Controlled synthesis of azobenzene functionalized homo and copolymers via direct acyclic diene metathesis polymerization

Article abstract of DOI:10.1016/j.polymer.2014.02.021

A powerful and efficient strategy for obtaining a series of azobenzene functionalized linear unsaturated polyolefins is described, which involves the first design and synthesis of three different α,ω-diene monomers with different reactivity of acrylates and terminal double bonds and their subsequent acyclic diene metathesis (ADMET) polymerization under mild reaction conditions. Different ruthenium based metathesis catalysts and conditions were tested to optimize the ADMET polymerization of these monomers. Besides, this polycondensation method is also reported allowing for the synthesis of alternating and diblock azobenzene-containing copolymers with molecular weight control by using the selectivity of olefin cross-metathesis between acrylates and terminal olefins. The resulting polymers showed well-defined molecular weights, reasonable polydispersity index, and reversible photoresponsive behavior. This special combination of the benefits of metathesis polymerization and azobenzene is a highly versatile system for materials in many applications.

Full text of DOI:10.1016/j.polymer.2014.02.021

Polymer 55 (2014) 1681e1687
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Controlled synthesis of azobenzene functionalized homo and
copolymers via direct acyclic diene metathesis polymerization
*
Liang Ding , Mengyu Xu, Jingjing Wang, Yang Liao, Jun Qiu
School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224051, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A powerful and efficient strategy for obtaining a series of azobenzene functionalized linear unsaturated
Received 14 November 2013
Received in revised form
16 January 2014
Accepted 5 February 2014
Available online 14 February 2014
polyolefins is described, which involves the first design and synthesis of three different
a,u-diene
monomers with different reactivity of acrylates and terminal double bonds and their subsequent acyclic
diene metathesis (ADMET) polymerization under mild reaction conditions. Different ruthenium based
metathesis catalysts and conditions were tested to optimize the ADMET polymerization of these
monomers. Besides, this polycondensation method is also reported allowing for the synthesis of alter-
nating and diblock azobenzene-containing copolymers with molecular weight control by using the
selectivity of olefin cross-metathesis between acrylates and terminal olefins. The resulting polymers
showed well-defined molecular weights, reasonable polydispersity index, and reversible photo-
responsive behavior. This special combination of the benefits of metathesis polymerization and azo-
benzene is a highly versatile system for materials in many applications.
Keywords:
Acyclic diene metathesis polymerization
Azobenzene
Linear polyolefins
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
peripheral groups [8]. Aoshima reported the synthesis of co-
polymers with various types of azobenzene groups via living
Polymers containing azobenzene (azo)-type chromophores
have drawn considerable attention in recent years due to their
combined two intriguing properties of a nonlinear optical property
and photoresponsive transecis isomerization [1]. Their photo-
responsive properties stem from the photoinduced fast and
reversible isomerization between the trans and cis isomers of the
azo groups upon exposure to UV or visible light, which triggers
significant changes in their physicochemical properties and can
lead to photoinduced reorientation of azo groups in the polymers
[2]. Many potential applications have been proposed for these
polymers, such as optical data storage, liquid crystal displays, op-
tical switching, nonlinear optical devices, surface relief gratings,
and photomechanical systems [3e7].
So far, a great number of azo-polymers with various architec-
tures have been developed through diverse methods. Wang syn-
thesized a series of hyperbranched azo-polymers by two-stage azo-
coupling reactions, in which a hyperbranched precursor azo-
polymer was prepared by step-growth polymerization of an AB2
monomer through azo-coupling reaction and then was further
modified by post-polymerization azo-coupling reaction at the
cationic polymerization and studied the photoresponsive behavior
of the obtained copolymers [9]. Living free radical polymerization,
such as atom transfer radical polymerization [10] and reversible
addition-fragmentation chain transfer polymerization [11] in
particular has been successfully used to synthesize azo-polymers
with well-defined structures. Besides, ring-opening metathesis
polymerization [12] and click chemistry [13] approaches were also
utilized in preparation of azo-chromophore-containing polymers.
Acyclic diene metathesis (ADMET) polymerization is a quanti-
tative reaction tolerating many functional groups, yields only the
desired linear polymer, and releases ethylene as an outgrowth [14].
Usually, symmetric
a,u-dienes are employed to obtain polymers
with a defined repeat unit structure. Wagener has explored the use
of this polymerization technique to elegantly synthesize precision
polyolefins containing functional groups [15]. A different approach
was reported by Slugovc, in which alternating copolymers were
synthesized by taking advantage of the high selectivity of the cross-
metathesis reaction between acrylates and terminal alkenes [16].
Meier has broadened the applications of this technique, and
introduced the concept of a selective head-to-tail ADMET poly-
merization using a monomer containing both a terminal double
bond and an acrylate. Such monomers polymerize with high cross-
metathesis selectivity, opening access to different polymer archi-
* Corresponding author. Tel.: þ86 515 88298872.
E-mail address: dl1984911@ycit.edu.cn (L. Ding).
tectures, if
a selective and irreversible chain transfer agent
http://dx.doi.org/10.1016/j.polymer.2014.02.021
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

1682
L. Ding et al. / Polymer 55 (2014) 1681e1687
(mono or multifunctional acrylate) is added [17]. The chain transfer
agent allows controlling the molecular weight and direct func-
tionalization of the ADMET polymer by reacting selectively with
one of the end groups (terminal double bond). This approach
has led to a wide range of unsaturated and saturated polymers
bearing different functionalities with the possibility to modify the
polyolefin architectures by placing precise functional groups
throughout the backbone [18,19]. However, to the best of our
knowledge, up to now, only two classes of azo-polymers synthe-
sized via ADMET polymerization based on more defined polymeric
structures (hyperbranched and long-chain highly branched poly-
mers) have been reported [18b,19c]. Therefore, the development of
new azo-containing polyolefins bearing reactive substituents
(particularly those with high reactivity under mild conditions) is of
significant importance (Scheme 1).
Herein, we reported a variable approach to prepare several azo-
chromophore bearing linear polymers based on ADMET polymer-
ization of three types of azobenzene monomers using different
Grubbs catalysts. The molecular weights varied for the different
monomers and catalysts applied but in all cases high molecular
weight polymers were obtained and thus empirically the molecular
weight can be adjusted, which confirmed the advantages by using
ADMET polymerization as an ideal synthetic strategy for linear azo-
polymers. This is the first report on the ADMET polymerization to
azo-polymers with a variable backbone which cannot be prepared
by other approaches.
(CH2Cl2) and chloroform (CHCl3) from calcium hydride, tetrahy-
drofuran (THF) and toluene from sodium/benzophenone.
2.2. Characterization
UVevis absorption spectra were measured on a UVe7504PC
(756 MC) spectrometer. UV irradiation was carried out with 8 W UV
lamp with wavelength at 365 nm. Irradiation by visible light was
performed using a 23 W Philips day light bulb (>400 nm). 1H
(500 MHz) and 13C (125 MHz) NMR spectra were recorded using
tetramethylsilane as an internal standard in CDCl3 on a Bruker DPX
spectrometer. Relative molecular weights and molecular weight
distributions were measured by gel permeation chromatography
(GPC) equipped with a Isocratic HPLC pump, a refractive index
detector, and a set of Waters Styragel columns (7.8 ꢀ 300 mm, 5
mm
bead size; 103, 104, and 105 A pore size). GPC measurements were
carried out at 35 ꢁC using THF as the eluent with a flow rate of
1.0 mL/min. The system was calibrated with polystyrene standards.
Elemental analysis (EA) was conducted with an Elementar Vario EL.
LC/MS measurements were performed with an Agilent technology
1200 series at 45 ꢁC using H2O or CH3CN as a mobile phase,
ꢀ
50 m ꢀ 4.6 mm ꢀ 3.5
mm diffused column.
2.3. Monomer synthesis
2.3.1. Synthesis of 2-(4-nitro-phenoxy)-ethanol (1)
4-Nitrophenol (6.95 g, 50.0 mmol), potassium carbonate (27.6 g,
200.0 mmol), and 150 mL of DMF were charged into a 500 mL
Schlenk flask. The reaction mixture was heated at 80 ꢁC for 6 h
under nitrogen allowing for the potassium salt formed. A solution
of 2-bromoethanol (9.38 g, 75.0 mmol) in 50 mL of DMF was then
added dropwise to the above mixture. After 24 h of stirring at 50 ꢁC,
the reaction mixture was poured into 3 L water and the crude
product was precipitated out, and further purified by recrystalli-
zation from ethyl acetate to give a yellow crystalline powder 1
2. Experimental section
2.1. Materials
4-Nitrophenol (99%), 2-bromoethanol (98%), stannous chloride
dehydrate (SnCl2$2H2O) (98%), 10-undecenoic acid (99%), acrylic
acid (99%), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hy-
drochloride (EDCI$HCl, >99%), [1,3-bis(2,4,6-trimethylphenyl)-2-
imidazolidinylidene]dichloro(o-isopropoxyphenylmethylene)
ruthenium (HoveydaeGrubbs second generation catalyst) (98%),
potassium carbonate (99%), benzylidene[1,3-bis(2,4,6-trimethyl
phenyl)-2-imidazolidinylidene] dichloro(tricyclohexylphosphine)
ruthenium (Grubbs second generation catalyst) (98%), benzyli-
deneebis(tricyclohexylphosphine) dichlororuthenium (Grubbs
first generation catalyst) (98%), 4-dimethylaminopyridine (DMAP)
(98%), phenol (99þ%), phenylphosphoric acid (>99%), and sodium
nitrite (99%) were purchased from Energy Chemical and used as
received without purification. Solvents were distilled over drying
agents under nitrogen prior to use: N,N-dimethyl formamide (DMF)
was freshly distilled and dried by sieves, methylene chloride
(6.16 g, 88.6% yield). 1H NMR (CDCl3):
d (ppm) 8.10 (d, 2H, o-ArHe
NO2), 7.06e7.01 (m, 2H, m-ArHeNO2), 4.14 (m, 2H, HOCH2CH2O),
3.92e3.89 (m, 2H, HOCH2CH2O). 13C NMR (CDCl3):
d (ppm) 166.8,
141.7, 126.2, 115.1, 77.5, 65.6.
2.3.2. Synthesis of 2-(4-amino-phenoxy)-ethanol (2)
To a round bottom flask were added compound 1 (5.49 g,
30 mmol) and SnCl2$2H2O (67.5 g, 300 mmol). These materials
were dissolved in 120 mL of dry alcohol and refluxed under argon
for 24 h. The resulting clear solution was cooled to room tem-
perature and then poured into an aqueous NaOH solution (20%,
500 mL). The precipitate was collected by filtration and washed
with water. The product was extracted with CH2Cl2, and the so-
lution was dried over MgSO4 and filtrated. Removal of the solvent
and further purified through recrystallization from ethyl acetate
to afford a light-yellow solid 2 (4.72 g, 86.0% yield). 1H NMR
(CDCl3):
d (ppm) 6.65e6.57 (m, 2H, m-ArHeNH2), 6.33e6.24 (d,
2H, o-ArHeNH2), 4.15 (m, 2H, HOCH2CH2O), 3.96 (m, 2H,
HOCH2CH2O). 13C NMR (CDCl3):
d (ppm) 151.8, 147.2, 116.4, 114.9,
116.1, 76.3, 64.4.
2.3.3. Synthesis of 4-hydroxy-40-(2-hydroxy ethoxy) azobenzene
(3)
A solution of sodium nitrite (2.07 g, 30 mmol) in water (8 mL)
was added dropwise to the solution of compound 2 (4.59 g,
30 mmol) in 20 wt% aqueous HCl (15 mL) at 0 ꢁC and stirred for 1 h.
A cooled solution of phenol (3.38 g, 36 mmol) in NaOH aqueous
(50 mL) was then added dropwise to the former solution at 0 ꢁC and
stirred further for 4 h. The resulting mixture was poured into water
and the crude product was precipitated out by neutralizing the
Scheme 1. Monomers synthesized and ADMET polymerization in this study.

L. Ding et al. / Polymer 55 (2014) 1681e1687
1683
reaction mixture with sodium acetate solution, and further purified
2.3.8. Synthesis of asymmetric azobenzene diene monomer (M3)
by recrystallization from toluene to afford a red crystalline powder
Following the general procedure described above and using
azobenzene chromophore 5 and 10-undecenoic acid, M3 was ob-
tained as a red crystal with a yield of 83.2% after recrystallization
3 (6.90 g, 89.2% yield). 1H NMR (CDCl3):
d (ppm) 7.82e7.75 (m, 4H,
o-ArHeN]NeArH), 6.98e6.90 (m, 4H, m-ArHeN]NeArH), 4.13
(m, 2H, HOCH2CH2O), 3.98e3.95 (m, 2H, HOCH2CH2O). 13C NMR
from ethanol/ethyl acetate (2:1 v/v). 1H NMR (CDCl3):
d (ppm)
(CDCl3):
77.4, 65.6.
d
(ppm) 162.4, 157.8, 148.9, 144.4, 126.5, 124.2, 117.5, 116.1,
7.92e7.86 (m, 4H, o-ArHeN]NeArH), 7.03e6.95 (m, 4H, m-ArHe
N]NeArH), 6.42e6.39 (d, 1H, OCOCH]CH), 6.17e6.13 (d, 1H,
OCOCH]CH), 5.85e5.71 (m, 2H, OCOCH]CH þ CH2]CH), 5.08e
4.92 (m, 4H, CH2]CH), 4.55e4.46 (m, 4H, OCH2CH2OCO), 4.17e4.12
(m, 4H, OCH2CH2OCO), 2.24e2.19 (m, 2H, OCOCH2), 2.02e1.95 (m,
2H, CH2]CHCH2), 1.74e1.63 (m, 2H, OCOCH2CH2), 1.45e1.23 (m,
2.3.4. Synthesis of 4,40-bis(2-hydroxy ethoxy) azobenzene (4)
Following the general procedure described above and using
azobenzene chromophore 3, 4 was obtained as a red crystal with a
yield of 80.6% after recrystallization from ethanol. 1H NMR (CDCl3):
10H, CH2). 13C NMR (CDCl3):
d (ppm) 171.0, 165.8, 161.1, 144.2, 139.8,
131.4, 127.9, 123.6, 116.5, 114.4, 71.9, 67.4, 33.6, 30.4, 25.4. LC: single
peak observed. EI/MS: calcd. for C30H38N2O6: 522.6; Found: 522.7.
Anal. calcd for C: 68.97, H: 7.28, O: 18.39; Found C: 68.95, H: 7.30, O:
18.38.
d
(ppm) 7.84e7.81 (m, 4H, o-ArHeN]NeArH), 7.06e6.95 (m, 4H,
m-ArHeN]NeArH), 4.15e4.08 (m, 4H, HOCH2CH2O), 3.99e3.93
(m, 4H, HOCH2CH2O). 13C NMR (CDCl3):
d (ppm) 161.2, 144.6, 123.0,
114.7, 75.1, 63.3.
2.4. Representative ADMET polymerization procedure
2.3.5. Synthesis of 4-(2-hydroxy ethoxy)-40-(2-ethyoxy acrylate)
azobenzene (5)
In a nitrogen-filled Schlenk tube, a solution of the appropriate
catalyst (0.5 mol% to monomer) in 0.1 mL of degassed toluene with
three freezeevacuumethaw cycles was added to a solution of
monomer (M1 or M3, 0.5 mmol) in 0.4 mL of toluene degassed with
the same procedure. After the reaction mixture was stirred at 40e
60 ꢁC for 24 h, the polymerization was quenched by adding ethyl
vinyl ether with stirring for 30 min. The solution was precipitated
into an excess of methanol, and the precipitate was isolated
by filtration and dried under vacuum to give the ADMET
homopolymers.
Following the general procedure described above, and using
azobenzene chromophore 3 and acrylic acid 2-bromo-ethyl ester,
5 was obtained as a red crystal with a yield of 77.5% after recrys-
tallization from ethanol. 1H NMR (CDCl3):
d (ppm) 7.86e7.78 (m,
4H, o-ArHeN]NeArH), 7.02e6.86 (m, 4H, m-ArHeN]NeArH),
6.42e6.35 (d, 1H, OCOCH]CH), 6.18e6.10 (d, 1H, OCOCH]CH),
5.85e5.81 (m, 1H, OCOCH]CH), 4.55e4.26 (m, 6H,
OCH2CH2OCO þ HOCH2CH2O), 4.01e3.98 (m, 4H, HOCH2CH2O). 13
C
NMR (CDCl3):
d (ppm) 165.2, 161.5, 143.2, 130.9, 128.7, 123.3, 115.6,
74.5, 72.4, 67.2, 63.4.
2.4.1. Polymer 1 (P1)
The ADMET polymerization was carried out at 60 ꢁC for 24 h 1H
2.3.6. Synthesis of symmetric azobenzene diene monomer (M1)
NMR (CDCl3): d (ppm) 7.91e7.83 (m, o-ArHeN]NeArH), 7.02e6.91
To
a Schlenk flask, azobenzene chromophore, 4 (4.53 g,
(m, m-ArHeN]NeArH), 5.49e5.40 (m, CH]CH), 4.56e4.27 (m,
OCH2CH2OCO), 4.22e4.09 (m, OCH2CH2OCO), 2.04e1.93 (m,
OCOCH2), 1.86e1.69 (m, CH2]CHCH2), 1.43e1.11 (m, CH2).
15.0 mmol), 10-undecenoic acid (7.74 g, 42.0 mmol), and DMAP
(0.44 g, 3.6 mmol) were discharged and dissolved in 50 mL of dried
DMF (50 mL) under a nitrogen atmosphere, and then EDCI (6.89 g,
36.0 mmol) was added at 0 ꢁC with rapid stirring. The reaction
mixture was allowed to warm to room temperature and stirred for
an additional time. After 4 days, the reaction mixture was poured
into water and red solid precipitated. The crude product was
further purified by recrystallization from ethanol/ethyl acetate (1:1
v/v) to give a red crystal M1 (8.17 g, 85.9% yield). 1H NMR (CDCl3):
2.4.2. Polymer 3 (P3)
The ADMET polymerization was carried out at 40 ꢁC for 24 h. 1H
NMR (CDCl3): d (ppm) 8.03e7.85 (m, o-ArHeN]NeArH), 7.01e6.79
(m, m-ArHeN]NeArH þ OCOCH]CHCH2), 5.82e5.71 (d, OCOCH]
CHCH2), 4.49e4.17 (m, OCH2CH2OCO), 2.24e2.06 (m, OCOCH2),
1.81e1.65 (m, CH2]CHCH2), 1.58e1.19 (m, CH2).
d
(ppm) 7.86e7.77 (m, 4H, o-ArHeN]NeArH), 7.04e6.93 (m, 4H,
m-ArHeN]NeArH), 5.79e5.72 (m, 2H, CH2]CH), 5.01e4.89 (m,
4H, CH2]CH), 4.52e4.47 (m, 4H, OCH2CH2OCO), 4.19e4.11 (m, 4H,
OCH2CH2OCO), 2.20e2.13 (m, 4H, OCOCH2), 2.00e1.96 (m, 4H,
CH2]CHCH2), 1.73e1.68 (m, 4H, OCOCH2CH2), 1.43e1.21 (m, 10H,
2.5. General procedure for preparation of block copolymers via
ADMET polymerization
CH2). 13C NMR (CDCl3):
d (ppm) 171.2, 161.6, 143.9, 139.2, 124.3,
In a nitrogen-filled Schlenk tube, a solution of the appropriate
catalyst (1 mol% to total monomers) in 1 mL of degassed toluene
with three freezeevacuumethaw cycles was added to M1 or M3
and M2 degassed with the same procedure. After the reaction
mixture was stirred at 40 or 60 ꢁC for 24 h, the polymerization was
quenched by adding ethyl vinyl ether with stirring for 30 min. The
solution was precipitated into an excess of methanol, and the
precipitate was isolated by filtration and dried under vacuum to
give the ADMET copolymers.
116.8, 114.5, 72.7, 64.4, 33.1, 30.6, 25.4. LC: single peak observed. EI/
MS: calcd. for C38H54N2O6: 634.3; Found: 634.2. Anal. calcd for C:
71.92, H: 8.52, O: 15.14; Found C: 71.95, H: 8.53, O: 15.11.
2.3.7. Synthesis of symmetric azobenzene diene monomer (M2)
Following the general procedure described above and using
azobenzene chromophore 4 and acrylic acid, M2 was obtained as a
red crystal with a yield of 81.7% after recrystallization from ethanol.
1H NMR (CDCl3):
d (ppm) 7.88e7.82 (m, 4H, o-ArHeN]NeArH),
7.02e6.89 (m, 4H, m-ArHeN]NeArH), 6.44e6.37 (d, 2H, OCOCH]
CH), 6.16e6.12 (d, 2H, OCOCH]CH), 5.83e5.76 (m, 2H, OCOCH]
2.5.1. Copolymer 1 (P1e2)
The ADMET polymerization was carried out at 60 ꢁC for 24 h
CH), 4.49e4.40 (m, 4H, OCH2CH2OCO), 4.15e3.97 (m, 4H, OCH2
using M1 and M2 as monomers. 1H NMR (CDCl3):
d (ppm) 7.87e
-
CH2OCO). 13C NMR (CDCl3):
d
(ppm) 165.6, 161.8, 145.1, 130.5, 128.3,
122.6, 114.5, 71.8, 67.1. LC: single peak observed. EI/MS: calcd. for
22H22N2O6: 410.1; Found: 410.4. Anal. calcd for C: 64.39, H: 5.36, O:
23.41; Found C: 64.37, H: 5.40, O: 23.39.
7.75 (m, o-ArHeN]NeArH), 7.21e6.82 (m, m-ArHeN]Ne
ArH þ CH2CH]CHOCO), 5.86e5.77 (m, CH2CH]CHOCO), 4.30e
3.92 (m, OCH2CH2OCO), 2.05e1.86 (m, OCOCH2), 1.84e1.51 (m,
CH2]CHCH2), 1.50e1.09 (m, CH2).
C

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L. Ding et al. / Polymer 55 (2014) 1681e1687
2.5.2. Copolymer 3 (P3e2)
The ADMET polymerization was carried out at 40 ꢁC for 24 h
using M3 and M2 as monomers. 1H NMR (CDCl3):
d (ppm) 8.03e
7.79 (m, o-ArHeN]NeArH), 7.08e6.85 (m, m-ArHeN]Ne
ArH þ CH2CH]CHOCO), 5.86e5.77 (m, CH2CH]CHOCO), 4.53e
4.11 (m, OCH2CH2OCO), 2.20e1.98 (m, OCOCH2), 1.89e1.65 (m,
CH2]CHCH2), 1.52e0.91 (m, CH2).
3. Results and discussion
3.1. Monomer synthesis
Three different diene investigated in this study were synthe-
sized by a similar strategy to that reported by Ikeda, using the
classical methods such as Williamson coupling reaction, reduction
reaction, diazotizationecoupling reaction, and esterification
(Scheme 2) [20]. Dienes comprised of two “symmetric” electron-
rich alkenes (M1) or electron-poor acrylates (M2) were used as
monomers. A terminal alkene and an acrylate were also used as an
“asymmetric” example (M3). The crude products were purified by
recrystallization to provide pure azobenzene monomers with good
yields. Fig. 1 showed the 1H NMR spectra of monomers, we can
observe the appearance of resonance signals of terminal double
bonds (i, j, q, k, and p). Furthermore, the molecular weights,
structures, and purities of monomers were fully tested by LC/MS,
13C NMR spectroscopy, and elemental analysis. All of these points
affirmed the successful preparation of diene monomers (M1eM3)
with the expected structures.
Fig. 1. 1H NMR spectra for azobenzene monomers (a) M1, (b) M2, and (c) M3.
3.2. ADMET homopolymerization
temperature was not high enough for M1 to activate the reaction
efficiently, and sometimes unreacted monomer was recovered.
Therefore, the polymerization temperature was raised to 60 ꢁC,
where a quick increase in the viscosity of the reaction mixture was
observed, and evolution of ethylene. The polymeric materials iso-
lated were in the range of 15,000e23,000 (molecular weight, Mn)
and were readily soluble in THF. After a screening on different pa-
rameters as reported in Table 1, the optimum conditions for the
polymerization of M1 were found to be at 60 ꢁC, 0.5 mol% of C1, and
for 24 h polymerization time. The representative GPC trace in Fig. 2
showed the expected monomodal peak with a relatively narrow
molecular weight distribution of 1.51 and high molecular weight of
15,600. Moreover, Fig. 3a displayed the 1H NMR spectrum of P1
proving almost disappearance of the terminal double bonds (5.79e
5.72 and 5.01e4.89 ppm) and the formation of internal double
bonds (5.38 ppm). All other peaks were easily assigned for the
polymer as well as for the monomer. This also indicates that the
Motivated by the high functional group tolerance of the Grubbs-
type catalysts, we investigated the influence of different catalysts
(Grubbs first generation (C1), second generation (C2), and Hov-
eydaeGrubbs second generation (C3)) on the molecular weight and
the polydispersity for the ADMET polymerizations of the mono-
mers M1eM3 in toluene with stirring at different temperatures
(40e60 ꢁC). It has been found that ruthenium-based catalysts could
catalyze other non-metathetic reactions, such as the isomerization
of alkenes [21]. Although this type of reaction has been successfully
used to synthesize a wide range of organic products, it is a problem
when the product targeted is the one resulting from metathesis
[22]. In order to prevent the association between ruthenium and
azobenzene group and suppress the unwanted isomerization pro-
cess, the isomerization inhibitor, phenylphosphoric acid, with as
little as 5% was added to the polymerization mixture [23]. Table 1
lists the polymerization conditions and results. The first reactions
were carried out at 40 ꢁC, however, it was observed that the
Scheme 2. General protocol for the synthesis of azobenzene monomers for ADMET polymerization.

L. Ding et al. / Polymer 55 (2014) 1681e1687
1685
Table 1
Conditions for the ADMET polycondensation of monomersa and analytical data of
the resulting polymers.
Monomer Polymer Catalyst Yieldb (%) Temp (ꢁC) Mn,GPC PDIc Mn,NMR
c
M1
P1a
P1b
P1c
P1d
P3a
P3b
P3c
C1
C2
C3
C2
C1
C2
C3
92
93
88
68
84
88
96
60
60
60
40
40
40
40
15,600 1.51 17,200d
22,300 1.93
16,800 1.65 18,100d
7900 1.52
13,500 1.66 12,400e
17,700 1.82
16,900 1.55 17,500e
e
e
M3
e
a
Polymerization conditions: [M] ¼ 1.0 mol/L, polymerization temperature ¼ 40
or 60 ꢁC, [Grubbs catalyst] ¼ 5.0 ꢀ 10ꢂ3 mol/L.
b
Obtained gravimetrically from the dried polymer.
Number average molecular weight (Mn) and polydispersed index (PDI) were
c
determined by gel permeation chromatography in THF relative to monodispersed
polystyrene standards.
d
Mn,NMR ¼ n ꢀ M(1) ꢂ (n ꢂ 1) ꢀ M(ethylene) was obtained by 1H NMR spectroscopy,
where n ¼ (Sr/2H): (Sj/4H), M(1) ¼ 634, and M(ethylene) ¼ 28.
e
Mn,NMR ¼ m ꢀ M(3) ꢂ (m ꢂ 1) ꢀ M(ethylene) was obtained by 1H NMR spectros-
copy, where m ¼ (Sb/4H): (Sq/2H), M(3) ¼ 522, and M(ethylene) ¼ 28.
isomerization process can be suppressed completely by addition of
phenylphosphoric acid to the reaction mixture.
Fig. 3. 1H NMR spectra for ADMET homopolymers (a) P1 and (b) P3.
For monomer M2 which functionalized with two relatively
electron-deficient olefinseacrylates, when treated with Grubbs’
catalyst, electron-deficient olefins do not homodimerize (or do so
very slowly), and almost no polymeric materials were obtained.
This is attributed to the so-called “negative neighboring group ef-
fect” [16,24]. To avoid suffering this effect, the number of methy-
lene spacers between these functionalities is at least two for
successful metathesis to occur [25], despite in some classical
catalyst systems, reactivity has been observed in systems with only
one methylene spacer present [26]. Consequently, the acrylate
functionality is always failed to (homo)polymerize but could
actualize ADMET polymerization with alkene overwhelmingly in
the manner of the cross-metathesis reaction [16e18,27].
Monomer M3 containing both a terminal alkene and an acrylate
functional group would polymerize only by head-to-tail additions
[17b]. Then, we focused our investigations on finding the ADMET
polymerization conditions giving the highest terminal olefin-to-
acrylate metathesis selectivity in the polymerization of M3. The
internal olefin peak of the generated ADMET polymer P3 at
5.38 ppm in 1H NMR spectrum (Fig. 3b) was not observed, indi-
cating that ADMET polymerization between terminal alkenes
almost did not happen, and a high selectivity can be achieved when
using 0.5 mol% of C3. Besides, GPC curve of P3 (Fig. 4) revealed a
molecular weight of 16,900 and a molecular weight distribution of
1.55 with complete monomer conversion (Table 1).
3.3. ADMET copolycondensation
Once the ADMET polymerization conditions were established,
we decided to investigate the possibility of constructing co-
polymers by using different diene monomers. The electron-poor
acrylates would react only with electron-rich alkenes because ac-
rylates have a very low tendency to dimerize during olefin
metathesis, i.e. diacrylates do not homopolymerize under the re-
action conditions used, but it can be considered as a copolymeri-
zation monomer [16]. On the basis of this principle, two types of
copolymers were synthesized using ADMET polymerization.
Treatment of a 1:1 mixture of M1 (diene) with M2 (diacrylate)
using ruthenium catalyst yields the corresponding alternating
copolymer P1e2 in high yields. Typical polymerization results are
summarized in Table 2. The structure of P1e2 can be easily
Fig. 2. GPC traces of homopolymer P1 and copolymer P1e2 with different molecular
Fig. 4. GPC traces of homopolymer P3 and copolymer P3e2 with different molecular
weights.
weights.

1686
L. Ding et al. / Polymer 55 (2014) 1681e1687
Table 2
new peak at 6.96 ppm and a doublet at 5.81 ppm (Fig. 5a). These
changes indicate that ADMET copolymerization had been accom-
plished successfully.
Characteristics of copolymers via ADMET polymerization using HoveydaeGrubbs
second generation catalyst.a
Polymer Yieldb (%) Temp (ꢁC) [M1]/[m2] ([M3/[M2]) Mn,GPC PDIc Mn,NMR
c
Since a mono or difunctional acrylate would react only with one
chain end of the polymer (acrylates have a very low tendency to
dimerize during olefin metathesis, as discussed above), it can be
considered as a selective chain stopper [17]. For this purpose, we
chose M2 (diacrylate) as a selective chain stopper in the ADMET
polymerization of M3 (a terminal alkene and an acrylate). Varying
the [Monomer]/[chain stopper] ratio should allow the synthesis of
different molecular weights of copolymer. The reaction of M3 and
M2 in 4:1e32:1 molar ratio yielded the block copolymers. GPC
curve showed the obvious increase in the molecular weight as the
molar ratio enhance (Fig. 4). The 1H NMR analysis of P3e2 (Fig. 5b)
exhibited a perfect match between the backbone and end group
signals, giving the expected repeating unit ratio of 32:1
(Mn,theo ¼ 32 ꢀ [M(3) ꢂ M(ethylene)] þ M(2) ¼ 16,200).
P1e2a 96
P1e2b 85
60
60
60
40
40
40
40
1:1
0.95:1
1.05:1
4:1
8:1
16:1
32:1
14,200 1.83 12,900d
9300 2.06
20,800 1.95
3500 1.98
5700 1.87
9900 1.82
e
e
e
e
e
P1e2c
88
P3e2a 89
P3e2b 86
P3e2c
92
P3e2d 93
18,600 1.75 16,500e
a
Copolymerization conditions: [total monomers] ¼ 1.0 mol/L, polymerization
temperature ¼ 40 or 60 ꢁC, [total monomers]/[Grubbs catalyst] ¼ 100:1.
b
Obtained gravimetrically from the dried polymer.
Number average molecular weight (Mn) and polydispersed index (PDI) were
c
determined by gel permeation chromatography in THF relative to monodispersed
polystyrene standards.
d
Obtained by 1H NMR spectroscopy: Mn,NMR
¼
[(Ss/1H): (Sa/
2H)] ꢀ [M(1) þ M(2) ꢂ M(ethylene)], where M(1) ¼ 634, M(2) ¼ 410, and M(ethylene) ¼ 28
are the molecular weights of M1, M2 and ethylene, respectively.
e
Obtained by 1H NMR spectroscopy: Mn,NMR ¼ 2k ꢀ [M(3) ꢂ M(ethylene)] þ M(2)
,
3.4. Photoresponsive behaviors of the azo main-chain polyolefins
where 2k ¼ (Ss/2H): (Sq/2H), M(2) ¼ 410, M(3) ¼ 522, and M(ethylene) ¼ 28.
The photoresponsive properties of the representative azo main-
chain polymer were first investigated in THF. By irradiating the
diluted solution by UV light of 365 nm, the studied polymer solu-
tion underwent trans to cis photoisomerization until a photosta-
tionary state was eventually reached (Fig. 6a). The intensity of the
* transition band around 362 nm decreased, whereas that of
the n / * transition band around 450 nm slightly increased.
p
/
p
p
Irradiation of the above polymer solution at the photostationary
state with visible light led to the cis to trans back-isomerization
process (Fig. 6b). However, the finally recovered absorbance of
trans-isomer was lower than that before UV irradiation with the
recovery of the trans-isomer being around 89%, just as previously
reported by others [18b,28].
Its real cause is not totally clear yet but might be that, at the
same wavelength where cis to trans photochemical back-
isomerization was performed, there was also a weak absorption
from the trans-isomer, which eventually led to an equilibrium with
cis to trans and trans to cis isomerizations taking place under the
same visible light after most of the cis-isomer returned to the trans-
isomer [29]. Nevertheless, the photoisomerization became
completely reversible upon the subsequent cycles of UV and visible
light irradiation (Fig. 7).
Fig. 5. 1H NMR spectra for ADMET copolymers (a) P1e2 and (b) P3e2.
4. Conclusion
A variable approach to several azobenzene functionalized linear
polyolefins is reported based on ADMET polymerization using
different Grubbs catalysts. The molecular weights vary for the
different monomers and catalysts applied but in all cases high
molecular weight polymers were obtained and thus empirically the
determined using 1H NMR spectroscopic analysis, since olefinic
protons for copolymerization units have a distinct chemical shift
from the starting materials and homocoupled units. P1 displays a
multiplet at 5.38 ppm (Fig. 3a), whereas alternating units produce a
Fig. 6. UVevis spectral changes in dependence of time for the solution of polymer in THF with a concentration of 20 mg/mL. (a) Absorbance changes with UV light irradiation
(365 nm) and (b) upon irradiating the solution at the photostationary state with visible light.

L. Ding et al. / Polymer 55 (2014) 1681e1687
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The authors thank the National Natural Science Foundation of
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of Yancheng Institute of Technology for financial support of this
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