Synthesis and association behavior of telechelic poly(N- isopropylacrylamides) with azobenzene end groups

Article abstract of DOI:10.1080/15421406.2010.495899

A telechelic poly(N-isopropylacrylamide) (PNIPAM) with azobenzene end-groups (telechelic Az-PNIPAM) was prepared by reversible addition-fragmentation chain transfer (RAFT) free radical polymerization of NIPAM. This polymer self-assembles in cold water for

Full text of DOI:10.1080/15421406.2010.495899

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Synthesis and Association Behavior of
Telechelic Poly(N-isopropylacrylamides)
with Azobenzene End Groups
Norihito Ishii a , Rodolphe Obeid b , Xing-Ping Qiu b , Jun-ichi Mamiya
a , Tomiki Ikeda a & Françoise M. Winnik b
a Chemical Resources Laboratory, Tokyo Institute of Technology ,
Midori-ku, Yokohama, Japan
b Department of Chemistry and Faculty of Pharmacy , University of
Montréal, Succursale Centre Ville , Montreal, Quebec, Canada
Published online: 11 Nov 2010.
To cite this article: Norihito Ishii , Rodolphe Obeid , Xing-Ping Qiu , Jun-ichi Mamiya , Tomiki
Ikeda & Françoise M. Winnik (2010) Synthesis and Association Behavior of Telechelic Poly(N-
isopropylacrylamides) with Azobenzene End Groups, Molecular Crystals and Liquid Crystals, 529:1,
60-70
To link to this article: http://dx.doi.org/10.1080/15421406.2010.495899
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Mol. Cryst. Liq. Cryst., Vol. 529: pp. 60–70, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.495899
Synthesis and Association Behavior of Telechelic
Poly(N-isopropylacrylamides) with Azobenzene
End Groups
NORIHITO ISHII,1 RODOLPHE OBEID,2
XING-PING QIU,2 JUN-ICHI MAMIYA,1
TOMIKI IKEDA,1 AND FRANC¸ OISE M. WINNIK2
1Chemical Resources Laboratory, Tokyo Institute of Technology,
Midori-ku, Yokohama, Japan
2Department of Chemistry and Faculty of Pharmacy, University of
´
Montreal, Succursale Centre Ville, Montreal, Quebec, Canada
A telechelic poly(N-isopropylacrylamide) (PNIPAM) with azobenzene end-groups
(telechelic Az-PNIPAM) was prepared by reversible addition-fragmentation chain
transfer (RAFT) free radical polymerization of NIPAM. This polymer self-
assembles in cold water forming nanoparticles with a hydrodynamic radius (RH)
of 8 nm and an aggregation number of 29 chains. The thermoresponsive behavior
of Az-PNIPAM in water was investigated by turbidimetry and light scattering. In
addition, the photoresponsive behavior of aqueous Az-PNIPAM was investigated
by monitoring the changes with temperature of the solution and transmittance at
650 nm before and after irradiation at 366 nm.
Keywords Azobenzene; liquid-crystalline molecules; reversible addition-
fragmentation chain transfer (RAFT) polymerization; telechelic poly(N-isopropyl-
acrylamide)
Introduction
Stimuli-responsive polymer gels and micelles have been studied extensively [1–3]. In
particular, poly(N-isopropylacrylamide)s (PNIPAMs) and its derivatives have been
explored due to their unique thermo-responsive properties in water [4–6]. One of
the most interesting properties of PNIPAMs is the heat-induced change in its solu-
bility in water. PNIPAM dissolves in cold water by forming hydrogen bonds
between water molecules and the polymer amide groups. The solubility of PNIPAM
is high in cold water but it decreases upon increasing the temperature. Water
becomes a poor solvent for PNIPAM above 32ꢀC. This useful property can be
exploited in applications such as drug delivery system and cosmetics [7]. Winnik
et al. reported the synthesis and solution properties of telechelic hydrophobically
Address correspondence to Tomiki Ikeda, Chemical Resources Laboratory, Tokyo Insti-
tute of Technology, R1-11, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. Tel.:
þ81 45 924 5240; Fax: þ81 45 924 5275; E-mail: tikeda@res.titech.ac.jp
60

Telechelic PNIPAM with Azobenzene End Groups
61
modified PNIPAMs [8,9], which form flower-like micelles in cold aqueous solution.
The micelles exist as isolated entities in dilute cold aqueous solution and form large
particles above the cloud-point temperature [10]. The azobenzene chromophore,
which exhibits photochromism and liquid-crystalline properties, has been studied
extensively as a molecule able to trigger changes in the properties of materials in
response to photoirradiation [11–13] as a result of the reversible photoisomerization
of the azo linkage. We report here the synthesis of a telechelic PNIPAM with azo-
benzene end-groups obtained by reversible addition-fragmentation chain transfer
(RAFT) polymerization of NIPAM and further end-group transformation. The
effect of the azobenzene end-groups on the self-assembly of the polymers in water
and the photo=temperature responsive properties of the polymers in water were
investigated by physico-chemical methods.
Experimental
Instrumentation and Measurements
1
All H NMR spectra were recorded on a Lambda-300 (300 MHz) spectrometer.
Gel permeation chromatography (GPC) was performed with a Japan Spec-
troscopy Co., model DG-980-50 system equipped with a refractive index detector
and three Shodex columns (K803 þ K804 þ K805) eluted with DMF at 40ꢀC. The
molecular weight values were determined using a calibration curve obtained with
PMMA standards. High sensitivity differential calorimetry (HS-DSC) measure-
ments were performed on a VP-DSC microcalorimeter (MicroCal Inc.) at an
external pressure of ca. 180 kPa. The cell volume was 0.517 ml. The heating rate
was 1ꢀC=min. The polymer concentration was 1.0 g=l, unless otherwise specified.
The solutions for analysis were degassed at 25ꢀC for 20 min. They were equili-
brated at 10ꢀC for 1 h before initiation of the heating process. The samples were
heated from 10 to 80ꢀC and cooled back immediately to 10ꢀC. Four consecutive
heating and cooling scans were performed in all cases. Data were corrected
for instrument response time and analyzed using the software supplied by the
manufacturer.
UV-Vis spectra were recorded with a JASCO, V-550 UV-visible spectrophot-
ometer, which was used also for turbidity measurements, which yielded the cloud
points (Tcp) of polymer solutions. The Tcp values were determined by spectrometric
detection of the changes in turbidity (650 nm) of aqueous polymer solutions (1.0 g=l)
heated at a constant rate (1ꢀC=min). The cloud points reported are the temperatures
corresponding to a 50% decrease of the solution transmittance.
Static (SLS) and dynamic light scattering (DLS) experiments were performed on
a CGS-3 goniometer (ALV GmbH) equipped with an ALV=LSE-5003 multiple-s
digital correlator (ALV GmbH), a He-Ne laser (k ¼ 633 nm), and a C25P circulating
water bath (Thermo Haake). SLS experiments yield the apparent weight-average
molar mass (Mw,app) and the z-average root-mean square radius of gyration (RG)
of scattering objects in dilute solution, based on the angular dependence of the excess
absolute scattering intensity, known as the excess Rayleigh ratio R(q, c) given by
Eq. (1):
Kðc ꢁ cmic
Rðq; cÞ
Þ
1
ffi
þ 2A2ðc ꢁ cmic
Þ
ð1Þ
M
w;appPðHÞ

62
N. Ishii et al.
where c is the polymer concentration, cmic is the concentration of micellization onset,
q is the scattering vector (q ¼ (4pn=k) sin(H=2), A2 is the second virial coefficient, n is
the refractive index of the solvent, k is the wavelength of the light in the vacuum, and
H is the scattering angle (30ꢀ–150ꢀ). The scattering constant is K ¼ 4p2n2 (dn=dc)2=
NAk4, where dn=dc is the refractive index increment and NA is Avogadro’s number.
The dn=dc of the derivatized PNIPAM was 0.074 ml=g for aqueous solutions at 24ꢀC
and a wavelength of 690 nm. In Eq. (1), it is assumed that the contribution of a single
polymer chain to the scattering intensity is negligible, compared to that of the
micelles. Data recorded for solutions of concentration c were analyzed according
to the Zimm equation, assuming that the macromolecules are in a swollen con-
formation. In this case, the particle scattering function is P(H) ¼ 1 ꢁ (q2RG2 )=3,
where RG is the radius of gyration. Since (q2RG2 )=3 < < 1, it may be assumed that
1=[1 ꢁ (q2RG2 )=3] ffi 1 þ (q2R2G)=3. Thus, Eq. (1) becomes
ꢀ
ꢁ
Kðc ꢁ cmic
Rðq; cÞ
Þ
1
R2
ffi
1 þ G q2 þ 2A2ðc ꢁ cmic
Þ
ð2Þ
Mw;app
3
The apparent mass of a polymer (Mw,app) in a solution of concentration c was
obtained by extrapolation of the scattered intensity R(q, c)=(c ꢁ cmic) to q ¼ 0.
The apparent radius of gyration of the scattering objects in a solution of concen-
tration c was obtained by a mean-square linear fit of the inverse of the scattered
intensity versus q2 (see Eq. (2)).
In DLS experiments, one measures the normalized time autocorrelation function
of the scattered intensity, which can be expressed in terms of the autocorrelation
function of the concentration fluctuations. In our experiments, the relaxations had
always a diffusive character with a characteristic time (s) inversely proportional
to q2. A cumulant analysis was applied to obtain the diffusion coefficient (D) of
the scattering objects in solution. Extrapolation of the first reduced cumulant
1=(sq2) to q ¼ 0 yields the value of D, which is related to the average hydrodynamic
radius RH of the scattering objects by Eq. (3):
kBT
D ¼
ð3Þ
6pgsRH
where gS the viscosity of the solvent, kB is the Boltzmann constant, and T is the
absolute temperature.
Materials
4-Cyano-40-(6-bromohexyloxy)azobenzene was prepared following a reported pro-
cedure [14] starting from 4-nitrobenzonitrile. Water was deionised with a Millipore
Milli-Q system.
Synthesis
Synthesis of 4-Cyano-40-(6-iodohexyloxy)azobenzene (I6Az) (Scheme 1).
4-132#Cyano-40-(6-bromohexyloxy)azobenzene (1.0 g, 2.6 mmol) and sodium
iodide (3.9 g, 26 mmol) were dissolved in acetone (40 ml). The mixture was heated

Telechelic PNIPAM with Azobenzene End Groups
63
Scheme 1. Synthetic routes of I6Az.
at reflux with stirring for 3 h. It was cooled to room temperature and the acetone was
removed under vacuum. The solid residue was dissolved in chloroform, washed with
water, dried over anhydrous MgSO4, filtered. The solvent was removed from the
filtrate with a rotary evaporator, yielding solid, which was recrystallized from
hexane=CH2Cl2 (8=2 v=v) Yield: 1.0 g, 88%.
1H NMR (CDCl3) ppm, d 1.59 (m, CH2ICH2CH2), 1.89 (m, CH2ICH2 and
-OCH2CH2), 3.24 (t, CH2I), 4.08 (t, -OCH2), 7.02 (d, Ar-H), 7.80 (d, Ar-H), 7.95
(d, Ar-H).
Polymerization. A solution of the chain transfer agent (synthesized according to
references [15,16]) (DEGDIM, 124 mg, 0.22 mmol), the initiator (AIBN, 3.5 mg,
0.022 mmol), and the monomer (NIPAM, 2.0 g, 18 mmol) in 1,4-dioxane (10 ml)
was placed in a round-bottom flask with rubber septa. The solution was
deoxygenated by bubbling nitrogen for 30 min at room temperature. The reaction
flask was placed in an oil bath preheated to 65ꢀC. The polymerization was
allowed to proceed for 4 h under constant magnetic stirring. At the end of this
polymerization, the solution was cooled to room temperature. The polymer,
a,x-di(isobutylthiocarbonylthio)-poly(N-isopropylacrylamide), was isolated by
precipitation in diethyl ether. It was purified further by two consecutive
reprecipitations from THF into diethyl ether. Yield: 1.90 g, 88%.
1H NMR (CDCl3) ppm, d 1.01 (d, -CH2CH(CH3)2), 1.13 (s, -NHCH(CH3)2),
1.27–2.30 (multipeaks, polymer backbone protons), 3.15 (s, -SCH2), 3.61 (s,
=
=
-OCH2), 3.95 (s, -NHCH), 4.16 (s, -C( O)OCH2), 6.40 (bs, -C( O)NH).
Preparation of Telechelic a,x-Dimercapto Poly(N-isopropylacrylamide)s
(SH-PNIPAMs). n-Butylamine (0.78 g, 0.16 mmol) and tris(2-carboxyethyl)
phosphine hydrochloride (TCEP, trace amount), were added to a solution of
a,x-di(isobutylthiocarbonylthio)-poly(N-isopropylacrylamide) (0.70 g 0.2 mmol) in
THF (10 ml) (Scheme 2). The resulting solution was stirred for 1 h at room
temperature under a nitrogen atmosphere. The product was recovered by
precipitation in diethyl ether and purified by two consecutive reprecipitations from
THF into diethyl ether. Yield: 0.71 g. 1H NMR (CDCl3) ppm, d 1.12 (s,
-NHCH(CH3)2), 1.30–2.20 (multiplets, polymer backbone protons), 3.67 (s,
=
-OCH2), 3.96 (s, -NHCH), 4.15 (s, -C( O)OCH2), 6.40 (bs, -C( O)NH).
=
Preparation of Telechelic a,x-Diazobenzene PNIPAM. Sodium carbonate (1.7 g,
16 mmol) was added to a solution of I6Az (0.062 g, 0.14 mmol, 1.2 equiv.) and

64
N. Ishii et al.
Scheme 2. Synthetic routes of telechelic Az-PNIPAM.
telechelic SH-PNIPAM (1.2 g, 0.12 mmol) in distilled DMF (10 ml). A crystal of
TCEP was added to the mixture to prevent disulfide bond formation. The
solution was stirred under nitrogen for 24 h. The polymer was recovered by
precipitation in diethyl ether and purified by two consecutive reprecipitations from
THF into diethyl ether. Yield: 0.99 g, 79%.
1H NMR (CDCl3) ppm, d 1.14 (s, -NHCH(CH3)2), 1.30–2.30 (multiplets, poly-
=
mer backbone protons), 3.67 (s, -OCH2), 3.97 (s, -NHCH), 4.17 (s, -C( O)OCH2),
=
6.40 (bs, -C( O)NH), 6.98 (d, Ar-H), 7.74 (m, Ar-H), 7.93 (m, Ar-H).
Examination of the Concentration of the Onset of Micellization (cmic
)
The cmic was determined using Nile Red. Solutions containing a constant Nile Red
concentration (10 mM) and varying amounts of polymer (1.8 Â 10ꢁ4 to 3.0 g=l) were
prepared, starting from a stock solution (3.0 g=l). UV-visible absorption spectra of
the solutions were recorded from 500 to 650 nm.
Photoresponsive Behavior
The transmittance at 650 nm of aqueous solutions of the telechelic Az-PNIPAM
(0.2 g=l) was measured as a function of temperature before and after irradiation of
the solutions at 366 nm (1 mW=cm2).
Results and Discussion
Synthesis of Telechelic Diazobenzene PNIPAM (Az-PNIPAM)
The telechelic diazobenzene PNIPAM (Az-PNIPAM) was prepared in three steps, as
depicted in Scheme 2. First, NIPAM was polymerized at 65ꢀC in dioxane in the
presence of the difunctional CTA diethylene glycol di(2-(1-isobutyl)sulfanylthio-
carbonylsulfanyl-2-methylpropionate (DEGDIM) and AIBN acting as an initiator.
The polymerization was left to proceed to high conversion (ꢂ95%) [8]. The

Telechelic PNIPAM with Azobenzene End Groups
65
Figure 1. 1H NMR spectrum of telechelic iBu-PNIPAM (top) and telechelic Az-PNIPAM
(bottom).
[NIPAM]=[DEGDIM] molar ratio in the polymerization mixture was adjusted to
attain a polymer of molecular weight ꢂ10,000 g=mol. The GPC elution profile of
the polymer was monomodal and symmetrical and the polydispersity index of the
polymer was 1.08. The structure of the polymer was ascertained by analysis of its
1H NMR spectrum in CDCl3 (Fig. 1, top). It is possible to calculate the number
1
average molecular weight from the H NMR spectrum of the polymer, using the
value of the integral of the signals due to Hc (3.97 ppm) and He (3.67 ppm) [8].
The number average molecular weight was determined also from the UV-vis spec-
trum of the polymer, which exhibits a strong absorbance centered at 310 nm ascribed
to the thiocarbonylthio moiety, which can be used to calculate the Mn value
(Mn ¼ w=cCTA), where w is the weight of polymer (in g) and cCTA is the amount of
thiocarbonylthio residues in the sample (in mole). The values of the number average
1
molecular weight from H NMR spectrum and the UV–vis spectrum are in good
agreement with the value obtained by GPC (Table 1).
Next, the isobutylthiocarbonylthio termini of the polymer were converted to
thiols by aminolysis, using a 10-molar excess of n-butylamine, with respect to thio-
carbonylthio groups, in order to ensure completeness of the reaction. Aminolysis
proceeded rapidly at room temperature, reaching near completion within 1 h, as con-
firmed by the disappearance of the absorbance at 310 nm in the UV-vis spectrum of
the polymer. Finally, the telechelic dithiol PNIPAM was reacted with 4-cyano-40-
(6-iodohexyloxy)azobenzene in DMF containing sodium carbonate. The evolution

66
N. Ishii et al.
Table 1. Number average molecular weight
Mn (g=mol)
GPCa
10500
NMRb
11500
UV-visc
11800
Mw=Mn (GPC)a
1.08
aGPC condition: Flow rate 1 ml=min, Solvent DMF, Temperature 40ꢀC.
bMn ¼ [Hc]=[He] Â 4 Â Mo þ MDEGDIM
.
cMn ¼ w=cCTA
.
of the reaction was assessed by measuring the residual thiol concentration in aliquots
of the reaction mixture taken at various time intervals. A reaction time of 24 h was
sufficient to ensure completion of the reaction. The level of end-group functionaliza-
tion was 99.5%, as determined by UV-vis spectroscopy.
Self-Assembly of Telechelic Az-PNIPAM in Water
Spectroscopic and light scattering experiments were carried out in order to confirm
the presence of micellar assemblies in aqueous solutions of telechelic Az-PNIPAM,
to characterize the assemblies in terms of size, aggregation number and concen-
tration of the onset of micellization (cmic) (Table 2), and to assess the effect of
changes in solution temperature on the polymer self-assembly. The cmic of the tele-
chelic Az-PNIPAM in water was determined by UV-vis spectroscopy using Nile
red as absorption probe. Nile red is poorly soluble in water. However in micellar
solution, the dye is solubilized within the hydrophobic core of the micelles, and,
consequently, the dye absorbance increases significantly. The absorbance at
582 nm of mixed solutions of Nile red (10 mM) and telechelic Az-PNIPAM was
measured as a function of polymer concentration for solutions kept at 10ꢀC. The
absorbance remained very low for solutions of cpol < 0.03 g=l. It increased sharply
as the polymer concentration exceeded this value, reaching a plateau value for
c > 0.06 g=l. The cmic value of the telechelic Az-PNIPAM in water (0.056 g=l) was
determined from a plot of the absorbance at 582 nm vs cpol. It was taken as the
concentration of the onset of absorbance increase (Fig. 2). The size of micelles
and their aggregation number in water (1.0 g=l) at 8ꢀC were 8 nm and 29 polymer
chains, as determined, respectively by DLS and SLS.
Temperature Dependence of the Self-Assembly of Telechelic Az-PNIPAM in Water
Aqueous solutions of telechelic Az PNIPAM became turbid in response to an
increase in temperature. Their cloud point temperature (Tcp ¼ 21ꢀC) was determined
Table 2. cmic and light scattering data for solution of a telechelic Az-PNIPAM
sample in dilute aqueous solution
Nagg
SLS
RHðnmÞ
T ¼ 8ꢀC
8.0
cmic (g=l)
Mw,app (g=mol)
2.9 Â 105
RG (nm)
RG=RH
0.056
ꢂ29
17.5
1.6

Telechelic PNIPAM with Azobenzene End Groups
67
Figure 2. The concentration of the onset of micellization using the absorption spectra of Nile
red as a probe.
by measuring the changes in turbidity (650 nm) of an aqueous telechelic Az PNI-
PAMs solution (1.0 g=l) heated at a constant rate (1ꢀC=min). The Tcp value reported
here is the temperature for which the solution transmittance falls below 50%
(Fig. 3a). The heat-induced phase transition of a telechelic Az-PNIPAM aqueous
solution was monitored also by microcalorimetry. A thermogram recorded for an
aqueous solution of telechelic Az-PNIPAM (1.0 g=l) (Fig. 3b) presents a sharp
endotherm with a maximum (TM) at 25.5ꢀC and a transition enthalpy (DH) of
1.25 kcal=mol NIPAM units. The temperature of the endotherm onset (10ꢀC) corre-
sponds well to the cloud point value recorded by turbidimetry, as observed pre-
viously [17]. The changes in the micellar size as a function of solution temperature
Figure 3. Changes in transmittance as a function of temperature of the telechelic Az-PNIPAM
in water (a) and temperature dependence of the specific heat capacity of the telechelic
Az-PNIPAM in water upon heating and cooling at a rate of 1ꢀC=min (b).

68
N. Ishii et al.
Figure 4. Changes in the absorption spectra of the telechelic Az-PNIPAM in water (0.14 g=l)
upon photoirradiation (1 mW=cm2) at 366 nm.
were monitored by DLS measurements. The hydrodynamic radius of the micelles
underwent a sharp increase as the solution temperature reached 21ꢀC, signaling
the formation of an intermicellar network. For T ¼ 25ꢀC, the size of the aggregates
exceeded the limit of detection (ꢂ800 nm) of the light scattering system employed.
Further heating of the solution triggered a significant shrinking of the aggregates,
which had a hydrodynamic radius of 270 nm and narrow size distribution for
T ¼ 34ꢀC, a temperature for which collapsed PNIPAM chains have a tendency to
form compact mesoglobules [17].
Figure 5. Changes in transmittance of probe light in solution of the telechelic Az-PNIPAM as
a function of temperature before and after irradiation at 366 nm.

Telechelic PNIPAM with Azobenzene End Groups
69
Photoresponsive Behavior of the Telechelic Az-PNIPAM in Water
Photoirradiation (k ¼ 366 nm) of an aqueous telechelic Az-PNIPAM solution
resulted in a decrease of the p-pà band of the azobenzene chromophore centered
at 352 nm and an increase of the n-pà transition band at 257 and 446 nm (Fig. 4),
confirming the occurrence of the trans-cis photoisomerization of the azobenzene
moiety in the telechelic Az-PNIPAM aqueous solution. Next, the cloud point of tele-
chelic Az-PNIPAM (0.2 g=l) in aqueous solution was measured before and after
irradiation at 366 nm (Fig. 5). The temperature, at which transmittance started to
decrease, was nearly the same before and after photoirradiation. However, the trans-
mittances at 40ꢀC of the two samples were different, which may be taken as an
indication of a change in the morphology of the aggregates formed by the polymers
carrying different isomers of azobenzene end-groups.
Conclusions
By using RAFT polymerization we prepared a telechelic PNIPAM of narrow
molecular weight distribution modified with an azobenzene moiety on each chain
end. The telechelic Az-PNIPAM in water above 0.056 g=l forms micellar assemblies.
The size of the micelle and the aggregation number of this polymer in water (1.0 g=l)
were 8 nm and 29 polymer chains at 8ꢀC. Upon heating above the cloud point
(21ꢀC), micelles associate into larger objects (1,400 nm) as the solution temperature
reaches 25ꢀC: this temperature corresponds to the temperature of the phase tran-
sition of telechelic Az-PNIPAM in water. Further heating of the solution triggered
a shrinking of the aggregate size, which is 270 nm at 34ꢀC. Furthermore, The tem-
perature, at which transmittance started to decrease, was nearly the same before
and after photoirradiation. However, the final values of transmittance were different
between the two samples. These results indicate that Az-PNIPAM in water shows
the photoresponsive behavior. Further work is in progress aimed at progresses of
the photoresponsiveness and liquid-crystalline property.
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