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



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


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


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.



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


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


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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Tandem RAFT Polymerization and Click Chemistry: An Efficient Approach to Surface Modification

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