Synthesis and decrosslinking of networked polymers having zwitterion structure consisted by cyclic amidine and isothiocyanate

Article abstract of DOI:10.1002/pola.29485

We have already found that the polymers, which are obtained by the polymerization of 4-vinylphenyl isothoiocyanate after the zwitterion formation with cyclic amidines, are networked through the ionic interaction among the zwitterions becoming insoluble to various solvents. We report here on the results of the reaction of nucleophilic reagents such as amines and alcohols with the zwitterionic adduct to investigate about the decrosslinking through the resolution of ionic interactions. In the model reactions of amines and alcohols with the zwitterion compounds, which were consisted of the phenyl isothiocyanate and cyclic amidines, the reaction of nucleophilic reagents and zwitterionic adducts having methyl group at the 2-position of the amidine proceed quantitatively. Based on the model reaction, such nucleophilic addition was applicable to decrosslinking reaction of the networked polymers containing the zwitterion structure in the side-chain.

Full text of DOI:10.1002/pola.29485

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Synthesis and Decrosslinking of Networked Polymers Having Zwitterion
Structure Consisted by Cyclic Amidine and Isothiocyanate
Akira Yamauchi, Yoshiaki Yoshida, Takeshi Endo
Molecular Engineering Institute, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka 804-8550, Japan
Correspondence to: T. Endo (E-mail: endo.takeshi328@mail.kyutech.jp)
Received 9 July 2019; Revised 20 August 2019; accepted 22 August 2019; published online 30 September 2019
DOI: 10.1002/pola.29485
ABSTRACT: We have already found that the polymers, which are
obtained by the polymerization of 4-vinylphenyl isothoiocyanate
after the zwitterion formation with cyclic amidines, are
networked through the ionic interaction among the zwitterions
becoming insoluble to various solvents. We report here on
the results of the reaction of nucleophilic reagents such as
amines and alcohols with the zwitterionic adduct to investigate
about the decrosslinking through the resolution of ionic interac-
tions. In the model reactions of amines and alcohols with the
zwitterion compounds, which were consisted of the phenyl iso-
thiocyanate and cyclic amidines, the reaction of nucleophilic
reagents and zwitterionic adducts having methyl group at the
2-position of the amidine proceed quantitatively. Based on the
model reaction, such nucleophilic addition was applicable to
decrosslinking reaction of the networked polymers containing
the zwitterion structure in the side-chain. © 2019 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57,
2131–2137
KEYWORDS: zwitterion structure; decrosslinking reaction; poly-
mer reaction; radical polymerization; copolymer; zwitterion
adduct; crosslinking; decrosslinking; thiourea; thiourethane
the reaction of isothiocyanate with amines or alcohols,8 and
the zwitterion compounds that are synthesized from the
cyclic amidines and isothiocyanete described above can be
considered as the intermediate compounds in the reaction
of isothiocyanate where cyclic amidines are used as cata-
lyst. Thus, the authors consider that the changes in the side
chain to thioureas and/or thiocarbamates are made possi-
ble to the polymers that have zwitterions consisted of cyclic
amidines and isothiocyanates. Accordingly, the bridge
structures of the polymer, with the bridge being formed via
the ionic interaction among zwitterions, are debridged
accompanying the polymer reaction. In view that a large
number of study is under way, with the environmental load
in mind, of reversible and/or irreversible and thermal
and/or optical processes of decrosslinking, and here-
reported way of decrosslinking should be worthy of
attention.9–12 The work reported here aimed to the
decrosslinking, by way of the polymer reaction, in the poly-
mers possessing zwitterions consisted by cyclic amidines
and isothiocyanates in the side chain. The study, as the
model of the polymer reaction, of the reaction of amines
and alcohols onto simple zwitterion structures, was
followed by the detailed examination of the reaction behav-
ior therein, and then the attempt was made to decrosslink
via polymer reaction in the homopolymers and the styrene-
copolymers thereof to achieve the decrosslinking.
INTRODUCTION Zwitterions, unlike normal ions, characteristi-
cally possess covalently bonded pair of cation and anion in
their structure and stay stable without getting affected by
harsh environments such as the graded electrical field under
the electrophoresis.1–3 The polymers that possess zwitterion
structure also display such characteristics as high polarity and
electrical neutrality despite their high polarity and possession
of ion pair, and thus they are expected to be useful as
antibacterial material, flame-resistive material, lubricant,
emulsifier, anti-freeze material, and electrical material.4–6 To
the present authors’ knowledge, however, the hitherto publi-
shed reports are overwhelmed by those related with betaines
and few works have been done on other zwitterion structures.
Present authors have reported on the basis of the syn-
thesizability of novel zwitterion structured materials by way
of the 1:1 reaction of highly nucleophilic cyclic amidines with
isothiocyanates which are heterocumurenes having cumula-
tive double bond, that the polymers which possess novel zwit-
terions on their side chain can be synthesized by way of the
radical polymerization of the zwitterion compounds that pos-
sess vinyl group.7 The synthesized polymers form strongly
crosslinked structure through the parallel ionic interaction
among zwitterions and improvement on thermal stability and
solvent resistivity resulted. The cyclic amidines that the
authors used to form zwitterions are known to function as the
catalyst in the synthesis of thiourea and/or thiocarbamate via
Additional supporting information may be found in the online version of this article.
© 2019 Wiley Periodicals, Inc.
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Measurements
Nuclear magnetic resonance (NMR) spectra were recorded on
a JEOL JME-ECS 400 instrument (JEOL, Tokyo, Japan) using
tetramethyl silane as an internal standard (400 MHz for 1H
NMR and 100 MHz for 13C NMR). The infrared absorption
(FTIR) spectra were measured by using Nicolet iS10 spec-
trometer (Thermo Fisher Scientific, Tokyo, Japan).
Materials
Phenyl isothiocyanate (PI), lithium chloride (LiCl), trifluoroacetic
acid (CF3COOH), 1-butyl-3-methylimidazolium hexafluorophos-
phate (IL), and 2,20-azobis(2,4-dimethyl-4-methoxy)valeronitrile
(V-70) were purchased from Tokyo Chemical Industry (Tokyo,
Japan). Styrene (St), aniline (An), n-hexylamine (HA), piperidine
(Pi), benzyl alcohol (BA), isopropyl alcohol (IPA), dimethyl sulfoxide
(DMSO), and acetonitrile (MeCN) were purchased from Wako Pure
Chemical Industries (Osaka, Japan). N-methyltetrahydropyrimidine
(MTHP),
1-methyl-2-methyltetrahydropyrimidine
(DMTHP),
4-vinylphenyl isothiocyanate (VPI), and the zwitterion compound
from PI with MTHP or DMTHP (PI-MTHP or PI-DMTHP) were syn-
thesized according to the reported procedures.7,13–15 The zwitterion
compound, which consisted of DMTHP and VPI (VPI-DMTHP), was
polymerized by using V-70 as radical initiator (poly(VPI-DMTHP)).
The VPI-DMTHP and St were also polymerized by using V-70 as rad-
ical initiator (poly(St-co-VPI-DMTHP)). These synthetic procedures
were summarized in the Supporting Information and shown in
Scheme 1.7
SCHEME 1 Synthesis of (a) poly(VDI-DMTHP) and (b) poly(St-co-
VPI-DMTHP). [Color figure can be viewed at wileyonlinelibrary.com]
yield of 77% (0.017 g, 0.07 mmol). 1H NMR (CDCl3): δ
7.21–7.36 (m, 10H, Ph), 4.66 (d, J = 2.7 Hz, 2H, S C CH2 Ph).
13C NMR (CDCl3): δ 140.95, 135.38, 131.32, 129.63, 128.64,
127.71, 127.39, 127.07, 125.83, 65.37 FTIR (neat): 3126,
3029, 2974, 2160, 1972, 1593, 1541, 1494, 1405, 1333, 1309,
1286, 1221, 1187, 1075, 1052, 1016, 967, 920, 904, 850,
Synthesis of 1,3-Diphenyl-2-Thiourea by the Reaction of
PI-DMTHP and Aniline
836, 815, 748, 732, 684 cm−1
.
After placing PI-DMTHP (0.023 g, 0.09 mmol) and dehydrated
acetonitrile (0.5 mL) into a two-necked round bottom flask
with a three-way stopcock, aniline (0.008 g, 0.09 mmol) was
added at room temperature under nitrogen. After stirring for
2 h at room temperature, the solvent was removed using a
rotary evaporator and the residue was purified by silica gel
column chromatography (hexane/ethyl acetate with v/v of
92/8), and white solid of 1,3-diphenyl-2-thiourea was
obtained with the yield of 84% (0.018 g, 0.076 mmol). 1H
NMR (CDCl3): δ 7.18–7.44 (m, 6H) 13C NMR (CDCl3): δ 180.09,
137.12, 129.72, 127.26, 125.37 FTIR (neat): 3263, 3092,
3057, 1601, 1590, 1552, 1498, 1451, 1399, 1347, 1318, 1309,
1297, 1244, 1182, 1071, 1029, 1001, 931, 761, 749, 705,
Polymer Reactions by Nucleophilic Reagents onto Poly(St-
co-VPI-DMTHP), a Typical Example
After placing poly(St-co-VPI-DMTHP) (0.014 g, 0.09 mmol,
St/VPI-DMTHP = 70/30) and dehydrated DMSO (0.5 mL) into
a two-necked round bottom flask with a three-way stopcock,
amines (3 equiv) or alcohols (5 equiv) were added. After stir-
ring for 24 h at room temperature, the objective adducts were
obtained by reprecipitation in acetonitrile.
Poly(St-co-VPI-An)
From aniline (0.007 g, 0.08 mmol), yield: 94% (0.013 g,
0.08 mmol), FTIR (neat):3267, 2918, 1651, 1593, 1510, 1493,
699 cm−1
.
1435, 1312, 1258, 1199, 1015, 950, 872, 831, 756, 699 cm−1
.
Poly(St-co-VPI-HA)
Synthesis of N-Phenylthiocarbamic Acid Benzyl Ether by
the Reaction of PI-DMTHP and Benzyl Alcohol
After placing PI-DMTHP (0.023 g, 0.09 mmol) and dehydrated
acetonitrile (0.5 mL) into a two-necked round bottom flask
From n-hexylamine (0.008 g, 0.08 mmol), yield: 89% (0.011 g,
0.07 mmol), FTIR (neat): 3289, 3024, 2921, 2848, 1651, 1597,
1511, 1493, 1436, 1415, 1312, 1259, 1195, 950, 798,
756, 699 cm−1
.
with
a
three-way stopcock, benzyl alcohol (0.012 g,
0.09 mmol) was added at room temperature under nitrogen.
After stirring for 2 h at room temperature, the solvent was
removed using a rotary evaporator and the residue was puri-
fied by silica gel column chromatography (hexane/ethyl ace-
tate with v/v of 92/8), and white solid of N-
phenylthiocarbamic acid benzyl ether was obtained with the
Poly(St-co-VPI-Pi)
From piperidine (0.007 g, 0.08 mmol), yield: 90% (0.007 g,
0.05 mmol), FTIR (neat): 3273, 3024, 2920, 2849, 1651, 1593,
1511, 1493, 1435, 1309, 1259, 1198, 1016, 951, 872,
831, 757, 699 cm−1
.
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Poly(St-co-VPI-BA)
From benzyl alcohol (0.015 g, 0.14 mmol), yield: 90%
(0.006 g, 0.04 mmol), FTIR (neat):3266, 3025, 2919, 2850,
1648, 1593, 1510, 1492, 1451, 1433, 1416, 1373, 1330, 1308,
1259, 1197, 1109, 1018, 950, 872, 832, 749, 699 cm−1
.
Poly(St-co-VPI-IPA)
From isopropyl alcohol (0.008 g, 0.14 mmol), yield: 93%
(0.007 g, 0.05 mmol), FTIR (neat): 3280, 3024, 2923, 2850,
1651, 1598, 1509, 1492, 1434, 1379, 1333, 1309, 1259, 1197,
1017, 952, 872, 820, 802, 758, 699 cm−1
.
SCHEME 2 (a) Synthesis of 1,3-diphenyl-2-thiourea from PI-
DMTHP and aniline. (b) Synthesis of N-phenylthiocarbamic acid
benzyl ether from PI-DMTHP and benzyl alcohol. [Color figure
can be viewed at wileyonlinelibrary.com]
RESULTS AND DISCUSSION
The authors first examined the nucleophilic reaction of aniline
(An) and zwitterion adduct (PI-MTHP), which was composed
of MTHP and phenyl isothiocyanate (PI) as the model reaction
for the polymer reaction. Thus, 1 equiv of An was added as
nucleophilic reagent to the PI-MTHP and the reaction was car-
ried out at ambient temperature for 2 h [Scheme 1(a–i)]. The
1H NMR spectrum (Fig. 1) of the resulted crude mixture indi-
cated that the mixture is a complicated mixture and thus the
occurrence of the reactions other than the intended one took
place. Specifically, characteristic peaks are observed at around
8 ppm, and as we know that the amidine we used in the reac-
tion undergoes in basic condition the ring opening and results
in formamide derivative under ambient moisture [Fig. 1(a)].14
As the peak of the formamide generated by this ring opening
the ratio of 1:1 indicating that the reaction took place quanti-
tatively [Fig. 2(a)]. It was confirmed that the isolation of the
thiourea compound can be achieved by the yield of 84% after
the purification using silica gel column chromatography. The
reaction using benzyl alcohol (BA) was examined in a similar
way. BA was added by 1 equiv to PI-DMTHP in acetonitrile
and then the mixture was stirred for 2 h at room temperature
1
[Scheme 2(b)]. The H NMR spectrum of the resulted mixture
showed that the mixture was composed of the DMTHP and
the intended thiocarbamate compound alone proving that is
the reaction to proceed quantitatively like the one for aniline
[Fig. 2(b)]. The authors achieved the isolation of the
thiocarbamate compound with 77% yield by way of the silica-
gel column chromatography.
1
was observed in H NMR spectrum [Fig. 1(b)], the occurrence
of ring opening of the amidine ring is suggested to take place
in the reaction with aniline.
Accordingly, 1 equiv of aniline was added to PI-DMTHP and
then the mixture was stirred for 2 h at room temperature
[Scheme 2(a)]. The solvent was removed and the resulted
crude mixture was examined by 1H NMR, which showed the
presence of the intended thiourea compound and DMTHP by
The synthesis of thiourea and thiocarbamate from PI-DMTHP
is explained by two kinds of routes, which are reaction of the
dissociated isothiocyanate from zwitterion adduct and nucleo-
philes (route i) and the direct reaction of zwitterion adduct
and nucleophiles (route ii). We have already demonstrated
that the zwitterion adduct was decomposed by heating at
1
60 ꢀC. Based on the previous report, we measured H NMR of
PI-DMTHP before and after heating to investigate the revers-
ible reaction of PI and DMTHP (Supporting Informa-
tion Fig. S1). The spectrum of PI-DMTHP after heating showed
the complicated signals in the aromatic region around 7.5–-
7.0 ppm, whereas the signals arisen from DMTHP were clearly
observed [Supporting Information Fig. S1(c)]. Next, the crude
mixture after heat treatment was reacted again at room tem-
perature for 24 h. However, 1H NMR spectrum after re-
reaction almost agreed with that after heat treatment
[Supporting Information Figs. S1(c) and S1(d)]. These results
indicated that not only reversible reaction of PI and DMTHP
but also direct reaction of PI-DMTHP and nucleophiles did not
proceed at all because the dissociated isothiocyanate from
zwitterion adduct was degraded and/or inactive. Therefore,
the authors inferred the ring-opening mechanism of PI-MTHP
as shown in Figure 3(a). As the first step, the nucleophilic ani-
line attacks the 2-position of MTHP because the imine carbon
is fairly electrophilic than the carbon of thiocarbonyl group.
Then, the ring opening takes place when the hydrolysis of the
FIGURE 1 (a) Reaction of zwitterionic adduct and aniline. (b) 1H
NMR Spectrum of crude product obtained from PI-MTHP
and An. [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 2 1H NMR spectra of the crude product of the reaction of (a) PI-DMTHP and aniline, and (b) that of the reaction of PI-DMTHP
and benzyl alcohol. [Color figure can be viewed at wileyonlinelibrary.com]
amidine occurs resulting in the generation of formamide
under ambient moisture. On the other hand, the nucleophilic
addition of aniline takes place on the thiocarbonyl carbon of
PI-DMTHP, resulting in the occurrence of thiourea compound
and its source compound of cyclic amidine structure, because
the nucleophilic attack of aniline to the position-2 of amidine
is inhibited owing to the steric hindrance of methyl group.
[Fig. 3(b)].
reason of this failure that bridge structure that is formed via
ionic interaction among the zwitterions is very dense and thus
the dissipation of the reagent aniline into the polymer net-
work is inhibited.
Briefly stated, poly(VPI-DMTHP), which is the polymer having
the zwitterion structure consisted of cyclic amidine and iso-
thiocyanate did not undergo the polymer reaction to debridge,
presumably because the bridging through ionic interaction
among the zwitterions is so dense. We then inferred that the
polymer reaction in the system with decreased density of the
zwitterion, namely the reaction in general-purpose polymer
may proceed as expected because the relative density of the
bridging is lower. The copolymer obtained by the copolymeri-
zation of VPI-DMTHP and styrene (St) display strong bridging
arising from ionic interaction even when the content of the
vinyl monomer having the zwitterion structure is as low as
5 mol % and is known to give out the polymer which has the
network and is insoluble to various solvents. Namely, the
authors conferred that the decrease in the density of the
bridge structure with respect to that in the homopolymer may
allow for the polymer reaction with aniline enabling the poly-
mer reaction that accompany the decrosslinking reaction. Two
types of the polymer obtained by the copolymerization of the
monomers with the ratios of St:VPI-DMTHP = 50:50 and
70:30 were examined for their polymer reaction (Scheme 3);
1 equiv. of aniline was added to the DMSO solutions of the
two types of poly(St-co-VPI-DMTHP) and let the reaction go
for 24 h. The result for the 50:50 copolymer was similar to
As we verified by using cyclic amidine having methyl group at
2-position that the model reaction proceed quantitatively, the
reaction of amines and alcohols with the polymer bearing a
zwitterion moiety on its side-chain (poly(VPI-DMTHP)), which
was obtained by the polymerization of 4-vinylphenyl iso-
thoiocyanate after the zwitterion formation with cyclic ami-
dine having methyl group at 2-position (DMTHP), was
investigated next. The polymer reaction using aniline was first
examined; 1 equiv of aniline with respect to the amount of
zwitterion was added and let the reaction proceed in DMSO
for 24 h. Incidentally, poly(VPI-DMTHP) is insoluble to DMSO
and the experiment was carried out in the polymer dispersion.
The system remained nonuniform after 24 h of reaction and
the inspection using the FTIR spectra measured prior to and
after the reaction indicated that the polymer did not undergo
the reaction at all (Supporting Information Fig. S2). All of the
supplemental experiments, that is, raise in the reaction tem-
perature to 50 ꢀC, addition of 5 equiv of aniline instead of
1 equiv, the use of aniline as solvent, and so on, failed to make
the reaction proceed (Table 1). The authors infer as the
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disappeared completely [Fig. 4(b)]. The polymer obtained by
the reaction looked as being dissolved in DMSO, but the GPC
and/or NMR measurements failed to give any significant sig-
nal at all and thus the formation of adducts by way of hydro-
gen bond is suggested. Thus, the homopolymer and
copolymers of 4-vinylphenyl isothiocyanate without the zwit-
terion moiety were utilized as alternative polymers to estimate
the molecular weight and dispersity of polymers containing the
zwitterion structure, which exhibited Mn = 6,500 (Mw/Mn = 1.5)
and Mn = 12,000 (Mw/Mn = 1.5), respectively (Supporting
Information Table S1).
Next, the reaction of poly-(St-co-DMTHP), (St/VP-DMTHP = 70/
30) with 3 equiv of benzyl alcohol was carried out in DMSO
for 24 h at room temperature. The comparison of the FTIR
FIGURE 3 Proposed mechanism of nucleophilic addition of
aniline into (a) PI-MTHP and (b) PI-DMTHP. [Color figure can be
viewed at wileyonlinelibrary.com]
TABLE 1 Reaction of Poly(VP-DMTHP) and An
Entrya
Aniline
Temperature
Resultb
1
2
3
4
5
1.0 equiv
5.0 equiv
5.0 equiv
Excessc
50 ꢀC
r.t.
50 ꢀC
No reaction
No reaction
No reaction
No reaction
No reaction
r.t.
50 ꢀC
Excessc
a Condition: in DMSO for 24 h.
b Determined by FTIR measurement.
c Aniline was used as solvent.
the one obtained for the homopolymer as revealed by the
FTIR spectrum measured after the reaction, and thus the reac-
tion did not proceed at all [Fig. 4(a)]. The copolymer that was
synthesized by the monomer ratio of 30:70, on the other
hand, the reaction system changed from heterogeneous to
homogeneous after the reaction and the FTIR spectrum of the
resultant displayed the near-disappearance of the peaks that
sit near those arising from the isothiocyanate part of the zwit-
terion. Furthermore, the reaction with the increase of the ani-
line quantity to 3 equiv with respect to that of the zwitterion
gave the product in which the FTIR peak due to the isocyanate
FIGURE 4 FTIR spectra of the polymer reaction of poly(St-co-
VPI-DMTHP) and aniline, (a) x/y = 50/50 and (b) x/y = 30/70.
[Color figure can be viewed at wileyonlinelibrary.com]
SCHEME
3 Polymer reaction of poly(St-co-VPI-DMTHP) and
aniline. [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 5 FTIR spectra of the polymer reaction of poly(St-co-VPI-
DMTHP). [Color figure can be viewed at wileyonlinelibrary.com]
spectrum measured after the reaction with that taken before-
hand showed the decrease in the intensity of the peaks arising
from the isothiocyanate, but the existence of remnant zwitter-
ion was suggested by the residual intensity of the absorption
peak. The remnant peak disappeared when reaction was per-
formed by increasing the amount of benzyl alcohol to 5 equiv
indicating the quantitativeness of the reaction (Fig. 5). The
polymer derived here has thiocarbamate group deriving from
the reaction with benzyl alcohol and it also appeared to be
dissolved in DMSO, significant signals were not obtained on
the results of both GPC and NMR measurements suggesting
the occurrence of the association through hydrogen bonding.
The polymers after repreciptation were completely insoluble
in not only typical solvents but also high polar solvents, acidic
solution, and ionic liquid, whereas the polymers containing
ratio of St/VP-DMTHP = 70/30 before and after nucleophilic
SCHEME 4 Polymer reaction of poly(St(30)-co-VPI-DMTHP(70))
and various nucleophiles. [Color figure can be viewed at
wileyonlinelibrary.com]
addition with aniline and benzyl alcohol were partly soluble
and swelling in DMSO (Table 2).
The last system reported in this article is the polymer reaction
of the amines and alcohols of different kind with poly(St-co-
VPI-DMTHP) containing ratio of St/VP-DMTHP = 70/30. The
examined amines are hexyl amine (HA) which is aliphatic
amine and piperidine (Pi) which is secondary amine. Also, the
examined alcohols are isopropanol which is bulky secondary
alcohol and benzyl alcohol which is aromatic alcohol
(Scheme 4). The reaction conditions are like the ones used in
the aforementioned systems; 3 equiv of amine or 5 equiv of
alcohol was added, DMSO was used as solvent, and reactions
were carried out at room temperature for 24 h. Then, the
reaction solutions became homogeneous after 24 h and the
FTIR spectra indicated that the reaction proceeded quantita-
tively. Thus, quantitative procedure of the reaction was rev-
ealed even for sluggish nucleophilic agents owing to their
bulky substituents such as secondary amines and secondary
alcohols.
TABLE 2 Reaction of Poly(VP-DMTHP) and An
Solvent
A
B
C
D
E
DMF
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
O
O
X
X
X
X
X
X
O
O
X
X
X
X
X
X
O
O
X
DMF(10%LiCl)
H2O
CF3COOH
1.0 M HClaq.
DMSO
CONCLUSIONS
DMSO (100 ꢀC)
IL
The authors reported in this article the study of the
decrosslinking in the networked polymer by the polymer reac-
tion of the polymers having the zwitterion structure formed
by cyclic amidines and heterocumurenes in their side chains
and the nucleophilic reagents such as amines and alcohols.
The model reaction of aniline and the zwitterion compound
which composed by phenyl isothiocyanate and cyclic amidine
demonstrates the importance of the presence of methyl group
A: poly(VPI-DMTHP).
B: poly(St(50)-co-VPI-DMTHP(50)).
C: poly(St(30)-co-VPI-DMTHP(70)).
D: poly(St-co-VPI-An).
E: poly(St-co-VPI-BA).
IL: 1-butyl-3-methylimidazolium hexafluorophosphate.
O: partly soluble.
X: completely insoluble.
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alcohols were used as the nucleophilic reagents. Therefore,
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