Preparation of α,ωtelechelic hexyl acrylate polymers with -OH, -COOH, and -NH2 functional groups by RAFT

Article abstract of DOI:10.1002/pola.24082

The design, synthesis, and use of two new, stable, functionalzed chain transfer agents (CTA's) containing OH and amine end groups for the RAFT polymerization is reported: 2-hydroxyethoxy-carbonylphenylmethyl dithiobenzoate and 2-(2-(tert-butoxycarbonyl)et

Full text of DOI:10.1002/pola.24082

Preparation of a,x-Telechelic Hexyl Acrylate Polymers with AOH, ACOOH,
and ANH2 Functional Groups by RAFT
´
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NORMA A. CORTEZ-LEMUS, RODOLFO SALGADO-RODRIGUEZ, ANGEL LICEA-CLAVERIE
Centro de Graduados e Investigacio´ n del Instituto Tecnolo´ gico de Tijuana, Apartado Postal 1166, Tijuana, B. C. 22510, Me´xico
Received 13 January 2010; accepted 16 April 2010
DOI: 10.1002/pola.24082
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The design, synthesis, and use of two new, stable,
functionalized chain transfer agents (CTA’s) containing OH
and amine end groups for the RAFT polymerization is
reported: 2-hydroxyethoxy-carbonylphenylmethyl dithioben-
zoate and 2-(2-(tert-butoxycarbonyl)ethylamino)-2-oxo-1-phe-
nylethyl benzodithioate, respectively. The RAFT polymerization
of n-hexyl acrylate (HA) using those CTA’s, were compared to
several other functionalized dithiobenzoate esters reported in
the literature containing COOH and Ester groups. The per-
formances of the dithiobenzoates were compared in terms of
kinetics and molecular weight distribution control. Good con-
trol in polymerization of n-hexyl acrylate with a linear increase
of Mn with conversion mantaining polydispersity indices (PDI)
below 1.1 was obtained by use of the new functionalized
CTA’s developed and also by use of some other CTA’s tested,
to produce well-defined linear polymers with one specific
chain-end functionality: AOH, ACOOH or Amine. Using a post-
polymerization reaction with functionalized azocompounds in
a 5 to 1 ratio, a,x-telechelic polymers, with AOH or ACOOH
as functional group at the second end were obtained. By
using this synthetic strategy a,x-homotelechelic and heterote-
lechelic polymers were readily prepared. The chemical avail-
ability of functional end-groups in the telechelics was
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demonstrated by reaction with methacrylic anhydride.
2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
3033–3051, 2010
KEYWORDS: a,x-telechelics;
acrylates;
dithiobenzoates;
dithioesters; reversible addition fragmentation chain transfer;
synthesis; telechelics
INTRODUCTION Reversible addition-fragmentation chain
transfer (RAFT) polymerization has been in the focus of
many research groups. Numerous publications have been
reported on the mechanism, parameters effecting perform-
ance or the application of method of synthesis to the prepa-
ration of well-defined macromolecular structures.1,2 RAFT
polymerization has been one of the most promising recent
advances in controlled free-radical polymerization because of
its compatibility with a wide range of monomers and its ver-
satility; all this makes RAFT polymerization an ideal method
for synthesis of polymeric biomaterials, such as drug deliv-
ery systems, implants, contact lenses, vascular grafts, dental
materials, and parts of artificial organs, which due to strict
regulations imposed on all pharmaceuticals and other health
related products have to be of well-controlled molecular
weight and high purity beside other specific properties.3–5
As the popularity of the method has grown, more new RAFT
agents have been synthesized and used for polymerization of
various monomers. As a consequence of the RAFT process,
nearly all polymer chains bear a thiocarbonylthio group at
one chain end and the R substituent at the other end. An
additional feature of this technique is the possibility to
design polymeric chains with specific chain-end functional-
ities. This end-group functionalization enables the subse-
quent synthesis of complex architectures starting with tele-
chelic polymers (telechelics). Telechelics are polymers
containing reactive groups on one end (semitelechelics) or
both ends (a,x-telechelics) and are widely used in colloidal
applications, catalysis, drug delivery systems, surface modifi-
cation, as compatiblizers, for block copolymer synthesis,
preparation of polymer networks, as crosslinking agents, in
nanotechnology applications and for conjugation with bioac-
tive molecules.6–9 There are different techniques to prepare
telechelic polymers via a variety of reaction mechanisms of
different complexity due to functional groups present in
reacting compounds and due to inherent steric effects of
reactions with macromolecules. Lai et al.10 prepared a,x-tele-
chelic polymers in one step using symmetrical trithiocarbon-
ates. One limitation of this technique is the poor results with
methacrylic monomers, due to the relatively low stability of
the leaving group compared to the one of a methacrylic radi-
cal. Haddleton et al.11 reported the use of molecules bearing
a hydroxyl group that can be transformed into initiators for
copper mediated ATRP living radical polymerization. This
strategy lead to the synthesis of block copolymers including
blocks that cannot be formed through conventional free
Additional Supporting Information may be found in the online version of this article. Correspondence to: A. Licea-Claverı´e (E-mail: aliceac@
tectijuana.mx)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3033–3051 (2010) 2010 Wiley Periodicals, Inc.
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a,x-TELECHELIC HEXYL ACRYLATE POLYMERS, CORTEZ-LEMUS, SALGADO-RODRIGUEZ, AND LICEA-CLAVERIE
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
radical polymerization; to the synthesis of polymers showing
specific functionalities at their chain-ends, to star polymers,
etc. Other common pathway to introduce functional groups
into polymers is the use of bifunctional initiators in conven-
tional free radical polymerization. A variety of bifunctional
initiators with carboxyl, hydroxyl, nitrile, and isocyanate
groups have been used for this purposes. However, the num-
ber of introduced functional groups is very difficult to con-
trol in addition to the fact that polydispersity is normally
very high. An alternative strategy has been to introduce by
synthesis different functional groups in the leaving/reinitiat-
ing R group of chain transfer agents used for RAFT-polymer-
ization. In this way, both polydispersity and the number of
functional groups introduced (normally one) are controlled.
efficiently reduced to hydroxy end group. Telechelic poly-
mers with primary amine end-groups have the advantage
with respect to polymers with other reactive functionality
(hydroxy or carboxy) because of their greater reactivity.2
Therefore there has been a number of previous reports aim-
ing the synthesis of polymers with primary amine end-
groups.33–39 Either the amino group is obtained after reac-
tion of a semitelechelic polymer containing azido38 or
alquene36 end-groups with amine functionalized reagents; or
the amino functionality was introduced during conventional
polymerization by use of amine functionalized chain transfer
agents33 or by means of ‘‘living’’ radical polymerization as a
phthalimido group by ATRP35 or by RAFT39 followed by
hydrazinolysis to yield the desired free amino end group. For
the present report two new AOH and amine functionalized
dithioesters were developed and their ability for the RAFT
polymerization of n-hexyl acrylate (HA) as a model monomer
were tested and compared to other frequently used RAFT
agents containing functional groups. The capability of these
new dithioesters for controlled polymerization of styrene
and methacrylates is currently under investigation in our
laboratory and is subject of a future report. In this study, the
amine functionality is introduced by RAFT as t-BOC pro-
tected group which can be easily removed afterward by use
of trifluoroacetic acid as reported for ATRP polymerization.40
Furthermore, an in-depth study on the postpolymerization
reaction of semitelechelic polyHA with functionalized
azocompounds is reported, aiming to minimize the azoexcess
and to demonstrate the chemical availability of the
introduced end-groups in a,x-telechelics for further
transformations.
A
variety of end-functionalized semitelechelics can be
obtained using this strategy; however for specific applica-
tions, the thiocarbonyl-thio moiety introduced by RAFT,
needs to be removed. For the removal of the thiocarbonyl-
thio moiety several strategies are under investigation, this
include: aminolysis,12–16 thermal treatment,17–20 reaction
with a functionalized azoinitiator in excess,21 reaction with
oxidizing agents22,23 and very recently, a reaction with a
combination of azoinitiator and a peroxide for styrenic and
acrylic polymers.24 A disadvantage of aminolysis and thermal
treatment methods is the formation of thermal labile disul-
fide bonds as side reaction resulting in increasing the molec-
ular weight by chain-chain coupling.25 To overcome this
problem, several groups have reported an approach employ-
ing aminolysis of the thiocarbonyl-thio end-group while
simultaneously reacting the resulting thiol with other com-
pounds. Theato et al.26 reports aminolysis in the presence of
S-3-butynyl methane thiosulfonate leading to acetylene func-
tionalized polymers for further use in ‘‘click chemistry.’’ This
method is of advantage compared with conjugations with
functional maleimides, where isolation of terminal thiols is
often required. Other approach is the aminolysis of the thio-
carbonyl-thio end-group in the presence of pyridyldisulfide-
bearing27 or ene-bearing compounds,28–30 leading to simulta-
neous protection/functionalization of the created thiols. This
methodology exploits thiol-ene chemistry with a range of
enes and yields successfully conjugates to build macromono-
mers and more complex polymer architectures.31 Perrier
et al.21 proposed a reaction that leads simultaneously to the
full removal of the thiocarbonyl thio end group from the
polymeric chains and introduction of chain-end functional-
ities on the polymers. The technique allows functionalized
polymers to be prepared directly from a difunctional chain
transfer agent with only one postpolymerization reaction
using an excess of azoinitiator. The strategy involving azoini-
tiators is reported to result in side reactions including dis-
proportionation of the polymer chains when only a slight
excess of azocompound is used, therefore a large excess
(20:1) is suggested,20 which is impractical for several appli-
cations. Other method for the conversion of polymers capped
by thiocarbonyl thio group into hydroxy terminated poly-
mers was reported by Barner-Kowollik et al.32 The reaction
involves dithioester-capped polymers and azoinitiator in tet-
rahydrofuran yielding hydroperoxide functionalities that are
EXPERIMENTAL
Reagents and Methods
Unless otherwise stated, all materials were purchased from
commercial sources and used without further purification.
All air and moisture sensitive compounds were manipulated
using standard Schlenk techniques under a dry nitrogen
atmosphere. Solvents (Fermont, Me´xico) were distilled prior
to use; n-hexyl acrylate (HA, Aldrich) was purified by passing
through an inhibitor remover column for HQ (Aldrich). 2-
Bromoacetic acid, carbon disulfide, phenylmagnesium bro-
mide (3.0 M solution in diethyl ether), a-bromophenyl acetic
acid, potassium ferricyanide (III), benzyl chloride, were pur-
chased from Aldrich. Thionyl chloride and sodium thiosulfate
were purchased from Aldrich. 4,40-Azobis(4-cyanovaleric
acid) (azo-8, Aldrich, 75%) was recrystallized from methanol,
and 4,40-azobis(4-cyanopentanol) was prepared according to
the method described by Clouet et al.41 5-Hydroxy-2-penta-
none, sodium cyanide, tert-butyl 2-aminoethylcarbamate was
prepared according to a published procedure.42 Gel permea-
tion chromatography (GPC) was performed on a Varian 9002
chromatograph equipped with two mixed-bead columns in
series (Phenogel 5 linear and Phenolgel 10 linear) and two
detectors: refractive index (Varian RI-4) and a triangle light
scattering detector (MINI-DAWN, Wyatt). The measurements
were performed in THF at 35 ꢀC and monodisperse
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ARTICLE
FIGURE 1 Synthetic scheme for the prepa-
ration of 2-hydroxyethoxy-carbonylphenyl-
methyl dithiobenzoate (CTA-2).
polystyrene standards were used for calibration. Reported
dn/dc values for poly(n-butylacrylate) in THF43 were used
for the molecular weight evaluations of polyHA. Mass spec-
troscopy (MS) mode (20 eV) was performed on a Hewlett–
described in detail with spectroscopic information as Sup-
porting Information:
Synthesis of 2-Hydroxyethoxy-carbonylphenylmethyl
Dithiobenzoate
The preparation of this CTA-2 was conceived as a three steps
method shown in Figure 1.
1
Packard 5989A equipment. H (500 or 200 MHz) NMR spec-
tra were recorded on a Varian Inova 500 (500 MHz) or on a
Varian Gemini (200 MHz) spectrometers and are reported in
ppm using TMS as internal standard. Data are reported as:
(b, broad; s, singlet; d, doublet; t, triplet; q, quartet; m, mul-
tiplet; dt, double triplet and combinations like bs, broad sin-
glet, etc.; coupling constant(s) in Hz, integration). 13C (125
or 50 MHz) NMR spectra were recorded on a Varian Inova
500 (125 MHz) or on a Varian Gemini (50 MHz) spectrome-
ters. Chemical shifts are reported in ppm from tetramethylsi-
lane (TMS), with the solvent resonance employed as the in-
ternal standard (CDCl3 at 77.0 ppm). Fourier transform
infrared spectroscopy (FTIR) was performed on a Perking
Elmer, 1600 Series FTIR. UV spectroscopy was performed on
a Varian CARY 100. Elemental analysis were performed by
NuMega Labs in San Diego, CA.
Synthesis of 2-(Tetrahydro-2H-pyran-2-yloxy)
Ethyl a-Bromophenylacetate
In a 500-mL-round-bottom glass and magnetic stirring, a so-
lution of a-bromophenylacetic acid (10.97 g, 50 mmol) in an-
hydrous CH2Cl2 (100 mL) was added. DMAP (0.61 g, 5
mmol) and 2-(Tetrahydro-2H-pyran-2-yloxy) ethanol (8.05 g,
55 mmol) were added. Then the reaction mixture was cooled
ꢀ
down to 0 C and DCC (11.35 g, 55 mmol)ꢀwas added drop-
wise. The solution stirred for 10 min at 0 C and then over-
night at RT. Precipitated urea was then filtered off and the
filtrate was concentrated removing the solvent under vac-
uum. The residue was dissolved in ethyl ether (40 mL) and
filtered again. The solvent was removed by evaporating and
the ester purified by flash chromatography on silica gel (70–
230 mesh), using hexanes/ethyl acetate (9:1)v as eluent. The
product was a colorless oil (16.5 g, yield 96%).
SYNTHESIS OF CTA’S
Seven different functionalized chain transfer agents (CTA’s)
were synthesized for this investigation: 2-cyano-5-hydroxy-
pentan-2-yl benzodithioate (CTA-1), 2-hydroxyethoxy-carbon-
ylphenylmethyl dithiobenzoate (CTA-2), 4-cyanopentanoic
acid dithiobenzoate (CTA-3), 2-phenyl-2-(phenylcarbono-
thioylthio)acetic acid (CTA-4), 2-(phenylcarbonothioylthio)-
acetic acid (CTA-5), 2-(2-(tert-butoxycarbonyl)ethylamino)-2-
oxo-1-phenylethyl benzodithioate (CTA-6) and 2-phenyl-2-
(phenylcarbonothioylthio)acetate (CTA-7). CTA’s 2 and 6 are
reported for the first time (below) while the synthesis of the
other five CTA’s is adapted from literature reports and is
FTIR (Film, cmꢁ1): 3064, 3032 (Aromatic CAH stretch),
2944, 2871 (Aliphatic CAH stretch), 1748 (C¼¼O Stretch);
1496 (C¼¼C ring stretch), 1454, 1439, 1384, 1352, 1279,
1258, 1212, 1138, 1128 (Asymmetric CAOAC stretch). 1H
NMR (200 MHz, CDCl3, d in ppm): 7.54–7.6 (m, 2H, Ar-H),
7.3–7.40 (m, 3H, Ar-H), 5.41 (s, 1H, CH), 4.6 (dt, J ¼ 8.4, 3.4
Hz, 1H, O-CH-O), 4.37 (m, 2H, ACOOCH2A), 3.91–3.47 (m,
4H, CH2 of THP), 1.4–1.9 (complex m, 6H, 3 ꢂ CH2 of THP).
13C NMR (50 MHz, CDCl3, d in ppm): 168.63 (C¼¼O), 136.18,
136.16, 129.66, 129.19, 129.09 (C of Ar), 99.01 (OACHAO),
65.90 (COOCH2), 65.08 (OACH2 of THP), 62.24
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 Synthetic scheme for
the preparation of 2-(2-(tert-
butoxycarbonyl)ethylamino)-2-
oxo-1-phenylethyl
thioate (CTA-6).
benzodi-
(CH2AOACH), 47.11 (SACHA(C¼¼O)), 30.72, 25.74, 19.45 (3
ꢂ CH2 of THP).
the solid cupric sulfate was filtered off and the solid residue
was washed with anhydrous ethyl ether (2 ꢂ 40 mL). The
filtrate was concentrated and then purified by flash column
chromatography on sı´lica gel (70–230 mesh) (hexanes:ethy-
lacetate (9:1)v. The CTA was obtained as a red oil (8.64 g,
65% yield).
Synthesis of 2-(Tetrahydro-2H-pyran-2-yloxy)
Ethoxy-carbonylphenylmethyl Dithiobenzoate
In a 250-mL-three-necked round bottom glass equipped with
condenser, gas inlet, and magnetic stirring, a solution of phe-
nyl magnesium bromide (13.24 mL, 40 mmol) in THF (3.0
M) was added to a flask containing dry THF (100 mL). The
solution was cooled to 0 ꢀC and then carbon disulfide (3.02
g, 40 mmol) was added dropwise. After 4 h, 2-(tetrahydro-
2H-pyran-2-yloxy) ethyl a-bromophenylacetate (BrEOTHP)
(15 g, 44 mmol) was added to the reaction mixture. The
temperature was rise to 85 ꢀC and maintained for 24 h.
Then the reaction mixture was cooled down and water (50
mL) was added. The organic product was extracted with
ethyl ether (3 ꢂ 50 mL) and dried with anhydrous magne-
sium sulfate, then concentrated under vacuum. The crude
product was purified by flash column chromatography on
silica gel (70–230 mesh) with hexanes:ethyl acetate (7:3)v,
obtaining a red oil (14.49 g, 88% yield).
FTIR (Film, cmꢁ1): 3446 (OAH stretch); 3060, 3030 (Aro-
matic CAH stretch), 2953, 2880 (Aliphatic CAH stretch),
1737 (C¼¼O stretch), 1589, 1495, 1446 (C¼¼C ring stretch),
1372, 1280, 1233 (C¼¼S stretch), 1159 (CAC (¼¼O)AO
stretch), 1081, 1046, 1027 (CAO stretch), 578 (CAS), 564,
1
498. H NMR (500 MHz, CDCl3, d in ppm): 8.00–8.06 (dt, J ¼
8.3, 1.4 Hz, 2H, ASC(Ar-H)SA), 7.36–7.60 (m, 8H, Ar-H), 5.75
(s, 1H, CH), 4.42 (m, 1H, ACOOCH2A), 4.27 (m, 1H,
ACOOCH2A), 3.85 (m, 2H, ACH2AOH), 1.79 (bs, 1H, AOH).
13C NMR (125 MHz, CDCl3, d in ppm): 226.40 (CS2), 169.48
(C¼¼O), 144.21, 133.36, 133.28, 129.59, 129.49, 129.20,
128.86, 127.37 (C of Ar), 68.12 (COOCH2), 61.40 (CH2AOH),
59.27 (CH). ELEM. ANAL. for C17H16O3S2 Calc.: C, 61.42; H,
4.85; S, 19.29; Found: C, 61.43; H, 4.95; S, 18.9.
FTIR (Film, cmꢁ1): 3060, 3030 (Aromatic CAH stretch),
2943, 2871 (Aliphatic CAH stretch), 1740 (C¼¼O stretch);
1590, 1496, 1445 (C¼¼C ring stretch), 1384, 1352, 1279,
1232 (C¼¼S stretch), 1202, 1181, 1159 (CAC (¼¼O)AO
stretch), 1138, 1127 (Asymmetric CAOAC stretch), 1080,
578 (CAS). 1H NMR (500 MHz, CDCl3, d in ppm): 8.00–8.06
(dt, J ¼ 8.4, 1.1 Hz, 2H, ASC(Ar-H)SA), 7.30–7.60 (m, 8H,
Ar-H), 5.78 (s, 1H, CH), 4.59 (dt, J ¼ 20.4, 3.3 Hz, 1H,
OACHAO), 4.2–4.5 (m, 2H, ACOOCH2A), 3.91–3.46 (m, 4H,
CH2 of THP), 1.4–1.9 (complex m, 6H, 3 ꢂ CH2 of THP). 13C
NMR (125 MHz, CDCl3, d in ppm): 226.40 (CS2), 169.20
(C¼¼O), 144.31, 133.78, 133.19, 129.59, 129.40, 129.21,
128.86, 127.35 (C of Ar), 98.96 (OACHAO), 65.68 (COOCH2),
65.18 (OACH2 of THP), 62.17 (CH2AOACH), 59.25 (SACH
A(C¼¼O)), 30.74, 25.77, 19.37 (3 ꢂ CH2 of THP).
Synthesis of 2-(2-(tert-Butoxycarbonyl)ethylamino)-2-
oxo-1-phenylethyl benzodithioate
The preparation of this CTA-6 was conceived as a two steps
method shown in Figure 2.
tert-Butyl 2-(2-Bromo-2-phenylacetamido)
ethylcarbamate
a-bromophenyl acetic acid (2 g, 9.3 mmol) was dissolved in
benzene (20 mL); added thionyl chloride (0.67 mL, 9.3
ꢀ
mmol) and the mixture was heated under reflux to 80 C for
5 h. The solvent was removed under reduced pressure to
yield 2-bromo-2-phenylacetyl chloride (yellowish oil). The
resulting product was immediately dissolved in CH2Cl2 (30
mL) and the mixture was then cooled to 0 ꢀC and an equi-
molar amount of triethylamine (0.94 g, 9.3 mmol) was added
to the solution; afterward tert-butyl 2-aminoethylcarbamate
(1.49 g, 9.3 mmol), prepared according to a published proce-
dure,15 was added slowly. The solution was allowed to warm
to room temperature and stirred overnight. The solvent was
then removed under reduced pressure and the residue chro-
matographed on silica gel (70–230 mesh), with ethyl aceta-
te:CH2Cl2 (1:9)v to obtain tert-butyl 2-(2-bromo-2-phenylace-
tamido)ethylcarbamate. The product (1.5ꢀ g, 4.2 mmol; 45%
yield) was a white powder; mp 114–116 C.
Synthesis of 2-Hydroxyethoxy-carbonylphenylmethyl
Dithiobenzoate
In a 250-mL-bottom-glass equipped with condenser, gas inlet
and magnetic stirring, a solution of 2-(tetrahydro-2H-pyran-
2-yloxy) ethoxy-carbonylphenylmethyl dithiobenzoate (CTA-
OTHP; 16.69 g, 40 mmol) in methanol (125 mL) was poured
and cupric sulfate pentahydrate (2 g, 8 mmol), was added.
The mixture was stirred at room temperature for 12 h. Then
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ARTICLE
FIGURE 3 Structures of chain
transfer agents (CTA’s) and azo
initiators used in polymerization
of HA.
FTIR (KBr, cmꢁ1): 3356, 3317 (NAH stretch), 3063 (Aro-
matic CAH stretch), 2977, 2938 (Aliphatic CAH stretch),
1693 (C¼¼O Stretch), 1646, 1535, 1449 (C¼¼C ring stretch),
1366, 1280, 1250, 1168 (CAO stretching). 1H NMR (200
MHz, CDCl3, d in ppm): 7.25–7.27 (m, 5H, Ar-H), 5.63 (s,
1H, CH), 4.9 (br, 1H, HNC(O)O), 3.26–3.32 (m, 4H,
HNCH2CH2NH), 1.35 (s, 9H, C(CH3)3). 13C NMR (50
(C¼¼S stretch), 1167; 1H NMR (200 MHz, CDCl3, d in ppm):
7.96–7.91 (m, 2H, S ¼ CAr-H), 7.44–7.19 (m, 8H, Ar-H), 6.8
(br, 1H, HNAC¼¼O), 5.60 (s, 1H, CH), 4.8 (br, 1H, HNAC(O)O),
3.18–3.38 (m, 4H, HNACH2ACH2ANH), 1.35 (s, 9H,
C(CH3)3). 13C NMR (50 MHz, CDCl3, d in ppm): 168.61
(ACONA), 144.14, 134.81, 132.86, 129.0, 128.95, 128.69,
128.41, 127.09 (C of Ar), 59.82, 41.24 (ACH2), 40.0 (ACH2),
28.37 (AC(CH3)3). MS-EI m/e [M]þ 398 (1), 374 (1), 341
(1), 297 (1), 270 (37), 253 (7), 209 (13), 178 (4), 149 (8),
121 (100), 57 (60). ELEM. ANAL. for C22H26N3O3S2 Calc.: C,
61.37; H, 6.09; N, 6.51; Found: C, 60.94; H, 6.38; N, 6.51.
MHz, CDCl3,
d in ppm): 167.61 (ACONHA), 156.54
(ACAOAC(CH3)3), 137.06, 128.72, 128.59, 128.55, 128.05,
127.5 (C of Ar), 50.62 (AC(CH3)3), 41.36, 39.5, 28.07. MS-EI
m/z [M]þ 356 (1), 301 (8), 277 (10), 257 (7), 214 (6), 169
(15), 148 (37), 87 (81), 57 (100).
POLYMERIZATION PROCEDURE
2-(2-(tert-Butoxycarbonyl)ethylamino)-2-oxo-1-phenyl-
ethyl Benzodithioate
Preparation of Semitelechelic PolyHA via RAFT Method
All semitelechelic polyHA’s were prepared using the same
methodology; the only changes were the use of specific CTA’s
and functional group matching azocompounds. Therefore,
CTA-1, CTA-2, and CTA-6 were used with azo 9; while CTA-3,
CTA-4, CTA-5, and CTA-7 were used with azo 8 (see Fig. 3
for chemical structures). The reaction conditions, purification
and characterization of the products were the same. As an
example we describe in detail an experiment using CTA-2
and azo 9.
Phenyl magnesium bromide (1.32 mL, 4.0 mmol) 3.0 M in
diethyl ether was pleased in a Schlenk flask equipped with
condenser under nitrogen flow. Dry THF (10 mL) were
transferred to the flask. The reaction mixture was then
cooled to 0 ꢀC and carbon disulfide (0.302 g, 4.0 mmol)
introduced dropwise, via a degassed syringe. The addition of
de CS2 led to a color change from green-gray to red. The
reaction mixture was allowed to reach room temperature
and react for 1 h. After this time tert-butyl 2-(2-bromo-2-
phenylacetamido)ethylcarbamate (1) (1.57 g, 4.4 mmol) was
added to this reaction mixture dissolved in the minimum
amount of anhydrous tetrahydrofuran (2–3 mL) via degassed
syringe. The reaction mixture was stirred at RT for 24 h.
Then, water (10 mL) was added, and the organic layer was
extracted with diethyl ether (3 ꢂ 15 mL). The combined or-
ganic extracts were washed with water and dried over mag-
nesium sulfate. The solvent was then evaporated and the
obtained product purified by column chromatography on
silica gel (70–230 mesh), with a CH2Cl2:ethyl acetate (9:1)v
mixture as an eluent. The compound obtained was a pink-or-
Hexyl acrylate (HA) (1 g, 6.4 mmol), 4,40-azobis(4-cyanopen-
tanol) (azo 9) (0.005 g, 0.02 mmol), (2-hydroxyethoxy-car-
bonylphenylmethyl dithiobenzoate) (CTA-2) (0.033 g, 0.1
mmol) and 1,4-Dioxane (4.0 mL) were mixed in a glass vial.
This mixture was transferred to a 20 mL ampoule containing
a magnetic stir bar. The oxygen was removed using 5 freeze-
thaw evacuation cycles, and the ampoule was sealꢀed with
flame under vacuum. The solution was heated to 70 C in an
oil bath with magnetic stirring. At designated time, the poly-
merization was stopped by cooling to room temperature.
The polymerization yield was obtained gravimetrically by
adding a fivefold excess of cold methanol, a nonsolvent for
polyHA. The polymer product was purified by dissolution in
the minimum amount of ethyl ether followed by adding a
fivefold excess of cold methanol and decanting. This
ꢀ
ange solid (0.85 g, 44% yield); mp 156–158 C.
FTIR (KBr, cmꢁ1): 3314 (NAH stretch), 3065 (Aromatic CAH
stretch), 2975, 2932 (Aliphatic CAH stretch), 1682 (C¼¼O
Stretch), 1663, 1450 (C¼¼C ring stretch), 1365, 1276, 1249
´
´
a,x-TELECHELIC HEXYL ACRYLATE POLYMERS, CORTEZ-LEMUS, SALGADO-RODRIGUEZ, AND LICEA-CLAVERIE
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
procedure was repeated three times to remove residual
monomer followed by drying under vacuum to constant
weight.
and CH2, backbone), 1.20–1.80 (bm, A[CH2]4A of the hexyl
acrylate polymer), 0.80–1.0 (H bs, ACH3 of the hexyl acrylate
polymer).
Preparation of Dihydroxy Homotelechelic PolyHA
Preparation of Hydroxy-amine Heterotelechelic PolyHA
The procedure requires two steps, in the first step a t-BOC
protected amino semitelechelic polyHA (0.3 g, 0.035 mmol,
Mn ¼ 8369 g molꢁ1); prepared using 2-(2-(tert-butoxycarbo-
nyl)ethylamino)-2-oxo-1-phenylethyl benzodithioate (CTA-6)
was dissolved in 1,4-dioxane (5 mL) and poured into a 10-
mL-Schlenk flask equipped with condenser and magnetic
stirrer. 4,40-azobis(4-cyanopentanol) (0.044 g, 0.175 mmol)
was added in a 5:1 ratio. The solution was purged with ar-
gon and the temperature was raised to 70 ꢀꢀC. The reaction
was performed under argon for 10 h at 70 C. After cooling
down to RT the solvent was removed in vacuum. Hexane
(5 mL) was added to the mixture and the precipitated
formed was eliminated. The polymer was purified by dissolu-
tion in the minimum amount of ethyl ether followed by add-
ing a fivefold excess of cold methanol and decanting. This
procedure was repeated three times to remove residues
from the CTA. Finally, the polymer was dried under vacuum
at RT. The product was a colorless viscous liquid at room
temperature (0.27 g, Mn ¼ 7939 g molꢁ1, 90% yield).
To a solution of hydroxy semitelechelic PolyHA (0.58 g,
0.0528 mmol of Mn ¼ 10,980 g molꢁ1) in 1,4-Dioxane (10
mL) in a Schlenk tube; 4,40-azobis(4-cyanopentanol) (0.066
g, 0.264 mmol) was added in a 5:1 ratio. The solution was
purged withꢀ argon for 10 min and the temperature was
raised to 70 C. The reaction was performed under argon for
10 h at 70 ꢀC. After cooling down to RT, the solvent was
removed in vacuum. Hexane was added to the mixture and
the precipitated formed was eliminated. The polymer was
purified by dissolution in the minimum amount of ethyl
ether followed by adding a fivefold excess of cold methanol
and decanting. This procedure was repeated three times to
remove CTA residues. Finally, the telechelic polymer was
dried under vacuum at RT. The product is a colorless viscous
liquid at room temperature (0.5 g, 86.2% yield of Mn
¼
10,100 g molꢁ1).
Semitelechelic-OH: 1H NMR (500 MHz, CDCl3, d in ppm):
7.98–7.82 (bd, SC(Ar-H)S, phenyl protons from thiocarbonyl
thio ester), 7.55–7.27 (m, other phenyl protons from CTA 2),
4.25–4.15 (m, ACOOCH2A of CTA 2), 3.85–4.20 (bm,
ACOOCH2A of the hexyl acrylate polymer), 3.45–3.80 (m,
ACH2AOH of CTA 2), 2.40–1.80 (bm, ACH and CH2, back-
bone), 1.20–1.80 (m, A[CH2]4A of the hexyl acrylate poly-
mer), 0.80–1.0 (bs, ACH3 of the hexyl acrylate polymer).
Telechelic-OH: 1H NMR (500 MHz, CDCl3, d in ppm): 7.55–
7.27 (m, phenyl protons from CTA 2), 4.18–4.25 (m,
ACOOCH2A of the CTA 2),), 3.85–4.20 (m, ACOOCH2A of the
hexyl acrylate polymer), 3.45–3.80 (m, ACH2AOH of CTA 2
and ACH2OH of AZO), 2.40–1.80 (bm,ACH and CH2, back-
bone), 1.20–1.80 (bm, A[CH2]4A of the hexyl acrylate poly-
mer), 0.80–1.0 (bs, ACH3 of the hexyl acrylate polymer).
Semitelechelic-tBOC-amino: 1H NMR (200 MHz, CDCl3, d in
ppm): 8.03–7.84 (bd, SC(Ar-H)S, phenyl protons from thio-
carbonyl thio ester), 7.58–7.27 (m, other phenyl protons
from CTA 6), 3.85–4.20 (bm, ACOOCH2A of the hexyl acry-
late polymer), 3.05–3.40 (HNACH2ACH2ANH protons from
CTA 6), 2.40–1.80 (bm,ACH and CH2, backbone), 1.20–1.80
(m, A[CH2]4A of the hexyl acrylate polymer), 1.41 (bs,
AC(CH3)3 from CTA 6), 0.80–1.0 (bs, ACH3 of the hexyl acry-
late polymer).
Telechelic-OH, tBOC-amino: 1H NMR (200 MHz, CDCl3, d in
ppm): 7.34–7.27 (m, other phenyl protons from CTA 6),
3.85–4.20 (bm, ACOOCH2A of the hexyl acrylate polymer),
3.05–3.40 (HNACH2ACH2ANH protons from CTA 6), 2.40–
1.80 (bm,ACH and CH2, backbone), 1.20–1.80 (m, A[CH2]4A
of the hexyl acrylate polymer), 1.41 (bs, AC(CH3)3 from CTA
6), 0.80–1.0 (bs, ACH3 of the hexyl acrylate polymer).
Preparation of Dicarboxylic Acid Homotelechelic PolyHA
To a solution of carboxylic acid semitelechelic polyHA (0.25
g, 0.037 mmol of Mn ¼ 6771 g molꢁ1) in 1,4-dioxane (10
mL) in a Schlenk tube 4,40-azobis(4-cyanopentanoic acid)
(0.051 g, 0.185 mmol) was added in a 5:1 ratio. The reaction
conditions, purification and characterization of the product
was the same as for the preparation of dihydroxy telechelic
polyHA. The product is a colorless viscous liquid a room
temperature (0.24 g, 96% yield of Mn ¼ 6414 g molꢁ1).
Deprotection of t-BOC
In the second step, a solution of the previous product
(hydroxy t-BOC-amine telechelic polyHA, 0.27 g, Mn ¼ 7939
g molꢁ1) was dissolved in anhydrous CH2Cl2 (5 mL), poured
into a 20-mL-round bottom flask and cooled down to 0 ꢀC.
Trifluoroacetic acid (TFA, 15 equiv, 0.039 mL) was dropped
in slowly, and then the mixture was allowed to warm to
room temperature and stirred for additional 2 h. The solvent
was removed in vacuum to give the hydroxyl-amine tele-
chelic polyHA as its TFA salt in quantitative yield. The prod-
uct was dissolved in CH2Cl2 (5 mL) and sodium hydroxide
solution (5 mL, 1 M) was added and vigorously mixed. After
separation of the aqueous layer, the product was obtained by
removal of solvent in vacuum from organic layer. The final
product is a colorless viscous liquid at room temperature
(0.15 g, Mn ¼ 7939 g molꢁ1, 90% yield).
Semitelechelic-COOH: 1H NMR (200 MHz, CDCl3, d in ppm):
8.05–7.92 (bd, SC(Ar-H)S, phenyl protons from thiocarbonyl
thio ester), 7.36–7.62 (m, other phenyl protons from thiocar-
bonyl thio ester), 3.85–4.20 (bm, ACOOCH2A of the hexyl ac-
rylate polymer), 2.62–2.40 (bm, ACH2CH2 from CTA 3),
2.40–1.80 (bm, CH and CH2, backbone), 1.20–1.80 (bm,
A[CH2]4A of the hexyl acrylate polymer), 0.80–1.0 (H bs,
ACH3 of the hexyl acrylate polymer).
Telechelic-COOH: 1H NMR (200 MHz, CDCl3, d in ppm):
3.85–4.20 (bm, ACOOCH2A of the hexyl acrylate polymer),
2.62–2.40 (bm, ACH2CH2 from CTA 3), 2.40–1.80 (bm, CH
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ARTICLE
Telechelic-OH, Amine: 1H NMR (200 MHz, CDCl3, d in ppm): (2-(2-(tert-butoxycarbonyl)ethylamino)-2-oxo-1-phenylethyl
1H NMR (CDCl3, d in ppm): 7.34–7.27 (m, other phenyl pro- benzodithioate (CTA-6), respectively. The synthesis of CTA-2
tons from CTA 6), 3.85–4.20 (bm, ACOOCH2A of the hexyl is based on a Grignard reaction involving the product of phe-
acrylate polymer), 2.40–1.80 (bm, ACH and CH2, backbone), nylmagnesium bromide and carbondisulfide with BrEOTHP,
1.20–1.80 (m, A[CH2]4A of the hexyl acrylate polymer), which was obtained in the first step by conventional esterifi-
0.80–1.0 (bs, ACH3 of the hexyl acrylate polymer).
cation of a-bromophenylacetic acid (10.97 g, 50 mmol) with
2-(tetrahydro-2H-pyran-2-yloxy) ethanol using DCC; to
obtain 2-(tetrahydro-2H-pyran-2-yloxy) ethoxy-carbonylphe-
nylmethyl dithiobenzoate (CTA-OTHP). CTA-2 was obtained
as a red semisolid compound by deprotection of CTA-OTHP
in methanol with cupric sulfate pentahydrate in a good
global yield of 65%. Following an analogous synthetic route
CTA-6 was prepared by Grignard reaction involving the prod-
uct of phenylmagnesium bromide and carbondisulfide with
tert-butyl 2-(2-bromo-2-phenylacetamido)ethylcarbamate),
which was synthesized by acylation of mono-t-BOC protected
ethylene diamine with 2-bromo-2-phenylacetyl chloride in
the first step. The product (2-(2-(tert-butoxycarbonyl)ethyla-
mino)-2-oxo-1-phenylethyl benzodithioate (CTA-6) was
obtained in acceptable global yield of 44%. For both CTA’s
the characteristic singlet peaks of R-group’s are observed at
a chemical shift of ꢃ 5.65 ppm (1H NMR) for methine hydro-
gen (NMR spectra in Supporting Information).
Reaction of Dihidroxy Telechelic PolyHA with
Methacrylic Anhydride
To a solution of dihydroxy telechelic polyHA (0.25 g, 0.048
mmol of Mn ¼ 5200 g molꢁ1) in dry dichloromethane (5
mL) methacrylic anhydride (0.018 g, 0.12 mmol) and trie-
thylamine (4 equiv) were added. The reaction was carried
out during 7 days at room temperature. After this time the
solvent was removed in vacuum. Hexane (5 mL) was added
to the mixture and the precipitated formed was eliminated.
The polymer was purified by dissolution in the minimum
amount of ethyl ether followed by adding a fivefold excess of
cold methanol and decanting. This procedure was repeated
three times to remove residues. Finally, the polymer was
dried under vacuum at RT. The product was a colorless vis-
cous liquid a room temperature (0.18 g, Mn ¼ 5150 g molꢁ1
,
78% yield).
Telechelic-methacrylate: 1H NMR (500 MHz, CDCl3, d in
ppm): 7.35–7.27 (m, phenyl protons from CTA 2), 5.94 (bs,
AC¼¼CHaHb), 5.51 (bs, AC¼¼CHaHb), 4.35–4.15 (m,
ACOOCH2A of CTA 2 and ACH2COOA of ester end group),
3.85–4.20 (m, ACOOCH2A of the hexyl acrylate polymer),
3.45–3.80 (m, ACH2AOH of CTA 2 and ACH2OH of AZO),
2.40–1.80 (bm, ACH and CH2, backbone), 1.20–1.80 (bm,
A[CH2]4A of the hexyl acrylate polymer), 1.84 (bs, CH3 of
the methacrylic end group), 0.80–1.0 (bs, ACH3 of the hexyl
acrylate polymer).
RAFT Polymerization of n-Hexylacrylate
One important goal in this investigation was to explore the
effect of different R substituents in dithiobenzoate esters, on
the outcome of polymerizations of HA and to examine the
effect of varying the functionality introduced in the leaving
group, if any, in the RAFT polymerization of HA. The variety
of functional chain transfer agents of the dithiobenzoate fam-
ily tested (Fig. 3) can produce a variety of semitelechelic
polymers with specific functionalities at their chain-ends.
RESULTS AND DISCUSSION
The following RAFT chain transfer agents: CTA-1 (2-cyano-5-
hydroxypentan-2-yl benzodithioate),2,3,11 CTA-3 (4-cyano-4-
Synthesis of New Functionalized CTA’s
Carboxylic acid containing RAFT chain transfer agents based (phenylcarbonothioylthio)pentanoic acid)2,49 and CTA-7
on dithiobenzoates and trithiocarbonates have been used to (methyl 2-phenyl-2-(phenylcarbonothioylthio)acetate)3,50 has
prepare a wide variety of polymers containing a carboxylic been proved successful in a number of RAFT polymerization
acid end-group.1,2,44-46 Perhaps the most widely used is 4- systems. Among these, cyanopentanoic dithiobenzoate CTA-3
cyanopentanoic acid dithiobenzoate (CTA-3)47 because of its was particularly chosen because it is one of the most studied
solubility in a wide variety of polar solvents including water RAFT agents for aqueous polymerization of monomers con-
(pH > 8) and the commercial availability of 4,40 azobis-(cya- taining functional groups.2,44,45 The synthesis of CTA-4 has
novaleric acid) (ACVA), a free radical initiator yielding been reported by Legge et al.,50 although, it has not being
exactly the same initiating R group than the leaving/re-ini- tested as a RAFT chain transfer agent. CTA-5 (2-(phenylcar-
tiating group contained in the dithiobenzoate (CTA-3). How- bonothioylthio)acetic acid) was used by Choe et al.51,52 for
ever, the shelf lifetime of this CTA-3 is limited due to hydro- the polymerization of styrene and by Pichot et al.3 for the
lysis and light sensitivity.47,48 We wish to describe herein, polymerization of N-acryloylmorpholine and they found that
the synthesis of two new functionalized RAFT chain transfer the CTA-5 was not efficient enough to control the polymer-
agents (CTA’s), containing
a leaving/reinitiating group ization. The new CTA’s 2 and 6 have similarities in their
(R, attached to sulfur) yielding a secondary carbon radical structure with CTA-4 and CTA-7 since they generate by the
by fragmentation in the RAFT-equillibrium containing either fragmentation of the CTA a secondary radical (ꢄR) with a
a functional OH group (CTA-2) or a t-BOC protected amine bulky phenyl group that may add resonance stability to the
group (CTA-6), respectively. It is worth to mention that the radical, but they include another functional groups (OH and
chemical structure next to dithiobenzoate can make a big dif- t-BOC protected amine) which are important for the prepara-
ference for stability of a CTA.12,49 The synthetic schemes are tion of telechelic polymers, as we will discuss later. Steric
described in Figure 1 for 2-hydroxyethyl 2-phenyl-2-(pahe- hindrance contributes also to the R radical stability and it is
nylcarbonothioylthio)acetate (CTA-2) and in Figure 2 for reported to play a major role in dithioester efficiency.53
´
´
a,x-TELECHELIC HEXYL ACRYLATE POLYMERS, CORTEZ-LEMUS, SALGADO-RODRIGUEZ, AND LICEA-CLAVERIE
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 RAFT Polymerization of HA in 1,4-Dioxane in the Presence of CTA’s 1-7 and Azo Initiators 8 and 9 at 70 8C Using a Molar
Ratio [HA]0:[CTA]0:[AZO]0 of 320:5 :1
CTA
1
RAFT Agent
Time, (h)
12
Initiator
Azo 9
Conv. (%)a
50
Mn, GPC (g molꢁ1 b
)
Mw/Mn
Mn, calc.c (g molꢁ1
)
8195
1.021
5257
2
6
12
12
Azo 9
Azo 9
25.4
56
6042
1.094
1.012
2868
6021
8225
3
4
12
12
Azo 8
Azo 8
57.4
44
9698
7332
1.001
1.012
6010
4681
7
12
Azo 8
Azo 8
46
14
7365
1.049
1.110
4895
1610
5
9
14,900
a
Conversion calculated by mass of isolated polymer using methanol.
Molecular weight and PDI values determined by GPC.
Calculated molecular weights based on eq 1.
b
c
A summary of experimental conditions and the molecular
characteristics (molecular weights, polydispersities, and per-
cent conversions) of the polymers obtained by RAFT polymer-
izations of HA in the presence of several functionalized
dithioesters under similar reaction conditions are shown in
Table 1. The obtained polymers have very low polydispersities
(PDI range between 1.00 and 1.10) as determined by GPC (Ta-
ble 1), and the elution profiles were monomodal and symmet-
rical (Fig. 4). A comparison with the calculated molecular
weights according to the simple eq 13 shows that the molecu-
lar weights determined by GPC were always higher than the
calculated but this may result from inaccuracy in conversion
determination gravimetrically using a nonsolvent given the
fact that polyHA was difficult to isolate because it is a viscous
liquid at room temperature. Therefore, the difference can be
attributed to the loss of material during polymer purification.
In any case, the calculated molecular weight by 100% con-
version would be around 10,000 g molꢁ1 an the measured
molecular weights by GPC are, with the exception of the
experiment using CTA-5, close to that value. The best match
is given by using CTA-3; additionally the highest conversion
and lowest PDI was obtained using this CTA (see Table 1
and Fig. 4). It is also evident that CTA-5 provided poor con-
trol in RAFT polymerization resulting in formation of chains
of high molecular weight by low conversion. All functional-
ized dithioesters were effective in controlling the molecular
weight; possess similar reactivities and provided a narrow
polydispersity excepting CTA-5. This CTA-5 does not have a
good leaving group (carboxymethyl), corresponding to a pri-
mary carbon centered radical. CTA’s 1 and 3 produce in the
fragmentation step of RAFT equilibrium a tertiary carbon
radical, stabilized by the cyano group and were evidently
very effective in controlling the RAFT polymerization of HA.
ꢀ
ꢁ
½Mꢅo
The novel CTA’s 2 and 6, together with CTA’s 4 and 7 pro-
duce in RAFT equillibrium a secondary leaving group (radical
Mn ¼
ꢂ Conversion ꢂ Mmon þ MCTA
(1)
½CTAꢅo
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ARTICLE
the polymerization. It seems that the polymerization rate is
slowed down at high conversion and that the molecular
weight comes to a limiting value (compare with Fig. 5 at
high conversion); however, this downward curvature in ki-
netic plots of radical polymerization is reported to be com-
mon in nearly all controlled free-radical polymerization
methods at high conversions.46,53,54 It is also important to
note that the limiting molecular weight value is very close to
the theoretical molecular weight at 100% conversion.
The first requirement for an effective CTA is that it should
have a high transfer constant relative to the monomers being
polymerized, which means a high rate of addition and a suit-
able leaving group for the propagating radical. The second
important quality of an effective CTA is the ability of forming
intermediates that fragment rapidly, giving no side reactions.
It is expected that the R-group structure of the CTA’s plays
an important role in the RAFT polymerization rate of HA
since polymerization rate should decrease with decreasing
radical stability of the R group in CTA (leaving/re-initiating
radical R): tertiary R> secondary R > primary R (see
Scheme 1). Thus, the rate of RAFT polymerization using CTA-
1 and CTA-3 is expected to be very similar and faster than
using CTA’s 2, 4, 6, and 7; while the polymerization rate
using CTA-5 is expected to be the slowest for RAFT polymer-
ization among the CTA’s tested in this study. Table 3 shows
that in fact for polymerization times of 12 h the use of CTA-
1 and CTA-3 resulted in higher conversions (50 and 54.4%,
respectively) than for the polymerizations using all other
CTA’s tested with one important exception, the use of CTA-6
resulted in a conversion of around 56% in 12 h. This may
result from the bulky structure of the R group in CTA-6 facil-
itating the rapid fragmentation. The conversions at 12 h
obtained by using CTA’s 4 and 7 are very similar (44 and
46% respectively), while the polymerization rate using CTA-
5 and CTA-2 were the slowest. The polydispersity values
reflect the quality of the RAFT process, since a high polydis-
persity is an indicator of the deviation from a controlled-rad-
ical (living) system. It is evident that the use of CTA-2
FIGURE 4 Gel permeation chromatograms (GPC-traces) of
poly(n-hexyl acrylate) prepared by RAFT polymerization using
several functionalized CTA’s. Experimental conditions in Table 1.
R) containing a bulky phenyl group adjacent to the carbon-
radical.
The presence of a phenyl group is anticipated to increase the
stability of the generated radical. Their ability to control the
polymerization of hexyl acrylate was also good.
We decided to study in detail the kinetics of HA RAFT poly-
merization using CTA-2. These results are summarized on
Table 2 and Figure 5, which show the evolution of Mn and
PDI with monomer conversion. The ability of the CTA-2 to
control the molecular weight and give narrow polydispersity
for poly(HA) is evident as shown by a linear increase of Mn
with conversions up to 65% and a low PDI (<1.1) thorough
the course of the reaction. The pseudo first order kinetic
plot in Figure 6 do not shows a very good linearity thorough
TABLE 2 RAFT Polymerization of HA in 1,4-Dioxane in the Presence of CTA 2 and Azo Initiator 9 at 70 8C Using a Molar Ratio
[HA]0:[CTA]0:[AZO]0 of 640:5:1
Entry
Time (h)
Conv. (%)a
Mn, GPC (g molꢁ1 b
)
Mw/Mn
Mn, calc. (g molꢁ1 c
)
1
2
3
4
5
6
7
8
9
a
2
4
8.8
22.0
33.5
47.6
52.9
64.1
62.9
71.1
70.5
1921
8020
1.007
1.004
1.001
1.012
1.069
1.033
1.053
1.060
1.100
2089
4725
6
10,830
14,700
17,350
18,100
18,830
19,490
20,920
7021
8
9837
10
12
14
16
18
10,895
13,131
12,892
14,529
14,409
Conversion calculated by mass of isolated polymer using methanol.
Molecular weight and PDI values determined by GPC.
Calculated molecular weights based on eq 1.
b
c
´
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a,x-TELECHELIC HEXYL ACRYLATE POLYMERS, CORTEZ-LEMUS, SALGADO-RODRIGUEZ, AND LICEA-CLAVERIE
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Evolution of molecular weight for RAFT polymerization of HA in 1,4-dioxane in the presence of CTA-2 and Azo-9 at 70
ꢀC using a molar ratio [HA]0:[CTA]0:[AZO]0 of 640:5:1. A: GPC chromatograms. B: Evolution of Mn and PDI with conversion.
resulted in a controlled RAFT polymerization although the
polymerization rate is as low as for using CTA-5, a noncon-
trolling dithiobenzoate RAFT agent: Using CTA-2, at 36.4%
conversion Mn ¼ 9836 g molꢁ1with polydispersity 1.00 was
obtained; while using CTA-5 at 35% conversion Mn ¼ 23,480
g molꢁ1, with polydispersity 1.30 (Table 3) was obtained. It
should be also mentioned that the secondary carbon radicals
that are produced after fragmentation of CTA’s 2, 4, 6, and 7
are further stabilized by resonance of the phenyl group,
which may allow them to increase their fragmentation rate
constant yielding good controlling RAFT agents. In all cases,
the expelled radicals (ꢆR) are able to reinitiate the polymer-
ization of HA since no polymerization inhibition was
observed.
and ratios using the freshly synthesized CTA and after 1 year
of storage in freezer. It is evident that the results obtained
after 1 year of storage are very similar to those originally
obtained demonstrating the good stability of these type of
CTA’s making clear another advantage of their use in RAFT
polymerization; among them are CTA 2 and CTA 6, the two
new CTA’s reported in this study for the first time.
Synthesis of a,x-Telechelic Polymers
In the literature, numerous reports claiming telechelic poly-
mer synthesis have been published, without thoroughly char-
acterizing the resulting polymer regarding the functional
end-groups. Many examples of affirmations based on the the-
oretical chemistry involved in the synthesis, or nonwell-
adapted analytical techniques, can be found. The inherent
low concentration of end groups and the possibility of side
The synthesized new CTA’s 2 and 6 are good RAFT agents
for the polymerization of HA, successfully controlling its mo-
lecular weights and distribution. The application of these
new CTA’s to the RAFT polymerization of other classes of
monomers like styrene and methylmethacrylate is subject of
a separate report.55
Stability of chain transfers agents (CTA’s) for RAFT polymer-
ization is an important factor that has not been addressed
extensively. The most commonly used dithioester compounds
are known to be sensitive to hydrolysis. For example, CTA’s 1
and 3 are used frequently, but they are unstable even in
freeze storage; in our experience CTA 1 is more hydrolytic
unstable, after a month of synthesis it starts to decompose
under storage conditions. Possibly the electron withdrawing
ACN group in a-position to the dithioester bond is responsi-
ble for faster hydrolysis. After 2 months in freeze storage
degradation of CTA 3 starts, as evidenced by the appearance
of a singlet at 1.86 ppm attributed to the methyl group
(CH3A(CCN)AS(C¼¼S)A) in it’s 1H NMR spectrum. In con-
trast CTA’s 2, 4, 6, and 7 showed good hydrolytic stability
under freeze storage conditions. After 1 year of synthesis
CTA’s 2, 4, and 6 showed good performance in polymeriza-
tion of HA both in molecular weight control and in low poly-
dispersity. Table 4 shows polymerization results of HA using
CTA’s 2, 4, and 6 under exactly the same synthetic conditions
FIGURE 6 Pseudofist-order kinetic plot with time for polymer-
ization of HA ([HA]0 ¼ 1.18 M in 1,4-dioxane) at 70 ꢀC with Azo-
9 in the presence of CTA 2. [HA]0:[CTA]0:[AZO]0 of 640:5:1.
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SCHEME 1 RAFT equilibrium and analysis of leaving/reinitiating groups from CTA’s.
reactions with other functional groups within the polymer,
make reactions with high efficiency and selectivity necessary
for successful and specific polymer modification.
radical exchange method but using a ratio of 5:1 (azo:PHA)
to minimize the excess of azo compound. The addition of
azo-9 to a solution of the semitelechelic PHA was accompa-
nied by a rapid color change from pink to colorless, indicat-
ing removal of the dithiobenzoate moiety of the RAFT end
group. After purification, the extent of the reaction was
monitored by 1H NMR. For lower molecular weight poly-
mers, end group transformations can be conveniently fol-
lowed by NMR.
Telechelic Polymers with Hydroxyl End-Groups
To achieve the synthesis of a,x-hydroxyl end-functionalized
telechelic polymers we followed a two step synthetic path
(Scheme 2). In the first step either CTA-1 or CTA-2 in con-
junction with azo 9 were used for the polymerization of HA
to yield a semitelechelic poly(n-hexylacrylate) (PHA) with a
terminal AOH group. Use of the azo cyanopentanol free-radi-
cal initiator (azo 9) is crucial to guarantee the semitelechelic
AOH endgroups in PHA, since the initiator moiety and the R
leaving/re-initiating group in CTA-1 and CTA-2 contain both
a OH group. In the second step, the second AOH end-group
was introduced while simultaneously removing the
dithioester functional group following the Perrier et al.21
The large excess of azo compound used in the Perrier21
method is reported to be necessary to avoid termination by
disproportionation in the case of polystyrene. It should be
noted that the accuracy of the analysis of end group signals
was limited in some cases by the poor signals obtained in the
spectrum. In our experiments, no signals in the olefinic region
(between 5 and 6 ppm) of the 1H NMR spectrum, which
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 RAFT Polymerization of HA in 1,4-Dioxane in the Presence of CTA’s 1–7 and Azo Initiators 8 and 9 at 70 8C Using a Molar
Ratio [HA]0:[CTA]0:[AZO]0 of 320:5:1
Conv. (%)a
Mn, GPC
Mw/Mn
Mn, Calc.c
b
Entry
RAFT Agent
Time (h)
1
2
3
4
4
3120
6723
8195
1.33
1.06
1.021
664
7
29.6
50
3220
5257
12
4
5
6
7
9
12.4
25.4
36.4
54.4
4128
6042
9836
10,730
1.29
1.094
1.00
1.00
1570
2868
3966
5763
12
14
16
8
4
–
5103
5687
6915
9698
11,510
1.10
1.13
1.05
1.00
1.00
–
9
6
35.3
37.3
54.4
59.3
3803
4003
5710
6200
10
11
12
8
12
16
13
14
12
20
44
60
7332
9425
1.012
1.004
4681
6278
15
16
17
7
13.6
19
14,900
16,950
23,480
1.11
1.11
1.30
1570
2108
3712
9
16
35
18
19
20
6
16
42
56
4988
8369
8225
1.06
2027
4623
6021
10
12
1.028
1.012
21
22
12
20
46
56
7365
8637
1.049
1.020
4895
5893
a
Conversion calculated by mass of isolated polymer using methanol.
Molecular weight and PDI values determined by GPC.
Calculated molecular weights based on eq 1.
b
c
would be evidence for termination by disproportionation,
could be detected, even if we used a molar excess of azo-9 of
only 5:1. (Compare for this the NMR Spectra in Fig. 7).
band at 346 nm and weak absorption at 500 nm (Fig. 8);
which allows one to use UV–vis spectroscopy as a simple
procedure for identification of the presence of the dithioester
moiety in the resulting polymers. As a result of the radical
exchange process used, the absorption due to the dithioester
is absent, indicating that the radical exchange was complete
and that the second AOH functional group was introduced;
compare absorption spectra before and after radical
exchange reaction in Figure 8.
Furthermore, as can be seen in Figure 7 the aromatic signals
in the 1H NMR spectrum of the semitelechelic polyHA
between 7.4 and 8.0 ppm, disappeared after reaction with
the excess of azo initiator. This proves further the complete
removal of the thiocarbonyl thio end moiety from the poly-
meric chain; while the aromatic protons attributed to the
phenyl ring adjacent to the dithioester in the CTA could be
clearly identified close to 7.3 ppm [Fig. 7(a,b)] indicating the
presence of the RAFT end group.
Telechelic Polymers with Carboxylic Acid End-Groups
For synthesizing a,x-carboxylic acid end-functionalized tele-
chelic polymers we followed a two step synthetic path simi-
lar to that described in Scheme 2, but using a CTA and an
initiator containing the carboxylic acid moiety. In the first
step, CTA-3 in conjunction with azo 8 were used for the po-
lymerization of HA to yield a semitelechelic PHA with a ter-
minal ACOOH group. Use of CTA-3 in conjunction with the
azo cyanopentanoic acid free-radical initiator (azo 8) was
In addition to NMR, UV–vis spectroscopy was also used as a
simple procedure for the characterization of the resulting
polymers. The absorbance of the C¼¼S group in visible band
is a valuable tool to determine the amount of active
dithioester end groups when the semitelechelic polymers are
analyzed. The dithioester moiety has a strong absorption
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TABLE 4 Outcome of Polymerization of HA Using Freshly
Synthesized CTA’s 2, 4 and 6 and After a Year of Storage at
70 8C Using a Molar Ratio [HA]0:[CTA]0:[AZO]0 of 320:5:1
expansion of spectrum) while the aromatic signals of the
semitelechelic PHA between 7.4 and 8.0 ppm disappeared af-
ter the radical exchange reaction evidencing the removal of
thiocarbonyl thio moiety.
Mn (g molꢁ1)/PDI Mn (g molꢁ1)/
Heterotelechelic Polymers with Hydroxyl and
Freshly
PDI
Amino End-Groups
CTA
Synthesized
After a Year
To achieve the synthesis of telechelic polymers containing an
amino end-group, the functional initiator approach to synthe-
size amino telechelics causes some complications since
amine compounds participate in chain-transfer reactions.
Accordingly, the reaction between thiocarbonyl thio groups
and primary amines is well known to occur rapidly at ambi-
ent temperatures leading irreversibly to thioamides and thi-
ols.18 Therefore the introduction of amine end-groups to
polymers by RAFT is accomplished by using an amine-group
protected approach. For the preparation of polystyrene with
amine end-group a series of phthalimido-functional RAFT
agents (CTA’s) were developed and used followed by a
reduction of the tritiocarbonate moiety with tributylstannane
and hydrazinolysis to obtain amine end-group.11,39 Despite
the fact that the phthalimido strategy was proven to be suc-
cessful for polystyrene, the application of this method to
other type of polymers relies on adapting the kinetics of the
phthalimido-CTA to other monomers, which is reported to
yield in some cases bimodal distributions of molecular
weight as in the case of butyl acrylate.57 For the investiga-
tion reported here, we followed another strategy: the amine
group is protected by the well known t-BOC group40 and is
located in the CTA-6 (Fig. 3) in the leaving/re-initiating R
group of the CTA far apart from the sulfur atom of the
dithioester, affecting only slightly the RAFT kinetics. We pre-
pared amine-hydroxy end-functionalized hetero-telechelic
polymers of hexyl acrylate using the strategy described in
Scheme 3. In the first step CTA-6 in conjunction with azo 9
were used for the polymerization of HA to yield a semitele-
chelic PHA with a terminal ANHt-BOC group. Figure 10(a)
9469/1.001
8465/1.008
9698/1.007
10030/1.006
8369/1.020
8607/1.004
chosen to guarantee the same initiating and re-initiating R-
Group. Using this strategy it is ensured that all polymer
chains contain the same end-group with the carboxylic acid
moiety independently of RAFT polymerization kinetics. How-
ever, when an azoinitiator is used containing a different R-
group than the one evolving from the CTA it will results in a
minor fraction (depending on kinetics) of polymer chains
derived from the initiator and a major fraction of polymer
chains derived from the CTA’s leaving/re-initiating R-group.56
Figure 9 shows the 1H NMR spectrum of the semitelechelic
polymer (‘‘Telechelic Polymers with Hydroxyl End-Groups’’
section) and of the telechelic polymer after radical exchange
reaction (‘‘Telechelic Polymers with Carboxylic Acid End-
Groups’’ section). No signals in the olefinic region (between
1
shows H NMR spectrum of this semitelechelic polymer. It is
important to note the presence of the signals of t-BOC group
(proton 11 at 1.4 ppm) and the aromatic protons 16–18
between 7.2 ppm and 8.0 ppm. In the second step, the AOH
1
5 and 6 ppm) of the H NMR spectrum can be observed (see
SCHEME 2 Preparation of dihydroxy-terminated homotelechelic poly(n-hexyl acrylate).
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 (a) 1H NMR spectra
(500 MHz) of hydroxy-semitele-
chelic poly(n-hexyl acrylate) and
(b) dihydroxy-terminated tele-
chelic poly(n-hexyl acrylate).
FIGURE 8 UV/vis absorbance spectrum of CTA-2 (OH), semitelechelic polyHA (OH) before and after radical exchange reaction with
Azo-9 to yield telechelic polyHA (OH): (a) Region between 300 and 600 nm (in all cases c ¼ 1.0 ꢂ 10ꢁ4 mol/L). (b) Region between
ꢁ3
400 and 600 nm. (in all cases c ¼ 3.0 ꢂ 10
mol/L).
FIGURE 9 1H NMR spectra (200
MHz) of (a) carboxylic acid-sem-
itelechelic poly(n-hexyl acrylate)
and (b) dicarboxylic acid-termi-
nated telechelic poly(n-hexyl
acrylate).
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ARTICLE
SCHEME 3 Preparation of amine-hydroxy-terminated heterotelechelic poly(n-hexyl acrylate).
FIGURE 10 (a) 1H NMR spectra (200 MHz) of t-BOC protected amine semitelechelic poly(n-hexyl acrylate). (b) 1H NMR spectra of
t-BOC protected amine, hydroxy-telechelic poly(n-hexyl acrylate). (c) 1H NMR spectra of amine, hydroxy-terminated telechelic
poly(n-hexyl acrylate) prepared after t-BOC deprotection reaction with TFA.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 5 Telechelic Polymers with Different Functional End Groups Using Functional Azo Initiators
Semitelechelic Polymer
Telechelic Polymer
Mn ¼ 8131, PDI¼ 1.028
Mn ¼ 6771, PDI¼ 1.004
Mn ¼ 7954, PDI¼ 1.007
Mn ¼ 6414, PDI¼ 1.006
Mn ¼ 8708, PDI¼ 1.004
Mn ¼ 7967, PDI¼ 1.009
Mn ¼ 5030, PDI¼ 1.00
Mn ¼ 4900, PDI¼ 1.00
end-group was introduced following the radical exchange
strategy described for the other telechelics using azo 9 in
fivefold excess. Figure 10(b) shows H NMR spectrum of the
exchange reaction in a 5:1 ratio. In all cases the molecular
weight decreases slightly after the radical exchange reaction
in accordance with the substitution of the dithiobenzoate
moiety with a lower molecular weight radical from the azo
compound used. No increase in the PDI was observed.
1
telechelic product evidencing the removal of the dithio moi-
ety as can be seen in the aromatic region between 7.2 and
8.0 ppm. Aromatic protons at 8.0 ppm disappeared and the
other two aromatic signals (between 7.3 and 7.6 ppm)
became less complex. The final step is the removal of t-butyl
groups in the polymer (deprotection of amine) this was
accomplished by the use of CF3COOH in CH2Cl2. 1H NMR
spectrum in Figure 10(c) shows that the signal attributed to
t-butyl groups disappeared indicating the deprotection of the
amine end group.
Chemical Availability of End Groups in Telechelics
The presence of end groups by the synthetic strategy used is
demonstrated sufficiently. Further derivations using the a, x
bifunctional polymers can be achieved using classical syn-
thetic organic chemistry. Moreover, the synthetic strategy
used could serve as an efficient way to introduce functional
moieties to polymers for further bioconjugations or hydrogel
preparations.5,31 In this work, we show an esterification
reaction between the hydroxyl end groups of dihydroxy tele-
chelic polymers and methacrylic anhydride. The analysis of
In addition to the structural evidences by NMR in each step
of the synthetic strategy for the preparation of the different
telechelic polymers discussed, GPC analysis showed that no
significant change in molecular weight or polydispersity
resulted from the dithioester group removal through the rad-
ical exchange method (Table 5). This is an additional advant-
age of the radical exchange method for removing the
dithioester moiety as compared with other methods like ami-
nolysis12–16 or thermal treatment17-20,39 that may result in
polymer to polymer coupling increasing the polydispersity
and altering the molecular weight. Table 5 show results of
molecular weights (Mn) and polydispersity (PDI) of several
semitelechelic and telechelic polymers prepared in this work
with diverse end group functionalizations using the radical
1
the product by H NMR spectra allows identifying the signals
of alkene protons between 5 and 6 ppm of the methacrylate
(Fig. 11). The long reaction at room temperature was needed
to avoid methacrylate radical reaction and to allow complete
conversion of the hydroxyl groups.
CONCLUSIONS
We have described the synthesis and uses of two new func-
tionalized chain transfer agents (CTA’s) containing OH and
protected amine groups and demonstrated its usefulness to
control the synthesis of telechelic polymers from hexyl acry-
late. Good control over molecular weight distributions of the
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FIGURE 11 Introduction of alkene moieties into dihydroxy-telechelic poly(n-hexyl acrylate): (a) Reaction scheme, (b) 1H NMR spec-
trum (500 MHz) of dimethacrylate-telechelic poly(n-hexyl acrylate).
obtained polymers shows that these dithioesters possesses
similar reactivities and provided a narrow polydispersity as
compared to more known dithioester derivatives of 4-ciano
pentanoic acid and analogue CTA’s. The developed type of
CTA’s have the advantage of the longer shelf lifetime because
they are not prone to hydrolysis like the other type of CTA’s
used. Using a postpolymerization reaction with functional-
ized azocompounds a,x-telechelic polymers, with AOH or
ACOOH as functional group at the second end were obtained
adapting a methodology proposed initially by Perrier et al.
but using only a fivefold excess of azo compounds instead of
20-fold excess as suggested in his report. This is important
to save precious functionalized azocompound in this applica-
tion. It was demonstrated that the strategy using only a five-
fold excess of CTA was effective to obtain homotelechelic
poly(n-hexylacrylate) with end functional OH and COOH
groups and also to obtain heterotelechelic poly(n-hexylacry-
late) with end functional OH and NH2 groups. It was also
demonstrated that the telechelic polymers prepared can be
further derivatized to introduce other functionalities,
like alkene groups as shown in a reaction of telechelic
dihydroxy-poly(n-hexylacrylate) with methacrylic anhydride
opening the door for the versatile thiol-ene chemistry. The
developed CTA’s are potentially useful for the preparation of
other telechelic polymers that is, other acrylates, methacry-
lates, and styrenes.
The authors acknowledge the guidance and support by C.J.
Hawker for the first synthesis of CTA-2, performed by R.S.-R.
during an appointment as Postdoctoral fellow at the Center on
Polymer Interfases and Macromolecular Assemblies (CPIMA).
This investigation was supported by the National Council of
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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