Multiresponsive spiropyran-based copolymers synthesized by atom transfer radical polymerization

Article abstract of DOI:10.1021/ma101331u

Multiresponsive spiropyran-based random copolymers were synthesized by atom transfer radical polymerization. The polymers were prepared by the copolymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) with the photochromic monomer 1′,3′,3′-trimethyl-6- methacryloyloxyspiro(2H-1-benzopyran-2,2′-indoline) (SP). The solvatochromic, pH, temperature, and light responsive behavior of the PDMAEMA-co-PSP polymers in solution was monitored by UV/vis spectroscopy. The copolymers exhibited reverse photochromism and stabilized the planar zwitterionic form of the chromophore, leading to negative solvatochromism upon increasing the polarity of the solvent, in contrast to a poly(methyl methacrylate)-co-PSP analogue which stabilized a nonpolar photoisomer and exhibited negligible sensitivity to the polarity of the surrounding medium, before irradiation. Moreover, the PDMAEMA-co-PSP copolymers exhibited a reversible pH-responsive character in aqueous media; the addition of a strong acid induced the SP-to-merocyanine (MC) isomerization, the formation of [MC-OH]⁺ and [SP-NH]⁺ species, and the disappearance of the H-type aggregates, whereas the initial SP moieties were recovered in alkaline media. The chromophore content and the photoinduced MC-to-SP isomerization affected the transition temperature of the PDMAEMA-co-PSP polymers. The lower critical solution temperature (LCST) of the copolymers increased with their content in hydrophilic MC moieties, while the MC-to-SP photoinduced isomerization decreased significantly the LCST due to the hydrophobic character of the SP units. Finally, the copolymers exhibited a first-order photoinduced bleaching of the chromophore units in water and acetonitrile, with a slower decoloration rate in the aqueous medium due to the effective stabilization of the bipolar MC form in the polar environment.

Full text of DOI:10.1021/ma101331u

Macromolecules 2010, 43, 7073–7081 7073
DOI: 10.1021/ma101331u
Multiresponsive Spiropyran-Based Copolymers Synthesized by Atom
Transfer Radical Polymerization
Demetra S. Achilleos and Maria Vamvakaki*
Institute of Electronic Structure and Laser Foundation for Research and Technology - Hellas, P.O. Box 1527,
711 10 Heraklion, Crete, Greece, and Department of Materials Science and Technology, University of Crete,
P.O. Box 2208, 710 03 Heraklion, Crete, Greece
Received June 15, 2010; Revised Manuscript Received July 28, 2010
ABSTRACT: Multiresponsive spiropyran-based random copolymers were synthesized by atom transfer
radical polymerization. The polymers were prepared by the copolymerization of 2-(dimethylamino)ethyl
methacrylate (DMAEMA) with the photochromic monomer 10,30,30-trimethyl-6-methacryloyloxyspiro-
(2H-1-benzopyran-2,20-indoline) (SP). The solvatochromic, pH, temperature, and light responsive behavior
of the PDMAEMA-co-PSP polymers in solution was monitored by UV/vis spectroscopy. The copolymers
exhibited “reverse photochromism” and stabilized the planar zwitterionic form of the chromophore, leading
to “negative solvatochromism” upon increasing the polarity of the solvent, in contrast to a poly(methyl
methacrylate)-co-PSP analogue which stabilized a nonpolar photoisomer and exhibited negligible sensiti-
vity to the polarity of the surrounding medium, before irradiation. Moreover, the PDMAEMA-co-PSP
copolymers exhibited a reversible pH-responsive character in aqueous media; the addition of a strong acid
induced the SP-to-merocyanine (MC) isomerization, the formation of [MC-OH]þ and [SP-NH]þ species,
and the disappearance of the H-type aggregates, whereas the initial SP moieties were recovered in alkaline
media. The chromophore content and the photoinduced MC-to-SP isomerization affected the transition
temperature of the PDMAEMA-co-PSP polymers. The lower critical solution temperature (LCST) of the
copolymers increased with their content in hydrophilic MC moieties, while the MC-to-SP photoinduced
isomerization decreased significantly the LCST due to the hydrophobic character of the SP units. Finally, the
copolymers exhibited a first-order photoinduced bleaching of the chromophore units in water and
acetonitrile, with a slower decoloration rate in the aqueous medium due to the effective stabilization of the
bipolar MC form in the polar environment.
Introduction
storage,4 sensors,5-7 microfluidics,8,9 smart surfaces,10-15 and
bioseparation.16
“Bistable” compounds that can be interconverted, under
certain external stimuli, to two thermodynamically stable states
of different color have attracted considerable attention lately.1
The interconversion can be induced by different stimuli and is
defined as photochromism, thermochromism, electrochromism,
and solvatochromism.1 Photochromism is the light-induced
reversible intramolecular rearrangement between two isomeric
conformations with different absorption spectra.2 Organic mole-
cules exhibiting reversible photochromism include azobenzenes,
spiropyrans (SPs), spirooxazimes, and naphthopyrans3 with the
former being the most extensively studied family of photo-
chromic compounds. Lately, SP molecules, which consist of
two orthogonal π-networks that are connected by a tetrahedral
carbon center, have also attracted great interest.1 Upon UV
irradiation, the nonpolar, colorless SP molecule undergoes het-
erolytic cleavage of the C-O bond to form the polar, colored
cisoid isomer of merocyanine (MC) (see Figure S1, Supporting
Information). The cisoid form then isomerizes spontaneously to
the thermodynamically stable transoid MC form, which is an
extended π-conjugated system and absorbs in the visible region.
The MC isomer can be converted back to the original SP
state either thermally or upon irradiation with visible light.
The SP-to-MC interconversion has been investigated for use in
various applications, such as in data recording and optical data
The photochromism of SP depends on the polarity of the
surrounding medium (solvent or polymer matrix), the solution
pH, andthe presence of ions.17 Media of low polaritypromote the
photoinduced isomerization of MC to SP.18 Moreover, polar
media favor the formation of the zwitterionic MC form over the
quinoidal isomer.15 Both of these MC forms exhibit solvatochro-
mism, which is changes in the position and the intensity of their
absorption bands induced by variations in the polarity of the
surrounding medium. In particular, when the oxygen is in the
ionic form, the zwitterionic MC exhibits negative solvatochro-
mism upon increasing the polarity of the medium, i.e., a shift of
the λmax of MC to shorter wavelengths,15 accompanied by band
broadening and attenuation of the extinction coefficient.19 How-
ever, if the MC oxygen possesses a quinodial character, the MC
isomer exhibits positive solvatochromism and the λmax is red-
shifted.20 The solution pH also affects the SP-to-MC transforma-
tion; low solution pH favors the isomerization of SP to MC and
leads to the protonation of the open form, which is accompanied
by changes in the absorption spectra of the chromophore, while
the SP isomer is recovered in alkaline media.21,22 The MC form
can also bind metal cations or undergo nucleophilic addi-
tion by CN-, which lead to the appearance of blue-shifted MC
bands.5,23-25
Photochromic copolymers carrying spiropyran side groups
have attracted particular attention lately.26-30 The chemical
linking of the SP moieties can been achieved by two different
*To whom correspondence should be addressed. E-mail: vamvakak@
iesl.forth.gr.
r 2010 American Chemical Society
Published on Web 08/12/2010
pubs.acs.org/Macromolecules

7074 Macromolecules, Vol. 43, No. 17, 2010
Achilleos and Vamvakaki
methods: the covalent attachment of photosensitive dyes to
preformed polymer chains31 or the copolymerization of photo-
sensitive compounds carrying polymerizable groups with other
vinyl monomers.32,33 These materials allow to overcome many of
the limitations inherent in traditional SP doped polymers such as
the phase separation of the colorant and the retardation of the
decoloration of the open form.32,34,35 Matsushima et al. showed
that the immobilization of the SP groups onto a polymer back-
bone suppressed their aggregation and reduced their photo-
degradation.36 This is particularly attractive for electronic appli-
cations that require high contents of photosensitive compounds
and stable performance.37 Besides, the decoloration rate has been
shown to depend on the rigidity, the free volume, and the polarity
of the polymeric matrix.19,38 For example, a low-Tg oligomer
enhanced the photoswitching rate due to the flexibility of the
polymer matrix.39 At high SP contents the attachment of the
photochromic side groups via a long, flexible spacer40 minimized
the retardation of the decoloration which was observed in short
spacer systems due to the lack of free volume.41,42
enzymes, and polyanionic drugs, are very important and open up
new applications of these materials in the areas of chemical and
biochemical gates, gene transfer, programmed adsorption of
proteins, and drug delivery.
Experimental Section
Materials. The monomer DMAEMA (98%, hydrophilic and
ionizable), the ligand N,N,N0,N00,N00-pentamethyldiethylene-
triamine (99%, PMDETA), the initiator ethyl 2-bromoiso-
butyrate (98%), triethylamine (99%, TEA), the metal salt
copper(I) bromide (99.9%, CuIBr), and chloroform (99.8%,
CDCl3) were purchased from Aldrich, Germany. The monomer
MMA (99%, hydrophobic), dichloromethane (99.8%, DCM),
and methacryloyl (97%) and acryloyl (96%) chlorides were
purchased from Fluka, Germany. The photochromic spiro-
pyran molecule, 10,30,30-trimethyl-6-hydroxyspiro(2H-1-benzo-
pyran-2,20-indoline) (99%, HBPS), was purchased from Acros
Organics, Germany, and was used without further purification.
Toluene (99.7%) and tetrahydrofuran (THF, 99.9%) were
The introduction of light-sensitive moieties within responsive
polymers has led to the development of sophisticated multi-
responsive systems.26 Spiropyran monomer repeat units have
been incorporated within temperature-responsive poly(N-iso-
propylacrylamide) (PNIPAM) chains. The photochromic mole-
cules influenced the phase transition behavior of PNIPAM and
reduced its cloud point by more than 10 ꢀC.29 On the other hand,
PNIPAM-SP microgels were shown to alter their volume phase
upon light irradiation.43 Sumaru et al. reported that the sensiti-
vity of a PNIPAM-co-PSP copolymer to light irradiation and
temperature changes was dependent on the solution pH.26
Recently, the photoinduced hydration of the polymer was found
to be effective only at low temperatures when the PNIPAM
component was sufficiently hydrophilic.44 This property was
applied to develop a colorimetric thermometer based on a
PNIPAM-co-PSP copolymer, which upon UV irradiation ex-
hibited a continuous bathochromic shift as the temperature
increased. The change in the absorption was attributed to the
zwitterionic-to-quinodal MC isomerization, induced by the less-
polar environment which surrounded the chromophore moieties
at higher temperatures.28
€
purchased from Riedel-de Haen, Germany.
Methods. The monomers were passed through basic alumina
columns to remove inhibitors. The polymerization solvent,
THF, was dried by refluxing it over potassium for 3 days. All
reagents used for the monomer synthesis were stirred over
CaH2, were freshly distilled under vacuum, and were kept under
a dry nitrogen atmosphere until use. All glassware was dried
overnight at 160 ꢀC prior to use.
Synthesis of the Spirobenzopyran Methacrylate Monomer.
The synthesis involved an esterification reaction to produce a
spiropyran-based molecule carrying a polymerizable methacryl-
ate moiety. In a typical reaction, HBPS (1.0 g, 3.4 mmol) was
transferred in a 250 mL flask and was dried under vacuum for
2 h. Dry dichloromethane (29 mL) was injected in the reaction
flask under nitrogen. Next, TEA (1.4 mL, 0.010 mol) was added
followed by the dropwise addition of methacryloyl chloride
(0.50 mL, 5.2 mmol). The reaction was allowed to proceed for
3 days at room temperature (RT). The triethylamine bromide
salt produced was removed by filtration under nitrogen, while
the solvent and the excess amounts of the reagents were elimi-
nated under reduced pressure. The product (1 g, yield: 85%) was
1
dried in a vacuum oven at RT and was characterized by H
NMR.
In this study, we report the synthesis of poly[2-(dimethyl-
amino)ethyl methacrylate]-co-poly[10,30,30-trimethyl-6-methacryl-
oyloxyspiro(2H-1-benzopyran-2,20-indoline)] (PDMAEMA-co-
PSP) copolymers that are sensitive to the solvent polarity,
temperature, pH, and light irradiation. Four different PDMAE-
MA-co-PSP copolymers in which the content of the photosensi-
tive moieties was varied between 1.3 and 10 mol % were prepared
by atom transfer radical polymerization (ATRP). A poly(methyl
methacrylate)-co-PSP (PMMA-co-PSP) copolymer containing
19 mol % SP was also synthesized by the same method. The
effect of the polarity of the comonomer on the isomerization
mechanism of the spiropyran groups was evaluated bycomparing
the solvatochromic behavior of the PDMAEMA-co-PSP and
PMMA-co-PSP copolymers. To the best of our knowledge, this is
the first time that “reverse photochromism” of the chromophore
units, induced by the polarity of the comonomer, is reported.
The reversible acid-catalyzed ring-opening process of the photo-
sensitive component of the PDMAEMA-co-PSP copolymers in
aqueous media was studied. The influence of the copolymer
composition and the photoinduced MC-to-SP isomerization on
the phase transition temperature of the PDMAEMA-co-PSP
copolymers was investigated. Finally, the photoinduced deco-
loration rates of the chromophore moieties of the PDMAEMA-
co-PSP copolymers were determined in solvents of different
polarity. The multiresponsive behavior of the PDMAEMA-co-
PSP copolymers in addition to their cationic character at low pH,
which can induce attractive electrostatic interactions with DNA,
The following peaks were identified in the proton nuclear
magnetic resonance (1H NMR) spectrum of the synthesized
monomer (500 MHz, CDCl3) (see Figure S2, Supporting In-
formation): δ 1.16 (3H, H-30), 1.31 (3H, H-30), 2.05 (3H, H-11)
2.73 (3H, H-10), 5.70 (1H, =CH2, H-9), 5.73 (1H, H-3), 6.31
(1H, =CH2, H-10), 6.52 (1H, H-4), 6.71 (1H, H-70), 6.79-6.85
(4H, H-5, H-50, H-7, H-8), 7.08 (1H, H-40), 7.18 (1H, H-60).
Polymerizations. A typical polymerization procedure fol-
lowed for the synthesis of the copolymers is described below
(see Scheme 1). The initiator ethyl 2-bromoisobutyrate (14.7 μL,
9.87 ꢀ 10-5 mol), 10,30,30-trimethyl-6-methacryloyloxyspiro-
(2H-1-benzopyran-2,20-indoline) (0.080 g, 2.21 ꢀ 10-4 mol),
and DMAEMA (0.147 g, 9.35 ꢀ 10-4 mol) were transferred in
a 100 mL round-bottom flask containing 6 mL of dry THF.
The reaction was set under a nitrogen atmosphere, and subse-
quently, the ligand PMDETA (124 μL, 5.94 ꢀ 10-4 mol) and
CuBr (0.0141 g, 9.83 ꢀ 10-5 mol) were added under continuous
stirring. The polymerization flask was inserted in an oil bath at
60 ꢀC, and the polymerization was allowed to proceed for 24 h.
The product was isolated by precipitation in hexane and was
dried under vacuum at RT. This procedure resulted in a
PDMAEMA-co-PSP random copolymer containing 10 mol %
PSP, with an Mn of around 5900 g/mol and Mw/Mn equal to 1.56
by gel permeation chromatography (see entry 4, Table 1) at 84%
yield. The other three PDMAEMA-co-PSP copolymers were
prepared by varying the comonomer ratio at a fixed initia-
tor concentration. A similar procedure was followed for the

Article
Macromolecules, Vol. 43, No. 17, 2010 7075
Scheme 1. Reaction Scheme for the ATRP Copolymerization of 10,30,30-Trimethyl-6-methacryloyloxyspiro(2H-1-benzopyran-2,20-indoline)
with DMAEMA and MMA
Table 1. GPC and 1H NMR Characterization Data of the
Spiropyran-Based Copolymers
The photoinduced bleaching of the chromophore moieties of
the PDMAEMA-co-PSP1 copolymer was studied in solvents of
different polarity by UV/vis spectroscopy in the wavelength
range 250-850 nm. A 5 ꢀ 10-2 wt % solution of the copolymer
in water and in acetonitrile was prepared, and the samples were
irradiated with visible light utilizing a Nd:yttrium aluminum
garnet (YAG) laser operating at the second harmonic, λ = 532
nm (B.M. Industries, Series 5000). The ∼100 mJ laser beam was
focused in the cuvette to ensure the homogeneous irradiation of
the sample. The decoloration rate, kobs, of the chromophore
units of the copolymer was calculated by fitting the experimental
data to the following equation:45
GPC results
composition by 1H
Mw/Mn NMR (mol % SP)
copolymer
Mn
Mw
PDMAEMA-co-PSP1
PDMAEMA-co-PSP3
PDMAEMA-co-PSP6
PDMAEMA-co-PSP10 5900
PMMA-co-PSP19 4200
8900 13 000
6600 10 000
8300 12 300
1.46
1.51
1.48
1.56
1.84
1.3
3.0
6.0
10.0
19.0
9 200
7 700
synthesis of the PMMA-co-PSP copolymer which was recovered
by precipitation in water.
lnðAt - AeÞ ¼ - kobst þ lnðA0 - AeÞ
ð1Þ
Characterization of the Copolymers. Gel Permeation Chro-
matography (GPC). The molecular weights (MWs) and the
molecular weight distributions (MWDs) of the random copoly-
mers were determined by GPC using a mixed-D and a mixed-E
column. The mobile phase was THF, delivered at a flow rate of
1 mL min-1. The refractive index signal was measured using a
Shodex RI-101 refractive index detector. The calibration curve
was based on six narrow MW linear polystyrene standards
where A0, At, and Ae are the absorbance at the λmax of the MC
form at time 0, t, and ¥ (taken as the absorbance of the visible
irradiation photostationary state), respectively.
Results and Discussion
Synthesis of the Random Copolymers by ATRP. Random
copolymers carrying photochromic spiropyran moieties
were synthesized by ATRP. A photosensitive molecule ex-
hibiting a singlet excited state was chosen instead of that
with a triplet state, i.e., nitrospiropyrans, because the latter
accelerates the photodegradation process.46 The polymeriz-
able spiropyran molecule was prepared first. The successful
ranging from 1310 to 299 000 g mol-1
.
1H NMR Spectroscopy. The composition of the copolymers
was determined by H NMR spectroscopy using a 500 MHz
1
Avance Bruker NMR spectrometer. The solvent used was
CDCl3 containing tetramethylsilane (TMS) which served
as an internal reference. The absolute molecular weight of
the PMMA-co-PSP copolymer was also determined by 1H
NMR.
UV/vis Absorption Studies. The solvatochromism of the
copolymers in a range of solvents of different polarity was
monitored by UV/vis spectroscopy. The absorption spectra of
3.3 ꢀ 10-2 wt % copolymer solutions in toluene, chloroform,
tetrahydrofuran, dichloromethane, and water were recorded
using a Lambda 25 Perkin-Elmer UV/vis spectrophotometer
in the wavelength range 250-850 nm.
The effect of the addition of acid or base on the absorption
properties of the PDMAEMA-co-PSP3 copolymer in aqueous
media was studied. A 1.2 ꢀ 10-2 wt % aqueous solution of the
photosensitive copolymer was prepared by dissolving the ap-
propriate amount of polymer in Milli-Q water prefiltered
through a 0.2 μm syringe filter. The UV/vis spectra of the
aqueous solution were recorded upon the addition of 2.4 μL
aliquots of 0.1 M HCl or NaOH solution.
The temperature-induced phase transition behavior of the
PDMAEMA-co-PSP copolymers was investigated by recording
the absorbance of 1 wt % aqueous solutions of the copolymers
at 750 nm, while the solution temperature was raised from 25 to
90 ꢀC at a heating rate of 1 ꢀC/min.
1
synthesis was confirmed by H NMR spectroscopy due to
the absence of the peak at 4.26 ppm attributed to the -OH
group of the precursor alcohol and the appearance of two
new peaks at 5.7 and 6.3 ppm assigned to the protons of
the carbon-carbon double bond and a peak at 2.04 ppm
which corresponds to the methyl group of the methacryloyl
moiety.47 The peaks attributed to the aromatic protons next
to the ester group were also shifted to lower fields following
the esterification reaction and verified the synthesis of the
monomer.
Next, the dye monomer was copolymerized with DMAE-
MA or MMA by ATRP. Four different copolymers of
10,30,30-trimethyl-6-methacryloyloxyspiro(2H-1-benzopyran-
2,20-indoline) with DMAEMA and one with MMA were
prepared in THF (see Table 1). The presence of the peaks due
to both monomer repeat units and the significant broadening
1
of the peaks in the H NMR spectra of the products (see
Figure S3, Supporting Information) verified the synthesis
of the copolymers. The absolute molecular weight of the
PMMA-co-PSP copolymer was calculated by H NMR by
1
ratioing the peak integrals at 3.59 ppm attributed to the

7076 Macromolecules, Vol. 43, No. 17, 2010
Achilleos and Vamvakaki
Figure 1. UV/vis absorption spectra of the PDMAEMA-co-PSP3 (a) and PMMA-co-PSP (b) copolymers in solvents of different polarity: toluene
(- -), chloroform ( ), tetrahydrofuran (;), dichloromethane (---), and water (- -).
3
3 3 3
3 3
methoxy protons of the PMMA and at 5.7 ppm due to the
proton of the benzopyran phenyl group next to the spiro
tetrahydral linkage over that at 4.1 ppm, which corresponds
to the methylene protons of the initiator. The actual mole-
cular weight of the copolymer was found 1360 g/mol, which
is in good agreement with the theoretical value of 1840 g/mol,
suggesting the “controlled/living” character of the poly-
merization. However, the actual molecular weights of the
PDMAEMA-co-PSP copolymers could not be determined
observed in toluene at 558 nm was attributed to the planar
zwitterionic MC form. The formation of the colored zwitter-
ionic MC form before irradiation was attributed to the
“reverse photochromism”, that is, the stabilization of the
open zwitterionic form of the chromophore in polar media
before irradiation, induced by the polar DMAEMA como-
nomer units.54,55 Upon increasing the solvent polarity a
shift of the MC peak to shorter wavelengths was observed
from 558 nm for toluene (SPP scale = 0.655) to 555 nm
for chloroform (SPP scale = 0.786), 551 nm for THF (SPP
scale = 0.838), and 549 nm for DCM (SPP scale = 0.876),
while the lowest value was found for water (539 nm, SPP
scale=0.962). This phenomenon, which is known as “nega-
tive” solvatochromism, is related to the stabilization of the
planar zwitterionic MC form in the solvent medium and
is common in spiropyrans disposing electron-withdrawing
groups on the benzopyran moiety, known as pull-type
photochromic compounds, such as the molecule used in this
study.20 In polar media, the ground state of the bipolar
colored form56 is stabilized more effectively compared to
the excited state (μground > μexcited).15 This results in a greater
energy gap between the two MC states and to a hypsochro-
mic shift of the absorption bands. A peak with an absorption
maximum at 524 nm was also observed in the spectrum in
toluene and was attributed to SP-MC and intramolecular
MCn stacks which are of H-type and thus blue-shifted
compared to the free MC.34 These aggregation phenomena
are strongly enhanced in systems with high chromophore
contents and under conditions which stabilize the planar
zwitterionic MC molecules.57 The H-stacks exhibited a
“negative” solvatochromism similar to that of the free MC
form, and a blue shift of the peak was observed upon
increasing the polarity of the solvent (524 nm for toluene,
521 nm for chloroform, 517 nm for THF, 515 nm for DCM,
and 506 nm for water). The “negative” solvatochromism of
the photochromic species of the PDMAEMA-co-PSP3 co-
polymer upon increasing the polarity of the medium was also
observed visually as a small change in the color of the
solution from light pink in toluene to light orange in water
(see Figure 1a inset).
1
by H NMR due to the overlap of the peaks attributed to
the methylene protons of the initiator and the methylene
protons next to the ester group of the DMAEMA monomer
repeat units at 3.95-4.35 ppm (see Figure S3). The apparent
molecular weights (Mn’s) and the molecular weight distribu-
tions (Mw/Mn’s, PDIs) of the copolymers were determined
by GPC (see Table 1). The Mn’s of the photoresponsive
copolymers were found between 4200 and 8900 g/mol, while
their PDIs ranged between 1.46 and 1.84, which are lower
than those reported for spiropyran-based copolymers syn-
thesized by conventional free radical polymerization.26,48,49
The slightly higher molecular weight distributions of the
copolymers, compared to those reported for the controlled
polymerization of styrenic and methacrylate monomers,50,51
are attributed to the low molecular weights of the poly-
mers and the polymerization of highly functional monomers
(DMAEMA and SP) and are similar to those reported in the
literature for PMMA-co-PSP copolymer analogues synthe-
sized by ATRP.30
The copolymer composition (see Table 1) was calculated
from the ratio of the peak integrals attributed to the methyl-
ene protons next to the ester group of DMAEMA (see
Figure S3, H-13) and the proton of the benzopyran phenyl
group next to the spiro tetrahydral linkage (see Figure S3,
H-3) for the PDMAEMA-co-PSP copolymers, while the
integrals of the peaks corresponding to the methoxy protons
of MMA and the benzopyran protons of SP next to the spiro
tetrahydral linkage were ratioed to calculate the composition
of the PMMA-co-PSP copolymer.
Solvatochromic Behavior of the Copolymers. The solvato-
chromic behavior of the copolymers in solvents of different
polarity was examined before irradiation. The polarity of
the solvent has been shown to influence the isomerization
of the photochromic molecules between the different MC
forms and the thermal fading of the MC isomers to the
SP form.52,53 Figure 1a shows the UV/vis spectra for five
PDMAEMA-co-PSP3 copolymer solutions in toluene, chlo-
roform, THF, DCM, and water. The absorption band
In contrast to the PDMAEMA-co-PSP copolymer, the
absorption properties of the PMMA-co-PSP analogue were
not influenced by the solvent medium before irradiation (see
Figure 1b). The absorption bands observed in toluene at
296 and 323 nm (shoulder) were attributed to the closed
form of the spiropyran molecule. The two halves of the
photochromic molecule in the closed form are topologically

Article
Macromolecules, Vol. 43, No. 17, 2010 7077
Scheme 2. Reversible Protonation/Deprotonation of the PDMAEMA-co-PSP Copolymers in Acidic/Basic Media
orthogonal; hence, the π-absorption spectrum consists of
two localized transitions rather than a single delocalized
transition for the molecule as a whole.58 The former intense
peak (296 nm) was attributed to the indoline part, and the
latter shoulder (323 nm) was assigned to the chromene
moiety. However, no peak assigned to the free MC form at
558 nm was observed for this copolymer. This was attributed
to the nonpolar character of the MMA comonomer which
does not stabilize the open MC form and thus favors the
“normal photochromism”. The very low intensity band at
∼500 nm was attributed to the formation of a few H-stacks
as discussed above for the PDMAEMA-co-PSP copolymer.
However, a new peak at 366 nm was observed attributed to
a nonpolar MC isomer. It is noted that this peak was not
observed for the PDMAEMA-co-PSP copolymers, suggest-
ing that the nonpolar photoisomer was stabilized by the less
polar MMA units. These species showed negligible sensiti-
vity to the polarity of the surrounding medium and the λmax
of the peak remained at 366 nm, while a yellow color polymer
solution was obtained (see Figure 1b inset) for all solvents.
The above results suggest that the polarity of the comonomer
affects the solvatochromic properties of the polymer by the
selective stabilization of a particular photoisomer. The co-
lored MC form is stabilized by polar comonomers, resulting
in copolymers which exhibit “reverse photochromism” and
“negative solvatochromism”, whereas “normal photochro-
mism” is favored by comonomers of lower polarity. The sta-
bilization of specific chromophore isomers by tuning the
polarity of the comonomer can be very promising for the
development of materials with high specificity in terms of
optical properties.
and the closed SP form is recovered upon the addition
of base.22 Figure 2a shows the absorption spectra of an
aqueous solution of the PDMAEMA-co-PSP3 copolymer
upon the gradual addition of acid. The protonation of both
the DMAEMA (pKa = 7.0)59 and the MC (pKa = 6-7)26
monomer repeat units (see Scheme 2) was taken into account
for the addition of a stoichiometric amount of HCl to fully
protonate the copolymer. For low acid contents (<9.6 μL)
the intensity of the absorption band at 296 nm, assigned to
the closed SP form, decreased gradually, while the intensity
of the absorption peak at 538 nm, which corresponds to the
open MC form, increased (see Figure 2a inset), suggesting the
acid-catalyzed ring-opening of SP to the MC isomer.60,61
Further addition of HCl (>9.6 μL) resulted in the decrease of
the intensity of the peak at 538 nm (see Figure 2a inset) and
the appearance of a new absorption band at 423 nm attri-
buted to the protonated colored MC form.62,63 This is
consistent with the protonation of the open MC species
and the formation of the trans-[MC-OH]þ form at higher
acid concentration. In a competing process, the closed SP
species became protonated as evidenced by the appearance
of an absorption peak at 316 nm attributed to the protonated
closed SP form. It is also noted that the intensity of the peak
attributed to the MC-MC H-type stacks (498 nm) decreased
upon the addition of acid. The suppression of aggregate
formation via intramolecular interactions in acidic media
was attributed to two synergetic effects: the first is the
protonation of the phenolate anions of the open MC form
which led to a positive net charge on the photochromic side
groups, and the second is the protonation of the tertiary
amine groups of DMAEMA, both of which resulted in repul-
sive forces along the polymer chain and prohibited the for-
mation of stacks. The acid-induced process discussed above
was fully reversible; addition of base resulted initially to the
deprotonation of the [MC-OH]þ and [SP-NH]þ species,
signified by the decrease of the intensity of the peaks at 423
and 316 nm, respectively (see Figure 2b), while at higher base
concentration the closed SP species were recovered due to the
MC-to-SP isomerization, leading to an increase of the in-
tensity of the band at 296 nm and a simultaneous decrease of
the MC peak at 538 nm (see Figure 2b inset).
Acidochromic Properties of the Spiropyran-Based Copoly-
mers. The effect of the solution pH on the SP-to-MC
isomerization of the chromophore units of the PDMAE-
MA-co-PSP3 copolymer was also investigated. The closed
SP form isomerizes to the open MC form and becomes
protonated upon the addition of acid leading to the for-
mation of [MC-OH]þ species. A competitive process also
takes place, that is the protonation of the closed SP moieties
leading to the formation of the positively charged [SP-NH]þ
species (see Scheme 2). Both processes are fully reversible,

7078 Macromolecules, Vol. 43, No. 17, 2010
Achilleos and Vamvakaki
Figure 3. Absorption intensities of the chromophore species in an
aqueous solution of the PDMAEMA-co-PSP3 copolymer as a function
of the ratio Reff = [HCl]/[DMAEMA þ SP monomer units] (a) and
(1 - Reff) = [NaOH]/ [DMAEMA þ SP monomer units] (b): SP (2),
MC (b), and [MC-OH]þ (0).
Figure 2. UV/vis absorption spectra of an aqueous PDMAEMA-co-
PSP3 copolymer solution upon the addition of acid (HCl) (a) and
following the neutralization of (a) with base (NaOH) (b).
resulted in a completely reversible process; first, the neutra-
lization of the protonated MC species by NaOH and the
formation of the open MC moieties took place for 0<1 -
Figures 3a and 3b summarize the absorbance intensity
changes for the PDMAEMA-co-PSP3 copolymer solution
upon the addition of acid and base, respectively. The con-
centration of acid in the solution is normalized over the
total concentration of ionizable species (DMAEMA and SP)
defined as Reff = [HCl]/[DMAEMA þ SP monomer units].
Reff represents the theoretical fraction of protonated
DMAEMA and SP monomer repeat units, if one assumes
that all Hþ from the added HCl protonate the DMAEMA
and SP species of the copolymer, and takes values in the
range 0 < Reff < 1 (see Figure 3a).64 For the addition of
base (see Figure 3b) 1 - Reff = [NaOH]/[DMAEMA þ SP
monomer units] is defined as the fraction of the deprotonated
ionizable groups (DMAEMA and SP), assuming that all
OH- from the added NaOH deprotonate the monomer
repeat units, and takes values in the range 0<(1 - Reff)<1.
As shown in Figure 3a, the addition of HCl resulted initially
(0 < Reff < 0.4) in a decrease of the SP species accompanied
by a simultaneous increase of the MC moieties due to the
acid-induced isomerization of SP to the open MC form.
Further addition of acid (0.4 < Reff <1) gave rise to a sharp
increase of the intensity of the [MC-OH]þ species accom-
panied by a decrease of the intensity of the MC units,
suggesting the protonation of the latter moieties. On the
other hand, Figure 3b shows that the addition of base
Reff < 0.6, followed by the isomerization of MC to the closed
SP species in strong alkaline media (0.6<1 - Reff<1).
The reversible acidochromic behavior of the PDMAE-
MA-co-PSP3 copolymer in water was also observed visually.
At high acid concentration the formation of the [MC-OH]þ
moieties was evidenced by the appearance of a yellow color
attributed to the absorption of the protonated MC form at
423 nm, while upon addition of base the deprotonation of the
[MC-OH]þ species resulted in a light red color attributed to
a low concentration of remaining MC species in the solution.
Temperature-Induced Phase Transition of the PDMAE-
MA-co-PSP Copolymers. PDMAEMA is a well-known
temperature-sensitive polymer which exhibits a LCST in
the range between 32 and 46 ꢀC.59,65 In this study we
investigated the effect of the photosensitive comonomer on
the LCST of the PDMAEMA-co-PSP copolymers. The
phase transition behavior of the four copolymers containing
1.3, 3, 6, and 10 mol % chromophore units was studied in
water. Figure 4 shows the absorbance at 750 nm for the four
copolymer solutions in the temperature range 30-90 ꢀC.
The PDMAEMA-co-PSP1 copolymer exhibited a LCST of
44 ꢀC. Furthermore, an increase of the chromophore content
of the copolymer to 3 mol % resulted in a 14 ꢀC increase of

Article
Macromolecules, Vol. 43, No. 17, 2010 7079
Figure 4. Turbidimetry vs temperature curves obtained for aqueous
solutions of the PDMAEMA-co-PSP copolymers at neutral pH:
Figure 5. UV/vis absorption spectra of an aqueous PDMAEMA-co-
PSP1 copolymer solution upon successive irradiation with visible light.
Inset (a) shows the decay of the absorption at the MC maximum (λ =
540 nm) as a function of irradiation time. Inset (b) is the rate constant
plot for the first-order decoloration reaction of the photochromic MC
moieties of the copolymer.
PDMAEMA-co-PSP1 (;), PDMAEMA-co-PSP3 ( ), PDMAE-
3 3 3
MA-co-PSP6 (---), PDMAEMA-co-PSP10 (- -), PDMAEMA-co-
3 3
PSP1 following irradiation with visible light (- - -). The cloud points
were determined from tangents constructed from the turbidimetry
curves.
function of irradiation time. After 32 min of irradiation the
absorbance at 540 nm reached a plateau, suggesting that the
chromophores attained a photostationary state.66 Similar
results were obtained upon irradiating a PDMAEMA-co-
PSP1 copolymer solution in acetonitrile with visible light (see
Figure S4, Supporting Information); the intensity of the
absorption bands at 509 nm (see Figure S4, inset (a)) and
542 nm (see Figure S4, inset (b)) attributed to the H-type MC
stacks and the MC isomer, respectively, decreased progres-
sively with irradiation time. However, the photostationary
state was reached only after 17 min of irradiation in aceto-
nitrile (see Figure S4, insets (a) and (b)). The rate constants
for the visible bleaching, kobs, of the colored MC moieties of
the copolymer in water and in acetonitrile were calculated by
fitting the experimental data to eq 1 (see Figure 5, inset (b)).
kobs in water was calculated to be 1.8 ꢀ 10-3 s-1, whereas
in acetonitrile the photobleaching process exhibited a higher
kinetic value of 3.4 ꢀ 10-3 s-1. The lower kinetic constant
found in water suggests that the ring-closing process is
suppressed in the polar aqueous environment (SPP scale =
0.962) due to the effective stabilization of the MC isomer.
The latter results in a higher energy barrier for the MC-to-SP
isomerization which requires an energerically unfavored
rotation around the central double bond of the mole-
cules.15,18 On the contrary, acetonitrile is a less polar solvent
(SPP scale = 0.895) compared to water, and thus, the MC
form is less effectively stabilized and the MC-to-SP isomer-
ization is energetically favored, resulting in a higher kinetic
value. Similar results have been reported in the literature
for spiropyran molecules in organic solvents of different
polarity.15
the LCST, while for chromophore contents above 6 mol %
the LCST was eliminated and the copolymers remained
soluble in the aqueous medium over the whole temperature
range. The increase in the LCST of the copolymer with the
chromophore content was attributed to the hydrophilic open
MC form of the photosensitive moieties the presence of
which was verified by UV/vis spectroscopy (data not shown).
The polar MC species increase the hydrophilicity of the
copolymer and hence the LCST of the polymer.43 Sumaru
and co-workers reported that the MC-to-SP conversion in
thermally responsive PNIPAM-co-PSP copolymers can be-
come very important in the dark as the temperature increases
and can hinder the tuning of the LCST of the copolymers.26
However, the stabilization of the open MC form by the polar
DMAEMA comonomer units in the PDMAEMA-co-PSP
copolymers is believed to retard the thermal fading of the
chromophore units in the dark and allow tuning the phase
transition temperature of the copolymers. Moreover, irra-
diation of the PDMAEMA-co-PSP1 copolymer with visible
light resulted in the significant decrease of the LCST to 34 ꢀC
(see Figure 4), which is lower compared to that reported in
the literature for a PDMAEMA homopolymer of similar
molecular weight.59,65 This is attributed to the hybrophobic
SP moieties formed upon irradiation of the copolymer with
visible light (see discussion below), which increase the hydro-
phobicity of the copolymer.43 Therefore, the photoinduced
MC-to-SP isomerization of the chromophore units of the
copolymers can be conveniently used to tune their phase
transition behavior, which could be particularly advanta-
geous for certain applications such as catalyst recovery,
waste capture, and others.
Photoresponsive Behavior of the PDMAEMA-co-PSP Co-
polymer. Figure 5 shows the UV/vis spectra for an aqueous
PDMAEMA-co-PSP1 copolymer solution upon successive
visible light irradiation. A gradual decrease of the intensity of
the absorption band at 540 nm, attributed to the bipolar MC
isomer, was observed due to the isomerization of MC to the
colorless SP form. A simultaneous decrease of the intensity
of the absorption band at 507 nm, attributed to the H-type
MC aggregates, was also observed, upon irradiation with
visible light, suggesting the effective MC-to-SP isomeriza-
tionof theMCmoieties ofthe aggregates. Inset (a)inFigure 5
shows the intensity of the absorption band at 540 nm as a
Conclusions
In this study, we report the synthesis of multiresponsive
spiropyran-based copolymers by ATRP. The polarity of the
comonomer affected the stabilization of a certain isomer of
the photosensitive moieties and thus the “normal” vs “reverse”
photochromism of the PSP-based copolymers. In particular,
DMAEMA, a polar comonomer, induced “reverse photochro-
mism” and stabilized the planar zwitterionic form which exhi-
bited a hypsochromic shift upon increasing the solvent polarity,
known as “negative solvatochromism”. On the other hand, the less
polar MMA comonomer, exhibited “normal” photochromism

7080 Macromolecules, Vol. 43, No. 17, 2010
Achilleos and Vamvakaki
and stabilized a nonpolar photoisomer, before irradiation, which
was not affected by the polarity of the solvent. The PDMAEMA-
co-PSP copolymers also exhibited a reversible pH-induced SP-
to-MC isomerization and protonation/deprotonation of the MC
form accompanied by a color change of the polymer solution. A
strong influence of the copolymer composition and the photo-
induced MC-to-SP isomerization of the chromophore moieties
on the phase transition behavior of the PDMAEMA-co-PSP
copolymers was found, with an increase of the LCST with the
hydrophilic MC chromophore content and a decrease of the
transition temperature following the photoinduced MC-to-SP
isomerization of the chromophores due to the hydrophobic
character of the SP moieties. Finally, the PDMAEMA-co-PSP
copolymers exhibited a photoinduced decoloration, upon irra-
diation with visible light, which was faster in acetonitrile com-
pared to water due to the more effective stabilization of the
bipolar MC isomer in polar media. These multisensitive polymers
create new perspectives for the development of multiresponsive
materials for biological and environmental applications.
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Supporting Information Available: Schematic representa-
tion of the reversible photoinduced SP-to-MC isomerization; 1H
NMR spectra of the 10,30,30-trimethyl-6-methacryloyloxyspiro-
(2H-1-benzopyran-2,20-indoline) monomer and the PDMAE-
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co-PSP1 copolymer solution in acetonitrile upon successive
irradiation with visible light. This material is available free of
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