Anionic ring-opening polymerization of a five-membered cyclic urethane derived from d-glucosamine

Article abstract of DOI:10.1002/pola.29511

The anionic ring-opening polymerization of a five-membered cyclic urethane, 2-amino-4,6-O-benzylidene-2-N,3-O-carbonyl-2-deoxy-α,d-glucopyranoside (MBUG), which was prepared from naturally abundant d-glucosamine, was examined. Potassium tert-butoxide (t-B

Full text of DOI:10.1002/pola.29511

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Anionic ring-opening polymerization of a five-membered cyclic
urethane derived from D-glucosamine
Osamu Haba , Yosuke Akashika
Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
Correspondence to: O. Haba (E-mail: haba@yz.yamagata-u.ac.jp)
Received 29 July 2019; Revised 4 September 2019; accepted 6 September 2019
DOI: 10.1002/pola.29511
ABSTRACT: The anionic ring-opening polymerization of a five-
membered cyclic urethane, 2-amino-4,6-O-benzylidene-2-N,3-O-
carbonyl-2-deoxy-α,D-glucopyranoside (MBUG), which was
prepared from naturally abundant D-glucosamine, was exam-
ined. Potassium tert-butoxide (t-BuOK) was the most effective
initiator among the evaluated bases and produced polyure-
thane with the Mn of 7800 without any elimination of CO2. The
equimolar reaction of MBUG and t-BuOK in the presence of
CH3I produced N-methylated MBUG and suggested that the
initiation reaction involves proton abstraction from the NH
group. This N-methylated compound did not undergo the poly-
merization. Therefore, the mechanism of propagation in the
ROP of MBUG should involve the proton abstraction and nucle-
ophilic substitution of the resulting amide anion. © 2019 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019
KEYWORDS: anionic polymerization; ring-opening polymeriza-
tion; cyclic urethane; polyurethane; glucosamine
during the ROP was noted.13 Wooley and co-workers later
reported that the polymerization of MBCG should be controllable
using organocatalyst systems.14 On the other hand, such an ROP
did not occurred with an MBCG isomer derived from α,D-man-
nnopyranoside11 and cis-cyclohexane-1,2-diyl carbonate.12 Thus,
the unusual high polymerizability of MBCG may be caused by
the ring-strain of MBCG, whose carbonate ring connects to a
cyclic structure in a trans fashion.
INTRODUCTION Polyurethanes are generally prepared by the
polyaddition reaction of diisocyanates with diols. Höcker and
his co-workers proposed that the ring-opening polymerizations
(ROPs) of cyclic urethanes are useful alternatives for the poly-
urethane synthesis.1–3 This type of polymerization has the
potential to copolymerize with the other cyclic monomers such
as lactones, cyclic carbonates, lactams, and so on. However, the
polymerizability of the cyclic urethanes usually depends on
their ring size and initiators. Hall and Schneider mentioned
that the five-membered cyclic urethane decomposes upon
heating in the presence of sodium or sodium hydride, although
six-membered cyclic urethanes are ROP polymerizable.4 The
cationic polymerizations of six-2,3 or seven-membered1 cyclic
urethanes produce linear polyurethanes, while there are no
reports about polyurethane syntheses by the ROP of five-
membered cyclic urethane.
Crich and Vinod reported the synthesis of 2-amino-4,6-O-
benzylidene-2-N,3-O-carbonyl-2-deoxy-α,D-glucopyranoside
(MBUG) from the naturally abundant D-glucosamine.15 MBUG
can be regarded as a urethane analog of MBCG. Considering
the results of the polymerization of MBCG, MBUG seems to also
have the potential to polymerize in spite of the thermodynami-
cally unfavorable nature of the five-membered cyclic urethanes
(Scheme 1). In this study, we examined the anionic polymeri-
zations of MBUG. We will also report the structure of the reac-
tion products and the reaction mechanisms.
Similarly, the five-membered cyclic carbonate is believed to be
an unsuitable monomer to produce polycarbonates. The ROP of
such a cyclic carbonate proceeds only at a relatively higher tem-
perature and causes elimination of a significant amount of CO2
to give carbonate/ether copolymers.5–10 However, our investiga-
tion about the anionic ROP of a five-membered cyclic carbonates,
EXPERIMENTAL
Measurements
The infrared (IR) spectra were recorded using a HORIBA FT-
210 spectrometer. The 1H and 13C NMR spectra were mea-
methyl
4,6-O-benzylidene-2,3-O-carbonyl-α,D-glucopyranoside
(MBCG),2,3 its alternatives derived from α,D-galactopyranoside11
and trans-cyclohexane-1,2-diyl carobonate,12 showed that the
polymerization proceeded at 60 ꢀC without any elimination of
CO2 and produced a polycarbonate containing no polyether
repeating units, although the presence of backbiting reaction
1
sured in CDCl3 by a JEOL JNM-ECX 400 (400 MHz for H and
100 MHz for 13C) spectrometer at room temperature. The
chemical shift values were in ppm downfield from tetra-
methylsilane (0.0 ppm) and CDCl3 (77.0 ppm) used as the
internal standards for the 1H and 13C measurements,
© 2019 Wiley Periodicals, Inc.
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atmosphere. A 1.0 M solution of t-BuOK in THF (3.4 ml,
3.4 mmol) was added and the solution was stirred for 20 min.
Water (73 μl) was added to quench the reaction and volatiles
were removed under reduced pressure. The aqueous residue
was extracted by CHCl3. The organic layer was washed with
water, dried over MgSO4, and evaporated to dryness. The resi-
due was purified on a silica-gel column with CHCl3/EtOAc
(6/1, v/v) to give a white solid. Yield 0.40 g (36%). 1H-NMR
(in CDCl3, 400 MHz): δ(ppm) = 7.52–7.36 (m, 5H, Ph), 5.61
(s, 1H, CHPh), 5.00 (d, J1-2 = 2.7 Hz, 1H, H-1), 4.64 (dd,
J3-2 = 11.3 Hz, J3-4 = 10.0 Hz, 1H, H-3), 4.30 (dd, 2J = 9.3 Hz,
O
n
HN
O
NH
O
O
O
MeO
O
MeO
O
O
Ph
Ph
O
O
MBUG
poly(MBUG)
SCHEME 1 ROP of MBUG.
respectively. The assignments were made by using 1H-1H
COSY and 13C-1H HETCOR spectra. The number-average (Mn)
and weight-average (Mw) molecular weights were estimated
by size-exclusion chromatography (SEC) using a Tosoh DP-
8020 GPC and polystyrene gel columns (Tosoh TSK gels
G2500H, G3000H, G4000H, and GMH) in THF using a calibra-
tion curve of polystyrene standards. The thermal gravimetric
analyses (TGA) and differential scanning calorimetry (DSC)
results were recorded by a Seiko SSC/5200 (DSC 220) instru-
ment at the heating/cooling rate of 10 ꢀC/min under a nitro-
gen atmosphere.
J6eq-5
= 3.9 Hz, 1H, H-6eq), 4.04 (dd, J4-3 = 10.0 Hz,
J4-5 = 8.2 Hz, 1H, H-4), 3.90–3.85 (m, 2H, H-5 and H-6ax), 3.52
(s, 3H, OCH3) 3.43 (dd, J2-1 = 2.7 Hz, J2-3 = 11.3 Hz, 1H, H-2)
2.85 (s, 3H, N-CH3). 13C-NMR (in CDCl3, 100 MHz):
δ
(ppm) = 158.9 (C O), 136.7, 129.3, 128.4, 126.2 (Ar), 101.5
(PhCH), 97.2 (C-1), 80.2 (C-4), 73.6 (C-3), 68.7 (C-6), 65.8
27
(C-5), 63.8 (C-2), 55.9 (OCH3), 30.3 (N-CH3). [α]D = +68.7ꢀ
(c 1.0, CHCl3).
Methyl 4,6-O-Benzylidene-2-(Methoxycarbonylamino)-
3-O-(N-Propylcarbamoyl)- 2-Deoxy-α,D-
Glucopyranoside (3)
Materials
To
a solution of methyl 2-amino-4,6-O-benzylidene-3-O-(N-
propylcarbamoyl)-2-deoxy-α,D-glucopyranoside15 (1.0 g, 3.0 mmol)
in THF (20 ml), a mixture of n-propylisocyanate (0.30 g, 3.5 mmol)
and dibutyltin dilaurate (2.7 g, 4.3 mmol) was added. The resulting
solution was stirred at 50 ꢀC for 24 h and then concentrated under
reduced pressure. The residue was dissolved in ethyl acetate,
washed with water, and dried over anhydrous MgSO4. The solvent
was removed under reduced pressure to give a brown solid, which
was purified on a silica gel column with CHCl3/EtOAc = 3/1 (v/v).
The resulting white solid was recrystallized from CHCl3/n-hexane.
MBUG was prepared from D-glucosamine hydrochloride
according to the procedure reported by Critch and Vinod.15
Methyl 4,6-O-benzylidene-α,D-glucopyranoside (4)16 and methyl
4,6-O-benzylidene-2,3-bis-O-ethoxycarbonyl-α,D-glucopyranoside
(5)17 were prepared by the reported procedures. N,N-
Dimethylformamide (DMF, Kanto Chemicals) was distilled from
CaH2 under reduced pressure. Potassium tert-butoxide (t-BuOK,
1.0 M in THF; Sigma-Aldrich), lithium tert-butoxide (t-BuOLi,
1.0 M in THF; Sigma-Aldrich), n-butyllithium, (1.6 M solution in
n-hexane, Kanto Chemicals), t-butyllithium (1.6 M solution in
n-pentane, Kanto chemicals), and lithium diisopropylamide
(1.0 M solution in n-hexane/THF, Kanto Chemicals) were pur-
chased and used as received. 1,9-Diazabicyclo[4.3.0]undec-7-ene
(DBU) was obtained from Kanto Chemicals and distilled from
CaH2 under reduced pressure. All other chemicals were commer-
cially available and used without further purification.
1
Yield 0.45 g (36%). H-NMR(in CDCl3, 400 MHz): δ(ppm) = 7.46–
7.25 (m, 5H, Ph), 5.50 (s, 1H, CHPh), 5.30 (d, JNH-2 = 9.6 Hz, 1H,
NH), 5.15 (t, J3-2 = J3-4 = 10.1 Hz, 1H, H-3), 4.85 (t, J = 5.0 Hz, 1H,
Pr-NH), 4.72 (d, J1-2 = 3.2 Hz, 1H, H-1), 4.25 (dd, J6eq-5 = 4.9 Hz, 2J =
10.1 Hz, 1H, H-6eq), 3.94 (dt, J2-1 = 3.2 Hz, J2-3 = 10.1 Hz,
J2-NH = 9.6 Hz, 1H, H-2), 3.84 (dd, J5-6eq = 4.6 Hz, J5-6ax = 9.62 Hz,
1H, H-5) 3.76 (t, J6ax-5 = 9.6 Hz, J6ax-6eq = 10.1 Hz, 1H, H-6ax),
3.70–3.60 (m, 4H, H-4, COOCH3.), 3.37 (s, 3H, OCH3), 3.10–2.94 (m,
2H, NHCH2), 1.50–1.35 (m, 2H, NHCH2CH2), 0.80 (t, J = 7.3 Hz, 3H,
NHCH2CH2CH3). 13C-NMR (in CDCl3, 100 MHz): δ (ppm) = 156.9
(C O), 156.3 (C O), 137.1, 129.1, 128.2, 126.4 (Ar), 101.7 (PhCH),
Polymerization of MBUG
A typical procedure was as follows: In a test tube equipped
with a three-way stopcock and filled with nitrogen, MBUG
(0.41 g, 1.3 mmol) was dissolved in DMF (0.8 ml, 3.0 molÁl−1).
The solution was cooled to 0 ꢀC, and then an initiator, t-BuOK
(1.0 M solution in THF, 0.10 ml, 0.10 mmol), was added via a
syringe. The solution was stirred at this temperature for 12 h.
Acetic acid (12 μl) was added to quench the reaction, and the
solution was poured into an ice-cold methanol (50 ml). The
white precipitate was collected by filtration, reprecipitated
from CHCl3-methanol and dried in vacuo at room temperature.
Yield 0.30 g (74%). Mn = 7800 (SEC).
TABLE 1 Anionic ROP of MBUG at 0 ꢀC for 12 ha
Yieldb (%)
Mnc × 10−3
Mw/Mnc
[α]D
d
Runs
Initiator
1
2
3
4
5
t-BuLi
n-BuLi
LDA
60
45
27
74
56
1.9
1.6
1.4
7.8
0.4
1.6
1.4
1.3
1.2
1.0
+67
+68
+70
+42
+85
t-BuOK
DBU
Methyl 2-(N-Methylamino)-4,6-O-Benzylidene-2-N,3-O-
Carbonyl-2-Deoxy-α,D-Glucopyranoside (6)
a [MBUG]0 = 3.0 mol•l−1, [MBUG]0/[I]0 = 25, solvent: DMF.
b Methanol-insoluble fraction.
c Estimated by SEC eluted with DMF based on polystyrene calibration.
To a solution of MBUG (1.0 g, 3.4 mmol) in THF (50 ml), CH3I
(2.0 mol, 33 mmol) was added at 0 ꢀC under a nitrogen
d Measured in DMF at 25−30 ꢀC, c = 1.0.
2
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The results are listed in Table 1. With DBU (Run 5), the
methanol-insoluble product was obtained in 56% yield, but
this product was only a low-molecular-weight material. When
t-BuLi, n-BuLi, or LDA was used (Runs 1–3), the polymeriza-
tion proceeded heterogeneously, and the yields and the num-
ber averaged molecular weights (Mn’s) were low. The
polymerization with t-BuOK (Run 4) proceeded homoge-
neously and produced a polymer having relatively large Mn of
7400 in 74% yield. When t-BuOLi was used, the polymeriza-
tion solution become heterogeneous and the yield of the prod-
uct was also low (ca. 30%). So the heterogeneous
polymerization in Runs 1–3 might be due to the poor solubil-
ity of lithium salt of some intermediate anions. Thus, among
the tested initiators, t-BuOK was the most suitable for the
polymerization of MBUG.
[M+Na]+
[M+H]+
2000
3000
4000
Mass (m/z)
5000
6000
FIGURE 1 MALDI-TOF MS spectrum of poly(MBUG) (Run 4).
HABA was used as the matrix.
Figure 1 shows MALDI-TOF MS spectra of the polymers
obtained with t-BuOK (Run 4). The spectrum shows two sets
of peaks. Because the difference of m/z values in one set was
almost 22, each peak should be attributed to [M + H]+ and [M
+ Na]+. The m/z value differed by 308–310 Da intervals
between each set. This interval agreed with the molecular
weight of MBUG (307.1). If the elimination of CO2 occurs dur-
ing the polymerization, the molecular weight of the repeating
units should be 263.3. Because such an interval was not
observed, the elimination of CO2 did not occur during the
polymerization, and the polymer should have carbonyl groups
in a quantitative amount.
99.5 (C-1), 79.5 (C-4), 70.6 (C-3), 68.9 (C-6), 62.9 (C-5), 55.4 (C-2),
55.0 (OCH3), 52.3 (COOCH3), 42.8 (NHCH2), 23.0 (NHCH2CH2), 11.1
27
(NHCH2CH2CH3). [α]D = +20ꢀ (c 1.0, CHCl3).
RESULTS AND DISCUSSION
Anionic Polymerization of MBUG
The anionic polymerization of MBUG was carried out at 0 ꢀC
for 12 h in DMF. As initiators, t-BuLi, n-BuLi, LDA, t-BuOK,
and DBU were used. The polymerization mixture was poured
into methanol, and the precipitate was collected by filtration.
For the polymer without any decarbonylation, head-to-tail ure-
thane (HT) and head-to-head carbonate-urea (HH) structures
O
O
NH
HN
HN
O
NH
O
MeO
O
O
OMe
O
MeO
and/or
MeO
O
O
O
O
O
Ph
O
O
HT
HH
MBUG
MeO
O
MeO
O
HN Pr
O
NH OH
NH
O
i
MeO
O
O
for HT
for HH
MeO
O
Ph
Ph
O
Ph
O
O
3
2
EtO
O
OEt
HO
MeO
OH
O
O
ii
O
O
O
O
MeO
O
Ph
O
O
4
5
SCHEME 2 Possible repeating units for poly(MBUG) and syntheses of the corresponding model compounds. Conditions: (i) Pr-NCO,
SnBu2(lau)2, THF, 50 ꢀC, 24 h, (ii) EtOCOCl, pyridine, THF, rt.
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MeO
O
MeO
7
HN Pr
9'
EtO
O
MeO
7
OEt
9'
100
80
60
40
20
0
9
9
NH
O
O
O
3
5
3
O
O
2
4
2
4
O 8
O 8
1
1
O
Ph
O 5
Ph
O
O
6
6
3
5
CDCl
3
Run 4
0
100
200
T (°C)
300
400
500
5
5
7 2
3
6
MeOCO
FIGURE 3 DSC trace in the second heating/cooling scan and
4
3
9,9'
TGA trace of poly(MBUG) (Run4). Each trace was recorded at
ΔT = 10 ꢀC min−1
.
4,6-O-benzylidene-α,D-glucopyranoside (4) and ethyl chloro-
carbonate. Figure 2 shows the 13C NMR spectra of the polymer
obtained in Run 4 and the model compounds. Only one car-
bonyl peak at 155.9 ppm was observed in the spectrum of the
poly(MBUG), which corresponded to those of 3 at 156.9 and
156.3 ppm rather than those of 5 at 154.0 and 154.1 ppm.
The carbon at the 3-position of poly(MBUG) (71.4 ppm) also
Et
5
4
2 3
7
6
9,9'
160 155 150 80 75 70 65 60 55 50
agreed with that of
3 (70.6 ppm) rather than that of
/ppm
5 (72.7 ppm). In addition, the previously reported polycarbon-
ate, poly(MBCG), showed the carbonyl carbon and C-3 at
153.3 and 72.9 ppm, respectively, which could not be
observed in the spectrum of poly(MBUG). The poly(MBUG)
obtained in Run 4, therefore, should contain no carbonate
units, and mainly consists of the head-to-tail urethane repeat-
ing units.
FIGURE 2 13C NMR spectra of poly(MBUG) (Run 4), and model
compounds 3 and 5, measured in CDCl3 at rt.
might be possible (Scheme 1). In order to further confirm the
polymer structure, two model compounds 3 and 5 for the HT
and HH units, respectively, were prepared (Scheme 2). The for-
mer was prepared from a reaction of methyl 2-methoxy-
carbonylamino-4,6-O-benzylidene-2-deoxy-α,D-glucopyranoside
(2), which is an intermediate in the synthesis of MBUG, with
n-propyl isocyanate in the presence of dibutyltin dilaulate. The
latter was previously prepared17 from the reaction of methyl
Table 2 shows the results of the polymerization of MBUG by
varying the polymerization conditions such as the tempera-
ture and time. For the temperature (Runs 6, 4, 11, 12), the
polymerization at 0 ꢀC gave the highest Mn (7800) in the best
yield (74%). At the lower temperature (Run 6), the
TABLE 2 Anionic ROP of MBUG with t-BuOKa
Yieldb
%
Mnc × 10−3
Mw/Mnc
[α]D
d
Runs
Temp. (ꢀC)
Time (h)
6
–30
0
12
1
60
48
72
75
70
74
33
13
5.0
3.8
4.9
5.1
5.3
7.8
3.4
2.1
1.2
1.1
1.2
1.2
1.2
1.2
1.2
1.2
+44
+45
+43
+44
+41
+42
+27
+33
7
8
0
3
9
0
6
10
4
0
9
0
12
12
12
11
12
30
60
a [MBUG]0 = 3.0 mol•l−1, [MBUG]0/[t-BuOK]0 = 25, solvent: DMF.
c Estimated by SEC eluted with DMF based on polystyrene calibration.
b Methanol-insoluble fraction.
d Measured in DMF at 25–30 ꢀC, c = 1.0.
4
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O
O
O
O
O
O
NH
O
N
HN
O
O
O
OMe
route I
route II
O
O
O
O
OMe
O
O
OMe
Ph
Ph
Ph
O
O
MBUG
SCHEME 3 Possible initiation mechanism for ROP of MBUG.
polymerization was too slow and considerable amounts of the
unreacted monomer remained. The higher temperature (Runs
11–12) caused a decrease in both the Mn and polymer yield,
which might be the result of polymer decomposition. By vary-
ing the polymerization time at 0 ꢀC (Runs 7–10, 4), the Mn
and yield increased within 3 h and remained constant around
Mn of 5000 and 70% yield. Thus, the polymerization at 0 ꢀC
was almost completed within 3 h.
The absence of t-butyl groups therefore denies route I. In
addition, this equimolar reaction between MBUG and t-BuOK
was carried out in the presence of 1 equivalent of CH3I as an
anion trapper. This reaction gave the N-methyl derivative 6 in
36% yield after purification by column chromatography. This
result also suggests that the initiation reaction requires pro-
ton abstraction from the NH group to form the amide anion.
The same polymerization condition as Run 4 was employed
for 6, but no methanol-insoluble product was obtained, and
the unreacted 6 was quantitatively recovered from the reac-
tion mixture. Next, the polymerization of 6 was examined in
the presence of 20 mol % t-BuOK and MBUG. If the proton
abstraction is required only in the initiation reaction, the initi-
ation should occur between t-BuOK and MBUG and then the
propagation of 6 should proceed. The methanol-insoluble frac-
tion from the resulting mixture, however, consisted only of
poly(MBUG), and the unreacted 6 was quantitatively recov-
ered (Scheme 4). These results should indicate that the poly-
merization of MBUG requires proton abstraction during both
the initiation and propagation reactions.
Figure 3 shows the TGA and DSC traces of the resulting
poly(MBUG) (Run 4). The 5% weight loss temperature (Td5
)
was 264 ꢀC. The DSC trace did not show any significant phase
and glass transitions within the range of −50 to 210 ꢀC. Thus,
poly(MBUG) should be an amorphous polymer with a rela-
tively high heat resistance.
Polymerization Mechanism
For the initiation reaction of anionic polymerization of the
cyclic urethanes, the two mechanisms shown in Scheme 3
might be possible. One is a nucleophilic addition-elimination
reaction of nucleophiles toward the urethane carbonyl group
(route I). Another involves an abstraction of the NH proton
(route II), which is generally proposed for the anionic ROPs of
lactams. In order to confirm which route the polymerization
of MBUG initiates, an equimolar reaction of MBUG with
t-BuOK was carried out. Figure 4 shows the 13C NMR spec-
trum of the obtained chromatographically pure product. The
product showed no t-butyl groups around 30 ppm in the 13C
NMR spectrum. If the reaction proceeds according to route I,
the product should be the 2-N-t-butoxycarbonyl compound.
Based on the aforementioned results, the polymerization
mechanism of MBUG was considered as shown in Scheme 5.
Table 3 lists the pKa values of the related compounds in
DMSO that appeared in the literature. Because the cyclic ure-
thane (pKa = 20.8) could be regarded as a stronger acid than
t-butanol (pKa = 32.2), the initiator, t-BuOK, should react with
MBUG to form the cyclic amide ion (I). The other MBUG
undergoes nucleophilic addition by I to produce a dimer anion
(II) in which one MBUG unit retains a cyclic structure and the
other has the ring-opened units. In this step, C O scission
(to give IIa) and C N scission (to give IIb) might be possible.
CDCl3
~
O
H3C
N
O
t-BuOK (4 mol%),
No conversion
C=O
H3CO
O
O
C, 12 h
O
6
t-BuOK (20 mol%),
Poly(MBUG) + 6
6 + MBUG (20 mol%)
C, 12 h
200
150
100
50
0
FIGURE 4 13C NMR spectrum of the product from the equimolar
reaction between MBUG and t-BuOK.
SCHEME 4 Reactions of 6 with t-BuOK in the presence/absence
of MBUG
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O
HN
O
O
O
O
O
O
O
N
O
O
HN
HN
O
N
O
N
O
O
HN
O
>
O
+
O
I
O
O
O
O
O
II b
O
II a
HN
O
O
O
O
O
O
O
O
HO HN
O
N
O
N
O
N
O
O
HO HN
O
N
+
O
O
O
O
IV
III
O
HN
O
O
O
O
O
O
O
O
O
N
O
HO HN
O
NH
O
O
N
O
NH
O
O
O
HO HN
O
N
n
+
O
O
O
V
SCHEME 5 Plausible mechanism for the anionic polymerization of MBUG with t-BuOK. The structure of MBUG is partly omitted
except for pyranose and the urethane rings.
Because alcohols (pKa ~ 32) are stronger acids than amines
(pKa ~ 44), the alkoxide anion IIa should be preferable. The
anion IIa also abstracted the proton from the other MBUG to
produce again the cyclic anion I, which attacks the carbonyl
group of the dimer III to give a ring-opened urethane anion
IV. Because its pKa should be estimated to be 24 and IV
should be regarded as a weaker base than alkoxides, this step
should favor the C N scission. Because the acidity of the
cyclic urethane is considered to be higher than the linear one,
the anion IV abstracts the proton from the monomer to pro-
duce the cyclic anion I again. The propagation reaction should
occur between the cyclic terminal group and I, and the
resulting polyurethane should have one urea and one cyclic
urethane unit at the initiation and propagation terminals,
respectively. The m/z values of [M + H]+ peaks in Figure 1
almost equal to simple multiples of 307.1 (molar mass of
MBUG), which should support these terminal structure.
TABLE 3 pKa values in DMSO of the related compounds
Compound
pKa
References
18
20.8
Summary
Anionic polymerization of the five-membered cyclic urethane
derived from D-glucosamine, MBUG, was examined. The poly-
merization with t-BuOK at 0 ꢀC formed polyurethane having a
relatively higher Mn without any decarbonylation in spite of
the thermodynamically unfavorable nature known for the
five-membered cyclic urethane. The mechanistic investigation
indicates that the polymerization proceeds by a proton
abstraction mechanism. The resulting polyurethane did not
decompose below 260 ꢀC and showed no glass-transition tem-
perature below 210 ꢀC. Because MBUG could be prepared
24.2
18
32.2
44.0
19
20
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ORIGINAL ARTICLE
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