Synthesis of phosphinodiselenoic acid esters and their application as RAFT agents in styrene polymerization

Article abstract of DOI:10.1016/j.tetlet.2008.06.098

Phosphinodiselenoic acid esters are synthesized from the reaction of chlorodiphenylphosphine and aryl- or alkyl-magnesium bromide in the presence of selenium powder. They are employed as RAFT agents in thermally initiated, styrene polymerization. The phos

Full text of DOI:10.1016/j.tetlet.2008.06.098

Tetrahedron Letters 49 (2008) 5137–5140
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of phosphinodiselenoic acid esters and their application
as RAFT agents in styrene polymerization
Jeonju Moon a, Hyungoog Nam a, Sangseop Kim b, Jeonga Ryu b, Changhun Han b,
Chanhong Lee b, Sunwoo Lee a,
*
a Department of Chemistry, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea
b Petrochemicals and Polymers R&D, LG Chem, Ltd/Research Park, 104-1, Moonji-dong, Yuseong-gu, Daejeon 305-380, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphinodiselenoic acid esters are synthesized from the reaction of chlorodiphenylphosphine and aryl-
or alkyl-magnesium bromide in the presence of selenium powder. They are employed as RAFT agents in
thermally initiated, styrene polymerization. The phosphinodiselenoic acid esters 6a and 6b showed some
degree of control over the radical polymerization of styrene.
Received 28 May 2008
Revised 19 June 2008
Accepted 20 June 2008
Available online 25 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Phosphinodiselenoic acid esters
Selenium
Grignard reagent
RAFT agent
Living radical polymerization
Styrene polymerization
Conventional radical polymerization has been used widely in
industry because of its simple procedure and water-friendliness,
along with the functional group tolerance of monomers.1 However,
control of the polymerization is very difficult due to the very short
average lifetime of a radical. In order to overcome such problems,
three main types of controlled radical polymerization have been
reported: nitroxide-mediated polymerization (NMP),2 atom trans-
fer radical polymerization (ATRP)3 and reversible addition frag-
mentation transfer polymerization (RAFT).4 The latter, which has
recently been reported by Rizzardo et al., allows the synthesis of
polymers with well-defined molecular weight, polydispersities
and architectures, and can be applied to all monomers at low tem-
perature.5 The RAFT process has been found applicable in both
homogeneous and heterogeneous media.
The general structure of RAFT agents is based on the dithiocar-
boxylate moieties 1 (Fig. 1). However, Gigmes et al. reported that
dithiophosphinate esters 2 were used as RAFT agents instead of
dithioesters.6 Subsequently, Coote et al. suggested that phosphin-
odithiolate may have only limited use in controlling free-radical
polymerization based on high-level, ab initio calculations.7 Re-
cently, Murai et al. reported the synthetic method of P-chiral phos-
phinodiselenoic esters 3 bearing a P@Se double bond and a P–Se
single bond.8 We expected the phosphinodiselenoic acid esters to
Ph
Me
S
C
S
P
Se
P
R'
R
Ph
Ph
S
Ph
R
Se
Z
S
1
2
3
Figure 1. RAFT agents.
show better activity than phosphinodithiolate, because the bond
energy of carbon–selenium is lower than that of carbon–sulfur.9
This expectation stimulated us to synthesize the phosphinodisele-
noic acid esters and use them as RAFT agents. In the presence of
phosphinodiselenoic acid esters, the polymerization of styrene
proceeded in a similar way6 in the presence of phosphinodithio-
lates (Scheme 1). The propagating radical is added to the selenium
atom of the P@Se double bond and led to the phosphoranyl radical.
This radical compound fragments the new propagating radical and
generates the phosphinoselenium-capped polymer.
In this Letter, we report the synthesis of phosphinodiselenoic
esters and demonstrate their efficiency as a RAFT agent in the poly-
merization of styrene. To synthesize the phosphinodiselenoic es-
ters, we reacted the starting material chlorodiphenylphosphine
(ClPPh2) with selenium in toluene at 120 °C for 3 h (Scheme 2).
The black brown suspension mixture was converted to clear solu-
tion, and thin layer chromatography (TLC) showed only one spot.
The desired product, diphenylphosphinoselenoic chloride (4), was
* Corresponding author. Tel.: +82 62 530 3385; fax: +82 62 530 3389.
E-mail address: sunwoo@chonnam.ac.kr (S. Lee).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.098

5138
J. Moon et al. / Tetrahedron Letters 49 (2008) 5137–5140
Initiation
Thermal
initiation
Styrene
Pn
Pn
chain transfer
Pn
Se
P
Se
P
Se
P
R'
R'
+
R'
+
Pn
Ph
R
Se
Ph
Se
Ph
Se
R
R
3
reinitiation
R'
+
Pm
Styrene
chain equilibration
Pn
Pn
Se
P
Se
Se
Pm
Pm
P
P
Ph
Se
+
Pm
Ph
Se
+
Pn
Ph
Se
R
R
R
Styrene
Styrene
termination
R'
+
Pm
dead polymer
Scheme 1. Proposed mechanism of styrene polymerization in the presence of phosphinodiselenoic acid esters 3.
R SeMgCl
Se
P
Se
P
Se / THF
5a : R = Ph
R MgCl
Cl
+
Cl
120 oC, 3h
5b : R = 4-MeOC6H4
5c : R = 2,4,6-Me3C6H2
5d : R = tBu
THF,
0 oC
4
30 min
THF, rt
1h
Se
6a : R = Ph; 91%
6b : R = 4-MeOC6H4; 74%
6c : R = 2,4,6-Me3C6H2; 67%
6d : R = tBu; 83%
P
R
Se
Scheme 2. Synthesis of diphenylphosphinodiselenoic (Ed-confirm spelling) acid esters.
obtained in almost quantitative yield and used in the next
step without further purification. The addition of the reaction
mixture of aryl- or alkyl-magnesium chloride (e.g., PhMgCl,
4-MeOC6H4MgCl, 2,4,6-Me3C6H2MgCl, tBuMgCl) and elemental
selenium, which formed the magnesium selenolate 5, to phos-
phinoselenoic chloride afforded the corresponding diphenyl-
phosphinodiselenoic acid esters 6 in moderate to good yields.
Diphenylphosphinodiselenoic acid phenyl ester (6a) was obtained
in 91% yield. More sterically hindered aryl-magnesium chlorides
such as 2,4,6-Me3C6H2MgCl afforded the desired product 6c with
lower yield than the others. All products were purified by column
chromatography and were stable under air and moisture condition.
They were all characterized by nuclear magnetic resonance (NMR:
1H, 13C, 31P), and mass and elemental analyses.10
Next, styrene was polymerized with diphenylphosphinodisele-
noic acid esters as RAFT agents using thermal initiation. The purity
of all RAFT agents was established by NMR analysis prior to use.
RAFT styrene polymerization was performed at 126 °C without
any solvent.11 The molar ratio between the RAFT agent and styrene
was kept constant at 400 in all the polymerization processes. The
molar mass characteristics of the polymers were determined by
gel permission chromatography. The molar mass evolutions were
studied as a function of the conversion rate and time. In addition,
in order to check their effectiveness as RAFT agents, we also ob-
tained the polymerization data using the monocarboxyl-termi-
nated trithiocarbonate 7. The results are summarized in Table 1.
Figures 2 and 3 illustrate the trends of Mn as a function of time
and conversion rate of the polystyrenes.

J. Moon et al. / Tetrahedron Letters 49 (2008) 5137–5140
5139
PDI
Table 1
Evolution of number-average molecular weight of polystyrene versus conversion rate of styrene using RAFT agents
RAFT agent
Time (h)
Conversion (%)
Mn (g molÀ1
)
0.5
1
2
3
4
3.5
7.7
22.0
35.9
48.7
58.7
3970
7130
15,300
16,900
19,700
21,800
1.6
2.5
2.6
2.6
2.6
2.6
Se
P
Se
5
6a
Se
0.5
1
2
3
4
3.0
6.9
22.0
35.9
47.3
46.3
2630
5840
15,600
20,000
21,600
25,500
1.8
2.2
2.4
2.5
2.5
2.5
OCH3
P
Se
5
6b
H3C
Se
CH3
0.5
1
2
3
4
5.3
10.76
22.1
35.5
47.9
60.4
3740
4880
7910
11,800
14,800
18,100
1.8
2.0
2.8
3.8
4.8
4.8
Se
P
CH3
5
6c
0.5
1
2
3
4
4.3
7.7
17.3
27.2
37.9
46.7
16,700
18,100
25,100
30,800
37,200
39,500
1.8
2.0
2.2
2.3
2.3
2.2
Se
P
Se
5
6d
0.5
1
1.5
2
2.5
3
14.0
29.8
45.9
54.3
65.4
75.6
6527
12,561
16,966
19,065
21,796
23,890
1.1
1.1
1.1
1.1
1.1
1.1
S
C
C12H26
OH
S
S
O
7
No RAFT agent
0.5
1
2
11.2
18.5
42.0
121,500
160,800
174,520
4.7
5.3
5.8
As shown in Table 1 and Figures 2 and 3, the number-average
molecular weight increased linearly with time and degree of
monomer conversion. The increasing trends with phosphinodisele-
noic acid esters 6 were similar to those of RAFT agent 7. Amongst
them, 6a and 6b showed very similar trends in styrene polymeriza-
tion. However, the polymerization performed in the presence of
the phosphinodiselenoic acid esters 6 was somewhat slower than
that performed with 7. In 3 h, the phosphinodiselenoic acid esters
6 afforded a conversion yield below 36%, compared to over 75% for
the trithiocarbonate 7. The 6d showed a low polymerization rate.
6a
40000
35000
30000
6a
6b
6c
6d
6b
6c
6d
7
80
60
40
20
0
7
25000
20000
15000
10000
5000
0
0
1
2
3
4
5
0
10
20
30
40
50
60
70
80
Time (h)
conversion (%)
Figure 2. Trend of the conversion of thermally initiated RAFT polystyrenes as a
Figure 3. Trend of the number-average molar mass of thermally initiated RAFT
function of time.
polystyrenes as a function of conversion rate.

5140
J. Moon et al. / Tetrahedron Letters 49 (2008) 5137–5140
1555–1560; P@S bond dissociation energy (96 kcal/mol), P@Se bond
dissociation energy (75 kcal/mol) see: (b) Capps, K. B.; Wixmerten, B.; Bauer,
A.; Hoff, C. D. Inorg. Chem. 1998, 37, 2861–2864.
The number-average molecular weight of polystyrene using alkyl
ester derivative 6d was higher, and the polymerization rate was
lower than those with the aryl ester derivatives (6a, 6b and 6c)
and RAFT agent 7. The polydispersity indices of polystyrene using
6a and 6b were 1.6–2.5, whilst that of polystyrene using 6c was
4.8. These results suggested that phosphinodiselenoic acid esters
6a and 6b can act as RAFT agents, and exhibited living character
in the polymerization of styrene.
We synthesized diphenylphosphinodiselenoic acid esters at
high yields from chlorodiphenylphosphine. They were very stable
towards air and moisture. To the best of our knowledge, this is
the first report of their use as RAFT agents in styrene polymeriza-
tion. Amongst them, the phosphinodiselenoic acid esters 6a and
6b showed living character in the polymerization of styrene.
10. Synthesis of phosphinodiselenoic acid esters: To
a suspension of elemental
selenium (870 mg, 11.0 mmol) in THF (50 mL) was added 11.0 mL of
phenylmagnesium chloride (1.0 M in THF, 11 mmol) at 0 °C, and the mixture
was stirred at that temperature for 30 min. This mixture was added to 3.14 g of
diphenylphosphinoselenoic chloride (10.5 mmol) in THF (30 mL) dropwise
over a period of 10 min at 0 °C with vigorous stirring. The reaction mixture was
raised to room temperature and stirred for 1 h. The desired product spot
appeared in TLC (eluent hexane/EtOAc = 2:1, Rf = 0.45). After removal of the
solvent, the crude mixture was purified by column chromatography on silica
gel to afford 4.01 g (9.54 mmol, 91%) of 6a. 1H NMR (CDCl3, 300 MHz) d 7.88
(ddd, J = 14.7, 7.8, 1.8 Hz, 4H), 7.48–7.30 (m, 9H), 7.18 (dd, J = 7.8, 7.2 Hz, 2H);
13C NMR (CDCl3, 75 MHz) d 136.85 (d, J = 2.9 Hz), 13.277 (d, J = 66.8 Hz), 132.07
(d, J = 11.2 Hz), 131.78 (d, J = 3.2 Hz), 129.51 (d, J = 2.3 Hz), 129.37 (d,
J = 13.3 Hz), 126.24 (d, J = 6.3 Hz); 31P NMR (CDCl3, 121.5 MHz)
d 43.50
(366.4, 774.4 Hz); MS(EI) m/z: 422 (M+); Anal. Calcd for C18H15PSe2: C, 51.45;
H, 3.60. Found: C, 51.74; H, 4.00. Compound 6b (74%); 1H NMR (CDCl3,
300 MHz) d 7.88 (ddd, J = 12.0, 7.8, 1.8 Hz, 4 H), 7.49–7.36 (m, 6H), 7.27 (dd,
J = 8.7, 2.1 Hz, 2H), 6.72 (d, J = 8.7 Hz, 2H), 3.74 (s, 3H); 13C NMR (CDCl3,
75 MHz) d 160.86 (d, J = 2.9 Hz), 138.40 (d, J = 2.8 Hz), 132.79 (d, J = 65.9 Hz),
132.07 (d, J = 11.2 Hz), 131.72 (d, J = 3.5 Hz), 128.35 (d, J = 13.3 Hz), 116.45 (d,
J = 5.7 Hz), 114.79 (d, J = 2.6 Hz), 55.17; 31P NMR (CDCl3, 121.5 MHz) d 43.29
(373.2, 771.5 Hz); MS(EI) m/z: 452 (M+); Anal. Calcd for C19H17OPSe2: C, 50.69;
H, 3.81. Found: C, 51.02; H, 4.07. Compound 6c (67%); 1H NMR (CDCl3,
300 MHz) d 7.90 (ddd, J = 14.7, 8.4, 1.8 Hz, 4H), 7.45–7.36 (m, 6H), 6.88 (s, 2H),
Acknowledgement
This work was funded by Fundamental R&D Program for Core
Technology of Materials of Korean Ministry of Knowledge
Economy.
2.24 (d, J = 2.4 Hz, 3H), 2.19 (s, 6H); 13C NMR (CDCl3, 75 MHz)
d 144.81
References and notes
(d,J = 3.8 Hz), 139.78 (d, J = 3.8 Hz), 134.31 (d, J = 66.0 Hz), 131.87 (d,
J = 10.9 Hz), 131.53 (d, J = 3.2 Hz), 128.97 (d, J = 2.9 Hz), 128.25 (d,
J = 13.0 Hz), 125.98 (d, J = 6.3 Hz), 24.78, 21.06; 31P NMR (CDCl3, 121.5 MHz)
d 39.51 (380.9, 774.3 Hz); MS(EI) m/z: 464 (M+); Anal. Calcd for C21H21PSe2: C,
54.56; H, 4.58. Found: C, 54.83; H, 4.92. Compound 6d (83%); 1H NMR (CDCl3,
300 MHz) d 8.01 (ddd, J = 14.7, 7.8, 2.1 Hz, 4H), 7.47–7.42 (m, 6H), 1.57 (td,
J = 6.0, 1.2 Hz, 9H); 13C NMR (CDCl3, 75 MHz) d 133.97 (d, J = 66.9 Hz), 132.14
(d, J = 11.2 Hz), 131.50 (d, J = 3.2 Hz), 128.33 (d, J = 12.9 Hz), 54.29 (d,
J = 4.0 Hz), 32.97 (d, J = 3.8 Hz); 31P NMR (CDCl3, 121.5 MHz) d 30.92 (403.0,
760.9 Hz); MS(EI) m/z: 402 (M+); Anal. Calcd for C16H19PSe2: C, 48.02; H, 4.79.
Found: C, 47.94; H, 4.88.
1. Hawker, C. J. Acc. Chem. Res. 1997, 30, 373–382.
2. Konn, C.; Morel, F.; Beyou, D. L.; Chaumont, P.; Bourgeat-Lami, E.
Macromolecules 2007, 40, 7464–7472.
3. Tsarevsky, N. V.; Matyjaszewski, K. Chem. Rev. 2007, 107, 2270–2299.
4. (a) Mccormick, C. L.; Lowe, A. B. Acc. Chem. Res. 2004, 37, 312–325; (b) Barner-
Kowollik, C.; Cavis, T. P.; Heuts, J. P. A.; Stenzel, M. H.; Vana, P.; Whittaker,M. J.
Polym. Sci., Part A: Polym. Chem. 2003, 41, 365–375.
5. (a) Chong, B. Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. H.
Macromolecules 1999, 32, 2071–2074; (b) Mayadunne, R. T. A.; Rizzardo, E.;
Chiefari, J.; Moad, G.; Thang, S. H. Macromolecules 1999, 32, 6977–6980.
6. Gigmes, D.; Berin, D.; Marque, S.; Guerret, O.; Tordo, P. Tetrahedron Lett. 2003,
44, 1227–1279.
7. Hodgson, J. L.; Green, K. A.; Coote, M. L. Org. Lett. 2005, 7, 4581–4584.
8. Kimura, T.; Murai, T. J. Org. Chem. 2005, 70, 952–959.
9. C–S bond energy (269 kJ/mol), C–Se bond energy (251 kJ/mol) see: (a)
Yamazaki, S.; Mizuno, W.; Yamabe, S. J. Chem. Soc., Perkin Trans. 1 1991,
11. Polymerization: RAFT agent 6 (0.48 mmol) and styrene (20.0 g, 192.0 mmol)
were added in a Schlenk flask. The reaction mixture was degassed by freeze
and thaw cycles and sealed under nitrogen. Bulk thermally initiated
polymerization was conducted at 126 °C. During the reaction, a sample was
taken under nitrogen atmosphere and quenched in cold water, and then
diluted with methylene chloride. The polymer was purified by precipitation
from methylene chloride solution into methanol.