Cyclopolymerization of α,ω-heterodifunctional monomers containing styrene and maleimide moieties

Article abstract of DOI:10.1002/pola.27004

A series of α,ω-heterodifunctional monomers with styrene (St) and maleimide moieties bridged by a varied length of oligo-ethylene glycol (OEG) linkers were synthesized. Cyclopolymerizations of these monomers through reversible addition-fragmentation chain transfer-mediated alternating radical copolymerization between intramolecular St and maleimide moieties were investigated. For the monomers with three or more ethylene glycol (EG) units, their cyclopolymerizations can be realized properly in low monomer feeding concentrations, affording well-defined cyclopolymers with crown ether encircled in their main chains. Importantly, the cyclopolymerizations of monomers with six or seven EG units in the presence of KPF₆ could be enhanced by the supramolecular effects between the OEG linkers and the potassium metal ion. Thus, the monomer feeding concentration could be largely improved, which may benefit preparation of the cyclopolymers with high degrees of copolymerization. 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 330-338 The cyclopolymerizations of α,ω-heterodifunctional monomers with styrene and maleimide moieties bridged by oligo-ethylene glycol linkers can be realized in low monomer feeding concentrations. The cyclopolymerizations of monomers with six or seven ethylene glycol units in high concentrations could be enhanced by the supramolecular effects between the linkers and the potassium metal ion, both of which afford well-defined cyclopolymers with crown ether encircled in their main chains. Copyright

Full text of DOI:10.1002/pola.27004

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Cyclopolymerization of a,x-Heterodifunctional Monomers
Containing Styrene and Maleimide Moieties
Lei Zou,1 Jian’an Liu,2 Ke Zhang,1 Yongming Chen,1 Fu Xi1
1Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, China
2Beijing Mass spectrometry Center, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, China
Correspondence to: Y. Chen (E-mail: ymchen@iccas.ac.cn)
Received 1 August 2013; accepted 28 October 2013; published online 15 November 2013
DOI: 10.1002/pola.27004
ABSTRACT: A series of a,x-heterodifunctional monomers with
styrene (St) and maleimide moieties bridged by a varied length
of oligo-ethylene glycol (OEG) linkers were synthesized. Cyclo-
polymerizations of these monomers through reversible addi-
tion–fragmentation chain transfer-mediated alternating radical
copolymerization between intramolecular St and maleimide
moieties were investigated. For the monomers with three or
more ethylene glycol (EG) units, their cyclopolymerizations can
be realized properly in low monomer feeding concentrations,
affording well-defined cyclopolymers with crown ether
encircled in their main chains. Importantly, the cyclopolymeri-
zations of monomers with six or seven EG units in the pres-
ence of KPF6 could be enhanced by the supramolecular effects
between the OEG linkers and the potassium metal ion. Thus,
the monomer feeding concentration could be largely improved,
which may benefit preparation of the cyclopolymers with high
C
V
degrees of copolymerization.
2013 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2014, 52, 330–338
KEYWORDS: cyclopolymerization; host-guest systems; reversible
addition-fragmentation chain transfer polymerization (RAFT)
of cyclization by bulky substituents were reported by
Holmes and coworkers12,13 and Prata and coworkers.14 In
these systems, two styrene (St) moieties were covalently
linked to a directing template possessing a high degree of
steric hindrance, which would help keeping the two poly-
merizable moieties in close proximity and orienting them
during propagation. Pasini and coworkers reported the
chemically more straightforward scaffolds.15 They simply
covalently linked two 4-styryle substituted moieties to the
1,3-diol moiety of pentaerythritol skeletons and afforded
structurally stable cyclopolymers with a high degree of cycli-
zation and good degrees of polymerization (DP). Recently,
they reported an easily obtained new styryl difunctional mal-
onate, which was suitably designed for efficient cyclopolyme-
rization.16 Liu and coworkers designed diacrylate monomers
bridged with an 1,2,3-triazole unit. Because the conjugated
structure of 1,2,3-triazole fixed two acryloyloxy groups in a
plane favorable for ring-closing, the formation of larger rings
with high cyclization efficiency was succeeded.17 Endo and
coworkers described the synthesis of polyacryamide contain-
ing 10- or 11-membered rings as repeating cyclic units by
cyclopolymerization of an optically active bisacrylamide
derived from a-pinene, which was benefited from the com-
pulsive conformation arrangement of the two acrylamide
groups in the monomeric structure.18 Recently, they realized
INTRODUCTION The polymerization of a,x-difunctional
monomers generally affords branched or even crosslinked
structures, resulting from random chain propagation process
between intramolecular and intermolecular polymerizable
groups. The initial report of Butler and coworkers during the
free-radical polymerization of diallyl quaternary ammonium
salts which resulted water soluble linear polymers rather
than crosslinked quaternary ammonium polymers opened
the field of research which is now referred as cyclopolymeri-
zation.1,2 During the process of cyclopolymerization of
difunctional monomers, intramolecular and intermolecular
chain propagations proceed alternatively, affording well-
defined polymers with cyclic moieties along the backbones.
Through cyclopolymerization, wide varieties of difunctional
monomers have been converted to unique cyclopolymers.
Significantly, cyclopolymerization of carefully designed multi-
functional monomers has become a very effective method for
the synthesis of sequence-regulated polymers.3–5
At the very beginning, the majority of researches were con-
fined in the cyclopolymerization of monomers typically pro-
ceeded via formation of thermodynamically stable five- or
six-membered rings before or during the period of polymer-
ization process.6–11 Then, cyclopolymerizations proceeded
via various mechanisms were discovered. The enhancements
Additional Supporting Information may be found in the online version of this article.
C
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SCHEME 1 Cyclopolymerization of styrene-maleimide containing a,x-heterodifunctional monomers.
the controlled radical cyclopolymerization of bismethacrylate
monomer through quantitative 19-membered ring forma-
tion.19 They attributed the monomer conformations favorable
for ring-closing to the cyclohexane ring and the urethane
groups straining with steric direction and hydrogen bonds.
Very recently, Zhu et al. reported ruthenium-catalyzed cas-
cade metathesis cyclopolymerization of bisnorbornenes with
flexible linkers.20 They presumed that intramolecular p–p
interactions between the pending aryl groups of the mono-
mer may take place to direct the ring closure process. There
were also many reports on the cyclocopolymerization of flex-
ible divinyl ether monomers with maleic anhydride (MAn)
based on that vinyl ether and MAn were prone to polymer-
ization following an alternating fashion.2
cyclopolymerization of monomers with longer OEG linkers
was benefited from the pseudo-host–guest interaction
between the OEG linkers and metal ion in higher concentra-
tions. The supramolecular effect induced the conformation
change of the monomer and thus pushed the St and malei-
mide moieties close to enhance the intermolecular propaga-
tion. Through this strategy, well-defined polymers with crown
ether encircled in their main chains could be prepared with
convenience. This is of great importance to the researches of
crown ether containing polymers which are very useful func-
tional materials or sensors.31–37 For instance, in early days,
Kakuchi and coworkers had developed a series of cyclopoly-
merization strategies of difunctional monomers bearing an
OEG chain to produce polymers with crown cavities for inves-
tigating the relevant metal ions binding properties.38–40 How-
ever, they did not employ or design any other effects, which
would enhance the cyclopolymerization process and therefore
failed to prepare the required well-defined polymers with effi-
ciency and convenience.
It is well known that radical copolymerizations of MAn and St
or their derivatives tend to copolymerize alternatively.21–23
This group has applied this chemistry to build novel poly-
meric architectures.24–29 Very recently, Jia et al. reported the
cyclocopolymeriztion of MAn and distyryl flexible monomers
in high dilute concentration and the cyclopolymers with
crown ether side units were obtained owing to preferential
alternating copolymerization effect between St and MAn moi-
eties.30 However, the concentration was too low and the syn-
thesis of polymers with high DP was difficult.
EXPERIMENTAL
Materials
Diethylene glycol (99%, J&K), triethylene glycol (99%, Alfa
Aesar), 4-vinylbenzyl chloride (90%, Acros), maleimide
(98%, J&K), KPF6 (99%, Acros), and PEG (Mn 5 300, Acros)
were used as received without further purification. Other
reagents were commercialized chemicals and used as
received. All solvents were analytical grade and purified
through appropriate method. Chain transfer reagent cumyl
dithiobenzoate (CDB) was synthesized according to ref. 41.
SEG, S2EG, and S3EG were synthesis according to ref. 42.
In this study, we investigated the polymerization of a,x-heter-
odifunctional monomers of which St and maleimide moieties
were covalently bridged by a series of OEG linkers (Scheme 1).
In one aspect, when the monomers have enough length of
OEG linkers, which do not hamper the folding of monomer
configuration during the polymerization process for the intra-
molecular interaction of St and maleimide moieties, cyclopoly-
merization of the monomers underwent smoothly due to the
alternating copolymerization tendency between St and malei-
mide moieties in low concentrations. More interestingly, the
Measurements and Instruments
1H and 13C NMR spectra were obtained on a Bruker 400
MHz instrument. High-resolution mass spectrometry (HRMS)
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was carried out using a Waters micromass GCT mass spec-
trometer. Size exclusive chromatography system equipped
with a Waters 515 HPLC pump, three Waters Styragel col-
umns (HT2, HT3, and HT4), a Rheodyne 7725i sampler, and
a Waters 2414 refractive-index detector was performed to
determine the average molecular weights and molar-mass
dispersities of all the samples. Polystyrene standards were
used to calibrate the SEC system and THF was used as the
eluent at a flow rate of 1 mL/min at 35 ꢀC. Matrix-assisted
laser desorption ionization time-of-flight mass spectrometry
was performed on a Bruker Biflex III spectrometer equipped
with a 337 nm nitrogen laser. 4-Hydroxy-cyanocinnamic acid
was used as the matrix. The mass spectra were acquired in
linear mode or positive reflector mode at an acceleration
voltage of 119 kV. External mass calibration was performed
using a standard peptide mixture.
was purified by column chromatography (petrol ether/ethyl
acetate, 2/1 (v/v)) as a viscous liquid (3.6 g, yield 87%).
1H NMR (400 MHz, CDCl3, d (ppm)) 2.82 (s, 2H), 3.61–3.70
(m, 12H), 4.54 (s, 2H), 5.23 (d, 1H, J 5 9 Hz), 5.24 (s, 2H),
5.74 (d, 1H, J 5 17.6 Hz), 6.47 (s, 2H), 6.71 (q, 1H, J1 5 9
Hz, J2 5 17.6 Hz), 7.29 (d, 2H, J 5 8.5 Hz), 7.38 (d, 2H, J 5
8.5 Hz).
Synthesis of SnEGM
A general procedure was given with S3EGM as an example.
S3EGMF (4.1 g, 0.01 mol) was dissolved in 100 mL of tolu-
ene and 10% 2,6-di-tꢀert-butylphenol was added. The mixture
was refluxed at 110 C for 12 h. Then, toluene was removed
under vacuum and the residue was subjected to column
chromatography (petrol ether/ethyl acetate, 2/1 (v/v)). The
product S3EGM was obtained as a viscous liquid (3.4 g, yield
99%).
Synthesis of FM
Maleimide (2.0 g, 0.02 mol) was suspended in 50 mL of tolu-
ene and furan (3.0 mLꢀ, 0.044 mol) was added. The mixture
was then stirred at 90 C for 12 h. After cooled to room tem-
perature, the product was obtained by filtration and washed
with small mounts of toluene several times as a white pow-
der (3.1 g, yield 95%).
1H NMR (400 MHz, CDCl3, d (ppm)) 3.59–3.74 (m, 12H),
4.54 (s, 2H), 5.23 (s, 1H, J 5 10 Hz), 5.74 (d, 1H, J 5 17.6
Hz), 6.66 (s, 2H), 6.70 (q, 1H, J1 5 10 Hz, J2 5 17.6 Hz),
7.29 (d, 2H, J 5 8.5 Hz), 7.38 (d, 2H, J 5 8.5 Hz). 13C NMR
(400 MHz, CDCl3, d (ppm)) 37.27, 67.95, 69.58, 70.22, 70.74,
70.81, 73.03, 113.90, 126.35, 128.07, 134.26, 136.71, 137.06,
138.12, 170.77. HRMS (EI) m/z calculated for C19H23NO5
[M1] 345.1576, found 345.1580.
1H NMR (400 MHz, DMSO-d6, d (ppm)) 2.83 (s, 2H), 5.10 (s,
2H), 6.51 (s, 2H), 11.13 (bs, H).
Polymerization
Synthesis of SnEGT
A general procedure was given by taking the polymerization
of S6EGM in higher concentration in the presence of KPF6 as
an example. S6EGM (0.24 g, 0.5 mmol), CDB (9.3 mg, 0.034
mmol), AIBN (1.2 mg, 0.007 mmol), CH3CN (5 mL), and
KPF6 (0.18 g, 1.0 mmol) were charged into a polymerization
tube. The tube was sealed after degassed by three freeze-
pump-thaw cycles and then was immersed in a 60 ꢀC oil
bath for 10 h. After exposing to air, CH3CN was removed
under vacuum and the residue was re-dissolved in 20 mL of
CH2Cl2. Then, the CH2Cl2 solution was wash with water for
three times and dried over MgSO4. After concentrated to
suitable volume, the solution was precipitated in large
amount of diethyl ether for three times. The precipitates
were centrifuged and dried under vacuum. The polymer was
obtained as pink glass.
A general procedure was given with S3EGT as an example.
To a 100-mL round-bottomed flask containing a solution of
S3EG (2.66 g, 0.01 mol) and Et3N (1.51 g, 0.015 mol) in 30
mL of THF, a solution of tosyl chloride (2.85 g, 0.015 mol) in
20 mL of THF was slowly added over 30 min with stirring.
The mixture was stirred at room temperature for 24 h. Then,
H2O (1.0 mL) was added and the mixture was stirred for
another 4 h. The mixture was diluted with 200 mL of water,
and extracted with 100 mL of dichloromethane three times.
The dichloromethane extracts were combined and washed
with 100 mL of brine for three times, then dried over
MgSO4, and concentrated. The product was purified by col-
umn chromatography [petrol ether/ethyl acetate, 4/1 (v/v)]
as a viscous liquid (3.8 g, yield 91%).
1H NMR (400 MHz, CDCl3, d (ppm)) 2.43 (s, 3H), 3.60–3.70
(m, 10H), 4.15 (t, 2H, J 5 4.7 Hz), 4.54 (s, 2H), 5.23 (d, 1H, J
5 9 Hz), 5.74 (d, 1H, J 5 17.6 Hz), 6.71 (q, 1H, J1 5 9 Hz, J2
5 17.6 Hz), 7.28–7.33 (m, 4H), 7.38 (d, 2H, J 5 8.5 Hz), 7.79
(d, 2H, J 5 8.5 Hz).
RESULTS AND DISCUSSION
Synthesis of the Monomers
In order to investigate intramolecular cyclopolymerization of
monomers bearing a,x-terminated St and maleimide, we syn-
thesized a series of a,x-heterofunctional monomers bridged
with different number of EG units. For the monomers with
n 5 1, 2, 3, they were synthesized straight forward as shown
in Scheme 2. 4-Vinylbenzyl chloride reacted with one termi-
nal of ethylene glycol, diethylene glycol, and triethylene gly-
col, respectively by Williams etherification reaction and the
obtained SnEG (n 5 1, 2, 3) reacted with tosyl chloride to
activate another terminal. Then furan-protected maleimide
(FM) was introduced by substitution reaction to SnEGT in
Synthesis of SnEGMF
A general procedure was given with S3EGMF as an example.
A mixture of the S3EGT (4.2 g, 0.01 mol), FM (1.7 g, 0.01
mol), K2CO3 (6.9 g, 0.05 mol), and 5% 2,6-di-tert-butylphe-
nol in 15 mL of DMF was stirred at 50 ꢀC for 12 h. Then,
the mixture was diluted with 200 mL of water, and extracted
with 100 mL of CH2Cl2 for three times. The CH2Cl2 extracts
were combined, washed with 100 mL of brine for three
times, then dried over MgSO4, and concentrated. The product
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SCHEME 2 Synthetic route of a,x-heterodifunctional monomers of styrene and maleimide bridged with different number of ethyl-
ene oxide units.
high yield. The final products were obtained by refluxing to
remove the furan unit quantitatively. For the monomers of
n 5 6 and 7, they were prepared by the same procedure
from commercially available PEG (Mn 5 300) which contains
six to nine ethylene glycol units. The samples were fractio-
nated by column chromatography at SnEGT step to give the
S6EGT and S7EGT. The structures of these monomers were
traces at higher concentration were very broad (Entries 3
and 4 in Table 1), demonstrating that the polymerization in
high concentration lost control.
The polymer obtained in 0.025 M (Entry 2 in Table 1) was
further characterized by NMR spectrometry. In Figure 2(A),
the proton resonances appeared as very broad peaks. The
signal between 6.5–7.8 ppm was attributed to aromatic
rings, the signal at 4.5 ppm to benzylic CH2, the signal at 3.7
ppm to the OEG linker and signal at 8 ppm to the moieties
of RAFT reagent. The integration ratio of b/c was 1/2, con-
sistent with the polymer structure from S3EGM. The broad-
ening of the resonance signals implied some extent of
conformation rigidity of the polymer chain. Importantly,
there were hardly any resonances between 5 and 6.5 ppm,
which were assigned to double bonds, demonstrating that
the vinyl groups were most likely consumed. This precluded
the liner structure in Chart 1(C). In the 13C NMR spectrum
[Fig. 2(B)], the signal of the aromatic “next to polymer chain”
carbon appeared at 136–139 ppm. This was perfectly con-
sistent with an alternative sequence of St-maleimide
copolymer.18
1
identified by H NMR, 13C NMR and HRMS as given in Exper-
imental section.
Cyclopolymerization of Monomers with Short OEG
Linkers
Reversible addition–fragmentation chain-transfer (RAFT)-
mediated radical polymerizations of the monomers with rela-
tively short OEG linkers were first conducted to investigate
whether their cyclopolymerizations could be realized by intra-
molecular interaction of St and maleimide moieties. It is
expected that the concentration of monomers is the most
critical factor to give the intramolecular cyclopolymer
[Chart 1(A)]. In a higher concentration, intermolecular copoly-
merization should be easily to occur and the products would
have branches [Chart 1(B)] and pendent unreacted vinyl units
[Chart 1(C)] and thus they would be ill-controlled.
Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrum of the polymer obtained in
0.025 M (Entry 2 in Table 1) was also obtained for further
characterization of the product structure. As demonstrated
in Figure 3(A), two series of peaks were observed. The inter-
val of both series was about 345, which was the mass of
monomer S3EGM. The peak series with higher mass (marked
with *) can be assigned to be mS3EGM 3 DP 1 mCDB 1 mNa1
5 345.2 3 DP 1 272 1 23 that was consistent with the
macromolecules with S3EGM repeating units being capped
with one Ph(C5S)S and one cumyl moieties. Another series
(marked with !) can be assigned to be mS3EGM 3 DP 1 119
1 mNa1 5 345.2 3 DP 1 119 1 23. The mass was about
153 smaller than the former, which was the mass of the CDB
moiety of Ph(C5S)S. These peaks could be the chains gener-
ated in the ionization process of the former series by the
With this consideration, the feed ratio of monomer/CDB/
AIBN was kept at 15/1/0.2 and the monomer feeding con-
centration was varied to optimize the condition (Table 1).
The polymerizations were conducted at 60 C in 1,4-dioxane
and all entries gave soluble polymers.
ꢀ
For the polymerizations of S3EGM, the feeding concentra-
tions were 0.0125, 0.025, 0.061, and 0.10 M. As shown in
Table 1 (Entries 1–4), the Mw/Mn increased with increase of
feeding concentration and the Mw/Mns from the lower con-
centrations, 0.0125 and 0.025 M, were 1.25 and 1.31, imply-
ing that polymerizations could be controlled. By size
exclusive chromatography (SEC) traces as shown in Figure
1(A), the eluent peaks of the Entries 1 and 2 in Table 1,
were relatively narrow and symmetric. As contrast, the
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CHART 1 Three of possible structures of the polymerization of S3EGM: (A) linear structure, (B) branched structure, and (C) struc-
ture with unconsumed double bonds.
fraction of Ph(C5S)S terminals. Therefore, both peak series
detected from the MALDI-TOF mass spectrum were attrib-
uted to the linear cyclopolymers [Chart 1(A)].
ducted. From the SEC curves of the resultant polymers [Fig.
1(B)] in different concentrations, we can see that the cyclo-
polymerization behavior of S2EGM was similar to that of
S3EGM except it needs a lower concentration to give a well-
controlled cyclopolymerization. In similar feed concentration,
the polymerization of S3EGM (0.025 M, Entry 2 in Table 1)
was under control while that of S2EG (0.02 M, Entry 6 in
Table 1) gave much higher Mw/Mn of 1.80. With decrease of
the feeding concentrating to 0.01 M, the polymerization of
S2EG seemed to be controlled as shown by SEC traces [Fig.
1(B)] and gave the Mw/Mn of 1.34. However, minor branched
structure could be found from the MALDI-TOF mass spec-
trum of resultant polymer from the concentration of 0.01 M,
which was different from that of S3EGM (Supporting Infor-
mation Fig. S1). For the monomer with only one ethylene
glycol unit (SEGM), the cyclopolymerization at both concen-
trations, 0.01 and 0.02 M, gave very broad and multimodel
polydispersed SEC traces [Fig. 1(C)]. It demonstrated that
well-controlled cyclopolymerization of SEGM could not be
realized by simply reducing the feeding concentration.
In the MALDI-TOF mass spectrum of the polymer obtained in
0.1 M [Entry 4 in Table 1, Fig. 3(B)], the major peaks were
still consistent with the linear structure capped with one
Ph(C5S)S and one cumyl moieties. However, minor peaks
(marked with ᭜) could be found at higher mass region.
They can be figured out as mS3EGM 3 DP 1 mCDB 3 21
mNa1 5 345.2 3 DP 1 272 3 2 1 23. Namely, these peaks
indicated the structure capped with two CDB moieties, a
branched structure as shown in Chart 1(B).
By the evidences from 1H NMR, 13C NMR spectrum, and
MALDI-TOF mass spectrum, we came to the conclusion that
the polymers obtained in low concentration should be a
strictly alternate linear cyclopolymer [Chart 1(A)]. Other kinds
of structures would be not consistent with either a structure
1
without double bonds as demonstrated by H NMR, a strictly
alternate structure as demonstrated by 13C NMR or a linear
structure as demonstrated by MALDI-TOF mass spectrum.
The phenomenon in polymerizations of these monomers can
be attributed to the influence of both feeding concentration
and length of linkers. In lower concentrations, there was
mainly intramolecular interaction of St and maleimide
Subsequently, polymerizations of the monomers with two
and one ethylene glycol linkers, S2EGM and SEGM, were con-
TABLE 1 Results of the Polymerization of S3EGM, S2EGM, and SEGM in 1,4-Dioxane
Monomer
Feed Ratio
Conversion
Entries
Monomer
Concentration (M)
(Monomer/CDB/AIBN)
(Weight) (%)
Mn
Mw/Mn(SEC)a
a
1
2
3
4
5
6
7
8
S3EGM
S3EGM
S3EGM
S3EGM
S2EGM
S2EGM
SEGM
0.0125
0.025
0.061
0.1
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
81
3,300
4,000
4,300
8,700
2,600
7,600
–
1.25
1.31
1.78
>3
56
67
43
0.01
63
1.34
1.80
–
0.02
63
0.01
ꢁ60b
SEGM
0.02
ꢁ60b
–
–
a
Determined by SEC.
b
There may be deviations in the data, because of difficulties in precipitations of polymers.
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opportunity of the intermolecular interaction between St and
maleimide units from different monomers increases, thus the
branched structure resulted.
Cyclopolymerization of Monomers with Longer OEG
Linkers
From the polymerizations of S3EGM, we can see that well-
defined cyclopolymers could be obtained by intramolecular
interaction of St and maleimide moieties of the monomers.
However, a well-controlled cyclopolymerization still occurred
at low monomer concentration, and it is not convenient for
preparation of the cyclopolymers with high DP. To improve
the monomer feeding concentration with controlled cyclopo-
lymerization, there should be additional effects to enhance
the cyclopolymerization. Those monomers with relatively
longer OEG linkers such as six EG units (S6EGM) and seven
EG units (S7EGM) could be regarded as pseudocrown ethers.
If host–guest interactions between crown ethers and metal
ion could also occur between such open chain structures
and metal ions, the configuration change of these monomers
FIGURE 1 SEC elution curves of the resultant polymers given
by RAFT-mediated radical copolymerization of (A) S3EGM, (B)
S2EGM, and (C) SEGM in different concentrations.
moieties of the monomers and, thus, intramolecular chain
propagation was dominant during the period of polymeriza-
tion process, providing that the OEG linkers were long and
soft enough for the intramonomeric folding to achieve linear
chain propagation. The shorter the linker was, more diffi-
culty for monomer folded for intramolecular propagation.
Therefore, lower concentration was necessarily for intramo-
lecular chain propagation against intermolecular chain prop-
agation. This was the reason for that S3EGM was likely to
intramolecular cyclcopolymerize than that of S2EG in the
similar concentration. However, when the linker was so
short, such as in SEGM, that the molecule cannot fold to
facilitate the intramolecular interaction of St and maleimide
moieties, cyclopolymerization became difficult even at highly
diluted concentrations. With increase of concentration, the
FIGURE 2 (A) 1H NMR (B) 13C NMR spectra in CDCl3 of result-
ant polymers of S3EGM in low concentration (0.025 M) (Entry
2 in Table 1).
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structure. The host–guest interactions would be observed by
an up- or downfield shift of the corresponding signals, which
is a common phenomenon in host–guest chemistry. From the
1H NMR spectra of S6EGM before and after the addition of
KPF6 (1 equiv), we can see the signals belong to oxyethylene
protons obviously divided from multiply peaks at 3.46–3.62
ppm to the signals at 3.40, 3.49, 3.53, and 3.60 ppm. This
clearly indicated the occurrence of host–guest interaction
between the monomer and the metal ion. Furthermore, the
signal shifts of the protons of terminal groups could also be
found. The protons of styryl moiety appeared at 5.24, 5.79,
7.30, 7.43, and 4.49 ppm and shifted to 5.25 (0.01 ppm
downfield shift), 5.80 (0.01 ppm downfield shift), 7.31 (0.01
ppm downfield shift), 7.45 (0.02 ppm downfield shift), and
4.52 ppm (0.03 ppm downfield shift) after the addition of
K1. And that of the maleimide moiety appeared at 6.75 ppm
and shifted to 6.77 ppm (0.02 ppm downfield shift). This
result indicated the relevant configuration change of the two
polymerizable functional groups of the monomer motivated
by the host–guest interaction between the pseudocrown
ether and the metal ion. The host–guest interaction between
the monomer and the metal ion seemed to be a 1/1 (mono-
mer/metal ion) stoichiometry since the chemical shifts
remained unchanged on adding more metal ion up to 2
equiv (Supporting Information Fig. S2).
We then carried out the RAFT copolymerization of S6EGM
(Table 2). There was no difference between the polymeriza-
tions of S6EGM to S3EGM in the absence of the metal ions.
In the lower concentrations such as 0.02 M, the polymeriza-
tions seem to be controlled (Entry 1 in Table 2, Fig. 5). In
the 1H NMR spectrum (Supporting Information Fig. S3) of
the resultant polymers, every signal appeared very broad.
Nearly no resonances between 5 and 6.5 ppm that were con-
tributed from the double bonds could be found. The signal
at 136–139 ppm in the 13C NMR spectrum indicated the
alternative sequence of the resultant polymer (Supporting
Information Fig. S4). In the MALDI-TOF mass spectrum (Sup-
porting Information Fig. S5), the distribution of the molecu-
lar ions was very symmetrical with the interval of 477.5,
which was consistent with the mass of monomer S6EGM.
The mass of every peak (marked with *) could be figured
FIGURE 3 (A) MALDI-TOF mass spectra of resultant polymer of
polymerization of S3EGM in low concentration (0.025 M) and
(B) in high concentration (0.1 M).
would take place and two polymerizable terminals would
become close to each other to facilitate the supramolecular
effects,
which
may
no
doubtless
benefit
the
cyclopolymerization.
The 1H NMR spectra of S6EGM with and without potassium
ions (Fig. 4) were recorded to check the supramolecular
TABLE 2 Results of the Polymerization of S6EGM in CH3CN under Different Conditions
Monomer
Concentration
(M)
Feed Ratio
(Monomer/CDB/
AIBN)
Conversion
(NMR)a (%)
Mw/
b
b
Entries
Mn
Mn
1
2
3
4
5
0.02
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
15/1/0.2
50
2,300
4,200
2,800
2,500
3,100
1.26
>2
0.05
ꢁ80c
85
0.051K1
0.11K1
0.161K1
1.25
1.27
1.62
81
ꢁ70c
a
Calculated according to the integral areas of resonance of ACH2OA attached to the styrene moieties in NMR spectrum.
Determined by SEC.
There may be deviations in the conversion of monomer calculated by NMR because of signals of unconsumed terminals in branched structure.
b
c
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FIGURE 4 1H NMR spectra of (A) S6EGM and (B) S6EGM with
1 equiv KPF6.
out as mS6EGM 3 DP 1 mCDB 1 mNa1 5 477.5 3 DP 1 272
1 23. This was consistent with the polymer being capped
with one Ph(C5S)S and one cumyl moieties, a linear struc-
ture from the cyclopolymerization of S6EGM. However, when
increasing the concentration to 0.05 M, the copolymers with
Mw/Mn >2 were obtained (Entry 2 in Table 2, Fig. 5), indi-
cating that the polymerization lost control.
FIGURE 6 SEC profiles at different conversions in the cyclopo-
lymerization of S6EGM. Feed ratio: S6EGM/CDB/AIBN/KPF6
100/1/0.2/200.
5
groups of the same monomer got close to each other. This
kind of effects had been reported previously by others in the
selective ion sensing with squaraine tethered bichromo-
phoric pseudocrown ether systems.43 Therefore, the intramo-
lecular interactions of St and maleimide moieties from the
same diheterofunctional monomer were enhanced and the
intermolecular interactions were suppressed even at a higher
concentration. Thus, intramolecular propagation can proceed
alternatively, affording well-defined cyclocopolymers.
From the SEC traces of the resultant polymers in the presence
of KPF6 (Entries 3 and 4 in Table 2, Fig. 5), we could see that
the polymerization of S6EGM in the higher concentrations up
to 0.1 M in the presence of KPF6 resulted well-controlled poly-
mers compared to those resulted in the absence of the salt.
Also, the result of MALDI-TOF mass spectrum was similar to
that obtained from the polymers at low concentration (Sup-
porting Information Fig. S6). Thus, the linear structure was
obtained at higher concentration in the presence of metal ion.
When the OEG linker was further elongated to seven EG
units (S7EGM), the cyclopolymerization could also be
assisted by supramolecular effects between the OEG linker
and K1 ion (Supporting Information Table S1). As we could
see from SEC profiles of polymerization of S7EGM under dif-
ferent conditions (Supporting Information Fig. S7), the poly-
merization in higher concentration (0.05 M) in the presence
of KPF6 could result well-controlled polymers compared to
those resulted in the absence of the salt. However, the supra-
molecular effect no longer affected so much at the concentra-
tion of 0.1 M, which was not so effective than that in the
polymerization of S6EGM at the same concentration. It could
be explained by weaker interaction between the monomer
with seven EGs and K1 ion. The up- or downfield shifts of
the corresponding signals from 1H NMR of the monomer
after the addition of KPF6 (Supporting Information Fig. S8)
were not so obvious, compared to that of S6EGM. The situa-
tion may be improved by the use of more suitable metal ion.
We believed that the polymerization behavior of S6EGM in
higher concentration could be explained by the host–guest
interaction of metal ion and monomer as we can see from
the 1H NMR spectra (Fig. 4). The interaction induced the
preorganization of the monomer conformation. When the
monomer adopted this structure, the two polymerizable
With aid of K1 templating, we easily obtained the cyclopoly-
mers with Mn up to 18,900 g/mol (dash dot line of Fig. 6),
which had not been achieved in the reports of other kinds of
cyclopolymerization. From the polymerization profiles of
experiment with a large feed ratio of RAFT polymerization of
S6EGM in 0.1 M (S6EGM/CDB/AIBN/KPF6 5 100/1/0.2/
200) (Fig. 6), we can see that the cyclopolymerization pro-
ceeded smoothly. Molecular weights increased gradually with
FIGURE 5 SEC profiles of polymerization of S6EGM under dif-
ferent conditions.
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16 A. K. Sharma, C. Cornaggia, D. Pasini, Macromol. Chem.
Phys. 2010, 211, 2254–2259.
increase of the monomer conversion while distributions
remained narrow.
17 C. Li, H. Liu, Y. Li, Macromol. Chem. Phys. 2011, 212, 1050–
1055.
CONCLUSIONS
18 A. Nagai, B. Ochiai, T. Endo, Macromolecules 2005, 38,
2547–2549.
We have synthesized a series of difunctional monomers with
St and maleimide moieties covalently linked by varied length
of OEG linkers. The cyclopolymerizations of these monomers
under different conditions were carried out. In low concentra-
tions, the cyclopolymerizations of those monomers with three
or more EG units could be realized properly, affording well-
defined polymers with crown ether encircled in their main
chain. In high concentrations, the polymerizations of these
monomers resulted branched polymers as indicated by above
spectroscopic methods. However, with aid of supramolecular
interaction, the cyclopolymerizations of monomers with six or
seven EG units in the presence of KPF6 in high concentrations
underwent smoothly. These findings will be helpful to the
synthesis of functional materials through cyclopolymerization
with convenience by elevating the feeding concentration.
19 B. Ochiai, Y. Ootani, T. Endo, J. Am. Chem. Soc. 2008, 130,
10832–10833.
20 L. Zhu, N. T. Lin, Z. Y. Xie, S. L. Lee, S. L. Huang, J. H.
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23 J. F. Lutz, Nat Chem 2010, 2, 84–85.
24 G. H. Deng, Y. M. Chen, Macromolecules 2004, 37, 18–26.
25 Y. H. Zhang, Z. Z. Xu, X. K. Li, Y. M. Chen, J. Polym. Sci.,
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A: Polym. Chem. 2010, 48, 3515–3522.
Financial support by the National Science Foundation of
China (21090353 and 51203171) is greatly acknowledged.
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30 Y. G. Jia, L. Y. Liu, B. Lei, J. Li, X. X. Zhu, Macromolecules
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