Synthesis of sulfur-rich polymers: Copolymerization of episulfide with carbon disulfide by using [PPN]Cl/(salph)Cr(III)Cl system

Article abstract of DOI:10.1021/ja076056b

The completely alternating copolymerization of propylene sulfide with carbon disulfide was first achieved. Copolymerization by using anionic initiator [Ph₃P=N=PPh₃]Cl gave poly(propylene trithiocarbonate). The addition of chromium complex was found to accelerate the copolymerization. The obtained copolymer has a completely alternating structure with high regioregularity and showed large refractive index of 1.781-1.782. Copyright

Full text of DOI:10.1021/ja076056b

Published on Web 11/14/2007
Synthesis of Sulfur-Rich Polymers: Copolymerization of Episulfide with
Carbon Disulfide by Using [PPN]Cl/(salph)Cr(III)Cl System
Koji Nakano, Go Tatsumi, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received August 11, 2007; E-mail: nozaki@chembio.t.u-tokyo.ac.jp
Table 1. Copolymerization of Propylene Sulfide with CS2 by Using
[PPN]Cl/Complex 3 Systema
Introduction of sulfur atoms into polymer chain confers attractive
features, such as high optical and thermal properties, excellent
chemical resistance, and heavy-metal recognition ability on poly-
mers. For example, sulfur-enriched polymers have high refractive
index, advantageous for plastic lenses.1 Some synthetic routes were
reported to give polymers with sulfur atoms in the main chain, such
as polythioethers,2 polythiosulfites,3 poly(di)thiocarbonates,4 and
polythioesters.5 Polytrithiocarbonates have also been synthesized
from CS2 with dihalides and/or dithiols.4h,6 However, the favorable
formation of five-membered cyclic trithiocarbonates resulted in
unsuccessful or inefficient synthesis of polytrithiocarbonates with
the alternating C2 unit and trithiocarbonate unit [-C-C-S-
C(dS)-S-]n, which would be classified into one of the most sulfur-
enriched polymers. Synthesis of such poly(ethylene trithiocarbon-
ate)s has also been attempted through the copolymerization of
episulfide with CS2. The pioneering work on the copolymerization
was reported by Soga et al. in 1970s.7,8 The copolymerization in
the presence of CdEt2, ZnEt2, or Hg(SBu)2 was found to give the
copolymer. However, the activity was low and the resulting
copolymer contained a large amount of thioether linkages. We
herein report the first successful completely alternating copolym-
erization of episulfide with CS2.
The copolymerization of propylene sulfide (PS) with CS2 (1.0
equiv to PS) was investigated in the presence of anionic initiator
(PS/initiator ) 500) because (1) episulfides are known to polymerize
via anionic or coordination anionic mechanism2 and (2) CS2 also
reacts with anionic species or inserts into metal-thiolate bond,7a
resulting in the formation of dithiocarboxylate moiety. The use of
the combination of thiol and DBU (1,8-diazabicyclo[5.4.0]undec-
7-ene), which is effective initiator system for the living episulfide
ring-opening polymerization,2c gave only cyclic propylene trithio-
carbonate (2). In contrast, the copolymerization in the presence of
[PPN]Cl (0.20 mol %, [PPN]+ ) [Ph3PdNdPPh3]+) for 5 h yielded
the completely alternating copolymer, poly(propylene trithiocarbo-
nate) (1), while the selectivity for the copolymer was low [yield of
1 + 2 ) 16%, 1/2 ) 37/63, Mn ) 11600 (g‚mol-1), Mw/Mn ) 1.4]
(Table 1, run 1). The use of tetrabutylammonium chloride also gave
the copolymer (Table 1, run 2). However, the use of phosphonium
salt [Ph3PMe]Cl resulted in no polymerization.
initiator/
catalyst
yield of
Mn
(g·mol-
1
c
c
run
PS/CS2
1
+
2 (%)b
1/2b
)
Mw/Mn
1
2
3
4
5
6
7e
1/1
1/1
1/1
1/0.5
1/0.7
1/2
[PPN]Cl
Bu4NCl
16
5
37/63
42/58
80/20
48/52
71/29
88/12
92/8
11600
ndd
1.4
ndd
1.4
1.6
1.6
1.4
1.3
[PPN]Cl/3
[PPN]Cl/3
[PPN]Cl/3
[PPN]Cl/3
[PPN]Cl/3
60
44
52
53
90
20300
11000
15400
18200
42600
1/2
a Reaction conditions: PS (10 mmol), [PPN]Cl or Bu4NCl (0.20 mol
%), [complex 3 (0.20 mol %)], CS2 at 25 °C for 5 h. b Determined on the
basis of 1H NMR spectroscopy of the crude product by using dodecane as
an internal standard. c Determined by gel permeation chromatography using
a polystyrene standard. d nd ) not determined. e Premixed complex from
[PPN]Cl and complex 3 was used.
The copolymer/cyclic trithiocarbonate selectivity (1/2) depends
on the molar ratio of PS and CS2. A larger amount of CS2 tends to
give higher copolymer selectivity (Table 1, runs 3-6). We conclude
that the use of 2 equiv of CS2 to PS is optimal for both high
selectivity and good catalytic activity. The organic salt [PPN]Cl
has low solubility to the reaction mixture, probably leading to the
slow initiation. To meet this problem, the premixed complex was
prepared by treatment of [PPN]Cl and complex 3 in THF which
dissolves both compounds. The resulting premixed complex showed
higher solubility in a mixture of PS/CS2. In such a condition, the
simultaneous initiation should be possible, resulting in higher
activity (yield of 1 + 2 ) 90%, 5 h, Table 1, run 7). Furthermore,
the highest selectivity for the copolymer was also attained (1/2 )
92/8).
Copolymerization rate and the copolymer selectivity were
improved by adding a metal complex with salen-type ligand. The
screening revealed that (salph)CrCl complex 3 [(salph)H2 ) bis-
(3,5-di-tert-butylsalicylidene)-1,2-diaminobenzene] was the most
effective catalyst.9 Thus, the copolymerization by using a combina-
tion of [PPN]Cl and complex 3 at 25 °C for 5 h resulted in higher
conversion of PS to give copolymer 1 in higher selectivity [yield
of 1 + 2 ) 60%, 1/2 ) 80/20, Mn ) 20300 (g‚mol-1), Mw/Mn )
1.4] (Table 1, run 3). A blank reaction showed that complex 3 alone
does not copolymerize PS with CS2.
The obtained alternating copolymer is highly soluble in CS2, and
slightly soluble in CHCl3 and THF, while its solubility to the other
common organic solvents is quite low. The main chain sequence
was confirmed by 1H and 13C NMR spectroscopy (see the
Supporting Information). No signal was assignable to the thioether
moiety, demonstrating that the sequence was completely alternating.
The 13C NMR spectrum of the obtained copolymer suggests high
head-to-tail regioregularity based on analogous speculation from
the 13C NMR spectrum of poly(propylene carbonate).10 The 13C
NMR signal of each carbon splits into two or three peaks (Figure
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10.1021/ja076056b CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
cation is less able to catch the free anionic-chain end, and thus, the
copolymerization with [PPN]Cl alone yielded a larger amount of
cyclic trithiocarbonate (Table 1, compare run 1 with run 3).
In conclusion, we have reported the alternating copolymerization
of episulfide with CS2 by using [PPN]Cl as initiator and chromium
complex as activator. The obtained copolymer has a completely
alternating sequence and high refractive index. Further studies are
directed toward the elucidation of polymerization mechanism and
improvement in catalytic performance.
Acknowledgment. We are grateful to Prof. Takashi Kato (The
University of Tokyo) for DSC measurements. We also thank
Mitsubishi Gas Chemical Co., Inc. for refractive index measure-
ments. This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas “Advanced Molecular Transformations
of Carbon Resources” from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Figure 1. Comparison of 13C NMR signals of poly(propylene trithiocar-
bonate)s from (a) (R)-propylene sulfide (90% ee) and (b) rac-propylene
sulfide (in CDCl3 + CS2, 125 MHz).
Supporting Information Available: Experimental details, a pro-
posed polymerization mechanism, and NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
1b). This is likely to reflect the atactic structure of the main chain
on the basis of the fact that the copolymer from enantio-enriched
PS (90% ee) gives sharp and almost undivided peaks for each of
the carbons (Figure 1a). The thermal analyses of the copolymer
obtained from rac-PS demonstrated that it has glass transition
temperature around 25 °C and decomposes rapidly to cyclic
trithiocarbonate 2 at around 200 °C.11 Such thermal behavior is
similar to that of poly(propylene carbonate).12 Refractive index (nD)
of the obtained copolymer 1 was measured to be 1.78 (22 °C, cast
film), which is classified into the largest values among those of
the reported sulfur-containing polymers.1
Although the reaction mechanism is not clear yet, we speculate
that copolymerization takes place via a similar process to the
alternating copolymerization of epoxide with CO2 (see the Sup-
porting Information):13 Chromium complex should work as Lewis-
acidic center to activate an episulfide. Both of two axial chloride
ligands on the starting chromate complex [(salph)CrCl2][PPN]
would be capable to initiate the polymerization.13d
Preformation of cyclic trithiocarbonate 2 followed by subsequent
ring-opening polymerization is unlikely. Treatment of 2 with a
mixture of [PPN]Cl and complex 3 in CS2 did not afford the
polymeric materials. Furthermore, the reaction of PS with CS2 under
the optimized condition in the presence of cyclic 1,2-butylene
trithiocarbonate gave only pure copolymer 1 and the starting cyclic
carbonate was quantitatively recovered. Accordingly, it was mani-
fested that the copolymer was produced via the direct alternating
copolymerization rather than via cyclic trithiocarbonate formation.
Cyclic trithiocarbonate formation should be via the back-biting
from the anionic thiolate-chain end [-CH2-CHCH3-S-]11 rather
than from the anionic trithiocarbonate-chain end [-SC(dS)-S-].
As mentioned above, the higher concentration of CS2 is effective
to suppress the formation of 2 (Table 1, runs 3-6). Under the higher
CS2 concentration, CS2 insertion should be accelerated, decreasing
the concentration of the thiolate-chain end and increasing the
concentration of the trithiocarbonate-chain end. This corresponds
to the cyclic trithiocarbonate formation mainly from the thiolate-
chain end. We think that the chromium center is also important to
suppress the formation of cyclic trithiocarbonate: the chromium
center effectively catches the free anionic-chain end which would
be more favorable to back-biting than the resulting chromium-
bound-chain end. On the other hand, the non-coordinating organic
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