Cationic ring-opening polymerization of optically active N-substituted cyclic thiourethanes

Article abstract of DOI:10.1021/ma049114b

N-substituted cyclic thiourethanes [4(S)-(methoxycarbonyl)-N-benzyl-1,3- oxazolidine-2-thione (BnS_L), 4(S)-(methoxycarbonyl)-N-benzoyl-1,3- oxazolidine-2-thione (BzS_L), and 4(S)-(methoxycarbonyl)-N-acetyl-1,3- oxazolidine-2-thione (A

Full text of DOI:10.1021/ma049114b

7538
Macromolecules 2004, 37, 7538-7542
Cationic Ring-Opening Polymerization of Optically Active N-Substituted
Cyclic Thiourethanes
Atsu sh i Na ga i, Bu n go Och ia i, a n d Ta k esh i En d o*
Department of Polymer Science and Engineering, Faculty of Engineering,
Yamagata University 4-3-16 J onan, Yonezawa, Yamagata 992-8510, J apan
Received May 6, 2004; Revised Manuscript Received J uly 25, 2004
ABSTRACT: N-substituted cyclic thiourethanes [4(S)-(methoxycarbonyl)-N-benzyl-1,3-oxazolidine-2-thione
(Bn SL), 4(S)-(methoxycarbonyl)-N-benzoyl-1,3-oxazolidine-2-thione (BzSL), and 4(S)-(methoxycarbonyl)-
N-acetyl-1,3-oxazolidine-2-thione (AcSL)] were synthesized from L-serine methyl ester hydrochloride and
were polymerized by using methyl trifluoromethanesulfonate to obtain the corresponding well-defined
polythiourethanes. The molecular weight of the polymers can be controlled by the ratio of the monomers
to the initiator and the molecular weight distributions are narrow (Mw/Mn < 1.15) similar to the previously
reported polymerization of a cyclic thiourethane [4(S)-(methoxycarbonyl)-1,3-oxazolidine-2-thione (SL)].
The polymerization rates are on the order of SL > Bn SL > BzSL > AcSL, which agrees well with the
nucleophilicity of the thiocarbonyl moieties of the monomers (SL, Bn SL BzSL, and AcSL). The Cotton
effects in the CD spectra of the polymers from the N-substituted monomers exhibit almost inverse shape
with that of poly(SL) and the specific rotations’ signs also inversed, suggesting the idea that poly(SL) and
N-substituted polymers take different high order structures.
In tr od u ction
ture, and the substituent will affect the polymerization
behavior. Herein, this paper reports the cationic ring-
opening polymerization behavior of N-substituted cyclic
thiourethanes in detail and the chiroptical properties
of the obtained polymers.
Sulfur-containing polymers have been given attention
due to their optical and thermal properties.1 Although
most of these polymers such as polythioester, polysul-
fide, polysulfone, and polythiocarbonate have been
synthesized by polyaddition or polycondensation of
bifunctional monomers,2 several recent works have
demonstrated successful ring-opening polymerizations
of cyclic sulfur compounds giving these polymers.3-7 For
example, we have reported cationic ring-opening po-
lymerization of dithiocarbonates and six- or seven-
membered monothiocarbonates affording polydithiocar-
bonates and polythiocarbonates8 and found that living
polymerization can be accomplished by aid from a stable
carbenium cation or neighboring group participation.9
This technique provided us versatility in designing
polymer architecture and solution to overcome an obvi-
ous limitation of traditional methods, which are insuf-
ficient to give polymers with controlled molecular weight
and complete head-to-tail structure. Further polythio-
urethane with head-to-tail structure has been also
achieved by cationic ring-opening polymerization of
cyclic thiourethane.10 More recently, we have reported
that living cationic ring-opening polymerization of an
optically active cyclic thiourethane (SL) derived from
L-serine by methyl trifluoromethanesulfonate (TfOMe)
affords a chiral polythiourethane with narrow molecular
weight distribution (Mw/Mn < 1.14) in quantitative
yield11 and that the living nature is not spoiled under
air and moisture by using a water-resistant cationic
initiator.12 Furthermore, the obtained chiral polythio-
urethane is supposed to take a secondary structure
based on intramolecular hydrogen bonds.13 If the hy-
drogen atoms in poly(SL) are replaced by substituents
without electron-accepting moieties, the resulting poly-
mers are expected to take a different secondary struc-
Exp er im en ta l P a r t
Mea su r em en t . 1H (270 MHz) and 13C NMR (67.5 MHz)
spectra were recorded on a J EOL J NM-LA-270 spectrometer,
using tetramethylsilane (TMS) as an internal standard in
CDCl3 and DMSO-d6. FT-IR spectra were obtained with a
J ASCO FT/IR-210 spectrometer. Specific rotations ([R]D) were
measured on a J ASCO DIP-1000 digital polarimeter equipped
a sodium lamp as a light source. Circular dichroism (CD)
spectra were measured on a J ASCO J -720 spectropolarimeter.
Number-average molecular weight (Mn) and molecular weight
distribution (Mw/Mn) were estimated by size-exclusion chro-
matography (SEC) using a Tosoh HPLC HLC-8020 system
equipped with four consecutive polystyrene gel columns [TSK-
gels (bead size, exclusion limited molecular weight); RM (13
µm, >1 × 107), R4000H (10 µm, >1 × 106), R3000H (7 µm, >1
× 105), and R2500H (7 µm, >1 × 104)]; and refractive index
and ultraviolet detectors at 40 °C. The system was operated
at a flow rate of 1.0 mL/min, using N,N-dimethylformamide
(DMF) solution (5.0 mM lithium bromide and 5.0 mM phos-
phoric acid) as an eluent. Polystyrene standards were em-
ployed for calibration. All monomer conversion was determined
from the 1H NMR spectroscopy.
Ma ter ia ls. 4(S)-(Methoxycarbonyl)-1,3-oxazolidine-2-thione
(SL)11 and (S)-N-benzylserine methyl ester14 were synthesized
according to the previously reported method. Methyl trifluo-
romethanesulfonate (TfOMe) (Aldrich Chemical, Co., >99%),
triethylamine (Tokyo Kasei Kogyo, Co., >99%), and dichlo-
romethane (CH2Cl2) were distilled over CaH2 before use.
Tetrahydrofuran (THF) was distilled over sodium. Other
reagents were used as received.
4(S)-(Met h oxyca r b on yl)-N-b en zyl-1,3-oxa zolid in e-2-
th ion e (Bn SL). Thiophosgene (23.8 g, 207 mmol) in dry THF
(200 mL) was slowly added to a solution of N-benzyl-L-serine
(43.4 g, 207 mmol) and triethylamine (41.9 g, 414 mmol) in
dry THF (600 mL) at 60 °C under nitrogen. The mixture was
stirred for 3 h and was stirred at room temperature for 12 h.
Triethylamine hydrochloride was removed by filtration and
* Correspondence should be addressed to this author. Tele-
phone: +81-238-26-3090. Fax: +81-238-26-3090. E-mail: tendo@yz.
yamagata-u.ac.jp.
10.1021/ma049114b CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/31/2004

Macromolecules, Vol. 37, No. 20, 2004
Cationic Ring-Opening Polymerization 7539
the solvent was evaporated in vacuo. The residue was purified
by silica gel column chromatography eluted with ethyl acetate/
n-hexane (1/1 ) v/v). Recrystallization from a mixed solvent
[THF/n-hexane (2/1 ) v/v)] gave Bn SL (40.1 g, 77%) as a white
powder. [R]25D ) 35.0 ° (c ) 0.1 g/dL, in CH2Cl2). Mp ) 101.3-
(-CH2-C5H6), 127.87, 128.13, 128.45, 135.86 (-C6H5), 168.08
(-SCONH-), 169.12 (-COOCH3) ppm. IR (KBr): 3456, 1743-
(-COOCH3), 1651 (-SCONH-), 1404, 1311, 1180, 1072, 987,
710 cm-1
.
P oly(BzSL). Yield: quantitative, light green solid. Mn
)
1
8500, Mw/Mn ) 1.11. 1H NMR (DMSO-d6): δ ) 2.21 (initiating
end, S-CH3), 2.88-3.79 (5H, -CH2- and -OCH3), 4.73-5.06
(1H, -CH<), 7.16-8.04 (5H, -C6H5) ppm. 13C NMR (DMSO-
d6): δ ) 30.7 (-CH2-), 52.9 (-OCH3), 59.6 (-CH<), 128.60,
129.02, 132.95, 133.88 (-C6H5), 168.42 (-SCONH-), 171.28
(-NHCO-C6H5), 172.12 (-COOCH3) ppm. IR (KBr): 3394,
1751 (-COOCH3), 1702 (-NHCO-C6H5), 1658 (-SCONH-),
101.8 °C. H NMR (CD2Cl2): δ ) 3.70 (s, 3H, -OCH3), 4.32-
4.37 (m, 1H, -CH2-), 4.54-4.59 (3H, -CH<, -CH2-, and
-CH2-C6H5), 5.31-5.42 (m, 1H, -CH2-C6H5), 7.33-7.38 (5H,
-C6H5) ppm. 13C NMR (CD2Cl2): δ ) 51.47 (-CH2-C6H5),
53.06 (-OCH3), 59.90 (-CH<), 65.55 (-CH2-), 128.98, 129.04,
129.48, 135.09 (-C6H5), 169.55 (-COOCH3), 189.39 (-OCSNH-)
ppm. IR (KBr): 1743 (-COOCH3), 1481 (CdS), 1450, 1357,
1304, 1211, 971, 701 cm-1. Anal. Calcd for C12H13NO3S: C,
57.35; H, 5.21; N, 5.57; S, 12.76. Found: C, 57.54; H, 5.19; N,
5.64; S, 12.72.
1296, 1203, 1126, 694 cm-1
.
P oly(AcSL). Yield: quantitative, colorless solid. Mn ) 6600,
1
Mw/Mn ) 1.09. H NMR (DMSO-d6): δ ) 2.21 (initiating end,
4(S)-(Meth oxyca r bon yl)-N-ben zoyl-1,3-oxa zolid in e-2-
th ion e (BzSL). Benzoyl chloride (14.2 g, 112 mmol) in dry CH2-
Cl2 was slowly added to a solution of SL (15.0 g, 93.0 mmol)
and pyridine (9.6 g, 121 mmol) at 0 °C under nitrogen. The
mixture was allowed to reach room temperature, and then
water was added with stirring. The organic phase was dried
over MgSO4, and the solvent was evaporated under reduced
pressure. The residue was purified by silica gel column
chromatography eluted with ethyl acetate/n-hexane (1/1 ) v/v).
Recrystallization from a mixed solvent [ethyl acetate/n-hexane
S-CH3), 2.18-2.44 (3H, -CH3), 2.74-3.81 (5H, -CH2- and
-OCH3), 4.72-5.10 (1H, -CH<) ppm. 13C NMR (DMSO-d6):
δ ) 24.5 (-COCH3), 29.90 (-CH2-), 52.76 (-OCH3), 59.46
(-CH<), 170.00 (-SCONH-), 171.50 (-NHCO-CH3), 173.34
(-COOCH3) ppm. IR (KBr): 3370, 1751 (-COOCH3), 1703(-
NHCO-CH3), 1658 (-SCONH-), 1373, 1250, 1203, 1003 cm-1
.
Meth yla tion of SL d er iva tives w ith TfOMe. A typical
procedure is shown as follows. A solution of SL (0.15 g, 0.77
mmol) in CD2Cl2 (0.8 mL) was placed in an NMR tube under
nitrogen atmosphere. The tube was sealed after the addition
of TfOMe (93 µL, 0.85 mmol), and the mixture was stirred for
1.0 min at room temperature. The iminothiocarbonate triflate
salt (Me-SL) from SL and TfOMe was characterized by 1H
(2/1 ) v/v)] gave BzSL (23.4 g, 95%) as a colorless solid. [R]25
D
) -28.9 ° (c ) 0.1 g/dL, in CH2Cl2). Mp ) 87.2-88.0 °C. 1H
NMR (CD2Cl2): δ ) 3.76 (s, 3H, -OCH3), 4.56-4.60 (m, 1H,
-CH2-), 4.77-4.81 (m, 1H, -CH<), 5.21-5.26 (m, 1H, -CH2-
), 7.40-7.74 (5H, -C6H5) ppm. 13C NMR (CD2Cl2): δ ) 53.98
(-OCH3), 60.81 (-CH<), 70.05 (-CH2-), 128.72, 130.13,
133.45, 133.59 (-C6H5), 168.97 (-COOCH3), 170.98 (-NHCO-
C6H5), 186.51 (-OCSNH-) ppm. IR (KBr): 1751 (-COOCH3),
1682 (-NHCO-C6H5), 1442 (-OCSNH-), 1373, 1311, 1250,
1
NMR, 13C NMR, and IR spectroscopy. H NMR (CD2Cl2): δ )
2.76 (s, 3H, -SCH3), 3.85 (s, 3H, -OCH3), 5.20-5.39 (3H,
>CH- and -CH2-), 11.92 (broad s, 1H, dHN+) ppm. 13C NMR
(CD2Cl2): δ ) 14.53 (-SCH3), 54.23 (-OCH3), 60.07 (>CH-),
77.75 (-CH2-), 167.82 (-COOCH3), 183.96 (-CdHN+) ppm.
IR (CD2Cl2): 2962, 1751 (-COOCH3), 1581 (-CdHN+), 1481,
1219, 1188, 964, 910, 733, 694 cm-1. Anal. Calcd for C12H11
-
1442, 1357, 1281, 1241, 1165, 1034, 957, 918, 640 cm-1
.
Me-Bn SL. 1H NMR (CD2Cl2): δ ) 2.79 (s, 3H, -SCH3), 3.67
(s, 3H, -OCH3), 4.89 (s, 2H, -CH2-C6H5), 5.00-5.45 (3H,
-CH< and -CH2-), 7.41 (5H, -C6H5) ppm. 13C NMR (CD2-
Cl2): δ ) 15.09 (-SCH3), 52.51 (-CH2-C6H5), 54.07 (-OCH3),
63.32 (>CH-), 77.29 (-CH2-), 129.96, 130.08, 130.39, 130.73
(-C6H5), 167.35 (-COOCH3), 183.03 (-CdHN+) ppm. IR (CD2-
Cl2): 3502, 3039, 2963, 1751 (-COOCH3), 1566 (-CdHN+),
NO4S: C, 54.26; H, 4.18; N, 5.28; S, 12.09. Found: C, 54.21;
H, 4.11; N, 5.28; S, 11.90.
4(S)-(Met h oxyca r b on yl)-N-a cet yl-1,3-oxa zolid in e-2-
th ion e (AcSL). The same procedure was followed as described
for BzSL using acetyl chloride (8.71 g, 111 mmol). Recrystal-
lization from a mixed solvent [ethyl acetate/n-hexane (2/1 )
v/v) gave AcSL (17.3 g, 92%) as a colorless solid. [R]25D ) -29.0
° (c ) 0.1 g/dL, in CH2Cl2). Mp ) 68.0-68.4 °C. 1H NMR (CH2-
Cl2): δ ) 2.81 (s, 3H, -COCH3), 3.79 (s, 3H, -OCH3), 4.53
(dd, J ) 4.05 and 5.97 Hz, 1H, -CH2-), 4.64 (m, 1H, -CH<),
5.15 (dd, J ) 4.05 and 5.40 Hz, 1H, -CH2-) ppm. 13C NMR
(CD2Cl2): δ ) 25.97 (-COCH3), 53.64 (-OCH3), 60.00 (-CH<),
69.61 (-CH2-), 169.26 (-COOCH3), 171.68 (-NHCOCH3),
186.26 (-OCSNH-) ppm. IR (KBr): 1758 (-COOCH3), 1712
(-NHCOCH3), 1419 (-OCSNH-), 1373, 1311, 1227, 1180,
1041, 980, 957 cm-1. Anal. Calcd for C7H9NO4S: C, 41.37; H,
4.46; N, 6.89; S, 15.78. Found: C, 41.24; H, 4.46; N, 6.88; S,
15.78.
1435, 1412, 1265, 1157, 1034, 964, 926, 710, 640, 517 cm-1
.
Me-BzSL. 1H NMR (CD2Cl2): δ ) 2.74 (s, 3H, -SCH3), 3.67
(s, 3H, -OCH3), 5.43-5.46 (m, 1H, -CH2-), 5.73-5.87 (3H,
-CH< and -CH2-), 7.53-7.90 (5H, -C6H5) ppm. 13C NMR
(CD2Cl2): δ ) 16.17 (-SCH3), 54.05 (-OCH3), 62.94 (>CH-),
79.40 (-CH2-), 129.52, 130.07, 130.68, 135.16 (-C6H5), 167.10
(-COOCH3), 167.52 (-NHCO-C6H5), 189.79 (-CdHN+) ppm.
IR (CD2Cl2): 3070, 1751 (-COOCH3), 1658 (-NHCO-C6H5),
1604 (-CdHN+), 1543, 1473, 1442, 1381, 1288, 1234, 1165,
1026, 972, 895, 717, 640, 517 cm-1
.
Me-AcSL. 1H NMR (CD2Cl2): δ ) 2.49 (s, 3H, -SCH3), 2.73
(-CH3), 3.91 (s, 3H, -OCH3), 5.50 (dd, J ) 3.78 and 5.13 Hz,
1H, -CH2-), 5.63 (m, 1H, -CH<), 5.84 (dd, J ) 4.05 and 6.21
Hz, 1H, -CH2-) ppm. 13C NMR (CD2Cl2): δ ) 15.89 (-SCH3),
23.67 (-CH3), 54.87 (-OCH3), 61.64 (>CH-), 80.14 (-CH2-
), 167.66 (-COOCH3), 170.11 (-NHCO-CH3), 188.98 (-Cd
HN+) ppm. IR (CD2Cl2): 3032, 2962, 1751 (-COOCH3), 1713
(-CdHN+), 1442 (-NHCO-CH3), 1389, 1281, 1234, 1165,
Ca t ion ic P olym er iza t ion of SL Der iva t ives. A typical
procedure is shown as follows. Dry CH2Cl2 (6.0 mL) and 3.04
mol % of TfOMe were introduced to a polymerization tube
containing SL (0.48 g, 3.0 mmol) subsequently. The resulting
mixture was stirred at 30 °C for 8 h under nitrogen. The
reaction proceeded homogeneously. After being quenched with
triethylamine (0.2 mL), the resulting mixture was poured into
ethyl ether (300 mL) to precipitate a polymer. The polymer
was collected by filtration with suction and dried under
vacuum. PolySL was obtained as a colorless solid quantita-
tively. Mn ) 6100, Mw/Mn ) 1.13. 1H NMR (DMSO-d6): δ )
2.21 (initiating end, S-CH3), 2.89-3.11 (1H, -CH2-), 3.17-
3.37 (1H, -CH2-), 3.55-3.76 (3H, -OCH3), 4.21-4.41 (1H,
>CH-), 8.79-9.00 (1H, -NH-) ppm. 13C NMR (DMSO-d6):
δ ) 29.5 (-CH2-), 53.0 (-OCH3), 54.37(>CH-), 163.89
(-SCONH-), 167.38 (-COOCH3) ppm. IR (KBr): 3301, 1743
1034, 980, 640 cm-1
.
Resu lts a n d Discu ssion
Con tr olled Ca tion ic Rin g-Op en in g P olym er iza -
tion of N-Su bstitu ted Cyclic Th iou r eth a n es (Rn SL).
The cationic ring-opening polymerization of N-substi-
tuted cyclic thiourethanes (Bn SL, BzSL, and AcSL) was
carried out using TfOMe (3.04 mol %) as an initiator at
30 °C in CH2Cl2 under nitrogen to give polymers (poly-
(Bn SL), poly(BzSL), and poly(AcSL)) in quantitative
yields (Scheme 1). In all cases, the polymerization
proceeded smoothly accompanying selective isomeriza-
tion of the thiocarbonyl groups into the carbonyl groups.
(-COOCH3), 1658 (-SCONH-), 1512, 1203 cm-1
.
P oly(Bn SL). Yield: quantitative, colorless solid. Mn ) 8200,
1
Mw/Mn ) 1.04. H NMR (DMSO-d6): δ ) 2.22 (initiating end,
S-CH3), 3.36-3.67 (3H, -OCH3), 4.05-4.92 (3H, -CH< and
-CH2-), 7.05-7.28 (5H, -C6H5) ppm. 13C NMR (DMSO-d6):
δ ) 29.4 (-CH2-), 52.3 (-OCH3), 53.18 (>CH-), 60.67

7540 Nagai et al.
Macromolecules, Vol. 37, No. 20, 2004
Sch em e 1. Ca tion ic Rin g-Op en in g P olym er iza tion
of RSL
The molecular weight distributions (Mw/Mn) of the
obtained polymers were narrow (Mw/Mn < 1.15) and the
number-average molecular weight (Mn) (poly(Bn SL) )
8200, poly(BzSL) ) 8500, and poly(AcSL) ) 6600)
agreed well with that expected from the feed ratios of
[Rn SL]/[TfOMe] (Mncalcd ) 8300, 8700, and 6700, respec-
tively).
We expected that this polymerization of the N-
substituted monomers would proceeds through con-
trolled fashion as well as the previously reported
polymerization of SL, and therefore, the controlled
nature of the polymerization of these monomers with
TfOMe was examined by polymerization under various
[RSL]/[TfOMe] ratios. Regardless of the ratios, the
polymers with narrow molecular weight distributions
were obtained in quantitative yields and the SEC
profiles showed unimodal peaks. The relationship be-
tween [RSL]/[TfOMe] ratios and the molecular weight
F igu r e 1. Mn and Mw/Mn vs feed ratio ([RSL]/[TfOMe])
[conditions: solvent, CH2Cl2 (0.5 M); temperature, 30 °C;
[RSL]/[1] ) 11.5-56.5; conversion of RSL ) 100%].
1
estimated by both from SEC and H NMR was linear,
and the Mn of the polymers agreed well with the
expected one (Figure 1). To be convinced whether the
polymerization of N-substituted monomers proceeds
without termination, we examined the relationship
between Mn, Mw/Mn, conversion, and reaction time on
the polymerizations of their monomers at 30 °C. Mn of
the polymers obtained increased with the monomer
conversion, maintaining the linear relationship and the
narrow unimodal peaks, as shown in Figure 2. These
data would seem to indicate that termination and chain
transfer reactions are negligible in the present poly-
merizations.
Th e Kin etic Stu d ies on Ca tion ic P olym er iza tion
of SL a n d N-Su bstitu ted Cyclic Th iou r eth a n es. The
polymerization rates of SL and N-substituted thioure-
thanes in CH2Cl2 (0.5 M) were examined at 30 °C using
TfOMe (3.04 mol %). Figure 3 shows the time-conver-
sion (a) and first-order time-conversion (b) plots. The
monomer conversions obey the first-order kinetic equa-
tion (i.e., -d[monomer]/dt ) kobs[monomer][propagating
species]), indicating that the polymerization proceeded
without termination. For the polymerization of SL, the
observed rate coefficient kobs was estimated to be 8.44
× 10-3 L‚mol-1‚s-1 under the assumption that initiation
occurred quantitatively and the concentration of the
propagating end was constantly equal to the initial
concentration of TfOMe (0.015 mol‚L-1). This kobs value
is 1.8 times larger than that for the polymerization of
Bn SL (kobs ) 4.58 × 10-3 L‚mol-1‚s-1). As well, the kobs
F igu r e 2. Conversion-Mn and conversion-Mw/Mn in the
polymerization of RSL with TfOMe in CH2Cl2 at 30 °C.
[TfOMe]0 ) 0.015 M; [RSL]0/[TfOMe]0 ) 32.9.
Keeping in mind that cyclic endo-iminothiocarbonate
triflate salts obtained by initiating reactions are highly
stable,12 we prepared the iminothiocarbonate triflate
salts from the monomers with TfOMe and elucidated
the electronic character by 13C NMR and IR spectros-
copy to discuss the kinetic results (Scheme 2 and Table
1). Both the 13C NMR and IR spectra indicate that the
electron densities of the thiocarbonyl groups in the cyclic
thiourethanes are in the order of the rates of polymer-
ization. Namely, the nucleophilicities of sulfur atoms
will also be on the order of SL > Bn SL > BzSL > AcSL.
of Bn SL is 1.5 times larger than that of BzSL (kobs
)
For cyclic endo-iminothiocarbonate triflate salts, the 13
C
NMR spectra revealed that the electrophilicites of the
methylene groups, which are attacked by sulfur atoms
of monomers, are on the order of AcSL < BzSL < Bn SL
3.12 × 10-3 L‚mol-1‚s-1), and kobs of BzSL is 3.3 times
larger than that of AcSL (kobs ) 0.93 × 10-3 L‚mol-1‚s-1
)
(Figure 3b).15

Macromolecules, Vol. 37, No. 20, 2004
Cationic Ring-Opening Polymerization 7541
Ta ble 1. Sp ectr oscop ic Da ta of Mon om er s (RSL) a n d
Im in iu m Sa lts fr om RSL a n d TfOMe
monomer (RSL)
iminothiocarbonate
CdS
triflate saltsa
b
b
run
R
δ/ppm
cm-1 c
O-CH2-C< δ/ppm
1
2
3
4
H
190.73
189.34
186.51
186.26
1504
1481
1442
1419
77.75
77.29
79.40
80.14
Bn
Bz
Ac
a
Conditions: Reactions of monomers (0.5 mmol) and TfOMe
(0.6 mmol, 68 µL) were carried out in CD2Cl2 (0.7 mL) at room
b
temperature for 1.0 min. Observed in 13C NMR spectra (CD2Cl2).
c Observed in IR spectra (KBr).
F igu r e 4. CD spectra of polythiourethane (c ) 0.1 g/dL, in
CH2Cl2): (a) poly(SL), (b) poly(Bn SL), (c) poly(BzSL), and poly-
(AcSL).
) -127.0 °), and poly(AcSL) (Mn ) 3400, Mw/Mn ) 1.15,
F igu r e 3. Time-conversion (a) and first-order time-conver-
sion (b) plots for the polymerization of RSL with TfOMe in CH2-
Cl2 at 30 °C. [TfOMe]0 ) 0.015 M; [RSL]0/[TfOMe]0 ) 32.9.
[R]25 ) -213.2 °). The specific rotation of poly(SL)
D
showed positive value in contrast to those of poly(Bn SL),
poly(BzSL), and poly(AcSL). The Cotton effect of the
thiourethane moiety in poly(SL) at 227 nm is positive,
whereas the Cotton effects of the thiourethane moieties
in poly(Bn SL), poly(BzSL), and poly(AcSL) are negative
as are the specific rotations as well. This difference may
be attributed to the presence of substituents at the
nitrogen atoms that leads to construction of secondary
structure based on steric factors, compared to that of
poly(SL), which is based on hydrogen bonds between the
carbonyl and NH moieties. We speculated that the
polythiourethanes from the N-substituted monomers
take different secondary structures from that of poly-
(SL) regulated by hydrogen bonds (e.g., these polymers
have inversed helix sense).17
Sch em e 2. Ad d ition of TfOMe to RSL
≈ SL. Although the electron densities of iminothiocar-
bonate triflate salts from Bn SL and SL are identical,
the nucleophilicty of the thiocarbonyl group of SL is
higher than that of Bn SL. The electronic character of
the propagating species and the monomers described
here would have reflected in the actual rates of poly-
merization (SL > Bn SL > BzSL > AcSL).
Con clu sion s
In this study, we synthesized N-substituted cyclic
thiourethanes (Bn SL, BzSL, and AcSL) and conducted
the cationic ring-opening polymerizations to obtain the
corresponding polythiourethanes. The molecular weight
of the obtained polymers can be controlled by the ratio
of the monomers to the initiator ([RSL]/[TfOMe]) and
the molecular weight distributions are narrow (Mw/Mn
< 1.15). These polymerization rates are on the order of
SL > Bn SL > BzSL > AcSL, which depends on the
nucleophilicity of the thiocarbonyl moieties of the mono-
CD Sp ectr a of Th e Obta in ed P olym er s. To char-
acterize the chiroptical properties of the obtained poly-
mers, we evaluated the CD spectra and specific rotations
([R]25D) of the polymers. Figure 4 illustrates the CD
spectra of poly(SL) (Mn ) 3000, Mw/Mn ) 1.13, [R]25
62.4 °), poly(Bn SL) (Mn ) 3600, Mw/Mn ) 1.14, [R]25
)
D
)
D
-99.6 °), poly(BzSL) (Mn ) 3500, Mw/Mn ) 1.15, [R]25
D

7542 Nagai et al.
Macromolecules, Vol. 37, No. 20, 2004
Yoshii, K.; Nagai, D.; Endo, T. J . Polym. Sci., Part A: Polym.
Chem. Submitted for publication.
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mers. The studies on the specific rotations and the CD
spectra suggest that secondary structure of poly(SL) is
different from that of the N-substituted polymers, e.g.,
inverse helix sense.
(10) Mukaiyama, T.; Kuwajima, I.; Mizui, K. J . Org. Chem. 1966,
Refer en ces a n d Notes
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>
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MA049114B