Anionic ring-opening polymerization of a five-membered cyclic carbonate having a glucopyranoside structure

Article abstract of DOI:10.1002/pola.26538

Anionic ring-opening polymerizations of methyl 4,6-O-benzylidene-2,3-O- carbonyl-α-D-glucopyranoside (MBCG) were investigated using various anionic polymerization initiators. Polymerizations of the cyclic carbonate readily proceeded by using highly active initiators such as n-butyllithium, lithium tert-butoxide, sodium tert-butoxide, potassium tert-butoxide, and 1,8-diazabicyclo[5.4.0]undec-7-ene, whereas it did not proceed by using N,N-dimethyl-4-aminopyridine and pyridine as initiators. In a polymerization of MBCG (1.0 M), 99% of MBCG was converted within 30 s to give the corresponding polymer with number-averaged molecular weight (M_n) of 16,000. However, the M_n of the polymer decreased to 7500 when the polymerization time was prolonged to 24 h. It is because a backbiting reaction might occur under the polymerization conditions. Copyright

Full text of DOI:10.1002/pola.26538

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Anionic Ring-Opening Polymerization of a Five-Membered Cyclic
Carbonate Having a Glucopyranoside Structure
Motohisa Azechi, Kozo Matsumoto, Takeshi Endo
Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820-8555, Japan
Correspondence to: T. Endo (E-mail: tendo@moleng.fuk.kindai.ac.jp)
Received 27 September 2012; accepted 15 December 2012; published online 9 January 2013
DOI: 10.1002/pola.26538
ABSTRACT: Anionic ring-opening polymerizations of methyl 4,6-
O-benzylidene-2,3-O-carbonyl-a-D-glucopyranoside (MBCG)
with number-averaged molecular weight (Mn) of 16,000. How-
ever, the Mn of the polymer decreased to 7500 when the poly-
were investigated using various anionic polymerization initia-
tors. Polymerizations of the cyclic carbonate readily proceeded
by using highly active initiators such as n-butyllithium, lithium
tert-butoxide, sodium tert-butoxide, potassium tert-butoxide,
and 1,8-diazabicyclo[5.4.0]undec-7-ene, whereas it did not pro-
ceed by using N,N-dimethyl-4-aminopyridine and pyridine as
initiators. In a polymerization of MBCG (1.0 M), 99% of MBCG
was converted within 30 s to give the corresponding polymer
merization time was prolonged to 24 h. It is because a
backbiting reaction might occur under the polymerization con-
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2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2013, 51, 1651–1655
KEYWORDS: anionic polymerization; five-membered cyclic car-
bonate;
glucopyranoside;
polycarbonates;
ring-opening
polymerization
INTRODUCTION Aromatic polycarbonates, which have high
thermal stability and high mechanical strength, are known as
one of typical engineering plastics produced industrially in a
large scale. On the other hand, aliphatic polycarbonates had
been paid less attention in the past because of their lower
thermal and mechanical stability compared with the aromatic
polycarbonates. In these days, however, the aliphatic polycar-
bonates are getting more attention because they have good
biodegradability and biocompatibility.1–5 In recent years, ring-
opening polymerizations of cyclic carbonates have been
widely studied by many researchers to synthesize polycar-
bonates. Particularly, ring-opening polymerizations of cyclic
carbonates with organometals, metal alkoxides, or organo-
bases have become one of the most promising methods to
obtain well-defined polycarbonates.6–16 It is well known that
anionic ring-opening polymerizations of six- and seven-mem-
bered cyclic carbonates such as cyclotrimethylene carbonates
and cyclotetramethylene carbonates easily proceed under
mild conditions. However, anionic ring-opening polymeriza-
tions of five-membered cyclic carbonates usually require
more vigorous conditions with higher temperature, and pro-
ceed with an elimination of carbon dioxide to provide poly-
carbonates containing ether units in the main chain. There-
fore, it was considered that polycarbonates solely composed
of carbonate repeating units could not be prepared by anionic
polymerizations of five-membered cyclic carbonates.
cyclic carbonate containing glucopyranoside, methyl 4,6-O-
benzylidene-2,3-O-carbonyl-a-D-glucopyranoside
(MBCG).17
Although MBCG is a five-membered cyclic carbonate, the ani-
onic polymerization of MBCG was induced by potassium
tert-butoxide (t-BuOK) and 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) at below 30 ꢀC, and proceeded without an
elimination of carbon dioxide to give the corresponding poly-
carbonate simply consisted of a carbonate repeating unit.
However, details of the anionic polymerization behaviors
have been not studied so far.
In this article, we report the effects of polymerization condi-
tions (initiator, solvent, time, and monomer concentration)
on the anionic ring-opening polymerization of MBCG
(Scheme 1).
EXPERIMENTAL
Materials
Tetrahydrofuran (THF) was distilled over sodium benzophe-
none ketyl. N,N-Dimethylformamide (DMF) was distilled over
calcium hydride (CaH2) under reduced pressure. MBCG was
prepared from methyl a-D-glucopyranoside, benzaldehyde di-
methyl acetal, and ethyl chloroformate according to the liter-
ature (Scheme 2).18,19 Methyl a-D-glucopyranoside was pur-
chased from Sigma-Aldrich (St. Louis, MO) and recrystallized
from methanol before use. Benzaldehyde dimethyl acetal was
purchased from Sigma-Aldrich. Ethyl chloroformate was pur-
chased from Tokyo Chemical Industry (Tokyo, Japan).
n-Butyllithium (n-BuLi) was purchased from Kanto Kagaku
Previously, we have reported in a short communication on
the anionic ring-opening polymerization of five-membered
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SCHEME 1 Anionic ring-opening polymerization of MBCG.
SCHEME 2 Synthesis of MBCG.
(Tokyo, Japan). Lithium tert-butoxide (t-BuOLi) in THF solu-
tion (1 M) and sodium tert-butoxide (t-BuONa) in THF solu-
tion (2 M) were purchased from Sigma-Aldrich. t-BuOK was
purchased from Wako Pure Chemical Industry (Osaka,
Japan). DBU was purchased from Wako Pure Chemical Indus-
try and distilled over CaH2 under reduced pressure. N,N-Di-
methyl-4-aminopyridine (DMAP) was purchased from Wako
Pure Chemical Industry and recrystallized from n-hexane
before use. Pyridine was purchased from Wako Pure Chemi-
cal Industry and distilled over CaH2 under reduced pressure.
tograph model HLC-8220GPC equipped with Tosoh TSKgel
Super AW 2500 columns (6.0 mm I.D. ꢁ 15 cm), TSKgel
Super AW 3000 columns (6.0 mm I.D. ꢁ 15 cm), and
TSKgel Super AW 4000 columns (6.0 mm I.D. ꢁ 15 cm), using
DMF containing 10 mM lithium bromide as an eluent at the
flow rate of 0.5 mL/min at 40 ꢀC. The molecular weight
calibration curve was obtained with polystyrene standards.
Anionic Ring-Opening Polymerizations of MBCG
A typical polymerization procedure is described below.
MBCG (0.308 g, 1.0 mmol) was placed in a Schlenk reaction
tube. The flask was evacuated and then filled with nitrogen
gas. THF (0.7 mL) was added to the flask under nitrogen
atmosphere. t-BuONa THF solution (20 lL, 1 M) was added
in a stream of nitrogen to initiate polymerization. After the
prescribed time, polymerization was terminated with two
drops of acetic acid. The polymerization solution was poured
into 100 mL of methanol. The resulting precipitate was
Measurements
Nuclear magnetic resonance (NMR) measurements were
recorded on JEOL JNM ECS 400 in DMSO-d6 with tetramethyl-
silane as an internal standard. Number-average molecular
weight (Mn) and polydispersity (Mw/Mn) were estimated from
size exclusion chromatography performed on a Tosoh chroma-
TABLE 1 Effect of Initiator on Anionic Polymerizations of
MBCGa
TABLE 2 Effect of Monomer Concentration on Anionic
Polymerizations of MBCGa
Run
Initiator
Yieldb (%)
Mn
Mw/Mn
c
c
c
c
Run Initiator
[MBCG]0 (M) Yieldb (%) Mn
Mw/Mn
1
2
3
4
5
6
7
a
n-BuLi
t-BuOLi
t-BuONa
t-BuOK
DBU
98
13,700
5,700
1.56
2.62
2.18
2.02
1.44
1
2
3
4
5
6
7
n-BuLi
1.0
0.5
89
88
76
77
83
43
8
5,700 1.70
5,200 1.62
7,800 1.58
7,100 1.75
7,700 1.81
6,000 1.49
3,600 1.24
81
96
12,100
8,600
t-BuONa 1.0
88
0.5
71
11,000
0.25
1.0
0.5
Pyridine
DMAP
No polymerization
No polymerization
DBU
Polymerization conditions: [MBCG]0 ¼ 3.0 M, [MBCG]0/[initiator]0
¼
a
b
c
25, r.t., 12 h, in THF.
b
Polymerization conditions: [MBCG]0/[initiator]0 ¼ 25, r.t., 12 h, in THF.
Methanol-insoluble part.
Methanol-insoluble part.
c
Estimated by GPC analysis (eluent: DMF, polystyrene standards).
Estimated by GPC analysis (eluent: DMF, polystyrene standards).
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SCHEME 3 Plausible mechanism of initiation reaction with DBU.
collected by centrifugation and purified by reprecipitation
from THF–methanol systems two times. The obtained poly-
mer was dried at 60 C in vacuo to afford 96% yield. The Mn
tion.17 To evaluate the effect of an initiator in detail, we exam-
ined the anionic polymerization with various initiators such as
n-BuLi, t-BuOLi, t-BuONa, t-BuOK, DBU, DMAP, and pyridine at
room temperature for 12 h in THF (3 M). The conditions and
results are summarized in Table 1. Using n-BuLi, t-BuOLi, t-
BuONa, t-BuOK, and DBU as initiators, the anionic polymeriza-
tion smoothly proceeded to obtain the corresponding pol-
y(MBCG)s in 71–98% yield. The number-averaged molecular
weights (Mns) were in a range from 5700 to 13,700. The dis-
persities of the obtained polymers (Mw/Mn ¼ 1.44–2.62) were
broader than we expected. The broader dispersity may be due
to an occurrence of backbiting reaction. This will be discussed
later. On the other hand, the anionic polymerizations of MBCG
initiated with DMAP and pyridine did not proceed and
resulted in the quantitative recovery of the monomer. This is
presumably because the nucleophilicity of DMAP or pyridine
is lower than that of n-BuLi, t-BuOLi, t-BuONa, t-BuOK, and
DBU. These results suggest that the reactivity of the five-mem-
bered cyclic carbonate (MBCG) toward ring opening might
strongly depend on the nucleophilicity of the initiator.
ꢀ
and Mw/Mn were 12,100 g/mol and 2.18, respectively.
1H NMR (DMSO-d6, 400 MHz): d ¼ 3.06–3.65 (br, 3H, CH3),
3.62–4.94
(br,
2H,
CH3AOACHACHA
and
CH3AOACHACHACHA), 4.12–4.34 (br, 2H, ACHACHHAOA
and ACHACHACH2AOA), 4.68–4.78 (br, 1H,
ACHACHHAOA), 4.87–5.26 (br, 2H, ACHAOACH3 and
ACHACHACH2AOA), 5.47–5.70 (br, 1H, ACHAPh), and
7.03–7.49 (br, 5H, Ph). 13C NMR (DMSO-d6, 400 MHz): d ¼
56.0 (CH3), 62.6 (ACHACH2A), 68.1 (ACH2AOA), 74.4
(O¼¼CAOACHACHAOAC¼¼O), 77.9 (ACHACHACH2A), 97.0
(ACHAOACH3), 100.8 (ACHAPh), 126.3 (Ar), 128.8 (Ar),
129.6 (Ar), 137.0 (Ar), and 153.8 (C¼¼O).
RESULTS AND DISCUSSION
Anionic Polymerization of MBCG with Various Initiators
In the previous work, the anionic polymerizations of MBCG
were carried out with t-BuOK and DBU in 3 M monomer solu-
Anionic Polymerization of MBCG at Various Monomer
Concentrations
We examined the effect of monomer concentration on the
polymerization behavior. The polymerizations were carried
TABLE 3 Anionic Polymerizations of MBCG with t-BuONaa
c
d
d
Run [MBCG]0/[Initiator]0 Yieldb (%) Mn theo Mn
Mw/Mn
1
2
3
50
100
200
86
89
89
15,260 12,700 1.99
30,210 18,100 2.08
57,960 20,200 1.82
a
b
c
Polymerization conditions: [monomer]0 ¼ 0.25 M, r.t., 2 h, in THF.
Methanol-insoluble part.
Mn
¼ (molecular weight of MBCG) ꢁ ([MBCG]0/[initiator]0) ꢁ (con-
theo
FIGURE 1 Time dependence of the polymerization of MBCG in
version of MBCG).
d
THF and DMF.
Estimated by GPC analysis (eluent: DMF, polystyrene standards).
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TABLE 4 Time Dependence of Mn of Poly(MBCG)a
other hand, the yield and Mn of the polymer obtained by
DBU decreased as the monomer concentration decreased.
This implies that the polymerization mechanism initiated
with DBU may be different from those initiated with n-BuLi
and t-BuONa. In the DBU-induced anionic polymerization of
the cyclic carbonate, a zwitterion intermediate 1 was formed
by a nucleophilic addition of DBU to the cyclic carbonate as
the initiation step (Scheme 3). However, the reaction is in
equilibrium and regenerates the monomer and the initiator.
Therefore, the polymerization behavior may be highly sus-
ceptible to the monomer and initiator concentrations. As a
result, the polymer yield and Mn widely varied with the con-
centrations in the polymerization using DBU as an initiator.
b
b
Run
[MBCG]0 (M)
1.0
Time
Mn
Mw/Mn
1
30 s
16,000
11,700
10,300
7,800
6,400
8,300
7,700
7,700
7,700
7,500
1.99
2.08
1.82
1.58
1.71
1.85
1.79
1.75
1.81
1.78
2
20 min
1 h
3
4c
12 h
24 h
30 s
5
6
0.25
7
20 min
1 h
8
9d
10
a
12 h
24 h
Anionic Polymerization of MBCG with DBU
Polymerization conditions: [MBCG]0/[initiator]0 ¼ 25, r.t., in THF.
Estimated by GPC analysis (eluent: DMF, polystyrene standards).
Run 3 in Table 2.
Figure 1 shows the time conversion of MBCG initiated
with DBU in DMF and in THF. Using DMF as a solvent, the
polymerization of MBCG proceeded rapidly and MBCG was
consumed within 24 h. In contrast, it took 72 h that the
monomer was quantitatively consumed in the polymeriza-
tion in THF. We consider that this marked difference in
the polymerization rates is due to the difference in the
reactivities of the polymer chain ends. The anionic polymer
chain end was more activated in DMF than in THF.
Comparing with the molecular weights of the polymers,
the polymer obtained in DMF had lower molecular weight
(Mn ¼ 4800) than that of the polymer obtained in THF
(Mn ¼ 11,400). This is presumably because chain transfer
including backbiting occurred more frequently in DMF
than in THF.
b
c
d
Run 5 in Table 2.
out with n-BuLi, t-BuONa, and DBU as initiators at room
temperature for 12 h in THF. As shown in Table 2, by using
n-BuLi and t-BuONa, the polymers with moderate Mns of
5200–7800 were obtained in high yield (76–89%). In these
cases, yield and molecular weight of the obtained polymers
did not depend much on the monomer concentrations. In
particular, the polymerization initiated with t-BuONa pro-
ceeded even in 0.25 M solution to obtain the corresponding
poly(MBCG) in 83% yield. The polymerization activity of
n-BuLi and t-BuONa was the highest in our results. On the
FIGURE 2 MALDI-TOF MS spectra of poly(MBCG) obtained with t-BuONa.
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Anionic Polymerization of MBCG with t-BuONa
mers. Presumably, this may be caused by the ring strain of
carbonate ring connected to a bicyclic structure of glucopy-
ranoside. Using DBU as an initiator, the polymerization rate
in DMF was faster than that in THF. It was supposed that
the zwitterion intermediate 1 was more favorably formed
between MBCG and DBU in DMF than in THF, which was sol-
vated and stabilized by DMF. When the polymerization was
performed with t-BuONa in 1 M solution, the molecular
weight of the polymer decreased as time progressed. MALDI-
TOF MS analysis of the obtained polymer revealed that cyclic
polymers were produced by a backbiting reaction. We expect
that the polycarbonate having glucopyranoside structure syn-
thesized here can be a novel biocompatible or biodegradable
material in the future.
As mentioned above, the polymerization of MBCG with t-
BuONa gave the corresponding polymer in high yield as
shown in Table 2. Then, we examined the effect of initiator
amount on the polymerization in detail under this polymer-
ization condition (Table 3). In all [MBCG]0/[initiator]0 ratios
examined here ranging from 50 to 200, poly(MBCG) could be
obtained in high yield. In particular, the condition of run 3
([MBCG]0/[initiator]0 ¼ 200) gave the polymer in 89% yield,
whose molecular weight was the highest (Mn ¼ 20,200). It is
worth nothing that the Mns were always lower than the Mn
theo. This is presumably because backbiting reaction occurred
during the polymerization. To further examine the polymer-
ization, we checked time-evolved polymerization behavior
using t-BuONa as an initiator. Table 4 shows the results of
the polymerization with t-BuONa at room temperature in
THF. In the case of 1 M solution, MBCG was quantitatively
consumed within 30 s. Thus, the polymerization rate with t-
BuONa was much faster than that with DBU (Table 2, run 6).
The number-averaged molecular weight of the polymer
obtained at 30 s was 16,000. However, the molecular weight
decreased to 6400 when the polymerization time was pro-
longed to 24 h. Hence, the backbiting reaction might occur
from a growing alkoxide chain end to carbonate group in the
same polymer chain. The polymerization smoothly proceeded
even in more diluted solution (0.25 M) to give a polymer in
93% conversion for 30 s. In this case, however, the molecu-
lar weight of the polymer was almost constant as the poly-
merization time evolved. Thus, it was assumed that backbit-
ing reaction tends to take place more frequently in the
concentrated polymerization condition. To check the struc-
tures of the obtained polymers in detail, we carried out
MALDI-TOF MS measurements. Figure 2 shows the MALDI-
TOF MS spectra of the polymers obtained in 1.0 and 0.25 M
solutions. In the spectra of Figure 2(a), two series of signals
(l and ~) were observed. Both series had the same peak
intervals of 308 m/z corresponding to the molecular weight
of the monomer repeating unit. Judging from their signal
positions, these spectra are assigned to be a linear structure
(~) and a cyclic structure (l) formed by backbiting reac-
tion, respectively. The signals of a cyclic structure were
observed even in the polymer obtained in 0.25 M solution
[Fig. 2(b)], though the molecular weight did not decrease.
Thus, in both concentrations, the obtained polymers con-
tained cyclic structure, which were formed by backbiting
reaction. It suggests that the backbiting reaction might be in-
evitable in the polymerization of MBCG under the present
conditions.
REFERENCES AND NOTES
1 L. S. Nair, C. T. Laurencin, Prog. Polym. Sci. 2007, 32,
762–798.
2 H. Wang, J. H. Dong, K. Y. Qiu, Z. W. Gu, J. Polym. Sci. Part
A: Polym. Chem. 1998, 36, 1301–1307.
3 K. J. Zhu, R. W. Hendren, K. Jensen, C. G. Pitt, Macromole-
cules 1991, 24, 1736–1740.
4 A. C. Albertsson, M. Eklund, J. Appl. Polym. Sci. 1995, 57,
87–103.
5 F. Suriano, R. Pratt, J. P. K. Tan, N. Wiradharma, A. Nelson,
Y. Y. Yang, P. Dubois, J. L. Hedrick, Biomaterials 2010, 31,
2637–2645.
6 W. J. Carothers, F. J. V. Natta, J. Am. Chem. Soc. 1930, 52,
314–326.
7 G. Rokicki, Prog. Polym. Sci. 2000, 25, 259–342.
8 K. Soga, S. Hosoda, Y. Tazuke, S. Ikeda, J. Polym. Sci. Part
A: Polym. Chem. 1977, 15, 219–229.
9 L. Vogdanis, B. Martens, H. Uchtmann, F. Hensel, W. Heitz,
Macromol. Chem. 1990, 191, 465–472.
10 T. Takata, H. Matsuoka, T. Endo, Chem. Lett. 1991, 12,
2091–2094.
11 T. Takata, M. Kanamaru, T. Endo, Macromolecules 1996, 29,
2315–2317.
12 J. Matsuo, F. Sanda, T. Endo, J. Polym. Sci. Part A: Polym.
Chem. 1997, 35, 1375–1380.
13 M. Murayama, F. Sanda, T. Endo, Macromolecules 1998, 31,
919–923.
14 J. Matsuo, K. Aoki, F. Sanda, T. Endo, Macromolecules
1998, 31, 4432–4438.
15 J.-C. Lee, M. H. Litt, Macromolecules 2000, 33, 1618–1627.
16 F. Sanda, J. Kamatani, T. Endo, Macromolecules 2001, 34,
1564–1569.
17 O. Haba, H. Tomizuka, T. Endo, Macromolecules 2005, 38,
3562–3563.
SUMMARY
18 A. S. Khomami, T. J. Forcada, C. Wilson, D. A. R.
Sanders, N. R. Thomas, Org. Biomol. Chem. 2010, 8,
1596–1602.
We investigated on the anionic ring-opening polymerization
of MBCG in detail under various conditions. The polymeriza-
tions initiated with n-BuLi, t-BuOLi, t-BuONa, t-BuOK, and
DBU smoothly proceeded to give the corresponding poly-
19 W. M. Doane, B. S. Shasha, E. I. Stout, C. R. Russell, C. E.
Rist, Carbohydr. Res. 1967, 4, 445–451.
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