Reaction of cyclic tetrathiophosphates with carboxylic acids as a means to generate dithioesters and control radical polymerization by RAFT

Article abstract of DOI:10.1002/anie.200250770

Radical control: Controlled free-radical polymerization of styrene and methyl (meth)acrylate is achieved by using Davy reagents or P₄S₁₀ in conjunction with benzoic acid, which leads to in situ formation of dithioesters that act as chain-transfer agents and thus control the reaction. In the case of P₄S₁₀/PhCOOH, d[thiobenzoic acid is the key intermediate in this process (see scheme).

Full text of DOI:10.1002/anie.200250770

Angewandte
Chemie
Controlled Radical Polymerization
thiocarbonylthio groups (Scheme 1)[4] without isolatingany
intermediates.
Styrene was first polymerized in the bulk at 1108C in the
presence of P S and PhCOOH in a one-pot procedure. After
Reaction of Cyclic Tetrathiophosphates with
Carboxylic Acids as a Means to Generate
Dithioesters and Control Radical Polymerization
By RAFT**
4
10
50 h this afforded a polystyrene (PS) sample with M =
n
ꢀ
1
5800 gmol and a polydispersity index (PDI) of 1.4 (34%
conversion). A similar reaction involvingstyrene, P S , and
4
10
PhCOOH was also performed at 608C with azobis(isobutyr-
onitrile) (AIBN), as a radical source. This vastly improved
Alex DurØault, Yves Gnanou,* Daniel Taton,*
Mathias Destarac, and FrØdØric Leising
control over the polymerization: a PS sample with M =
n
Substantial research has been devoted in recent
years to controlled radical polymerization
(
CRP).[1] The CRP methods combine the advan-
tages of truly “living” systems with regard the
quality of the polymers formed with the inherent
ease of radical processes.[ Amongthe CRP
1]
systems,
nitroxide-mediated
polymerization
[
2]
(
(
NMP), atom-transfer radical polymerization
ATRP), and reversible addition fragmentation
[
3]
2
3
4
[
4]
Scheme 1. Z=alkyl or aryl, OR, NR R , SR .
chain transfer (RAFT) have been extensively
investigated. However, all of these methods have
3300 gmol 1 and PDI = 1.11 was obtained after 16 h (19%
conversion). To explain these differences, we hypothesized
that the reaction between PhCOOH and P4S10 generated
dithiobenzoic acid (DTBA) as an intermediate (Scheme 2).
Dithiobenzoic acid is usually prepared by reaction of
ꢀ
drawbacks that limit their industrial development. For
example, NMP often requires elevated temperatures and is
[
2]
only applicable to a limited number of monomers, and
ATRP, though very powerful, is ineffective for monomers that
can poison the catalyst (e.g., acrylic acid, acrylamide);
moreover, the removal of metal ions from polymers is an
unresolved issue on the industrial scale.[ As to the RAFT
process, the preparation of the correspondingchain-transfer
agents suffers from drawbacks, such as the use of carbon
disulfide or iodine, two toxic reagents.[ This situation shows
the necessity to improve CRP methods before they can be
implemented in industry.
bromobenzene with magnesium and subsequent treatment
with CS and aqueous HCl.
add to substituted alkenes to form dithioesters. Dependingon
the electron density of the alkene, the addition can either
result in a Michael-type reaction (path B) or follow a
3]
[10,11]
Dithioic acids, such as DTBA,
2
4]
[
12,13]
Markownikoff-type mechanism (path A).
Therefore, it
is likely that the reaction between PhCOOH and P4S10 first
generated DTBA, which subsequently added to styrene to
give phenylethyl dithiobenzoate (path A in Scheme 2). Such a
dithioester, formed in situ, then acted as a chain-transfer
agent (CTA) and controlled the RAFT polymerization of
styrene.
Next, P4S10 and PhCOOH were heated together in
toluene, and the product formed was treated with NaH and
bromopropionyl bromide. Propionyl dithiobenzoate was
formed (path D in Scheme 2).
Herein we describe the preparation of dithioesters by
reaction of Davy reagents or P4S10 with benzoic acid. These
dithioesters serve to control in situ the free-radical polymer-
ization of styrene and alkyl (meth)acrylates. Davy reagents
and P4S10 are commercially available cyclic tetrathiophos-
phates that are widely employed for the thionation of
carbonyl groups and the preparation of thioamides, thiopep-
[
5–9]
tides, thiolactams, and dithioesters.
Our aim is to provide
an efficient, single-step procedure to access well-defined
polymers by generating RAFT agents in situ. A major
advantage of our approach is that polymerization can be
initiated and controlled by chain-to-chain transfer of w-
To account for the role of AIBN in the polymerization
carried out at 608C, a mixture of PhCOOH, P S , and AIBN
was heated in toluene at 1108C (no monomer was added).
4
10
After workup, cyanopropan-2-yl dithiobenzoate was recov-
1
ered, as demonstrated by H NMR spectroscopy. This
[
*] Dr. Y. Gnanou, Dr. D. Taton, Dr. A. DurØault
Laboratoire de Chimie des Polym›res Organiques
ENSCPB - UniversitØ Bordeaux 1
dithioester was formed by the radical path C in
[
14,15]
Scheme 2.
Cyanopropan-2-yl dithiobenzoate being ge n-
erated under these conditions methyl methacrylate (MMA)
was added, and its polymerization was initiated with AIBN at
16 Avenue Pey-Berland, 33607 Pessac (France)
Fax: (+33)5-5684-8487
E-mail: gnanou@enscpb.fr
taton@enscpb.fr
6
08C (Table 1). Figure 1 shows size exclusion chromatogra-
phy (SEC) traces of samples at different reaction times: the
molar mass of poly(methyl methacrylate) (PMMA) evolved
linearly with increasingmonomer conversion, and PDI values
remained below 1.3, indicatinga controlled polymerization.
Styrene was also polymerized in the presence of Davy-R
reagents (Davy reagents carrying an R alkyl group) in
Dr. D. Taton, Dr. M. Destarac, Dr. F. Leising
RHODIA, Centre de Recherches d'Aubervilliers
5
2 rue de la Haie Coq, 93 308 Aubervilliers cedex (France)
**] Financial support by Rhodia is gratefully acknowledged. RAFT
reversible addition fragmentation chain transfer.
[
=
Angew. Chem. Int. Ed. 2003, 42, 2869 – 2872
DOI: 10.1002/anie.200250770
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2869

Communications
Scheme 2.
Table 1: Polymerization of MMA in toluene at 608C with P S , PhCOOH,
4
10
and AIBN.[
a]
t [h]
Conversion [%]
M [gmolꢀ1]
PDI
n
0
1
2
3
.62
11.2
18.2
32.3
42.5
6300
9200
14900
18700
42700
1.3
1.25
1.2
1.19
1.25
.25
.25
.17
1
7.17
100
ꢀ1
[
5
a] [PhCOOH]=0.06 molLꢀ1;
.82 molL .
[AIBN]=0.0135 molL
;
[MMA]=
ꢀ
1
conjunction with PhCOOH. It was first verified that the
reaction of a Davy-R reagent with PhCOOH effectively
Figure 1. SEC traces (RI) of samples obtained by polymerizing MMA in
toluene at 608C with P S , AIBN, and PhCOOH, see Table 1;
a) t=0.62 h, b t=1.25 h, c) t=2.25 h, d) t=3.17 h, e) t=17.17 h.
4
10
[
7–11]
yields the dithioester PhC(¼S)SR (Scheme 3).
wards, styrene and AIBN were added to
a medium containingbenzyl dithioben-
zoate formed in situ, and the mixture was
heated to 608C (Table 2). A continuous
increase in molar mass with increasing
monomer conversion was observed by
SEC. This indicates that methyl, phenyl,
and benzyl dithiobenzoates formed in situ
controlled the polymerization of styrene.
The PDI of the PS samples decreased in
the followingorder: R = benzyl > R =
phenyl > R = methyl, which is related to
the rate of fragmentation of the homolytic
After-
[
4]
group R. We also noted that the Mn
Scheme 3.
2
870
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Angew. Chem. Int. Ed. 2003, 42, 2869 – 2872

Angewandte
Chemie
Table 2: Polymerization of styrene in toluene at 608C with Davy-R
Davy-benzyl reagent was prepared as previously reported in
reagents and PhCOOH.
ref. [1617] Polymerization reactions were performed after degassing
by freeze–thaw cycles. NMR spectra were recorded on a Bruker
AC200 NMR spectrometer. SEC was performed in THF at 258C
(flow rate 1 mLmin ) on a Varian apparatus equipped with
refractive index (Varian)/UV dual detection and fitted with TSK
column pack (G3,000HXL, G4,000HXL, G2,000HXL) or on a fast
SEC column (PSS SDV linear M 5m 8 ” 300 mm). Calibration was
performed with linear PS standards.
R
t [h]
Conversion [%]
M [gmolꢀ1]
PDI
n
ꢀ1
CH3
17.83
7
50
17.5
37.5
22
2600
15000
2450
2740
1200
1.14
1.72
1.25
1.53
1.1
1
1
18
Ph
44.5
41.5
7.5
CH Ph
2
2
3
45
4000
1.05
Synthesis of cyanoisopropyl dithiobenzoate (Scheme 2, path C):
ꢀ
3
ꢀ3
P4S10 (580 mg, 1.3 ” 10 mol), PhCOOH (670 mg, 5.5 ” 10 mol),
ꢀ2
AIBN (2.5 g, 1.5 ” 10 mol), and toluene (30 mL) were heated at
108C for 1 h. The mixture was poured into cyclohexane and the
filtrate was purified by chromatography (silica gel in CH Cl and
1
values did not match those expected from the ratio of
monomer] to [PhCOOH] used. This result was attributed to
2
2
[
alumina with hexane/diethyl ether 9/1).
1
H NMR (200 mHz, CDCl ):
3
incomplete formation of the dithioester from the reaction of
Davy-R reagents with PhCOOH. However, all experiments
were reproducible.
Finally, this method was applied to the synthesis of block
copolymers. Figure 2 shows the SEC trace of a poly(methyl
acrylate) (PMA) precursor synthesized by generating benzyl
dithiobenzoate in situ and the subsequently formed PMA-b-
PS diblock copolymer.
d = 1.9 (6H, s, 2 ” CH ), 7.4 (2H, dd, m-ArH), 7.6 (1H, dd, p-ArH),
3
1
3
7
4
2
.9 ppm (2H, d, o-ArH); C NMR (50 mHz, CDCl ): d = 27 (CH ),
2.3 (C(CN)), 120.6 (CN), 127.3, 129.2, 133.5, 145.1 (ArC),
23.75 ppm (C¼S); MS: m/z (%): 221 (11), 153 (16), 121 (85), 105
3
3
(68), 77 (100).
Synthesis of propionyl dithiobenzoate (Scheme 2, Path D): P S
7.3 g, 1.64 ” 10 mol), PhCOOH (2 g, 1.64 ” 10 mol), and toluene
4
10
ꢀ2
ꢀ2
(
(30 mL) were heated at 1108C for 1 h. The mixture was washed with
water at pH 10. After washingwith diethyl ether, the aqueous phase
was reacidified. After addition of acetonitrile, the solution was
treated with NaH. Methyl 2-bromopropionate bromide was added to
this suspension and the mixture stirred for 20 h at room temperature.
After concentration, the solution was washed with water and the
product purified by column chromatography on silica gel in CH Cl /
2
2
1
heptane (9/1). H NMR (200 mHz, CDCl ): d = 1.67 (3H, d, CH ),
3
3
3
.76 (3H, s, COOCH ), 4.77 (1H, q, CSSCHCH ), 7.2–8.1 ppm (5H,
3
3
1
3
m, Ph); C NMR (50 mHz, CDCl ): d = 127–145 (Ar), 224 ppm (C¼
3
S); MS: m/z (%): 240 (8), 207 (7), 121 (100), 77 (31).
Polymerization of MMA with P4S10 and PhCOOH (Table 1):
ꢀ
4
ꢀ4
PhCOOH (67 mg, 5.5 ” 10 mol), P4S10 (58 mg, 1.31 ” 10 mol),
ꢀ
3
and AIBN (250 mg, 1.52 ” 10 mol) were stirred at 1108C in toluene
ꢀ2
(
3 mL) for 1 h. After coolingto 60 8C, MMA (6 mL, 5.24 ” 10 mol)
ꢀ
4
and AIBN (20 mg, 1.22 ” 10 mol) were added. The mixture was
stirred for 24 h. The product was analyzed by SEC after removal of
MMA by evaporation (Table 2).
Synthesis of benzyl dithiobenzoate (Scheme 3) and its use to
control the polymerization of styrene (Table 2): Davy-benzyl reagent
ꢀ4
ꢀ3
(
370 mg, 8.25 ” 10 mol), PhCOOH (207 mg, 1.65 ” 10 mol), and
toluene (15 mL) were heated at 1108C for 1 h. Two-thirds of this
mixture was withdrawn for analysis after purification by chromatog-
Figure 2. SEC traces (RI) of a PMA obtained by polymerizing MA in
toluene at 608C with Davy-benzyl reagent and PhCOOH (a) and PMA-
b-PS diblock copolymer after chain extension (b).
1
raphy on silica gel (CH Cl ). H NMR (200 mHz, CDCl ): d = 4.62
2
2
3
1
3
(
2H, s, CSSꢀCH Ph), 7.2–8.1 ppm (10H, m, Ph); C NMR (50 mHz,
2
CDCl ): d = 42.88 (SCH Ph), 127–145 (Ar), 224 ppm (C¼S); MS: m/z
3
2
(
%): 244 (13), 228 (25), 121 (22), 105 (100), 91 (37), 77 (38). The
remainingthird of the mixture was cooled to 60 8C and then styrene
In summary, the use of P4S
10
or Davy reagents in
ꢀ2
ꢀ4
(6 mL, 5.24 ” 10 mol) and AIBN (17.2 mg, 1.05 ” 10 mol) were
conjunction with PhCOOH proved versatile in controlling
the free-radical polymerization of styrene and (meth)acry-
lates under simple conditions. Dithioesters are generated
in situ and serve as CTAs in a RAFT process. Dithiobenzoic
acid formed by reaction of P4S10 with PhCOOH is the key
intermediate in the synthesis of dithioesters. Work is in
progress to apply this simple method to the preparation of
star polymers.
added. The solution was analyzed by SEC after removal of styrene by
evaporation.
Received: December 13, 2002
Revised: February 12, 2003 [Z50770]
Keywords: block copolymers · phosphorus · polymerization ·
.
radical reactions · sulfur
Experimental Section
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10
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(
late, and methyl acrylate (Aldrich) were distilled from CaH . The
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www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2871

Communications
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