Tuning the color switching of naphthopyrans via the control of polymeric architectures

Article abstract of DOI:10.1039/b617865k

A range of polymerisable photochromic naphthopyrans were prepared via the synthesis of an acrylate moiety on either the naphthalene ring (R) and/or the di-aryl (Ar) rings on the 3-position of the naphthopyran. The monomers obtained (NA) were copolymerised

Full text of DOI:10.1039/b617865k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Tuning the color switching of naphthopyrans via the control of polymeric
architectures{{
Wilasinee Sriprom, Marjorie Ne´el, Christopher D. Gabbutt, B. Mark Heron and Se´bastien Perrier*
Received 7th December 2006, Accepted 5th March 2007
First published as an Advance Article on the web 13th April 2007
DOI: 10.1039/b617865k
A range of polymerisable photochromic naphthopyrans were prepared via the synthesis of an
acrylate moiety on either the naphthalene ring (R) and/or the di-aryl (Ar) rings on the 3-position
of the naphthopyran. The monomers obtained (NA) were copolymerised with methyl
methacrylate (MMA) and methyl acrylate (MA) by the RAFT (reversible addition fragmentation
chain transfer) process in order to control the structure of the polymeric chains and accurately
control the number of photochromic molecules per chain. Films were prepared with the
synthesized photochromic polymers and their photochromic properties were assessed. We found
that the fading rate of the photochromic dye was slower in a matrix of PMMA (higher Tg) than in
a matrix of PMA (lower Tg). By producing block copolymers of PMMA-b-poly(MA-co-NA), the
fading rate was increased, and the production of PS-b-poly(MA-co-NA) further enhanced the
speed of switching, as phase separation occurred between the poly(MA-co-NA) block and the PS
block. This study presents an example of tuning the photochromic properties of a dye via
variation of the architecture of its polymeric support.
Photochromic materials have great potential in various
applications such as ophthalmic lenses,6 erasable optical
Introduction
storage,7–9 optical switching7,9,10 and nonlinear devices.11 For
Photochromism is a reversible transformation of a chemical
system which is induced at least in one direction by
practical applications of these materials, the photochromic
electromagnetic radiation.1 When irradiated with ultraviolet
compounds are required to be incorporated into a polymer
(UV) light, photochromic compounds undergo a reversible
media to form film, sheets, or bead. To date, photochromo-
change between two molecular structures resulting in a change
phores are uniformly incorporated into, or applied to, a
from colourless to coloured. Photochromism can be divided
polymer matrix by several methods, including the dissolution
into two areas depending on how the dye reverts to the
of photochrome in the monomer before free radical polymer-
colourless form. Dyes which revert by light are defined as P
isation, dissolution/dispersion within the host polymer, or
types whereas those which revert by thermal radiation are T
insertion as a separate layer between adjacent layers of the
types.1 There are many organic molecular families exhibiting
polymer matrix.12 The use of photochromic monomers that
photochromism, including spiropyrans, spirooxazines,
can covalently bond to a polymer chain by copolymerisation is
naphthopyrans (chromenes), fulgides, diarylethenes and azo
an alternative approach that offers a number of advantages
compounds. In all these main classes of organic photochromic
such as prevention of the leaching of the photochrome, thus
compounds, naphthopyran is one of current great interest
reducing risks of toxicity, and prevention of phase separation
because of the breadth of colour generated, absence of
between the polymer matrix and the photochrome. This
background colour, fine control over fading kinetics, good
approach has been widely studied for a range of spirooxazine,
thermal reversibility and good resistance to photochemical
spirobenzopyran, and diarylethene derivatives, which have
fatigue. The photochromism of naphthopyrans was first
been copolymerised with monomers such as styrene, methyl
reported by Becker and Michl in 1966.2 The photochromic
methacrylate (MMA), methacrylonitrile, and 2-vinylnaphthal-
reaction of the naphthopyrans involves initial heterolytic
ene.13–17 Naphthopyran monomers have however not
cleavage of C–O bonds on irradiation with UV light generating
received a great deal of attention, in spite of their significant
two coloured ring-opened structures, a trans–cis (TC) and a
trans–trans (TT) merocyanine isomer (Scheme 1). The TC
isomer is thermally cyclised at low temperatures over time
while the TT species is more stable and its ring-closing only
occurs on irradiation with visible light.3–5
Department of Colour and Polymer Chemistry, University of Leeds, UK
LS2 9JT. E-mail: s.perrier@leeds.ac.uk; Fax: +44 113 343 2947;
Tel: +44 113 343 2932
{ This paper is part of a Journal of Materials Chemistry issue
highlighting the work of emerging investigators in materials chemistry.
{ Electronic supplementary information (ESI) available: Synthesis
Scheme 1 Generic photochromic process for naphthopyran derivatives.
J. Mater. Chem., 2007, 17, 1885–1893 | 1885
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advantages over other photochromic classes. To the best of
our knowledge, there exists only patent data describing the
synthesis and use of polyalkoxylated naphthopyrans as
monomers.18 An additional advantage of incorporating a
photochromic species into a polymer by covalent bonds is
the opportunity to finely tune the kinetics of the switching
by modifying the polymer structure. Indeed, Evans et al.
were among the first researchers to report the local
modification of a photochromic moiety environment by
attaching the dye to a polymer support in order to tune its
switching properties.16,19,20 In recent work, they showed that
attaching a photochrome to an oligomer chosen for its
physical properties would enhance the kinetics of colour
switching. Their approach covers the incorporation
of oligomers onto photochromic dyes via chemical
modification, or living radical polymerisation initiated by
spirooxazine.16,19–22
(CPDB) was synthesized following a published procedure.29
A
typical synthesis of a naphthopyran acrylate derivative is
described below.33 Air-and moisture sensitive compounds were
manipulated using standard Schlenk techniques under a
nitrogen atmosphere.
Differential scanning calorimetry (DSC)
The thermal properties of the photochromic block copolymers
were studied by differential scanning calorimetry (DSC) using
a TA Instrument DSC 2010 thermal analyzer under a nitrogen
atmosphere (200 cm3 min21). For measurement of the glass
transition temperature (Tg), about 5–10 mg samples in an
aluminium holder were heated from 20 to 150 uC, then cooled
to 210 uC and reheated to 250 uC. The heating and cooling
rate was set to 10 uC min21. An empty aluminium pan was
used as reference. The Tg value was determined from the onset,
end and inflection of
a step transition using the TA
In the past decade, living radical polymerisations (LRP)23–25
have been very promising for high scale applications in
polymer chemistry because of the combination of undemand-
ing reaction conditions and the industrial friendly nature of
radical polymerisation with the control of molecular
weight, polydispersities and architectures of the living pro-
cesses. To date, the most important techniques of LRP are
nitroxide-mediated polymerisation (NMP),26 atom transfer
radical polymerisation (ATRP),27,28 reversible addition frag-
mentation chain transfer (RAFT)29 polymerisation, and
macromolecular architecture design by interchange of
xanthates (MADIX)30 polymerisation. RAFT and MADIX
have the same mechanism process and are based on the
reversible reaction of a chain transfer agent (CTA), usually a
thiocarbonyl thio derivative, with the propagating species. The
tolerance to a wide range of polymerisable monomers and
solvent under a wide range of reaction conditions (i.e. bulk,
solution, suspension, and emulsion) make RAFT and MADIX
two of the most versatile living polymerisation processes. In
addition, the RAFT technique can also produce multiblock
copolymers with controlled molecular weight and low PDIs as
well as a variety of polymeric architectures such as star,
networks (co)polymers, hyperbranched, and comb/brush
copolymers.31,32 Since the reaction proceeds with retention of
the thiocarbonyl thio end group, an AB di-block copolymer
can be synthesized by the simple addition of a second
monomer.
Instruments Thermal Advantage Universal Analysis software.
Size exclusion chromatography (SEC)
The number-average molecular weight (Mn) and polydispersity
index (Mw/Mn)) were determined using SEC equipped with a
LC 1120 HPLC pump ((Polymer Laboratories, UK), a
MIDAS (type 830) auto sampler (Spark Holland,
Netherlands), a differential refractive index (DRI) detector
(Shodex, RI-101), a 5.0 mm bead-size guard column (50 6
7.5 mm) and two PLgel 5.0 mm MIXED-C columns (300 6
7.5 mm) in series (Polymer Laboratories, UK). THF was used
as the eluent at a flow rate of 1 mL min21 at ambient
temperature and toluene was used as a flow rate marker. The
SEC system was calibrated with polystyrene standards
(Polymer Laboratories, UK) with molecular weights ranging
from 580 to 7 500 000 g mol21
.
UV/vis spectrophotometry
The photochromic properties of the polymeric films and
solution were analyzed by using a SPECORD S100 UV/vis
spectrophotometer. Samples were placed in the spectrophot-
ometer and their absorbance was measured from 400 nm to
700 nm. The samples were then irradiated to a constant
intensity with UV light (365 nm) at room temperature for
1 minute to ensure the sample has reached steady state
coloration. The absorbance decay was then measured in the
dark in the same wavelength range every second, for 5 minutes.
The wavelength of the maximum absorption (lmax) of the ring-
opened naphthopyran derivative (coloured) form was obtained
by plotting the UV spectrum of the sample. Moreover, the time
taken for the intensity of the photogenerated colour to
decrease to half of its original value (half-life, t1/2) was
determined by recording successive spectra and plotting the
lmax as a function of time. The background spectrum of the
slides was automatically subtracted from the samples.
This work investigates the use of the RAFT process to
control the number of naphthopyran moieties incorporated in
a polymer and design polymeric architecture to control the
photochromic properties.
Experimental
General data, materials
All solvents, monomers, and other reagents were purchased
from Aldrich at the highest purity available. Methyl metha-
crylate (MMA, 99%), styrene (Sty, 99%) and methyl acrylate
(MA, 99%) were filtered before utilization through a basic
alumina (Brockmann I) column, to remove the radical
inhibitor. Azobis(isobutyronitrile) (AIBN, 99%) was recrys-
tallised twice from ethanol. 2-(2-Cyanopropyl)dithiobenzoate
1H NMR spectroscopy
1H NMR (400 MHz) spectra were recorded on a Bruker 400
UltraShield spectrometer at 25 uC and d-chloroform was used
as solvent.
1886 | J. Mater. Chem., 2007, 17, 1885–1893
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Typical synthesis of naphthopyran acrylate derivatives—
synthesis of 9-acryloyloxy-[3,3-bis(4-methoxyphenyl)]-3H-
naphtho[2,1-b]pyran (1)
final product was isolated by precipitation in cold hexane.
Conversions were calculated via 1H NMR and molecular
weight distributions were analysed via SEC.
3,3-Bis(4-methoxyphenyl)-9-hydroxy-3H-naphtho[2,1-b]pyran.
A mixture of 2,7-dihydroxynaphthalene (6.0 g, 37.5 mmol) and
1,1-di(4-methoxyphenyl)prop-2-yn-1-ol (10.0 g, 37.3 mmol) in
toluene (600 mL) was heated at reflux for 45 minutes. Then
alumina (20.0 g) was added and the mixture was heated at reflux
for 4 hours. The mixture was filtered hot and the residue washed
with EtOAc. The filtrate was evaporated and the residue
suspended in hexane and 10 drops of EtOAc. The suspension
was filtered. Further hexane and EtOAc were added to the solid
under heating (100 uC), but the solid didnot dissolved entirely. The
mixture was heated in order to concentrate it and then allowed to
cool at room temperature. Then the mixture was filtered to give a
pale brown solid, (3.33 g, 14%). The filtrate was concentrated and
the residue suspended in hexane and 10 drops of EtOAc. The
suspension was filtered to give the title compound as a red-brown
solidwithayieldof35%(5.38 g).Rf: 0.14 (EtOAc–hexane 1 : 1). 1H
NMR(CDCl3, 400.13 MHz, 300 K) d: 7.60 (d, 1H, Jortho =8.8 Hz),
Typical copolymerisation procedure: copolymerisation of
MMA with 9-acryloyloxy-[3,3-bis(4-methoxyphenyl)]-3H-
naphtho[2,1-b]pyran (1) in toluene
MMA (2.1510 g, 21.5 mmol), 1 (0.0992 g, 0.214 mmol), CPDB
(0.0471 g, 0.215 mmol), AIBN (0.0175 g, 0.107 mmol) and
toluene (1.9789 g) were mixed in a Schlenk tube. Prior to
sealing, the mixture was carefully degassed by five freeze–
pump–thaw cycles under high vacuum. The reaction was then
heated at 60 uC and samples were taken at various times and
quenched in ice. Conversions were calculated via 1H NMR and
molecular weight distributions were analysed via SEC.
Typical block copolymerisation procedure: block
copolymerisation of PS, MA and 9-acryloyloxy-[3,3-bis(4-
methoxyphenyl)]-3H-naphtho[2,1-b]pyran (1)
Polystyrene (PS) homopolymer (6 500 g mol21, PDI = 1.11)
was used as macro chain transfer agent for the random
copolymerisation of MA with 1 (135 MA : 1 1 : 1 PS : 0.5
AIBN; 50 vol% toluene at 60 uC for 24 h) following a similar
experimental setup as that described above. After precipita-
tion, poly(St-b-poly(1-co-MA)) was analysed by 1H NMR and
GPC to characterise the composition and molecular weight
distribution. The dithiobenzoate end group was removed from
the polymer by the addition of a large excess of AIBN (20 eq)
to a toluene solution of the polymer, followed by heating at
80 uC for 2.5 hours. The polymer was recovered by precipita-
tion in cold hexane and isolated as a pale yellow solid. The pale
yellow colour is thought to be due to naphthopyran units
remaining in their open-ring form after polymerisation.
7.56 (d, 1H, Jortho = 8.8 Hz), 7.37 (m, 4H), 7.25 (d, 1H, Jmeta
=
2.4Hz), 7.14(d,1H,3J= 10.0Hz), 7.01 (d, 1H, Jortho =8.8Hz), 6.90
(dd, 1H, Jortho = 8.8 Hz, Jmeta = 2.4 Hz), 6.83 (m, 4H), 6.17 (d, 1H,
3J = 10.0 Hz), 4.99 (s, OH), 3.77 (s, 3H)
9-Acryloyloxy-[3,3-bis(4-methoxyphenyl)]-3H-naphtho[2,1-b]
pyran (1). To a suspension of 3,3-bis(4-methoxyphenyl)-9-
hydroxy-3H-naphtho[2,1-b]pyran (5.0 g, 23.34 mmol) in
MeCN (230 mL) cooled to ca. 210 uC, DBU (7.7 mL,
51.54 mmol) was added in a single portion. The solution was
stirred for 20 minutes and then acryloyl chloride (5.2 mL,
63.99 mmol) dissolved in MeCN (25 mL) was added dropwise
to the mixture. The mixture was stirred until room temperature
for 28 hours. MeCN was removed and then water and DCM
were added. The aqueous layer was extracted with DCM. The
organic layers were washed with a solution of HCl 2 M,
combined, dried over Na2SO4 and concentrated. The yellow
solid obtained was chromatographed (SiO2: EtOAc–hexane
3 : 7) to yield the title compound (yield: 98%) as an orange
solid. Rf: 0.24 (EtOAc–hexane 3 : 7). 1H NMR (CDCl3,
400.13 MHz, 300 K) d: 7.73 (d, 1H, Jortho = 8.8 Hz), 7.68
(d, 1H, Jmeta = 2.0 Hz), 7.36 (m, 4H), 7.17 (d, 1H, 3J =
Film formation
Polymers were dissolved in THF or dichloromethane to
produce a viscous solution which was spread with a K-bar
number 8 or with a spin coater Sheen Spinner (I.C.I Patter N)
on microscope glass slides. The films were left to dry at room
temperature.
10.0 Hz), 7.14 (d, 1H, Jortho = 8.8 Hz), 7.09 (dd, 1H, Jortho
=
Results and discussion
8.8 Hz, Jmeta = 2.0 Hz), 6.84 (m, 4H), 6.65 (dd, 1H,
Naphthopyran acrylate derivatives synthesis
2
3
3Jtrans = 17.2 Hz, Jgem = 1.2 Hz), 6.37 (dd, 1H, Jtrans
=
=
17.2 Hz, 3Jcis = 10.4 Hz), 6.19 (d, 3J = 10.0 Hz), 6.05 (dd, 3Jcis
2
10.4 Hz, Jgem = 1.2 Hz), 3.77 (s, 6H)
The naphthopyran derivatives were synthesised as described in
supporting information. In order to test the steric effect of the
polymeric chain on the ring opening kinetics of the naphtho-
pyran, a linker was introduced between the photochromic dye
and the backbone (Scheme 2, 2 and 4 vs. 1 and 3). In addition,
the attachment point between the naphthopyran moiety and
the polymeric backbone was varied from the naphthalene ring
(Scheme 2, 1 and 2) and the di-aryl rings on the 3-position of
the naphthopyran (Scheme 2, 3 and 4).
Typical homopolymerisation procedure: polymerisation of
styrene (St) by the RAFT process
CPDB (1 eq), St (from which inhibitor was previously removed
by filtration through activated basic alumina) (500 eq) and
toluene (50 wt%) were mixed and put in a series of sample
tubes. The oxygen was removed from the reaction mixtures by
pumping nitrogen gas into the mixture for 5 minutes, whilst
allowing the oxygen to be released through an escape needle.
The sealed tubes were then heated to 110 uC, and removed at
different times. Reactions were quenched in ice and the PS
Statistical copolymerisation
The naphthopyran acrylate (NA) derivatives were copoly-
merised with MMA and MA as illustrated in Scheme 3, using
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polymerisations. It is also noteworthy that MMA polymerisa-
tion proceeds faster than MA polymerisation, as previously
observed in RAFT. The molecular weights increase linearly
with conversion (Fig. 3), although the values are slightly higher
than anticipated. Fig. 3 shows a rapid increase in molecular
weight early in the reaction, suggesting a loss of control.
However, the polymerisation becomes rapidly controlled, as
illustrated by the linear evolution of molecular weight with
conversion. This loss of control early in the reaction can be
explained by the slow reaction of the CTA with the
propagating radicals.34 Nevertheless, the molecular weight
remains reasonably close to that expected, within the calibra-
tion error of the GPC system, with polydispersities below 1.2
at all time during polymerisation, thus implying reasonably
good control over the polymeric chain growth. The final
polymers had similar molecular weights (y11 000 g mol21 for
methacrylate derivatives and 7 000–8 500 g mol21 for acrylate
derivatives), and all polymers had a content of y1 mol%
naphthopyran in their final structure. In the case of the
acrylate copolymerisation, we noticed that the naphthopyran
acrylate was incorporated early in the reaction, as 1 mol%
content was reached when polymerisations were stopped as
early as 71% monomer conversion. It is also noteworthy that
the presence of a complex molecule such as the naphthopyran
moiety does not affect either the rate, or the control, of the
polymerisation, illustrating the robustness of the RAFT
process towards functional groups. Finally, our kinetic and
molecular weight data show that, within experimental error,
there is no difference between the copolymerisation of
naphthopyran acrylate derivatives with and without a linker.
In order to test the influence on the photochromic properties
of the attachment point between the naphthopyran and the
polymeric backbone, monomers 3 and 4 were also copoly-
merised with MA by RAFT (Scheme 4). The polymerisation
kinetics were similar to those of 1 and 2 (data not shown), and
Table 1 summarises the reaction conditions and the final
molecular weight and polydispersities of the copolymers of
MA with 1, 2, 3 and 4. A similar degree of photochrome
incorporation in the final copolymer as that observed with 1
and 2 was achieved, whilst keeping a very good control over
the molecular weight and polydispersity of the copolymer.
Scheme 2 Naphthopyran acrylate (NA) derivatives used in this
study.
CPDB as chain transfer agent to mediate the polymerisations.
CPDB is a well known chain transfer agent for RAFT
polymerisation, and has been shown to control very well both
acrylate and methacrylate polymerisations.31,32 We chose to
use RAFT polymerisation rather than a conventional free
radical polymerisation process, as RAFT not only allows the
control of the polymer molecular weight, but it also permits
control of the amount of naphthopyran moieties introduced in
each polymeric chain. The incorporation of the photochrome
into the polymer backbone was estimated by 1H NMR, by
comparing the characteristic signals of the naphthopyran
derivative (e.g. Hd, Fig. 1) with those of the (meth)acrylate
polymer (Table 1).
Both compounds 1 and 2 were copolymerised with MMA
and MA. Table 1 summarises the reaction conditions and the
details of the final product. Each polymerisation was followed
kinetically and the molecular weight evolution was
plotted versus conversion. Fig. 2 shows the linear evolutions
of ln(1/(1 2 C)) with time, thus implying a pseudo-first order
polymerisation kinetic, characteristic of living radical
Scheme 3 Copolymerisation of 1 and 2 with MMA and MA mediated by RAFT.
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Fig. 1 1H NMR spectrum of 9-[3-(acryloyloxy)propoxy]-3,3-bis(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran copolymerised (2) with methyl acrylate.
Table 1 Final conversion, molecular weight and polydispersity data for the copolymerisation of MMA or MA with compounds 1, 2, 3 and 4
Naphthopyran
acrylate (NA)
Ratio of naphthopyran
incorporated (%)b
Sample
Comonomer
Timea/min
Conversionb (%)
Mnc/g mol21
PDIc
A
B
C
D
E
F
a
1
2
1
2
3
4
MMA
MMA
MA
MA
MA
1200
1220
2665
2600
3535
2130
99
98
77
83
88
71
11 900
10 800
7 500
8 200
8 500
7 000
1.10
1.08
1.16
1.10
1.34
1.12
1.1
1.2
1.0
1.1
0.9
0.9
MA
Time of the polymerisation carried out with the ratio comonomer (MMA or MA) : NA : CPDB : AIBN = 100 : 1 : 1 : 0.5 in 50 vol% of
toluene, under nitrogen, at 60 uC. Determined by 1H NMR (using CDCl3 as solvent) peak intensity ratio. Estimated by SEC using THF as
eluent and PMMA standards. Determined by integrating the characteristic H of the acrylate moiety (acr) of the photochrome and the H
characteristic of the naphthopyran (char) (d at d = 6.19 ppm) and the following equation: (Ichar/(Iacr + Ichar)) .
b
c
d
Thermal analyses
The Tg of all polymers were determined by differential
scanning calorimetry (DSC). A similar Tg (98 uC) was obtained
for both samples A and B, which is below the expected value
for PMMA homopolymers (105–120 uC).35 This can be
explained by the presence of the bulky naphthopyran moiety
in the PMMA chains, which disturbs the order of the chains,
and therefore increases their mobility, independently of the
presence of a linker. On the other hand, polymers C, D, E and
F, exhibited a similar Tg (Tg = 7 uC), which is close to that
expected from a poly(methyl acrylate). It is noteworthy that
the presence of a linker did not seem to affect the overall Tg of
the polymer.
Photophysics
In a polymeric structure, factors such as the formation of
naphthopyran mesomers, the aggregation of naphthopyrans
due to the intermolecular interactions, and the intramolecular
Fig. 2 Pseudo-first-order rate plot for the copolymerisaton of MMA
with 1 ($) and 2 (&), and MA with 1 (.) and 2 (m), mediated by
CPDB in toluene at 60 uC.
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absorbance in the same wavelength range every second, for
5 minutes. The time taken for the intensity of the photo-
generated colour to decrease to half of its original value (half-
life, t1/2) was determined by recording successive spectra and
plotting the lmax as a function of time. The data in Table 2
record the wavelength of maximum absorption (lmax) of the
ring-opened species formed when the polymeric films are
irradiated to a constant intensity with UV light (365 nm) at
20 uC. Additionally, the time taken for the intensity of the
photogenerated colour to decrease to half of its original value
(half-life or bleach rate, t1/2) is presented (Fig. 4). When
comparing samples A and B to C and D, respectively, we
observe that the speed of switching is higher for the films based
on PMA (10–12 s) than that of the films based on PMMA
(over 5 min). The photochromism of polymer A, B, C and D
was also tested in a solution of toluene. In all cases, the
colourless solutions developed intense colorations under UV
light at 365 nm, and regained very rapidly their initial state
after ceasing irradiation. This faster fade rate was expected in
solution, as the hindrance of the solvent-swollen polymeric
chains is greatly diminished.
Fig. 3 Molecular weight and PDI evolution with monomer conver-
sion for the copolymerisation in toluene of MMA with 1 ($) and 2
(&), MA with 1 (.) and 2 (m), mediated by CPDB. Lines represent the
…
theoretical molecular weight for the polymerisation of MMA ( ) and
The photochromic decoloration rates can also be studied by
MA (- - -).
using a mathematical model. We found that a biexponential
equation fits best the experimental data:19 A(t) = A1e2k t
+
1
A2e2k t + Ath, where A(t) is the optical density at lmax, A1 and
A2 are contributions to the initial optical density A0, k1 and k2
are the rates of the fast and slow components, respectively, and
Ath is coloration when time approaches infinity. The values for
2
interactions of photochromes with other part of the macro-
molecule may affect the kinetics and the spectral properties of
the system. The samples were irradiated to a constant intensity
with UV light at room temperature, prior to measuring their
Scheme 4 Copolymerisation of 3 and 4 with MA mediated by RAFT.
Table 2 Wavelength of maximum absorption and half-life time of the polymers prepared in this study
Sample
Composition
lmax/nm
t1/2/s
k1/min21
k2/min21
A1
A2
Ath
A
B
C
D
E
F
G
H
I
Poly(MMA98.9%-co-11.1%
Poly(MMA98.8%-co-21.2%
)
)
483
463
439
437
450
480
428
443
441
.300
.300
10
12
10
10
145
165
100
0.080
0.099
0.421
0.444
0.455
0.568
0.181
0.161
0.256
0.007
0.008
0.013
0.007
0.013
0.013
0.012
0.012
0.011
0.24
0.27
0.47
0.47
0.48
0.49
0.36
0.36
0.45
0.31
0.27
0.11
0.07
0.12
0.16
0.40
0.40
0.45
0.53
0.56
0.32
0.41
0.35
0.31
0.41
0.43
0.32
Poly(MA99%-co-11%
)
Poly(MA98.9%-co-21.1%
Poly(MA99.1%-co-30.9%
Poly(MA99.1%-co-40.9%
)
)
)
PMMA-b-poly(MA99.1%-co-10.9%
PMMA-b-poly(MA99.2%-co-20.8%
PS81%-b-poly(MA18%-co-11%
)
)
)
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observe that the covalent link of the photochrome to the
polymeric backbone seems to slow the fading rate. This effect
may be explained by the variation of properties of the polymer
on the photochrome that may affect the naphthopyrans more
than the spiropyrans.
Block copolymerisation
As a living radical polymerisation system, RAFT offers the
potential to produce block copolymers by sequential addition
of monomers. In order to facilitate the switching of the
naphthopyran moiety, we aimed to incorporate the photo-
chromic moiety into a block of low Tg (PMA), with a second
block made of PMMA, of higher Tg, which would allow higher
rigidity of the overall material, thus facilitating film formation.
AB block copolymers poly(MMA)-b-poly(MA-co-NA)), with
NA = 1 or 2, were therefore synthesized (Scheme 5). Initially,
the homopolymerisation of MMA (70 MMA : 1 CPDB : 0.5
AIBN, 50 vol% toluene, 60 uC) was undertaken as described
above, and led to a PMMA of Mn = 7 700 g mol21 (PDI =
1.13). After purification by precipitation, the PMMA block
was used as macro chain transfer agent for the random
copolymerisation of MA with 1 and 2 (30 MA : 1 NA : 1
PMMA : 0.5 AIBN). After precipitation, PMMA-b-P(MA-co-
1), G, (Mn = 13 300 g mol21, PDI = 1.18), and PMMA-
b-P(MA-co-2), H, (Mn = 12 000 g mol21, PDI = 1.18) were
analysed by NMR and GPC (Table 3), to confirm their
compositions and molecular weights. By measuring the fading
rates of block copolymers G and H by UV, as described above,
we found that both rates were half-way between those of the
MMA and MA copolymers (see Fig. 4 and Table 2). However,
the processing of the sample (film formation) was close to that
of the PMMA copolymers. Therefore, a good compromise was
reached between rigidity of the material, provided by the
PMMA matrix, and rapid colour switching, favoured by the
PMA environment of the naphthopyran.
Fig. 4 Photochromic thermal decoloration rates of
a range
of copolymers-naphthopyran conjugates. See Table 2 for sample
identification.
the constants for each system are given in Table 2. We
observed that the incorporation of the naphthopyran deriva-
tive into a methyl acrylate matrix gave a larger increase in the
fast component k1 of the decoloration relative to the PMMA
system. Higher t1/2 and k1 for a PMMA matrix can be justified
by the high Tg of PMMA when compared to PMA. Indeed, at
temperatures below Tg, the movement of the pendant
photochrome is restricted, and the ring-closing process is
slowed. On the other hand, as the Tg of PMA is far closer to
room temperature, the acrylate chains are more mobile and
facilitate the switching, resulting in faster kinetics. It is also
noteworthy that for both PMMA and PMA copolymers the
samples showed a small absorbance in the visible spectrum,
due to daylight. This phenomenon is more intense in the case
of PMA films, which show a pale yellow colour. Although low
Tg of the polymeric support favours rapid photochromic
switching, it leads to processing issues in the film making.
Indeed, PMA films were difficult to obtain, and were rather
unstable by comparison to PMMA-based films. When
comparing the films made from samples C and D with E and
F, respectively, it appears that the attachment point of the
photochrome to the backbone does not affect extensively the
kinetics. Finally, comparing sample A with B, C with D and E
with F, shows that the presence of a linker does affect the
fading rate in the case of the methacrylate copolymers, but
does not seem to influence the fading rate of the acrylate
copolymers (Fig. 4 and Table 2). The presence of a linker keeps
the naphthopyran away from the polymer backbone, and in
the cases of steric hindrance, this affects directly the fading
rate. In the case of the acrylate polymers, the kinetics of ring
closing is too fast to be accurately quantified. Previous studies
have shown that the kinetic behaviour of photochromic
spiropyran groups covalently bonded to a polymer main chain
is very similar to that of spiropyrans incorporated into a
polymer matrix.36,37 In the case of naphthopyrans, however,
when comparing A and B with samples in which 1 and 2,
respectively, have been mixed in a matrix of PMMA, we
In order to improve the fading rate whilst keeping good
process properties of the final polymer, a block copolymer PS-
b-poly(MA-co-1) (I) was synthesized. PS-b-PMA block copo-
lymers have been shown to phase separate,38 and upon phase
separation, we expected the photochrome to be embedded in a
PMA environment, therefore allowing for rapid colour
switching, whilst keeping the rigidity of the PS matrx (Tg
(PS) y 110 uC). The same procedure as that described above
was used, with the initial synthesis of a PS macro chain
transfer agent via homopolymerisation of styrene mediated by
CPDB (Mn = 6 500 g mol21, PDI = 1.10), followed by block
1
extension with MA and 1. H NMR spectroscopy and GPC
analyses demonstrated the successful synthesis of a block
copolymer of PS, MA and 1. The molar ratio of styrene, MA
and 1 in the block copolymer (Mn = 9 600, PDI = 1.21) were
calculated from 1H NMR, and were similar to those of the
PMMA-b-poly(MA-co-NA) copolymers. Moreover, as shown
in Fig. 5, successful block copolymerisation was demonstrated
by a shift to higher molecular weights of the block copolymer
when compared to the macro CTA. The GPC trace of the
block copolymer showed a characteristic shoulder on the peak
at high molecular weight, indicating the occurrence of chain
branching during acrylate polymerisation.39
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Scheme 5 RAFT polymerisation of MMA followed by chain extension with MA and 1.
Table 3 Final conversion, molecular weight and polydispersity data for the block copolymerisation of PMMA with MA and 1 or 2
Naphthopyran
acrylate
Ratio of naphthopyran
incorporated (%)c
Sample
Time/min
Conversiona (%)
Mnb/g mol21
PDIb
PMMA
—
1
2
1375
3935
3920
3920?
98
91
90
91
7 700
13 300
12 000
9 600
1.13
1.18
1.18
1.21
—
G
H
K
a
0.9
0.8
1.0
1
Determined by 1H NMR (using CDCl3 as solvent) peak intensity ratio. Estimated by SEC using THF as eluent and PMMA standards.
Determined by integrating one of the H of the acrylate moiety (acr) of the photochrome and the H characteristic of the naphthopyran (char)
(d at d = 6.19 ppm) and making the calculation: (Ichar/(Iacr + Ichar)).
b
c
The phase separation of the block copolymer was demon-
strated by DSC analyses, which showed the occurrence of two
Tgs, at 5 uC and 119 uC, close to the Tg of PMA (7 uC) and PS
(100 uC). The t1/2 of the block copolymer (K, Table 2) was
measured at 100 seconds, a lower value than that of G,
confirming that phase separation occurring between the PS
and P(MA-co-1) blocks allows for the confinement of the
naphthopyran moiety in a matrix of low Tg PMA, thus
decreasing the fading rate.
Conclusion
Naphthopyran acrylate derivatives were synthesised and
incorporated into poly(methyl methacrylate) and poly(methyl
acrylate) chains via radical polymerisation. The use of RAFT
polymerisation allowed the control of the number of naphtho-
pyran units introduced in the polymers. Fading rate was
Fig. 5 GPC trace of PS (- - -) and PS-b-poly(MA-co-1) (—).
1892 | J. Mater. Chem., 2007, 17, 1885–1893
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coatings, films and plastics, US Pat. 99-US20663 2000015629,
19990909, 2000.
19 R. A. Evans, T. L. Hanley, M. A. Skidmore, T. P. Davis,
G. K. Such, L. H. Yee, G. E. Ball and D. A. Lewis, Nat.Mater.,
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20 R. A. Evans and G. K. Such, Aust. J. Chem., 2005, 58(12),
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21 G. K. Such, R. A. Evans and T. P. Davis, Macromolecules, 2006,
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shown to be dependent on the matrix surrounding the
naphthopyran and the presence of a linker between the
photochrome and the polymeric backbone. The use of living
radical polymerisation to produce block copolymers that
phase separate allowed us to encapsulate the naphthopyran
derivative into a low Tg polymer local environment, therefore
enhancing its fading rate, whilst keeping a continuous matrix
of high Tg, to allow easy processing of the polymer into films.
22 G. K. Such, R. A. Evans and T. P. Davis, Mol. Cryst. Liq. Cryst.,
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26 C. J. Hawker, A. W. Bosman and E. Harth, Chem. Rev., 2001,
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