Photochromic polymer conjugates: The importance of macromolecular architecture in controlling switching speed within a polymer matrix

Article abstract of DOI:10.1021/ma901830b

Naphthopyran-poly(n-butyl acrylate) conjugates with different geometries were assembled using ATRP. First, within a rigid lens matrix, an investigation of the photochromic behavior of various poly(n-butyl acrylate), p(n-BA), homopolymers showed that midplacement of a single dye moiety, made possible using a Y-branching difunctional photochromic initiator, gave superior fade kinetics per chain length of conjugated polymer compared to end-functionalized homopolymers. Furthermore, having the dye pendant from the chain opposed to directly within the chain was also found to be advantageous. Fading kinetics became faster when chain length was increased, except in the case of linear random copolymers made by copolymerization of n-butyl acrylate with a naphthopyran acrylate. A gradient copolymer made with a nonphotochromic difunctional initiator and a naphthopyran methacrylate displayed superior kinetics. Films consisting of ABA triblock copolymers, incorporating the photochromic in the middle of a soft p(n-BA) section, gave slower switching speeds compared to lens samples, with responses that were highly tunable and dependent on the amount of soft section inhabited by the photochromic moiety.

Full text of DOI:10.1021/ma901830b

Macromolecules 2010, 43, 249–261 249
DOI: 10.1021/ma901830b
Photochromic Polymer Conjugates: The Importance of Macromolecular
Architecture in Controlling Switching Speed within a Polymer Matrix
Francesca Ercole,†,‡,§ Nino Malic,†,‡ Simon Harrisson,†,‡ Thomas P. Davis,§ and
Richard A. Evans*,†,‡,§
†CSIRO Molecular & Health Technologies, Bag 10, Clayton VIC 3169, Australia, ‡The Cooperative Research
Centre for Polymers, 8 Redwood Drive, Notting Hill, VIC 3168, Australia, and §Centre for Advanced
Macromolecular Design, School of Chemical Sciences and Engineering, The University of New South Wales,
Sydney, NSW 2052, Australia
Received August 17, 2009; Revised Manuscript Received September 28, 2009
ABSTRACT: Naphthopyran-poly(n-butyl acrylate) conjugates with different geometries were assembled
using ATRP. First, within a rigid lens matrix, an investigation of the photochromic behavior of various
poly(n-butyl acrylate), p(n-BA), homopolymers showed that midplacement of a single dye moiety, made
possible using a Y-branching difunctional photochromic initiator, gave superior fade kinetics per chain
length of conjugated polymer compared to end-functionalized homopolymers. Furthermore, having the dye
pendant from the chain opposed to directly within the chain was also found to be advantageous. Fading
kinetics became faster when chain length was increased, except in the case of linear random copolymers made
by copolymerization of n-butyl acrylate with a naphthopyran acrylate. A gradient copolymer made with a
nonphotochromic difunctional initiator and a naphthopyran methacrylate displayed superior kinetics. Films
consisting of ABA triblock copolymers, incorporating the photochromic in the middle of a soft p(n-BA)
section, gave slower switching speeds compared to lens samples, with responses that were highly tunable and
dependent on the amount of soft section inhabited by the photochromic moiety.
Introduction
restricted in their movement, known as the matrix effect.7 In an
attempt to overcome this drawback, industry has focused on the
Photochromic dyes are well-known for their commercial
application in light-sensitive ophthalmic lenses due to their ability
to undergo a reversible color change when irradiated with
ultraviolet (UV) light.1 Naphthopyran dyes, also known as
chromenes,2 are noteworthy examples, being able to thermally
decolorize in the absence of irradiation, to develop a broad range
of intense colors depending on their electronic substitution and to
withstand fatigue on prolonged exposure to light.
The photocoloring process takes place via the heterolytic
cleavage of the C-O bond in the pyran moiety, resulting in a
distribution of isomeric open forms (merocyanines) which are
colored due to their extended conjugation and quasi-planar
structures. In order to convert from a closed form to an
open form, or the reverse, the molecules must undergo a
large intramolecular rotation to become coplanar-like in their
colored (open) form or orthogonal in their clear (closed) forms
(Figure 1).3,4
Practical application in ophthalmic eyewear requires that the
photochromic compounds be incorporated into the polymer
host/substrate using one of several strategies: the process of
imbibing integrates the molecules within the top layers of the
host;5 casting-in can be carried out by dissolution or dispersion of
the dyes into the host monomers prior to curing the material; or a
separate layer of the dyes can be incorporated as a film on top of,
in between, or in adjacent layers of the host.6 However, when
photochromic molecules are incorporated into a polymer matrix,
their switching speed is found to be considerably slower com-
pared to what can be achieved in solution. This is because within
the limited free volume of the plastic host the molecules are
bulk host material by using plasticizers, by improving the method
of dye application, and by developing faster dyes. However, for
applications such as spectacles, where hardness and abrasion
resistance are important, the components and properties of the
substrate cannot be compromised in order to achieve a desired
improvement in the rate of coloration and fade.
The control of photochromic switching speed in a rigid host
can now be achieved using engineered photochromic-polymer
conjugates.8 This approach has been developed byususing a cast-
in method, where an optimized host matrix is left unmodified in
order to bring about changes to photochromic performance.
Instead, the dye that is added to the host matrix composition has
a polymer/oligomer bound directly to it in order to bring about
critical changes to its local environment, i.e., what immediately
surrounds the dye within the host matrix. A local fluid environ-
ment can be introduced around the dye by conjugation to a
soft oligomer, such as p(n-BA), allowing greater freedom of
movement during molecular rotation which results in faster
switching.9
Specialized dye-polymer conjugates can be constructed using
controlled radical polymerization techniques, such as atom
transfer radical polymerization (ATRP). This is mainly carried
out with the approach of growing well-defined polymers (with
controlled molecular weights, Mn, and polydispersity indexes,
PDI) from photochromic inititiators.10-13 An important feature
of the technology is that tuning of the fade speed of the
photochromic material can be achieved by adjusting both the
Tg of the polymer and the chain length of the tail. It has recently
been demonstrated that the geometry of the conjugate can also
affect performance. Larger and more tunable improvements have
been observed for the midplacement of the dye within p(n-BA),
*Corresponding author. E-mail: richard.evans@csiro.au.
r 2009 American Chemical Society
Published on Web 11/02/2009
pubs.acs.org/Macromolecules

250 Macromolecules, Vol. 43, No. 1, 2010
Ercole et al.
Figure 1. Photochromic transition between clear (closed) and colored (open) forms of a 2H-naphthopyrans.
Scheme 1. Synthesis of Photochromic Control Compounds 2 and 4
Figure 2. Photochromic dye-p(n-BA) conjugate strategies: (A) end-
placement of the dye; (B, C) midplacement of the dye; (D) random
copolymer; and (E) gradient copolymer with pendant photochromic
groups, made with photochromic acrylate and methacrylate.
a low-Tg polymer, vs the end placement of a dye.14,15 This
Y-branching approach was developed using an aromatic central
scaffold which allows the dye to be attached centrally, and then
initiator sights located on opposite sides of the aromatic ring
allow the radial growth of two polymer arms.
We believe that chain architecture has an important role in
determining the level of encapsulation that can be provided by
polymer conjugation and its ability to affect the local environ-
ment around the photochromic. The present paper extends this
concept by comparing several motifs for applying p(n-BA)
conjugation to a naphthopyran photochromic, as depicted in
Figure 2. End-functional photochromic-polymer conjugates (A)
were assembled using a monofunctional photochromic initiator.
Midfunctional photochromic-polymer conjugates (B) and (C)
were explored using difunctional photochromic initiators. In the
case of (B) this was approached using a symmetrical, difunctional
initiator which incorporated an aliphatic scaffold, and with (C),
two arms were grown biradially from opposite sides of a central
photochromic molecule. Photochromic functional monomers
were used for the synthesis of random copolymers (D) and
gradient copolymer (E), containing pendant dye moieties. ATRP
was used to polymerize these structures. Their photochromic
performances were then evaluated with reference to one another
and to those of unconjugated controls when cast within a rigid
host matrix.
Figure 3. Evolution of polydispersity (O) and molecular weight (b)
with conversion for the atom transfer radical polymerization of n-butyl
acrylate with initiator 5 at 90 °C in benzene where [monomer]/
[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[5]=100:1:2:1. Calculated molec-
ular weight (---) by NMR conversion using the equation Mn
=
((monomer MW) ꢀ ([monomer]/[5]) ꢀ (NMR conversion)) þ (MW
of 5). Polymerization times ranged from 2 to 12.8 h.
Results and Discussion
Hydroxy-functional naphthopyrans 1 and 316 were synthesized
in high yield, as described in previous publications,10,14,17 and
then converted to the red-coloring isobutyrate derivatives 2 and 4
via simple esterification routes as shown in Scheme 1. With the
aim of scrutinizing the effect of polymer conjugation on the
performance of the dye, these electronically equivalent controls
served as reference points for our tests.
Photochromic ATRP initiators 5 and 7 were also prepared by
esterification of these hydroxy-functionalized precursors with
2-bromoisobutyryl bromide (Scheme 2). In the case of photo-
chromic initiator 6, the aliphatic branching agent, 2,2-bis-
(hydroxymethyl)propionic acid,18 was first converted to 2,2-
bis(2-bromo-2-methylpropanoyloxymethyl)propanoic acid,19-21
Lastly, given the promising results obtained for Y-branching
approaches (B) and (E), these p(n-BA) structures were extended
to make ABA-type triblock copolymers with isobornyl acrylate
(IBA), where the photochromic moiety resides within the soft
p(n-BA) central portion. These block copolymers were made into
films and their photochromic behavior was compared to one
another and to those of dye-p(n-BA) conjugates that were cast in
the rigid host matrix.

Article
Macromolecules, Vol. 43, No. 1, 2010 251
Scheme 2. Atom Transfer Radical Polymerization Synthesis of Naphthopyran-Poly(n-butyl acrylate) Conjugates Displaying Molecular Weights of
Purified Samples Tested in Survey (dNbpy = 4,40-Dinonyl-2,20-bipyridine)
using 2-bromoisobutyryl bromide. The corresponding acid chlor-
ide was then coupled to the hydroxy-functionalized naphtho-
pyran 1.
This can be attributed to long chain branching which occurs
at higher conversions in polyacrylates as a result of back-
biting and scission processes.22
Naphthopyran-Polymer Conjugate Synthesis. ATRP was
used to synthesize various naphthopyran-p(n-BA) conju-
gates of increasing molecular weight (Mn, g/mol), as depicted
in Scheme 2. All homopolymerizations were carried out in
solution at 90 °C with CuBr and 4,40-dinonyl-2,20-bipyridine
(dNbpy) as the catalyst system. In order to gauge the viability
of these conditions, evolution plots were generated
(Figures 3, 4, and 5). These displayed a linear progression
of Mn with conversion, with values similar to those expected
theoretically for a controlled radical process and with low
PDIs (e1.2) throughout. Furthermore, first-order kinetic
plots for each of these polymerizations showed a near-linear
relation with time (refer to Supporting Information). The
specific characteristics of the polymerizations that generated
the conjugates are shown in Table 1 and the final Mn values
of purified samples which were subsequently tested are
displayed in Scheme 2. The corresponding GPC traces
(shown in Supporting Information) display a high mole-
cular weight shoulder for end-function polymers 8d and 8e.
Copolymerizations of naphthopyran acrylate 11 with
n-BA were carried out using the same polymerization con-
ditions (catalyst system, temperature, and solvent; see
Figure 6) to yield 13a-13c. Made only from acrylates, these
copolymers are expected to contain a random distribution of
naphthopyrans on each polymer chain. The final composi-
tion and molecular weights of each conjugate are shown in
Scheme 3, their GPC traces and first-order kinetic plots are
displayed in the Supporting Information, and their specific
polymerization characteristics are shown in Table 1. Con-
versions were found to be no higher that 50%, even after 8 h,
and in each case the remaining quantity of unreacted photo-
chromic acrylate proved difficult to remove from the bulk
polymer. In contrast, the purification of polymers made out
of photochromic initiators was far less problematic since all
photochromic moieties were utilized in the polymerization
process.
A small proportion of naphthopyran methacrylate 12
was copolymerized with n-BA using difunctional ATRP

252 Macromolecules, Vol. 43, No. 1, 2010
Ercole et al.
Table 1. Polymerization Characteristics of Naphthopyran-Poly-
(n-butyl acrylate) Conjugates
time conv [monomer]/ exp theor
(min)b (%)c [initiator]
d
e
Mn PDI
sample
typea
Mn
8a
8b
8c
8d
homopolym
60
20
36
48
68
88
15
19
24
34
47
22
22
29
38
51
20
27
48
67
130
100
100
100
130
200
200
200
200
200
100
200
200
200
200
150
150
150
135
3360 3332 1.18
5710 5211 1.10
8150 6769 1.10
11600 9303 1.12
18700 15183 1.16
4900 4611 1.14
5800 5498 1.13
7100 6854 1.12
10200 9551 1.11
13900 12781 1.11
3820 3672 1.16
5940 6492 1.20
7830 8210 1.17
10300 10645 1.15
14500 13900 1.12
4270 4040 1.23
5950 5386 1.17
11500 9423 1.15
14400 12000 1.13
homopolym 240
homopolym 360
homopolym 770
homopolym 960
homopolym
homopolym
homopolym 140
homopolym 200
homopolym 340
homopolym
homopolym
homopolym
homopolym 140
homopolym 210
copolymer
copolymer
copolymer
copolymer
8e
10a
10b
10c
10d
10e
9a
9b
9c
9d
9e
75
90
Figure 4. Evolution of polydispersity (O) and molecular weight (b)
with conversion for the atom transfer radical polymerization of n-butyl
acrylate with initiator 6 at 90 °C in benzene where [monomer]/
[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[6]=200:1:2:1. Calculated molec-
60
60
80
ular weight (---) by NMR conversion using the equation Mn
=
((monomer MW) ꢀ ([monomer]/[6]) ꢀ (NMR conversion)) þ (MW
13a
13b
13c
14a
60
130
490
120
of 6). Polymerization times ranged from 1 to 3.5 h.
a Homopolym. = poly(n-butyl acrylate) homopolymer as shown in
Scheme 2; copolymer = poly(n-butyl acrylate)-co-poly(naphthopyran)
as shown in Scheme 3. b Polymerizations for 8a-8e performed with
n-butyl acrylate at 90 °C in benzene with [monomer]/[5] as indicated and
[CuBr]/[4,40-dinonyl-2,20-bipyridine] = 1:2; polymerizations for 9a-9e
performed with n-butyl acrylate at 90 °C in benzene with [monomer]/[6]
as indicated and [CuBr]/[4,40-dinonyl-2,20-bipyridine] = 1:2; polymer-
izations for 10a-10e performed with n-butyl acrylate at 90 °C in benzene
with [monomer]/[7] as indicated and [CuBr]/[4,40-dinonyl-2,20-
bipyridine] = 1:2; polymerizations for 13a-13c performed with n-butyl
acrylate and 11 (98:2 by mole, respectively) at 90 °C in benzene with
[monomers]/[ethyl 2-bromoisobutyrate] as indicated and [CuBr]/[4,40-
dinonyl-2,20-bipyridine] = 1:2; polymerization for 14a performed with
n-butyl acrylate and 12 (99:1 by mole, respectively) at 90 °C in benzene
with [monomers]/[BBMP] as indicated and [CuBr]/[4,40-dinonyl-2,20-
bipyridine] = 1:2. c Determined by 1H NMR analysis of polymerization
mixture. d Crude Mn value, determined by GPC with THF as eluent with
poly(n-butyl acrylate) equivalents obtained using Mark-Houwink
parameters on a PS calibration. e Calculated based on monomer con-
version plus initiator molecular weight (example 1H NMR spectra
displayed in Supporting Information).
Figure 5. Evolution of polydispersity (O) and molecular weight (b)
with conversion for the atom transfer radical polymerization of
n-butyl acrylate with initiator 7 at 90 °C in benzene where [mono-
mer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[7] = 200:1:2:1. Calcu-
lated molecular weight (---) by NMR conversion using the equa-
tion Mn = ((monomer MW) ꢀ ([monomer]/[7]) ꢀ (NMR con-
version)) þ (MW of 7). Polymerization times ranged from 1.3 to
5.7 h.
initiator, n-butyl 2,2-bis(2-bromo-2-methylpropanoyloxy-
methyl)propanoate (BBMPP) (Scheme 3), to yield 14a. It
was expected that an earlier and complete uptake of the
naphthopyran monomer would occur since in this case its
reactivity ratio in the copolymerization would be greater
than that of n-BA.23 Consequently, the crude polymerization
mixture was found to contain no unreacted monomer by the
end of the polymerization time (67% conversion). The
difunctional nature of the ATRP initiator BBMPP also
means that the naphthopyran moieties reside in the central
portion of the polymer, within a Y-branched structure,
depicted as “E” in Figure 2.
As shown in Scheme 4, several Y-branched midfunctional
naphthopyran-p(n-BA)-conjugates were synthesized using
difunctional initiator 6 and then chain extended with IBA to
produce 15a-15c. The initial homopolymerizations with n-
BA were carried out in solution analogously to 9a-9e and
are labeled as p(n-BA) macroinitiator in Table 2. Block
copolymer 16a was made directly from conjugate 14a as in
Scheme 5. These ABA triblock copolymers, with naphtho-
pyran/s contained within the middle soft section, were
fabricated into films for testing.
The composition of the polymers was ascertained by
1H NMR, with specific examples displayed in the Suppor-
ting Information. Resonance peaks corresponding to
the photochromic functionality as well as the initiator scaf-
fold are evident in each. These aspects are confirmed by
Figure 6. Evolution of polydispersity (O) and molecular weight (b)
with conversion for the atom transfer radical copolymerization of
n-butyl acrylate with monomer 11 (2%) at 90 °C in benzene, where
[monomers]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[ethyl 2-bromoiso-
butyrate initiator] = 150:1:2:1. Calculated molecular weight (---) by
NMR conversion using the equation Mn = {(n-butyl acrylate MW) ꢀ
([n-butyl acrylate]/[ethyl 2-bromoisobutyrate]) ꢀ (NMR conversion)}
þ {(11 MW) ꢀ ([11]/[ethyl 2-bromoisobutyrate]) ꢀ (NMR conversion)}
þ (MW of ethyl 2-bromoisobutyrate). Polymerization times ranged
from 1 to 8.2 h.
comparison of the spectrum of the conjugate with that of
the initiator. The GPC traces showed successful block ex-
tension (Figures 7-10), a process that was made possible by

Article
Macromolecules, Vol. 43, No. 1, 2010 253
Scheme 3. Atom Transfer Radical Polymerization Synthesis of Naphthopyran-Poly(n-butyl acrylate) Copolymers Displaying Molecular Weights and
Compositions of Purified Samples Tested in Survey (NP = Naphthopyran Monomer, dNbpy = 4,40-Dinonyl-2,20-bipyridine)
the living nature of ATRP. The polymers displayed
good film processing properties and phase separated
on annealing, as evidenced by DSC analysis which
showed two separate glass transition temperatures (Tg) for
each.
biexponential equation4,24 was used to analyze the thermal
ring closure kinetics of each sample:
AðtÞ ¼ A1e-k t þ A2e-k t þ Ath
1
2
Evaluation of Photochromic Performance. Cast-in Lenses.
The development of intense color in a photochromic test
sample, either a cured lens or a cast film, is induced by
incident irradiation with UV light. Upon cessation of the UV
light the samples decolorize spontaneously in the dark due to
the thermal back-reaction which occurs as the naphthopyran
open form undergoes ring closure. Spectrokinetic properties
can be studied by monitoring absorption density with time at
the λmax of the colored form. The sample is continuously
irradiated for 1000 s, and then the decoloration kinetics
of the sample are investigated in the dark at 20 °C upon
cessation of UV irradiation. The following empirical
where A(t) is the optical density at λmax of the open form; A1
and A2 are the contributions to the initial optical density A0;
k1 and k2 are exponential decay rate constants of fast and
slow components respectively and Ath is the residual colo-
ration (offset).
The parameter A0 which is also presented is the absor-
bance level achieved after 1000 s of continuous irradia-
tion. This equation has been used frequently to repre-
sent and compare the decoloration behavior of both
spirooxazines11-13 and naphthopyrans10,14,17 within solid
media and has consistently fitted our decoloration
curves with correlation coefficients (R) greater than 0.99.

254 Macromolecules, Vol. 43, No. 1, 2010
Ercole et al.
Evaluation of T1/2 values, which is the time taken for the
sample to fade to half of initial absorbance value, provides
insight into the overall kinetics. The photochromic proper-
ties of the lens samples are displayed in Table 3, and those of
the cast films comprising the block copolymers are shown in
Table 4.
monomer formulation at a set concentration level (1.5 ꢀ
10-7 mol of photochromic conjugate per gram of host matrix
composition). This lens monomer formulation comprised
poly(ethylene glycol) 400 dimethacrylate (PEGDMA) and
2,20-bis(4-methacryloxyethoxy)phenylpropane (EBPDMA),
1:4 weight ratio, and with 0.4 mass % azobis(isobu-
tyronitrile) (AIBN). The formulations were mixed thoro-
ughly and thermally cured in a mold to produce opti-
cally clear test samples (as described in the Experimental
Details).
Photochromic lens samples were prepared by adding the
photochromic conjugates (or controls) individually to a lens
Scheme 4. Atom Transfer Radical Polymerization Synthesis of
Naphthopyran ABA Triblock Copolymers 15a-15c (with Poly-
(isobornyl acrylate) as the A Block Sections and Poly(n-butyl acrylate)
as the B Block Section), with Molecular Weights and Compositions of
Purified Samples Testeda
Both end-functional and midfunctional naphthopyran-
p(n-BA) conjugates displayed superior decoloration rates
compared to their controls (Table 3). For each type the rates
mildly increased with molecular weight of conjugated tail
Scheme 5. Atom Transfer Radical Polymerization Synthesis of
Naphthopyran ABA Triblock Copolymer 16a (with Poly(isobornyl
acrylate) as the A Block Sections and (Naphthopyran)-co-Poly(n-butyl
acrylate) as the B Block Section), Displaying Molecular Weight and
Composition of Purified Sample Tested (NP = Naphthopyran Mono-
mer, n-BA = n-Butyl Acrylate, IBA = Isobornyl Acrylate)
a (i) Polymerization of n-butyl acrylate with initiator 6 at 90 °C in
benzene; (ii) block extension of purified poly(n-butyl acrylate) macro-
initiator with isobornyl acrylate at 90 °C in benzene; refer to Table 2 for
specific polymerization characteristics. NP = naphthopyran monomer,
n-BA = n-butyl acrylate, IBA = isobornyl acrylate.
Table 2. Polymerization Characteristics of Naphthopyran-Poly(n-butyl acrylate)-b-Poly(isobornyl acrylate) Triblock Copolymers
time (min)a
conv (%)b
[monomer]/[initiator]
exp Mn
theor Mn
PDI
Tg (°C)e
c
d
type
p(n-BA) macroinitiator
15a
p(n-BA) macroinitiator
15b
p(n-BA) macroinitiator
60
250
80
250
170
250
220
52
61
35
67
58
42
40
100
154
200
150
200
144
150
7270
30000
9740
36700
16500
28400
27000
7530
26800
9825
30500
15759
28800
28000
1.13
1.24
1.19
1.22
1.14
1.17
1.17
-46, 47
-48, 48
15c
16a
-41, 74
-44, 80
a For 15a polymerization of n-butyl acrylate with initiator 6 at 90 °C in benzene, where [monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[6] =
100:1:2:1, and block extension of purified poly(n-butyl acrylate) macroinitiator with isobornyl acrylate at 90 °C in benzene, where
[monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[macroinitiator]=154:1:2:1; For 15b polymerization of n-butyl acrylate with initiator 6 at 90 °C in
benzene where [monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[6]=200:1:2:1, and block extension of purified poly(n-butyl acrylate) macroinitiator
with isobornyl acrylate at 90 °C in benzene, where [monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[macroinitiator]=150:1:2:1; For 15c polymeriza-
tion of n-butyl acrylate with initiator 6 at 90 °C in benzene, where [monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[6]=200:1:2:1, and block exten-
sion of purified poly(n-butyl acrylate) macroinitiator with isobornyl acrylate at 90 °C in benzene, where [monomer]/[CuBr]/[4,40-dinonyl-2,
20-bipyridine]/[macroinitiator]=144:1:2:1; For 16a block extension of p(n-butyl acrylate) macroinitiator 14a (see polymerization characteristics shown
in Table 1) with isobornyl acrylate at 90 °C in benzene where [monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[macroinitiator]=150:1:2:1. b Determined
by 1H NMR analysis of polymerization mixture. c For p(n-butyl acrylate) macroinitiator determined by GPC with THF as eluent with poly(n-butyl
acrylate) equivalents obtained using Mark-Houwink parameters on a PS calibration; for block copolymers estimated by 1H NMR of purified samples.
d Calculated based on monomer conversion plus macroinitiator molecular weight. Example 1H NMRs displayed in the Supporting Information. e Two
Tg values determined by DSC; refer to Experimental Details.

Article
Macromolecules, Vol. 43, No. 1, 2010 255
Figure 7. Overlaid and normalized GPC traces of each step of block
copolymer formation. Peak A=midfunctional naphthopyran-poly(n-
butyl acrylate) macroinitiator; peak B=naphthopyran-poly(n-butyl
acrylate)-b-poly(isobornyl acrylate) triblock copolymer, 15a (shown in
Scheme 4).
Figure 10. Overlaid and normalized GPC traces of each step of block
copolymer formation for 16a (as shown in Scheme 5).
Therefore, two polymer chains per photochromic moiety
provides better encapsulation within the rigid host matrix.
Furthermore, placing the photochromic pendant from the
chain, as 9a-9e, gives superior performance compared to
directly placing it within the chain, as 10a-10e.
This is also reflected in the major rate constant k1, which,
when compared directly to that of the control, is markedly
higher for midfunctional conjugates 9a-9e (Figure 12). The
values in fact start to approach twice the value of the control
(k1=0.595 min-1) and this effect arises at a relatively modest
molecular weight of conjugated tail (e.g., 9d with Mn 10 500
g/mol, k1=1.116 min-1). Coloration values, A0, of samples
prepared from conjugates 8a-8e, 9a-9e, and 10a-10e,
which have exactly one dye moiety per polymer chain, can
be directly compared to that of their controls since they all
contained equivalent concentrations of dye. As seen in
Table 3, the values for these conjugates were 1.1-1.3 times
greater than their controls. This is in agreement with pre-
vious work which showed that a higher level of coloration
was associated with samples that displayed faster kinetics.14
Naphthopyran-p(n-BA) linear random copolymers
13a-13c showed decay speeds comparable to the midfunc-
tional (homopolymer) conjugates, 10a-10e; however, per
chain length their performance was found to be superior to
that of end-functional conjugates, 8a-8e. This is also in
agreement with previous results which compared various
naphthopyran copolymers with end-functional conjugates10
and can be ascribed to greater association of pendant
naphthopyran moieties with neighboring n-BA units within
the copolymer. Interestingly, increases in chain length were
correlated with a tuning down of response (k1 goes down
from 13a to 13c). This is most likely due to increasing
naphthopyran units per chain along the series that would
decrease chain mobility. Therefore, from the point of view of
incomplete uptake of monomer units during the copolymer-
ization and an inability to regulate the exact location of the
dye in the chain, this approach is less attractive overall
compared to Y-branching architecture (9a-9e) which pro-
vides both a precise and well-encapsulated location for the
dye in the chain.
Figure 8. Overlaid and normalized GPC traces of each step of block
copolymer formation. Peak A=midfunctional naphthopyran-poly(n-
butyl acrylate) macroinitiator; peak B=naphthopyran-poly(n-butyl
acrylate)-b-poly(isobornyl acrylate) triblock copolymer, 15b (shown in
Scheme 4).
Figure 9. Overlaid and normalized GPC traces of each step of block
copolymer formation. Peak A=midfunctional naphthopyran-poly(n-
butyl acrylate) macroinitiator; peak B=naphthopyran-poly(n-butyl
acrylate)-b-poly(isobornyl acrylate) triblock copolymer, 15c (shown in
Scheme 4).
showing that chain length can be used to finely tune the rate
of decay. We have found that when a short and hard block
segment is inserted near a photochromic, followed by a
lubricating section, this allows a much wider range of switch-
ing speeds to be accessed.25 This has also been the case when
an aromatic Y-branching linker is used to synthesize mid-
functional p(n-BA) conjugates.14 In light of these results, the
level of tunability appears to be dependent on the nature of
the linker near the photochromic moiety.
Copolymer 14a also incorporates a Y-branched structure
where the naphthopyran moieties reside in the central por-
tion of the polymer, as depicted in Figure 2 and Scheme 3. Its
kinetics within the host matrix were in fact superior to those
of all the linear copolymers, 13a-13c, indicating that this
system offers better encapsulation per chain length.
As seen in Figure 11, a comparison of the decoloration
performance of end-functionalized conjugates 8a-8e with
that of mid-functionalized conjugates, 9a-9e and 10a-10e,
shows that when the photochromic is positioned in the center
of the chain, it responds favorably with lower T1/2 values and
with an overall improved performance per chain length.
Hypsochromic (blue) shifts in wavelength of the colored
form were seen for the photochromic conjugates in the host
matrix compared to the controls. This can be attributed to
effective shielding of naphthopyran moieties, as a result
of conjugation to p(n-BA), from the more polar matrix
environment that comprises substantial PEG units. This is

256 Macromolecules, Vol. 43, No. 1, 2010
Ercole et al.
Table 3. Photokinetic Analysis of the Decoloration of Poly(n-Butyl acrylate)-Naphthopyran Conjugates and Their Corresponding Controls in
Host Matrix (PEGMA:EBPDMA 1:4 Mass Composition)a
k1 (min-1
)
A1
k2 (min-1
)
A2
Ath
t1/2 (s)d
b
c
sample
λmax (nm)
Mn
A0
2
8a
8b
8c
8d
8e
4
10a
10b
10c
10d
10e
2
9a
9b
9c
9d
500
500
496
496
495
495
500
492
492
492
492
492
500
497
495
495
495
495
495
495
495
495
0.93
1.02
1.06
1.11
1.11
1.19
0.90
1.06
1.09
1.07
1.04
1.09
0.92
1.11
1.08
1.21
1.19
1.21
0.80
1.01
1.76
1.92
0.6095
0.7356
0.7920
0.8192
0.8801
0.8543
0.5943
0.8151
0.8483
0.8800
0.8782
0.8961
0.5950
1.0319
1.0267
1.0647
1.1159
1.1168
0.9216
0.8642
0.8406
0.9802
0.6415
0.7117
0.7321
0.7368
0.7124
0.7396
0.6334
0.7461
0.7594
0.7608
0.7677
0.7658
0.6630
0.7352
0.7374
0.7356
0.7439
0.7453
0.7435
0.7351
0.7544
0.7316
0.0699
0.0743
0.0864
0.0844
0.7138
0.0556
0.0763
0.0878
0.0634
0.0551
0.0534
0.0723
0.0856
0.0740
0.0774
0.0648
0.0505
0.0658
0.0414
0.0736
0.0887
0.1727
0.0975
0.0478
0.0331
0.0303
0.0555
0.0253
0.1127
0.0318
0.0213
0.0202
0.0194
0.0206
0.1004
0.0237
0.0238
0.0231
0.0185
0.0171
0.0248
0.0361
0.0333
0.0513
0.2394
0.2226
0.2216
0.2229
0.2260
0.2225
0.2346
0.2096
0.2080
0.2063
0.2005
0.2037
0.2184
0.2297
0.2307
0.2304
0.2268
0.2285
0.2174
0.2189
0.1999
0.2120
123.0
88.5
79.5
76.5
75.0
73.2
130.0
75.0
70.5
67.5
66.0
66.0
120.0
62.0
62.0
59.0
56.0
56.0
66.0
73.2
70.5
65.0
2780
5800
8000
11300
18100
4900
5800
7100
10200
13900
3090
6090
7330
10500
15200
4100
5600
12100
15300
9e
13a
13b
13c
14a
a Samples initially irradiated at 350-400 nm for 1000 s, then decoloration monitored at λmax of the colored form (predetermined by wavelength scan of
colored form) at 20 °C in the dark for 4800 s. b Molecular weight (Mn, g/mol) of purified conjugates estimated from GPC analysis: poly(n-butyl acrylate)
equivalents obtained using Mark-Houwink parameters on a PS calibration. c Measured absorbance intensity at onset of thermal decoloration period.
d Time taken for the initial absorbance value, A0, to decay to half its original value.
Table 4. Photokinetic Analysis of the Decoloration of Naphthopyran Tribock Copolymer Films
k1 (min-1
)
A1
k2 (min-1
)
A2
Ath
t1/2 (s)d
b
c
sample
λ
max (nm)
Mn
[n-BA]/[IBA]
A0
15a
15b
15c
16a
488
488
488
488
30000
36700
28400
27000
1: 2
1: 1.9
2: 1
1.19
2.00
1.42
1.27
0.5215
0.6680
0.9743
0.7360
0.5418
0.6410
0.7860
0.7542
0.0646
0.0739
0.0249
0.0804
0.1996
0.1345
0.0149
0.0515
0.2180
0.1954
0.1968
0.1756
178.0
110.0
60.0
2.1: 1
80.0
a Samples initially irradiated at 350-400 nm for 1000 s, then decoloration monitored at λmax of the colored form (predetermined by wavelength scan of
colored form) at 20 °C in the dark for 4800 s. b Molecular weight (Mn, g/mol) of purified block copolymers estimated from 1H NMR. c Measured
absorbance intensity at onset of thermal decoloration period. d Time taken for the initial absorbance value, A0, to decay to half its original value.
Figure 12. Thermal decoloration comparison: k1(conjugate)/k1-
(control) of naphthopyran-poly(n-butyl acrylate) conjugates (8a-8e,
9a-9e, 10a-10e, 13a-13c, and 14a) when cast in the host matrix
PEGDMA:EBPDMA (1:4).
Figure 11. Thermal decoloration comparison: measured T1/2 values (in
seconds) of naphthopyran-poly(n-butyl acrylate) conjugates (8a-8e,
9a-9e, and 10a-10e) vs their corresponding molecular weight when
cast in the host matrix, PEGDMA:EBPDMA (1:4).
63 and 52 s for its corresponding propionate derivative.10,17
The ability of the photochromic-polymer conjugates in a
rigid host to surpass solution kinetics is unlikely, strongly
suggesting that the Y-branching technology presented here,
which achieved a T1/2 value of 56 s (for 9d and 9e), has
reached a limit. Extra branching points are unlikely to offer
additional kinetic benefits.
In conclusion, these results show that by changing the
arrangement and geometry of units comprising the photochro-
mic-polymer conjugate, one can further tune the switching
behavior within a rigid host, and this is possible without making
modifications to the bulk host material.
particularly evident for conjugates 10a-10e which displayed
the largest shift (492 nm) indicating a significant insula-
tion and encapsulation effect. However, given that these
structures were second best overall in terms of enhancing
kinetics, the weight of two polymer chains radiating from a
central, nonpendant photochromic moiety seems to offset
this benefit. Overall, Y-branched structures 9a-9e, which
contain two polymer arms per midfunctional and pendant
photochromic moiety, displayed the fastest kinetics and the
most impressive colorabilities.
In previous studies the T1/2 value for the naphthopyran
control dye 2 measured in toluene at 20 °C was found to be

Article
Macromolecules, Vol. 43, No. 1, 2010 257
Figure 14. Structures of monomers of thermally curable host matrix
formulation.
The film comprised of 16a, which also contained a high
proportion of p(n-BA), was second in speed to 15c (T1/2 80 s
vs T1/2 60 s). Furthermore, 14a, which is the precursor to 16a,
displayed a higher coloration value, A0, in the lens compared
to all the conjugates, indicating more than one naphthopyr-
an moiety is present per chain. As described above, the
photochromic moieties of this conjugate reside within the
central part of the polymer with proximity to one another,
which increases their local rigidity and reduces the mobility
of polymer chains. With reference to Table 3, this is sup-
ported by the fact that the decolorization behavior of 14a
within the lens matrix is a lot slower than conjugate 9e (T1/2
65 s vs T1/2 56 s), even though both have approximately the
same molecular weight.
Figure 13. Thermal decoloration curves (monitored in the dark at
20 °C after UV irradiation for 1000 s) of naphthopyran-p(n-butyl
acrylate)-b-p(isobornyl acrylate) block copolymers. Refer to Schemes 4
and 5 and Table 4 for sample description.
Photochromic Films: ABA Triblock Copolymers. The tun-
ing of naphthopyran switching in bulk films has already been
achieved using block copolymers synthesized using reversi-
ble addition-fragmentation chain transfer (RAFT) poly-
merization.26 These structures were linear block copolymers
made with naphthopyran monomers copolymerized into the
soft poly(methyl acrylate) sections of the block copolymers.
The hard sections comprised either polystyrene or poly-
(methyl methacrylate), both optically clear, high-Tg poly-
mers. Controlled radical polymerization techniques can be
used to synthesize hard-soft block copolymers that phase
separate with the photochromic encapsulated into the low-
Tg environment of the soft phase. The soft local environment
allows switching speed to be enhanced while the continuous
high-Tg matrix maintains structural integrity at higher than
ambient temperatures. This approach was explored in this
paper using ATRP with the formation of ABA triblock
copolymers with a promising arrangement and choice of
building units for photochromic kinetics where Y-branched
naphthopyran-p(n-BA) macroinitiators were block ex-
tended with p(IBA) (Schemes 4 and 5). An ideal system of
encapsulation for the dye was afforded by its midplacement
within the fluid phase of p(n-BA), and the surroun-
ding p(IBA) was chosen as the hard section of the block
copolymers because of its high Tg, hardness, com-
parable properties to poly(methyl methacrylate) and poly-
styrene, and ease of polymerization.27 Films of the block
copolymers were prepared by casting followed by annealing
for 16 h. The photochromic properties of films comprised
of 15a-15c and block copolymer 16a, the latter made from
Y-branched p(n-BA) macroinitiator 14a, are presented in
Table 4.
Lastly, these studies display an overall tendency of the
casting-in method to provide faster kinetics in comparison to
the films. The macroinitiators used to synthesize block
copolymers 15a-15c had the same structures and similar
Mn values (7270-16 200 g/mol) as 9c-9e (Mn 7330-15 200
g/mol). However, the corresponding block copolymer films
of 15a-15c displayed slower kinetics (k1=0.52-0.97 min-1
)
compared with the lens kinetics displayed by conjugates
9c-9e (k1 = 1.06-1.12 min-1). Furthermore, comparing
the kinetics of 14a in the lens matrix (k1=0.98 min-1) with
that of the film 16a (k1 = 0.74 min-1), the same trend is
displayed. The lens host is a continuous cross-linked network
with a measured Tg of ∼120 °C,9 so chain mobility within the
local environment of the photochromic rests heavily on the
ability of p(n-BA) to partition away from the host
matrix. This will be enhanced by the discontinuity and
incompatibility between the two components. However, in
the case of the films, the block copolymer comprises the
bulk matrix, where the ability of the two phases to effectively
separate is critical and therefore highly dependent on proces-
sing conditions. In order to bring kinetics between the
two matrices closer together, this aspect could be further
examined.
Conclusion
The overlaid thermal decolorization curves of the block
copolymer films, shown as Figure 13, clearly show that a
high level of tuning is achievable using this approach, and it
is the composition of the block copolymers that is primarily
implicated; the fastest decolorization time was achieved for
15c (T1/2 60 s), which contained the highest proportion of
p(n-BA) and the highest quantity of n-BA repeat units and
the slowest time was achieved by the 15a (T1/2 178 s) which
contained the lowest proportion of p(n-BA) and also the
lowest quantity of n-BA repeat units. As distinct from the
casting-in method described above, here the photochromic
conjugates themselves comprise the matrix. The extent of
separation of the photochromic moieties from the hard
p(IBA) component is therefore highly dependent on the
amount of p(n-BA) encapsulating them.
ATRP was used to synthesize naphthopyran-polymer con-
jugates of a variety of architectures. First, within a lens rigid
matrix, an investigation of various p(n-BA) homopolymers
showed that a midfunctional placement of the dye, made possible
with a difunctional photochromic initiator, gave superior kinetics
per chain length of conjugated polymer. Moreover, it was
preferable to have the dye pendant from the chain as opposed
to directly within the chain. Analysis of naphthopyran-p(n-BA)
copolymers showed that having the dye pendant from the chain
as a monomer was also attractive; however, the ability to tune
response via chain length was not possible using random copo-
lymers made with a naphthopyran acrylate. A better approach
was to use a gradient copolymer system with a nonphotochromic
difunctional initiator that allowed total incorporation of the

258 Macromolecules, Vol. 43, No. 1, 2010
Ercole et al.
naphthopyran methacrylate within the middle portion of the
polymer.
polymer). The compositional ratio of the copolymers (including
triblocks) was calculated by 1H NMR via the integrated
peak intensity ratio of naphthopyran vs that of the polymers.
All other spectra were recorded on a Bruker Av400 spectro-
meter.
Positive ion EI mass spectra were run on a ThermoQuest
MAT95XL mass spectrometer using ionization energy of 70 eV.
Accurate mass measurements were obtained with a resolution of
5000-10000 using PFK as the reference compound.
Thermal analysis by differential scanning calorimetry (DSC)
was performed in order to determine the Tg of the triblock
copolymers. This was carried out using a Mettler Toledo
DSC821 machine with temperature and heat flow calibrated
using indium and zinc as reference substances. Samples
(∼10 mg) were heated under nitrogen from -50 to 150 °C at
10 °C/min. The Tg values were taken from the midpoints of the
heat flow changes observed in the second heat cycle.
Photochromic Analysis. Under continuous UV irradiation,
the photochromic responses of the samples (cured lenses or cast
films) were analyzed on a light table composed of a Cary 50
spectrophotometer to measure the absorbance and a 160 W
Oriel xenon lamp as an incident light source. A series of two
filters (Edmund Optics 320 cutoff and bandpass filter U-340)
were used to restrict the output of the lamp to a narrow band
(350-400 nm). The samples were maintained at 20 °C and
monitored at their maximum absorbance of the colored form
for a period of 1000 s. Then the thermal decoloration was
monitored in the absence of UV irradiation for a further 6000 s.
Synthesis of 6-Hydroxy-2-(4-(2-hydroxyethoxy)phenyl)-2-
phenyl-2H-naphtho[1,2-b]pyran-5-carboxylate (3). The complete
procedure for synthesis of the title compound can be derived
from what is already reported in the literature.17 4-Hydroxy-
benzophenone was first converted to 4-(2-hydroxyethox-
y)benzophenone using bromoethanol and Na2CO3. This was
then converted to 1-(2-hydroxyethoxy)phenyl)-1-phenyl-prop-
2-yn-1-ol using 2 mol equiv of (trimethylsilyl)acetylene and n-
butyllithium (1.6 M in hexane) with respect to the benzophe-
none. The title compound was then derived from 1-(2-hydro-
xyethoxy)phenyl)-1-phenyl-prop-2-yn-1-ol and methyl 1,4-
dihydroxynapthalene-2-carboxylate, after purification by col-
umn chromatography (silica gel 60, ethyl acetate/hexane) as a
bright yellow solid (2.3 g, 75%). 1H NMR (400 MHz, d6-
acetone) δ: 3.80-3.83 (m, 2H, CH2CH2OH), 3.87 (t, 1H,
J 5.80 Hz, OH), 4.00 (t, 2H, J 4.85 Hz, CH2CH2OH), 4.06 (s,
pyran-CH, COOCH3), 6.38 (d, J 10.0 Hz, 1H, pyran-CH), 6.87
(apparent d, J 8.8 Hz, 2H, ArH), 7.22-7.26 (m, 1H, ArH),
7.31-7.35 (m, 2H, ArH), 7.47 (apparent doublet, J 8.8 Hz, 2H,
ArH), 7.52 (d, J 10.0 Hz, 1H, pyran-CH), 7.56-7.63 (m, 3H,
ArH), 7.75-7.79 (m, 1H, ArH) 8.32 (d, J 8.4 Hz, 1H, ArH), 8.42
(d, J 8.4 Hz, 1H, ArH), 12.17 (s, 1H, ArOH). 13C NMR (100
MHz, d6-acetone) δ: 53.8, 62.1, 71.3, 82.7, 104.0, 115.3, 115.6,
123.5, 125.2, 125.5, 126.7, 128.0, 128.4, 128.9, 129.5, 129.6,
129.7, 130.2, 131.6, 138.3, 142.6, 146.9, 157.9, 160.2, 173.6. Mass
Spec (EI): m/z 468.1 ([M]þ 86%), 436.1 (84), 391.1 (31), 363.1
(20), 289.1 (21), 268.1 (39), 223.1 (27), 207.1 (25), 191.1 (39),
181.0 (26), 165.1 (23), 147.0 (25), 131.0 (36), 121.0 (24), 119.0
(28), 105.0 (25), 77.0 (29), 69.1 (100). Mass Spec (HR, EI): m/z
468.1568 (C29H24O6 requires 468.1573).
The formation of copolymers with the photochromic encap-
sulated in the middle of a Y-branched central soft section was
made possible using an ABA triblock geometry. Their described
films showed enhanced tuning of response dependent on the
overall proportion of soft section inhabited by the photochromic.
It is interesting that regardless of the method used to assemble
a photochromic material, such as casting in a lens or film
formation, the ability to manipulate polymer architecture is
appealing because it provides an extra avenue to control photo-
chromic properties, beyond chain length and rigidity (Tg).
Experimental Details
Materials. The synthesis of methyl 6-hydroxy-2-(4-methoxy-
phenyl)-2-phenyl-2H-naphtho[1,2-b]pyran-5-carboxylate (1),
methyl 6-(isobutyryloxy)-2-(4-methoxyphenyl)-2-phenyl-2H-
naphtho[1,2-b]pyran-5-carboxylate (2), methyl 6-(2-bromo-
2-methylpropanoyloxy)-2-(4-methoxyphenyl)-2-phenyl-2H-
naphtho[1,2-b]pyran-5-carboxylate (5), methyl 6-(acryloyl)-2-
(4-methoxyphenyl)-2-phenyl-2H-naphtho[1,2-b]pyran-5-carbo-
xylate (11), and methyl 6-(methacryloyl)-2-(4-methoxyphenyl)-
2-phenyl-2H-naphtho[1,2-b]pyran-5-carboxylate (12) has been
described by us previously in the literature.10 2,2-Bis(2-bromo-
2-methylpropanoyloxymethyl)propanoic acid was prepared
using a procedure adapted from the literature.19-21 All chemi-
cals (reagents and solvents) used for synthesis were of high
purity and used as received unless otherwise stated. n-Butyl
acrylate (99þ% purity, Aldrich) was purified by passing
through aluminum oxide 90, activated basic (0.063-0.200 nm,
Merck) to remove inhibitors and then flash vacuum distilled
prior to use. Isobornyl acrylate (tech grade, Aldrich) was
purified by passing through aluminum oxide 90, activated basic
(0.063-0.200 nm, Merck) to remove inhibitors. All reagents
were purchased from Aldrich Chemical Co. unless otherwise
stated. All chromatography was performed using silica gel
(Kieselgel Merck 60, 0.040-0.063 mm), and TLC was perfor-
med on Merck Silica 60F254 plates.
General Experimental Measurements. Gel permeation chro-
matography (GPC) was performed on a Waters 515 HPLC
pump and Waters 717 Plus Autosampler equipped with Waters
2414 refractive index detector and 3 ꢀ Mixed-C (7.5 mm ꢀ 300
mm, 5 μm particle size, linear molecular weight range
200-2 000 000) and 1 Mixed E PLgel column (7.5 mm ꢀ 300
mm, 3 μm particle size, linear molecular weight range up to
30 000) from Polymer Laboratories. Tetrahydrofuran (THF)
with a flow rate of 1.0 mL min-1 was used as eluent at 22 ( 2 °C.
Molecular weights for p(n-BA)-naphthopyran conjugates were
calculated via calibration with narrow polydispersity polystyr-
ene standards (Polymer Laboratories) ranging from 600 to 7.5 ꢀ
106 g/mol. Molecular weights (Mn) were converted to p(n-BA)
equivalents using Mark-Houwink parameters28 on the PS
calibration. Number (Mn)- and weight-average (Mw) molecular
weights were evaluated using Waters Millennium/Empower
software. A third-order polynomial was used to fit the log M
vs time calibration curve, which was linear across the molecular
weight ranges.
1H (400 MHz) and 13C NMR (100 MHz) spectra were
obtained with a Bruker Av400 spectrometer at 25 °C. Spectra
were recorded for samples dissolved in deuterated solvent, and
chemical shifts are reported as parts per million from external
tetramethylsilane. Monomer conversions were obtained from
the 1H NMR spectra. The resonances integrated to obtain
conversions for n-BA polymerizations were the vinyl peaks
at 5.8, 6.2, and 6.4 ppm (monomer only) and the OCH2- peaks
at 3.9-4.1 ppm (monomer and polymer). The resonances
integrated to obtain conversions for IBA polymerizations were
the vinyl peaks at 5.8, 6.2, and 6.4 ppm (monomer only) and
the isobornyl -CH- peak at 4.60-4.85 ppm (monomer and
Synthesis of Methyl 6-(Isobutyryloxy)-2-(4-(2-(isobutyryloxy)-
ethoxy)phenyl))-2-phenyl-2H-naphtho[1,2-b]pyran-5-carboxy-
late (4). To an ice-cooled solution of 6-hydroxy-2-(4-(2-hydro-
xyethoxy)phenyl)-2-phenyl-2H-naphtho[1,2-b]pyran-5-carboxy-
late 3 (0.2 g, 0.43 ꢀ 10-3 mol) and triethylamine (TEA) (0.36
mL, 2.57 ꢀ 10-3 mol) in dry dichloromethane (CH2Cl2) (10 mL)
was added dropwise isobutyryl chloride (0.18 mL, 1.72 ꢀ 10-3
mol) under argon. The solution was stirred with ice cooling for
half an hour and was then left to stir for an additional 12 h at
room temperature. The solvent was evaporated under vacuum,
and the residue redissolved in diethyl ether (Et2O) (30 mL) and
washed successively with 0.5 M HCl, water, aqueous NaHCO3,

Article
Macromolecules, Vol. 43, No. 1, 2010 259
water, and brine. The organic layer was dried with MgSO4 and
the solvent evaporated under vacuum. The crude product was
purified by column chromatography (silica gel 60, CH2Cl2),
acid19-21 and 1-butanol using the same general procedure as
above and isolated as a clear oil after purification by column
chromatography (silica gel 60, 4:1 CH2Cl2/hexane) (1 g, 67%).
1H NMR (400 MHz, d6-acetone) δ: 0.92 (t, J 7.4 Hz, 3H, CH3),
1.35 (s, 3H, CH3), 1.36-1.45 (m, 2H, CH2), 1.61-1.68 (m, 2H,
CH2), 1.92 (s, 12H, 4 ꢀ CH3), 4.15 (t, J 6.5 Hz, 2H, CH2), 4.36 (q,
J 11.0 Hz, 4H, 2 ꢀ CH2) 13C NMR (100 MHz, (100 MHz, d6-
acetone) δ: 13.9,18.0, 19.7, 30.9, 31.3, 47.3, 56.9, 65.6, 67.2,
171.2, 172.8, 205.9.
General Procedure for ATRP of n-BA with Naphthopyran
Initiator 5. A stock solution containing n-BA (8.73 g, 68.10 ꢀ
10-3 mol, 5 M), naphthopyran initiator 5 (400 mg, 6.81 ꢀ 10-4
mol), and dNbpy ligand (556.5 mg, 1.36 ꢀ 10-3 mol) was
prepared in benzene (3.4 g). 3 mL aliquots were added to
ampules containing CuBr (22.4 mg, 1.56 ꢀ 10-4 mol); the final
molar ratio of n-BA:5:ligand:CuBr was 100:1:2:1. The ampules
were then degassed with three freeze-pump-thaw cycles,
sealed, and then heated at 90 °C in a thermostated oil bath for
2-12.8 h. The final polymers were purified by (1) evaporation of
excess monomer over a gentle stream of N2, (2) dissolution of the
crude mixtures into CH2Cl2, (3) precipitation into methanol, (4)
decanting of supernatant liquid, (5) column chromatography
(silica gel 60, 1:1 CH2Cl2/Et2O) to remove residual catalyst, and
(6) removal of solvent under vacuum.
General Procedure for ATRP of n-BA with Naphthopyran
Initiator 7. A stock solution containing n-BA (9.56 g, 74.58 ꢀ
10-3 mol, 5 M), naphthopyran initiator 7 (285.8 mg, 3.73 ꢀ 10-4
mol), and dNbpy ligand (304.8 mg, 7.46 ꢀ 10-4 mol) was
prepared in benzene (6.95 g). 3 mL aliquots were added to
ampules containing CuBr (9.4 mg, 6.54 ꢀ 10-5 mol); the final
molar ratio of n-BA:7:ligand:CuBr was 200:1:2:1. The ampules
were then degassed with three freeze-pump-thaw cycles,
sealed, and then heated at 90 °C in a thermostated oil bath for
1.3-5.7 h. The final polymers were purified as described above.
General Procedure for ATRP of n-BA with Naphthopyran
Initiator 6. A stock solution containing n-butyl acrylate (8.59
g, 67.04 ꢀ 10-3 mol, 4 M), naphthopyran initiator 6 (285.8 mg,
3.35 ꢀ 10-4 mol), and dNbpy ligand (273.9 mg, 6.70 ꢀ 10-4 mol)
was prepared in benzene (6.25 g). 3 mL aliquots were added to
ampules containing CuBr (9.4 mg, 6.54 ꢀ 10-5 mol); the final
molar ratio of n-BA:6:ligand:CuBr was 200:1:2:1. The ampules
were then degassed with three freeze-pump-thaw cycles,
sealed, and then heated at 90 °C in a thermostated oil bath for
1-3.5 h. The final polymers were purified as described above.
ATRP Synthesis of p(NP)-co-p(BA) Copolymers 13a-13c.
A 10 mL stock solution of n-BA (5.65 g, 44.1 ꢀ 10-3 mol,
98 mol %), naphthopyran acrylate 11 (443 mg, 9.00 ꢀ 10-4 mol,
2 mol %), ethyl 2-bromoisobutyrate initiator (58.52 mg, 3.00 ꢀ
10-4 mol), and dNbpy ligand (245.3 mg, 6.00 ꢀ 10-4 mol) was
prepared in benzene. 2.5 mL aliquots were added to ampules
containing CuBr (10.8 mg, 7.50 ꢀ 10-5 mol); the final molar
ratio of monomers:initiator:ligand:CuBr in each ampule was
150:1:2:1. The ampules were then degassed with three free-
ze-pump-thaw cycles, sealed, and then heated at 90 °C in a
thermostated oil bath for 1-8.2 h. The final polymers were
purified as above except column chromatography (silica gel 60)
was carried out using a gradient column (DCM/hexane f
DCM) to remove residual catalyst and unreacted naphthopyran
monomer.
1
giving the product as a crunchy pink solid (200 mg, 77%). H
NMR (400 MHz, d6-acetone) δ: 1.07-1.10 (m, 6H, 2 ꢀ CH3),
1.35-1.37 (m, 6H, 2 ꢀ CH3), 2.47-2.55 (m, 1H, CH(CH3)2,
2.96-3.04 (m, 1H, CH(CH3)2, 3.92 (br s, 3H, COOCH3),
4.19-4.23 (m, 2H, CH2CH2OAr), 4.35-4.38 (m, 2H,
CH2CH2OAr), 6.47 (d, J 10.0 Hz, 1H, pyran-CH), 6.91-6.98
(m, 3H, ArH), 7.28-7.82 (m, 10H, Ar-H), 8.45 (d, J 8.6 Hz, 1H,
ArH). 13C NMR (100 MHz, d6-acetone) δ: 19.1, 19.3, 34.4, 34.7,
52.9, 63.3, 66.9, 83.6, 114.2, 115.1, 121.1, 121.8, 123.04, 123.2,
127.0, 127.3, 128.3, 128.4, 128.7, 128.8, 129.0, 129.1, 130.0,
137.8, 140.0, 145.9, 146.6, 159.3, 166.3, 175.4, 176.9.
Synthesis of Methyl 6-(2-Bromo-2-methylpropanoyloxy)-
2-(4-(2-bromo-2-methylpropanoyloxy)ethoxy)phenyl))-2-phenyl-
2H-naphtho[1,2-b]pyran-5-carboxylate (7). This compound was
synthesized from 3 and 2-bromoisobutyryl bromide using the
same general procedure as above and isolated as a crunchy pink
solid after purification by column chromatography (silica gel 60,
1:1 Et2O/hexane) (523 mg, 80%). 1H NMR (400 MHz, d6-
acetone) δ: 1.86 (s, 6H, 2 ꢀ CH3), 2.13 (s, 6H, 2 ꢀ CH3), 3.94
(s, 3H, COOCH3), 4.24-4.29 (m, 2H, CH2CH2OAr), 4.46-4.51
(m, 2H, CH2CH2OAr), 6.49 (d, J 10.0 Hz, 1H, pyran-CH),
6.91-7.0 (m, 3H, ArH), 7.24-7.76 (m, 9H, ArH), 8.0-8.05 (m,
1H, ArH), 8.46-8.50 (m, 1H, ArH). 13C NMR (100 MHz, d6-
acetone) δ: 31.7, 31.9, 53.9, 57.3, 65.9, 67.4, 84.5, 115.0, 116.0,
121.8, 122.5, 123.7, 123.9, 127.8, 128.1, 128.8, 129.2, 129.8,
129.8, 129.9, 130.9, 138.6, 140.2, 146.5, 147.9, 160.0, 166.8,
171.1, 172.6.
Synthesis of Naphthopyran 2,2-Bis(2-bromo-2-methylpropa-
noyloxymethyl)propionate Initiator (6). Oxalyl chloride (579
mg, 4.60 ꢀ 10-3 mol) was added dropwise via a syringe to a
solution of 2,2-bis(2-bromo-2-methylpropanoyloxymethyl)pro-
panoic acid19-21 (984 mg, 2.28 ꢀ 10-3 mol) in dry CH2Cl2 (10
mL), followed by one drop of DMF. The reaction was allowed
to reach completion after stirring for 2.5 h at room temperature.
The excess oxalyl chloride was then removed on the rotary
evaporator by stripping with several portions of dichloroethane
to give the corresponding acid chloride, 2,2-bis(2-bromo-2-
methylpropanoyloxymethyl)propanoyl chloride, as a yellow
oil that was used without further purification. The acid chloride
was diluted in a small amount of dry CH2Cl2 (3 mL) and added
dropwise to a solution of 6-hydroxy-2-(4-methoxyphenyl)-2-
phenyl-2H-naphtho[1,2-b]pyran-5-carboxylate,
1 (906 mg,
2.07 ꢀ 10-3 mol), TEA (340 μL, 3.42 ꢀ 10-3 mol), and DMAP
(13 mg, 1.06 ꢀ 10-4 mol) in dry CH2Cl2 (10 mL) at 0 °C and
under argon. After stirring for 1 h at 0 °C the temperature was
raised to 25 °C, and the reaction was allowed to reach comple-
tion overnight. The solvent was evaporated under vacuum, and
the residue redissolved in diethyl ether (Et2O) (30 mL) and
washed successively with 0.5 M HCl, water, aqueous NaHCO3,
water, and brine. The organic layer was dried with MgSO4 and
the solvent evaporated under vacuum. The product was purified
by column chromatography (silica gel 60, Et2O/hexane) and
isolated as light pink crystals (1.40 g, 80%). 1H NMR (400 MHz,
d6-acetone) δ: 1.62 (s, 3H, CH3), 1.98 (s, 12H, 4 ꢀ CH3), 3.75 (s,
3H, Ar-OCH3), 3.98 (s, 3H, COOCH3), 4.62 (q, J 11.3 Hz, 4H,
2 ꢀ CH2), 6.49 (d, J 10.0 Hz, 1H, pyran-CH), 6.88-6.90 (m, 2H,
ArH), 6.95 (d, 1H, J 10.0 Hz, pyran-CH), 7.26-7.30 (m, 1H,
ArH), 7.35-7.38 (m, 1H, Ar-H), 7.46-7.48 (m, 2H, Ar-H),
7.55-7.58 (m, 2H, Ar-H), 7.63-7.73 (m, 2H, Ar-H),
7.88-7.90 (m, 1H, ArH), 8.46-8.48 (m, 1H, ArH). 13C NMR
(100 MHz, d6-acetone) δ: 19.0, 31.7, 31.7, 48.7, 54.0, 56.3, 57.8,
67.7, 84.5, 114.9, 115.2, 122.0, 122.4,123.9, 124.1, 127.7, 128.1,
128.5, 129.2, 129.7, 129.7, 129.8, 129.9, 131.0, 138.0, 139.9,
146.6, 147.9, 161.0,167.0, 172.2, 172.2.
ATRP Synthesis of p(NP)-co-p(BA) Copolymer 14a. A stock
solution of n-BA (2.25 g, 17.6 ꢀ 10-3 mol, 99 mol %), naphtho-
pyran methacrylate 12 (90 mg, 1.78 ꢀ 10-4 mol, 1 mol %),
BBMPP initiator (64.25 mg, 1.32 ꢀ 10-4 mol), and dNbpy
ligand (107.6 mg, 2.63 ꢀ 10-4 mol) was prepared in benzene
(1.25 g). This solution was transferred to an ampule containing
CuBr (18.9 mg, 1.32 ꢀ 10-4 mol); the final molar ratio of
monomers:initiator:ligand:CuBr in each ampule was 135:1:2:1.
The ampules were then degassed with three freeze-pump-thaw
cycles, sealed, and then heated at 90 °C in a thermostated oil
bath for 2 h. The final polymer was purified by (1) evaporation
Synthesis of n-Butyl 2,2-Bis(2-bromo-2-methylpropanoyloxy-
methyl)propanoate (BBMPP). This compound was synthesized
from 2,2-bis(2-bromo-2-methylpropanoyloxymethyl)propanoic

260 Macromolecules, Vol. 43, No. 1, 2010
Ercole et al.
of excess monomer over a gentle stream of N2, (2) dissolution of
the crude mixtures into CH2Cl2, (3) precipitation into methanol,
(4) decanting of supernatant liquid, (5) column chromatography
(silica gel 60, 1:1 CH2Cl2/Et2O) to remove residual catalyst, and
(6) removal of solvent under vacuum.
then heated at 90 °C in a thermostated oil bath for 2 h 50 min.
The final polymer was purified by (1) evaporation of excess
monomer over a gentle stream of N2, (2) dissolution of the crude
mixtures into CH2Cl2, (3) precipitation into methanol, (4)
decanting of supernatant liquid, (5) column chromatography
(silica gel 60, 1:1 CH2Cl2/Et2O) to remove residual catalyst, and
(6) removal of solvent under vacuum. Block extension was
carried out by making a solution of the purified p(n-BA)
macroinitiator (672 mg, 4.10 ꢀ 10-5 mol), IBA (1.28 g, 6.15 ꢀ
10-3 mol, 3.2 M), and dNbpy ligand (33.5 mg, 8.20 ꢀ 10-5 mol)
in benzene (0.85 g) and adding this to an ampule containing
CuBr (5.88 mg, 4.10 ꢀ 10-5 mol); the final molar ratio of
[monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[macroinitiator]=
144:1:2:1. The ampule was then degassed with three freeze-
pump-thaw cycles, sealed, and then heated at 90 °C in a thermo-
stated oil bath for 4 h 10 min. The final polymers were purified by
(1) dissolution of the crude mixtures into CH2Cl2, (2) precipitation
into methanol, (3) decanting of supernatant liquid, (4) column
chromatography (silica gel 60, 1:1 CH2Cl2/Et2O) to remove residual
catalyst, and (5) removal of solvent under vacuum.
ATRP Synthesis of Triblock Copolymer 16a, (n-butyl)-[(p-
(NP)0.01-co-p(n-BA)0.99)58-b-p(IBA)28]2. A solution of macroini-
tiator 14a (681 mg, 3.98 ꢀ 10-5 mol), IBA (1.24 g, 5.97 ꢀ 10-3
mol, 3.2 M), and dNbpy ligand (32.51 mg, 7.96 ꢀ 10-5 mol) in
benzene (0.53 g) was added to an ampule containing CuBr (5.71
mg, 3.98 ꢀ 10-5 mol); the final molar ratio of [monomer]/
[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[macroinitiator] = 150:1:
2:1. The ampule was then degassed with three freeze-
pump-thaw cycles, sealed, and then heated at 90 °C in a
thermostated oil bath for 4 h 10 min. The final polymers were
purified by (1) dissolution of the crude mixtures into CH2Cl2, (2)
precipitation into methanol, (3) decanting of supernatant liquid,
(4) column chromatography (silica gel 60, 1:1 CH2Cl2/Et2O) to
remove residual catalyst, and (5) removal of solvent under
vacuum.
ATRP Synthesis of Triblock Copolymer 15a, NP-[p(n-BA)25-
b-p(IBA)54]2. A stock solution containing n-BA (1.55 g, 12.08 ꢀ
10-3 mol, 4 M), naphthopyran initiator 6 (103 mg, 1.21 ꢀ 10-4
mol), and dNbpy ligand (98.7 mg, 2.42 ꢀ 10-4 mol) was
prepared in benzene (1.13 g) and added to an ampule containing
CuBr (17.3 mg, 1.21 ꢀ 10-4 mol); the final molar ratio of
n-BA:6:ligand:CuBr was 100:1:2:1. The ampule was degassed
with three freeze-pump-thaw cycles, sealed, and then heated at
90 °C in a thermostated oil bath for 1 h. The final polymer was
purified by (1) evaporation of excess monomer over a gentle
stream of N2, (2) dissolution of the crude mixtures into CH2Cl2,
(3) precipitation into methanol, (4) decanting of supernatant
liquid, (5) column chromatography (silica gel 60, 1:1 CH2Cl2/
Et2O) to remove residual catalyst, and (6) removal of solvent
under vacuum. Block extension was carried out by making a
solution of the purified p(n-BA) macroinitiator (654 mg, 9.16 ꢀ
10-5 mol), IBA (2.86 g, 13.74 ꢀ 10-3 mol, 3.2 M), and dNbpy
ligand (74.9 mg, 1.83 ꢀ 10-4 mol) in benzene (1.22 g) and adding
this to an ampule containing CuBr (13.1 mg, 9.16 ꢀ 10-5 mol);
the final molar ratio of [monomer]/[CuBr]/[4,40-dinonyl-2,20-
bipyridine]/[macroinitiator]=154:1:2:1. The ampule was then
degassed with three freeze-pump-thaw cycles, sealed, and then
heated at 90 °C in a thermostated oil bath for 4 h 10 min. The
final polymers were purified by (1) dissolution of the crude
mixtures into CH2Cl2, (2) precipitation into methanol, (3)
decanting of supernatant liquid, (4) column chromatography
(silica gel 60, 1:1 CH2Cl2/Et2O) to remove residual catalyst, and
(5) removal of solvent under vacuum.
ATRP Synthesis of Triblock Copolymer 15b, NP-[p(n-BA)35-
b-p(IBA)65]2. A stock solution containing n-BA (4.03 g, 31.40 ꢀ
10-3 mol, 4 M), naphthopyran initiator 6 (134 mg, 1.57 ꢀ 10-4
mol), and dNbpy ligand (128 mg, 3.14 ꢀ 10-4 mol) was prepared
in benzene (2.93 g). A 3 g aliquot was added to an ampule
containing CuBr (9.37 mg, 6.53 ꢀ 10-5 mol); the final molar
ratio of n-BA:6:ligand:CuBr was 200:1:2:1. The ampule was
degassed with three freeze-pump-thaw cycles, sealed, and then
heated at 90 °C in a thermostated oil bath for 1 h 20 min. The
final polymer was purified by (1) evaporation of excess mono-
mer over a gentle stream of N2, (2) dissolution of the crude
mixtures into CH2Cl2, (3) precipitation into methanol, (4)
decanting of supernatant liquid, (5) column chromatography
(silica gel 60, 1:1 CH2Cl2/Et2O) to remove residual catalyst, and
(6) removal of solvent under vacuum. Block extension was
carried out by making a solution of the purified p(n-BA)
macroinitiator (460 mg, 4.76 ꢀ 10-5 mol), IBA (1.49 g, 7.14 ꢀ
10-3 mol, 3.2 M), and dNbpy ligand (38.9 mg, 9.52 ꢀ 10-5 mol)
in benzene (0.63 g) and adding this to an ampule containing
CuBr (6.83 mg, 4.76 ꢀ 10-5 mol); the final molar ratio of
[monomer]/[CuBr]/[4,40-dinonyl-2,20-bipyridine]/[macroinitiator]=
150:1:2:1. The ampule was then degassed with three freeze-
pump-thaw cycles, sealed, and then heated at 90 °C in a
thermostated oil bath for 4 h 10 min. The final polymers were
purified by (1) dissolution of the crude mixtures into CH2Cl2,
(2) precipitation into methanol, (3) decanting of supernatant
liquid, (4) column chromatography (silica gel 60, 1:1 CH2Cl2/
Et2O) to remove residual catalyst, and (5) removal of solvent
under vacuum.
Preparation of Photochromic Lens Samples. The naphthopyran-
p(n-BA) conjugates (or controls) were individually dissolved in a
standard industrial lens formulation made up of 1:4 weight ratio of
poly(ethylene glycol) (400) dimethacrylate and 2,20-bis((4-metha-
cryloxyethoxy)phenyl)propane (specific monomer structures given
below) with 0.4 mass % AIBN. The samples were then cured
at 80 °C for 16 h in a mold to give optically clear test samples of
equivalent thickness (∼2.4 mm). They were each doped at equiva-
lent concentrations of 1.5 ꢀ 10-7 mol of photochromic-polymer
conjugate per gram of lens formulation. This concentration was
chosen in order to maintain optical densities in a meaningful
detector range for photochromic kinetic tests.
Preparation of Photochromic Films. Block copolymers were
dissolved in toluene (3 M) and then cast onto glass slides. The
films were left to dry at room temperature for 8 h and then dried
in a vacuum oven at 100 °C overnight.
Acknowledgment. F.E. thanks the CRC for Polymers for
funding the research in association with CAMD, School of
Chem. Eng. and Ind. Chem., UNSW and CSIRO Molecular
and Health Technologies for providing laboratory space. F.E.
also acknowledges the assistance of Dr. Jonathan Campbell
(UNSW) for the setup and maintenance of photochromic testing
equipment. T.P.D. acknowledges the receipt of a Federation
fellowship from the ARC.
Supporting Information Available: Example 1H NMR
spectra of naphthopyran-polymer conjugates and photochro-
mic initiators, polymerization kinetic plots, and GPC traces.
This material is available free of charge via the Internet at http://
pubs.acs.org.
ATRP Synthesis of Triblock Copolymer 15c, NP-[p(n-BA)60-
b-p(IBA)30]2. A stock solution containing n-butyl acrylate (4.03
g, 31.40 ꢀ 10-3 mol, 4 M), naphthopyran initiator 6 (134 mg,
1.57 ꢀ 10-4 mol), and dNbpy ligand (128 mg, 3.14 ꢀ 10-4 mol)
was prepared in benzene (2.93 g). A 3 g aliquot was added to an
ampule containing CuBr (9.37 mg, 6.53 ꢀ 10-5 mol); the final
molar ratio of n-BA:6:ligand:CuBr was 200:1:2:1. The ampule
was degassed with three freeze-pump-thaw cycles, sealed, and
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