Anionic ring-opening polymerization of cyclic 1,3-dithiocarbonate and thermal depolymerization

Article abstract of DOI:10.1016/j.reactfunctpolym.2016.01.005

A new anionic ring-opening polymerization (ROP) was used to synthesize polydithiocarbonates from cyclic dithiocarbonates. The polymerization was optimized in the presence of dibenzo-18-C-6 and NaH/EtOH, and carried out at 50°C to suppress any retropolymer

Full text of DOI:10.1016/j.reactfunctpolym.2016.01.005

Reactive and Functional Polymers 100 (2016) 37–43
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Anionic ring-opening polymerization of cyclic 1,3-dithiocarbonate and
thermal depolymerization
⁎
Su Jin Jeon, Min-young Jung, Jung Yun Do
Department of Chemistry Education, Pusan National University, Pusan 609-735, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A new anionic ring-opening polymerization (ROP) was used to synthesize polydithiocarbonates from cyclic
dithiocarbonates. The polymerization was optimized in the presence of dibenzo-18-C-6 and NaH/EtOH, and car-
ried out at 50 °C to suppress any retropolymerization. Six- and seven-membered cyclic dithiocarbonates
underwent ROP to afford the corresponding polymers. Thermal depolymerization of the prepared polymers gen-
erated a dimer, which was found to be a back-biting mechanism according to 1H nuclear magnetic resonance
(NMR) spectroscopy. The characteristic structure of the linear polymer was defined by 1H NMR and 13C NMR
spectroscopy, and compared with the monomer. The ROP at 80 °C exhibited the formation of a polymer contam-
inated by a dimer. Thermal analysis of the polymers by differential scanning calorimetry and thermogravimetric
analysis revealed a melting process at 85–110 °C and thermal instability accompanying a 5% weight loss at
150–230 °C.
Received 18 November 2015
Received in revised form 4 January 2016
Accepted 4 January 2016
Available online 8 January 2016
Keywords:
Poly(dithiocarbonate)
Cyclic 1,3-dithiocarbonate
Anionic ROP
Depolymerization
Back-biting
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
reported the synthesis of a poly(dithiocarbonate) from the cationic ROP
of 1,3-oxathiolane-2-thione (–S–CS–O–) and polymer instability caused
The driving force of ring-opening polymerization (ROP) is the ring
strain of a cyclic monomer that is dependent on the ring size and steric
hindrance of the constituent atoms or groups [1]. The negative Gibb's
free energy (ΔG) of ROP is the sum of the enthalpy (ΔH) factor, including
strain relief, and negative entropy change (ΔS) during polymerization
[2–3]. Polymerization is accompanied by a decrease of entropy due to
the loss of translational freedom of the monomers. The contribution of
entropy to the free energy is associated with temperature (ΔG =
ΔH − TΔS). Hence, a reverse polymerization, retropolymerization, can
occur at higher temperatures [4]. Various cyclic monomers have been de-
veloped for polycarbonates, polyamides, polyesters, and polyolefins
[5–7]. For example, the ROP of an epoxide is initiated by anionic or
cationic reagents [8]. Nylon 6 is produced by the thermal ROP of ε-
caprolactam. Five- and six-membered cyclic carbonates are polymerized
to polycarbonates under ionic initiators [3,9]. Sulfur analogs of the cyclic
carbonates have attracted attention for sulfur-containing polycarbonates,
which exhibit good thermal and optical properties as well as chemical
and biological resistance [10–11]. Several types of cyclic thiocarbonates
were proposed for the formation of poly(thiocarbonate)s through ROP,
such as cyclic trithiocarbonate (–S–CS–S–), cyclic 1,3-oxathiolane-2-
thione (–S–CS–O–), and cyclic thione carbonate (–O–CSO–) [12–15],
whose polymerization was initiated by cationic reagents. On the contrary,
there is no report related to the ROP of cyclic dithiocarbonates (–S–CO–
S–), such as 1,3-dithiolan-2-one and 1,3-dithian-2-one. T. Endo's group
by a back-biting reaction [16–18]. The isomeric transformation of 1,3-
oxathiolane-2-thione to 1,3-dithiolan-2-one was irreversible under
cationic ROP conditions, where the formed cyclic dithiocarbonate was
unreactive. Anionic polymerization of a six-membered cyclic mono-
thiocarbonate (–O–CO–S–) was previously reported to occur via the
carbonyl–sulfur bond cleavage [19]. Therefore, a similar polymerization
reaction is expected from the cyclic dithiocarbonate. The anionic ROP
might involve several steps, such as nucleophilic attack on a carbonyl,
ring cleavage, and reproduction of a sulfur anion for polymer growth.
The anionic ROP should be initiated at a certain temperature because of
the possible depolymerization at higher temperatures. The ring size of a
cyclic dithiocarbonate monomer will contribute to both polymerization
and depolymerization.
2. Experimental
2.1. Materials and characterization
All reagents were purchased from Sigma-Aldrich Corporation, and
the reagent-grade solvents were dried when necessary and purified
by vacuum distillation. 1H nuclear magnetic resonance (NMR) and 13
C
NMR spectroscopy experiments (Bruker AM-300 spectrometer) were
conducted to characterize their molecular structure. The mass spectra
were recorded on an Agilent 1200LC/1100 MSD SL mass spectrometer.
A Magna-IR 750 spectrometer (Nicolet Instrument Co., USA) recorded
the Fourier transform infrared (FTIR) spectra. Thermal analysis was per-
formed on a Mettler Toledo DSC 822e and TAG/SDTA 851e, with a
⁎
Corresponding author.
E-mail address: jydo@pusan.ac.kr (J.Y. Do).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.01.005
1381-5148/© 2016 Elsevier B.V. All rights reserved.

38
S.J. Jeon et al. / Reactive and Functional Polymers 100 (2016) 37–43
heating rate of 10 °C/min under nitrogen atmosphere. Molecular
weights of polymers were determined by Agilent PL-20 using a polysty-
rene standard. Elemental analysis was run on a Vario EL elemental
analyzer.
further reaction without purification. 1H NMR (300 MHz, CDCl3) δ
(ppm) = 1.43 (t, 3 H, J = 7.2 Hz), 4.15 (s, 2 H), 4.32 (s, 2 H), 4.64 (q,
2 H, J = 7.2 Hz); 13C NMR (75 MHz, CDCl3) δ (ppm) = 13.9, 43.0,
48.2, 17.4, 195.9 (C = O), 212.9 (C = S).
A solution of 5 (1.00 g, 4.7 mmol) and KBr (2.81 g, 23.5 mmol) in ac-
etone (14 mL) was refluxed for 24 h. The cooled mixture was filtered to
remove solids and concentrated under reduced pressure. The organic
residue was purified by SiO2 column chromatography using ethyl
acetate:hexane (1:4, v/v) as eluent to obtain product 6 with 77% yield.
1H NMR (300 MHz, CDCl3) δ (ppm) = 3.85 (s, 4 H); 13C NMR
(75 MHz, CDCl3) δ (ppm) = 39.8, 190.5 (SC = O), 198.1 (C = O); IR
(NaCl, cm−1): νmax 1728 (C = O), 1630, 1381, 1237, 1150, 1104,
1040, 930, 878. C4H4O2S2 (148.21): C 32.42, H 2.72, S 43.27, O 21.59;
Found C 32.42, H 2.77, S 43.19, O 21.62.
1,3-Dithiane-2,5-dione (6) (1.48 g, 10.0 mmol) was placed in a two-
necked round-bottom flask equipped with the Dean–Stark apparatus
and charged with benzene (30 mL) and molecular sieve (4 A, 1 g). To
the mixture were added ethylene glycol (0.56 mL, 10.0 mmol) and p-
toluene sulfonic acid (0.10 g, 0.5 mmol), and refluxed for 12 h. The
cooled mixture was diluted with ethyl acetate (30 mL) and washed
with water. The organic layer was concentrated and purified by SiO2
column chromatography using ethyl acetate:hexane (1:4, v/v) as eluent
to obtain product 7a with 85% yield.
2.2. Syntheses of cyclic 1,3-dithiocarbonates
2.2.1. Preparation of five-membered cyclic dithiocarbonate (3) and thiol
(4a)
A solution of 2-methylallyloxybenzene (1.48 g, 10.0 mmol) and
meta-chloroperoxybenzoic acid (m-CPBA, 77%, 2.69 g) in anhydrous
CH2Cl2 (30 mL) was stirred for 12 h at 20 °C. The mixture was diluted
with CH2Cl2 (100 mL) and washed with aqueous sodium bicarbonate.
The organic solution was dried over anhydrous MgSO4, filtered, and
concentrated. The residue was purified by column chromatography
using ethyl acetate:hexane (1:7, v/v) as eluent to obtain epoxy product
1 with 78% yield. 1H NMR (300 MHz, CDCl3) δ (ppm) = 1.51 (s, 3 H),
2.72 (d, 1 H, J = 4.5 Hz), 2.86 (d, 1 H, J = 4.5 Hz), 3.93 (d, 1 H, J =
10.2 Hz), 4.04 (d, 1 H, J = 10.2 Hz), 7.00 (m, 3 H), 7.30 (m, 2 H); 13C
NMR (75 MHz, CDCl3) δ (ppm) = 18.3, 51.7, 55.4, 71.3, 114.5, 121.0,
129.4, 158.5.
To a solution of the epoxide 1 (1.64 g, 10.0 mmol) dissolved in tet-
rahydrofuran (THF, 30.0 mL) were successively added CS2 (1.21 mL,
20.0 mmol) and catalytic amount of LiBr (0.09 g). After stirring for
20 h at 20 °C, the resulting mixture was diluted with CH2Cl2
(100 mL) and washed with water. The organic solution was dried
over anhydrous MgSO4, filtered, and concentrated. The residue was
purified by column chromatography using ethyl acetate:hexane
(1:4, v/v) as eluent to obtain product 2 with 50% yield. 1H NMR
(300 MHz, CDCl3) δ (ppm) = 1.72 (s, 3 H), 3.36 (d, 1 H, J =
11.1 Hz), 3.82 (d, 1 H, J = 11.1 Hz), 3.99 (d, 1 H, J = 10.2 Hz), 4.20
(d, 1 H, J = 10.2 Hz), 6.80–7.05 (m, 3 H), 7.22–7.35 (m, 2 H); 13C
NMR (75 MHz, CDCl3) δ (ppm) = 22.9, 41.3, 70.1, 96.6, 114.7,
122.0, 129.7, 157.9, 211.1 (C = S).
A mixture of 2 (1.20 g, 5.0 mmol) and ZnCl2 (0.14 g) in chloroben-
zene (15 mL) was heated for 6 h at 80 °C. The cooled mixture was dilut-
ed with CH2Cl2 (50 mL) and washed with water. The organic solution
was dried over anhydrous MgSO4, filtered, and concentrated. The or-
ganic residue was purified by SiO2 column chromatography using
ethyl acetate:hexane (1:4, v/v) as eluent to obtain product 3 with 65%
yield. 1H NMR (300 MHz, CDCl3) δ (ppm) = 1.84 (s, 3 H), 3.57 (d, 1 H,
J = 12.3 Hz), 3.85 (d, 1 H, J = 12.3 Hz), 4.04 (d, 1 H, J = 9.3 Hz), 4.38
(d, 1 H, J = 9.3 Hz), 6.97–7.08 (m, 3 H), 7.37 (m, 2 H); 13C NMR
(75 MHz, CDCl3) δ (ppm) = 23.6, 42.9, 60.1, 71.3, 114.6, 121.6, 129.6,
158.0, 196.5(C = O). C11H12 S2O2 (240.03): C 54.97, H 5.03, S 26.68, O
13.31; Found C 54.96, H 4.99, S 26.62, O 13.43.
7a: 1H NMR (300 MHz, CDCl3) δ (ppm) = 3.26 (s, 4 H), 4.04 (s, 4 H);
13C NMR (75 MHz, CDCl3) δ (ppm) = 40.2, 65.4, 107.9, 194.7 (C = O);
C6H8O3S2 (192.26): C 37.48, H 4.19, S 33.36, O 24.97; Found C 37.46, H
4.25, S 33.28, O 25.01.
Similarly, 7b was prepared using propylene glycol with 65% yield.
7b: 1H NMR (300 MHz, CDCl3) δ (ppm) = 1.84 (m, 2 H), 3.40 (s, 4 H),
4.00 (t, 4 H, J = 6.0 Hz); 13C NMR (75 MHz, CDCl3) δ (ppm) = 25.2, 38.8,
61.1, 117.3, 198.1 (C = O); C7H10O3S2 (206.28): C 40.76, H 4.89, S 31.09,
O 23.27; Found C 40.75, H 4.88, S 31.18, O 23.19.
To a solution of 2-phenylthio-1,3-propanediol (1.84 g, 10.0 mmol)
[20] in CH2Cl2 (40 mL) were added successively triethylamine
(3.2 mL, 24 mmol) and methanesulfonyl chloride (3.1 mL, 22 mmol)
at 0 °C. After stirring for 12 h, the reaction mixture was diluted with
CH2Cl2 (30 mL), followed by washing with water and brine. The organic
solution was dried over MgSO4 and concentrated under reduced pres-
sure to obtain a dimesylated product (2.99 g). This product and thiourea
(1.9 g, 25.0 mmol) were dissolved in 2-propanol (40 mL), and refluxed
for 24 h. Then, aqueous NaOH (10%, 10 mL) was added to the mixture
and refluxed for additional 24 h. The reaction temperature was reduced
to 25 °C and acidified to pH = 1 by adding aqueous HCl (1 M). The
resulting mixture was extracted with CH2Cl2 and washed with brine.
After drying with MgSO4, the solvents were removed under reduced
pressure and purified by column chromatography to produce 4-
phenylthio-1,2-dithiolane (8) with 75% yield. 1H NMR (300 MHz,
CDCl3) δ (ppm) = 3.02 (br, 4 H), 3.63 (m, 1 H), 7.20–7.35 (m, 3 H),
7.36–7.52 (m, 2 H); 13C NMR (75 MHz, CDCl3) δ (ppm) = 44.2, 52.9,
125.7, 128.1, 129.5, 132.7, 133.2; IR (NaCl, cm−1): νmax 3629, 2955,
1586, 1479, 1434.
A mixture of 4-phenylthio-1,2-dithiolane and NaBH4 (0.38 g,
10.0 mmol) in THF/methanol (30 mL, 1/1) was stirred for 2 h. The re-
action mixture was acidified to pH = 1 by adding aqueous HCl (1 M),
and the organic material was extracted with CH2Cl2 and washed with
brine. The concentrated organic residue was dissolved in CH2Cl2
(20 mL), and N,N-carbonyl diimidazole (1.79 g, 11.0 mmol) was
added at 25 °C. After stirring for 12 h, the resulting mixture was con-
centrated and purified by SiO2 column chromatography using ethyl
acetate:hexane (1:10, v/v) as eluent to obtain product 9 with 42%
yield. 1H NMR (300 MHz, CDCl3) δ (ppm) = 3.08 (m, 2 H), 3.48 (m,
2 H), 3.95 (m, 1 H), 7.33 (m, 3 H), 7.53 (m, 2 H); 13C NMR (75 MHz,
CDCl3) δ (ppm) = 30.5, 44.2, 123.7, 128.3, 128.6, 129.5, 132.1,
194.0 (C = O); IR (NaCl, cm−1): νmax 2931, 2854, 1643 (C = O),
1443, 876. C10H10OS3 (242.38): C 49.55, H 4.16, S 39.69, O 6.60;
Found C 49.65, H 4.43, S 39.61, O 6.31.
A solution of 3 (0.50 g, 2.1 mmol) and morpholine (0.92 g,
10.5 mmol) in ethanol (6 mL) was stirred for 2 h at 20 °C. The resulting
mixture was concentrated under vacuum to remove volatile materials
to yield a thiol product (4a).
4a: 1H NMR (300 MHz, CDCl3) δ (ppm) = 1.46 (s, 3 H), 2.31 (s, 1 H,
−SH), 3.50 (s, 2 H), 3.54 (m, 4 H), 3.62 (m, 2 H), 3.94 (dd, 2 H, J =
9.3 Hz, J = 8.9 Hz), 6.89–6.99 (m, 3 H), 7.27 (m, 2 H); 13C NMR
(75 MHz, CDCl3) δ (ppm) = 25.1, 41.0, 45 (m), 47.3, 66.6, 75.5, 114.8,
121.3, 129.6, 158.5, 166.7(C = O); IR (NaCl, cm−1): 2963, 2924, 2858,
2549 (w), 1653 (s), 1596, 1500, 1404, 1239, 1206, 1113. C15H21NS2O3
(327.46): C 55.02, H 6.46, N 4.28, S 19.58, O 14.66; Found C 55.16, H
6.34, N 4.22, S 19.56, O 14.72.
2.2.2. Preparation of six-membered cyclic dithiocarbonates (6, 7a, 7b, 9)
To a solution of potassium O–ethyl xanthate (1.31 g, 9.9 mmol) in
water (12 mL) was slowly added 1,3-dichloroacetone (1.67 g,
10 mmol) at 0 °C. After stirring for 1 h, the obtained solid product was
filtered and washed with water. It was then dissolved in CH2Cl2
(30 mL) and dried over anhydrous MgSO4. The organic solution was fil-
tered and concentrated to produce 5 with 65% yield, which was used for

S.J. Jeon et al. / Reactive and Functional Polymers 100 (2016) 37–43
39
2.2.3. Preparation of seven-membered cyclic dithiocarbonate (11)
To a solution of potassium O–ethyl xanthate (1.30 g, 10.0 mmol) in
water–THF (30 mL, 1/1, v/v) was slowly added 1,2-bis(chloro-
methyl)benzene (3.50 g, 20 mmol) at 0 °C. After stirring for 12 h, the mix-
ture was diluted with ethyl acetate and washed with water.
H 5.55, S 27.26, O 26.91. Liquid chromatography–mass spectrometry
(LC–MS): calculated exact mass: 474.05, found 473.01 [M–H+].
2.4. Depolymerization of poly(dithiocarbonate) (7P-1)
The organic layer was dried over anhydrous MgSO4, filtered, and
concentrated. The residue was purified by column chromatography
using hexane as eluent to obtain epoxy product 10 with 80% yield
Sodium ethoxide (15 mg) was added to a solution of the polymer
7P-1 (0.40 g) in THF (4 mL) and refluxed for 0.5 h. The cooled mixture
was concentrated and the residue was purified by column chromatogra-
phy using ethyl acetate:hexane (1:7, v/v) as eluent to produce 7a (5%
yield) and 13 (55% yield).
1
(2.0 g). H NMR (300 MHz, CDCl3) δ (ppm) = 1.43 (t, 3 H, J =
6.9 Hz), 4.55 (s, 2 H), 4.71 (s, 2 H), 4.66–4.77 (q, 2 H, J = 6.9 Hz),
7.30–7.45 (m, 4 H); 13C NMR (75 MHz, CDCl3) δ (ppm) = 13.7, 37.2,
43.8, 70.1, 128.2, 129.1, 130.4, 130.8, 133.9, 135.9, 213.2 (C = S); IR
(NaCl, cm−1): νmax 1604, 1491, 1451, 1359, 1219, 1111, 1044.
13: 1H NMR (300 MHz, CDCl3) δ (ppm) = 3.28 (s, 8 H), 4.08 (s, 8 H);
13C NMR (75 MHz, CDCl3) δ (ppm) = 40.4, 65.5, 108.2, 195.0 (SC = O);
C12H16O6S4 (384.52): C 37.48, H 4.19, S 33.36, O 24.97; Found C 37.51, H
4.25, S 33.11, O 25.13. LC–MS: calculated exact mass: 383.98, found
382.95 [M–H+].
A solution of 10 (2.0 g, 7.7 mmol) and KBr (4.58 g, 38.5 mmol) in ac-
etone (23 mL) was refluxed for 12 h. The mixture was spontaneously
concentrated by heating and diluted with ethyl acetate (30 mL). After
refluxing for 24 h, the cooled mixture was mixed with water (30 mL)
and extracted with ethyl acetate. The organic layer was dried over anhy-
drous MgSO4, filtered, and concentrated. The residue was purified by
column chromatography using ethyl acetate:hexane (1:10, v/v) as elu-
ent to produce cyclic dithiocarbonate (11) with 35% yield (0.53 g). 1 H
NMR (300 MHz, CDCl3) δ (ppm) = 4.28 (s, 4 H), 7.37 (s, 4 H); 13C
NMR (75 MHz, CDCl3) δ (ppm) 34.2, 128.4, 129.1, 136.5, 190.8 (C =
O); IR (NaCl, cm−1): νmax 3026, 2972, 2919, 2850, 1698, 1605 (s),
1488, 1442, 938, 823, 763. C9H8OS2 (196.29): C 55.07, H 4.11, S 32.67,
O 8.15; Found C 55.01, H 4.11, S 32.61, O 8.27.
3. Results and discussion
3.1. Synthesis of cyclic 1,3-dithiocarbonate monomers
A five-membered cyclic dithiocarbonate 2 was prepared through the
O/S isomerization of the corresponding 5-alkyl-1,3-oxathiolane-2-
thione [17]. Oxidation of 2-methylallyloxybenzene with m-CPBA
yielded an epoxide, which was coupled with CS2 in the presence of
LiBr catalyst to generate cyclic 1,3-oxathiolane-2-thione 1. The acid-
mediated isomerization of 1 using ZnCl2 or trifluoromethanesulfonic
acid afforded cyclic dithiocarbonate 3 with 65% yield. A six-membered
cyclic dithiocarbonate (6) was prepared from 1,3-dichloroacetone
(Fig. 1). Monoalkylation of 1,3-dichloroacetone was achieved using
aqueous potassium ethyl xanthate to produce 5, and successive
intramolecular cyclization occurred accompanying elimination of
chloroethane to obtain the cyclic product 6. When KBr was added to 5
in acetone under reflux, the cyclization was accelerated to give a prod-
uct with 88% yield. This was an improvement of the previous synthesis
in terms of the high dilution condition and low synthetic yield [21].
Mesylation of 2-phenylthio-1,3-propanediol [20] followed by alkylation
with thiourea afforded a cyclic disulfide (8). The formation of an S–S
bond was confirmed by 1H NMR and FTIR spectroscopy with no obser-
vation of a specific SH band at around 2550 cm−1. Further, oxidation
of 2-phenylthio-1,3-propanedithiol occurred during the reaction. The
NaBH4 reduction of 8 and cyclocarbonylation with N,N-carbonyl
diimidazole generated a six-membered cyclic dithiocarbonate (9).
Monoalkylation of 1,2-bis(chloromethyl)benzene with potassium
ethyl xanthate, followed by thermal cyclization afforded a seven-
membered cyclic dithiocarbonate (11). The structure of the prepared
cyclic dithiocarbonates was clearly established by 1H NMR spectra.
The carbonyl group of S–C(O)–S was observed in the 13C NMR spectrum
and at 1600–1700 cm−1 in the FTIR absorption spectrum.
2.3. Polymerization of cyclic dithiocarbonates
General procedure: All glassware were dried and washed with N2
flow before use. A catalyst solution (NaOEt, 0.025 M) was prepared.
NaH (60% in mineral oil, 120 mg, 3.0 mmol) was placed in a 250-mL
round-bottom flask under N2 condition and charged with anhydrous
benzene (20 mL). Ethanol (175 μL, 3.0 mmol) was slowly added at
room temperature and diluted with additional benzene (100 mL) to
yield 0.025 M-NaOEt stock solution. Cyclic dithiocarbonate 7a (1.0 g,
5.2 mmol) and dibenzo-18-crown-6 (190 mg, 0.52 mmol) were placed
in a 25-mL flask purged by N2, and the catalyst solution (4.2 mL) was
added at room temperature under N2 condition. The resulting solution
was stirred for 24 h at 50 °C and then cooled. The mixture was loaded
on a SiO2-packed column and treated with eluents ethyl acetate and
chloroform to remove all organics except the polymer, which was iso-
lated by passing THF. The polymer solution was concentrated and pre-
cipitated in methanol to obtain poly(dithiocarbonate) (7P-1) with 65%
yield.
7P-1: (65% isolation) 1H NMR (300 MHz, CDCl3) δ (ppm) = 3.38 (s,
4 H), 4.04 (s, 4 H); 13C NMR (75 MHz, CDCl3) δ (ppm) = 36.4, 66.7,
108.4, 188.1 (C = O).
7P-2: (75% isolation) 1H NMR (300 MHz, CDCl3) δ (ppm) = 1.72 (m,
2 H), 3.47 (s, 4 H), 2.17 (s), 3.92 (m, 4 H); 13C NMR (75 MHz, CDCl3) δ
(ppm) = 25.6, 36.1, 62.1, 117.3, 188.1 (C = O).
3.2. Anionic ROP
11P: (12-h polymerization, 88% isolation) 1H NMR (300 MHz, CDCl3)
δ (ppm) = 4.28 (s, 4 H), 7.20–7.21 (m, 2 H), 7.26–7.29 (m, 2 H); 13C
NMR (75 MHz, CDCl3) δ (ppm) = 32.1, 128.2, 130.8, 134.8, 188.1
(C = O); IR (NaCl, cm−1): νmax 2926, 1712 (w), 1631 (s), 1448, 870 (s).
Sodium hydride (60%, 5 mg) was placed in a 25-mL flask and
charged with anhydrous benzene (2.0 mL) and ethanol (0.10 mL). To
this solution was added cyclic dithiocarbonate (7a, 1.0 g, 5.2 mmol) dis-
solved in benzene (2.0 mL). The resulting mixture was stirred for 24 h at
70 °C. The mixture was concentrated and purified by column chroma-
tography with ethyl acetate:hexane (1:7, v/v) as eluent to afford prod-
uct 12 (1.15 g) with 95% yield. 1H NMR (300 MHz, CDCl3) δ (ppm) =
1.32 (t, 6 H, J = 7.1 Hz), 3.22 (d, 4 H, J = 8.1 Hz), 3.30 (d, 4 H, J =
8.1 Hz), 4.08 (s, 8 H), 4.27 (q, 4 H, J = 7.1 Hz); 13C NMR (75 MHz,
CDCl3) δ (ppm) = 25.2, 30.1, 35.9, 66.1, 67.7, 107.8, 187.8 (C = O).);
C16H26O8S4 (474.64): C 40.49, H 5.52, S 27.02, O 26.97; Found C 40.29,
Anionic polymerization of a six-membered cyclic monothiocarbonate
was previously reported to occur through selective carbonyl–sulfur bond
cleavage and efficiently at room temperature [19]. A similar anionic poly-
merization was attempted with five-membered cyclic dithiocarbonates
such as 1,3-dithiolane-2-one and 5,5-dialkyl-1,3-dithiolane-2-one (3).
Polymerization was not observed and the monomer was recovered
when it was attempted at various temperatures from 0 to 80 °C with an-
ionic initiators such as NaOEt, KOtBu, and morpholine. The reaction of 3
and an excess amount of morpholine in ethanol afforded a thiol (4a) as
the sole product at room temperature. Nucleophilic attack of morpholine
on a carbonyl and subsequent C–S bond cleavage may occur in two ways.
Fast isomerization between 4a and 4b leads to the formation of more sta-
ble 4a. This was deduced from a mechanistic study of an O-analog cyclic
carbonate [22]. Nucleophilicity of 3°-SH is weak due to steric crowding,
which is insufficient to propagate for the polymerization of 3. 1,3-

40
S.J. Jeon et al. / Reactive and Functional Polymers 100 (2016) 37–43
Fig. 1. Synthesis of cyclic dithiocarbonates with various ring sizes.
Dithiolane-2-one, accompanying the formation of 1°-SH, was also
unreactive under the anionic ROP condition. The back-biting reaction of
a primary mercapto anion occurs preferentially to regenerate a cyclic
dithiocarbonate (Fig. 2).
disulfide 12 was presumably formed through an oxidative coupling re-
action of the corresponding thiols, which was driven by O2. The result
indicates that an effective carbonyl attack of the ethoxy anion and C–S
bond cleavage takes place as expected. The ethoxy group of 12 was sup-
plied by the excess ethanol. Nevertheless, no reaction occurred when
solid NaOEt was used as a catalyst. Therefore, a stock solution containing
NaOEt (0.025 M) was prepared from the reaction of NaH and ethanol in
equivalent quantities in anhydrous benzene. Polymerization was ob-
served when 7a was heated in the catalyst solution, and was stopped
after 12 h. Table 1 lists the conditions for the polymerization of 7a. Poly-
merization progressed slowly at temperatures below 70 °C. Structural
These studies of the anionic ROP were directed toward six-
membered cyclic dithiocarbonates. An initial study was performed
with a known cyclic dicarbonate (7b). On the contrary, polymer 6P-0
was not formed because of a reactive carbonyl group consuming the an-
ionic initiator, as suggested in Fig. 3. The selective carbonyl protection of
6 produced 7a and 7b. Two carbonyls of 6 were protected simultaneous-
ly with excess diols under an acid catalyst. The treatment of 6 with an
equivalent amount of a diol yielded a mono-protected product from se-
lective one-site protection. The reaction of 7a with catalyst NaOEt,
which was in situ prepared from NaH and ethanol in benzene, resulted
in the quantitative formation of a coupled product (12). The S–S bond of
analysis of the prepared polymer was performed by 1H NMR and 13
C
NMR spectroscopy, and compared with 7a. A peak at 3.26 ppm (CH2–
S) in the spectrum of 7a shifted to 3.38 ppm of 7P-1. The carbonyl car-
bon of 7a at 194.7 ppm moved to 188.1 in the 13C NMR spectrum of
Fig. 2. Ring-opening reaction of five-membered cyclic dithiocarbonates under ROP conditions.

S.J. Jeon et al. / Reactive and Functional Polymers 100 (2016) 37–43
41
Fig. 3. Anionic ring-opening polymerization of six- and seven-membered cyclic dithiocarbonates.
the polymer 7P-1. Methylene carbons (CH2–S) of the monomer and
polymer appeared at 40.2 and 36.4 ppm, respectively. Fig. 4 shows
two strong peaks assigned to two methylene protons from a main back-
bone and two weak peaks at 4.25 and 1.24 ppm assigned to an ethoxy
group attached to the polymer terminals.
A dimer product (13) was isolated together with polymer 7P-1
(Table 1, entry 4). 1H NMR analysis of the crude mixture after polymer-
ization revealed the formation of 13 and polymer with a 1:3 composi-
tion ratio. A similar result, that is, initiation of polymerization by
LiOEt, was presented in entry 5 of Table 1. A back-biting reaction was
considered to explain dimer 13, which was predominant at high tem-
peratures, and is discussed shortly. In order to suppress the side reac-
tion, polymerization was performed a lower temperature, despite the
low rate of polymerization. A phase transfer catalyst was introduced
in the polymerization process to overcome the rate problem. When
the polymerization exhibited a conversion of b10% at 50 °C, the addition
of dibenzo-18-C-6 improved it and the reaction was complete within
24 h. Furthermore, the polymer was isolated without the formation of
13. Similarly, the polymerization of 7b afforded a polymer 7P-2 under
similar anionic ROP conditions using NaH/EtOH/dibenzo-18-C-6 at
50 °C and the formation of a corresponding dimer was not observed.
In an attempt to polymerize 1,3-dithian-2-one, it was difficult to isolate
any distinct product except for the monomer. 4-Substituted cyclic
dithiocarbonate 9 with a thiophenyl group for UV detection resulted
in the formation of disulfide 8 with 30% yield without a polymer. Mono-
mer decomposition occurred through a decarbonylation under low-
temperature ROP conditions. This is in contrast to the result of six-
membered dithiocarbonate (7a,7b). Ring strain would contribute to
ROP polymerization. The 13C NMR analysis of the six-membered cyclic
monomers revealed a carbonyl carbon at N190 ppm and an upfield
shift toward 188.1 ppm after the ring-opening reaction. Therefore, fail-
ure of the polymerization of monomer 9 is probably due to the rapid
back-biting reaction. Moreover, steric contribution from 4,4-disubsti-
tuted cyclic dithiocarbonate (7a,7b) would retard the reaction at
50 °C. Polymerization of the seven-membered ring dithiocarbonate
with a large ring size was also examined. When the ROP of monomer
11 was performed using the anionic catalysts and a crown-ether, an
ROP polymer (11P) was obtained with 88% yield. Polymerization was
completed in 12 h without dimerization and decarbonylation. There-
fore, the anionic ROP of a cyclic dithiocarbonate is reversible and occurs
depending on the temperature and ring size.
Table 1
Polymerization results of 7a in various anionic ROP conditions.
Temp Time Products,
SM
Entry Catalyst1
Solvent
(°C)
(h)
(%)
(%)7
1
2
3
NaOEt
NaOEt2
KOtBu2
EtOH/C6H6 70
4
12
–
–
100
0
0
0
95
92
C6H6
THF
25–80 24
25–70 24
7P-1 +
4
NaH/EtOH3
C6H6
70
12
85
5
13(3/1)6
7P-1 +
13(3/1)
7P-1
3.3. Thermal properties of the prepared poly(dithiocarbonate)s
5
6
BuLi/EtOH4
C6H6
70
50
12
24
77
65
3
0
NaH/EtOH/18-C-65 C6H6
Observation of dimer 13 during the polymerization of 7a indicates
the occurrence of depolymerization, which is favorable at high temper-
atures. The back-biting attack of a terminal thiol group to the adjacent
carbonyls in the main backbone can generate monomer 7a and dimer
13 (Fig. 4). When polymer 7P-1 was treated with 0.1 equiv. NaOEt in
THF under reflux, dimer 13 was isolated with 55% yield together with
7a (5% yield). The structure of dimer 13 was confirmed by mass spec-
trometry providing the molecular weight. Thermal reaction of 7P-1 in
1
2
3
4
Polymerization was performed in solvent (ca. 20 wt./v.% of 7a) with catalyst (2 mol%).
Used as a powder.
A stock solution (0.025 M–NaOEt in benzene).
n-BuLi(1.0 M in hexane) was added to benzene containing ethanol to generate LiOEt
in situ.
5
Dibenzo-18-crown-6 (2 mol%) was used.
6
7
The ratio was determined by 1H NMR.
Recovered starting material (7a).

42
S.J. Jeon et al. / Reactive and Functional Polymers 100 (2016) 37–43
Fig. 4. 1H NMR spectra of (a) the ROP polymer 7P-1 recorded during thermal depolymerization for (b) 1 h and (c) 6 h leading to the partial formation of dimer 13, and (d) complete
depolymerization using NaOEt.
benzene under reflux resulted in a gradual increase in the formation of
dimer 13 with increasing reaction time. The conversion reached 60% in
6 h without NaOEt. Two peaks at 3.38 and 4.08 ppm in the 1 H NMR
spectrum of 13 were attributed to the methylene protons bound to S
and O, respectively, and separated from those of the polymer 7P-1.
The conversion was estimated from an integration ratio of 7P-1 and
13 (Fig. 4). The thermal behavior of the prepared polymers was exam-
ined by differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The DSC thermogram of polymer 7P-1 exhibited an en-
dothermic shift at 103 °C due to melting. Similarly, a melting process
was observed from polymers 7P-2 and 11P at 111 and 86 °C, respective-
ly. No crystallization accompanying an exothermic shift was observed in
the first scan and in the cooling step from high temperatures. Depoly-
merization might be severe around the melting temperature, which
prevented crystallization. The DSC curve showed no evidence of ther-
mal depolymerization because of the rapid heating at 10 °C/min. The
large endothermic shifts at 200 °C are difficult to explain and are com-
plicated by decomposition products. Polymer decomposition was re-
corded from TGA (Fig. 5). The polymer 7P-1 began to lose weight at
126 °C (onset temperature) and showed 10% weight loss at 171 °C;
the other polymers exhibited a higher Td (10%). Table 2 lists the thermal
behavior of the prepared polymers. The better thermal stability of
polymer 11P than 7P-1 and 7P-2 is partially understood by degradation
process involving a back-biting mechanism. This attack of a terminal
thiol group of 11P would occur via seven-membered cyclic approach,
which is kinetically unfavorable, contributing to the thermal stability,
while the other polymers (7P-1 and 7P-2) involve six-membered cyclic
approach resulting in fast thermal degradation (Supplementary data).
In addition, the polymer 7P-1 prepared at 70 °C exhibited larger molec-
ular weights than that at 50 °C. Therefore, high temperature increased
the reactivity of an intermediate mercapto anion for both polymeriza-
tion and depolymerization.
4. Conclusion
ROP of cyclic dithiocarbonates was demonstrated depending on ring
size. This polymerization competed with depolymerization via a back-
biting reaction, which occurred predominantly at N70 °C. Dibenzo-18-
C-6 was introduced to increase the reactivity of the anionic initiator,
which led to successful ROP at 50 °C without depolymerization. The for-
mation of a dimer during ROP confirmed the occurrence of depolymer-
ization, which was verified by a thermal reaction of the prepared
polydithiocarbonate. Six- and seven-membered cyclic dithiocarbonates
underwent polymerization, whereas the five-membered rings failed to
Fig. 5. Thermal properties of the prepared poly(dithiocarbonate)s by DSC (left) and TGA (right).

S.J. Jeon et al. / Reactive and Functional Polymers 100 (2016) 37–43
43
Table 2
Molecular weights and thermal properties of the poly(dithiocarbonate)s.
References
[1] O. Nuyken, S.D. Pask, Polymers 5 (2013) 361–403.
[2] S. Strandman, J.E. Gautrot, X.X. Zhu, Polym. Chem. 2 (2011) 791–799.
[3] J.-C. Lee, K.H. Litt, Macromolecules 33 (2000) 1618–1627.
[4] A. Duda, A. Kowalski, Thermodynamics and kinetics of ring-opening polymerization
(chapter 1), in: P. Dubois, O. Coulembier, J.-M. Raquez (Eds.), Handbook of Ring-
Opening Polymerization, Wiley-VCH Verlag, Weinheim, Germany 2009, pp. 1–51.
[5] T.F. Al-Azemi, K.S. Bisht, Macromolecules 32 (1999) 6536–6540.
[6] R. Jianming, X. Anguo, W. Hongwei, Y. Hailin, Des. Monomers Polym. 17 (2014)
345–355.
Polymers GPC results1
Mn(kDal) Mw(kDal) Mw/Mn Tm Td(onset) Td(5%) Td(10%)
Thermal properties (°C)
(7P-1)2 10.0
11.7
12.5
18.0
6.2
1.2
1.6
1.9
1.6
–
–
–
–
7P-1
7P-2
11P
7.8
9.3
3.8
103
111
86
126
205
230
151
226
245
171
240
269
1
The polymer was prepared at 70 °C in the anionic ROP condition without a crown
[7] J.M.G. Cowie, V. Arrighi, Polymers chemistry and physics of modern materials, third
ether. Other polymers were prepared at 50 °C using dibenzo-18-C6. Molecular
weights were measured by a gel-permeation chromatography using THF eluent with
a polystyrene standard.
ed. Taylor & Francis, Boca Raton, 2007 181–183.
[8] W.R. Ashcroft, Curing agents for epoxy resins (chapter 2), in: B. Ellis (Ed.), Chemis-
try and Technology of Epoxy Resins, Springer, Netherlands, Chapman & Hall 1993,
pp. 37–71.
[9] A. T, T. Takata, T. Endo, Macromolecules 30 (1997) 737–744.
[10] C. Bert, A. Celli, P. Marchese, E. Marianucci, C. Marega, V. Causin, A. Marigo, Polymer
48 (2007) 174–182.
[11] D.J. Darensbourg, S.J. Wilson, A.D. Yeung, Macromolecules 46 (2013) 8102–8110.
[12] C. Berti, A. Celli, E. Marianucci, M. Vannini, Eur. Polym. J. 41 (2005) 1812–1820.
[13] N. Nemoto, K. Yoshii, H. Kameshima, F. Sanda, T. Endo, J. Polym. Sci. A Polym. Chem.
41 (2003) 185–195.
provide a polymer likely due to the rapid depolymerization. Overall, an-
ionic ROP provides an alternative way to synthesize polydithiocarbonate
from stable cyclic dithiocarbonates.
Acknowledgments
[14] B. Ochiai, T. Endo, Prog. Polym. Sci. 30 (2005) 183–215.
[15] J. Hong, Q. Wang, Y. Lin, Z. Fan, Macromolecules 38 (2005) 2691–2695.
[16] W. Choi, F. Sanda, N. Kihara, T. Endo, J. Polym. Sci. A Polym. Chem. 35 (1997)
3853–3856.
[17] F. Sanda, T. Shinjo, W. Choi, T. Endo, Macromol. Rapid Commun. 22 (2001) 363–366.
[18] P. Hodge, React. Funct. Polym. 80 (2014) 21–32.
This subject was supported by Korea Ministry of Environment (MOE)
as “Chemical Accident Prevention Technology Development Project” and
by the Technological Innovation R&D Program of SMBA (S2127285).
[19] F. Sanda, J. Kamatani, T. Endo, Macromolecules 32 (1999) 5715–5717.
[20] P. Štacko, P. Šebej, A.T. Veetil, P. Klán, Org. Lett. 14 (2012) 4918–4921.
[21] O. Bergeot, C. Corsi, M.E. Qacemi, S.Z. Zard, Org. Biomol. Chem. 4 (2006) 278–290.
[22] C. Sklavounos, I.M. Goldman, D.E. Kuhla, J. Org. Chem. 45 (1980) 4239–4240.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.reactfunctpolym.2016.01.005.