Tandem RAFT Polymerization and Click Chemistry: An Efficient Approach to Surface Modification
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Transcript of Tandem RAFT Polymerization and Click Chemistry: An Efficient Approach to Surface Modification
Communication
2084
Tandem RAFT Polymerization and ClickChemistry: An Efficient Approachto Surface Modificationa
Rajesh Ranjan, William J. Brittain*
PS grafted silica nanoparticles have been prepared by a tandem process that simultaneouslyemploys RAFT polymerization and click chemistry. In a single pot procedure, azide-modifiedsilica, an alkyne functionalized RAFT agent and styrene are combined to produce the desiredproduct. As deduced by thermal gravimetric and elemental analysis, the grafting density ofPS on the silica in the tandem process is intermediate between analogous ‘‘grafting to’’ and‘‘grafting from’’ techniquesfor preparing PS brushes onsilica. Relative rates of RAFTpolymerization and clickreaction can be altered tocontrol grafting density.
Introduction
Click chemistry corresponds to an efficient and selective
reaction between alkynes and azides to form heteroatom
links.[1] These reactions employ mild reaction conditions
and simple work up procedures, but still proceed in high
yields. Click functionalized polymers can be used for
functionalization and modification of a variety of sub-
Rajesh Ranjan, William J. BrittainDepartment of Polymer Science, The University of Akron,Ohio, USAFax: 330-972-5290; E-mail: [email protected] J. BrittainCurrent Address: Bausch & Lomb, 1400 N. Goodman St.,Rochester, NY 14609, USA
a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.
Macromol. Rapid Commun. 2007, 28, 2084–2089
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
strates. Controlled radical polymerization has received
increasing attention in recent years.[2] Controlled radical
polymerization permits synthesis of polymers with pre-
determined molecular weight, low polydispersity, con-
trolled composition and functionality. Combining the
chain-end functionality control of living free radical
polymerization and the efficiency and diversity of click
chemistry is desirable. The utility of the click reaction has
been demonstrated in living radical polymerization.[3]
Among controlled free radical polymerizations, rever-
sible addition fragmentation transfer (RAFT) has arguably
the most important commercial significance because it
works with the greatest range of vinyl monomers.[4] The
Cu(I) catalyzed variant of the Huisgen 1,3-dipolar cycload-
dition of azides and alkynes is one of the most common
click reactions. We explored the possibility of combining
RAFT polymerization and click chemistry as a synthetic
strategy for surface modification. Furthermore we also
worked on the option of conducting tandem RAFT poly-
merization and click chemistry in one pot. We applied this
DOI: 10.1002/marc.200700428
Tandem RAFT Polymerization and Click Chemistry: . . .
strategy to the surface modification of silica nanoparticles
and controlling the grafting density of tethered polymer
chains.
Traditionally there are two main routes for surface
modification. Route I is the ‘‘grafting to’’ approach where
a preformed, end functionalized polymer couples to an
activated surface. Route II is the ‘‘grafting from’’ approach
that typically proceeds via propagation from a surface-
immobilized initiator. Because route II affords a higher
grafting density, this route is generally preferred.[5]
The use of RAFT polymerization for surface modification
of silica particles is not common.[6] Here we report a novel
approach for surface modification in which polymeriza-
tion and surface immobilization reactions proceed simul-
taneously. To the best of our knowledge, this is first report
where RAFT polymerization and click coupling reactions
are conducted in tandem, thus demonstrating mutual
compatibility. We applied this tandem approach tomodify
the surface of silica particles with polystyrene (PS). We also
altered the relative rate of both reactions to control
grafting density.
Experimental Section
Materials
Colloidal silica (D¼ 75–100 nm) dispersed (30 wt.-%) in
isopropyl alcohol was provided by Nissan Chemical. N-(3-
dimethylaminopropyl)-N0-ethylcarbodiimide hydrochloride
(EDC), 4-dimethylaminopyridine (DMAP), 2,20-azoisobut-
yronitrile (AIBN), N,N-dimethylformamide (DMF), toluene,
methanol, sodium azide, copper sulfate, copper(I) bromide,
(pentamethyl)diethylenetriamine (PMDETA), sodium asco-
rbate and Aliquat 336 (tricaprylmethylammonium chlo-
ride) were purchased from Aldrich. Styrene was purchased
fromAldrich Chemical andwas purified by passing through
a column of chemically activated alumina (Aldrich
150 mesh). 3-Bromopropyltrichlorosilane was purchased
from Gelest Inc. Unless otherwise specified all the chemi-
cals were used as received.
Instrumentation
Transmission electronic microscopy (TEM) was performed
using a FEI Technai 12 transmission electron microscope.
Thermogravimetric analysis (TGA) was performed on a TA
2950 instrument at a scan rate of 20 K �min�1 under
nitrogen atmosphere. Elemental analyses were obtained
from Galbraith Labs in Knoxville, TN. The molecular
weights of polystyrene were measured in THF (at 35 8Cwith a flow rate of 1 mL �min�1) by gel permeation
chromatography (GPC) using a Waters 501 pump, Waters
Macromol. Rapid Commun. 2007, 28, 2084–2089
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
HR4 and HR2 Styragel columns, a Waters 410 differential
refractometer, and a Viscotek 760A dual light scattering
and viscosity detector. Fourier transform infrared (FT-IR)
spectra were obtained on a Bruker Tensor 27 FT-IR spectr-
ometer using a diffuse reflection apparatus (Cricket1,
Harrick Scientific).
Synthesis of Alkyne Terminated ChainTransfer Agent (CTA)
The synthesis of S-1-dodecyl-S0-(a,a0-dimethyl-a00-acetic
acid)trithiocarbonate CTA was carried out according to a
previously reported method in the literature.[7] Trithio-
carbonate CTA (1 g, 2.73 mmol), EDC (0.783 g, 4.095 mmol),
DMAP (0.5 g, 4.095 mmol) and 10 mL dichloromethane
were added to a round bottom flask and the mixture was
stirred for several min under inert atmosphere. Propargyl
alcohol (0.5 mL, 8.9 mmol) was added and the mixture
stirred overnight at room temperature. The product was
washed with acidic water, water and brine several times.
After drying the product was obtained as a low melting
solid.1H NMR (300 MHz, CDCl3): d¼ 0.87 (t, –CH2CH3, 3H),
1.2–1.4(m, –CH2CH2, 20H), 1.65 (s, –C(CH3)–CH3, 6H), 2.5(s,–CH C, 1H), 3.25 (t, –CH2S, 2H), 4.7 (s, –CH2C, 2H).
13C NMR (300 MHz, CDCl3): d¼ 17 (CH2–CH3), 22
(CH2(CH3)–CH2), 24.5 (2C, CH3–C), 26–32 (9C, CH2(CH2)–
CH2), 37 (CH2(CH2)–S), 53 (C(CH3)–CH3), 55 (CH2(C)–O), 76
(CH C), 77 (C(CH2) CH), 173 (C––O), 222 (C––S).
Deposition of 3-Bromopropyltrichlorosilaneon Silica Nanoparticles
In a Schlenk flask, 6 g of dried silica and 45 mL of an-
hydrous toluene were added under inert atmosphere. This
content of the flask was sonicated for 30 min followed by
heating to 80 8C. A solution 5.5 mL of 3-bromopropyl-
trichlorosilane in 15 mL toluene was added dropwise and
the solution was heated at 80 8C for 18 h. The particles
were recovered by centrifugation at 3 000 rpm for 30 min.
The particles were redispersed in toluene and centrifuged;
this cycle was repeated 6 times to afford the modified
particles.
Synthesis of Azide Modified Silica Nanoparticles
Bromopropyl modified silica nanoparticles (5 g) and a satu-
rated solution of NaN3 (2 g of NaN3) in 100 mL DMF were
combined in a flask under inert atmosphere. The mixture
was stirred at 80 8C for 18 h. The particles were recovered by
centrifugation at 3000 rpm for 30 min. These particles
were redispersed in water and centrifuged; this cycle was
repeated three times to afford modified silica nanoparticles.
www.mrc-journal.de 2085
R. Ranjan, W. J. Brittain
Scheme 1. Surface modification of silica nanoparticles using tandem approach.
2086 �
Surface Modification of SilicaNanoparticles using Tandem RAFTPolymerization and ClickChemistry
The azide modified silica nanoparticles
(1 g), 0.3 g (0.718 mmol) alkyne termi-
nated CTA, 30 mL (0.26 mol) styrene,
30 mL toluene, 0.004 g (0.028 mmol)
CuBr, 0.010 mL (0.048 mmol) PMDETA
and 0.01 g (0.06 mmol) AIBN were
combined in a Schlenk flask. The flask
was subjected to three freeze-pump-
thaw cycles to remove oxygen. The
mixture was heated at 90 8C for 18 h
under inert atmosphere. The polymer-
ization was stopped by cooling the
flask in ice water. Silica particles were
separated after centrifugation of the
mixture. Alkyne terminated PS was
obtained from the solution by preci-
pitation in cold methanol. The silica
particles were redispersed in toluene and centrifuged. This
cycle was repeated 5 times to remove physisorbed
polymer. The silica particles were dried under reduced
pressure to afford PS modified silica particles.
Cleavage of Grafted Polymer from thePS Modified Silica Nanoparticles
The PSmodified silica nanoparticles (0.25 g) were dissolved
in 5 mL of toluene. Aliquat 336 (75 mg) was added as a
phase transfer catalyst. A 5% aqueous HF solution (5 mL)
was added and themixturewas stirred overnight (Caution:
HF is highly toxic and great care must be taken when
handling.). The organic layer was separated and the poly-
mer was isolated by precipitation in methanol.
Results and Discussion
For tandem RAFT polymerization and click chemistry, we
separately synthesized alkyne terminated RAFT CTA and
azidemodified silica nanoparticle (Scheme 1). Traditionally
click chemistry is performed using CuSO4/sodium ascor-
bate [as a source of Cu(I)] catalyst in water or a mixture of
water and polar solvent. We initially attempted simulta-
neous RAFT polymerization and click chemistry in the
presence of CuSO4/sodium ascorbate in DMF. But we did
not observe good control over the polymerization. We
speculate that the ascorbic acid may be acting as a radical
scavenger.[8] Another source of Cu(I) catalyst that has been
used in click chemistry of polymers is the CuBr/PMDETA
complex.[9] This complex is widely used in atom transfer
Macromol. Rapid Commun. 2007, 28, 2084–2089
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
radical polymerization (ATRP) where it undergoes a rever-
sible chain transfer reaction with alkyl halide. In the
absence of any alkyl halide group, we expect this copper
complex to be innocuous during RAFT polymerization.
Simultaneous RAFT polymerization and click chemistry
was performed in a flask containing styrene, azide-
modified silica, alkyne terminated RAFT CTA, CuBr/PMDETA
(in a catalytic amount) and toluene at 90 8C under inert
atmosphere for 18 h. The click (alkyne) functionalized CTA
participated in RAFT polymerization as well as the click
coupling reaction with azide-modified silica particles. At
any particular reaction time, CTA would be attached to
silica nanoparticles and present in solution. PS chains are
growing via RAFT polymerization whether the process is
occurring on the silica or in solution. At the reaction end
there will be PS attached to silica particles and free PS in
solution. PS modified silica nanoparticles were separated
by centrifugation and characterized by TEM (Figure 1), TGA
(Figure 2), FT-IR (Figure 3) and elemental analysis (Table 1).
The grafted PS brush provides high stability and dis-
persibility for the silica nanoparticles in organic solvents.
The transparency of the suspended PS modified silica
samples suggests reasonably good dispersion of these
particles in organic solvents. TEM images were also taken
to examine dispersion at a particle scale. Figure 1 shows
the TEM image of PS modified nanoparticles cast from a
dilute THF solution. TEM analysis indicated that silica
spheres were not agglomerated. These PS grafted silica are
expected to be well dispersed in most common good
solvents for PS.
Free PS was precipitated from methanol and found to
have a molecular weight of Mn ¼ 9 000 g �mol�1 and a
DOI: 10.1002/marc.200700428
Tandem RAFT Polymerization and Click Chemistry: . . .
Figure 3. IR spectra of (a) bare silica nanoparticle, (b) silica-bromide, (c) silica-azide, silica-PS by (d) ‘‘grafting to’’ approach,(e) tandem approach, (f) ‘‘grafting from’’ approach.
Figure 1. TEM image of PS modified silica nanoparticles.
polydispersity index of 1.09. The presence of an alkyne
group was confirmed by 13C NMR. These results are
consistent with the controlled nature of RAFT polymeriza-
tion. There are few reports on the use of trimethylsilyl
group as a protecting group for alkyne functionality during
living radical polymerization.[3b,3h] However Matyjas-
zewski et al.[3c] have polymerized styrene using an alkyne
terminated ATRP initiator without any protecting group.
Hawker et al.[10] directly utilized alkyne-functionalized
RAFT CTA for a RAFT based sequential polymerization. In
absence of any protecting group, we synthesized well
Figure 2. TGA analysis of (a) bare silica nanoparticle, (b) silica-bromide, (c) silica-azide, (d) silica-PS (Mn ¼9 000 g �mol�1) by‘‘grafting to’’ approach, (e) silica-PS (Mn ¼ 7 000 g �mol�1) bytandem approach, (f) silica-PS (Mn ¼ 12 000 g �mol�1) by ‘‘graftingfrom’’ approach.
Macromol. Rapid Commun. 2007, 28, 2084–2089
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
defined alkyne functionalized PS which indicates minimal
interference of the alkyne group in RAFT polymerization.
PS chains attached to silica were cleaved using HF to afford
PS with Mn ¼ 7000 g �mol�1 and a polydispersity of 1.31.
The molecular weight was similar for PS immobilized on
silica compared to PS formed in solution. PS grafted from
nanoparticle surfaces show slightly higher polydispersity
than those formed in solution. This may be due to
decreased accessibility of surface bound growing polymers
to monomer which would alter the kinetics of polymer-
ization relative to solution. Similar behavior has been
reported when surface initiated ATRP was done in pre-
sence of free ATRP initiator.[11]
This tandem approach was compared with ‘‘grafting to’’
and ‘‘grafting from’’ in terms of grafting density. For
the ‘‘grafting to’’ method, the isolated free PS with an
alkyne group was reacted with azide-modified silica.
For the ‘‘grafting from’’ method, alkyne terminated RAFT
CTA was coupled with azide-modified silica followed by
surface mediated RAFT polymerization in presence of free
CTA. For each method, grafting density was calculated
using elemental analysis and TGA.[12] (See Supporting
Information)
Grafting density obtained by tandem method was
in-between the grafting densities obtained by the ‘‘graft-
ing to’’ and the ‘‘grafting from’’ approaches (Table 1). In the
‘‘grafting to’’ method, polymer chains must diffuse to the
silica particles, while in the ‘‘grafting from’’ approach, only
low molecular weight monomer diffuses to the silica
surface. Consistent with the literature lower grafting
www.mrc-journal.de 2087
R. Ranjan, W. J. Brittain
Table 1. Grafting density of PS on silica nanoparticles as prepared by different grafting methods.
Synthetic
approachMn of
grafted PS
Elemental
analysis of Carbon
Grafting density by
elemental analysis
Grafting density
by TGA
g �molS1 wt.-% groups �nmS2 groups �nmS2
Grafting to 9 000 11.74 0.29 0.32
Tandem 7000 15.39 0.51 0.54
Grafting from 12000 30.02 0.68 0.68
2088 �
densities are seen for ‘‘grafting to’’.[13] The tandem process
of RAFT and click chemistry represents an intermediate
case in terms of steric aspects consistent with the inter-
mediate grafting density.
By changing the relative rates of the click reaction and
RAFT polymerization, this tandem method can be shifted
between ‘‘grafting to’’ and ‘‘grafting from’’ methods. For
example, with a faster click reaction, there would be a
higher probability that only RAFT CTA or RAFT CTA having
a few monomers would attach to silica nanoparticles and
it would be closer to the ‘‘grafting from’’ approach. Recent
kinetic studies indicate that the rate of the catalytic
process in the click reaction is generally second order in
copper.[14] We used Cu(I) concentration to change rate of
the click reaction and studied it’s effect on grafting density
and molecular weight. When 10 fold more Cu(I) was used
in the tandem process, grafting density increased sig-
nificantly (from 0.51 groups �nm�2 to 0.70 groups �nm�2)
and became more closer to the ‘‘grafting from’’ approach.
But there were no effects on molecular weight and poly-
dispersity of RAFT polymers. Therefore, it is possible to
control the length of a polymer brush (RAFT polymeriza-
tion) and grafting density (click reaction) of tethered
polymer brushes separately.
This tandem approach definitely has clear advantages
over the other two in terms of labor, time and cost.
It definitely saves the step of surface immobilization on
surface. This step requires a toxic solvent and the washing
of modified silica nanoparticles. The tandem approach
provides grafting density in the brush regime. In a typical
‘‘grafting from’’ approach there is a need to separately
synthesize surface bound RAFT CTA (mostly silane group
terminated RAFT CTA) and free CTA. But in this method
there is only one kind of CTA which performs as a free CTA
and surface bound CTA.
Conclusion
Click reaction is very specific reaction which doesn’t
interfere with RAFT polymerization. With suitable choice
of Cu (I) catalyst it is possible to do both RAFT poly-
merization and click chemistry together. Using this
Macromol. Rapid Commun. 2007, 28, 2084–2089
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
concept, we demonstrated an efficient method of surface
modification which has advantages of time and cost. We
also observed that, the grafting density of tethered chains
can be easily adjusted by changing their relative rates.
With high specificity of click reaction and versatility of
RAFT polymerization this tandem approach can easily
used to do many functionalization and postmodification
reactions for a variety of polymers.
Acknowledgements: The authors thank National Starch andChemical for their generous financial support.
Received: June 12, 2007; Revised: August 6, 2007; Accepted:August 6, 2007; DOI: 10.1002/marc.200700428
Keywords: click chemistry; nanoparticles; polystyrene (PS); re-versible addition fragmentation chain transfer (RAFT); silica
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