Photoswitchable ring-opening polymerization of lactide catalyzed by azobenzene-based thiourea

Article abstract of DOI:10.1039/c6cc04090j

The reactivity of a catalytic polymerization system using photoresponsive azobenzene-based thiourea/PMDETA as a catalyst could be switched between slow and fast states by alternating exposure to UV and ambient light, because the active site of azobenzene thiourea is blocked via intramolecular hydrogen bonding when the azobenzene thiourea transfers from the E isomer to the Z isomer under UV irradiation.

Full text of DOI:10.1039/c6cc04090j

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COMMUNICATION
Photoswitchable ring-opening polymerization of lactide catalyzed
by azobenzene-based thiourea†
Received 00th January 20xx,
Accepted 00th January 20xx
Zhongran Dai, Yaqin Cui, Changjuan Chen, and Jincai Wu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
because it is non-invasive, chromophore-selective, and provides
excellent temporal control.9 Photochemical switches for photo-
The reactivity of catalytic polymerization system with
photoresponsive azobenzene-based thiourea/PMDETA as
a
catalyst could be switched between slow and fast states by initiating polymerization processes and mediating small-molecule
alternating exposure to UV and ambient light, because the active
site of azobenzene thiourea is blocked via intramolecular
hydrogen bonding when the azobenzene thiourea transfers from E
isomer to Z isomer under UV irradiation.
organic transformations have been commonly used in recent
years.1,9,10 However, photoswitchable catalytic ROP (ring-opening
polymerization) of lactones were very rare. For example,
a
photoswitchable ROP of -caprolactone and -valerolactone using
ε
δ
dithienylethene-annulated N-heterocyclic carbene catalyst was
reported by Bielawski and a visible light regulated ROP using a
photoacid as organocatalyst was developed by Xu and Boyer,11 and
no photoswitchable ROP of lactide has been explored until now.
In recent years, the development of switchable catalytic
polymerization using external stimuli attracts much attention
because this kind of progress can be controlled and give desirable
polymers or block copolymers at will.1 External stimuli, including
coordination chemistry,2 redox processes,3 lights,4 and so on,5
usually induce the change in the state of the catalyst and then
control the polymerization progress. In the past several years, some
excellent works on redox-controlled switchable polymerization of
lactide were reported. For example, Diaconescu and co-workers
reported the activity of several group 4 metal alkoxide complexes
supported by ferrocene-based ligands can be controlled using redox
reagents during the ring-opening polymerization of L-lactide and ε-
caprolactone.6 Byers and co-workers reported redox-controlled
polymerization of lactide catalyzed by bis(imino)pyridine iron
bis(alkoxide) complexes, because bis(imino)pyridine iron
bis(alkoxide) complex could be “switched” on and off by reversibly
reducing and oxidizing the metal center.7 Furthermore, they applied
this redox switchable iron-based catalyst to synthesize block
copolymerization of lactide and various epoxides because epoxide
polymerization could be “switched off” upon in situ reduction of the
iron(III) catalyst and “switched on” upon in situ oxidation, which is
orthogonal to what was observed for lactide polymerization.8
Compared with the redox control, light was more ideal stimulus
CF3
CF3
S
S
hv
N
N
H
N
H
CF3
N
N
H
N
H
CF3
hv'
Switch
O
N
N
blocked active site
active site
N
Switch
N
O
O
O
E-1
Z-1
Chart 1 Photoswitchable azobenzene-based thiourea compound 1.
Poly (lactic acid) (PLA) is a biocompatible and biodegradable
polymer and has been widely applied in packaging, agricultural
materials, drug delivery, and medical devices.12 Although the ring-
opening polymerization catalyzed by metal complexes was an
effective method for the synthesis of polylactide,13 the
organocatalytic ring-opening polymerization of lactide also
attracted much attention because of it is more economical,
environmentally friendly, and it is not needed to removal metals.14
Among them, thiourea(TU)/amine base for the ROP of lactide
seems to be a good system because of convenient synthesis, high
activity, and excellent controllability, and the general mechanism
for the H-bonding catalysis in the ROP of lactide have been
proposed: the initiating or propagating alcohol and the carbonyl
group of lactide were activated by a H-bond acceptor (amine) and a
donor (thiourea), respectively.15 For example, Kiesewetter recently
reported a series of excellent works in this area.16 Herein a
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photoresponsive azobenzene-based thiourea catalyst
1 was
synthesized and applied in the photoswitchable ROP of rac-lactide.
Azobenzene-based thiourea compound
1 was synthesized
according to a reported procedure (Chart 1).17 Compound 1 has Z
and E isomers, which can be mutually transformed easily by
appropriate light irradiation. The E isomer of compound 1 has an
open active site of thiourea group for the coordination of lactide via
hydrogen bond interactions (Chart 1). While the thiourea group in Z
isomer can be blocked by a formation of intramolecular hydrogen
bonds with the nitro group. Consequently compound 1 as a catalyst
will be deactivated.
Fig. 1 Homonuclear-decoupled 1H NMR spectrum of PLA (entry 5, Table 1).
In order to study the effect of irradiation for the rate and
isoselectivity of the ROP of rac-Lactide, we firstly quantify their
1
Table 1: Polymerization of lactide catalyzed by 1/PMDETAa
photostationary states (PSS) via H NMR. The ratio of E-isomer and
O
Z-isomer of compound 1 with a concentration of 20 mM in CDCl3 is
more than 99/1. After irradiation with UV light (λ = 330-400 nm,
λmax = 365 nm) for 3 h, 34% Z-1 isomer was obtained (Fig. S4).
O
1
O
/ PMDETA
Bn
O
H
n
+
m BnOH
O
O
R.T.
O
n/m
O
O
Considering Z-isomer of compound 1 may be inactive in the ROP of
lactide due to the intramolecular hydrogen bonding interactions
between thiourea group and one O-atom of nitro group (Chart 1),
so the rate of polymerization could be switched by UV/ambient
irradiation through the process of E-Z/Z-E isomerization. For the
entr cataly monom [LA]0/[I]0 conv.(%)b t (h) Mn,calc
Mn,obsd
6200
PDI Pm
c
d
e
y
1
2
3
4
5
6
7
8
st
1
1
1
1
1
1
1
1
er
L-LA
L-LA
L-LA
L-LA
rac-LA
50:1
100:1
150:1
200:1
50:1
95
94
95
90
94
95
94
91
24
6900
1.04
1
1
1
1
48 13600
72 20600
120 26000
11000 1.03
15100 1.05
19000 1.04
polymerization of rac-lactide with
a 50:1:1:1 ratio of [rac-
LA]0/[1]0/[PMDETA]0/[BnOH]0 in ambient light, the conversion of
reaction reached to 94 % after 24 h. However, only 30 % lactide can
be converted for the same reaction under ultraviolet irradiation (Fig.
3a). The vast difference between the reaction rates (kamb = 2.08×10-
3 min-1 in ambient light vs kUV = 2.08×10-4 min-1 in UV light, Figure S5,
kamb/kuv = 10) shows the light has a great influence on the activity of
compound 1 indicating the formation of Z-isomer will decrease the
whole reaction rate. What is more, the isotaticities of polymers at
different conversions in this progress keep similar (Pm = 0.72-0.76,
Fig. 2) and are almost same to the reaction in ambient light.
Because Z-isomer and E-isomer should have different isoselectivies
unless by chance, this same isoselectivities should result from E-
isomer which may suggest the Z-isomer of compound 1 is non-
active for the ROP of lactide.
24
6900
6400
1.04 0.75
rac-LA 100:1
rac-LA 150:1
rac-LA 200:1
48 13800
72 20400
120 26300
10300 1.03 0.74
14700 1.03 0.73
18500 1.05 0.74
a Conditions (unless special instructions): [LA]0 = 1 M, [1] = [PMDETA] = 0.02 M; in
CDCl3. Lactide conversion determined by 1H NMR. g/mol, these values were
b
c
calculated from [Mlactide × [LA]0/[BnOH]0 × conversion yield + MBnOH
]
d g/mol, these
values were obtained from GPC analysis and calibrated by polystyrene standard
and corrected using the Mark-Houwink factor of 0.58.19 Determined by analysis
e
of all of the tetrad signals in the methine region of the homonuclear-decoupled
1H NMR spectra.18
Based on this hypothesis, the ROP reactions of L-LA and rac-LA
using compound 1 as a catalyst together with N,N,N',N'',N''-
pentamethyldiethylenetriamine (PMDETA) as a co-catalyst were
tested (Table 1). When
a
50:1:1:1 ratio of [L-
catalyst
LA]0/[1]0/[PMDETA]0/[BnOH]0 was used with
a
concentration of 0.02 M at room temperature (entry 1, Table 1), the
polymerization of L-LA within 24 h almost be finished, affording a
polymer with an expected molecular weight and low polydispersity
(PDI). The linear relationships between the molecular weights of the
polymers and the monomer-to-initiator ratio indicate the
polymerization remains controllable and exhibits characteristics of a
“living” polymerization (entries 1-4, Table 1 and Fig. S2). In the
1/PMDETA catalyzed ROP of rac-LA (entries 5-8, Table 1), the
azobenzene-based thiourea catalyst shows modest iso-slectivities of
Pm = 0.73-0.75 under controllable conditions with a living character
at room temperature. The microstructure analysis of a polymer (Pm
= 0.75, Fig. 1) via homonuclear decoupled 1H NMR spectroscopy
indicates that the isoselective mechanism for the ROP of rac-LA was
the chain-end control mechanism because the tetrad probabilities
agree well with the Bernoullian statistics.18
Fig. 2 Plot of Pm of final polymer vs conversion (%) for the ring opening polymerization
of lactide catalyzed by 1 ([rac-LA]0/[1]0/[PMDETA]0/[BnOH]0 = 50:1:1:1, [rac-LA] = 1 M).
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Furthermore, compound 1 as a catalyst could be switched from decreased when UV irradiation was applied for the reaction in the
“OFF” state to “ON” state within the course of the polymerization ambient light after 6 h (conversion: 91 % vs 75 % after 24 h, Fig. 3c).
by stopping UV irradiation. After 6 h in the UV light, the rate of In fact, the difference of activity for “switched off” and “switched
polymerization was increased by alternating UV to ambient light on” of this catalyst was not remarkable, which can be ascribed to
compared with the whole reaction in UV light (conversion: 33 % vs that the process of E-Z/Z-E isomerization was not instantaneous and
45 % after 28 h, Fig. 3b). On contrast, the reaction rate was thorough.
Fig. 3 (a) ROP of rac-lactide by 1 in the UV light (blue line) and ambient light (red line); (b) the reaction vessel was exposed to UV light 6 h and then was kept in ambient light (red
line); (c) the reaction vessel was exposed to ambient light 6 h and then was kept in UV light (blue line).
Cacciapaglia, S. Di Stefano and L. Mandolini, J. Am. Chem.
Soc., 2003, 125, 2224.
(a) J. Beswick, V. Blanco, G. De Bo, D. A. Leigh, U.
Lewandowska, B. Lewandowski and K. Mishiro, Chem. Sci.,
In conclusion, an azobenzene-based thiourea compound 1 as a
catalyst was successfully used in the ring-opening polymerization of
rac-lactide giving isotactic enriched polymer with desirable
molecular weights and narrow molecular weight distributions. In
particularly, the reactivity of this catalytic polymerization system
with photoresponsive azobenzene-based thiourea/PMDETA could
be switched between slow and fast states by alternating exposure
to UV and ambient light.
5
2015, 6, 140; (b) V. Blanco, D. A. Leigh, V. Marcos, J. A.
Morales-Serna and A. L. Nussbaumer, J. Am. Chem. Soc.,
2014, 136, 4905; (c) O. Coulembier, S. Moins, R. Todd and
P. Dubois, Macromolecules, 2014, 47, 486; (d) X. Tian, C.
Cassani, Y. Liu, A. Moran, A. Urakawa, P. Galzerano, E.
Arceo and P. Melchiorre, J. Am. Chem. Soc., 2011, 133
,
17934; (e) Y. Sohtome, S. Tanaka, K. Takada, T. Yamaguchi
and K. Nagasawa, Angew. Chem., Int. Ed., 2010, 49, 9254; (f)
A. Piermattei, S. Karthikeyan and R. P. Sijbesma, Nat.
This work was supported by National Natural Science Foundation
of China (No. 21271092, 21171078) and the Science Foundation of
Gansu Province of China (1308RJ2A121).
Chem., 2009, 1, 133.
6
X. Wang, A. Thevenon, J. L. Brosmer, I. Yu, S. I. Khan, P.
Mehrkhodavandi and P. L. Diaconescu, J. Am. Chem. Soc.,
2014, 136, 11264.
7
8
9
A. B. Biernesser, B. Li and J. A. Byers, J. Am. Chem. Soc.,
2013, 135, 16553.
A. B. Biernesser, K. R. Delle Chiaie, J. B. Curley and J. A.
Byers, Angew. Chem. Int. Ed., 2016, 55, 5251.
S. M. Guillaume, E. Kirillov, Y. Sarazin and J.-F. Carpentier,
Chem. Eur. J., 2015, 21, 7988.
Notes and references
1
V. Blanco, D. A. Leigh and V. Marcos, Chem. Soc. Rev., 2015,
44, 5341.
2
(a) C. Romain and C. K. Williams, Angew. Chem. Int. Ed.,
2014, 53, 1607; (b) H. J. Yoon, J. Kuwabara, J. H. Kim and C.
A. Mirkin, Science, 2010, 330, 66.
10 B. M. Neilson and C. W. Bielawski, ACS Catal., 2013,
3
,
3
(a) E. M. Broderick, N. Guo, C. S. Vogel, C. Xu, J. Sutter, J. T.
Miller, K. Meyer, P. Mehrkhodavandi and P. L. Diaconescu,
J. Am. Chem. Soc., 2011, 133, 9278; (b) E. M. Broderick, N.
Guo, T. Wu, C. S. Vogel, C. Xu, J. Sutter, J. T. Miller, T. Cantat
and P. L. Diaconescu, Chem. Commun., 2011, 47, 9897; (c)
E. M. Broderick, P. S. Thuy-Boun, N. Guo, C. S. Vogel, J.
Sutter, J. T. Miller, K. Meyer and P. L. Diaconescu, Inorg.
Chem., 2011, 50, 2870; (d) C. K. A. Gregson, V. C. Gibson, N.
J. Long, E. L. Marshall, P. J. Oxford and A. J. P. White, J. Am.
Chem. Soc., 2006, 128, 7410.
(a) M. Samanta, V. S. Rama Krishna and S. Bandyopadhyay,
Chem. Commun., 2014, 50, 10577; (b) T. Imahori, R.
Yamaguchi and S. Kurihara, Chem. Eur. J., 2012, 18, 10802;
(c) B. M. Neilson and C. W. Bielawski, J. Am. Chem. Soc.,
2012, 134, 12693; (d) J. Wang and B. L. Feringa, Science,
2011, 331, 1429; (e) D. Sud, T. B. Norsten and N. R.
Branda, Angew. Chem., Int. Ed., 2005, 44, 2019; (f) R.
1874.
11 (a) C. Fu, J. Xu and C. Boyer, Chem. Commun., 2016, 52
,
7126; (b) B. M. Neilson and C. W. Bielawski, Chem.
Commun., 2013, 49, 5453.
12 (a) J. W. Rhim, H. M. Park and C. S. Ha, Prog. Polym. Sci.,
2013, 38, 1629; (b) A. M. DiCiccio and G. W. Coates, J. Am.
Chem. Soc., 2011, 133, 10724; (c) C. M. Thomas, Chem. Soc.
Rev., 2010, 39, 165; (d) A. Morschbacker, Polym. Rev.,
2009, 49, 79.
13 (a) E. E. Marlier, J. A. Macaranas, D. J. Marell, C. R. Dunbar,
M. A. Johnson, Y. DePorre, M. O. Miranda, B. D. Neisen, C. J.
Cramer, M. A. Hillmyer and W. B. Tolman, ACS Catal., 2016,
4
6
, 1215; (b) C. Romain, Y. Zhu, P. Dingwall, S. Paul, H. S.
Rzepa, A. Buchard and C. K. Williams, J. Am. Chem. Soc.,
2016, 138, 4120; (c) V. Balasanthiran, C. Chatterjee, M. H.
Chisholm, N. D. Harrold, T. V. RajanBabu and G. A. Warren,
J. Am. Chem. Soc., 2015, 137, 1786; (d) X. Pang, R. L. Duan,
X. Li, Z. Q. Sun, H. Zhang, X. H. Wang and X. S. Chen, Polym.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Please do not adjust margins
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Chem., 2014, , 6857; (e) A. Pilone, K. Press, I. Goldberg, M.
Journal Name
5
Kol, M. Mazzeo and M. Lamberti, J. Am. Chem. Soc., 2014,
136, 2940; (f) M. Normand, T. Roisnel, J. -F. Carpentier
and E. Kirillov, Chem. Commun., 2013, 49, 11692; (g) I. Yu,
A. Acosta-Ramirez and P. Mehrkhodavandi, J. Am. Chem.
Soc., 2012, 134, 12758.
14 (a) X. Zhi, J. Liu, Z. Li, H. Wang, X. Wang, S. Cui, C. Chen, C.
Zhao, X. Li and K. Guo, Polym. Chem., 2016, 7, 339; (b) J.-B.
Zhu and E. Y.-X. Chen J. Am. Chem. Soc., 2015, 137, 12506;
(c) S. Venkataraman, V. W. L. Ng, D. J. Coady, H. W. Horn, G.
O. Jones, T. S. Fung, H. Sardon, R. M. Waymouth, J. L.
Hedrick and Y. Y. Yang, J. Am. Chem. Soc., 2015, 137, 13851.
15 (a) C. Thomasa and B. Bibal, Green Chem., 2014, 16, 1687;
(b) M. K. Kiesewetter, E. J. Shin, J. L. Hedrick and R. M.
Waymouth, Macromolecules, 2010, 43, 2093.
16 (a) O. I. Kazakov and M. K. Kiesewetter, Macromolecules,
2015, 48, 6121; (b) S. S. Spink, O. I. Kazakov, E. T.
Kiesewetter and M. K. Kiesewetter, Macromolecules, 2015,
48, 6127.
17 L. Osorio-Planes, C. Rodríguez-Escrich and M. A. Pericas,
Org. Lett., 2014, 16, 1704.
18 C. Bakewell, T. P. A. Cao, N. Long, X. F. Le Goff, A. Auffrant
and C. K. Williams, J. Am. Chem. Soc., 2012, 134, 20577.
19 A. Kowalski, A. Duda and S. Penczek, Macromolecules,
1998, 31, 2114.
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