A facile synthesis route to thiol-functionalized α,ω-telechelic polymers via reversible addition fragmentation chain transfer polymerization

Article abstract of DOI:10.1021/ma051035s

A straightforward procedure to α,ω-thiol telechelics was analyzed on the basis of reversible addition fragmentation chain transfer (RAFT) polymerization. It was observed that this technique allows α,ω- thiol functionalized polymers to be prepared directly

Full text of DOI:10.1021/ma051035s

Macromolecules 2005, 38, 8597-8602
8597
Notes
limited by the availability of hydroxyl-functionalized
telechelics as precursors for the reaction.7 Carrot and
co-workers extended the use of Sangers reagent to
include functional initiators for atom transfer radical
polymerization (ATRP) and ring-opening polymerization
which resolved the limitation of hydroxyl-functionalized
telechelics.8 Even so, by employing such an approach,
one is limited to the synthesis of monofunctional thiol
telechelics. To access R,ω-dithiol telechelics, an alternate
route must be considered. In this regard, few accounts
exist in the literature for the preparation of difunctional
thiol telechelics.1c For example, Kim and co-workers
recently reported the synthesis of R,ω-thiol-functional-
ized polystyrene telechelics by anionic polymerization
upon the reaction of R,ω-dilithiopolystyrene with eth-
ylene sulfide. While the authors report functionalization
yields of greater than 98 mol %, the procedure requires
the stringent conditions of anionic polymerization. A
milder approach involving controlled radical polymer-
ization (CRP) would be desirable since CRP methods
generally require less stringent conditions than anionic
polymerization (most reactions can be carried out under
inert atmosphere at mild temperatures). In addition,
CRP offers more versatility in the types of functional
monomers that may be polymerized under CRP condi-
tions.
Reversible addition fragmentation chain transfer po-
lymerization (RAFT) is an extremely versatile, con-
trolled free radical polymerization technique for the
synthesis of well-defined polymer architectures with
narrow polydispersity.9 The RAFT process is applicable
to a wide range of monomers and can be performed
under a broad range of conditions.10 The RAFT tech-
nique relies on a degenerative transfer mechanism in
which a chain transfer agent (CTA) (typically a thio-
carbonylthio compound SdC(Z)-SR) reacts with either
an initiator-derived primary radical or a propagating
polymer chain (P•), forming a new CTA and eliminating
R•, which is able to reinitiate the polymerization. The
transfer of the CTAs between dormant and active chains
effectively controls the polymerization (Scheme 1). The
nature of the RAFT mechanism allows one to produce
polymers that carry dithioester end groups which upon
reduction yields macromolecules bearing a thiol func-
tionality at the end of the polymer chain.11 Thus, by
employing a bifunctional CTA similar to that shown in
Scheme 2, one has a facile route for the synthesis of
polymers with R,ω-dithiol telechelic functionality, i.e.,
HS-(P)n-SH. Polymers bearing R,ω-thiol functionality
are of particular interest for the investigation of mul-
tiply bound polymer chains (MBPCs) in which both ends
of the polymer are surface-bound (surface attached
“polymer loop”), in contrast with the traditional singly
bound polymer brush.12 Such systems have the capabil-
ity to form surface-bound polymer loops on a substrate
and may exhibit drastically different interfacial proper-
ties (e.g., adhesion, wetting, biocompatibility, etc.) in
A Facile Synthesis Route to
Thiol-Functionalized r,ω-Telechelic Polymers
via Reversible Addition Fragmentation Chain
Transfer Polymerization
Derek L. Patton, Matthew Mullings,
Timothy Fulghum, and Rigoberto C. Advincula*
Department of Chemistry and Department of Chemical
Engineering, University of Houston,
Houston, Texas 77204-5003
Received May 19, 2005
Revised Manuscript Received August 10, 2005
Introduction
A general synthetic methodology to thiol-functional
telechelic polymers has been developed. This method,
based on the use of difunctional chain transfer agents
(CTAs) with reversible addition fragmentation chain
transfer (RAFT) polymerization, is a straightforward
and versatile procedure which produces well-defined
R,ω-thiol telechelic polymers in one step directly from
the products isolated after polymerization.
Telechelic polymers (R,ω-functionality) are generally
defined as relatively low molecular weight macromol-
ecules with selectively positioned functional groups at
both termini of any given chain. Such polymers have
been thoroughly studied within the realm of many
applications including precursors in the synthesis of
block copolymers, formation of polymer networks, and
use as cross-linking agents and as compatiblizers in
polymer blends.1 While the synthesis of telechelic
polymers containing functional groups such as hydroxy,2
carboxylic acid,3 and amine4 have been well documented
in the literature, far less attention has been given to
telechelics which contain a thiol functionality at the
chain ends. Thiols or mercaptans are widely used in
many areas of chemistry and materials science by
exploiting the specific interaction of the thiol with
metals such as gold, silver, and cadmium. In addition,
advances in technology in the areas of biochemistry and
microelectronics have further increased the need for a
straightforward approach to thiol-functional materials.5
Although there are many reports of polymers bearing
an ill-defined number of mercapto groups,6 only a few
examples have been reported for the preparation of well-
defined thiol-containing polymers, most of which ad-
dress macromolecules that contain a thiol at only one
end, i.e., mono- or heterofunctional telechelics. In most
cases, thiols are incorporated at the terminus of a
polymer by coupling a protected thiol with preexisting
functionalities at the end of the polymer chain. Trollsas
et al. employed this approach in which they reported
the synthesis of monofunctional thiol telechelics based
on Sangers reagent (2,4-dinitrofluorobenzene). Although
this method is carried out under mild conditions, it is
* Corresponding author. E-mail: radvincula@uh.edu.
10.1021/ma051035s CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/10/2005

8598 Notes
Scheme 1. The RAFT Mechanism
Macromolecules, Vol. 38, No. 20, 2005
1,4-diisopropenylbenzene (3.00 g, 0.0189 mol) in carbon tet-
rachloride (50 mL) was heated at 75 °C for 20 h. The volatiles
were removed under reduced pressure, and the residue was
triturated with 1:2 diethyl ether/hexanes to isolate the product
as a pink solid (41.1%). 1H NMR, δ (ppm): 1.99 (12H, s); 7.33
(m, 4H); 7.47 (m, 2H); 7.50 (s, 4H) and 7.86 (m, 4H). 13C
NMR: δ (ppm) in CDCl3: 226.92, 146.31, 142.63, 131.74,
128.12, 126.60, 126.34, 56.29, 28.10. Elemental analysis
calculated for C26H26S4: C, 66.91; H, 5.61; S, 27.48. Found:
C, 66.70; H, 5.56; S, 27.19.
1,4-Bis(thiobenzoylthiomethyl)benzene (CTA-2).14 Phe-
nylmagnesium bromide was prepared from bromobenzene
(5.94 g, 0.038 mol) and magnesium turnings (0.965 g, 0.0397
mol) in dry THF (75 mL). The solution was heated to 40 °C,
and carbon disulfide (2.88 g, 0.038 mol) was added dropwise
over 15 min while keeping the reaction temperature at 40 °C.
R,R-Dibromoxylene (5 g, 0.0189 mol) in 25 mL of dry THF was
then added dropwise over 15 min to the resulting dark brown
solution. The reaction temperature was maintained at 40 °C
for 1.5 h. Ice water (150 mL) was added, and the organic
products were extracted with ether (3 × 30 mL). The combined
organic phase was washed with water and brine and dried over
anhydrous magnesium sulfate. Removal of the solvent yielded
a red solid. The solid was recrystallized from ethanol to yield
Scheme 2. Schematic Representation for the
Synthesis of r,ω-Thiol Telechelics from a Difunctional
CTA
1
the product as a bright red solid (4.295 g, 55.2%). H NMR, δ
(ppm): 4.60 (4H, s); 7.34-7.60 (6H, m); 8.00 (4H, m). 13C
NMR: δ (ppm) in CDCL3: 227.6, 144.64, 134.63, 132.63,
132.47, 129.62, 128.35, 126.89, 41.74. Elemental analysis
calculated for C22H18S4: C, 64.35; H, 4.42; S, 31.23. Found:
C, 64.27; H, 4.32; S, 31.03.
General Procedure for Polymerization. All polymeriza-
tions were performed in a Schlenk apparatus under homoge-
neous conditions. The polymerizations were initiated by
addition of AIBN. Solutions of the respective monomer, initia-
tor, and RAFT agent were prepared in Schlenk tubes fitted
with septa and a stir bar. The mixtures were thoroughly
deoxygenated by 3-5 freeze-pump-thaw cycles, backfilled
with nitrogen, and placed in a thermostated water bath at
preset temperature. At specific time intervals, aliquots were
removed and quenched by lowering the temperature to 0 °C.
Unreacted monomer was removed under reduced pressure at
ambient temperatures or by precipitation and centrifugation.
The resulting residue was analyzed directly by gel permeation
chromatography (GPC).
Reduction of the Dithioester Termini by Aminolysis.
The R,ω-dithioester end-capped polymers were dissolved in
THF containing a few drops of aqueous sodium bisulfite
(Na2S2O4). The reaction mixture was purged of oxygen by
either bubbling with N2 for 15 min or by three successive
freeze-pump-thaw cycles. Propylamine (degassed) was then
added in excess via syringe, and the reaction was stirred
overnight under N2. Upon the addition of the propylamine, an
immediate color change from pink to yellow was observed
which is indicative of the loss of the dithioester end groups.
The reaction mixture was then added dropwise to 10-fold
excess of methanol, and the polymer was collected by centrifu-
gation or by filtration. This process was repeated 3-5 times
to obtain the pure R,ω-dithiol telechelics.
comparison to single-tethered polymer systems.13 In this
note, we report a straightforward method for the
preparation of R,ω-dithiol telechelics as demonstrated
by the synthesis of poly(lauryl methacrylate), poly-
(methyl methacrylate), and poly(styrene) containing
thiol functionalities at both ends of the polymer chain.
This is accomplished through the use of difunctional
CTAs and the RAFT technique which yields the difunc-
tional thiol telechelics in one reaction following polym-
erization. With minimal purification, R,ω-dithiol telech-
elics are achieved after the quantitative cleavage of the
dithioester via aminolysis. Improved thermal properties
were observed in the absence of the dithioester end
group. The approach presented in the current work
demonstrates an extremely facile route to R,ω-thiol
telechelics via controlled radical polymerization.
Experimental Section
Materials. Reagent chemicals were purchased from Aldrich
and used without further purification unless otherwise indi-
cated. 1,4-Diisopropenylbenzene was purchased form TCI.
Tetrahydrofuran (THF) used in synthesis was distilled from
sodium benzophenone. Styrene and methyl methacrylate
(MMA) were passed through a column of activated basic
alumina to remove the inhibitor, distilled, and stored at -20
°C. Lauryl methacrylate (LMA) was diluted with hexane and
washed three times with an aqueous solution of sodium
hydroxide (5 wt %) followed by three times with distilled water.
LMA was dried over MgSO4 and passed through a column of
neutral alumina prior to solvent removal. LMA was stored at
-20 °C. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized
twice from ethanol.
Characterization. NMR spectra were recorded on a Gen-
eral Electric QE-300 spectrometer at 300 MHz for 1H NMR
and 75 MHz for 13C NMR. FT-IR spectra were obtained on a
Digilab FTS 7000 equipped with a HgCdTe detector at
wavenumbers from 4000 to 600 cm-1. Gel permeation chro-
matography was carried out with a Viscotek 270 with a triple
detector array (RALS, IV, RI, or UV) equipped with 2 GM-
HHR-M and 1 GMHHR-L mixed bed ViscoGel columns with
THF as eluent at a flow rate of 1 mL min-1. A Viscotek VE
1122 GPC solvent pump, a Viscotek VE 7510 GPC degasser,
and Viscotek TriSEC software were also used. The thermal
properties of the polymers and copolymers were measured by
thermogravimetric analyses (TGA). The samples were heated
to 700 °C at a heating rate of 5 °C/min under a dry nitrogen
atmosphere in a TA Instruments 2950 thermogravimetric
analyzer.
1,4-Bis(2-(thiobenzoylthio)prop-2-yl)benzene (CTA-
1).14 A mixture of dithiobenzoic acid (6.12 g, 0.0397 mol) and

Macromolecules, Vol. 38, No. 20, 2005
Notes 8599
Table 1. Summary of the RAFT Polymerizations of LMA,
MMA, and Styrene
b
monomer
lauryl MA
time (h)
conv (%)a
Mn,theory
Mn,GPC
PDI
1
2
4
8
18.5
2
4
8
12
4
6
12
21
28
3.00
10.3
37.9
62.7
94.9
7.59
18.0
38.9
59.1
1.72
2.82
6.81
11.74
15.74
2 000
5 707
19 741
32 359
48 779
5 796
4 799
9 429
1.13
1.19
1.20
1.15
1.18
1.08
1.17
1.19
1.14
1.15
1.13
1.14
1.15
1.20
19 142
31 333
40 839
11 911
15 415
25 388
38 333
3 889
MMA
13 027
27 640
42 110
3 655
styrene
5 731
5 759
13 178
22 565
29 917
11 699
20 658
24 560
b
a Conversion was determined by NMR. Mn,theory ) [M]0/
[CTA]‚MWM‚conversion.
Figure 1. Evolution of molecular weight and PDI with
conversion for polymerization of (A) LMA/CTA1 (B) MMA/
CTA1. (9) Mn,GPC; (- - -) Mn,theory; (O) PDI. The inset shows the
kinetic plot for polymerization of (A) LMA and (B) MMA at
60 °C (2).
Figure 2. Size exclusion chromatograms for PLMA synthe-
sized in the presence of CTA-1 in benzene at 60 °C.
Results and Discussion
lymerization of MMA as shown in Figure 1B (inset).
Further evidence for a controlled radical polymerization
can be seen in Figure 2, which shows an example of the
stepwise evolution of MW for PLMA as determined by
gel permeation chromatography. Close observation of
the GPC traces reveal a slight low MW tail with
increasing reaction time which is also reflected by a
slight increase in the PDI (see Table 1). Styrene was
polymerized under similar conditions using CTA-2 and
yielded characteristics of a controlled radical polymer-
ization. A summary for the polymerization of LMA,
MMA, and styrene is also given in Table 1; however,
for purposes of brevity, the actual kinetic data for the
styrene polymerization are not shown (see Supporting
Information for details of styrene polymerization).
As a result of the widely accepted mechanism for
RAFT-mediated polymerization, the polymers obtained
by employing a difunctional CTA of the structure
presented here yield macromolecules containing a
dithioester at both termini of a given chain. Chong and
co-workers exploited this fact and illustrated the vi-
ability of difunctional CTAs where chain extension of a
difunctional polymeric dithioester compound resulted in
the formation of ABA triblock polymer architectures.15
While a small fraction of the polymer chains are
initiated by a radical derived from AIBN, the majority
of the chain ends bear the dithioester functionality
Lauryl methacrylate (LMA) and methyl methacrylate
(MMA) were polymerized in benzene in the presence of
AIBN as the free radical initiator and 1,4-bis(2-
(thiobenzoylthio)prop-2-yl)benzene (CTA-1) as the CTA
at 60 °C. To our knowledge, this represents the first
example of RAFT polymerization for LMA. The initiator:
CTA molar ratio was maintained at 1:1.6. The monomer:
CTA was such that a theoretical molecular weight of
51 350 g/mol was expected at 100% conversion. Under
these conditions, the ability of the difunctional CTA 1
to control the molecular weight and give narrow poly-
dispersity for LMA is evident as shown by the linear
increase of Mn with conversion and a low PDI (<1.2)
throughout the course of the reaction (Figure 1A). Table
1 shows a summary of the molecular weights, polydis-
persities, and percent conversions for the polymeriza-
tion. Figure 1A (inset) shows the kinetic plot for the
polymerization of LMA and illustrates first-order kinet-
ics as expected for a well-controlled radical polymeri-
zation. Similarly, the polymerization of MMA in the
presence of CTA-1 showed the characteristics of a
controlled radical polymerization with the observation
of a linear increase of Mn with conversion and the
maintenance of a low PDI over the course of the reaction
(Figure 1B). The experimentally observed values of Mn
deviated only slightly from the values predicted theo-
retically. First-order kinetics was obtained in the po-

8600 Notes
Macromolecules, Vol. 38, No. 20, 2005
Figure 4. UV-vis absorption spectra of PLMA synthesized
by RAFT-mediated polymerization before (s) and after (- - -)
reaction with propylamine.
Figure 3. 1H NMR spectra of PLMA prepared by RAFT-
mediated polymerization before (inset lower) and after (inset
upper) reaction with propylamine.
for the characterization of the resulting polymers (Fig-
ure 4). As a result of aminolysis, the absorption due to
the dithioester is absent, indicating, along with the
NMR result, a near-quantitative reduction of the
dithioester termini to yield R,ω-dithiol telechelics. In
contrast to the highly colored (pink) polymers prior to
aminolysis, the final product was observed as pristine
white in the case of PMMA and PS and colorless in the
case of PLMA.
assuming an efficient reinitiation from the leaving group
of the RAFT agent. The dithioester end-capped polymer
has been shown to be the major product by multiple
studies in which matrix-assisted laser desorption ion-
ization (MALDI) time-of-flight (TOF) mass spectrom-
etry16 and electrospray ionization mass spectrometry
(ESI-MS)17 were employed for end-group analysis of
RAFT-mediated polymers. Upon cleavage of the two
dithioester groups at the chain ends, one is left with
thiol functionalities as chain termini. Accordingly, the
reaction between dithioesters and primary amines is
well-known to occur rapidly at ambient temperatures
leading irreversibly to thioamides and thiols.18 While
aminolysis of dithioester-containing RAFT polymers is
not new, until now, the technique has mostly been used
as a probe for structural analysis and confirmation, i.e.,
chain branching and block formation.19 In addition,
Lima et al. used aminolysis to access a heterofunctional
telechelic as an intermediate for the synthesis of R,ω-
dihydroxy telechelic polymethacrylates via Michael ad-
dition of hydroxyethyl acrylate.3a As far as we know,
there are no accounts in the literature which describe
the use of aminolysis and difunctional CTAs to directly
access R,ω-dithiol telechelics. In the current work,
polymers synthesized using difunctional chain transfer
agents were subjected to aminolysis by reaction of the
dithioester termini with propylamine (see Scheme 2).
The addition of propylamine to a solution of the
dithioester end-capped polymers was accompanied by
a rapid color change from pink to yellow, indicating the
aminolysis of the RAFT end group. After purification,
To avoid the oxidative coupling of the thiol end
groups, a similar procedure to that used by Lima et al.
was adopted for the current work.3 Despite taking all
precautions, such as degassing the reaction mixture,
oxidative coupling of the thiol termini can still occur
upon treatment with a primary alkylamine. Lima et al.
showed that by addition of a small amount of antioxi-
dant in the form of aqueous sodium bisulfite (Na2S2O4)
the formation of disulfides which result from oxidative
coupling of thiols could be effectively suppressed. The
presence or absence of the disulfide species can easily
be observed by gel permeation chromatography in which
such species would appear at an elution volume corre-
sponding to a MW twice that of the original monofunc-
tional thiol end-capped polymer. For the difunctional
polymers presented here, the MW of the disulfide
species could be larger than 2 times due to the multi-
functionality. However, under the conditions described
above (addition of Na2S2O4), the presence of the oxida-
tive coupling products was not observed as shown by
the GPC traces for a poly(lauryl methacrylate) sample
in Figure 5. Furthermore, no significant change was
observed in the polydispersity of the pLMA before (PDI
1.15, Mn 48 125) and after (PDI 1.17, Mn 47 446)
aminolysis. A small decrease in the MW was observed
after aminolysis, which is consistent with the reduction
of the dithioester termini to thiol functional groups. The
exact value of the change in MW is difficult to resolve
by GPC since the change is relatively small in contrast
to the total MW of the polymer. Alternatively, in the
absence of sodium sulfite in the aminolysis procedure,
we explored other procedures for the reduction of the
disulfide products such as Zn/acetic acid (50 °C) in a
subsequent step following aminolysis.20 This reductive
system showed no advantages over the previously
described procedure in which sodium sulfite was em-
ployed.
1
the extent of the reaction was monitored by H NMR.
As shown in Figure 3, the aromatic protons of the
dithioester end groups could not be observed after the
reaction with propylamine within the sensitivity of the
NMR method. It should be noted that in the present
case NMR alone cannot be used to suggest the quantita-
tive reduction of the dithioester end group since the
signal attributed to the aromatic end group is extremely
low. Thus, in addition to NMR, UV-vis spectroscopy
was also used as a simple procedure for the character-
ization of the resulting polymers. The dithioester moiety
has a strong absorption band at 302 nm which allows
one to use UV-vis spectroscopy as a simple procedure

Macromolecules, Vol. 38, No. 20, 2005
Notes 8601
porting Information (Figure S3). Geometric parameters
such as grafting density (σ, chains/nm2) and interchain
spacing (R, 1/σ) can be extracted from the QCM experi-
ments and show a strong dependence on the MW of the
adsorbed material. A more detailed account of the
adsorption studies for the materials presented here will
be addressed in a forthcoming publication.
Conclusions
A straightforward procedure to R,ω-dithiol telechelics
has been developed on the basis of reversible addition
fragmentation chain transfer (RAFT) polymerization.
The technique allows R,ω-thiol functionalized polymers
to be prepared directly from a difunctional chain trans-
fer agent with only one postpolymerization reaction. The
dithioester termini were converted to thiols by way of
aminolysis in the presence of an antioxidant to yield the
difunctional polymers with narrow molecular weight
distributions. The R,ω-thiol telechelics showed greatly
improved thermal properties in the absence of the
dithioester end group. The present method offers the
advantages of mild reaction conditions associated with
RAFT polymerization and applicability to a wide range
of monomer functionalities. The adsorption of the R,ω-
dithiol telechelics on gold surfaces is currently under
investigation with an interest in the formation of
multiply bound polymer chain (MBPCs).
Figure 5. GPC chromatograms of PLMA before (s) and after
(- - -) aminolysis with propylamine in the presence of Na2S2O4.
The results show no evidence for the formation of disulfides.
Acknowledgment. The financial support from the
National Science Foundation (CHE-0304807 and CTS-
033012) is gratefully acknowledged. M.M. is also thank-
ful for financial support and participation in the NSF
REU program at the Center for Materials Chemistry
at the University of Houston. Technical support from
Viscotek Inc. and TA Instruments is also appreciated.
A special thanks to Yongbok Go for help with the
thermal analysis.
Figure 6. TGA traces of PMMA as synthesized by RAFT
(- - -) and the R,ω-thiol telechelic PMMA (s) obtained after
aminolysis.
Supporting Information Available: Details of styrene
polymerization. This material is available free of charge via
the Internet at http://pubs.acs.org.
The thermal stability of the R,ω-thiol telechelics was
determined by thermogravimetric analysis under nitro-
gen. Figure 6 shows a typical example of the thermal
properties observed for PMMA dithiol telechelics (Mn
) 14K). The lowered thermal stability (onset of weight
loss ∼200 °C) of the polymers synthesized by RAFT can
be attributed to the presence of the labile dithioester
moiety at the chain end. It should be noted that the
onset of thermal degradation should vary with molec-
ular weight since chain ends play a more important role
at lower molecular weights. However, once the dithioester
end groups are replaced by thiol functionalities at the
chain end, the onset of weight loss is first observed at
∼300 °C. Along with a loss of color in the polymer, the
absence of the dithioester functionality imparts greater
thermal stability in the R,ω-thiol telechelics.
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