Synthesis of multifunctional dithioesters using tetraphosphorus decasulfide and their behavior as RAFT agents

Article abstract of DOI:10.1021/ma030420j

The reaction of tetraphosphorus decasulfide (P₄S₁₀) with multicarboxylic acids and benzyl thiol afforded multifunctional dithioesters by a simple experimental procedure. The dithioesters were subsequently used as chain transfer agents to grow poly(iert-butylacrylate) and/or polystyrene chains in a controlled fashion by reversible addition-fragmentation chain transfer (RAFT). RAFT polymerizations of styrene were performed thermally at 110°C either in bulk or in toluene solution, while tert-butylacrylate was polymerized at 60°C in toluene in the presence of AIBN as the radical source. The multifunctional RAFT agents provided good control over the molar masses and polydispersities of the polymers formed, even though the presence of some linear dead chains generated by irreversible terminations could not be avoided. The contribution of the latter reactions was enhanced with the increase of viscosity at high conversion and with the shielding effect created by the polymeric arms around the RAFT centers of the core. Better control could be achieved when performing RAFT-mediated polymerizations in toluene solution as compared to bulk experiments. This methodology was applied to synthesize not only linear poly(iertbutylacrylate-b-styrene-b-tert-butylacrylate) triblock copolymers, but also three-arm polystyrene stars following an arm-first approach.

Full text of DOI:10.1021/ma030420j

Macromolecules 2004, 37, 5513-5519
5513
Synthesis of Multifunctional Dithioesters Using Tetraphosphorus
Decasulfide and Their Behavior as RAFT Agents
Alex Du r e´a u lt,† Da n iel Ta ton ,*,†,‡ Ma th ia s Desta r a c,‡ F r e´d e´r ic Leisin g,‡ a n d
Yves Gn a n ou *,†
Laboratoire de Chimie des Polyme`res Organiques, ENSCPB-CNRS-Universite´ Bordeaux 1, 16,
Avenue Pey Berland, 33 607 Pessac Cedex, France, and Rhodia, Centre de Recherches d’Aubervilliers,
52 rue de la Haie Coq, 93 308 Aubervilliers Cedex, France
Received August 8, 2003; Revised Manuscript Received May 5, 2004
ABSTRACT: The reaction of tetraphosphorus decasulfide (P4S10) with multicarboxylic acids and benzyl
thiol afforded multifunctional dithioesters by a simple experimental procedure. The dithioesters were
subsequently used as chain transfer agents to grow poly(tert-butylacrylate) and/or polystyrene chains in
a controlled fashion by reversible addition-fragmentation chain transfer (RAFT). RAFT polymerizations
of styrene were performed thermally at 110 °C either in bulk or in toluene solution, while tert-butylacrylate
was polymerized at 60 °C in toluene in the presence of AIBN as the radical source. The multifunctional
RAFT agents provided good control over the molar masses and polydispersities of the polymers formed,
even though the presence of some linear dead chains generated by irreversible terminations could not be
avoided. The contribution of the latter reactions was enhanced with the increase of viscosity at high
conversion and with the “shielding effect” created by the polymeric arms around the RAFT centers of the
core. Better control could be achieved when performing RAFT-mediated polymerizations in toluene solution
as compared to bulk experiments. This methodology was applied to synthesize not only linear poly(tert-
butylacrylate-b-styrene-b-tert-butylacrylate) triblock copolymers, but also three-arm polystyrene stars
following an “arm-first” approach.
Sch em e 1. Mech a n ism of th e RAF T P r ocess
In tr od u ction
An important development in the area of controlled/
living radical polymerization came about with the use
of thiocarbonylthio compounds, Z-C(dS)SR, as revers-
ible chain transfer agents (CTAs) in two independently
discovered processes.1-4 These have been named RAFT
for reversible addition fragmentation chain transfer1,2
and MADIX for macromolecular design via interchange
of xanthates.3,4 In these polymerizations (Scheme 1), the
CTA first reacts through its CdS double bond with
propagating oligomers, leading to a transient radical
(step 2). The fate of the latter species depends on both
the activating substituent (Z) and the leaving group (R)
of the CTA. In the ideal case, this radical intermediate
undergoes a â-scission generating a new thiocarbon-
ylthio compound and an expelled R° radical capable of
reinitiating the polymerization (steps 3 and 4). Increas-
ing the stability of the radical intermediate may cause
the polymerization to be retarded and/or inhibited, in
particular when low molar mass polymers are targeted;
this is a subject of debate in the current literature.5-14
A rapid chain-to-chain transfer of the ω-thiocarbonylthio
groups between dormant chains and radical active
species ensures a controlled/living growth of the chains
(step 5). A fast equilibrium relative to the rate of
propagation is highly desirable should polymers with
narrow molar mass distributions be required. For this
purpose, both the activating substituent Z and the R
leaving group of the CTA, Z-C(dS)-S-R, should be
properly chosen.1-25
mechanistic investigations,5-14 their implementations
to dispersed media2,4,26-29 or to homogeneous aqueous
solutions,30-32 and their utilization for macromolecular
engineering.10,33-41
Among the various types of RAFT agents reported to
date, including dithioesters, dithiocarbamates, trithio-
carbonates, and xanthates, only a few have been used
for star polymer synthesis.2,36-41 In contrast, numerous
reports on the preparation of star polymers by atom
transfer radical polymerization (ATRP)41-43 or, more
seldom, by nitroxide-mediated polymerization (NMP)44,45
can be found in the literature. However, recent reports
have shown the possibility to derive multiarm star
polymers by RAFT, through the use of multifunctional
dithioesters2,36 or trithiocarbonates.38-40 Two different
types of multifunctional CTAs can be contemplated for
use in a RAFT process: those implying an outward
growth of arms from the core and those involving the
reaction of linear chains with the functional core,
following the so-called “core-first” and “arm-first” ap-
proaches, respectively.2,36-40
Since their discovery, significant efforts have been
directed at establishing the scope and limitations of
RAFT and MADIX processes. These include kinetic and
* Corresponding authors. Telephone: 33-540-006987. Fax: 33-
540-008487. E-mail: taton@enscpb.fr, gnanou@enscpb.fr.
† ENSCPB-CNRS-Universite´ Bordeaux 1.
‡ Rhodia.
10.1021/ma030420j CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/29/2004

5514 Dure´ault et al.
Macromolecules, Vol. 37, No. 15, 2004
Sch em e 2. Syn th esis of th e Di- a n d Tr ifu n ction a l RAF T Agen ts
difunctional dithioester was followed except that benzene
tricarboxylic acid as a trifunctional precursor and dioxane as
Generally speaking, the procedures for synthesizing
RAFT agents, including multifunctional ones, suffer
from some disadvantages, such as the use of toxic
reagents (e.g., iodine, carbone disulfide) or procedures
that are not tolerant of functional groups.46-53 Here, we
demonstrate that the use of cheap and mild reactants
such as cyclic tetrathiophosphates (P4S10 and Davy
reagents) for the synthesis of a di- and a trifunctional
dithioester can substantially prevent some of the short-
comings associated with the synthesis of RAFT agents.
P4S10 and Davy reagents have been employed for the
thionation of carbonyl groups and the preparation of
thioamides, thiopeptides, thiolactames, and dithio-
esters.46-53 We have recently shown that dithioesters
can be obtained in just one step from the reaction of
P4S10 (or Davy reagents) with a carboxylic acid in the
presence of an alkyl bromide - or radical source - and
can be used in situ to control free-radical polymeriza-
tions by a RAFT-type mechanism.54 This report de-
scribes (i) the synthesis of multifunctional dithioesters
using P4S10, a thiol (RSH), and a multicarboxylic acid
precursor, and (ii) the subsequent utilization of these
dithioesters to prepare ABA-type triblock copolymers
and three-arm polystyrene stars by RAFT.
1
a solvent were employed (yield ) 23%). H NMR: δ 7.1-8.8
ppm (18H, m, Ar ); 4.6 ppm (6H, s, 3 × CSSCH2Ph). 13C NMR:
δ 43.3 ppm (SCH2Ph); 123-145 ppm (Ar ); 225 ppm (CdS).
MS: 476 (18%); 453 (22%); 330 (30%); 207 (60%); 91 (75%); 75
(100%).
Gen er a l P r oced u r e for P olym er iza tion Exp er im en ts.
All polymerizations were performed in a Schlenk apparatus
under homogeneous conditions. The required concentrations
of the RAFT agent, the monomer, and the initiator were mixed
with the solvent. The solutions were degassed by three freeze-
pump-thaw cycles and placed in a thermostated oil bath prior
to the polymerization. After a given time, unconverted mono-
mer was removed by evaporation under vacuum without
heating, and the residue was analyzed directly by SEC. In a
typical experiment, 10 mg (2.17 × 10-5 mol) of the difunctional
RAFT agent, 3 mL (2.05 × 10-2 mol) of tert-butyl acrylate, and
1.5 mg (9.15 × 10-6 mol) of AIBN were dissolved in 3 mL of
toluene. After degassing, the solution was heated at 60 °C for
7 h under nitrogen atmosphere. The polymer was obtained
with 10% of conversion, Mn ) 10 500 g mol-1; PDI ) 1.12. The
polymer compound (215 mg, 2.04 × 10-5 mol) was subse-
quently used as a macro-CTA for chain extension by adding
styrene (1 mL, 8.74 × 10-3 mol) and 4 mL of toluene. After
degassing, the solution was heated at 110 °C for 17 h under
nitrogen atmosphere. Conversion of the obtained polymer was
20% with Mn ) 20 200 g mol-1; PDI ) 1.20 (Figure 7).
Exp er im en ta l Section
Ch a r a cter iza tion . NMR spectra were obtained using a
Bruker AC200 NMR spectrometer. Apparent molar masses of
homopolymers and block copolymers were obtained by SEC
using a refractive index detector (Varian). SEC analysis was
performed in THF at 25 °C at a flow rate of 1 mL/min using
a refractometer (Varian) and a UV-visible spectrophotometer
(Varian) operating at 254 nm, and either 4 TSK columns
G5000HXL (9 µm), G4000HXL (6 µm), G3000HXL (6 µm), and
G2000HXL (5 µm) or a PSS column (SDV linear M 5µ 8 × 300
mm). Calibration was performed using linear polystyrene
standards. The actual molar masses of some samples (entries
10 and 11 in Table 1) were drawn from the response of a
multiangle laser light scattering detector (Wyatt Technology,
ASTRA Software) that was connected to the SEC line with a
laser source (He-Ne) operating at 633 nm. It was assumed
that the dn/dc values for polystyrene stars were the same as
that of linear polystyrene (dn/dc ) 0.183 cm3/g).43
Ma ter ia ls. All reagents were purchased from Aldrich. AIBN
(98%) was purified by recrystallization from a methanol
solution. All other reagents, P4S10 (97%), benzyl mercaptan
(98%), 1,3,5-benzene tricarboxylic acid (98%), 2,6-naphthalene
dicarboxylic acid (98%), were used as received. The solvents
(toluene, dioxane, 98%) were purified by distillation from
calcium hydride prior to use. Styrene (99%) and tert-butyl
acrylate (99%) were purified by distillation from calcium
hydride at reduced pressure and were stored in a glass buret
prior to use.
Synthesis of the Difunctional Dithioester (Scheme 2). In a
three-neck flask equipped with a magnetic stirrer and a reflux
condenser, P4S10 (2.05 g, 4.63 × 10-3 mol), 2,6-naphthalene
dicarboxylic acid (2 g, 9.25 × 10-3 mol), benzyl mercaptan (2.29
g, 1.85 × 10-2 mol), and toluene (100 mL) were introduced,
and the mixture was heated at 110 °C for 24 h. After filtration
and concentration, CH2Cl2 was added to the crude compound.
The mixture was filtered and then purified by two successive
columns chromatography on silica gel (one using CH2Cl2 as
the eluent, the other one using a mixture of 90% hexane and
10% CH2Cl2 as the eluting system). The product crystallized
at room temperature in CH2Cl2 (yield ) 19%). 1H NMR: δ 7.2-
8.5 ppm (16H, m, Ar ); 4.6 ppm (4H, s, 2 × CSSCH2Ph). 13C
NMR: δ 43.1 ppm (SCH2Ph); 126-143 ppm (Ar ); 227.2 ppm
(CdS). MS: 460 (23%); 337 (32%); 214 (10%); 170 (64%); 126
(20%); 91 (100%).
Resu lts a n d Discu ssion
To design novel multifunctional RAFT agents from
which star polymers could be grown, we resorted to a
synthetic methodology based on the use of P4S10 which
was first described by Davy et al.51 and subsequently
applied by Levesque et al.52 and Benicewicz et al.53 P4S10
(see its chemical structure in Scheme 2) is a mild and
versatile sulfunation agent of carbonyl groups.46-54 To
the best of our knowledge, however, there is only one
Syn th esis of th e Tr ifu n ction a l Dith ioester (Sch em e 2).
A procedure similar to the one used for the synthesis of the

Macromolecules, Vol. 37, No. 15, 2004
Synthesis of Multifunctional Dithioesters 5515
F igu r e 1. 1H NMR spectrum (CDCl3; 200 MHz) of (a) RAFT agent 1 and (b) RAFT agent 2.
F igu r e 2. SEC traces of PS samples obtained by polymerizing
styrene in bulk at 110 °C in the presence of the difunctional
RAFT agent 1 (entries 1-4 in Table 1).
F igu r e 3. SEC traces of three-arm star PS obtained by
polymerization of styrene in toluene at 110 °C in the presence
of RAFT agent 2 (entries 8-11 in Table 1).
reported example of multifunctional dithioesters being
synthesized by this procedure. This was described by
Levesque et al.52 who used P4S10, methanol, and difunc-
tional carboxylic acids. The difunctional dithioesters
thus obtained have not been used for RAFT polymeriza-
tions but in a step growth process for the preparation
of polythioamides. For the synthesis of our dithioesters,
we employed benzyl thiol, rather than methanol as did
Levesque et al.,52 and reacted it with P4S10 in the
presence of either a di- or a tricarboxylic acid because
benzyl dithiobenzoate derivatives thus formed are better
RAFT agents than their methyl version.2 In a RAFT
process, indeed, such compounds expel benzyl radicals
after fragmentation (Z ) -CH2Ph in Scheme 1): as
compared to primary methyl radicals, benzyl radicals
are not only substantially better leaving groups but they
also initiate styrene and acrylate polymerization more
efficiently,55 which is crucial for a successful RAFT
polymerization.2 Thus, benzyl thiol was reacted with
P4S10 in the presence of either 2,6-naphthalene dicar-
boxylic acid or 1,3,5-benzene tricarboxylic acid (Scheme
2). Purification by column chromatography afforded the
dithioesters with moderate yields and a high chemical
purity, as evidenced by 1H and 13C NMR and mass
spectroscopy. As an illustration, the 1H NMR spectra
of the di- and trifunctional dithioesters and their as-
signments are given in Figure 1.
In employing these di- and trifunctional RAFT agents,
one can expect that each of the thiocarbonylthio groups
serves as a transfer center for the growing radicals and
retains them to form either a linear difunctional chain
or a three-arm star polymer, respectively. A similar

5516 Dure´ault et al.
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 7. SEC traces of (a) a PtBA sample [MnSEC ) 10 500
g mol-1 PDI ) 1.12] obtained by polymerization of tert-
butylacrylate in toluene at 60 °C in the presence of the
difunctional RAFT agent 1 and (b) a PtBA-b-PS-b-PtBA sample
[Mn ) 20 200 g mol-1; PDI ) 1.20] obtained by polymerization
of styrene in toluene at 110 °C in the presence of a macro-
RAFT agent.
F igu r e 4. Evolution of molar masses with conversion for the
polymerization of styrene in the presence of RAFT agent 2
(entries 8-11 in Table 1).
multifunctional cores. One main advantage of the arm-
first strategy is that complications seen in the core-first
synthesis40-45 such as star-star and star-linear chain
couplings can be ruled out here as shown in Scheme 3.
Addition of an oligomer chain (Pn°) onto one of the
dithioester groups of 2 results in an intermediate radical
that will release a benzyl radical upon fragmentation,
allowing a new linear chain to be initiated. The latter
will eventually become one of the arms of a given star
after undergoing an addition-fragmentation transfer
reaction to its core. Thus, such arm-first stars only
contain branches in a dormant form. Samples prepared
under these conditions, however, include dead chains
produced by termination reactions, in this case R-benzyl
linear polymers, and their proportion can largely vary
with the conditions used. Such dead chains do not
contain any thiocarbonylthio group.
Another potential problem from such an arm-first
approach is the accessibility to the dithioester groups
carried by the core. As recently emphasized by Davis
et al.,39 the star core can be shielded by its polymeric
arms, its access by linear growing chains being limited
as the star arm grows. Consequently, linear chains will
experience a higher probability of termination, thus
increasing the concentration of dead polymers. This was
demonstrated in the case of the heptafunctional cyclo-
dextrin-based RAFT agent used by Davis et al.39 One
can however anticipate less complications with our di-
and trifunctional dithioesters.56
F igu r e 5. Ln[M]0/[M] as a function of time for the polymer-
ization of tert-butylacrylate in toluene at 60 °C in the presence
of the difunctional RAFT agent 1 (entries 12-15 in Table 1).
The data pertaining to the polymerizations of styrene
and tert-butylacrylate in the presence of the di- and the
trifunctional RAFT agents are summarized in Table 1.
Styrene was polymerized thermally at 110 °C (with no
added initiator), first in bulk, then in a toluene solution;
the polymerization of tert-butylacrylate was performed
in toluene solution at 60 °C, using AIBN as the radical
source. Figure 2 shows the refractometric response of
the SEC traces for polystyrene samples prepared in bulk
with the difunctional RAFT agent 1, at different reac-
tion times. The molar masses increase with the mono-
mer conversion. The SEC traces show a symmetrical
and narrow molar mass distribution for moderate
monomer conversions and its broadening at higher
conversions. As discussed above, this is likely due to the
continuous build-up of the concentration of dead chains
induced by the increasingly hindered access to RAFT
groups carried by the naphthalene moiety. Similar
F igu r e 6. SEC traces of PtBA samples obtained by polym-
erization of tert-butylacrylate in toluene at 60 °C controlled
by the difunctional RAFT agent 1 (entries 12-15 in Table 1).
approach to generate multiarm stars and star-block
copolymers has recently been reported by the Rizzardo
and the Davis groups. These authors synthesized tet-
rakis-(trithiocarbonate)40 and a heptakis(trithiocarbon-
ate)39 CTA using CS2-based chemistry to derive their

Macromolecules, Vol. 37, No. 15, 2004
Synthesis of Multifunctional Dithioesters 5517
Ta ble 1. Da ta for P olym er s Obta in ed by P olym er iza tion of Styr en e or ter t-Bu tyla cr yla te in th e P r esen ce of Com p ou n d 1
or 2a
RAFT
agent
monomer,b °C
(in toluene,
e
f
initiatorc
time
(h)
conv.d
(%)
Mntheo
(g mol-1
MnSEC
entry
1
(mol L-1
)
mol L-1
)
(mol L-1
thermal
)
)
(g mol-1
3200
)
PDIf
1.25
1g
St, 110
(bulk)
3.2
9
3930
(2.1 × 10-3
)
2
3
4
8.5
21.8
45.5
27
50
82
11 800
21 800
35 900
7420
10 800
15 600
1.29
1.41
1.57
5
2
St, 110
(bulk)
thermal
thermal
4
30
36 300
22 300
1.19
(7.5 × 10-3
)
)
6
7
22
46
61
93
74 000
113 000
42 600
53 200
1.45h
1.52h
8
2i
St, 110
14.5
24
21 000
14 500
1.20
(5.2 × 10-3
(4.37)
9
10
11
24
43.7
78
38
63
97
33 200
55 000
84 800
17 800
22 700
31 900
1.30
1.29
1.19
12
1j
tBA, 60
(4.35)
AIBN
1.8
2
1840
1820
1.36
(4.1 × 10-3
)
(2.2 × 10-3
)
13
14
15
5
19
28
6
50
70
5520
46 000
64 500
6600
41 500
54 900
1.21
1.28
1.31
a
All reactions were performed in Schlenk flasks that were placed in an oil bath at the desired temperature, after degassing by three
freeze-pump-thaw cycles. St ) styrene; tBA ) tert-butylacrylate. c AIBN ) 2,2′-azobis(2-cyanopropane). Conversions determined by
gravimetry after the evaporation of solvent and/or monomer. e Theoretical molar mass expressed as follows: Mntheo ) [monomer]/[RAFT
agent] × conversion(%) × molar mass of the monomer unit. This expression is based on both assumptions that the RAFT agent was
entirely consumed at the given conversion of the monomer and that the proportion of chains derived from AIBN is negligible. f Experimental
b
d
g
molar mass and polydispersity index obtained by SEC in THF using the RI detector (calibration with linear PS standards). See Figure
h
i
j
2. Bimodal distribution. See Figure 3: entry 10, MnLLS ) 46 000 g mol-1; entry 11, MnLLS ) 68 000 g mol-1
. See Figure 6.
Sch em e 3. Syn th esis of Ar m -F ir st Sta r s Usin g RAF T
Agen t 2
Styrene was then polymerized at 110 °C using toluene
as the solvent. We reasoned that the “shielding effect”
mentioned above was also due to the increasing viscosity
of the polymerization medium, which decreases the
chain’s mobility. The solution polymerizations of styrene
were more efficient than bulk experiments, in terms of
control over the molar masses and polydispersities
(PDIs). The linear increase of the molar masses as a
function of the monomer conversion and the PDIs
around 1.2 are indicative of a controlled growth of the
polymer chains (Figure 3). In addition, the core con-
sisting of the chromophoric thiocarbonylthio groups,
-S(CdS), could be detected by SEC using a UV detector
at 310 nm. At high conversions, however, a tailing
attributed to dead linear chains formed by recombina-
tion appeared in the low molar mass area. Nevertheless,
it was much less pronounced as compared to the
samples obtained in bulk, plausibly because of an
increased chain mobility due to the dilution. As ex-
plained above, these dead chains did not contain any
thiocarbonylthio moiety and could not be detected at 310
nm; as they were not taken into account in the calcula-
tion of the apparent Mn values delivered by UV, the
latter were higher than those obtained with the refrac-
tometric detector (see Figure 4). However, both evolve
linearly with the conversion of styrene. These experi-
mental molar masses (MnSEC)swhether generated by
UV or RIswere lower than those determined by laser
light scattering (MnLLS). Because of the compactness of
star molecules, SEC, when calibrated with linear PS
standards, only gives apparent Mn values for stars, in
contrast to LLS that allows access to absolute molar
masses.43 However, MnLLS values (see footnote i of Table
observations were made when the trifunctional RAFT
agent 2 was employed in the thermal bulk polymeriza-
tion of styrene (see entries 5-7 in Table 1). The molar
mass distribution was multimodal in this case, even at
moderate conversion (footnote h, Table 1).
1) did not perfectly match the theoretical values (Mntheo)
corresponding to the following expression: [monomer]/
[CTA] × conversion of the monomer, because of the
presence of the aforementioned linear dead chains that
decreased the overall molar mass of the star.

5518 Dure´ault et al.
Macromolecules, Vol. 37, No. 15, 2004
Sch em e 4. Syn th esis of a n ABA-typ e Tr iblock
Cop olym er Usin g RAF T Agen t 1
In all experiments, a short inhibition period was noted
with both RAFT agents 1 and 2. A similar observation
was made upon polymerizing styrene in the presence
of benzyl dithiobenzoate,2 (PhC(dS)SCH2Ph), which is
the monofunctional homologue of 1 and 2. Opposing
explanations have been proposed to account for this
phenomenon. Some authors have suggested that the
intermediate radical PnS-(Z)C•-SPm (Scheme 1) un-
dergoes a slow fragmentation and terminates reversibly,
the RAFT agent acting therefore as a radical sink,8,10-13
whereas others have counterproposed2,11-12 that PnS-
(Z)C•-SPm can be captured by propagating species Pn
•
in irreversible terminations thereby producing three-
arm stars. The short duration of the inhibition period
observed with our RAFT agents is an argument in favor
of a slow fragmentation of the transient radical species.
Irreversible terminations with propagating chains would,
indeed, have led to byproducts of higher molar masses
and would have been easily detected by LS/SEC.
To demonstrate that the samples obtained with the
RAFT agent 2 actually exhibit a star-type architecture,
one of them (entry 5, Table 1) was subjected to a “core
destruction” reaction. This could be achieved by reacting
that sample with diethylamine, as reported previously.2
As expected, this yielded PS chains with a lower molar
mass, Mn(RI) ) 13 000 g mol-1, than that of the parent
sample, Mn,RI ) 22 300 g mol-1 (Mntheo ) 36 300 g
mol-1).
One can note, however, a tailing in the low molar mass
area of the SEC of the triblock copolymer, likely due to
the formation of dead chains obtained by irreversible
termination reactions for the same reasons as those
explained above.
Work is in progress to exemplify this low cost and safe
procedure of dithioester synthesis through the prepara-
tion of various polymeric structures (block and graft
copolymers, organic/inorganic hybrid compounds, hy-
perbranched polymers) from properly selected carboxylic
acid-containing precursors.
The polymerization of tert-butylacrylate was also
performed in the presence of RAFT agent 1, using
toluene as the solvent and AIBN as the radical source
at 60 °C (entries 12-15 in Table 1). Free-radical
polymerizations of alkylacrylates carried out in the
presence of benzyl dithiobenzoate are relatively well-
controlled, although dithioesters with a better homolytic
leaving group can be selected for this class of mono-
mers.2,8 With the difunctional benzyl dithiobenzoate
derivative 1, no polymer was produced after 1 h of
polymerization. Figure 5 illustrates this inhibition
period with the plot of the evolution of Ln[M]0/[M] as a
function of time. This is consistent with the results of
Davis et al. who studied the polymerization of acrylates
in the presence of benzyl dithiobenzoate.8 Again, the
inhibition period can be ascribed to a slow fragmenta-
tion of the benzyl group during the first equilibrium
(Scheme 1), rather than to an inefficient initiation of
tert-butylacrylate by benzyl radicals or to irreversible
terminations. After this inhibition period, however, the
molar mass of the samples increased linearly with the
monomer conversion and PDIs were lower than 1.3,
confirming a controlled growth of the chains (see Figure
6).
Con clu sion
In this paper, emphasis was placed on the versatility
and simplicity of a new synthetic route to di- and
trifunctional thiocarbonylthio compounds based on the
reaction of P4S10 with a thiol and a carboxylic acid. The
di- and trifunctional dithioesters thus obtained subse-
quently served as multifunctional RAFT agents for the
synthesis of either ABA-type triblock copolymers or
three-arm stars. This was achieved following an arm-
first approach because the core itself was part of the
activating group of the RAFT agents, while the ho-
molytic leaving group generated linear growing chains.
This allowed us to overcome the complications of star-
star couplings often encountered in star polymer syn-
thesis by the core-first method. However, the accessi-
bility of the RAFT functionalities within the core of the
star was impeded in bulk experiments, presumably due
to the decreasing mobility of the linear chains as the
viscosity of the polymerization medium increased with
monomer conversion. This, however, could be minimized
by carrying out the polymerizations in toluene solutions.
These poly(tert-butylacrylate) (PtBA) samples subse-
quently served as difunctional macro-RAFT agents for
styrene polymerization, generating ABA-type triblock
copolymers in this way. This order of addition of the
two monomers led to the formation of a PS central block
and two PtBA external blocks (Scheme 4). The thermal
polymerization of styrene at 110 °C in the presence of
the macro-RAFT agent was carried out with toluene as
the solvent. The SEC traces of this chain extension
unambiguously showed the formation of the expected
PtBA-b-PS-b-PtBA block copolymer (Figure 7): the SEC
trace shifted from that of the poly(tert-butylacrylate)
precursor to the higher molar mass region after poly-
merization of styrene, meaning that the crossover
reaction between the two blocks occurred efficiently.
Ack n ow led gm en t. We are grateful to Rhodia for
supporting this research and for permission to publish
this work.
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