Angewandte
Chemie
halides are much less suitable for this reaction due to their
À
higher degree of SO2 X bond hydrolyzability and the lower
À1
À
À
À
Si X bond formation energy (Si Cl 456 kJmol , Si Br
À1
À1
À
343 kJmol , Si I 399 kJmol ). The exact mechanism of
the exchange has not been completely elucidated, but
Gembus et al. hypothesized that a sulfonyl ammonium
fluoride salt is formed by the reaction of an amine with the
sulfonyl fluoride, which can then react with the silyl ether to
form a sulfonate ester plus inert silyl fluoride.[24]
In order to explore SuFEx chemistry on surfaces, a sulfo-
nyl fluoride monomer, 3-(fluorosulfonyl)propyl methacrylate
(FSPMA) monomer, was synthesized in two steps from
sodium 3-(methacryloyloxy)propane-1-sulfonate (Supporting
Information). Next, p(FSPMA) brushes were grown from an
azo-based silane initiator monolayer using radical polymer-
ization initiated with UV light.[25] Brushes of ca. 25 nm were
grown by this method, and the solution polymer was isolated
and analyzed by GPC to obtain molecular weight data in
order to estimate the grafting density of the brushes (0.14
chains/nm2). The p(FSPMA) brushes demonstrated excellent
reactivity with silyl ethers in the presence of certain non-
nucleophilic amines, which is consistent with the SuFEx
reaction in solution, albeit with some interesting differences.
These brushes were then explored to examine the suitability
for efficient and highly specific postpolymerization modifica-
tion. Triazabicyclodecene (TBD), a new catalyst for this
reaction, was also investigated for its suitability for SuFEx.
Many of the chemical functional groups involved in the
most widely used click chemistries are either incompatible or
lead to complications with free radical polymerization. These
functionalities include unprotected terminal alkynes and
azides for CuAAC (CuI catalyzed azide/alkyne cycloaddi-
tion), thiols and alkenes for Michael addition, and dienes or
dienophiles for Diels–Alder. Thiol–ene/yne chemistry, which,
while not strictly a click reaction, shares several important
traits such as fast kinetics and modularity, and is also not
compatible with radical polymerization to high molecular
weight because of chain transfer and/or cross-linking. Sulfonyl
fluorides are tolerant of free radical polymerization condi-
tions, and we also illustrate that p(FSPMA) brushes are an
excellent platform to add some of these other reactive
moieties to surface through easily synthesized tert-butyldime-
thylsilyl (TBDMS) protected alcohol precursors (Scheme 1).
Figure 1 shows the grazing incidence attenuated total
reflection infrared spectrum (GATR-IR) of the p(FSPMA)
brushes grown by free radical polymerization along with
subsequent PPM with the different TBDMS precursors
shown in Scheme 1. The p(FSPMA) brushes (Figure 1A)
Figure 1. GATR-FTIR spectra of p(FSPMA) brush and PPM brushes.
A) p(FSPMA), B) propargyloxy brush, C) mercaptoethoxy brush, D) fur-
furyloxy brush. Spectra were vertically shifted for clarity; y axis corre-
sponds to spectrum (A).
À1
À
stretches. Also in each case, the S F stretch at 816 cm
disappears. We also observed the complete disappearance of
fluorine (0.7 keV) on the surface by electron dispersive X-ray
spectroscopy after each functionalization (Supporting Infor-
mation).
UV/Vis data was obtained by dipping p(FSPMA) brush
functionalized glass slides in solutions containing a catalyst
and silyl ether protected Disperse Red 1, rinsing with good
solvent, then monitoring the appearance of the Disperse
Red 1 dye peak at 486 nm. These studies established that in
the brush system, assuming pseudo-first-order kinetics, the
rates of reaction for the TMS (trimethylsilyl) and TBDMS
protected Disperse Red 1 (DR1) in the presence of DBU
were similar (kTMS = 0.001 sÀ1 vs
k
TBDMS = 0.0007 sÀ1)
(Figure 2). Rate constants were obtained during the portion
of the functionalization reaction where substrate absorbance
increased linearly with reaction time, before steric constraints
and lowered SO2F availability changed reaction dynamics
(Supporting Information). The similarity of DBU surface
reaction rate constants of TMS and TBDMS derivatives
contradicts other reports where the reaction is carried out in
homogeneous solution, and it was observed that SuFEx
reactions with TBDMS derivatives are generally less favor-
able and may require heating to start the reaction.[24,26] Also,
we observed when using TBD as a catalyst, the reaction rate
increased more than an order of magnitude (k’ = 0.038 sÀ1)
with the same TBDMS-DR1. The fact that TBD exhibits
more than an order of magnitude faster SuFEx kinetics than
DBU while still using the TBDMS derivative makes it
a superior choice as a catalyst for SuFEx. A brush reaction
rate constant of k’ = 0.038 sÀ1 is comparable to the rate
constant previously established for CuAAC in polymer
brushes (k’ = 0.02 sÀ1).[27] The functional group density of
dye molecules on the surface after reaction completion is
similar to what was obtained using activated ester brushes
display the spectral features apparent in the monomer, such as
À1
=
C O ester stretches at 1731 and 1236 cm , symmetrical and
=
À1
asymmetrical S O stretches of the sulfonyl fluoride at 1405
À1
À
and 1200 cm , and an S F stretch at 816 cm . After
immersion in acetonitrile (MeCN) with DBU or TBD and
a silyl ether protected molecule (propargyloxy-TBDMS,
mercaptoethoxy-TBDMS, furfuryloxy-TBDMS, Figure 1B–
D, respectively) at ambient temperature and open to atmos-
phere, spectral features of sulfonate ester formation appear.
À1
=
The S O symmetrical stretch completely shifts to 1338 cm ,
while the peaks at 1158 and 1038 cmÀ1 are assigned to the S O
À
Angew. Chem. Int. Ed. 2015, 54, 13370 –13373
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim