A simple technique to grow polymer brushes using in situ surface ligation of an organometallic initiator

Article abstract of DOI:10.1021/ja064549v

A simple approach is described here for the in-place synthesis of polymer brushes by surface-initiated polymerization. A cyano-terminated self-assembled monolayer on a gold surface was used to anchor a highly active cationic Pd organometallic initiator by ligand exchange. We grew ultrasmooth patterned poly(4-methoxystyrene) brushes with excellent thickness control at room temperature. Copyright

Full text of DOI:10.1021/ja064549v

Published on Web 09/16/2006
A Simple Technique To Grow Polymer Brushes Using in Situ Surface Ligation
of an Organometallic Initiator
Krishna D. Dronavajjala,# Ramakrishnan Rajagopalan,‡ Sundararajan Uppili,† Ayusman Sen,†
David L. Allara,*,†,‡ and Henry C. Foley*,†,#,‡
Department of Chemistry, Department of Chemical Engineering, and Materials Research Institute,
The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
Received June 27, 2006; E-mail: hcf2@psu.edu; dla3@psu.edu
Scheme 1. Steps Involved in the Synthesis of Patterned
Poly(4-methoxystyrene) Brushes
In recent years, research on grafted polymer brushes has resulted
in important technological applications in coatings, microelectronics,
colloid stabilization, sensors, and biomolecular adhesion.1 A very
useful way of preparing polymer brushes on surfaces is by direct
surface-initiated polymerization (SIP). In this technique, initiator
groups are bound to the substrate, and polymer chains of controlled
thickness, density, and desired functional groups are grown from
the substrate with introduction of a monomer. There are numerous
reports of SIP on substrates such as Au and SiO2 in which free
radical,2 atom transfer radical3 (ATRP), anionic,4 cationic,5 and ring-
opening metathesis6 (ROMP) mechanisms were used for the
polymerizations. Of these methods, living and controlled polym-
erization mechanisms give polymer chains of low polydispersity,
a prerequisite for the growth of smooth and uniform polymer
brushes. However, in the current state of the art, ROMP requires
the synthesis of complex initiator molecules to form a SAM on
the surface, ATRP involves a multicomponent initiator system, and
anionic mechanisms require low temperatures and long reaction
times (often many days). In our search to find the simplest and
reliable conditions for growing in-place, just-in-time highly uniform
polymer brush patterns, we have developed a new cationic SIP
method. In this report we describe a very simple method in which
a highly active cationic Pd organometallic complex is readily ligated
to specific sites on a simple SAM which allows efficient, highly
controlled growth of patterned polymer brushes at room temperature
in a period of hours.
Cationic Pd organometallic complexes have been shown to
initiate polymerization of various monomers, for example styrene
and its derivatives, and to copolymerize carbon monoxide and
ethylene.7 These initiators offer another route to the synthesis of
polymeric brushes if they can be properly anchored to a sur-
face. For this purpose we sought to anchor a Pd(II) complex8
([Pd(C2H5CN)4](BF4)2) on a self-assembled monolayer (SAM) by
a simple ligand-exchange method. We formed a SAM of a cyano-
terminated alkanethiol (16-mercaptohexadecanenitrile (MHN), see
Supporting Information) on a gold surface with the idea of binding
the initiator to the SAM surface. The tail group of the SAM
displaced one of the propanenitrile ligands on the Pd(II) catalyst,
thereby anchoring it to the substrate. This very simple ligand-
exchange reaction proceeds at room temperature, and hence it offers
a new and simple synthetic route for anchoring a catalyst on a
substrate.
formed on the Au substrate by using a method described elsewhere.9
Ellipsometry on the substrate indicated a SAM thickness of 2.1 (
0.1 nm. The chemical structure of the SAM substrate was verified
by reflection absorption infrared spectroscopy (see Supporting
Information). The presence of the methylene groups in hydro-
carbon chains making up the monolayer was made evident by the
peaks at 2920, 2850, 1470, and 1430 cm-1, which correspond to
CH2 asymmetric and symmetric stretches and the CH2 scissors
modes, respectively. We also detected the presence of the CN
tail group by the peak at 2250 cm-1, which corresponds to a CN
stretch.
The next step was to anchor the Pd(II) complex on the SAM
substrate. This was done by immersing the SAM substrate in a
solution containing 10 mM Pd(II) complex in nitromethane (ACS
Reagent Grade) for 12 h under an inert atmosphere. The surface
was then rinsed thoroughly with anhydrous dichloromethane to
ensure the removal of any physisorbed catalyst from the surface.
Successful immobilization of the Pd(II) complex on the surface
was confirmed by X-ray photoelectron spectroscopy (XPS). The
substrate showed a Pd 3d3/2 peak at a binding energy of 343.3 eV.
Using XPS, the number of catalytic sites on the surface was
calculated to be 0.34 ( 0.06 /nm2 (number of sites per unit area;
see Supporting Information).
Our first step was to form the SAM of MHN on the gold surface
(see Scheme 1). A layer of gold 200 nm thick was grown via
thermal evaporation on a Si(100) wafer that had been precoated
with 15 nm of chromium as an adlayer. The SAM of MHN was
† Department of Chemistry.
# Department of Chemical Engineering.
‡ Materials Research Institute.
9
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J. AM. CHEM. SOC. 2006, 128, 13040-13041
10.1021/ja064549v CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. (a) Atomic force microscopy image of a patterned poly(4-
methoxystyrene) brush and (b) line profile across the image in (a).
surfaces to perform cationic polymerizations leading to new types
of nanoscale materials and controlled formation of precision
thickness brushes in nanoscale geometries such as gaps, pores, and
channels for a range of applications, and such work is underway
in our laboratories at this time.
Figure 1. Infrared reflection spectroscopy of (a) polymer brush and (b)
monomer 4-methoxystyrene.
Acknowledgment. We thank the National Science Foundation
for the financial support of this research (NIRT contract no. DMI02-
10229) and DARPA.
The last step was to grow polymer brushes using the anchored
initiator on the substrate by placing it in a reaction flask and
subjecting it to flowing argon saturated with monomer at room
temperature for 24 h. Ellipsometry (632.8 nm laser) of the sample
showed a change in the ellipsometric parameters consistent with
growth of a 24 ( 1 nm film with a refractive index of 1.6 (bulk
poly(4-methoxystyrene) film). Infrared reflection spectroscopy
showed the signature of poly(4-methoxystyrene) characterized by
the presence of peaks at 1600-1500 cm-1 (benzene ring), 1200-
1100 cm-1 (methoxy group), and 3000-2800 cm-1 (methylene
stretches in the polymer backbone). The polymerization was further
confirmed by the absence of peaks at 988 and 900 cm-1, which
correspond to the out-of-plane bending mode of the R-C atom in
the vinyl group, which is present in the monomer (see Figure 1)
but not in the polymer. Atomic force microscopy images (see
Supporting Information) demonstrated that the polymer film was
very smooth, with a root-mean-square roughness of about 0.3 nm.
These results were completely reproducible.
In-place growth of polymer brushes was illustrated by synthesiz-
ing patterned polymer brushes (Scheme 1). A polydimethylsiloxane
stamp was coated with the MHN molecule and stamped on a bare
gold surface. The stamped substrate was then backfilled by
hexadecanethiol solution in ethanol to obtain a patterned nitrile-
terminated alkanethiol substrate. The procedure of anchoring the
Pd catalyst and conducting the polymerization reaction was the same
as that used for the initial experiments. The resulting patterned
polymer brushes were characterized by atomic force microscopy
(AFM; Figure 2). The profile in the AFM image indicates that ridges
of polymer brush developed having a width of just less than 1 µm
and a thickness of approximately 29 nm, consistent, within
experimental error, with the ellipsometric thickness.
Supporting Information Available: MHN synthesis, reflection
absorption infrared spectroscopy of MHN SAM on gold surface,
calculation of Pd coverage on the surface, and AFM of polymer brush.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) (a) Caster, K. C. Polymer Brushes 2004, 331-370. (b) Senaratne, W.;
Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427-2448.
(2) (a) Prucker, O.; Ruhe, J. Langmuir 1998, 14, 6893-6898. (b) Huang,
W.; Skanth, G.; Baker, Gregory L.; Bruening, M. L. Langmuir 2001, 17,
1731-1736. (c) Hyun, J.; Chilkoti, A. Macromolecules 2001, 34, 5644-
5652.
(3) (a) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macro-
molecules 1998, 31, 5934-5936. (b) Matyjaszewski, K.; Miller, P. J.;
Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T.
M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules
1999, 32, 8716-8724.
(4) (a) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J.
Am. Chem. Soc. 1999, 121, 1016-1022. (b) Advincula, R.; Zjou, Q.; Park,
M.; Wang, S.; Mays, J.; Sakellariou, G.; Pispas, S.; Hadjichristidis, N.
Langmuir 2002, 18, 8672-8684.
(5) (a) Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243-247. (b)
Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (c) Kim,
I.-J.; Faust, R. J. Macromol. Sci., Pure Appl. Chem. 2003, A40, 991-
1008.
(6) (a) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J.
Am. Chem. Soc. 1999, 121, 4088-4089. (b) Kim, N. Y.; Jeon, N. L.;
Choi, I. S.; Takami, S.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo,
R. G.; Whitesides, G. M.; Laibinis, P. E. Macromolecules 2000, 33, 2793-
2795. (c) Buchmeiser, M. R.; Sinner, F.; Mupa, M.; Wurst, K. Macro-
molecules 2000, 33, 32-39.
(7) (a) Sen, A.; Lai, T.-W. Organometallics 1982, 1, 415-417. (b) Lai, T.-
W.; Sen, A. Organometallics 1984, 3, 866-870.
(8) [Pd(C2H5CN)4](BF4)2 was prepared by dissolving [Pd(CH3CN)4](BF4)2
(bought from Strem Chemicals and used as received) in an excess of
propanenitrile, followed by evacuation. Acetonitrile (displaced by pro-
panenitrile) and excess propanenitrile were both removed by evaporation.
To ensure that this occurred, the sample was dried under dynamic vacuum
for several hours.
In conclusion, we have demonstrated that a highly active cationic
Pd(II) organometallic catalyst can be anchored to a surface by means
of ligand exchange with a SAM. We believe that this approach is
general and can be extended in order to anchor a whole class of
Pd and Rh organometallic catalysts7,10 with nitrile ligands on
(9) Frey, S.; Shaporenko, A.; Zharnikov, M.; Harder, P.; Allara, D. L. J. Phys.
Chem. B 2003, 107, 7716-7725.
(10) Connelly, N. G.; Draggett, P. T.; Green, M. J. Organomet. Chem. 1977,
140, C10-C11.
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