (protonated), and (d) pDMAEMA-b-
pHEMA pendent thiol polymer brush clicked with dodecyl isocyanate (protonated).
show a significant change in topography when DMAEMA is
in a deprotonated [Supporting Information Fig. S7(a-b,e-f)]
versus protonated state [Fig. 5(a,c)] (i.e., the differences in
hydrophobicity of the protonated DMAEMA and the o-NB
blocks are not significant). However, upon photodeprotection
and functionalization with dodecyl isocyanate [Fig. 6(b,d)],
the height images reveal an increase in roughness and more
pronounced domain-like topography. The phase images also
show a greater contrast between domains, particularly when
comparing samples containing an o-NB inner block and
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protonated DMAEMA outer block [Fig. 5(a)] with the equiva-
lent structure containing an inner block modified with do-
decyl isocyanate [Fig. 5(b)]. The observed changes in topog-
raphy and phase are likely derived not only from the greater
contrast in hydrophobicity among dodecyl and protonated
tertiary amine-containing blocks, but also from the final
solvent treatment with toluene before imaging. In compari-
son, samples containing dodecyl-pendent groups in the outer
block [Fig. 5(d)] show little rearrangement due to toluene
exposure, as the solvated block already dominates the inter-
face as in the case of the dodecyl-modified HEMA homopoly-
mer [Supporting Information Fig. S6(g-h)]. Water contact
angle measurements of the block copolymer surfaces, shown
as insets in Figure 5, also indicate the presence of a mixed
chemical composition at the brush-air interface. Samples
containing protonated DMAEMA as the outer block with
o-NB and dodecyl-modified inner blocks showed water con-
tact angles of 50.1ꢀ and 61.3ꢀ, respectively. These values are
much higher than the protonated pDMAEMA homopolymer
brush (13.5ꢀ), indicating that both the inner and outer
blocks contribute to the observed wettability. Similarly, the
reverse scenario, where protonated DMAEMA units formed
the inner block with the outer block consisting of o-NB and
dodecyl moieties showed water contact angles of 57.2ꢀ
and 63.6ꢀ, respectively—values slightly lower than either the
o-NB or dodecyl-modified homopolymers (Supporting
Information Fig. S8). Combined, these results show that PPM
of pendent thiols via thiol-click reactions can be successfully
used to tailor the functionality of complex polymer brush
architectures—including the inner and outer blocks of a
block copolymer brush surface.
functional groups in multiple dimensions, that is, laterally
via photolithography and vertically via architectural design
of block copolymer brushes; efforts in these directions are
ongoing.
ACKNOWLEDGMENTS
The authors acknowledge financial support from the National
Science Foundation (NSF CAREER DMR-1056817). Funding
from the Office of Naval Research (Award N00014-07-1-1057)
enabled the acquisition of the AFM equipment. RMH was
supported by the US Dept. of Education GAANN Fellowship
Program (Award #P200A090066). EAH was supported by a
NSF Graduate Research Fellowship (NSF DGE-0940712). We
thank Michael Blanton and Kevin Davis of the Rawlins research
group for help with confocal microscopy.
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We have demonstrated a postpolymerization surface modifi-
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protection chemistry of both o-nitrobenzyl and p-methoxy-
phenacyl thioethers. Addressing the protecting groups with
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and outer blocks of the block copolymer surface. The combi-
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controlled radical surface-initiated polymerization provides
an attractive and modular approach to tailor the chemical
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