Synthesis of thermo- and photo-responsive polysiloxanes with tunable phase separation: Via aza-Michael addition

Article abstract of DOI:10.1039/c7nj03177g

Two kinds of thermo- and photo-dual-responsive polysiloxanes (DRPSs) with functional pendent groups, N-isopropyl amides and azobenzene (Azo) or salicylideneaniline (SA), were synthesized through a facile, effective, and catalyst-free aza-Michael addition of poly(aminopropylmethyl-siloxane) with N-isopropyl acrylamide and N-azobenzene acrylamide or N-salicylaldehyde acrylamide. The chemical structrures of DRPSs were systematically characterized using FT-IR, H NMR and UV-Vis spectroscopy. The as-prepared DRPSs with lower Azo or SA contents exhibited lower critical solution temperature (LCST)-type phase transition in water, which is reversible and can be controlled by temperature and UV light. The effects of Azo and SA contents on the responsive properties of DRPSs are examined in detail. The LCST decreased with the increasing Azo or SA content. Once the content of Azo or SA reached up to 5.7% or 8.2%, respectively, DRPSs could not be dissolved in water even in an ice bath. Higher values of the LCST were measured after irradiation of the polymer solutions due to the higher polarity of cis-Azo and keto-SA conformation, induced by irradiation. The differences in cloud points between the irradiated and the non-irradiated DRPS aqueous solutions increased up to 3.4 °C and 9.8 °C when combined with 3.8% Azo and 5.8% SA units, respectively.

Full text of DOI:10.1039/c7nj03177g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Synthesis of thermo- and photo- responsive polysiloxanes with
tunable phase separation via aza-Michael addition
Kai Feng,a Shusheng Li,a,b* Linglong Feng,a and Shengyu Fenga*
Received 00th January 20xx,
Accepted 00th January 20xx
Two kinds of thermo- and photo- dual-responsive polysiloxanes (DRPSs) with functional pendent groups, N-isopropyl
amides and azobenzene (Azo) or salicylideneaniline (SA) were synthesized through a facile, effective, and catalyst-free aza-
Michael addition of poly(aminopropylmethyl-siloxane) with N-isopropyl acrylamide and N-azobenzene acrylamide or N-
salicylaldehyde acrylamide. The chemical structrures of DRPSs were systematically characterized by FT-IR, H NMR and UV-
Vis spectrum. The as-prepared DRPSs with lower Azo or SA contents exhibited lower critical solution temperature (LCST)-
type phase transition in water, which is reversible and can be controlled by temperature and UV light. The effect of Azo
and SA content on responsive properties of DRPSs are examinated in detail. The LCST decreased with the increasing Azo or
SA content. Once the content of Azo or SA reached up to 5.7% or 8.2% respectively, DRPSs can not dissolved in water even
in ice bath. Higher values for the LCST were measured after irradiation of the polymer solutions due to the higher polarity
of cis-Azo and keto-SA conformation, induced by irradiation. The differences of cloudy points between the irradiated and
the non-irradiated DRPSs aqueous solutions increased up to 3.4 °C and 9.8 °C when combined with 3.8% Azo and 5.8% SA
units, respectively.
DOI: 10.1039/x0xx00000x
www.rsc.org/
researches interest in polymers that show responsive
behaviours to multiple stimuli.9-12 In addition, polymers that
are responsive to light and temperature are very appealing
1 Introduction
Polysiloxane is an appealing material for clinical and medical
treatments, because of its outstanding physiological inertness,
non-toxicity, and biocompatibility.1,2 To satisfy smart clinical
and biomedical applications, such as controllable drug
delivery, smart bioactive surfaces, and selective bio-
and important for applications, because light is everlasting,
clean, non-invasive, a wide range of wavelengths, and features
high spatiotemporal resolution. Accordingly, there have been
several reports on thermo-responsive polymers that contain
photo-responsive moieties.13-18 Azobenzene (Azo) groups19-22
and salicylideneaniline (SA) moieties23,24 have been applied as
photochromic groups to establish photo-responsive polymers,
separation,
a few smart polysiloxanes with controllable
hydrophobic/hydrophilic property have been synthesized
through modification of polysiloxane matrix using guest smart
polymers or inlaying polysiloxane into amphipathic block
copolymers.3-6 However, the polysiloxane segments in these
polymers merely serve as accessories and show no stimuli-
responsive capacities due to their intrinsic hydrophobicity, and
these poysiloxanes are non-water-soluble, limited their usage
in smart applications. Therefore, it is highly desirable, yet
synthetically a challenge to the development of methodologies
to synthesize stimuli-responsive polymers with polysiloxane as
backbones. Recently, we designed and synthesized a series of
high thermo-responsive polysiloxanes with high sensitivity and
controllable phase separation behaviour, due to its flexible Si–
O–Si backbone, based on a facile, effective, and catalyst-free
aza-Michael addition.7,8
benefitting
from
their
photo-controlled
reversible
isomerization. The former show a reversible isomerization
from trans- to cis-configuration upon irradiation25-28 and the
latter exhibit isomerization from the enol- into the keto-form
upon irradiation with UV light.29-31 Polysiloxanes with photo-
responsive property have also been gaining attentions,32-37
however water-soluble ones with thermo- and photo-tunable
phase-separation have not been reported.
In this article, we present a facile method for synthesis
thermo- and photo- dual-responsive polysiloxanes (DRPSs)
featuring N-isopropyl amide (NIPAs) and Azo or SA groups, in
which, NIPAs served as thermo-responsive groups, while Azo
and SA served as the corresponding photo-responsive
moieties. The synthesis was successfully achieved via catalyst-
free aza-Michael addition of amino-containing polysiloxane
(PAPMS) with N-isopropyl acrylamide (NIPAAm), and N-
azobenzene acrylamide (AAM-Azo) or N-salicylaldehyde
acrylamide (AAM-SA). In specific, aza-Michael addition yields
no by-products and exhibits high function group tolerance and
high conversions at room temperature.38,39 The polysiloxanes
Besides responsive to a single stimulus, recent a few
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exhibit a lower critical solution temperature (LCST) in aqueous (10 mL) was added into the solution and refluxed for
solution, which was dependent on the amount of incorporated overnight. The solution was filtered and distilled under
NIPAs, and Azo or SA groups, and the isomerization state of reduced pressure and crude powder product was obtained.
the respective Azo or SA groups. The addition process and Then, the powder was dissolved in ethanol (50 mL) and treated
thermo- and light- controlled phase separation behavior of as- with salicylaldehyde (1.22 g, 10 mmol). Under vigorous stirring,
prepared polysiloxanes are discussed in details.
the mixture was refluxed for 3 h. The ethanol was removed
under reduced pressure and the residue was purified by silica
gel chromatography using acetic ether/Hexanes (1:1) as eluent
to yield AAM-SA (76 %). 1H NMR (400 MHz, DMSO-d6): δ 13.22
(1H, s), 10.31 (1H, s), 8.98 (1H, s), 7.78 (2H, d), 7.64 (1H, m),
7.42 (3H, m), 6.98 (2H, m), 6.45 (1H, m), 6.29 (1H, m), 5.79 (1H,
m). 13C NMR (100 MHz, DMSO-d6): δ 163.61, 162.51, 160.73,
143.60, 138.61, 133.46, 132.89, 132.26, 127.48, 122.37,
120.57, 119.56, 117.02 ppm.
2 Experiments
2.1 Materials
All chemicals and solvents were commercially available and
used as received unless otherwise stated. p-Nitroaniline,
acryloyl chloride, salicylaldehyde, 4-aminoazobenzene and N-
isopropylacrylamide of analytically pure were obtained from
Energy Chemical. 3-aminopropyl methyldiethoxysilane of
industrial grade was purchased from Shenzhen Changhao
Technology Company and used after distillation under reduced
pressure and were determined by 1H NMR. 2,4,6,8-
tetraaminopropyl-2,4,6,8-tetramethyl cyclotetrasiloxane (D4-
AP) was synthesized as described in the literature.7
2.4 Synthesis of N-azobenzene acrylamide (AAM-Azo)
4-aminoazobenzene (1.97 g, 10 mmol) and triethylamine (1.21
g, 12 mmol) were dissolved in CH2Cl2 (90 mL), and acryloyl
chloride (1.09 g, 12 mmol) was dissolved in another CH2Cl2 (10
mL). The latter was dropwise added into the former for 0.5 h
under ice-water bath and vigorous stirring for overnight. The
obtained mixture was filtered and the filtrate was
concentrated under reduced pressure and purified by silica gel
chromatography using acetic ether/Hexanes (2:3) as eluent to
2.2 Instrumentation
FT-IR spectra were recorded using a Bruker TENSOR 27 FT-IR
spectrometer instrument within the range of 4000-400 cm-1
1
1
working at 4 cm-1 resolution. H NMR spectra were recorded
yield AAM-Azo (86 %). H NMR (400 MHz, DMSO-d6): δ 10.54
(s, 1H), 7.92–7.85 (m, 6H), 7.62-7.54 (m, 3H), 6.55-6.29 (m,
1H), 6.35-6.29 (m, 1H), 5.85-5.78 (m, 1H). 13C NMR (100 MHz,
DMSO-d6): δ 163.75, 152.76, 149.29, 140.38, 131.00, 130.96,
129.21, 128.74, 124.17, 122.89, 120.14 ppm.
on a Bruker 400 MHz spectrometer in DMSO-d6. The molecular
weights and molecular weight distributions (Mw/Mn) of
samples were determined by gel permeation chromatography
(GPC) measurements performed on a Milford MA Waters 515
liquid chromatography equipped with
a Waters 2414
2.5 Synthesis of poly(aminopropylmethylsiloxane) (PAPMS)
and thermo- and photo-responsive polysiloxane (DRPSs)
refractive-index detector. Samples were run in THF at 40 °C at
a rate of 1 mL·min-1 with respect to polystyrene standards.
D4-AP (23.4 g, 0.05 mol) was injected with nitrogen into a
three-neck flask equipped with a mechanical stirrer. Then, KOH
(0.05 g, 0.2 wt.-%) and DMSO (0.23 g, 1.0 wt.-%) was added to
the mixture, and the temperature was increased to 50 °C. The
viscosity of the reaction mixture sharply increased after 1 h or
so. AP2-PSi2 (0.50 g, 2 mmol) was added rapidly to the flask,
and the reaction was equilibrated for 5 h. PAPMS was obtained
after the mixture cooled to room temperature and neutralized
by an equimolar acetic acid with KOH. Then, PAPMS (1.0 g),
NIPAAM (0.29 g, 30 mol.-%) and AAM-SA (0.095 g, 4 mol. %) or
AAM-Azo (0.09 g, 4 mol. %) were dissolved in DMF (10 mL),
and stirred for 24 h at 30 °C. Smart polysiloxanes were
obtained after the DMF was removed by reduced pressure and
vacuum drying. A viscous product was obtained. The contents
of NIPAs and Azo/SA in the obtained polysiloxane were
controlled by the feed molar ratios of NIPAAM and AAM-
SA/AAM-Azo in systems.
UV/vis
spectra
were
recorded
on
a
TU-9001
photospectrometer. LCSTs of the polysiloxnaes were
determined by turbidimetry in water at a concentration of 10
mg·mL-1. The optical transmittance of a light beam (λ=700 nm)
through the 1.0 cm sample quartz cell of the photo-
spectrometer was monitored as a function of temperature.
The heating rate was 0.5 °C·min-1. The LCST were defined as
the temperature at which a transmission of 50 % was
observed. The irradiation experiments were conduct with an
oriel instruments 500 W mercury lamp with a 365 nm filter in a
1 cm diameter quartz cell.
2.3 Synthesis of N-salicylaldehyde acrylamide (AAM-SA)
p-Nitroaniline (1.38 g, 10 mmol) and triethylamine (1.20 g, 12
mmol) were dissolved in THF (100 ml) and cooled to 0 °C and
stirring. Then, acryloyl chloride (1.18 g, 12 mmol) was added
into the solution and stirred for overnight. THF was
evaporated under reduced pressure, and the residue was
dissolved in acetic ether (100 mL) and washed with a saturated
aqueous solution of NaHCO3 (50 mL) for three times. The
organic layer was dried with magnesium sulfate, filtered, and
distilled under reduced pressure and yellow crude powder
product (1.76 g) was obtained. Then, the powder was
dissolved in a 5:1 mixture of ethanol and water (100 mL). Iron
powder (1.10 g), and saturated ammonium chloride solution
3 Results and discussion
Scheme 1 shows the synthetic scheme of DRPSs via catalyst-
free aza-Michael addition of PAPMS (molar weight of 11520
g·mol-1, about 97 Si-O units) with NIPAAm and AAM-Azo or
AAM-SA using DMF/ethanol (8:2) as solvent at room
temperature. The PAPMS was synthesized through the ring-
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opening polymerization of 2,4,6,8-Tetraaminopropyl-2,4,6,8-
tetramethyl cyclotetrasiloxane (D4-AP) using KOH as a catalyst,
DMSO as an accelerator and 1,3-bis(3-aminopropyl)-
tetramethyldislloxane (AP2-PSi2) as an end-capping reagent.7,8
AAM-Azo was synthesized by amidation of aminoazobenzene
with acryloyl chloride.40,41 AAM-SA was obtained by amidation
of
p-nitroaniline with acryloyl chloride and followed by a
reduction reaction with iron powder and a Schiff base reaction
with salicylaldehyde.42,43 These guest molecules, NIPAAm,
AAM-SA, and AAM-Azo, which are all α,β-unsaturated carbonyl
compounds possessing electron drawing group (C=O), served
as receptors of the aza-Michael addition. Meanwhile, amino
groups in PAPMS can serve as nucleophiles and bases. Thus,
the addition can be high-efficiently conducted at room
temperature without any catalyst, and shows good tolerant to
other active groups.44-46 Analogous aza-Michael reactions of
amino groups with acrylamide or acrylate derivatives have
been reported as novel polymer synthesis strategies, especially
for poly(amido amine)s47-49 and poly(ester amine)s50-54
respectively. In specific, the reaction yields no by-products and
exhibits high function group tolerance and high conversions at
room temperature, which provides a convenient and practical
method for the establishment of smart polysiloxane systems.
Primary amines add to electron poor alkenes that are
susceptible to nucleophilic attack to give secondary and
tertiary derivatives. The first product formed, the mono-
adduct, can react further to give the tertiary derivative, or bis-
adduct.55,56 Herein, DMF/ethanol (8:2) mixed solvent was
selected for the addition, because DMF was more conducive to
form a mono-addition product,57,58 in addition, the obtained
AAM-Azo and AAM-SA can be dissolved in DMF. Ethanol, a
general proton solvent, can promoted rapid proton transfer
and to stabilize charged intermediates and was considered as
good solvent to accelerate the Michael addition. Furthermore,
amino groups in PAPMS are much more than α,β-unsaturated
carbonyl compounds in molar, thus the bis-addition are
indiscernible in NMR.
As shown in Fig. 1, the obtained DRPSs can be dissolved in
water at lower temperature, and precipitated from solvent
when the temperature increased up to a certain value, namely
cloudy point. This phenomenon, analogous to reported
poly(NIPAAm)s,59-61 can be attribute to thermo-dependent
hydrogen bonding effect of the N-isopropyl groups in polymers
with water molecules, which become weak with increased
temperature. Simply, at low temperatures, the polysiloxane
chains are in the energetically favoured hydrated state, as
water molecules associated with amino groups form hydrogen-
bonded cage-like structures around the hydrophobic isopropyl
groups. As the temperature increases, the hydrogen bonding
becomes weaker. Above the cloudy point, intramolecular
hydrogen bonds between C=O and N-H groups in the chains
result in a compact and hydrophobic collapsed conformation
of polymer chains and polysiloxane precipitation from water.
The phase separation behaviour, that is to say the cloudy point
of the obtained polysiloxane is affected significantly to the
amount of attached Azo or SA and UV irritation, because of the
combination of Azo or SA groups in the polysiloxane as
pendant groups, which will be discussed in details in the later.
The polymer structures were analysed by 1H NMR, and FT-IR.
As shown in Fig. 2, the signals of α,β-unsaturated double
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bonds of acrylamide derivatives (NIPAAm, AAM-Azo and AAM- observed in as-prepared polymers, P0, P1b and P2b, indicating
SA) about at 4.2, 6.1, and 6.2 ppm62 totally vanished from the aza-Michael addition was successful conducted.
obtained DRPSs (taking P0a, P1b and P2b for examples),
The characteristic singles of aromatic rings of Azo and SA
indicating a complete conversion of aza-Michael addition. were not observed in FT-IR spectra due to their low molar
Meanwhile, peaks displayed at 2.2 and 2.8 ppm in obtained contents (about 4.0 %). However, in the UV/vis spectra of P0a,
polymers further justified that –NH–CH2–CH2–CO– groups P1b and P2b (Fig. 4), the characteristic absorptions at 324 nm
newly formed after the addition. Additionally, the of Azo66,67 and 311 nm of SA68 chromophore were observed in
characteristic signal of –CH(CH3)2 at 1.0 ppm and aromatic P1b and P2b, respectively, further indicating the success of
proton signals at the range of 7.0 to 8.0 ppm also indicated aza-Michael addition. Moreover, there was no absorption at
successful addition. The peaks at 0.0, 0.5, and 1.2 ppm were the range of 550 to 750 nm; hence, 700 nm was reasonably
assigned to –Si–CH3, –Si–CH2–, and –Si–CH2–CH2– selected for LCST measure.
respectively.63 The conversions of NIPAAm, AAM-Azo, and
As shown in Table 1 and Fig. 5A, the LCSTs of the prepared
AAM-SA into DRPSs were determined from the characteristic polysiloxanes in aqueous exhibited a strong dependence upon
signals of –CH(CH3)2 and aromatic ring relative to the signal of the content of incorporated Azo and SA groups. At a given
–Si–CH2– at 0.5 ppm. The results are listed in Table 1.
content of NIPAs (about 30 %), the LCST of the as-prepared
The FT-IR spectrum of polymer PAPMS, P0, P1b and P2b polysiloxanes decreased from 43.2 °C to 22.3 °C with increase
were shown in Fig. 3. The characteristic singles at 3,200–3,400 of Azo from 0.0 % to 3.8 %, and from 43.2 °C to 12.4 °C with
cm-1 belonged to the stretching vibration of NH2. The peaks at increase of SA from 0.0 % to 5.8 % respectively, because of the
1,258 cm-1, 1,090 cm-1, and 800 cm-1 assigned to the vibration hydrophobic character of Azo and SA. Usually, the LCST of a
features of Si-C, the asymmetric bending of Si–O–Si, and thermo-responsive polymer may be adjusted in a wide range
symmetric stretching of Si-O respectively.64,65 Besides, clearly through the copolymerization of more hydrophobic or more
visible singles at 3,058 cm-1 (overtone bands of stretching
vibration of NH), 1,660 cm-1 (stretching vibration of the C=O)
and 1,550 cm-1 (bending vibration of the NH) were also
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switched reversibly by UV irradiation.77 Irradiation of Azo
containing surfactants with UV-light with a wavelength of 360
nm resulted in more hydrophilic changes of the interfacial
properties.78-80 Chen et al. embedded Azo into hyperbranched
polyethylenimine terminated with isobutyramide and UV-
induced trans-to-cis isomerization increases the local polarity
of the supramolecular complexes, increasing LCST.81,82 Jochum
et al. prepared series of Azo contained polyacrylamides or
PNIPAAm, and higher LCST values were measured for UV-
irradiated solutions of the copolymers in comparison to the
non-irradiated one.82-84 SA showed
a change of color
accompanied by a change in the dipole moment after
irritation. An intramolecular proton-transfer reaction between
the enol-imine and keto-amine forms. Jochum et al. reported
LCST shift of up to 13 °C was found for poly(N-
cyclopropylacrylamide) copolymer containing 15.0 mol % of AS
groups.85 However, the phase separation occurred at a broad
range of temperatures. Compared with reported thermo- and
light- dual responsive polymers, the obtained DRPSs were
considered physiological inertness, non-toxicity, and
biocompatibility, and exhibit high sensitive (the phase
separation occurred within 0.3 °C) due to them flexible Si–O–Si
backbones, which reduced conformational energy during them
phase separation. Furthermore, the LCST values of DRPSs can
be controllable by pH, salty and physic or chemical interaction
with other hydrophobic/hydrophilic molecules. Accordingly,
within the temperature range of the LCSTs before and after
irradiation, an isothermal, light-induced precipitation of the
polysiloxanes was possible.
hydrophilic monomers compared to the original main
monomer. In addition, hydrophobic co-monomers that favour
segment-segment interactions in polymers decrease the
LCST.69-72 For the same reason, when more NIPAAm was added
from 30.3 % to 39.8 %, the LCST also decreased from 43.2 °C to
28.9 °C (P0a and P0b). Meanwhile, the as-prepared
polysiloxanes cannot be dissolved in water even at 0 °C, when
the amount of SA was increased up to 8.2 % or Azo up to 5.7
%.
The values of LCST of obtained polysiloxanes are also
affected by UV light. As shown in Fig. 5B, in both Azo and SA
modified polysiloxanes, higher LCST values were observed
during irradiation with UV light. In a given content of NIPAs
(30%), the LCST shift increased lineally from 0 °C to 3.4 °C and
from 0 °C to 9.8 °C, with the increasing content of Azo from 0.0
% to 3.8 % and SA from 0.0 % to 5.8 % respectively. The LCST
shift upon irradiation can be explained by the isomerization of
Azo and SA. After irritation by UV light at 365 nm, Azo
transforms from trans- to cis-configuration73,74 and SA from
the enol- to the keto-form75,76 (Fig. 6), accompanied by an
increase in dipole moment; and thus, an increased local
polarity is present at the polymer backbone, which resulted in
the increase of LCST. Analogous phenomenon has been
reported in non-polysiloxane materials. Theato et al.
immobilized Azo and SA onto glass, polycarbonate, and steel
surface and the wettability of substrates surface could be
Conclusions
In summary, we prepared two kinds of thermo- and photo-
responsive polysiloxane containing different amounts of NIPAs
and Azo/SA chromophores as side groups. The synthesis was
based on a catalyst-free aza-Michael addition of PAPMS with
NIPAAm and AAm-SA or AAm-Azo in a mild condition. The
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obtained DRPSs exhibited a LCST in aqueous solution, which 18 J. R. Kumpfer, and S. J. Rowan, J. Am. Chem. Soc., 2011, 133,
depended strongly on the amount of incorporated Azo or SA 12866–12874.
chromophore. Furthermore, the UV-light induced 19 N. Ishii, J. Mamiya, T. Ikeda, and F. M. Winnik, Chem. Commun.,
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This work was financially supported by the National Natural
Science Foundation of China (No. 21274080) and Special Fund
for Shandong Independent Innovation and Achievements
Transformation (No. 2014ZZCX01101).
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A graphical and textual abstract for the contents pages
Graphical
O
O
O
H
N
NH
N
P1
P2
N
R
R:
NH NH2 NH
H
N
N
Si
Si
Si
O
O
O
O
HO
j
k
i
Structure of thermo- and photo- dual-responsive polysiloxanes
Textual abstract
Two kinds of thermo- and photo- dual-responsive polysiloxanes were synthesized through a facile,
effective, and catalyst-free aza-Michael addition.