Modulating light-tunable acid sensitivity of a bioinspired polymer simply by adjusting the position of a single methoxy substituent

Article abstract of DOI:10.1002/pola.25057

This article describes a rhodopsin-inspired photosensitive polymer whose light-tunable acid sensitivity can be widely modulated simply by adjusting the position of a single methoxy substituent in the aromatic rings of cinnamyl groups. The well-defined pol

Full text of DOI:10.1002/pola.25057

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Modulating Light-Tunable Acid Sensitivity of a Bioinspired Polymer
Simply by Adjusting the Position of a Single Methoxy Substituent
Zhilin Liu,1,2 Yufang Tang,2 Nan Li,2 Lican Lu,2 Junjie Deng,1 Yuanli Cai1,2
1Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and
Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
2Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, and Key Laboratory of
Advanced Functional Polymeric Materials of College of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan
411105, China
Correspondence to: Y. Cai (E-mail: ylcai@suda.edu.cn)
Received 10 August 2011; accepted 9 October 2011; published online 7 November 2011
DOI: 10.1002/pola.25057
ABSTRACT: This article describes a rhodopsin-inspired photosen-
sitive polymer whose light-tunable acid sensitivity can be widely
modulated simply by adjusting the position of a single methoxy
substituent in the aromatic rings of cinnamyl groups. The well-
defined poly(5-ethyl-5-methacryloyloxymethyl-2-(p-methoxys-
tyryl)-[1,3]dioxane) (PEMpMSD) and poly(5-ethyl-5-methacryloy-
loxymethyl-2-(o-methoxystyryl)-[1,3]dioxane) (PEMoMSD) as
well as poly(5-ethyl-5-methacryloyloxymethyl-2-styryl-[1,3]diox-
ane) were synthesized via reversible addition-fragmentation
chain transfer (RAFT) process. The results demonstrated that the
para-methoxy substitution of EMpMSD monomer led to the
more shortened initialization period and rapid chain propagation
of RAFT process than 5-ethyl-5-methacryloyloxymethyl-2-styryl-
[1,3]dioxane monomer under mild visible light radiation at
25 ꢀC. The ortho-methoxy substitution of PEMoMSD led to high
degree of photoinduced Z-isomerization over 80%. Moreover,
the para-methoxy substitution of PEMpMSD led to the rapid hy-
drolysis of the cyclic acetal linkages in ambient acid media, while
the ortho-methoxy substitution of PEMoMSD slowed down this
hydrolysis. This hydrolysis slowed down on Z-isomerization par-
ticularly in PEMoMSD. These effects widely broadened the tuna-
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bility of the light-modulated acid sensitivity.
2011 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 495–508,
2012
KEYWORDS: biomimetic; reversible addition fragmentation
chain transfer (RAFT); stimuli-sensitive polymers; structure-
property relations
INTRODUCTION Stimulus-responsive polymers are playing
increasingly important roles in a variety of fields.1,2 Among
them, photosensitive polymers have various applications rang-
ing from reversible optical data storage,3,4 polymer viscosity
control,5 photomechanical actuation,6 bioactivity switching of
proteins,7 tissue engineering,8 drug delivery,9 etc. Recently,
the light-triggered aggregation and dissociation of polymers
were extensively studied.10–13 The majority of chemistry used
relied on the photoisomerization of azobenzene10,14–22 or
spiropyran23–25 moieties. Another viable chemistry was the
cleavage of photolabile groups. Zhao26–29 and other groups30–32
described the dissociation of polymer micelles via photocleav-
age of their pyrenylmethyl, o-nitrobenzyl, or (diethylamino)-
methylcoumarinyl groups. Stupp and coworkers33,34 described
the conversion of quadruple helix of polypeptides to single
fibers via photocleavage of o-nitrobenzyl groups.
various tunable properties.35–37 These multiple-response
behaviors are very commonly seen in nature. For example,
the natural rhodopsin is a photosensitive protein, whose 11-
cis retinal covalently binds to opsin through the acid-labile
Schiff base linkage. The primary event of visual perception is
the conversion of its 11-cis retinal chromophore to the all-
trans form on exposure to light. This light-induced isomeri-
zation effectively drives the cleavage of the acid-labile Schiff
base linkage.38–41
Inspired by these characters, our group42 recently proposed
a preliminary protocol of a photosensitive polymer whose
cinnamyl chromophores were directly linked with the acid-
labile cyclic acetal linkages. Photoisomerization of cinnamyl
did not only change the segmental mobility and extinction
coefficient of this polymer but also deactivated the acid sen-
sitivity of their neighboring cyclic acetal linkages. However,
this deactivation was restricted by the limited Z-isomeriza-
tion equilibrium. From both the academic and application
The multiple stimuli response of a polymer that combines
the light response with pH or thermosensitivity exhibited
Additional Supporting Information may be found in the online version of this article.
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2011 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of EMpMSD and EMoMSD monomers.
points of view, further optimization of this protocol toward
the wide light tunability and effective response like the natu-
ral rhodopsin is desirable.
confirmed the pure EHpMSD and EHoMSD compounds (Sup-
porting Information Fig. S1).
EMpMSD or EMoMSD monomers were synthesized via esteri-
fying EHpMSD or EHoMSD using methacryloyl chloride. As
shown in Figure 1, the integral ratio of the assigned proton
signals was in good agreement with the molar ratio of pro-
tons of EMpMSD or EMoMSD. Moreover, except for the signal
of CHCl3 impurity in CDCl3, no other impurities were
The cyclic acetal linkage was successfully used for the acid-
cleavable polymers.43–46 Moreover, Fre´chet and coworkers47–49
introduced the electron-donating methoxy in cyclic benzylidene
acetals to activate the acid sensitivity. Colson50 and Zhong51
used the trimethoxy substitution of cyclic benzylidene acetals
for pH-responsive expansile nanoparticles. More importantly,
the methoxy substitution of cinnamyl could effectively change
the equilibrium of Z-isomerization.52–54 The steric hindrance of
ortho-methoxy substitution in cinnamyl effectively lowered
down the torsional barriers.55,56
Accordingly, we expect that introducing a methoxy in the dif-
ferent position of aromatic rings of cinnamyl groups will
change the light-induced Z-isomerization thus tune the acid
sensitivity of their neighboring cyclic acetal linkages of our
previous prococol. This article describes our effort of the
structure optimization for broadening the light-tunable acid
sensitivity. To this end, the well-defined poly(5-ethyl-5-meth-
acryloyloxy-methyl-2-(p-methoxystyryl)-[1,3]dioxane) (PEMpMSD)
and poly(5-ethyl-5-methacryloyloxy-methyl-2- (o-methoxys-
tyryl)-[1,3] dioxane) (PEMoMSD) as well as the control
sample of poly(5-ethyl-5-methacryl-oyloxymethyl-2-styryl-
[1,3]dioxane) (PEMSD) were synthesized via reversible addi-
tion-fragmentation chain transfer (RAFT) process. The effect
of methoxy position on photoinduced Z-isomerization was
studied by 1H NMR analysis. The hydrolysis of polymers at
various degrees of Z-isomerization was monitored by UV–vis
spectroscopy and 1H NMR studies. The hydrolysis of cyclic
acetal linkages adjacent to either E-type units or Z-type units
1
was separately assessed by H NMR studies.
RESULTS AND DISCUSSION
Synthesis and Characterization of EMpMSD and EMoMSD
Monomers
As shown in Scheme 1, the hydroxyl compounds, 5-ethyl-5-
hydroxymethyl-2-(o-methoxystyryl)-[1,3]dioxane (EHoMSD)
and 5-ethyl-5-hydroxymethyl-2-(p-methoxystyryl)-[1,3]diox-
ane (EHpMSD) were selected as precursors of the targeted
monomers. EHoMSD was synthesized via condensation in
refluxing cyclohexane at 95 ꢀC, and EHpMSD was synthe-
sized via condensation in anhydrous methanol at 25 ꢀC,
according to the literature precedures.43–51 1H NMR studies
FIGURE 1 1H NMR spectra of (a) EMpMSD and (b) EMoMSD in
CDCl3.
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I365
¼ 600 lW cmꢁ2 could trigger Z-isomerization of
nm
these methoxy-substituted cinnamyl groups. However, no
visible light absorption was detectable. This provided the
possibility for these monomers to polymerize via RAFT pro-
58
ꢀ
cess under visible light radiation at 25 C.
RAFT Polymerization of EMpMSD Monomer Under
Visible Light Radiation at 25 8C
The visible light at mild intensity of I420 nm ¼ 150 lW cmꢁ2
was used for activating this RAFT process; this polymeriza-
tion proceeded at 25 ꢀC, using 2-cyanoprop-2-yl(4-fluoro)di-
thiobenzoate (CPFDB) as a chain transfer agent and (2,4,6-
trimethylbenzoyl) diphenyl-phosphine oxide (TPO) as a pho-
toinitiator (Scheme 2).
As shown in Figure 3(a), at the feed molar ratio of
[EMpMSD]0:[CPFDB]0:[TPO]0 ¼ 100:1:0.2 in 35 wt% THF,
the semilogarithmic kinetic curve linearly evolved with radia-
tion time up to 68% monomer conversions, suggesting the
constant and steady concentration of propagating radicals.
Moreover, as compared to the control monomer of 5-ethyl-5-
methacryloyloxymethyl-2-styryl-[1,3]dioxane (EMSD), the
para-methoxy substitution of EMpMSD monomer led to the
shortened initialization period,59,60 and accelerated chain
propagating in RAFT process, as indicated by the slope
increase of semilogarithmic curve. The GPC traces were rea-
sonably unimodal and symmetrical and gradually shifted to
the higher molecular weight side [Fig. 3(b)]. This suggested
the well-controlled behavior of this RAFT polymerization.
As shown in Figure 4(a), increasing [EMpMSD]0:[CPFDB]0:
[TPO]0 led to the shortened initialization period and accelerated
chain propagating. These results were observed in the visible
light-activating ambient RAFT process of glycidyl methacrylate61
or N-(2-methacryloyloxyethyl)pyrrolidone.62 At [EMpMSD]0:
[CPFDB]0:[TPO]0 ¼ 50:1:0.2 or 100:1:0.2, the semilogarithmic
kinetic curve linearly evolved. However, increasing this feed
molar ratio up to 150:1:0.2 or 200:1:0.2 led to the negative
deviation at relatively high conversions, presumably because of
the rapid viscosity increase as suggested by slowing down of
stirring bar and the broadened GPC traces (Supporting Informa-
tion Fig. S2).
FIGURE 2 UV–vis spectra of 0.4 mmol Lꢁ1 of THF solution of
(a) EMpMSD and (b) EMoMSD. Inset: 400 mmol Lꢁ1 of
EMpMSD (a) or EMoMSD (b).
detectable. This suggested the high purity of monomers.
Moreover, as shown in Figure 1(a), the integral ratio of
Id:Imþe:In ¼ 1:2:1 suggested the all E-type p-methoxy-substi-
tuted cinnamyl groups of EMpMSD monomer.57 As shown in
Figure 1(b), the integral ratio of Id:Ibþf:Ig ¼ 1:2:1 suggested
the all E-type o-methoxy-substituted cinnamyl groups of
EMoMSD monomer.57
As shown in Figure 2, the EMpMSD or EMoMSD solution
exhibited a strong UV absorption at k < 350 nm. UV light at
As shown in Figure 4b, the number-average molecular
weight (Mn) linearly increased with monomer conversions
SCHEME 2 RAFT polymerization of EMpMSD and EMoMSD monomers.
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As shown in Figure 5(a–c), no signal at d ¼ 5.56 ppm (one of
CH2¼¼CCH3 of monomers) was detectable, suggesting the com-
plete removal of monomers. More importantly, the integral
ratios of assigned proton were in good agreement with the
molar ratios of protons of the targeted polymers. This sug-
gested the intact molecular structures of these polymers. In
addition, the equal integral of d, e, and f signals for PEMpMSD
[Fig. 5(a)], Idþbþf:Ig:Ih ¼ 3:1:1 for PEMoMSD [Fig. 5(b)], and
the equal integral of b, c, and d signals for PEMSD [Fig. 5(c)]
suggested the all E-type chromophores in these polymers.57
1H NMR Evidence for the Effect of Methoxy Position on
Light-Induced Z-isomerization
On irradiation of PEMpMSD solution with the full-wave UV
light at I365 nm ¼ 600 lW cmꢁ2 in N2 gas atmosphere at 25
ꢀC [Fig. 6(a)], the signal at d ¼ 6.0 ppm (E-type p-
CH3OC6H4CH¼¼CH) attenuated. A new signal at d ¼ 5.6 ppm
(Z-type p-CH3OC6H4CH¼¼CH) appeared, and the signal at d ¼
4.9 ppm (OCHO of cyclic acetal linkages) shifted to the low
field. This suggested the Z-isomerization of the p-methoxy-
substituted cinnamyl groups.
The integral of signal OCHO was equal to the total integrals
of E-type and Z-type signals p-CH3OC6H4CH¼¼CH after
FIGURE 3 (a) Kinetic curves of RAFT polymerization of (n)
EMpMSD or (ꢃ) EMSD using CPFDB chain transfer agent and
TPO photoinitiator at [monomer]0:[CPFDB]0:[TPO]0 ¼ 100:1:0.2
in 35 wt% THF on irradiation with the mild visible light at I420
¼ 150 lW cmꢁ2 at 25 ꢀC; (b) GPC trace evolution of
nm
PEMpMSD under aforementioned conditions.
over [EMpMSD]0:[CPFDB]0:[TPO]0 ranging from 50:1:0.2 to
200:1:0.2. This indicated the controlled behavior of this
RAFT polymerization. Clearly, these Mn values were overesti-
mated by GPC analysis, presumably because of calibration
error caused by using polystyrene standards. Thus, Mn val-
ues of the polymers for the light-tunable acid-sensitivity
studies were evaluated by 1H NMR studies and calculated
according to the CPFDB chain ends [see inset of Fig. 5(a–c)].
EMoMSD polymerized very slowly in the aforementioned RAFT
process because of the poor solubility in THF, DMF, or other
commonly used solvents at 25 ꢀC. To ensure homogeneous so-
lution, this monomer was polymerized at [EMoMSD]0:[CPFD-
B]0:[AIBN]0 ¼ 100:1:0.3 in 80 wt% DMF and at the elevated so-
lution temperature of 70 ꢀC. Based on 1H NMR studies, 42.5%
EMoMSD monomer was polymerized in 4 h.
The samples of PEMpMSD and PEMoMSD with narrow distri-
butions and similar short chain lengths of degree of poly-
merization (DP) ꢂ 40 were selected for the light-tunable
acid-sensitivity studies (Table 1). As the control sample, a
well-defined PEMSD was also synthesized via RAFT polymer-
ization using CPFDB chain transfer agent and TPO photoini-
FIGURE 4 (a) Kinetic curves and (b) the evolution of number-
average molecular weight (Mn, solid) and polydispersity index
(Mw/Mn, hollow) in RAFT polymerization of EMpMSD using
CPFDB chain transfer agent and TPO photoinitiator at
[EMpMSD]0:[CPFDB]0:[TPO]0 of (n) 200:1:0.2, (ꢃ) 150:1:0.2, (~)
100:1:0.2, (!) 50:1:0.2 in 35 wt% THF under visible light radia-
tion at I420 nm ¼150 lW cmꢁ2 at 25 ꢀC.
ꢀ
tiator under visible light radiation at 25 C.
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irradiation for 12 h. This suggested that only Z-isomerization
occurred, that is, [2þ2] photocycloaddition that occurred
irreversibly in the polymeric cinnamic ester moieties63–66 or
reversibly in the polymeric coumarin moieties67,68 was negli-
gible. In addition, this Z-isomerization was a typical equilib-
rium reaction, similar to their small-molecular analogs.69
In contrast, as shown in Figure 6(b), the ortho-methoxy sub-
stitution in PEMoMSD led to the rapid attenuation of signal
at d ¼ 6.15 ppm (E-type o-CH3OC6H4CH¼¼CH) and rapid
increase of new signal at
d
¼
5.67 ppm (Z-type o-
CH3OC6H4CH¼¼CH). The new signal of Z-type o-
CH3OC6H4CH¼¼CH was remarkably larger than that of E-type
one after irradiation for 12 h, suggesting the significant shift
of this equilibrium to Z-type. In addition, the total integral of
E- and Z-type o-CH3OC6H4CH¼¼CH was equal to that of OCHO.
This suggested the negligible [2þ2] cycloaddition or other
side reactions. However, the GPC traces were slightly broad-
ened after irradiation for 12 h (Supporting Information Fig.
S3), presumably because of the side reaction induced by the
photolysis of CTA chain ends. Further studies are necessary
to clarify this phenomenon but beyond the scope of this
article.
The degrees of Z-isomerization of PEMpMSD, PEMoMSD, and
1
PEMSD were precisely assessed by H NMR studies (see Sup-
porting Information). As shown in Figure 7, PEMpMSD exhib-
ited the Z-isomerization kinetics comparable to the nonsub-
stituted PEMSD, with the equilibrium degree of Z-
isomerization of 66%, slightly higher than 62% of PEMSD.
The photoisomerization of PEMoMSD reached the equilib-
rium of 81% Z-type. This suggested that the ortho-methoxy
substitution favored the Z-type formation.
UV–vis Spectroscopic Evidence for the Effect of Methoxy
Position on Light-Tunable Acid Sensitivity
After adding the catalytic amount of hydrochloric acid in the
ꢀ
all E-type PEMpMSD solution at 25 C [Fig. 8(a)], the absorb-
ance of a new peak of p-methoxyphenylacrylaldehyde at kmax
¼ 315 nm increased rapidly, suggesting the rapid hydrolysis.
However, this absorbance increased much more slowly than
the former for the 61% Z-isomerized PEMpMSD sample [Fig.
8(b)], suggesting that Z-isomerization deactivated this hydro-
lysis. Similarly, Z-isomerization of PEMoMSD (Supporting In-
formation Fig. S4) or PEMSD (Supporting Information Fig.
S5) also slowed down this hydrolysis. This suggested that
the Z-type groups prevented their neighboring cyclic acetal
linkages from hydrolysis.
The degree of hydrolysis was assessed according to Lambert–
Beer law (see Supporting Information Fig. S6). As shown in
Figure 9(a), PEMpMSD was completely hydrolyzed in shortly
ꢄ60 min because of the electron-donating effect caused by the
para-methoxy substitution,47,70 which was quite shorter than
ꢄ540 min needed for the complete hydrolysis of PEMSD.
While the ortho-methoxy substitution of PEMoMSD slowed
down this hydrolysis, which was essentially complete in mod-
erately ꢄ180 min. Moreover, Z-isomerization remarkably
deactivated this hydrolysis [Fig. 9(b)]. After 1800 min, the
62% Z-type PEMoMSD sample hydrolyzed 46%, similar to
FIGURE 5 1H NMR spectra of the CDCl3 solutions of (a)
PEMpMSD, (b) PEMoMSD, and (c) PEMSD that were used for
the light-tunable acid-sensitivity studies.
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TABLE 1 Monomer Conversions Controlled in RAFT Processes and Molecular Structure Parameters of PEMpMSD, PEMoMSD, and
PEMSD that were Selected for Light-Tunable Acid-Sensitivity Studies
Monomer
Conversiona (%)
Mn,GPC
(kg molꢁ1
Mn,NMR
(kg molꢁ1
Sample
)
Mw/Mn
)
DPa
PEMpMSD
PEMoMSD
PEMSD
a
40.2
42.5
43.8
21.2
22.9
20.9
1.18
1.19
1.16
13.7
14.4
13.8
39
41
43
The monomer conversions and DP were assessed by 1H NMR studies.
FIGURE 6 The evolution of 1H NMR spectrum of 5.0 mg mLꢁ1 PEMpMSD (a), PEMoMSD (b), or PEMSD (c) on irradiation with full-
wave UV light of mercury lamp at I365 nm ¼ 600 lW cmꢁ2 in nitrogen gas atmosphere at 25 ꢀC.
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hydrolyzed more rapidly than the all E-type PEMSD [Fig.
13(a)], which was essentially complete in 65 min. Moreover,
the partially Z-isomerized PEMpMSD hydrolyzed rapidly at
the early stage and slowed down after E-type units were
completely hydrolyzed [Fig. 12(b)]. Their E-type units hydro-
lyzed at the comparable rate to the all E-type PEMpMSD
[Fig. 12(a)]. Meanwhile, their Z-type units hydrolyzed at the
comparable rate at different degrees of Z-isomerization,
more rapidly than Z-type units in PEMSD samples [Fig.
13(b)] but slower than their corresponding E-type units over
the range of [E]:[Z]¼72:28–39:61.
As shown in Figure 14(a), as varying the configuration from
100% E-type to 81% Z-type, the degree of hydrolysis of
PEMoMSD samples changed from 100% to 33% in 570 min,
implying the remarkable light-tunability of acid sensitivity.
As shown in Figure 14(b), the hydrolysis of E-type units was
accelerated, but the hydrolysis of Z-type units was slowed
FIGURE 7 The evolution of degrees of Z-isomerization of the
100% E-type PEMSD, PEMpMSD, and PEMoMSD on irradiation
with full-wave UV light of mercury lamp at I365
¼ 600 lW
nm
cmꢁ2 in nitrogen gas atmosphere at 25 ꢀC.
37% hydrolysis of the 60% Z-type PEMSD sample, while the
61% Z-type PEMpMSD sample hydrolyzed up to 91%.
1H NMR Evidence for the Effect of Methoxy Position on
Light-Tunable Acid Sensitivity
1H NMR was used to precisely evaluate this light-tunable
acid sensitivity. Both the all E-type and 61% Z-type
PEMpMSD samples were separately dissolved in acetone-d6
with the addition of catalytic amount of deuterated hydro-
chloric acid at 25 ꢀC. 1H NMR spectra were recorded at the
predetermined intervals.
As shown in Figure 10(a), after addingꢀhydrochloric acid in
the all E-type PEMpMSD solution at 25 C in shortly 10 min,
the signals of p-methoxyphenylacrylaldehyde at d ¼ 9.64,
7.69, and 7.03 ppm appeared, indicating the hydrolysis of
cyclic acetal linkages. In contrast, for the 61% Z-isomerized
PEMpMSD solution [Fig. 10(b)], p-CH3OC6H4CH¼¼CH signal of
Z-type units attenuated much more slowly than that of E-
type units. Moreover, the higher field signal at d ¼ 5.00 ppm
(OCHO of E-type units) attenuated rapidly and disappeared
in shortly 66 min, while the lower field signal of OCHO of Z-
type units at d ¼ 5.13 ppm was still detectable after hydro-
lyzing for 536 min. This suggested that the Z-type units
hydrolyzed more slowly than E-type units.
In contrast, for the all E-type PEMoMSD solution [Fig. 11(a)],
the signals at d ¼ 6.20 ppm (o-CH3OC6H4CH¼¼CH) and at d
¼ 5.03 ppm (OCHO) attenuated much more slowly than
those from the all E-type PEMpMSD [Fig. 10(a)], which were
still detectable in 168 min. More importantly, for the 81% Z-
isomerized PEMoMSD sample [Fig. 11(b)], the signal of E-
type o-CH3OC6H4CH¼¼CH disappeared in 535 min, and the
signal of Z-type o-CH3OC6H4CH¼¼CH was still very strong.
This evidenced that this Z-isomerization improved the stabil-
ity of cyclic acetal linkages against hydrolysis.
FIGURE 8 UV–vis spectra of 1,4-dioxane solutions of
PEMpMSD at [E]:[Z] ¼ 100:0 (a) or 39:61 (b), hydrolyzed under
the conditions: 0.25 g of 0.98 mol Lꢁ1 HCl solution was added
in 50 mL of 12.0 mg Lꢁ1 PEMpMSD solution, stirred at 25 ꢀC
for the predetermined intervals. (a) From bottom to upper: 0, 5,
10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 110, and
120 min; (b) from bottom to upper: 0, 15 30, 45, 60, 90, 120,
180, 240, 360, 480, 600, 720, 900, 1080, 1260, 1440, 1620, and
1800 min.
The 1H NMR assessment of degree of hydrolysis from either
E-type or Z-type units was described in Supporting Informa-
tion. As shown in Figure 12(a), the all E-type PEMpMSD
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ꢀ
ꢀ
tallized from ethanol at ꢁ20 C and dried in vacuo at 30 C.
TPO (97%; Runtec Chem. Co.), 2-ethyl-2-hydroxymethyl-1,3-
propane-diol (98%; Aldrich), and methacrylic chloride (97%;
Aldrich) were used as received. CDCl3, acetone-d6, and deu-
terated hydrochloric acid were purchased from Cambridge
Isotope Laboratory, and used as received. DMF, THF, 1,4-diox-
ane, and triethylamine were dried over metallic sodium, dis-
tilled prior to use. Methanol and ethyl ether were dried over
metallic magnesium and distilled prior to use. Acetone was
treated with K2MnO4 and anhydrous MgSO4 and distilled
prior to use. Chloroform was washed with distilled water,
dried over anhydrous CaCl2 overnight, and distilled prior to
use.
Visible Light Used for Activating RAFT
Polymerization at 25 8C
A mercury vapor lamp emitting separately at 254, 302, 313,
365, 405, 436, 545, and 577 nm was selected. JB400 filters
were used to filter out the shorter-wave UV light at k < 400
nm and adjust the light intensity. The light intensity was
measured using a UV-A radiometer that was equipped with a
420-nm sensor. Thus the visible light emitting at 405, 436,
545, and 577 nm at mild intensity of I420
¼ 150 lW
nm
cmꢁ2 was obtained for this purpose.
Synthesis of EHpMSD
p-Methoxyphenylacrylaldehyde (32.40 g, 0.20 mol) and 2-
ethyl-2-hydroxymethyl-1,3-propanediol (32.16 g, 0.24 mol)
were dissolved in 250 mL of anhydrous methanol in a 500
mL round-bottom flask. In this flask, 0.5 mL of 8.0 mol Lꢁ1
hydrochloric acid and 64 g dried 4 Å molecular sieves were
added. The mixture was stirred at 25 ꢀC for 48 h. Triethyl-
amine (2 mL) was added to neutralize this solution. The mix-
ture was filtrated, and methanol was removed by rotary
evaporation and dissolved in 200 mL chloroform. The solu-
tion was washed using 2 wt% Na2CO3, saturated NaCl solu-
tion, and distilled water until it was neutralized. The solution
was dried over anhydrous MgSO4 overnight. After filtration,
chloroform was removed by rotary evaporation. The crude
FIGURE 9 The hydrolysis kinetic curves of PEMSD, PEMpMSD,
and PEMoMSD monitored by UV–vis spectrophotometry under
the conditions described in Figure 8: (a) 100% E-type polymers,
(b) approximately 60% Z-type polymers.
down. Only 24% of Z-type units hydrolyzed in 570 min,
close to 22% hydrolysis of Z-type EMSD units in 540 min.
This indicated that this isomerization-induced deactivation of
hydrolysis was enough to neutralize the activation caused by
the electron-donating o-methoxy substitution.
ꢀ
product was recrystallized from ethyl ether at ꢁ20 C, dried
in vacuo overnight to afford white solid product.
Weight: 31.25 g; yield: 56%. 1H NMR (d, in CDCl3, ppm):
0.88 (3H, CCH2CH3), 1.24 (2H, CCH2CH3), 3.61 (2H, two pro-
tons of OCH2CCH2O), 3.82 (3H, C6H4OCH3), 3.95 (2H,
CH2OH), 4.05 (2H, two protons of OCH2CCH2O), 5.07 (1H,
OCHO), 6.10 (1H, C6H4CH¼¼CH), 6.72 (1H, C6H4CH¼¼CH),
6.87 (2H, 2 protons of C6H4CH¼¼CH), 7.36 (2H, two protons
of C6H4CH¼¼CH).
As far as the chemical mechanism was concerned, this light-
tunable hydrolysis was quite similar to the cleavage of the
Schiff base linkage of rhodopsin on exposure to light.38–41
Moreover, adjusting the position of a single methoxy in aro-
matic rings of cinnamyl groups widely changed the light-tun-
able acid sensitivity of their neighboring cyclic acetal link-
ages. This intriguing strategy may be essential in the
molecular design of novel polymers for photosensors and
light-tuned drug delivery.
Synthesis of EHoMSD
2-Ethyl-2-hydroxymethyl-1,3-propanediol (32.16 g, 0.24
mol), o-methoxyphenylacryl-aldehyde (32.40 g, 0.20 mol),
150 mL cyclohexane, and 0.5 mL of 8.0 mol Lꢁ1 hydrochloric
acid were charged in a 500 mL round-bottom flask that was
equipped with a water separator. The solution was refluxed
at 95 ꢀC for 4 h. Water was removed by azeotropic distilla-
tion. The solution was cooled down to 25 ꢀC. The oil phase
was separated and washed subsequently using 2 wt%
Na2CO3, the saturated solution of NaCl, and distilled water
EXPERIMENTAL
Materials
EMSD was synthesized according to our previous proce-
dures.42 CPFDB was synthesized according to the literature
procedures.71 2,20-Azobis(isobutyronitrile) (AIBN, 98%;
Aldrich), p-methoxyphenylacrylaldehyde, and o-methoxyphe-
nylacrylaldehyde (95%, Yuancheng Reagent Co.) were recrys-
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FIGURE 10 The evolution of 1H NMR spectrum of PEMpMSD at [E]:[Z] ¼ 100:0 (a) or 39:61 (b) on hydrolyzing in 1.0 mL of 6.0 mg
mLꢁ1 acetone-d6 solution with the addition of 13 mg of 0.25 mol Lꢁ1 deuterated hydrochloric acid at 25 ꢀC.
until it was neutralized. This solution was dried over anhy-
drous MgSO4 overnight. After filtration, the solvent was
removed by rotary evaporation. The crude product was puri-
fied through silica column using the mixed petroleum ether
and ethyl acetate eluent (3:1, v/v), recrystallized from ethyl
ether at ꢁ20 ꢀC, and dried in vacuo overnight to afford
white solid product.
mL dried flask. The flask was immersed in a water/ice bath
at 0 ꢀC. Methacryloyl chloride (10.03 g, 96 mmol) was dis-
solved in 75 mL of anhydrous THF and added dropwise to
thisꢀflask under stirring over 2 h. The mixture was stirred at
10 C overnight. The white ammonium salt was filtered out.
The solution was concentrated by rotary evaporation, dis-
solved in 150 mL ethyl ether, washed using 2 wt% Na2CO3,
saturated NaCl solution, and distilled water until it was neu-
tralized. The solution was dried over anhydrous MgSO4. After
filtration, the solvent was removed by rotary evaporation.
The crude product was purified through silica column using
the mixed eluent of petroleum ether and ethyl acetate (8:1,
v/v), recrystallized from mixed petroleum ether and ethyl ac-
Weight: 22.82 g; yield: 41%. 1H NMR (d, in CDCl3, ppm):
0.88 (3H, CCH2CH3), 1.24 (2H, CCH2CH3), 3.58 (2H, two pro-
tons of OCH2CCH2O), 3.82 (3H, C6H4OCH3), 3.96 (2H,
CH2OH), 4.03 (2H, two protons of OCH2CCH2O), 5.07 (1H,
OCHO), 6.28 (1H, C6H4CH¼¼CH), 6.86–6.94 (2H, C6H4CH¼¼CH
and one proton of C6H4CH¼¼CH), 7.06 (1H, one proton of
C6H4CH¼¼CH), 7.25 (1H, one proton of C6H4CH¼¼CH), 7.43
(1H, one proton of C6H4CH¼¼CH).
ꢀ
etate eluent (3:1, v/v) at ꢁ20 C, dried in vacuo overnight to
afford white solid product.
Weight: 11.63 g; yield: 42%. 1H NMR (d, in CDCl3, ppm):
0.86 (3H, CCH2CH3), 1.25 (2H, CCH2CH3), 1.98 (3H,
CH3C¼¼CH2), 3.63 (2H, two protons of OCH2CCH2O), 3.82
(3H, C6H4OCH3), 4.07 (2H, two protons of OCH2CCH2O), 4.48
Synthesis of EMpMSD
EHpMSD (22.24 g, 0.08 mol), triethylamine (12.12 g, 0.12
mol), and 150 mL of anhydrous THF were charged in a 500
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FIGURE 11 The evolution of 1H NMR spectrum of PEMoMSD at [E]:[Z] ¼ 100:0 (a) or 20:80 (b) on hydrolyzing in 1.0 mL of 6.0 mg
mLꢁ1 acetone-d6 solution with the addition of 13 mg of 0.25 mol Lꢁ1 deuterated hydrochloric acid at 25 ꢀC.
(2H, COOCH2), 5.06 (1H, OCHO), 5.57 (1H, one proton of
CH3C¼¼CH2), 6.06 (1H, C6H4CH¼¼CH), 6.12 (1H, one proton of
CH3C¼¼CH2), 6.75 (1H, C6H4CH¼¼CH), 6.87 (2H, two protons
of C6H4 CH¼¼CH), 7.36 (2H, two protons of C6H4 CH¼¼CH).
The synthesis of EMoMSD follows the same procedure except
for using EHoMSD and eluent of petroleum ether–ethyl ace-
tate (6:1, v/v).
Visible Light-Activated RAFT Polymerization of EMpMSD
Monomer at 25 8C
EMpMSD (3.46 g, 10.0 mmol), CPFDB (23.9 mg, 0.1 mmol),
TPO (7.0 mg, 0.02 mmol), and 1.88 g of anhydrous THF
were charged in a 25 mL flask and capped with rubber
septa. After bubbling with nitrogen gas for 40 min, the flask
was immersed in water bath at 25 ꢀC and irradiated with
visible light. Samples were collected using deoxygenated
syringes at predetermined intervals and quenched by expos-
ing to air and adding small amount of hydroquinone inhibi-
tor. One portion of sample was diluted in CDCl3 for 1H
NMR studies. Another portion was diluted in DMF for GPC
analysis. The resultant PEMpMSD was precipitated from
large excess of anhydrous methanol and dried in vacuo
overnight. The visible light-activating RAFT polymerization
of EMSD at 25 ꢀC was the same except for using EMSD
monomer.
1
Weight: 10.80 g; yield: 39%. H NMR (d, in CDCl3, ppm): 0.86
(3H, CCH2CH3), 1.25 (2H, CCH2CH3), 1.98 (3H, CH3C¼¼CH2),
3.63 (2H, 2 protons of OCH2CCH2O), 3.85 (3H, C6H4OCH3),
4.05 (2H, two protons of OCH2CCH2O), 4.48 (2H, COOCH2),
5.08 (1H, OCHO), 5.57 (1H, one proton of CH3C¼¼CH2), 6.12
(1H, one proton of CH3C¼¼CH2), 6.28 (1H, C6H4CH¼¼CH), 6.86–
6.95 (2H, C6H4CH¼¼CH and one proton of C6H4CH¼¼CH), 7.06
(1H, one proton of C6H4CH¼¼CH), 7.25 (1H, one proton of
C6H4CH¼¼CH), 7.43 (1H, one proton of C6H4CH¼¼CH).
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ysis. The photoisomerization of PEMSD or PEMoMSD was
the same except for using PEMSD or PEMoMSD.
UV–Vis Spectroscopic Studies on Light-Tunable Acid
Sensitivity of Polymers
The typical procedures were as follows. PEMpMSD was dis-
solved in 1,4-dioxane under stirring at 25 ꢀC and diluted to
12 mg Lꢁ1. A 100-mL round-bottom flask was charged with
50 mL of such solution. This flask was charged with 0.25 g
of 0.98 mol Lꢁ1 hydrochloric acid under stirring at 25 ꢀC.
UV–vis spectra were recorded at predetermined intervals.
1H NMR Studies on Light-Tunable Acid Sensitivity of
These Polymers
The typical procedures were as follows. PEMpMSD (6 mg)
was dissolved in 1.0 mL of acetone-d6 in an NMR sample
tube at 25 ꢀC. This tube is charged with 13 mg of 0.25 mol
Lꢁ1 deuterated hydrochloric acid. 1H NꢀMR spectra were
recorded at predetermined intervals at 25 C.
FIGURE 12 (a) The overall degree of hydrolysis of PEMpMSD
at [E]:[Z] ¼ 100:0, 72:28, or 39:61 as a function of hydrolysis
time under the hydrolyzing conditions described in Figure 10.
(b) The degrees of hydrolysis of E-type units (solid) or Z-type
units (hollow) of PEMpMSD as mentioned above.
RAFT Polymerization of EMoMSD Monomer at 70 8C
EMoMSD (1.73 g, 5.0 mmol), CPFDB (12.0 mg, 0.05 mmol),
AIBN (2.5 mg, 0.015 mmol), and anhydrous DMF (7.00 g)
were charged in a 25 mL round-bottom flask and capped
with rubber septa. After bubbling with nitrogen gas for 40
min, the flask was immersed in a thermostatic oil bath at 70
ꢀC for 4 h. The solution was exposed to air by adding small
amount of hydroquinone inhibitor. The resultant PEMoMSD
was precipitated from large excess of anhydrous methanol
and dried in vacuo overnight.
Photoinduced Z-Isomerization of PEMpMSD Under UV
Light Radiation
The typical procedures were as follows. PEMpMSD (0.10 g)
was dissolved in 20 mL of acetone under stirring in a 50-mL
quartz flask capped with rubber septa. The solution was
deoxygenated by purging with nitrogen gas for 30 min. The
flask was immersed in a water bath at 25 ꢀC and irradiated
with the full-wave UV light with intensity of I365 nm ¼ 600
lW cmꢁ2. The samples were collected at the predetermined
intervals. Acetone was removed under reduced pressure at
FIGURE 13 (a) The overall degree of hydrolysis of PEMSD at
[E]:[Z] ¼ 100:0 (~), 70:30 (^) or 40:60 (n) as a function of hy-
drolysis time under the hydrolyzing conditions as described in
Figure 10. (b) The degrees of hydrolysis of E-type units (solid)
or Z-type units (hollow) of the corresponding PEMSD as men-
tioned above.
1
ꢀ
25 C. The polymer was dissolved in CDCl3 for H NMR anal-
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EMpSMD and EMSD monomers were polymerized via RAFT
polymerization under visible light radiation at 25 ꢀC.
EMoMSD monomer was polymerized via RAFT polymeriza-
ꢀ
tion at 70 C because of the poor solubility.
The kinetic results demonstrated that the para-methoxy sub-
stitution led to the shortened initialization period and accel-
erated chain propagation in RAFT process under visible light
ꢀ
radiation at 25 C. This RAFT process was well controlled at
the low feed molar ratio of [EMpMSD]0: [CPFDB]0:[TPO]0
¼
50:1:0.2 or 100:1:0.2, and increasing this feed molar ratio
led to the ill-controlled polymerization. Thus the well-
defined PEMpMSD, PEMoMSD, and PEMSD samples with the
similar short chain lengths of DP ꢂ 40 were synthesized for
the light-tunable acid-sensitivity studies.
1H NMR studies evidenced the high degree of light-induced
Z-isomerization of the ortho-methoxy-substituted PEMoMSD,
which reached up to 81% at equilibrium. UV–vis spectro-
scopic studies demonstrated that the para-methoxy substitu-
tion of cinnamyl in PEMpMSD led to the rapid hydrolysis of
their neighboring cyclic acetal linkages, which was essen-
tially complete in shortly ꢄ60 min in ambient acidic media,
quite shorter than ꢄ540 min for the complete hydrolysis of
PEMSD. However, the ortho-methoxy substitution of
PEMoMSD slowed down this hydrolysis, which was essen-
tially complete in ꢄ180 min under the same conditions.
Moreover, Z-isomerization remarkably slowed down this hy-
drolysis process. Under such conditions for 1800 min, the
62% Z-type PEMoMSD sample hydrolyzed 46%, similar to
37% hydrolysis of the 60% Z-type PEMSD sample, while the
61% Z-type PEMpMSD sample hydrolyzed up to 91%.
Moreover, the degree of hydrolysis of PEMoMSD samples var-
ied from 100% to 33% in 570 min as changing the configura-
tion from 100% E-type to 81% Z-type; the hydrolysis of the
E-type units was accelerated but the hydrolysis of the Z-type
units was slowed down on this light-triggered Z-isomerization.
FIGURE 14 (a) The overall degree of hydrolysis of PEMoMSD
at [E]:[Z] ¼ 100:0 (~), 68:32 (!), 38:62 (^), or 20:80 (n) as a
function of hydrolysis time under the hydrolyzing conditions
as described in Figure 11. (b) The degrees of hydrolysis of E-
type units (solid) or Z-type units (hollow) of this PEMoMSD.
This wide light tunability of acid sensitivity was induced by
broadening the degree of light-triggered Z-isomerization and
also electrodonating effects caused by a methoxy substituent
in the different position of the aromatic rings of cinnamyl
groups. This intriguing strategy provided a new molecular
design of polymers for photosensors and light-tuned drug
delivery applications.
Analytical Techniques
GPC was performed on a PL-GPC120 setup being equipped
with a column set consisting of two PL gel 5 lm MIXED-D
columns (7.5 ꢅ 300 mm2, effective molecular weight range
of 0.2–400.0 kg molꢁ1), using DMF as an eluent that con-
ꢁ1
ꢀ
tained 0.01 M LiBr at 80 C, at a flow rate of 1.0 mL min
.
Polystyrene standards over the molecular weight range of
0.5–7500.0 kg molꢁ1 (PSS, Mainz, Germany) were used for
calibration. 1H NMR analyses were performed on a Bruker
AV-400 NMR spectrometer. Samples were scanned for 16
times at 25 ꢀC. UV–vis spectra were recorded on a Perki-
The authors thank National Natural Science Foundation of
China (20874081, 21074104), Research Fund for Doctoral Pro-
gram of Higher Education of China (200805300004), Scientific
Research Fund of Hunan Provincial Education Department
(10A116), and Priority Academic Program Development of
Jiangsu Higher Education Institutions for financial support of
this work.
ꢀ
nElmer Lambda 25 spectrometer at 25 C.
CONCLUSIONS
This article described a rhodopsin-inspired photosensitive
polymer whose light-tunable acid sensitivity was widely
modulated simply by adjusting the position of a methoxy
substituent in the aromatic rings of cinnamyl groups. The
para- or ortho-methoxy-substituted EMpMSD and EMoMSD
monomers as well as EMSD monomer were synthesized.
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