A method for preparing water soluble cyclic polymers

Article abstract of DOI:10.1016/j.reactfunctpolym.2013.11.006

A novel ring-closure method was developed to specifically focus on the preparation of water soluble cyclic polymers. The well-defined linear polymers were synthesized by a standard RAFT polymerization using a functional RAFT agent 1. The cyclic polymers were then obtained by virtue of an efficient bromomaleimide-thiol substitution reaction to ring-close the linear precursors. This technique is unique in that it not only produces various well-defined water soluble cyclic polymers with high efficiency and topology purity, but also employs the environmentally benign solvent, water, as the ring-closure reaction media.

Full text of DOI:10.1016/j.reactfunctpolym.2013.11.006

Reactive & Functional Polymers 80 (2014) 15–20
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
A method for preparing water soluble cyclic polymers
a
Shiyu Long a,b,1, Qingquan Tang a,1, Ying Wu a, Luoxin Wang b, Ke Zhang a, , Yongming Chen
⇑
a State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
b College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 1 December 2013
A novel ring-closure method was developed to specifically focus on the preparation of water soluble cyc-
lic polymers. The well-defined linear polymers were synthesized by a standard RAFT polymerization
using a functional RAFT agent 1. The cyclic polymers were then obtained by virtue of an efficient bromo-
maleimide-thiol substitution reaction to ring-close the linear precursors. This technique is unique in that
it not only produces various well-defined water soluble cyclic polymers with high efficiency and topology
purity, but also employs the environmentally benign solvent, water, as the ring-closure reaction media.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Cyclic polymer
Water soluble polymer
RAFT polymerization
Ring-closure technique
Bromomaleimide-thiol substitution reaction
1. Introduction
strategy can produce the cyclic polymer derivatives with complex
architectures, such as theta and eight shapes conveniently [15–18].
Cyclic polymers, as one of the oldest topological polymers, have
rejuvenated recently due to the significant achievements in cyclic
polymer synthesis [1–6]. With the endless molecular topology,
cyclic polymers have markedly different characteristics than their
linear counterparts, such as a smaller radius of gyration and hydro-
dynamic volume, lower melt viscosity, and higher thermostability.
To date, the known synthesis methods for cyclic polymers can be
generalized into two categories: the ring-expansion and ring-
closure strategies.
In the ring-expansion approach, the cyclic polymers are formed
by a continuous insertion of monomer units into the activated cyc-
lic chains. Because the cyclic polymers remain intact during the
whole ring expansion process, this strategy can produce pure cyclic
polymers with high molecular weight at concentrated solution and
even in bulk. However, the ring-expansion approach is hardly to
control the molecular weight and polydispersity of the resultant
cyclic polymers [7–11]. In addition, it is difficult to produce the
topological polymers derived from monocyclic polymers, such as
the theta, eight, and tadpole shape polymers.
However, the disadvantages are also obvious in this approach.
One of the main problems lies in the requirement of highly di-
luted coupling reaction condition to avoid the intermolecular reac-
tion. Usually, this concentration goes to 10ꢀ5 M, which means that
the preparation of 100 mg cyclic polymers needs around 1 L sol-
vent [12,14,19]. As the environmental pollution becomes a very
heavy topic nowadays, the utilization of huge amount of toxic or-
ganic solvents should be viewed as a significant limitation for pro-
ducing cyclic polymers. Nearly all of the current ring-closure
methods, however, are developed using the hazardous organic sol-
vents as reaction media. Resultantly, the unique ring-closure tech-
niques are urgently needed to produce cyclic polymer in the
environmentally benign reaction media, such as water. In addition,
since the natural bioorganic transformations are all carried out in
water, the water solubility is a fundamental prerequisite for cyclic
polymers when used in biology fields. From this point, it is again
necessary to develop efficient ring-closuring techniques specifi-
cally focusing on the preparation of water soluble cyclic polymers.
The critical issue for producing cyclic polymers in water is to
choose the right coupling reaction which has a high efficiency
using water as reaction media. In 2009, Baker and co-workers dis-
covered that bromomaleimides undergo rapid and highly efficient
substitution reaction with thiol group in aqueous solution [20].
Subsequently, they demonstrated that this reaction can be utilized
to prepare protein-polymer conjugates with high efficiency [21–
23]. Just recently, this chemistry was extended into polymer
synthesis field [24,25]. Robin et al. designed a novel dibromomalei-
mide reversible addition-fragmentation chain transfer (RAFT)
agent, various well-defined polymers with dibromomaleimide
end group were then produced by RAFT polymerization. The high
In the ring-closure method, cyclic polymers are prepared by
applying highly efficient coupling chemistry to end-functionalized
linear telechelic polymers. Especially when click chemistry is com-
bined with controlled polymerization, this approach has demon-
strated its power in the preparation of varied cyclic polymers
with controlled molecular weight and low polydispersity [12–
14]. In addition to the simple monocyclic topology, the ring-closure
⇑
Corresponding author. Fax: +86 10 62559373.
E-mail address: kzhang@iccas.ac.cn (K. Zhang).
These authors have equal contribution to this paper.
1
1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2013.11.006

16
S. Long et al. / Reactive & Functional Polymers 80 (2014) 15–20
reactivity of the end dibromomaleimide was demonstrated by
reacting with a model thiophenol chemical [24].
2.3. Preparation of RAFT agent 1
Inspired by this, we developed a unique ring-closure method for
the preparation of water soluble cyclic polymers by combining
RAFT polymerization and bromomaleimide-thiol substitution
reaction herein. Fig. 1 shows the main process. In the first step,
the well-defined water soluble linear polymers were synthesized
by RAFT polymerization using a functional RAFT agent 1. As the
second step, in the linear precursor water solution, the reducing
agent NaBH4 was used to cut the thiocarbonylthio group and re-
lease the thiol group, the cyclic polymers were then obtained
in situ by virtue of thebromomaleimide-thiol substitution reaction.
4-Cyano-4-((thiobenzoyl)sulfanyl)pentanol
(a)
(661 mg,
2.49 mmol), 2-dibromo-3-propylsulfanyl-maleimide (b) (520 mg,
2.08 mmol), and PPh3 (654 mg, 2.49 mmol) were dissolved in
anhydrous THF (10 mL). After cooled the mixture to 0 °C in an
ice/water bath, DIAD (502 mg, 2.49 mmol) was dropwised into
the solution and left it to stir for 30 min. The solution was then
warmed to room temperature and kept stirring for another 12 h.
After that, the solvent was removed under reduced pressure and
the crude mixture was purified by column chromatography (SiO2,
petroleum ether/ethyl acetate = 9/1) to afford the red oil product
(486 mg) with a yield of 47%.
2.4. Preparation of Linear PDMAM
2. Experimental
A mixed solution of DMAM (1.1 g, 11.11 mmol), RAFT agent 1
(53 mg, 0.11 mmol), and AIBN (3.5 mg, 0.02 mmol) was degassed
via three freeze–thaw–pump cycles. After stirring for 5.5 h at
60 °C, the reaction was terminated by exposing system to air.
The polymer was precipitated from an excess of mixture of ether
and petroleum ether (v/v = 1/3) three times. After drying overnight
in a vacuum oven at room temperature, the light red product was
obtained with a yield of 22.11%. The monomer conversion was
obtained from 1H NMR with 23.94%.
2.1. Materials
Bromobenzene, carbon disulfide (CS2), maleimide, bromine,
4,40-azobis(4-cyano-1-pentanol), 1-propanethiol, triphenylphos-
phine (PPh3), diisopropylazodicarboxylate (DIAD), and sodium
borohydride were purchased as regent grade from Aldrich, Acros,
Alfa Aesar and used as received. Ethyl acetate, diethyl ether, meth-
anol, chloroform, dichloromethane (DCM), tetrahydrofuran (THF)
were purchased as regent grade from Beijing Chemical Reagent
Co. and used as received unless otherwise noted. N-isopropylacryl-
amide (NIPAM) was purified by recrystallization in a mixture of n-
hexane and toluene. N,N-dimethylacrylamide (DMAM) was dried
over CaH2 and distilled before use. 2,20-Azoisobutyronitrile (AIBN)
was recrystallized from ethanol and stored at 4 °C. 2,3-Dibromo-
maleimide [26], 2-bromomaleimide [27], and 4-Cyano-4-((thi-
obenzoyl)sulfanyl)pentanol (ain Fig. 2) [28] were synthesized
according to the previous literature.
2.5. Preparation of cyclic PDMAM
After dissolving linear PDMAM (20 mg) in water (100 mL) 1 h,
NaBH4 (80 mg, 2 mmol) was added and the solution was kept stir-
ring for 24 h at room temperature. After that, the reaction solution
was concentrated by vacuum evaporation to produce the cyclic
PDMAM.
2.6. Preparation of linear PNIPAM
2.2. Preparation of 2-dibromo-3-propylsulfanyl-maleimide (b in Fig. 2)
A mixed solution of N-isopropylacrylamide (4 g, 35.34 mmol),
RAFT agent (176 mg, 0.35 mmol), AIBN (12 mg, 0.07 mmol), and
DMF (4 g) was degassed via three freeze–thaw–pump cycles. After
stirring for 24 h at 60 °C, the reaction was terminated by exposing
system to air. The polymer was precipitated from an excess
mixture of ether and petroleum ether (v/v = 1/3). After drying over-
night in a vacuum oven at room temperature, the light red product
was obtained with a yield of 17.21%. The monomer conversion was
obtained from 1H NMR with 19.74%.
A
methanol solution (20 mL) of sodium acetate (360 mg,
4.39 mmol) and propanethiol (304 mg, 4.00 mmol) was dropwised
into a methanol solution (16 mL) of 2,3-dibromomaleimide (2.04 g,
8 mmol)over 1 h. On completion of addition, the solution was stir-
red at room temperature for another 3 h. After that, the solution
was concentrated and the crude product was purified by column
chromatography (SiO2, petroleum ether/ethyl acetate = 3/1) to pro-
duce the yellow solid product (520 mg) with a yield of 52%.
Fig. 1. The preparation of linear water soluble polymers by RAFT polymerization and the corresponding cyclic polymers by bromomaleimide-thiol substitution reaction.

S. Long et al. / Reactive & Functional Polymers 80 (2014) 15–20
17
Fig. 2. Synthesis of functional RAFT agent 1.
2.7. Preparation of cyclic PNIPAM
After dissolving linear PNIPAM (20 mg) in the mixture of water
and methanol (v/v = 1/1, 100 mL) 1 h, NaBH4 (80 mg, 2 mmol) was
added and the solution was kept stirring for another 24 h at room
temperature. Subsequently, the reaction solution was concentrated
by vacuum evaporation to produce the cyclic PNIPAM.
2.8. Characterization
1H NMR spectra were recorded on a Bruker DMX400 spectrom-
eter at room temperature. Matrix-assisted laser desorption and
ionization time-of-flight (MALDI-TOF) mass spectrometry was per-
formed on a BrukerBiflex III spectrometer equipped with a 337 nm
nitrogen laser. Gel permeation chromatography (GPC) was per-
formed with a Waters 515 pump, a Waters 2414 refractiveindex
detector, and a combination of Styragel columns HT2, HT3, HT4,
and HT5, the effective molecular weight range being 100–10 k,
500–30 k, 5 k–600 k, and 50 k–4000 k. Analysis was carried out in
THF running with a flow rate of 1.0 mL/min at 35 °C. Polystyrene
standards were used for the calibration.
3. Results and discussion
Functional RAFT agent 1 was synthesized by an efficient mits-
unobu coupling reaction between 4-cyano-4-((thiobenzoyl)sulfa-
nyl)pentanol (a) and 2-dibromo-3-propylsulfanyl-maleimide (b)
(Fig. 2). The corresponding 1H NMR and peak assignments were
shown in Fig. 3A.
The agent 1 was then used to produce water soluble polymers
(the first step in Fig. 1). The preparation of poly(N,N-dimethyl-
acrylamide) (PDMAM) was firstly selected to demonstrated this
concept. Table 1 shows the RAFT polymerization condition of
PDMAM, in which the bulk polymerization of DMAM was per-
formed at 60 °C and the DMAM/RAFT Agent/AIBN ratio was used
as 100/1/0.2. After 5.5 h polymerization, a monomer conversion
of 23.94% was obtained from 1H NMR characterization of crude
polymerization solution. The GPC curve is shown in Fig. 4A (black
curve), in which a well-defined, monomodal, and symmetric peak
was observed with Mn and Mw/Mn of 3360 and 1.06, respectively.
Fig. 3B shows the 1H NMR spectrum of purified PDMAM, where
the three group peaks between 7.3 and 8.0 were inherent from
the RAFT agent 1, ascribing to the protons from the end benzene
ring. This indicates the success of RAFT polymerization and the
dithiobenzoate group was kept at the end of PDMAM polymer
chain.
The cyclic PDMAM was subsequently prepared by a bromoma-
leimide-thiol substitution reaction to close the linear precursor in a
highly diluted solution (the second step in Fig. 1). When dissolving
20 mg linear PDMAM in 100 mL water (the corresponding molar
concentration is around 6 ⁄ 10ꢀ5 M), an excess of reducing agent
NaBH4 (80 mg) was used to cut the dithiobenzoate group and re-
lease the –SH at the end of polymer chain. The bromomaleimide-
thiol substitution reaction was then carried out in situ to close
the linear PDMAM and produce the cyclic polymers.
Fig. 3. 1H NMR spectra of functional RAFT agent 1 (A), PDMAM (B), and PNIPAM (C)
in CDCl3.
The GPC and MALDI-TOF MS were used to verify the successful
cyclization. As shown in Fig. 4A and B, after the cyclization (red

18
S. Long et al. / Reactive & Functional Polymers 80 (2014) 15–20
Table 1
Table 2
Synthesis and characterization of linear PDMAM and PNIPAM.
GPC characterization of linear and cyclic PDMAM and PNIPAM.
d
d
a
a
Run
Monomera
Feed ratiob
Conv. (%)c
Mn
Mw/Mn
Run
Polymer
Mn
Mw/Mn
1
2
DMAM
NIPAM
100:1:0.2
100:1:0.2
23.94
19.74
3360
2290
1.06
1.07
1
2
3
4
Linear PDMAM
Cyclic PDMAM
Linear PNIPAM
Cyclic PNIPAM
3360
2420
2290
1900
1.06
1.06
1.07
1.06
a
b
c
RAFT polymerization was performed at 60 °C.
Initial molar ratio of monomer/RAFT Agent/AIBN.
Calculated from the 1H NMR spectrum.
a
THF was used as the eluent, and polystyrene standards were used for the
d
Calculated from GPC, in which THF were used as the eluent and polystyrene
calibration.
standards were used for the calibration.
GPC curve (red) shifted to lower molecular weight direction com-
pletely for the PDMAM with Mn of 6410 (Fig. S1A) (Table S2, Run
1 and 2). For the PDMAM with Mn of 12190 (Fig. S1B) (Table S2,
Run 3 and 4), however, only lower molecular weight part curve
shifted but the higher molecular weight part kept constant. These
results indicate that bromomaleimide-thiol substitution reaction
could be used to successfully ring-close the linear polymers with
molecular weightlower than around 10,000.
curve), the well-defined monomodal and symmetric peak shape
was preserved but the whole peak position shifted to lower molec-
ular weight direction completely compared to the linear precursor
(black curve). Table 2 shows the GPC results of cyclic and linear
PDMAM, where the same Mw/Mn and a Mn ratio of 0.72 between
cyclic and linear PDMAM indicates the success of the ring-closure
process from bromomaleimide-thiol substitution reaction [6,7,9].
To characterize the purity of the resultant cyclic topology, a log-
normal distribution model [16,29,30] was used to simulate the
molecular weight distributions of cyclic and linear PDMAM. As
shown in Fig. 4B, the fitting curves (dash) from the log-normal dis-
tribution model excellently matched with the measured GPC
curves, which demonstrated the high monocyclic topology purity
of the resultant cyclic polymers.
Fig. 5 shows the MALDI-TOF mass spectra of linear PDMAM (A)
and the cyclic counterparts (B). From the full spectra (left), the
absolute Mn of 3750 and 3360 were obtained for the linear and cyc-
lic PDMAM respectively. Comparing to the much smaller (0.72
times) apparent Mn of cyclic PDMAM than that of linear precursor
from GPC (Table 2), the similar absolute Mn indicated a more com-
pact molecular structure for cyclic PDMAM. This again confirmed
the successful formation of cyclic PDMAM. In addition, the detailed
descriptions of the MALDI-TOF mass spectra were shown in Fig. 5
(right). In the expanded spectrum of linear PDMAM (A), the
To explore the universality of this novel ring-closure technique,
the preparation of cyclic PNIPAM was used as another example. By
virtue of the agent 1, RAFT polymerization of NIPAM was carried
out in DMF solution at 60 °C (Table 1). A monomer conversion of
19.74% was obtained after 24 h reaction. Fig. 6A shows the GPC
curve (black) of the resultant PNIPAM, where a monomodal and
symmetric peak was obtained with Mn = 2290 and Mw/Mn = 1.07.
From 1H NMR spectrum of purified PNIPAM (Fig. 3C), the proton
signals from end benzene ring were clearly observed at 7.3–
8.0 ppm, inherited from the RAFT agent 1 (Fig. 3A). This indicated
that the well-defined linear PNIPAM was prepared by RAFT poly-
merization using agent 1. The 20 mg linear precursor was then dis-
solved into 100 mL mixed solvents of water and methanol (v/v = 1/
1), where the methanol was used to improve the PNIPAM solubility
in water. After cleaving the dithiobenzoate group by NaBH4, the
cyclic PNIPAM was obtained in situ by virtue of bromomalei-
mide-thiol substitution reaction. As shown in Fig. 6A and B, the
GPC curve of cyclic PNIPAM (red) inherited the monomodal and
symmetrical peak shape from its linear precursor (black), but the
peak position shifted to lower molecular weight direction com-
pletely. A Mn ratio of 0.83 was obtained between cyclic and linear
PNIPAM (Table 2), which is quite consistent with the value re-
ported in the literature [14,19]. The log-normal distribution model
was further used to investigate the topological purity of resultant
cyclic PNIPAM. As shown in Fig. 6B, the simulation curves accu-
rately fit the experimentally measured GPC curves, demonstrating
a high monocyclic topology purity of the resultant cyclic polymers.
These strongly supported the success the bromomaleimide-thiol
substitution ring-closure technique for the preparation of cyclic
PNIPAM.
marked peak at m/z = 3792.2 was ascribed to the linear PNIPAM33
/
Na+ (cal. = 3791.8). The corresponding cyclic counterpart was ob-
served in the expanded spectrum of cyclic PDMAM (B) with a
cleavage of propylthio group. As shown in Fig. 5B (right), the
marked peak at m/z = 3532.1 was assigned to cyclic PDMAM33-pro-
pylthio/K+ (cal. = 3531.7). Furthermore, a regular m/z interval of
99.3 was observed between the major peaks in both cases, which
corresponds to the molar mass of the DMAM monomer unit.
In addition, the molecular weight effect was explored on the
ring-closure efficiency of bromomaleimide-thiol substitution reac-
tion. To demonstrate this, the linear PDMAMs were synthesized
with different Mn of 6410 and 12190 (Table S1). Their correspond-
ing GPC curves show in Fig. S1 (black). After the cyclization, the
1.0
A
4
B
L PDMAM
C PDMAM
L PDMAM
C PDMAM
Simulation
0.8
3
0.6
0.4
0.2
0.0
2
1
0
32
33
34
35
36
37
38
39
3.2
3.4
3.6
3.8
Time (min)
LogMW
Fig. 4. (A) The GPC data (normalized RI intensity vs. time) for linear PDMAM (black) and cyclic PDMAM (red). (B) The GPC data (dwt/dlog (MW) vs. logMW) for linear PDMAM
(black), cyclic PDMAM (red), and the corresponding log-normal distribution simulation (blue dash). The arrow indicates the low molecular weight integration limit of the
used GPC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

S. Long et al. / Reactive & Functional Polymers 80 (2014) 15–20
19
Fig. 5. MALDI-TOF mass spectra for linear PDMAM precursor and the corresponding cyclic PDMAM.
4
1.0
0.8
0.6
0.4
0.2
0.0
A
B
L PNIPAM
C PNIPAM
L PNIPAM
C PNIPAM
Simulation
3
2
1
0
3.2
3.4
3.6
3.8
34
35
36
37
38
39
Time (min)
LogMW
Fig. 6. (A) The GPC data (normalized RI intensity vs. time) for linear PNIPAM (black) and cyclic PDMAM (red). (B) The GPC data (dwt/dlog (MW) vs. logMW) for linear PNIPAM
(black), cyclic PNIPAM (red), and the corresponding log-normal distribution simulation (blue dash). The arrow indicates the lowmolecular weight integration limit of the used
GPC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Conclusion
Acknowledgments
We developed a unique ring-closure method for the prepara-
tion of water soluble cyclic polymers in this paper. As the first
step, the functional RAFT agent 1 was designed and used to pro-
duce well-defined water soluble linear polymers, such as
PDMAM and PNIPAM, by a standard RAFT polymerization tech-
nique. At the second step, when dissolving the linear precursors
in the environmentally benign solvent water with a high dilu-
tion, the reducing agent NaBH4 was used to cleave the dith-
iobenzoate group and release the thiol group at the end of
polymer chain. The well-defined water soluble cyclic polymers
were then obtained by an in situ ring-closure process from the
efficient bromomaleimide-thiol substitution reaction. Since the
linear polymer precursors were produced by the standard RAFT
polymerization and the subsequent ring-closure process was car-
ried out at an extremely mild reaction condition, we expect that
this novel technique could become one of the basic tools for pro-
ducing various well-defined water soluble cyclic polymers. In
addition, inspired by the success for preparing monocyclic poly-
mers, the preparation of varied water soluble cyclic polymer
derivatives, such as tadpole and eight shape topological poly-
mers, are exploring in our group based on the same chemistry.
Generous support was primarily provided by Ministry of
Science and Technology of China (MOST, 2014CB932200) and Na-
tional Science Foundation of China (21374122 and 21090353). K.
Z. thanks the Bairen project from The Chinese Academy of Sciences
for support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.reactfunctp-
olym.2013. 11.006.
References
[1] K. Endo, Adv. Polym. Sci. 217 (2008) 121–183.
[2] B.A. Laurent, S.M. Grayson, Chem. Soc. Rev. 38 (2009) 2202–2213.
[3] H.R. Kricheldorf, J. Polym. Sci. Part A: Polym. Chem. 48 (2010) 251–284.
[4] T. Yamamoto, Y. Tezuka, Polym. Chem. 2 (2011) 1930–1941.
[5] J.N. Hoskins, S.M. Grayson, Polym. Chem. 2 (2011) 289–299.
[6] Z.F. Jia, M.J. Monteiro, J. Polym. Sci. Part A: Polym. Chem. 50 (2012) 2085–2097.
[7] C.W. Bielawski, D. Benitez, R.H. Grubbs, Science 297 (2002) 2041–2044.
[8] Y. Xia, A.J. Boydston, Y.F. Yao, J.A. Kornfield, I.A. Gorodetskaya, H.W. Spiess, R.H.
Grubbs, J. Am. Chem. Soc. 131 (2009) 2670–2677.

20
S. Long et al. / Reactive & Functional Polymers 80 (2014) 15–20
[9] K. Zhang, M.A. Lackey, J. Cui, G.N. Tew, J. Am. Chem. Soc. 133 (2011) 4140–
[22] F.F. Schumacher, M. Nobles, C.P. Ryan, M.E.B. Smith, A. Tinker, S. Caddick, J.R.
Baker, Bioconjugate Chem. 22 (2011) 132–136.
4148.
[10] K. Zhang, M.A. Lackey, Y. Wu, G.N. Tew, J. Am. Chem. Soc. 133 (2011) 6906–
6909.
[23] M.W. Jones, R.A. Strickland, F.F. Schumacher, S. Caddick, J.R. Baker, M.I. Gibson,
D.M. Haddleton, J. Am. Chem. Soc. 134 (2012) 1847–1852.
[24] M.P. Robin, M.W. Jones, D.M. Haddleton, R.K. O’Reilly, Acs Macro Lett. 1 (2012)
222–226.
[25] Y. Cui, Y.C. Yan, Y.M. Chen, Z.H. Wang, Macromol. Chem. Phys. 214 (2013) 470–
477.
[11] K. Zhang, G.N. Tew, Acs Macro Lett. 1 (2012) 574–579.
[12] B.A. Laurent, S.M. Grayson, J. Am. Chem. Soc. 128 (2006) 4238–4239.
[13] J.N. Hoskins, S.M. Grayson, Macromolecules 42 (2009) 6406–6413.
[14] X.P. Qiu, F. Tanaka, F.M. Winnik, Macromolecules 40 (2007) 7069–7071.
[15] N. Sugai, H. Heguri, K. Ohta, Q.Y. Meng, T. Yamamoto, Y. Tezuka, J. Am. Chem.
Soc. 132 (2010) 14790–14802.
[26] C. Marminon, A. Pierre, B. Pfeiffer, V. Perez, S. Leeonce, P. Renard, M.
Prudhomme, Med. Chem. 11 (2003) 679–687.
[16] D.E. Lonsdale, M.J. Monteiro, J. Polym. Sci. Part A: Polym. Chem. 49 (2011)
4603–4612.
[27] R. Vanel, F. Berthiol, B. Bessieres, C. Einhorn, J. Einhorn. Synlett. (2011) 1293–
1295.
[17] Y. Tezuka, Polym. J. 44 (2012) 1159–1169.
[28] S.H. Thang, Y.K. Chong, R.T.A. Mayadunne, G. Moad, E. Rizzardo, Tetrahedron
Lett. 40 (1999) 2435–2438.
[29] S.E. Cabaniss, Q.H. Zhou, P.A. Maurice, Y.P. Chin, G.R. Aiken, Environ. Sci.
Technol. 34 (2000) 1103–1109.
[18] F. Hatakeyama, T. Yamamoto, Y. Tezuka, Acs Macro Lett. 2 (2013) 427–431.
[19] J. Xu, J. Ye, S.Y. Liu, Macromolecules 40 (2007) 9103–9110.
[20] L.M. Tedaldi, M.E.B. Smith, R.I. Nathani, J.R. Baker, Chem. Commun. (2009)
6583–6585.
[30] C.A. Bell, Z.F. Jia, J. Kulis, M.J. Monteiro, Macromolecules 44 (2011) 4814–4827.
[21] M.E.B. Smith, F.F. Schumacher, C.P. Ryan, L.M. Tedaldi, D. Papaioannou, G.
Waksman, S. Caddick, J.R. Baker, J. Am. Chem. Soc. 132 (2010) 1960–1965.