Straightforward preparation of telechelic h-bonding polymers from difunctional trithiocarbonates and supramolecular block copolymers thereof

Article abstract of DOI:10.1021/ma200540u

We report an original strategy toward the straightforward preparation of precisely defined telechelic H-bonding polymers and the generation of supramolecular block copolymers thereof. Making use of an α,ω- functionalized symmetrical trithiocarbonate beari

Full text of DOI:10.1021/ma200540u

ARTICLE
pubs.acs.org/Macromolecules
Straightforward Preparation of Telechelic H-bonding Polymers From
Difunctional Trithiocarbonates and Supramolecular Block
Copolymers Thereof
Arthur Bertrand,†,‡,§ Senbin Chen,†,‡,§ Grꢀegoire Souharce,†,‡,§ Catherine Ladaviꢁere,†,‡,§ Etienne Fleury,
and Julien Bernard*,†,‡,§
†Universitꢀe de Lyon, Lyon, F-69003, France
‡INSA-Lyon, IMP, Villeurbanne, F-69621, France
§CNRS, UMR 5223, Ingꢀenierie des Matꢀeriaux Polymꢁeres, Villeurbanne, F-69621, France
S Supporting Information
b
ABSTRACT: We report an original strategy toward the straight-
forward preparation of precisely defined telechelic H-bonding
polymers and the generation of supramolecular block copoly-
mers thereof. Making use of an R,ω-functionalized symmetrical
trithiocarbonate bearing thymine groups at both chain ends, a
series of heterocomplementary H-bonding polymers were effort-
lessly generated by RAFT polymerization in a one step process.
The resulting telechelic macromolecules selectively interacted
with heterocomplementary R-DAP-functionalized chains to
afford supramolecular block copolymers in solution and in bulk.
These self-assemblies were evidenced by 1H NMR and rheological measurements. As a consequence of these associations, whereas
non functional homopolymer blends tended to microphase separate as observed by AFM analysis, H-bonding homopolymer blends
exhibited homogeneous microstructures in accord with the formation of supramolecular block copolymers promoting the
stabilization of the interface between the polymers.
’ INTRODUCTION
is the difficulty of specifically and quantitatively introducing
H-bonding moieties at both sides of a polymer chain. A generic
approach toward R,ω-functionalized macromolecules relies on the
postpolymerization functionalization of telechelic chains as pre-
viously reported by Binder, Meijer, Sijbesma, or Gong.5 Unfortu-
nately, this strategy usually suffers from incomplete yields and is
therefore limited to low molecular weight polymer chains.
Alternatively, H-bonding stickers can be straightforwardly
incorporated at the chain ends in the course of the polymeriza-
tion process through the use of functionalized initiators, chain
transfer agents or chain terminators.6 Yet, the huge majority of
the examples in the literature deal with the preparation of well-
defined R or ω-functionalized macromolecules. A major step
forward can be assigned to Weck and co-workers that recently
reported the first one-pot synthesis of homotelechelic and
heterotelechelic macromolecules from cyclooctene or norbor-
nene derivatives via the combination of ring-opening metathesis
polymerization (ROMP) with functional chain transfer agents
(CTA) and the supramolecular block copolymers thereof.7 One
restriction of this elegant strategy lies though in the use of the
Over the last few decades, our scientific community has
successfully extended the concept of self-assembling noncovalent
interactions to the field of polymer materials.1 In particular, the
development of hydrogen bonding recognition units by Lehn
together with others has paved the way to the design of supra-
molecular polymeric assemblies possessing a stimuli-responsive
nature and self-healing abilities.2 In analogy to covalent poly-
merizations, significant efforts have indeed gone into the devel-
opment of homoditopic self-complementary or heterocom-
plementary monomers (AꢀA or AꢀA/BꢀB), heteroditopic
heterocomplementary monomers (AB) or oligotopic monomers
(AB2, Am, m > 2) respectively capable to self-organize to give
birth to (AꢀA)n, (AꢀAꢀBꢀB)n, (AꢀB)n linear supramolecular
polymers and supramolecular hyperbranched polymers and
networks.3
In order to yield materials with valuable macroscopic proper-
ties, the preparation of tailor-made macromolecular counterparts
(macromonomers), that is to say, linear polymer chains bearing,
at their both extremities, H-bonding motifs promoting the
reversible formation of original supramolecular edifices such as
supramolecular homopolymers, block or multiblock copolymers
and cross-linked architectures is also highly desirable.4 However,
a bottleneck to expanding the scope of this new class of materials
Received: March 9, 2011
Revised:
April 13, 2011
Published: April 29, 2011
r
2011 American Chemical Society
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Scheme 1. General Strategy for the Synthesis of Supramolecular Block Copolymers
ROMP technique which is strictly limited to the polymerization
of strained monomers and in the multistep synthesis of the CTAs
and initiators.7,8
at 70 °C (flow rate: 0.7 mL.minꢀ1) and equipped with a Viscotek VE
1121 automatic injector, three columns (Waters HR2, HR1 and HR0.5),
and a differential refractive index detector (Viscotek VE3580). The
average molar masses of PnBuA, PS, PVAc, and PI were derived from a
calibration curve based on a series of PS standards ranging from 500 to
In this context, the RAFT process (reversible additionꢀ
fragmentation chain transfer), which is arguably the most
versatile controlled/living free radical polymerization technique,
with respect to monomer choice (vinyl esters, (meth)acrylates,
(meth)acrylamides, styrenics, isoprene, ...) and experimental
conditions is of particular interest. Indeed, the RAFT process
has been extremely valuable to the preparation of end-functional
polymers.6iꢀk,9 In the specific case of R,ω-functional trithiocar-
bonates possessing a Z group having a CꢀS cleavable bond,10 the
RAFT process gives the quite unique opportunity to grow well-
defined telechelic polymer chains in one step.
As part of our continuing studies on H-bonding RAFT-made
polymers, we report in the present contribution a new and simple
approach to the straightforward preparation of precisely defined
telechelic H-bonding polymers by RAFT polymerization and the
generation of supramolecular block copolymers thereof (see
Scheme 1). Herein we demonstrate the effectiveness of the
present strategy in synthesizing R,ω-thymine functionalized
telechelic polystyrenes, poly(n-butyl acrylate)s and polyiso-
prenes in a very simple manner from functional trithiocarbonates
and further investigate their subsequent self-assembly into
supramolecular block copolymers with (RAFT-made) macro-
molecular blocks bearing complementary 2,6-diamidopyridine
(DAP) hydrogen-bonding motifs.
50000 g molꢀ1
.
3
All MALDIꢀTOF mass spectra were obtained with a Voyager-DE
PRO (Applied Biosystems, Framingham, MA) equipped with a nitrogen
laser emitting at 337 nm with a 3 ns pulse duration. The instrument was
operated in linear or reflectron modes. The ions were accelerated under
a potential of 20 kV. The positive ions were detected in all cases. The
spectra were the sum of 300 shots and an external mass calibration of
mass analyzer was used (a mixture of peptides, Sequazyme, Applied
Biosystems, Framingham, MA). Samples were prepared by mixing 45 μL
of 1,8,9-anthracenetriol (dithranol, purchased by Sigma-Aldrich) at
10 g Lꢀ1 in THF with 5 μL of PnBuA solution (at 10 g Lꢀ1 in THF).
3
3
To enhance cationization of polymers, 5 μL of NaI (from Sigma-Aldrich,
at 10 g Lꢀ1 in acetone) were added to solutions. Finally, resulting
3
mixtures (0.5 μL) were spotted on the MALDI sample plate and air-dried.
Dynamic rheological measurements were performed on a Physica
MC301 (Anton Paar GmbH) stress controlled rheometer with 25 mm
cone and plate geometry. The temperature was adjusted by a Peltier
plate. Samples for the rheological measurements in solution were
prepared by dissolving selected polymers in CHCl3.
In order to perform atomic force microscopy measurements, silicon
substrates (approximately 1 ꢁ 1 cm2) were cleaned by ozonolysis
treatment. Samples were prepared by spin-coating from toluene solu-
tions (2800 rpm, 30 s), followed by drying during 72 h under vacuum.
AFM images were acquired in air at room temperature using a Nano-
scope IIIa Multimode (Digital Instruments/VEECO, CA). Intermittent
contact imaging (i.e., tapping mode) was performed at a scan rate of
1 Hz. Uncoated silicon probes with a resonant frequency between 280
and 405 kHz, a spring constant 20 and 80 N/m, length of 115ꢀ135 μm,
width of 30ꢀ40 μm and nominal tip radius of curvature less than 10 nm
are used. Images were displayed and analyzed using the Nanoscope
6.14R1 software.
’ EXPERIMENTAL SECTION
Materials. All reagents were purchased from Aldrich. All monomers
were filtered prior to use by passing through a column of basic aluminum
oxide to remove inhibitors. Unless otherwise stated, the other chemicals
were used without further purification. S,S0-Bis(R,R0-dimethyl-R00-acetic
acid)trithiocarbonate was synthesized according to a protocol found in
the literature.10 1-(ω-hydroxyundecyl)thymine was prepared as pre-
viously reported from 11-bromo-1-undecanol and thymine.6i Dithio-
benzoic acid was synthesized as previously reported.11 Unless otherwise
indicated the other chemicals were used without further purification.
Characterization Methods. 1H and 13C NMR spectra were
recorded on a Bruker AVANCE250 (250 MHz) or AVANCE400
(400 MHz) spectrometer using CDCl3 or CD2Cl2. ESI-MS spectra
(m/z) of CTAs and initiator were measured on a Thermo-FinniganMat
95XL. R,ω-thymine-functionalized PnBuA, PS and PI and 2,6-diamido-
pyridine-functionalized PS, PVAc and PnBuA were analyzed with a SEC
apparatus running in THF at 25 °C (flow rate: 1 mL.minꢀ1) or in DMF
Synthesis of CTA1. S,S0-Bis(R,R0-dimethyl-R00-acetic acid) trithio-
carbonate (0.51 g, 1.81 mmol), 1-(ω-hydroxyundecyl)thymine (1.18 g,
3.98 mmol), and triphenylphosphine (PPh3) (1.04 g, 3.97 mmol) were
dissolved in distilled THF in a three-neck round-bottom flask cooled
with an ice bath under argon. Diisopropyl azodicarboxylate (DIAD)
(0.81 g, 4.01 mmol) diluted in distilled THF was then added dropwise.
The solution was allowed to reach room temperature and stirred
overnight. The solution was then heated at 40 °C during 3 h. The
reaction mixture was subsequently cooled to room temperature, diluted
with dichloromethane (150 mL) and washed with distilled water (2 ꢁ
200 mL). The organic layer was collected and dried with MgSO4. The
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solvent was removed under reduced pressure. The crude product, a
yellow oil, was then purified by column chromatography, using dichlor-
omethane:ethanol (30:1) as eluent. Pure fractions were combined and
dried with MgSO4. The solvent was removed under reduced pressure
and the resulting yellow oil was placed in a fridge at 0 °C whereupon
it slowly crystallized (0.46 g, 29%). ESIꢀMS: calculated for
(C41H66N4O8S3þNa)þ: 862.2. Found: 861.2. 1H NMR (CDCl3, δ, ppm):
1.27 (br, ꢀ(CH2)9ꢀ, 36H), 1.65 (s, ꢀSꢀC(CH3)2ꢀCO, 12H), 1.92
(s, CH3ꢀCdCꢀ, 6H), 3.68 (t, ꢀNꢀCH2ꢀ, 4H), 4.06 (t,
ꢀCOOꢀCH2ꢀ, 4H), 6.98 (s, CH3ꢀCdCHꢀ, 2H), 8.59 (s, NH,
2H). 13C NMR (CDCl3, δ, ppm): 12.30, 25.15, 25.89, 26.41, 28.32,
29.07, 29.16, 29.38, 29.41, 48.51, 56.07, 66.14, 110.47, 140.48, 151.11,
164.63, 172.73, 218.25.
Synthesis of CTA2. CTA2 was synthesized in three steps from 2,6-
diaminopyridine (DAP). DAP (4.4 g, 40 mmol) and triethylamine (4.2 g,
40 mmol) were dissolved in dry THF (180 mL), and the solution was
cooled to 0 °C in an ice bath. A solution of butyryl chloride (4.2 g, 38
mmol) in THF (20 mL) was added dropwise over a period of 1 h, and
the reaction was allowed to proceed at 0 °C for another 3 h, before
warming to room temperature. The reaction mixture was filtered,
evaporated to dryness, and purified by column chromatography using
ethyl acetate:cyclohexane (3:2) as eluent. 1 was obtained as a white
powder (2.45 g, 51%).
To a solution of 1 (1 g, 5.2 mmol) and triethylamine (0.74 g,
7.3 mmol) in dichloromethane (100 mL) was slowly added 2-bromo-
propionylbromide (1.57 g, 7.3 mmol). The solution was stirred for 12 h,
the solvent removed under reduced pressure and the resulting orange
solid dissolved in ethyl acetate (100 mL) and successively extracted with
brine (150 mL), saturated sodium bicarbonate (100 mL), and 0.1 M
HCl (75 mL). The organic layer was collected and dried using MgSO4,
and the residue was purified by flash column chromatography on silica
(1:1 cyclohexane:ethyl acetate). The resulting white solid (2) was
vacuum-dried overnight (0.30 g, 18%).
Dithiobenzoic acid (15.60 g, 0.10 mol) was added to 2 (6.61 g, 0.02
mol) dissolved in THF (300 mL) and the reaction was stirred at 60 °C
for 15 h. The reaction solution was cooled to room temperature, washed
with brine, and then the organic layer was collected and dried using
MgSO4. The residue was purified by flash column chromatography on
silica (3:1 cyclohexane:ethyl acetate) to remove unreacted dithiobenzoic
acid and repurified by recrystallization (3:1 cyclohexane:ethyl acetate).
CTA2 was obtained as an orange solid (3.11 g, 40%). ESIꢀMS:
calculated for (C19H21N3O2S2 þ Na)þ: 410.5. Found: 410.0. 1H NMR
(CDCl3, δ, ppm): 0.99 (t, ꢀCH2ꢀCH3, 3H), 1.73 (d, ꢀSꢀCHꢀCH3,
3H), 1.75 (m, CH3ꢀCH2ꢀ, 2H), 2.34 (t, ꢀ(OdC)ꢀCH2ꢀ, 2H), 4.92
(q, ꢀSꢀCHꢀCH3ꢀ, 1H), 7.2ꢀ7.7 (t, 3,4,5-PyH, 3H), 7.63 (t, 4-PyH,
1H), 7.85 (d, 2,6-PhH, 2H), 7.90 (d, 3,5-PyH, 2H), 8.61 (s, NH, 2H). 13C
(CDCl3 δ, ppm): 13.12, 19.04, 38.30, 47.72, 11.78, 128.24, 130.87,
141.42, 147.72, 169.35, 172.23, 222.52.
Synthesis of CTA4. S,S0-Bis(R,R0-dimethyl-R00-acetic acid)trithiocar-
bonate (1 g, 3.54 mmol) and triphenylphosphine (PPh3) (2.05 g,
7.81 mmol) were dissolved in anhydrous butanol (50 mL) in a three
neck round-bottom flask cooled to 0 °C under argon. Diisopropyl
azodicarboxylate (DIAD) (1.60 g, 7.91 mmol) diluted in anhydrous
butanol (10 mL) was added dropwise. The solution was allowed to reach
room temperature and stirred overnight. The solution was then heated
at 40 °C during 3 h. The reaction mixture was cooled to room
temperature, diluted with dichloromethane (150 mL) and washed with
distilled water (2 ꢁ 250 mL). The organic layer was collected and dried
with MgSO4. The solvent was removed under reduced pressure. The
crude product, a yellow oil, was then purified by column chromatogra-
phy, using cyclohexane:ethanol (30:1) as eluent. Pure fractions were
combined, dried under MgSO4, and evaporated to dryness, giving CTA4
as a yellow oil (850 mg, 57%). ESIꢀMS: calculated for (C17H30O4S3 þ
Na)þ: 417.6. Found: 417.0. 1H NMR (CDCl3, δ, ppm): 0.92 (t,
CH3ꢀCH2ꢀ, 6H), 1.66ꢀ1.32 (m, CH3ꢀ(CH2)2ꢀCH2ꢀOꢀ, ꢀSꢀC
(CH3)2ꢀCO, 20H), 4.08 (t, ꢀCH2ꢀOꢀCdOꢀ, 4H). 13C NMR
(CDCl3, δ, ppm): 12.39, 18.19, 24.15, 29.42, 55.06, 64.86, 171.76,
217.27.
Synthesis of IThy. 4,40-Azobis(4-cyanopentanoic acid) (ACPA)
(0.46 g, 1.64 mmol), triphenylphosphine (0.94 g, 3.58 mmol) and
1-(ω-hydroxyundecyl)thymine (1.06 g, 3.58 mmol), were dissolved in
distilled THF, in a round-bottom flask cooled to 0 °C, under argon.
DIAD (0.72 g, 3.58 mmol) diluted in distilled THF was then added
dropwise. The solution was allowed to reach room temperature and
stirred for 1 day. The solution was diluted with dichloromethane
(150 mL) and washed with distilled water (2 ꢁ 200 mL). The organic
layer was collected and dried with MgSO4. The solvent was removed
under reduced pressure. 2/3 of the crude product were then purified by
column chromatography, using dichloromethane:ethanol (30:1) as
eluent. Pure fractions were combined, dried with MgSO4. IThy was
obtained as a white solid (72.5 mg, 8%). ESIꢀMS: calculated for
(C44H68N8O8 þ Na)þ: 860.0. Found: 859.1. 1H NMR (CDCl3, δ, ppm):
1.27 (br, ꢀ(CH2)9ꢀ, 36H), 1.62 (t,ꢀCH2ꢀCH2ꢀCOOꢀ, 4H), 1.72
(s, CH3ꢀCꢀCtN, 6H), 1.92 (s, CH3ꢀCdCꢀ, 6H), 2.47 (t, ꢀCH2ꢀ
COOꢀ, 4H), 3.67 (t, ꢀNꢀCH2ꢀ, 4H), 4.08 (t, ꢀCOOꢀCH2ꢀ, 4H),
6.98 (s, CH3ꢀCdCHꢀ, 2H), 9.29 (s, NH, 2H). 13C NMR (CDCl3, δ,
ppm): 12.30, 25.15, 25.89, 26.41, 28.32, 29.07, 29.16, 29.38, 29.41, 48.51,
56.07, 66.14, 110.50, 117.50, 140.43, 150.97, 164.41, 171.38.
General Procedure for Bulk Polymerization of n-Butyl
Acrylate (nBuA). Bulk polymerization of nBuA was carried out using
CTA1, CTA2, or CTA4 as RAFT agent, and IThy or AIBN as initiator.
For instance, CTA1 (90.9 mg, 1.1 ꢁ 10ꢀ4 mol), IThy (4.71 mg, 5.6 ꢁ
10ꢀ6 mol) and trioxane (78.5 mg, 8.7 ꢁ 10ꢀ4 mol, internal reference)
were dissolved in n-butyl acrylate (1.5 mL, 1.1 ꢁ 10ꢀ2 mol) and
transferred in a Schlenk tube, which was deoxygenated by five free-
zeꢀpumpꢀthaw cycles. The Schlenk tube was immerged in an oil bath
at 70 °C. The reaction was stopped by plunging the tube into liquid
nitrogen. Conversion was estimated by analyzing an aliquot of the
obtained mixture by 1H NMR, weighting the integral of vinyl protons of
the monomer (5.7ꢀ6.5 ppm) and the integral of trioxane (5.15 ppm).
The polymer was precipitated twice in methanol:water (1:1), and dried
under vacuum during 24 h. The molecular weight of the pure PnBuA was
finally determined by 1H NMR in CDCl3, from integration oftheprotons
of the polymer chains (ꢀCOOꢀCH2ꢀ, 2nH, δ = 4.03 ppm,
with n being the degree of polymerization) and of protons of the
thymine end-group (ꢀCdCHꢀN, 2H, δ = 6.97 ppm).
Synthesis of CTA3. 2 (0.60 g, 1.91 mmol) and O-ethyl xanthic acid
potassiumsalt (0.92 g, 5.73 mmol) were dissolvedin dryacetone (100 mL)
and the reaction was stirred overnight at room temperature. The white
precipitate (KBr) was filtered off. The solution was concentrated under
reduced pressure, and then precipitated twice in dichloromethane in order
to remove the excess of O-ethyl xanthic acid potassium salt. Finally, the
crude product was purified by column chromatography (9:1, dichloro-
methane:ethyl acetate). CTA3 was obtained as a slightly yellow paste (0.20
g, 30%). ESIꢀMS: calculated for (C15H21N3O3S2 þ Na)þ: 378.5. Found:
1
378.0. H NMR (CDCl3, δ, ppm): 0.98 (t, CH3ꢀCH2ꢀCH2ꢀCdO,
3H), 1.40 (t, CH3ꢀCH2ꢀOꢀ, 3H), 1.61 (d, ꢀSꢀCHꢀCH3, 3H), 1.73
(m, CH3ꢀCH2ꢀCH2ꢀCdO, 2H), 2.34 (t, CH3ꢀCH2ꢀCH2ꢀCdO,
2H), 4.47 (q, CH3ꢀCH2ꢀOꢀ, 2H), 4.64 (q, ꢀSꢀCHꢀCH3, 1H),
7.62ꢀ7.95 (m, PyH, 3H), 7.77 (s, NH, 1H), 8.55 (s, NH, 1H). 13C NMR
(CDCl3, δ, ppm): 13.79, 13.81, 16.26, 18.87, 39.73, 48.69, 71.26, 109.50,
109.85, 140.88, 149.23, 149.72, 169.42, 171.63, 213.35.
General Procedure for Bulk Polymerization of Styrene.
Bulk polymerization of styrene was carried out using CTA1, CTA2, or
CTA4 as RAFT agent, and IThy or AIBN as initiator. For instance,
CTA1 (40 mg, 4.8 ꢁ 10ꢀ5 mol), IThy (1.03 mg, 1.2 ꢁ 10ꢀ6 mol), and
trioxane (287 mg, 3.2 ꢁ 10ꢀ3 mol) were dissolved in styrene (2.2 mL,
1.9 ꢁ 10ꢀ2 mol), and then transferred in a Schlenk tube, which was
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Scheme 2. Synthesis of Thymine and DAP-functionalized CTAs (CTA1-3), and non-functionalized CTA (CTA4), and Thymine-
Functionalized Initiator (IThy)a
a Key: (a) PPh3, DIAD, CH2Cl2; (b)NEt3, CH2Cl2; (c) THF; (d) acetone.
deoxygenated by five freezeꢀpumpꢀthaw cycles. The Schlenk tube was
immersed in an oil bath at 80 °C. The reaction was stopped by plunging
the tube into liquid nitrogen. Conversion was estimated by analyzing an
aliquot of the obtained mixture by 1H NMR, weighting the integral of
vinyl protons of the monomer (5.7ꢀ6.5 ppm) and the integral of
trioxane (5.15 ppm). The polymer was isolated by precipitation in
methanol (2ꢁ) and dried under vacuum during 24 h. The molecular
weight of the pure PS was finally determined by 1H NMR in CD2Cl2,
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Table 1. Characteristics of the Thy and DAP-Functionalized Polymers Generated by RAFT Polymerization of Styrene, nBuA,
Isoprene, and VAc Mediated by CTA1, CTA2 or CTA3
sample
CTA
[M]/[CTA]
conva %
Mn,tha (g/mol)
Mn,NMRa (g/mol)
Mn,SECb (g/mol)
PDI
Thy-PnBuA-Thy1
Thy-PnBuA-Thy2
Thy-PnBuA-Thy3
Thy-PI-Thy1
Thy-PI-Thy2
Thy-PS-Thy1
Thy-PS-Thy2
Thy-PS-Thy3
Thy-PS-Thy4
PnBuA-DAP1
PnBuA-DAP2
PS-DAP1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
100
100
250
500
500
70
48
70
6
7000
9800
8000
10 000
2800
8700
11 000
3300
1.15
1.23
1.25
1.37
1.52
1.07
1.29
1.28
1.22
1.32
1.19
1.25
1.17
1.26
1.27
1.38
1.45
1.43
2700
21
29
20
35
21
24
29
28
7
8000
9300
12 000
14 000
3400
11 000
2300
12 400
2800
200
400
400
100
300
400
400
400
400
250
250
250
8000
14 400
15 600
14 600
4900
11 000
10 500
10 500
4800
9600
10 800
4100
11 100
4000
11 800
3000
10 700
5200
PS-DAP2
10
11
20
35
66
80
4700
3200
4000
PS-DAP3
4900
4000
5100
PS-DAP4
9000
8300
9400
PVAc-DAP1
PVAc-DAP2
PVAc-DAP3
7800
7200
9200
14 600
15 900
17 100
16 000
20 000
17 500
a Calculated by 1H NMR. b Evaluated by SEC in THF using polystyrene standards.
from integration of the aromatic protons of the polymer chains (Ar, 5nH,
δ = 6.4ꢀ7.4 ppm, with n being the degree of polymerization) and of
protons of the chain ends (ꢀNꢀCH2ꢀ, 4H, δ = 3.68 ppm).
’ RESULTS AND DISCUSSION
Synthesis of H-Bonding CTAs. The versatility and the
tolerance of the RAFT process in terms of functionality and
polymerization conditions facilitate the preparation of polymers
possessing H-bonding motif. To this end, three original H-bond-
ing CTAs were designed (see Scheme 2).
General Procedure for Solution Polymerization of Iso-
prene. Solution polymerization of isoprene was carried out using
CTA1, or CTA2 as RAFT agent, and dicumyl peroxide (DCP) as
initiator. For instance CTA1 (34.5 mg, 4.1 ꢁ 10ꢀ5 mol), DCP (0.55 mg,
2.0 ꢁ 10ꢀ6 mol) and isoprene (2.1 mL, 2.1 ꢁ 10ꢀ2 mol) were dissolved
in toluene (2.5 mL), and then transferred in a Schlenk tube, which was
deoxygenated by five freezeꢀpumpꢀthaw cycles. The Schlenk tube was
immersed in an oil bath at 120 °C. The reaction was stopped by plunging
the tube into liquid nitrogen. The mixture was then concentrated under
reduced pressure. Conversion was estimatedgravimetrically. The polymer
was isolated by precipitation in methanol (2ꢁ) and dried under vacuum
for 24 h. The molecular weight of the pure PI was finally evaluated by 1H
NMR (CDCl3) from relative integration of the protons of the PI
backbone (5.6ꢀ5.9 ppm for the ꢀCHdCH2 from the 1.2-addition
polymerization, 5.0ꢀ5.5 ppm for ꢀCHdC(CH3)ꢀ for the 1,4-addition
polymerization, and 4.4ꢀ5.0 ppm for the mixture of ꢀCHdCH2 (1.2-
addition) and ꢀC(CH3)dCH2 (3,4-addition)) and of characteristic
protons of the thymine end group ((CdCHꢀN, 1H, δ = 6.97 ppm).
General Procedure for Bulk Polymerization of Vinyl Acet-
ate (VAc). Bulk polymerization of styrene was carried out using CTA3
as RAFT agent, and AIBN as initiator. For instance, CTA3 (42 mg, 1.2 ꢁ
10ꢀ4 mol), AIBN(1.82 mg, 1.1 ꢁ 10ꢀ5 mol), andtrioxane (160 mg, 1.8ꢁ
10ꢀ3 mol) were dissolved in VAc (2.6 mL, 2.8 ꢁ 10ꢀ2 mol), and then
transferred in a Schlenk tube, which was deoxygenated by five free-
zeꢀpumpꢀthaw cycles. The Schlenk tube was immersed in an oil bath
at 80 °C. The reaction was stopped by plunging the tube into liquid
nitrogen. The polymer was precipitated twice into pentane to remove
unreacted monomer. Conversion was estimated by analyzing an aliquot
A symmetrical trithiocarbonate bearing a thymine moiety at
each extremity (CTA1) and thus capable to grow telechelic
H-bonding polymers in a one step process was targeted. We first
unsuccessfully attempted to adapt the procedure developed by
Leung et al.12 with disodium trithiocarbonate and a thymine
functionalized secondary alkyl halide under phase transfer con-
ditions. The introduction of the thymine moieties was then
undertaken through esterification reactions between preformed
trithiocarbonates and 1-(ω-hydroxyundecyl)thymine. Whereas
the reaction involving the diacid chloride functionalized CTA
and 1-(ω-hydroxyundecyl)thymine surprisingly failed at gener-
ating the expected compound, the coupling between 1-(ω-
hydroxyundecyl)thymine and the diacid functionalized CTA pro-
ceeded smoothly under Mitsunobu conditions to generate
CTA1. The pure product was finally isolated in moderate yield
(30%) owing to difficult separation from thymine mono-
functionalized CTA.
Aiming at investigating the association of R,ω-thymine func-
tionalized polymers with blocks bearing heterocomplementary
recognition unit, a dithiobenzoate (CTA2) and a xanthate
(CTA3) bearing a DAP moiety were also prepared with mod-
erate yields (30ꢀ40%) by coupling the alkylating agent 2
respectively with dithiobenzoic acid or O-ethyl xanthic acid
1
1
potassium salt. The H NMR spectra for the resulting sticker-
of the obtained mixture by H NMR, weighting the integral of vinyl
functionalized CTAs are given in Supporting Information.
Prior to undertaking any polymerization reaction, the capability
of CTA1 and CTA2 or CTA3 to associate through hydrogen
bonding interactions was examined by 1H NMR analysis in CDCl3
at 25 °C. The association of the heterocomplementary stickers was
protons of the monomer (5.7ꢀ6.5 ppm) and the integral of trioxane
(5.15 ppm). The molecular weight of the pure PVAc was finally
evaluated by 1H NMR (CDCl3) from relative integration of the protons
of the PVAc backbone (4.84 ppm, ꢀCH2ꢀCHꢀOꢀCdO) and of
protons of the chain end (7.5ꢀ8 ppm, PyH).
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Figure 1. (A) First-order kinetic plots and (B) evolution of the SEC traces for the bulk polymerization of styrene at 80 °C mediated by CTA1 using
IThy as initiator: [M]/[CTA] = 400, [CTA][Initiator] = 80. (C) First-order kinetic plots and (D) evolution of the SEC traces for the bulk
polymerization of nBuA at 70 °C mediated by CTA1 using IThy as initiator: [M]/[CTA] = 250, [CTA][Initiator] = 20. (E) First-order kinetic plots and
(F) evolution of the SEC traces for the polymerization of isoprene at 120 °C in toluene mediated by CTA1 using DCP as initiator: [M]/[CTA] = 500,
[CTA][Initiator] = 20.
undoubtedly evidenced by the downfield shift of the ꢀNHꢀ
thymine proton of CTA1 observed after addition of 1 equiv of
heterocomplementary CTA2/CTA3 ([Thy] = [DAP] = 2.5 ꢁ
10ꢀ2 mol) (from 8.74 to 10.71 ppm for CTA2, from 8.74 to 10.68
ppm for CTA3). Using titration experiments and a relevant
mathematical model,13 a binding constant consistent with reported
values (110 Mꢀ1) was determined (see Supporting Information).
Synthesis of Macromolecular Building Blocks. The RAFT
polymerizations were carried out at adequate [CTA]/[initiator]
ratio (typically 20/1, 40/1, or 80/1) to drastically limit the
number of nonfunctionalized or monofunctionalized chains
while maintaining reasonable polymerization rates. In addition,
R,ω-thymine functionalized trithiocarbonate mediated RAFT
polymerizations of nBuA and styrene were carried out in the
presence of a functionalized initiator (IThy) specifically designed
from ACPA initiator to exclusively release initiating radicals
possessing thymine moieties.
As shown in Table 1 and Figure 1, bulk polymerizations of
styrene and n-butyl acrylate in the presence of CTA1 resulted in a
good control. The telechelic polymers grew linearly with mono-
mer conversion, and the polydispersity indices remained below
1.3. Experimental molecular weights were in very good agree-
ment with those expected confirming that the number of chains is
dictated by the CTA concentration. Moreover a linear relation-
ship between ln(1/[1 ꢀ x]) vs time was observed indicating that
the concentration of propagating species remains constant
throughout the polymerization. Because of the poor solubility
of R,ω-thymine functionalized trithiocarbonate in the monomer,
the polymerization of isoprene was investigated in solution
(toluene). The polymerizations were carried out at 120 °C in
the presence of dicumyl peroxide. In contrast to styrene and
n-butyl acrylate, CTA1-mediated polymerization of isoprene
proceeded in a less controlled fashion (possibly due to chainꢀ
chain coupling) leading to polyisoprene possessing broader
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molecular weight distribution (PDI = 1.3ꢀ1.6). These results are
consistent with those of Jitchum and Perrier,14 who synthesized
polyisoprenes of similar molar mass in the presence of trithio-
carbonates. To demonstrate the full utility of this new H-bonding
CTA, we further investigated the ability to generate triblock
copolymers from the thymine functionalized homopolymers. As
an illustration, we intended to prepare PnBuA-b-PS-b-PnBuA
from a PnBuA macroRAFT agent. As can be seen in Figure 2,
the chain extension of PnBuA macroRAFT agent (Mn
=
SEC
15600 g molꢀ1, PDI = 1.13, in DMF) with styrene resulted in a
3
clear shift of the SEC peak toward higher molecular weight
region (Mn = 27800 g molꢀ1, Mn
= 34000 g molꢀ1
,
th
RMN
3
3
MnSEC = 28200 g molꢀ1, PDI = 1.19 in DMF) consistent with
3
the growth of the polystyrene blocks. Moreover, the absence of
low molecular weight tailing ascertained that nearly all the
PnBuA chains participate to the (co)polymerization process
underlining the excellent efficiency of the thymine functionalized
PnBuA macroRAFT agent.
The effective incorporation of the thymine functionalities at
both chain ends of the homopolymer was assessed by 1H NMR
analysis for the PnBuA and PI backbones, with the appearance of
characteristic thymine peaks at 6.97 and 8.03 ppm (relative to
ꢀCdCHꢀNꢀ and NH, respectively) and the excellent accord
1
between the experimental molecular weights calculated by H
NMR (from relative integration of polymer backbone protons
and the thymine protons) and theoretical values. At this point, it
is important to note that the functionalization of the polystyrene
chains could not be precisely determined in a similar manner due
to peak overlapping with the aromatic protons of the PS back-
bone. However, the presence of thymine moieties was evidenced
by 13C NMR (thymine peaks at 12.9, 110.8, 139.7, 152.0, 164.2).
A structural investigation was also achieved by mass spectro-
metry to confirm the presence of thymine end-groups in the
Figure 2. Overlaid SEC traces of (a) PnBuA macroRAFT agent and (b)
PS-PnBuA-PS triblock copolymer (using DMF as eluent).
Figure 3. Positive ion MALDIꢀTOF mass spectrum of Thy-PnBuA-Thy3 (dithranol matrix, NaI salt, linear mode), and enlargements of mass spectra
acquired in linear and reflectron modes (same m/z scale). Upper inset: MALDIꢀTOF mass spectrum of Thy-PnBuA-Thy3 after a chemical treatment by
hydrazine ([hydrazine]/[PnBuA] = 20, 1 h, THF).
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Figure 4. 1H NMR spectra at 25 °C in CDCl3 of a mixture of
Thy-PnBuA-Thy1 and PS-DAP3, with [Thy-PnBuA-Thy] = 3 ꢁ 10ꢀ3
mol Lꢀ1 and various equivalents of PS-DAP3: (a) 0 equiv; (b) 1 equiv;
(c); 2 equiv; (d) 4 equiv; (e) 8 equiv; (f) 16 equiv; (g) 24 equiv; (h) 32
equiv. Red symbol: thymine NH protons.
Figure 6. Bulk viscosities for a PnBuA-DAP2 (Mn
SEC
= 10700 g
SEC
= 11000 g molꢀ1
3
molꢀ1, PDI = 1.19) /Thy-PnBuA-Thy2 (Mn
,
3
PDI = 1.23) blend at 1/1 DAP/Thy molar ratio and a nonfunctionalized
PnBuA generated from CTA4 (Mn SEC = 11700 g molꢀ1, PDI = 1.16).
3
spectroscopy is very delicate and that the presence of a second
population resulting from hydrolysis has never been observed by
SEC whatever the initial molar mass of PnBuA.
This thiol-terminated population corresponding to the popula-
tion E in the enlargement of initial Thy-PnBuA-Thy3 mass spec-
trum was associated with 5 other distinct populations. The same
enlargement in reflectron mode confirmed that the main popula-
tion (peak A, Figure 3) matched the expected R,ω-thymine
functionalized telechelic PnBuA chains (e.g., m/z monoisotopic
masses: theo. 2014.1, found 2013.9). The simulated and experi-
mental isotopic patterns of these dormant species were in good
agreement (Figure 3). Concerning the population C, it was
assigned to H-terminated species coming from a fragmentation
process in the MALDI mass spectrometer (e.g., m/z monoiso-
topic masses: theo. 2054.3, found 2054.0), as previously
discussed.15 Minor populations (B and F peaks) located on each
side of the thiol-terminated population (e.g., m/z monoisotopic
masses: theo, 2086.3; found, 2086.0) probably correspond to Na-
adducts. (peak B, [A ꢀ H þ Na þ Na]þ, m/z average masses:
theo, 2037.7; found, 2036.1; peak F, [E ꢀ H þ Na þ Na]þ, m/z
average masses: theo, 2109.8; found, 2109.3 in linear mode.)
Finally, the identification of peak D peaks is unknown for this
moment.
Figure 5. Viscosities of chloroform solutions (325 g Lꢀ1) containing
3
both PS-DAP4 and Thy-PS-Thy2 (Mn SEC = 9400 g molꢀ1and Mn
=
SEC
3
11000 g molꢀ1, respectively) at a 1/1 DAP/Thy molar ratio or
3
nonassociating PS (Mn SEC = 9000 g molꢀ1).
3
polymer chains (Figure 3 for Thy-PnBuA-Thy3, see Supporting
Information for Thy-PS-Thy1). The MALDI mass spectrum of
Thy-PnBuA-Thy3, acquired in linear mode, disclosed a main
distribution of peaks separated by expected 128 mass units (i.e., a
nBuA repeat unit), and centered at 2400 m/z. Besides this series,
a second distribution at low m/z values was observed (dotted
line, Figure 3). Thanks to an aminolysis reaction of the same
Thy-PnBuA-Thy3 sample by hydrazine and a MALDI analysis of
the resulting sample, this second distribution was assigned to a
thiol-terminated PnBuA population (mass spectrum in upper
inset). As a result of this chemical treatment, a drastic decrease in
average molecular mass was also detected by SEC (before
treatment: 3300 g molꢀ1 and after 1870 g molꢀ1). The presence
To evaluate the efficiency of these H-bonding telechelic
polymers as building blocks for supramolecular architectures,
polymers bearing a heterocomplementary DAP group were
generated from bulk polymerization of nBuA and styrene
mediated by CTA2, and from bulk polymerization of VAc
mediated by CTA3. All polymerizations were well-controlled,
as evidenced by the good accordance between molecular weights
determined by SEC and NMR, and the theoretical values (see
Table 1). Moreover, narrow polydispersity indexes (from 1.17 to
1.45) were measured (See Supporting Information).
3
3
of this thiol-terminated population in Thy-PnBuA-Thy3 sample
was thus attributed without a doubt to chain cleavage events
(hydrolysis) during the purification process. Indeed, Thy-
PnBuA-Thy3 exhibits very low molar mass (well-suited for a
MALDIꢀTOF characterization) that enhances its susceptibility
toward hydrolysis during the purification process (difficult pre-
cipitations in methanol:water 1:1). At this point it is also impor-
tant to note that a quantitative analysis based on the proportions
of the different populations identified by MALDIꢀTOF mass
1
Characterization of Polymer Self-Assembly by H NMR.
The self-assembly of R-ω-thymine functionalized polymers
(Thy-PnBuA-Thy1, Thy-PS-Thy3, Thy-PS-Thy4 or Thy-PI-
Thy1) with R-DAP functionalized polymers (respectively PS-
DAP3, PS-DAP2, PVAc-DAP2, PS-DAP1) was subsequently
1
investigated by H NMR in CDCl3. Similar to the association
of corresponding H-bonding CTAs, increasing the concentration
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Figure 7. AFM images of mixture of (A): Thy-PI-Thy1 (Mn SEC = 12000 g molꢀ1) and nonfunctionalized PS (Mn SEC = 4000 g molꢀ1); (B): Thy-PI-
3
3
Thy1 (Mn SEC = 12000 g molꢀ1) and PS-DAP1 (Mn SEC = 5200 g molꢀ1).
3
3
of PS-DAP or PVAc-DAP block in solution of heterocomple-
mentary THY-PnBuA-Thy1, THY-PS-Thy3, Thy-PS-Thy4 and
THY-PI-Thy1 blocks led to a significant downfield shift of the
NH thymine proton (for instance with [DAP] = [Thy], from
8.04/8.05/7.98/7.95 to 8.80/8.67/8.73/8.52 ppm respectively)
demonstrating the formation of supramolecular block copoly-
mers (see Figure 4 for the association between Thy-PnBuA-Thy1
and PS-DAP3). Increasing the solution temperature up to 50 °C
was also accompanied by a significant upfield shift of the NH
thymine proton with respect to the thermoresponsiveness of the
hydrogen bonds (see Supporting Information). In agreement
with previous works on the self-assembly of THY/DAP func-
tionalized polymers in CDCl3, close values of association
constants (ranging from 75 to 100 Mꢀ1) were observed
whatever the nature of the R,ω-thymine telechelic macromo-
lecular block (See Supporting Information).6j
PS analogue generated from CTA4 (Mn
= 9000 g molꢀ1
,
SEC
3
PDI = 1,14). Viscosities were collected over temperatures
ranging from ꢀ15 °C to þ20 °C using a shear rate of 100 sꢀ1
(Figure 5). A maximum temperature of 20 °C was fixed,
combined with the use of a solvent trap cover, in order to avoid
evaporation of chloroform during measurements.
Supramolecular association was corroborated by significant
viscosity enhancements observed for the mixture of PS-DAP4
and Thy-PS-Thy2. As expected, owing to the weakening of the
hydrogen-bond association upon heating, the increase of tem-
perature strongly impacted the rheological behavior of the
1
solution, nicely complementing the H NMR study on the
thermo-reversibility of the system (See Supporting Information).
Bulk rheological measurements were also performed over a large
range of temperatures (from ꢀ15 to 50 °C using a shear rate of
10 sꢀ1) on associating and nonassociating PnBuA possessing
equivalent molecular weights (see Figure 6). Again, a drastic
increase of viscosity was detected for the blend of PnBuA-DAP2
and Thy-PnBuA-Thy2 confirming the effectiveness of the PnBuA
blocks self-assembly in bulk through H-bonding. Above 50 °C,
associating and non associating PnBuAs exhibited very similar
viscosities suggesting that nearly all the three-points hydrogen
bonded complex between DAP and thymine are disrupted.
Characterization of Polymer Self-Assembly by Rheologi-
cal Measurements. As a further test of the efficient formation of
supramolecular structures through H-bonding, comparative
rheological analyses were performed on chloroform solutions
(325 g Lꢀ1) containing either Thy-PS-Thy2 (Mn
= 11000
SEC
g mol , PDI = 1.29) and PS-DAP4 (MnSEC = 9400 g molꢀ1
ꢀ31
,
3
3
PDI = 1,27) at a 1/1 DAP/Thy molar ratio or a nonassociating
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Characterization of Polymer Self-Assembly by AFM. The
phase behavior of thymine and DAP end-functionalized polymer
blends was subsequently investigated. Thy-PI-Thy1/PS-DAP1
([Thy] = [DAP]) blends as well as Thy-PI-Thy1/nonfunctio-
nalized PS (generated from CTA4) blends (used as a reference,
same ratio PS/PI w/w) were prepared by spin-coating on a
silicon wafer from a solution in toluene (10% w/w). Morphol-
ogies of the polymer blends were then observed by Atomic Force
Microscopy (AFM) in tapping mode. Height imaging results
from the measurement of oscillation amplitude, giving informa-
tion on the topology, while phase imaging results from measure-
ment of the phase lag induced by variations in material prop-
erties, such as viscoelasticity or adhesion. As can be seen
(Figure 7), whereas Thy-PI-Thy1/nonfunctionalized PS blends
tend to microphase separate (Figure 7 A1/A2), functional Thy-
PI-Thy1/PS-DAP1 blends exhibit a much finer structure con-
sistent with the presence of supramolecular block copolymers
that ensures the stabilization of the interface between the
polymers (Figure 7 B1/B2).
spectrometry analyses, respectively, as well as Prof. Jannick
Duchet-Rumeau and Pierre Alcouffe for fruitful discussions
about microscopy.
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In conclusion, we have described a simple and straightforward
strategy toward the generation of precisely defined H-bonding
telechelic polymer chains and supramolecular block copolymers
thereof. Relying on the design of a novel Rꢀω-thymine functio-
nalized symmetrical trithiocarbonate capable to grow well-de-
fined telechelic polymer chains in one step, we produced an
extensive range of Rꢀω end-functional polymers with predict-
able molar mass and narrow molar mass distribution from
styrene, n-butyl acrylate and isoprene. In combination with
RAFT-made polymers exhibiting complementary recognition
units (DAP), these original telechelic macromolecular building
blocks afford the formation of supramolecular block copolymers
1
in solution as well as in bulk as clearly evidenced by H NMR,
rheological measurements and supramolecular blend phase
behavior investigations. These novel H-bonding telechelic poly-
mers constitute an attractive class of elementary blocks for
elaborating innovative materials such as supramolecular net-
works. Studies in this direction are currently in progress.
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra of the CTAs,
S
b
1H NMR temperature dependence investigation of the polymer
H-bonding association, THY-PS-THY1 ESI mass spectrum,
evolution of molar masses and PDI with conversion for the
polymerization of styrene, nBuA and isoprene mediated by
CTA1, and determination of Kassoc. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: julien.bernard@insa-lyon.fr.
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R. P.; Meijer, E. W. P.N.A.S 2006, 103, 11850–11855.
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38, 9518–9525.
(10) Lai, J. T.; Filla, D.; Shea, R. Macromolecules 2002,
35, 6754–6756.
’ ACKNOWLEDGMENT
The authors thank the French Ministꢁere de l0Enseignement
supꢀerieur et de la Recherche for A.B.’s grant and the CNRS for
financial support (ANR Jeune Chercheur SUPRABLOCK)
gratefully acknowledge Dr. Frederic Lortie and Frederic Delolme
(IBCP/CCMP Lyon) for their help in rheological and mass
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