A facile approach to catechol containing UV dismantlable adhesives

Article abstract of DOI:10.1016/j.polymer.2015.05.032

We report a facile synthetic approach to a new type of catechol containing UV dimantlable adhesive. A series of linear polymers containing pendent catechol moieties and main chain o-nitrobenzyl ester groups were synthesized by the Passerini multicomponent

Full text of DOI:10.1016/j.polymer.2015.05.032

Polymer 68 (2015) 270e278
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Polymer
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A facile approach to catechol containing UV dismantlable adhesives
Yao-Zong Wang, Lei Li, Fu-Sheng Du*, Zi-Chen Li*
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, College of Chemistry
and Molecular Engineering, Center for Soft Matter Science and Engineering, Peking University, Beijing 100871, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a facile synthetic approach to a new type of catechol containing UV dimantlable adhesive. A
series of linear polymers containing pendent catechol moieties and main chain o-nitrobenzyl ester
groups were synthesized by the Passerini multicomponent polymerization (MCP) of a di-o-nitro-
benzaldehyde, 1,6-diisocyanohexane, 3-(3,4-dihydroxyphenyl) propionic acid, and undecanoic acid. The
content of the catechol moieties was adjusted by varying the molar ratio of 3-(3,4-dihydroxyphenyl)
propionic acid to undecanoic acid. The thermal properties of these polymers were investigated, they are
stable up to 250 ꢀC, and the glass transition temperatures (Tg) are in the range of 17e70 ꢀC. Increasing the
catechol content will increase the Tg and slightly decrease the thermal stability. The pendent catechol
groups and the in situ formed o-nitrobenzyl ester linkages in the polymer main chain endow the polymer
with dual functions, substrate adhesiveness and UV degradation. The UV degradation process was
monitored by 1H NMR, GPC, UVeVis and FTIR. Lap shear strength tests revealed that the adhesion
performance of these polymers could be tuned by varying the catechol contents. UV irradiation could
cleave the polymer chains, thus decreasing the adhesion strength.
Received 7 February 2015
Received in revised form
3 May 2015
Accepted 16 May 2015
Available online 21 May 2015
Keywords:
UV degradation
Catechol
Adhesive
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
derivatives into polymers as new functional materials has been
pursued for many years, the synthetic pathway is rather compli-
Adhesives are such materials that can hold different substrates
together. Every year, quantities of adhesives are produced world-
wide to meet different demands in industry, medicine and daily
use. In fact, human beings have used natural glues such as starch or
hydrolyzed collagen long before, many of which are still being used
today [1]. However, with the development of adhesion technology,
more and more attentions have been paid to synthetic adhesives.
Because of the ease to finely tune the chemical compositions of
synthetic polymers, which is a major factor influencing the adhe-
sion strength, scientists can design better adhesives for different
purposes. One such good example is the mimicry of bio-adhesives
inspired by marine mussels [2e18]. These organisms are able to
attach themselves firmly to the underwater surface by secreting
adhesive proteins containing unique amino acid 3,4-
dihydroxyphenylalanine (DOPA) [19,20]. Studies have revealed
that DOPA moieties have extensive interactions with virtually any
substrates via metal chelation, oxidation and Michael adduct for-
mation etc [21e23]. Though the incorporation of DOPA or its
cated, often requiring complex synthetic routes and pro-
tectionedeprotection procedures [2e5].
On the other hand, facile debonding ability after use is another
aspect of adhesive research. This emerging area promises smart
adhesives with great industrial potentials not only in terms of
saving energy but also recycling materials. As can be expected,
dismantlable adhesives require external stimuli, such as heat
[24e34], light [35e37] and electrostatic interactions [38] to achieve
the on-demand disassembly. Among them, photo irradiation is
particularly attractive, because of its temporal and spatial control
[39e43], which ensures precise and selective removal of adhesives.
Ideally, this method provides an opportunity for easy operation and
minimal damage to the substrates. A common strategy toward light
responsiveness is the incorporation of o-nitrobenzyl ester groups in
polymers [44e46]. This photolabile structure undergoes photo-
cleavage and generates an o-nitrosobenzaldehyde and a carboxylic
acid upon UV irradiation [47,48], which in turn changes the mate-
rial properties.
However, it remains difficult to simultaneously introduce cate-
chol moieties and o-nitrobenzyl ester groups into polymer through
conventional synthetic procedures. An intriguing methodology is
the use of multicomponent reactions (MCRs) [49e54]. Unlike
* Corresponding authors.
E-mail addresses: fsdu@pku.edu.cn (F.-S. Du), zcli@pku.edu.cn (Z.-C. Li).
http://dx.doi.org/10.1016/j.polymer.2015.05.032
0032-3861/© 2015 Elsevier Ltd. All rights reserved.

Y.-Z. Wang et al. / Polymer 68 (2015) 270e278
271
traditional two component reactions, the fundamental superiority
2.2. General measurements and characterization
of MCRs is the simultaneous incorporation of more than two
starting materials into a single product, providing a chance to
circumvent the aforementioned problems.
1H NMR (300 MHz) spectra were recorded in CDCl3 or DMSO-d6
on a Varian Gemini 300 spectrometer with tetramethylsilane (TMS)
Passerini reaction is a typical isocyanide based multicompo-
nent reaction. First reported in 1921 [52], this three component
reaction can assemble a carboxylic acid, an isocyanide and an
as the internal reference for chemical shifts (d, ppm). Molecular
weights and polydispersity index (PDI ¼ Mw/Mn) of polymers were
measured with a gel permeation chromatography (GPC) equipped
with a 2414 refractive index detector, a Waters 1525 binary HPLC
pump, and three Waters Styragel HT columns (HT2, HT3, HT4). The
columns were thermostated at 35 ꢀC and THF was used as an eluent
at a flow rate of 1.0 mL/min. Calibration was made against linear
polystyrene standards. Thermal gravimetric analysis (TGA) was
carried out using a Q600-SDT thermogravimetric analyzer (TA Co.,
Ltd.) with nitrogen purging rate set at 50 mL/min. Measurements
were conducted from room temperature to 600 ꢀC at a heating rate
of 10 ꢀC/min. Calorimetric measurement was performed using a
Q100 differential scanning calorimeter (TA Co., Ltd.) with nitrogen
purging rate set at 50 mL/min. The program was set to finish two
cycles in a temperature range of ꢁ80 to 200 ꢀC at a heating/cooling
rate of 10 ꢀC/min. Data of the endothermic curve were acquired
from the second scan for each polymer sample. TA Universal
Analysis software was applied for data acquisition and processing
in the two measurements.
aldehyde into an
a-acyloxy amide in an atom-economic way.
Thanks to the mild reaction conditions and functional group
tolerance, our group [55e62] and others [63e69] have exploited
this reaction in many aspects such as polymerization and post-
functionalization. Our recent works demonstrated that the
multicomponent polymerization (MCP) of a diisocyanide, a dicar-
boxylic acid and an o-nitrobenzaldehyde was a straightforward
way to synthesize photocleavable polymers [59,60]. The key point
was the in-situ formed o-nitrobenzyl ester linkage in the polymer
main chain, this linkage can be cleaved upon UV irradiation. We
here report that two functionalities, catechol and o-nitrobenzyl
ester groups, can be simultaneously incorporated into a linear
polymer via the MCP of a di-o-nitrobenzaldehyde derivative, 1,6-
diisocyanohexane, and 3-(3,4-dihydroxyphenyl) propionic acid
(Scheme 1). The facilely obtained polymers show good adhesion
and UV debonding abilities, and they are expected to be used as
dismantlable adhesives.
2.3. Synthesis of 5-ethoxy-2-nitrobenzaldehyde
2. Experimental section
5-Hydroxy-2-nitrobenzaldehyde (1.6712 g, 10 mmol), bromo-
ethane (1.6346 g, 15 mmol) and potassium carbonate (1.6583 g,
12 mmol) were dissolved in DMF (20 mL). The reaction mixture was
heated at 30 ꢀC for 24 h. The excess bromoethane was evaporated
under reduced pressure and the residue was poured into water to
precipitate the product. After filtration, simply washing with water
for three times offered 5-ethoxy-2-nitrobenzaldehyde as a pale
yellow solid in 77% yield. Tm ¼ 63 ꢀC (Fig. S1).
2.1. Materials
The following chemicals were used as received: 5-hydroxy-2-
nitrobenzaldehyde (99%, Beijing Datianfengtuo Chem. Co., Ltd.),
bromoethane (A. R., Tianjin Guangfu Fine Chem. Res. Inst.), 1, 12-
dibromododecane (98%, Alfa Aesar), 1,6-diisocyanohexane (98%,
Alfa Aesar), tert-butyl isocyanide (98%, Alfa Aesar), 3-(3,4-
dihydroxyphenyl) propionic acid (98þ%, Alfa Aesar), undeca-
noic acid (98%, Alfa Aesar). All other chemicals were purchased
from Beijing Chem. Reagent Co. and used as received
unless otherwise noted. THF was refluxed with Na and distilled
before use.
2.4. Synthesis of di-o-nitrobenzaldehyde derivative monomer 1
1, 12-dibromo-dodecane (1.6407 g, 5 mmol), 5-hydroxy-2-
nitrobenzaldehyde (2.5068 g, 15 mmol), and potassium carbonate
Scheme 1. Synthetic strategy for the dismantlable adhesives.

272
Y.-Z. Wang et al. / Polymer 68 (2015) 270e278
(2.0729 g, 15 mmol) were dissolved in DMF (30 mL). The reaction
mixture was heated at 30 ꢀC for 24 h, and then poured into water.
The aqueous phase was extracted with CH2Cl2 for three times. After
evaporation of CH2Cl2, yellow oil was obtained which was solidified
after being cooled to 0 ꢀC. Simply washing the solid with cold
diethyl ether gave the di-o-nitrobenzaldehyde 1 as a pale yellow
solid in 52% yield. Tm ¼ 92 ꢀC (Fig. S2).
(40 mg/mL) was uniformly applied to the adherend. The adherend
contact area was 25.4 ꢂ 25.4 mm2. The specimen was clamped for
12 h and then dried in vacuo. The adhesion strength tests were
performed using an Instron 5567 testing machine with a 1 kN load
cell. Load was recorded as a function of displacement with a cross-
head speed of 2 mm/min. The maximum load was then divided by
the contact area to calculate the adhesion strength. For each sam-
ple, lap shear strength was measured at least seven times, averaged
and reported with error bars showing standard deviation. For the
typical UV dismantlable behavior, the specimen adhered by PA-0.4
was placed on a platform at a distance of 10 cm under the UV lamp.
After irradiation for 24 h, lap shear strength was tested. Data were
repeated for five times.
2.5. Synthesis of model compound 2
3-(3,4-Dihydroxyphenyl) propionic acid (182.18 mg, 1 mmol), 5-
ethoxy-2-nitro-benzaldehyde (195.17 mg, 1 mmol) and tert-butyl
isocyanide (99.756 mg, 1.2 mmol) were dissolved in THF (1 mL). The
homogeneous solution was heated to 40 ꢀC for 48 h. During this
period, the mixture gradually became heterogeneous, indicating
the formation of the product. Finally, the mixture was poured into
diethyl ether. The precipitated products were collected by filtration
and washed with diethyl ether for three times to get compound 2 as
a pale yellow solid in 80% yield. No Tm was detected for this
compound.
3. Results and discussion
3.1. Polymer synthesis and characterization
In our previous reports, we found that the Passerini MCP of a
diisocyanide, a dicarboxylic acid and an o-nitrobenzaldehyde could
efficiently generate UV degradable polymers, demonstrating that o-
nitrobenzaldehyde is a highly reactive aldehyde towards this
polymerization [59,60]. To examine whether the catechol group is
compatible with this polymerization process, we first carried out
the model Passerini reaction of 3-(3,4-dihydroxyphenyl) propionic
acid, 5-ethoxy-2-nitrobenzaldehyde and tert-butyl isocyanide.
Thus, these three compounds were dissolved in THF, and the
mixture was heated to 40 ꢀC with vigorous stirring. The reaction
mixture was homogeneous at the beginning, but it gradually
became heterogeneous. After 48 h, the mixture was poured into
diethyl ether, and the precipitate was collected, washed with
diethyl ether to get compound 2 in 80% yield (Fig. 1). Character-
ization by NMR spectrum confirmed the expected product, thus
demonstrating that the catechol group is compatible with the
Passerini reaction.
Encouraged by this model reaction, we synthesized the di-o-
nitrobenzaldehyde derivative monomer 1. 1, 12-Dibromo-dodecane
was used as a linker of the two o-nitrobenzaldehydes, because this
long alkyl chain can increase the polymer chain flexibility and thus
reduce the glass transition temperature (Tg) of the resulting poly-
mer. Simple treatment of 5-hydroxy-2-nitrobenzaldehyde with 1,
12-dibromododecane in DMF in the presence of potassium car-
bonate afforded 1 in 52% yield (Fig. S2). Then, the Passerini MCP
was conducted using monomer 1, 1,6-diisocyanohexane, and
undecanoic acid (molar ratio 1: 1: 2) in CHCl3 or THF. In both sol-
vents, polymers were obtained as expected as confirmed by 1H
NMR spectrum (Fig. 2), the polymers from CHCl3 showed slightly
higher molecular weight (Table 1).
2.6. General procedure for the Passerini MCP
In a polymerization tube containing a stir bar, carboxylic acid
(total
dihydroxyphenyl) propionic acid: undecanoic acid ¼ 0:10, 1:9,
2:8, 3:7, 4:6, 5:5, 6:4, 7:3) and di-o-nitrobenzaldehyde
1 mmol) with variable catechol feed ratios (3-(3,4-
1
(250.27 mg, 0.5 mmol) were dissolved in THF (1 mL). After adding
1,6-diisocyanohexane (68.1 mg, 0.5 mmol), the tube was sealed
under nitrogen. The initially heterogeneous mixture, due to the
poor solubility of di-o-nitrobenzaldehyde 1, became homogeneous
in about 1 h, indicating the gradual conversion of monomers. The
polymerization was conducted at 40 ꢀC for 48 h. Afterward, the
solution was immediately diluted with THF, and precipitated into
diethyl ether. The obtained polymer was dried in vacuum and kept
in a desiccator.
2.7. UV irradiated degradation of the polymers in solution
All the UV irradiation experiments were carried out under a
Spectroline SB-100P/F high-intensity UV lamp (100 W, 365 nm).
The solution of PA-0 in CDCl3 (10 mg/mL) was put on a platform at a
distance of 10 cm under the lamp. The degradation was monitored
by 1H NMR at 0 h, 2 h, 4 h, 6 h, 8 h and 18 h intervals. After irra-
diation for 18 h, the supernatant was also examined by GPC.
2.8. UV irradiated degradation of the polymer film
The bulk degradation of polymer film upon UV irradiation was
monitored by either UVeVis or FTIR spectrometer. The polymer
film was obtained by drop casting the solution of PA-0.4 in THF onto
the quartz plate or KBr disk. After removal of solvent via evapora-
tion, the sample was placed on a platform at a distance of 10 cm
under the UV lamp. UVeVis spectra were recorded on a Shimadzu
2101 UVeVis spectrometer. FTIR spectra were recorded by a Bruker
Vector22 FTIR spectrometer. The degradation of the bulk film was
monitored at 0 h, 2 h, 4 h, 6 h, 8 h, 10 h and 24 h intervals.
We next prepared seven polymers with variable catechol con-
tents to examine their adhesion properties. The interaction of
catechol moieties with many surfaces was well documented in the
literature [21e23]. Using 3-(3,4-dihydroxyphenyl) propionic acid as
a functional carboxylic acid could endow the obtained polymers
with pendant catechol groups, which would enhance adhesion.
Besides, we chose undecanoic acid as a comonomer not only to
change the content of pendant catechol groups, but also to further
adjust the Tg of the polymers. The polymerization of 1, 1,6-
diisocyanohexane, 3-(3,4-dihydroxyphenyl) propionic acid, and
undecanoic acid with different carboxylic acids feed ratios was
conducted in THF at 40 ꢀC for 3 days. Though CHCl3 seemed more
appropriate according to the aforementioned results with a slight
higher molecular weight of the final polymer, we had to choose THF
as the reaction medium, owing to the poor solubility of 3-(3,4-
dihydroxyphenyl) propionic acid in CHCl3. The obtained polymers
were characterized by 1H NMR (Fig. 3, Fig. S3). From the 1H NMR
2.9. Adhesion test
Adhesion properties were determined by lap shear strength test.
Glass plates (76.2 ꢂ 25.4 ꢂ 0.8 mm3) were chosen as substrates to
ensure the transmission of light. Prior to mount the adhesives, the
glass plates were washed with acetone and sonicated for three
times, and air-dried overnight. Polymer solution (50
mL) in THF

Y.-Z. Wang et al. / Polymer 68 (2015) 270e278
Fig. 1. 1H NMR spectrum of model compound 2.
Fig. 2. 1H NMR spectrum of PA-0 in CDCl3.
273
spectra, we could observe the gradual increase of proton signals
corresponding to the catechol groups (c, e, g, k, l) with the increase
of the feed ratio of 3-(3,4-dihydroxyphenyl) propionic acid. In
addition, the calculated catechol content in the polymer via the
comparison between the integration of methylene protons (k, l)
and methyl protons (t) was in good accordance with the feed ratio.
However, with more 3-(3,4-dihydroxyphenyl) propionic acid
incorporated, such as polymers PA-0.6 and PA-0.7, the solubility of
the obtained polymer decreased, and we had to use DMSO-d6
instead of CDCl3 to confirm the polymer structure by NMR mea-
surement (Fig. S3). The GPC data of these two polymers could not
be evaluated due to the poor solubility. Molecular weights of other
polymers are summarized in Table 1.
Thermal properties of all the polymers were investigated by TGA
and DSC (Table 1, Fig. 4 and Fig. S4). From the TGA traces, we could
see a two-stage decomposition, which was ascribed to the pyrolysis
Table 1
Characterizations of polymers obtained by Passerini MCP.
Entry
Catechol (%)
Feed
M
n (kDa)e
PDIe
Yield (%)f
Td (ꢀC)
Tg (ꢀC)
Lap shear strength (Mpa)
Polymerd
PA-0a
0
0
0
0
9.1
22.8
34.9
44.2
48.3
64.5
69.4
11.1
7.7
6.5
14.2
10.4
8.6
9.2
e
1.59
1.64
1.71
2.12
2.59
1.73
2.76
e
70
52
71
79
92
86
93
91
94
293
290
281
276
271
267
260
259
249
16.9
14.0
28.4
37.8
45.6
50.0
55.1
65.1
70.4
e
PA-0b
0.22 0.05
0.30 0.11
0.34 0.12
0.49 0.04
0.46 0.05
0.53 0.07
e
PA-0.1
PA-0.2
PA-0.3
PA-0.4
PA-0.5
PA-0.6c
PA-0.7c
10
20
30
40
50
60
70
e
e
e
a
Polymerization was carried out in CHCl3.
Polymerization was carried out in THF.
The Mn, PDI and lap shear strength were not determined due to the poor solubility of polymers.
The catechol contents in polymer were calculated by the integration of relevant 1H NMR spectra.
Measured by GPC in THF.
b
c
d
e
f
Determined by precipitation into diethyl ether and vacuum dryness.

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Y.-Z. Wang et al. / Polymer 68 (2015) 270e278
Fig. 3. 1H NMR spectra of polymers with varied catechol contents.
of the ester bond and amide linkage respectively. The calculated
value of the first-stage weight loss was 36%, which was in good
accordance with the theoretical value (33%), when the ester bond
was decomposed (Fig. 4(a)). Increase of catechol content could
lower the decomposition temperature and increase the Tg of the
polymers.
The peak of methine proton (a, 6.70 ppm) ascribed to the o-
nitrobenzyl ester group gradually disappeared within 18 h. Mean-
while the methylene protons (b, 2.44 ppm) adjacent to the pendant
ester bond gradually shifted to the higher field (c, 2.35 ppm),
indicating the photo-cleaved release of undecanoic acid. It should
be noted that after 2 h irradiation, the initial homogeneous solution
became heterogeneous, with a clear observation of brown solid.
The amount of this solid increased with irradiation time. Owing to
the poor solubility, we could not identify the chemical structure of
this solid. This phenomenon also revealed the intrinsic complexity
of this photochemical reaction. From the NMR spectra, we are quite
sure that the solids are the degradation products containing mainly
the aromatic fragments. Since the signals of methylene protons (d,
3.22 ppm) adjacent to the amide bonds were still present in the 1H
NMR spectra, though with a slight decrease of integration, we
anticipated that the polymer was degraded into three parts. As was
well known, o-nitrobenzyl ester could be cleaved by UV irradiation
and afford the corresponding carboxylic acid and o-nitro-
sobenzaldehyde [47,48]. In this work, we observed the formation of
undecanoic acid. The residue backbone should contain ɑ-keto
amide linkages (Scheme 2). According to the literature [70], this
3.2. UV irradiated degradation of copolymers
One distinctive feature of the obtained polymers is the photo-
degradability due to the incorporated o-nitrobenzyl ester groups.
Studies show that this structure can be facilely cleaved upon UV
irradiation [47,48]. Therefore, all the prepared polymers are ex-
pected to decompose via UV irradiation, providing the unique
debonding characteristic of the adhesives. Herein, the degradation
behavior of these polymers was characterized by 1H NMR, GPC,
UVeVis and FTIR analysis.
Initially, the UV degradation behavior of PA-0 in CDCl3 (10 mg/
mL) was monitored by 1H NMR measurements with different UV
irradiation time (365 nm, 100 w) (Fig. 5a). A regular change of the
corresponding proton signals could be observed.
Fig. 4. (a) One typical TGA curve of PA-0 polymerized in THF and (b) stacked DSC curves of polymers with varied catechol contents.

Y.-Z. Wang et al. / Polymer 68 (2015) 270e278
275
Fig. 5. (a) 1H NMR spectra of PA-0 upon UV irradiation. (b) GPC traces of PA-0 before and after UV irradiation.
structure could further undergo photochemical reaction to ensure
the degradation of the polymer backbone. GPC measurements
revealed the absence of high molecular weight products in the
supernatant of the PA-0 solution after UV irradiation (Fig. 5(b)).
Collectively, these data demonstrated that at least most part of the
polymer main chain had been photo-cleaved.
ester bond and the NeO stretching of nitro group, respectively.
Though the newly formed undecanoic acid could not be clearly
observed due to the overlay of the OeH stretching of carboxylic acid
and catechol above 3000 cmꢁ1, the regular change of FTIR spectra
revealed the photoreaction of polymer film upon UV irradiation.
These results confirm that these polymers can also be photo-
cleaved in the solid state.
Considering the practical application of adhesives, we also
studied the UV degradation behavior of these polymers in bulk. At
first, we measured the maximum absorption of catechol and o-
nitrobenzyl ester groups by comparing the UVeVis absorption
spectra of 3-(3,4-dihydroxyphenyl) propionic acid, PA-0 and PA-0.4
in THF (0.04 mg/mL) (Fig. 6(a)). Catechol group had a typical ab-
sorption peak at around 280 nm. PA-0 had a maximum absorption
at around 310 nm, which was ascribed to the o-nitrobenzyl ester
group. The two distinguishing peaks of PA-0.4 are consistent with
the presence of two functionalities. Since catechol did not show any
absorbance at around 365 nm, which was the operating wave-
length of the UV lamp, it would not inhibit the photoreaction of o-
nitrobenzyl ester group. When the thin film of PA-0.4 was irradi-
ated with a UV lamp for different time, a gradual decrease of the
absorption peak of o-nitrobenzyl ester at 310 nm was observed,
while new absorption bands at 400e500 nm gradually increased
with the formation of o-nitrosobenzaldehyde (Fig. 6(b)) [47,48].
Besides, FTIR analysis was also employed to monitor the UV
degradation of PA-0.4 in bulk (Fig. 7). After UV irradiation for 24 h,
we could notice the gradual decrease of peaks at 1747 cmꢁ1 and
1348 cmꢁ1, which were ascribed to the carbonyl stretching of the
3.3. Application to dismantlable adhesives
The adhesion performance of the polymers containing different
catechol groups was investigated by lap shear strength test. Glass
plates were chosen as adherends to ensure that the majority of light
can pass through. In this way, the subsequent UV dismantlable
process could be achieved by just putting a UV lamp upon the glass
plates. The adhesion test was conducted by uniformly placing the
polymer solution in THF between two substrates. After removal of
THF by evaporation in vacuo, the specimen was mounted on an
Instron testing machine and pulled to failure. The typical result for
PA-0.5 is shown in Fig. 8(a). With the extension of displacement,
the force gradually reached its maximum value. The sudden drop of
force indicated the failure of the specimen, and the maximum load
was divided by the overlapping contact area to calculate the
adhesion strength.
To examine the influence of catechol contents on the adhesive
performance, we measured the bulk adhesion strength for each of
the polymers, except for PA-0.6 and PA-0.7, due to their poor sol-
ubility in THF. Fig. 8(b) revealed that polymers with more catechol
groups displayed relatively larger adhesion strength, with that of
PA-0 being 0.22
0.05 MPa, while that of PA-0.5 being
0.53 0.07 MPa, 240% of PA-0. The adhesion strengths of PA-0.3,
PA-0.4, and PA-0.5 are quite similar, thus demonstrating that a
maximum plateau may be reached in PA-0.3. This DOPA derivative
has been incorporated into many synthetic adhesives [8e11,14,15],
and the amount of catechol contents in the polymer can influence
the adhesion performance dramatically. Theoretically, more cate-
chol groups can provide more interaction with the substrate, thus
enhance the adhesion force. Besides, an increase in catechol con-
tent also increases Tg. The modulus of adhesive layer, i. e. lap shear
strength could also be improved as well. However, in our case, the
observed adhesion strength did not increase constantly with the
increase of catechol groups. As documented in the literature [1], the
measured energy during separation of the adherends is a complex
function of the adhesion energy and the dissipative properties.
When a force is applied to the system, some energy is dissipated by
the chain movement. In the current study, too much catechol
groups could lead to the extensive increase of Tg, which in turn
Scheme 2. Proposed mechanism of photocleavage of PA-0 [47,48,70].

276
Y.-Z. Wang et al. / Polymer 68 (2015) 270e278
Fig. 6. (a) UVeVis absorption spectra of 3-(3,4-dihydroxyphenyl) propionic acid, PA-0 and PA-0.4 in THF (0.04 mg/mL). (b) UVeVis absorption spectra of bulk PA-0.4 recorded as a
function of UV exposure time (365 nm, 100 w).
Finally, PA-0.4 was selected as a representative example to test
the UV dismantlable properties of the adhesives. The reason we
chose PA-0.4 was because the adhesion strength of PA-0.4 was in
the middle of the maximum plateau, indicating a possible appro-
priate balance between catechol interaction and chain dissipation
during use. From the aforementioned 1H NMR results, the UV
degradation of PA-0 took about 18 h to complete. Thus, the spec-
imen adhered by PA-0.4 was irradiated for 24 h to ensure a full
degradation of the polymer. Upon UV irradiation, the initially
colorless adhesive became gradually brown (data not shown),
which was contributed to the conversion of nitro groups into
nitroso counterparts. The lap shear strength test of the UV
debonded specimen was carried out under the same condition as
mentioned above. From Fig. 8(c), an obvious change of adhesion
properties of PA-0.4 could be noticed. The adhesion strength after
UV irradiation (0.25
0.03 MPa) was 54% of the original value
(0.46 0.05 MPa), revealing the UV dismantlability of the polymers.
4. Conclusions
Polymers consisting of varied catechol moieties and o-nitro-
benzyl ester groups were successfully synthesized by the Passerini
MCP of
a di-o-nitrobenzaldhyde derivative monomer, 1,6-
diisocyanohexane, 3-(3,4-dihydroxyphenyl) propionic acid and
undecanoic acid. Incorporation of dual functionalities endowed the
polymers with adhesion and UV degradation properties. Adjusting
the catechol contents in the polymers could tune the adhesion
strengths to glass substrates. UV irradiation could cleave the
polymer main chain and deteriorate the corresponding adhesion
performance. It should be pointed out that the limitation of the
application should be only for transparent substrates and the ad-
hesive properties should be further improved by molecular design,
Fig. 7. FTIR spectra of PA-0.4 upon UV irradiation.
prevents the motion of molecular chains. The decrease of dissipa-
tion energy leads to the decreased adhesion strength. Therefore,
the results shown in Fig. 8(b) were the compromised results of
catechol interaction and chain movement.
Fig. 8. (a) One typical force versus displacement curve reported by the Instron testing machine, (b) Lap shear strength of adhesives with varied catechol contents and (c) lap shear
strength of PA-0.4 before and after UV irradiation.

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Acknowledgments
This work is financially supported by National Natural Science
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2015.05.032.
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