Synthesis of NMP/RAFT inifers and preparation of block copolymers

Article abstract of DOI:10.1002/pola.26081

Two different initiator/transfer agents (inifers) containing an alkoxyamine and a dithiobenzoate were synthetized and used to trigger out either reversible addition-fragmentation chain transfer (RAFT) polymerization or nitroxide-mediated polymerization (NMP). α-Dithiobenzoate-ω- alkoxyamine-difunctional polymers were produced in both cases which were subsequently used as precursors in the formation of block copolymers. This synthetic approach was applied to N-isopropylacrylamide (NIPAM) or polyethylene oxide methacrylate (EOMA) to form α,ω-heterodifunctional homopolymers via RAFT at 60°C which were chain extended with styrene by activating the alkoxyamine moiety at 120°C. Under such temperature conditions, it is proposed that a tandem NMP/RAFT polymerization is initiated producing a simultaneous growth of polystyrene blocks at both chain-ends. Self-assembled nanostructures of these amphiphilic block copolymers were evidenced by scanning electron microscopy.

Full text of DOI:10.1002/pola.26081

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Synthesis of NMP/RAFT Inifers and Preparation of Block Copolymers
Claude St. Thomas,1 Hortensia Maldonado-Textle,1 Antal Rockenbauer,2 Laszlo Korecz,2
Nora Nagy,2 Ramiro Guerrero-Santos1
1
ꢀ
Centro de Investigacion en Qu´ımica Aplicada, Blvd. Enrique Reyna No. 140, 25100, Saltillo, Mexico
2
ꢀ
Research Centre for Natural Sciences, Hungarian Academy of Sciences, Pusztaszeriut 59-67, H-1025 Budapest, Hungary
Correspondence to: R. Guerrero-Santos (E-mail: ramirog@ciqa.mx)
Received 20 January 2012; accepted 26 March 2012; published online
DOI: 10.1002/pola.26081
ABSTRACT: Two different initiator/transfer agents (inifers) con-
taining an alkoxyamine and a dithiobenzoate were synthetized
and used to trigger out either reversible addition-fragmentation
chain transfer (RAFT) polymerization or nitroxide-mediated po-
lymerization (NMP). a-Dithiobenzoate-x-alkoxyamine-difunc-
tional polymers were produced in both cases which were
subsequently used as precursors in the formation of block
copolymers. This synthetic approach was applied to N-isopro-
pylacrylamide (NIPAM) or polyethylene oxide methacrylate
(EOMA) to form a,x-heterodifunctional homopolymers via
RAFT at 60ꢀC which were chain extended with styrene by acti-
vating the alkoxyamine moiety at 120ꢀC. Under such tempera-
ture conditions, it is proposed that
a
tandem NMP/RAFT
polymerization is initiated producing a simultaneous growth of
polystyrene blocks at both chain-ends. Self-assembled nano-
structures of these amphiphilic block copolymers were evi-
C
V
denced by scanning electron microscopy.
2012 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000,
2012
KEYWORDS: block copolymers; dual controller; inifer; living po-
lymerization; nitroxide-mediated polymerization; reversible
addition fragmentation chain transfer (RAFT); reversible-deacti-
vation radical polymerization
INTRODUCTION The reversible-deactivation radical polymer-
ization (RDRP),1—vernacularly known as living free radical
polymerization—is an ensemble of techniques commonly
used for the synthesis of well-defined functional polymers
with a predetermined molecular weight (MW) and narrow
mentation of intermediate radicals, whereas NMP relies on
the reversible termination of propagating radicals with stable
free radicals to form thermolabile alkoxyamines (P-ONR1R2).6
Separately, RAFT and NMP offer the possibility to define accu-
rately MW in the synthesis of complex polymer architectures
-
-
molar mass distribution (D ¼ Mw/Mn). The general principle
with possible low D. Nevertheless, limitations associated with
involved in the control of the chain propagation reaction is a
temporal termination, or reversible-deactivation, induced by
a variety of rapid equilibrium reactions between propagating
radicals and dormant species. Albeit, in these processes, a
small fraction of chains is irreversibly deactivated by radi-
cal–radical reactions, the living behavior of the polymeriza-
tion is clearly observed by the MW increase with conversion
each system proscribe the use of some monomers and func-
tionalities. For instance, the synthesis of a triblock copolymer
entirely by RAFT with three different monomers has been
only rarely applied, as a single RAFT agent cannot effectively
control the polymerization of all three monomers at the same
level. Conversely, the list of monomers that can be used in
NMP is rather limited. The principle of formation of chains in
RAFT and NMP entirely differs, therefore, it is of no use mix-
ing a RAFT chain transfer agent (Z-(C¼¼S)SR) with an alkoxy-
amine to carry out combined RDRPs. This particular combi-
nation NMP/RAFT could turn out to be interesting if a single
molecule can be prepared with both functional groups, v.gr. a
dithiobenzoate bearing an alkoxyamine in the Z or R group
(T-Ph(C¼¼S)S-R) and Ph(C¼¼S)S-R-T, respectively, where T ¼
alkoxyamine). The last compound is more interesting, as it
can be used as chain transfer agent in a RAFT polymerization
to prepare a,x-heterodifunctional polymers (Z-(C¼¼S)S-poly-
mer-T). Afterward, an AB diblock copolymers can be formed
-
and low D values. RDRP includes degenerative-transfer poly-
merization methods as reversible addition-fragmentation
chain transfer (RAFT)2 and iodine-transfer polymerization
(ITP/RITP)3 and polymerizations based on reversible termi-
nation reactions as atom transfer radical polymerization
(ATRP)4 and nitroxide-mediated polymerization (NMP).5
These methods are based on different chemical reactions. For
instance, RAFT polymerization depends on a well-known
equilibrated interconversion of thiocarbonylthio compounds
Z-(C¼¼S)SR to a functional polymer Z-(C¼¼S)SP in the early
stages of the polymerization through the formation and frag-
Additional Supporting Information may be found in the online version of this article.
C
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RESULTS AND DISCUSSION
As shown in Scheme 1, the strategy for the synthesis of ini-
fer 1 comprises two steps: (a) the formation of the bromo
adducts14 by treating the oxoaminium bromide with 4-chlor-
ostyrene or styrene and, (b) the formation of the dithioben-
zoate group by treatment of bromo adducts with bromomag-
nesiumdithiobenzoate. The overall yield after column
purification was estimated to be 21%. Compounds differ by
the presence of chlorine atom in 1, which might provide a
greater stability and better control to polymerizations in
comparison with the nonsubstituted inifer 2. It should be
noted that 1 and 2 (seeing it as RAFT agents with R ¼ pri-
mary alkyl), would not anticipated to be effective because
primary alkyl radicals are considered as poor homolytic leav-
ing groups. However, the use of disubstituted monomers as
trans-b-methyl styrene instead of styrene or 4-chloro styrene
in the synthesis of more active chain transfer agents resulted
in unstable intermediaries giving low yields in step 1 or
undetectable formation of corresponding dithiobenzoates.
SCHEME 1 Synthesis of 2-(4-chlorophenyl)-2-(2,2,6,6-tetrame-
thylpiperidin-1-yloxy)ethyl-dithiobenzoate (1) and 2-phenyl-2-
(2,2,6,6-tetramethylpiperidin-1-yloxy)ethyl-dithiobenzoate (2).
via RAFT without altering the alkoxyamine group, which can
be lastly used to initiate NMP to achieve an additional chain
extension with monomer C. Alternatively, Z-(C¼¼S)S-polymer-
T can be heated in the presence of a monomer B to activate
the scission of T establishing the NMP equilibrium. In such
conditions a tandem NMP/RAFT polymerization would be
produced, as the x-thiocarbonylthio moiety could react with
propagating radicals leading straightforward to ABA block
copolymer.
1
Figure 1 and Supporting Information, Figure S1, show the H
NMR and 13C NMR of 1 and 2, respectively. Protons d, e, cor-
respond to diastereotopic protons adjacent to the thiocar-
bonyl group, whereas signals at 5.1 (f) ppm correspond to
benzylic protons; carbon m was assigned to the thiocarbonyl
group. The fragmentation pattern and Mþ in the GC/MS
spectrum (not shown in figures) gives a second piece of evi-
dence about the successful and unprecedented synthesis of
The combination of two RDRP methods as mentioned above
for the synthesis of block copolymers has been described
earlier. For instance, a Br-containing alkoxyamine was syn-
thetized and used as ATRP/NMP initiator to prepare self-
assembled monolayers on silicon wafers.7 The combination
of an ATRP initiator and a RAFT chain transfer agent has
been also succeeded. For instance, a dibromotrithiocarbonate
was prepared and used to conduct, independently or concur-
rently, ATRP and RAFT polymerization.8 Several chloro-xan-
thates were synthesized to combine RAFT polymerization of
N-vinyl pirrolidone (NVP) with ATRP of styrene and (meth)a-
crylates to prepare well-defined NVP9 or vinyl acetate10
block copolymers. More recently, a multiheterofunctional ini-
fer was prepared and used to prepare multihydrophilic block
copolymers.11 In these examples, heterodifunctional mole-
cules (containing two different groups) were applied to pre-
pare block copolymers from mechanistically incompatible
monomers, independently and selectively, without the need
of chain end transformation12 or protection steps. This unim-
olecular approach differs of the use of thiocarbonylthio com-
pounds as pseudohalogen initiators in ATRP.13
In this article, we present the synthesis of inifers 1 and 2 to
introduce the concept of NMP/RAFT dual controllers. We
demonstrate that such inifers are effective in the preparation
of heterodifunctional polymers, which were used forming
multiblock copolymers via tandem NMP/RAFT polymeriza-
tion using water soluble monomers as N-isopropylacrylamide
(NIPAM) or polyethylene oxide methacrylate (EOMA). At our
best knowledge, it is the first report on the application of a
NMP/RAFT inifer which clearly expand the possibilities of
macromolecular design.
FIGURE 1 1H (top) and 13C (bottom) NMR spectra of 2-(4-chloro-
phenyl)-2-(2,2,6,6-tetramethylpiperidin-1-yloxy)ethyl-dithioben-
zoate (1).
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FIGURE 2 (a) Time dependence of Ln([M]0/[M]), (b) Mn versus monomer conversion for RAFT polymerizations of styrene ([S]/[1]/
[AIBN] ¼ 150/1/0.5; (^), [S]/[2]/[AIBN] ¼ 150/1/0.5; (n), [S]/[1]/[AIBN] ¼ 650/1/1; (^) and [S]/[2]/[AIBN] ¼ 650/1/1 (h), T ¼ 60ꢀC), and
(c) SEC traces for the bulk RAFT polymerization of styrene in the presence 2; ([S]/[2]/[AIBN]¼150/1/0.5 T ¼ 60ꢀC).
such inifers. These compounds were subsequently evaluated
for the controlled polymerization of S and in the formation
of block copolymers composed of butyl acrylate (BA), t-
butyl acrylate (t-BA), acrylic acid (AA), NIPAM, and EOMA.
results reflect the fact that the dithiobenzoate moiety does
not have a good homolytical leaving group (R group) relative
to the attacking polystyryl macroradical. That may imply a
low fragmentation rate for the intermediate radical and a
-
greater possibility for side reactions. High D values also reflect
the fact that an equimolar ratio of inifer to initiator was used
to increase the polymerization rate when it was targeted high
MW ([S]/[Inifer]/[AIBN] ¼ 650/1/1). As it is anticipated that
the alkoxyamine group remains untouched during the RAFT
polymerization; the product of the RAFT polymerization
should be a heterodifunctional polystyrene (PS) (3).
RAFT Polymerization of Styrene with Inifers
1 and 2 as Chain Transfer Agents
The reaction conditions and results of the RAFT polymeriza-
tion of styrene with a molar ratio[S]/[Inifer]/[2,20-azobis(iso-
butyronitrile), AIBN] ¼ 150/1/0.5 and 650/1/1 are sum-
marized in Figure 2 (see also Supporting Information, Tables
S1 and S2). A linear development of Ln([M]0/[M])-vs.-time
and a gradual shift of size exclusion chromatography (SEC)
traces to higher MW was observed in all polymerizations
indicating the living character of the process. However, the
-
molar mass distribution (D) increases with conversion and
Mn is higher than the theoretical values determined from
SEC
the molar ratio of consumed monomer to inifer. These
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amine, 2-(methyl)-propionitrile groups as well as other pro-
duced during the MALDI-TOF-MS analysis as CH2¼¼CPhA,
CHPh¼¼CHA, chlorostyrene) were evaluated. The major
peak indicated in expanded region shown in Figure 4(b)
correspond to the series: CH2¼¼CHPhA(CH2ACHPh)nA
CH2ACH(C9H18NO)(PhCl) Naþ; consistent with the polymer
being capped with an alkoxyamine (detailed structure see *
in Table 1). The MW given by MALDI-TOF-MS for this series
-
is Mn ¼ 3597.98; Mw ¼ 3804.95; D ¼ 1.06. For n ¼ 29, the
experimental peak centered at 3453.19 m/z fits with the cal-
culated molar mass with Naþ (3453.04 g/mol). The small se-
ries indicated with symbol corresponds to the counterpart,
that is, a polymer end-capped by the dithiobenzoate group.
The molar mass calculated for n ¼ 32 is 3505.98 which is
consistent with the experimental m/z ¼ 3505.69. The struc-
tures assigned to series indicated with symbols ꢂ, &, ~, ^
also contains the dithiobenzoate group and the experimental
molar mass is in good agreement with the calculated (see
Table 1 and Fig. 4). For instance, the series identified with
the symbol ‘‘&’’ can be assigned to the structure
FIGURE 3 1H NMR of a PS (3) prepared by RAFT polymeriza-
tion in the presence of inifer 1 at 60ꢀC [S]/[1]/[AIBN] ¼ 150/1/
0.5. Mn ¼ 5130 g/mol (conversion ¼ 30%) and its correspond-
ing TGA thermogram (solid line) compared with an anionic PS
of comparable MW (dotted line).
S¼¼C(Ph)SA(CHPhACH2)nACH2ACH(C9H18NO)(PhCl)
Naþ.
The peak at m/z ¼ 3465.4 is attributed to this structure
with n ¼ 29, which fits with the calculated m/z ¼ 3464.96.
The MALDI-TOF-MS spectrum provides with a definitive evi-
dence of the structure of PS (3) prepared with inifer 1, even
though the degree of functionality cannot be assessed pre-
cisely. Nevertheless, on the base of the apparent intensity
area of the series ‘‘ꢂ, *, &,’’ it can be reasonably stated the
Mn
estimated from the 1H NMR spectra15 of samples
NMR
obtained at moderated or high conversion are rather close to
the Mn (See Supporting Information, Table S2). This result
SEC
strongly suggests that a major fraction of chains ([Mn
/
NMR
Mn
]
avgꢁ100 ¼ 90%) is functionalized with the dithioben-
SEC
zoate group and validates the structure of 3.
The structure of PS (3) was confirmed by the characteristics
signals of ortho protons in the thiobenzoate group and the
methyl protons in the alkoxyamine group as can be seen in
1
the H NMR spectrum of a sample prepared with a molar ra-
tio [S]/[1]/[AIBN] ¼ 150/1/0.5 at 60ꢀC (Fig. 3). The assign-
ments of resonances were confirmed by comparing the
chemical shifts of inifer 1. To further validate the sample
structure, we performed a TGA analysis. The inset in Figure
3, shows the corresponding thermogram obtained under
nitrogen atmosphere and the weight loss associated with the
first degradation event at 206ꢀC. The weight loss between
206 and 254ꢀC is 5.97%, which is consistent with the of
chain-ends content [(308/5130) ꢁ 100 ¼ 6.0% (w/w)]. The
thermal stability of this samples is compared with an anionic
-
PS (Mn ¼ 5300 g/mol, D ¼ 1.07). As can be seen, the hetero-
difunctional PS is less stable than the reference PS, with
thermal decomposition starting at about 300ꢀC, being com-
plete by 450ꢀC.
As an additional evidence for the retention of the RAFT-end
groups, matrix assisted laser desorption ionization time of
flight mass spectrometry (MALDI-TOF-MS) analysis for a PS
-
(3) (Mn ¼ 4,620; D ¼ 1.6), prepared in the presence of 1 at
60ꢀC (S]/[1]/[AIBN] ¼ 150/1/0.3) was conducted as shown
in Figure 4. Three main ion series can be distinguished [Fig.
4(a)] with various head and end-groups and a repeat unit of
104.06 Da corresponding to the molar mass of the styrene.
FIGURE 4 (a) MALDI-TOF-MS spectrum of a PS (3) (Mn ¼ 4620;
¼
1.6), prepared in the presence of 1
at 60ꢀC (S]/[1]/
-
D
To assign the polymer structure to these series, the molar
mass calculation of many possible structures combining dif-
ferent terminal groups (including the dithiobenzoate, alkoxy-
[AIBN]¼150/1/0.3). (b) Expanded region of the spectrum (a).
The peaks are labeled with their measured MW. The detailed
structures are shown in Table 1.
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TABLE 1 PS (3) with Various End-Groups Estimated by MALDI-TOF-MS in Figure 4
Symbol in Figure 4
Polymer Structure
Formula weight (*Na)
ꢂ
Mass (exp) ¼ 3435.23
Mass (calc) ¼ 3435.87
n ¼ 30
*
Mass (exp) ¼ 3453.19
Mass (calc) ¼ 3453.04
n ¼ 29
h
Mass (exp) ¼ 3465.40
Mass (calc) ¼ 3464.96
n ¼ 29
~
Mass (exp)¼ 3485.55
Mass (calc) ¼ 3485.95
n ¼ 29
Mass (exp)¼ 3505.69
Mass (calc) ¼ 3505.98
n ¼ 32
^
Mass (exp) ¼ 3520.16
Mass (calc) ¼ 3519.99
n ¼ 31
end-functionality is even higher that that calculated by 1H
NMR (>95%).
tions between TEMPO and 3, as well as, TEMPO and p-chlo-
ride atom were also tested but their contribution was found
not significant. The half-life time, for PT and TEMPO was cal-
culated to be 4.2 and 23.7 min, respectively. Interestingly, no
chain coupling was produced as evidenced by the compari-
son of SEC traces of samples before and after heating (not
Further evidence about the structure of PS (3) was obtained
from ESR measurements. Carefully purified samples were
dissolved in fresh distilled toluene, placed in an ESR tube,
degassed, sealed, and heated in an ESR spectrometer to dis-
sociate 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and its
accumulated concentration was followed in the medium (see
Fig. 5). The signal intensity increased rapidly to reach a max-
imum ([T]max). After this point, TEMPO concentration decay
was observed suggesting that side reactions occur involving
the macroradical 4 and TEMPO (see Scheme 2). A reshuffling
of the dithiobenzoate groups is also possible to form 5 and
6 during this treatment involving the formation of an inter-
mediate free radical PS-S-Cꢀ-S-PS. Various kinetic models
were tested to simulate the TEMPO evolution and decay
assuming the reversible dissociation of TEMPO furthermore,
different side reactions were assumed at elevated tempera-
ture. [T]max was calculated to be 1.411, whereas the initial
concentration of alkoxyamine [PT]0 was 1.90 (arbitrary
units). The decay is a second-order process described by
ꢃd[T]/dt ¼ 2k2[T] ꢁ [S], where S stands for an unknown
reductive substance. We obtained excellent fit for the kinetic
curves by adjusting k2, [PT]0, and [S]. The impact of reac-
FIGURE 5 Experimental and simulated concentration of
TEMPO during the decomposition of PS prepared via RAFT at
120ꢀC.
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inifers. However, experimental Mn values are higher than
those predicted all along. This deviation can be explained by
the fact that radical–radical coupling reactions of x-dithio-
benzoate or x-TEMPO propagating macroradicals (4 and 6,
respectively) leads up to new dormant polymers capable to
increase its size subsequently. This is a key difference with
monofunctional dormant species that increases the MW pro-
-
gression slope and D. In fact, chains might propagate at both
extremities and the coupling reactions may occur unavoid-
ably. High temperature can also be a potential drawback for
conducting RAFT polymerizations, as temperature may favor
dithiobenzoate decomposition. Nevertheless, the degree of
thiocarbonylthio functionalization of samples calculated by
SCHEME 2 Dissociation of difunctional PS (3) at 120ꢀC.
NMR was again very high ([Mn /Mn
]
avgꢁ100 ¼ 92%)
NMR
SEC
indicating that the thermal degradation of dithiobenzoate is
indiscernible. It is worth mentioning that the effect of elec-
tron-withdrawing substituent in inifer1 is revealed, as the
shown in figures). This may suggest that the radical–radical
termination of 4 and 6 is prevented by the high dilution
and/or retarded by the RAFT equilibrium.
Mn of samples prepared with this compound is closer to
SEC
the theoretical values compared with samples 17–22 pre-
pared with inifer 2.
Even though styrene RAFT polymerization performed in the
presence of inifers 1 or 2 yielded heterodifunctional PS (3)
-
The SEC chromatograms of polystyrenes prepared with 1
(corresponding to entries 11–16 in Supporting Information,
Table S3) are presented in Figure 6(c) to show the entire
shift of traces up to high MW. The tandem polymerization
described above might also yield heterodifunctional poly-
mers, which are suitable for applying in subsequent RAFT or
NMP/RAFT processes to prepare block copolymers. To test
its capability for reacting either as RAFT chain transfer agent
or as alkoxyamine, we performed preliminary chain exten-
sion reactions to prove its difunctionality. The results are
described below.
with broad molar mass distribution (D ¼ 1.3–2.1), the
method is suitable for the preparation of block copolymers
via RAFT or, via a concurrent NMP/RAFT polymerization.
Thus, it was also interesting to test the ability of such inifers
for the preparation of PS using the typical conditions for
NMP polymerization at high temperature in the absence of a
thermal initiator.
Styrene NMP Polymerization with 1 or 2
Inifers 1 and 2 were also tested as alkoxyamines in a bulk
polymerization of styrene at 120ꢀC using a degassed solution
containing a molar ratio [S]/[inifer] ¼ 200/1. Polymers were
precipitated twice in methanol obtaining reddish product
that give us a qualitative indication about its dithiobenzoate
functionalization. The thermal decomposition of inifers gen-
erates TEMPO and its corresponding carbon centered radical
as shown in Scheme 3. It was hypothesized that the styrene
initiation and the reversible cross-coupling of TEMPO with
polymeric radicals belonging to a NMP mechanism took
place. However, the addition of the dithiobenzoate moiety to
growing radicals cannot be excluded which can also activate
the RAFT process. Therefore, under NMP conditions, that is,
temperatures around 120ꢀC, a tandem NMP/RAFT polymer-
ization occurs. Obviously, this combined process might be
affected by secondary reactions such as coupling of inter-
mediate RAFT radicals with TEMPO and a partial loss of the
dithiobenzoate group through thermal degradation.16
Chain Extension
Heterodifunctional polystyrenes (3) prepared by RAFT in the
presence of 1 were retaken in styrene and heated at 120ꢀC
without any kind of initiator. For instance, a PS having a Mn
-
¼ 12,960 g/mol and D ¼ 1.8 was extended to Mn ¼ 26,330
-
g/mol, (D ¼ 1.5) in 3 h with a yield of 44%. This experiment
demonstrates that the alkoxyamine group contained in PS
(3) is still active. However, admitting that the RAFT process
is activated, it is impossible to determinate the degree of
extension at the a and x ends. To understand the possible
interferences between the RAFT and NMP, a PS (3) (Mn
¼
10,980 g/mol) was treated with AIBN/PPh3 in THF to trans-
form the thiocarbonylthio end group into a hydroxyl func-
tion.17 The PS was then chain extended via NMP activating
the remaining alkoxyamine group at 120ꢀC and compared
with the product prepared under the same conditions from
the unmodified PS (3). Unexpectedly, the monomer
Results of bulk styrene polymerization in the presence of ini-
fers 1 and 2 at 120ꢀC are summarized in Figure 6 (see Sup-
porting Information, Table S3). MWs increased with conver-
sion and the molecular mass distributions remained narrow
-
throughout the polymerization (D ¼ 1.2–1.5). The pseudo-
first-order kinetic plot exhibited a consistent consumption of
monomer as a function of time [Fig. 6(a)] but deviation from
linear first-order behavior is observed after 8 h of reaction,
likely due to the aforementioned side reactions. The Mn vs.
conversion plot [Fig. 6(b)] is approximately linear for both
SCHEME 3 Thermal decomposition of inifer 1.
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FIGURE 6 (a) Time dependence of Ln([M]0/[M]), (b) Mn versus monomer conversion for a NMP/RAFT polymerizations of styrene
([S]/[1]/ ¼ 200/1; (^), [S]/[2]/ ¼ 200/1; (n), T ¼ 120ꢀC), and (c) SEC traces for the samples obtained in the series ([S]/[1]/¼200/1.
conversion is substantially higher for the modified PS and its
MW increased twice as the PS (3) (See SEC traces in Sup-
porting Information, Fig. S3). These results indicate that the
NMP polymerization is altered by the presence of the dithio-
benzoate chain-end; however, this interference is not de-
ever, it is challenging to determinate the size of chain exten-
sion at each individual polymer end.
A possible way to clarify this issue is to prepare a heterodi-
functional polymer based on an appropriate diene (v.gr. iso-
prene) under RAFT conditions. The chain extension with sty-
rene and further oxidation (digestion) with OsO4 of
polydiene central block may provide valuable information
about the size of outer blocks. Some model molecules repre-
senting chain ends in PS (3) has been prepared and used as
a mixture in the styrene polymerization to determinate the
-
structive because D is unaffected.
Conversely, a PS prepared using NMP conditions in the pres-
-
ence of 1 (Mn ¼ 10,630 g/mol and D ¼ 1.5) was chain
extended at 60ꢀC using AIBN. This RAFT polymerization
yielded a PS with Mn ¼ 12,240 g/mol after 8 h of reaction.
As in this case, only the dithiobenzoate group could cause
such a chain growth and is put forward that the dithioben-
zoate group is well-preserved during NMP at 120ꢀC. SEC
traces of chain-extended polymers and their corresponding
prepolymers are presented in Supporting Information, Fig-
ures S2, S3 and data are quoted in Table S4. The results
above suggest that synthesis of block copolymers is possible
by alternating RAFT and NMP/RAFT polymerization; how-
-
effect on MW and D. The results will be developed in more
detail in a forthcoming communication.
Block Copolymers
Once we studied the formation of PS using RAFT or NMP
conditions, we sought to explore the use of inifers 1 and 2
for the synthesis of block copolymers composed of acrylic,
methacrylic, or acrylamide derivatives monomers. The
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TABLE 2 Conditions and results for the synthesis of BAB block copolymers through RAFT polymerization at 608C and tandem
NMP/RAFT polymerization at 1208C
M
Mnth
nSEC
-
D
Entry
Polymer
Inifer/M
Ratioe,f/inifer/AIBN
T (ꢀC)
Conv (%)
t (h)
(g/mol)
(g/mol)
1a
2b
3c
4d
a
PEOMA
2/EOMAe
PEOMA/S
1/NIPAM
PNIPAM/S
84
84
1.0
16.0
1.0
0.3
0.0
0.5
0.0
60
120
60
74
44
15
36
4
5
38,140
63,820
5,480
6,110
52,460
4,710
1.2
1.4
1.3
1.8
P(S-b-EOMA-b-S)
PNIPAM
270
88
15
7
P(S-b-NIPAM-b-S)
12.0
120
38,700
19,930
d
e
f
Methanol 50% (w/w).
THF 50% (w/w).
b
c
DMF 50% (w/w).
Benzene 50% (w/w).
Molar ratio for entries 1 and 3.
Weight ratio for entries 2 and 4.
‘‘RAFT-first’’ or the ‘‘NMP-first’’ approaches were the two
possible routes to form the a,x-heterodifunctional polymer,
that is, the first block. For the second block, the RAFT or
NMP conditions could lead to a structure AB or ABA, respec-
tively, provided that a tandem polymerization is produced at
120ꢀC. From an AB copolymer prepared entirely by RAFT
polymerization, we can use the alkoxyamine group extending
with a styrenic monomer to obtain a CABC tetrablock copol-
ymer. The experimental conditions and results of different
copolymers prepared using some of these synthetic possibil-
ities are summarized in Tables 2 and 3.
forming complex structures. In the case of homopolymers
quoted in Tables 2 and 3, it is also crucial to maintain high
dithiobenzoate-group fidelity, but if the second block is pre-
pared via NMP, the degree of functionality is less relevant.
Chain extension can be produced either at one or two
extremities depending of the selected experimental condi-
tions. For instance, chain extension of PNIPAM or PEOMA
with styrene using typical NMP conditions resulted in amphi-
philic block copolymers with excellent assembly behavior
(see Entries 2 and 4 in Table 2). Indeed, clear evidence
for vesicular structures was obtained by scanning electron
microscopy (SEM) observation, as shown in Figure 7(a) for
the P(S-b-NIPAM-b-S). Aggregation of these polymers was
induced via the dropwise addition of water to THF solutions
(a nonselective solvent) of the amphiphilic block polymers
forming spherical vesicles with sizes in the range of 300–
400 nm. The presence of holes in a number of the particles
clearly demonstrates the hollow nature of the particles and
their vesicular structure. Similar ‘‘open-mouth’’ vesicles were
recently reported by Davis and coworkers18 and Changez
et al.19 When P(S-b-EOMA-b-S) was self-assembled in the
mixed THF/water solvents, supramolecular associations were
also observed. However, in this case, small micelles sized
between 16 and 42 nm were formed as shown in Figure
7(b). Further investigations on the formation of nano-objects
are now in progress considering the effect of copolymers
We can see in Table 2 that polyEOMA (PEOMA) (Entry 1)
and Poly(N-isopropylacrylamide) (PNIPAM) (Entry 3) pre-
pared by RAFT polymerization using 2 and 1, respectively,
-
exhibit low D suggesting that in these cases the control is
favored by the use of appropriate [inifer]/[AIBN] molar ratio
and solvents. Measured Mn’s are significantly higher than the
calculated values, which might be explained by the fact that
it is expressed in PS equivalents. These polymers should be
a,x-heterodifunctionals, that is, at the x-end, they hold an
alkoxyamine, whereas at the a-end they should have dithio-
benzoate group. As it is known, the successful synthesis of
block copolymers via a living free radical process is pre-
cluded if the end-group fidelity is not secured in each block
formation step. As for most RDRP methods, it has been
established that the end-group fidelity continually decreases
during the polymerization process, the fact of having two
functional groups makes more versatile these homopolymers
-
composition, experimentally determined MW, D, and hydro-
phobic/hydrophilic ratios to clarify the potential of inifers.
TABLE 3 Conditions and results for the synthesis of CABC block copolymers through RAFT polymerization at 608C and tandem
NMP/RAFT polymerization at 1208C
Mn
Mnth
SEC
-
D
Entry
Polymer
Inifer/M
Ratiod,e/inifer/AIBN
T (ꢀC)
Conv (%)
t (h)
(g/mol)
(g/mol)
1a
2b
3b
4
PAA
1/AA
307
83.5
1
0.3
0.14
0
60
60
33
42
32
24
39
55
4
5
2
4
8
2
18,540
80,400
97432
19,670
30,800
47930
16,900
67,210
107,870
7,380
1.7
2.1
1.8
2.0
1.8
2.3
P(AA-b-BA)
P(S-b-AA-b-BA-b-S)
PtBA
PAA/BA
16.3
19
P(AA-b-BA)/S
2/tBA
81
120
65
217
1
0.9
0.01
0
5
6c
P(tBA-b-S)
PtBA /S
83.3
80
16.2
20
65
37,480
43,657
P(ClS-b-tBA-b-S-b-ClS)
P(tBA-b-S)ClS
120
a
d
e
Methanol 50% (w/w).
DMF 50% (w/w).
Toluene 50% (w/w).
Molar ratio for entries 1 and 4.
Weight ratio for entries 2,3,5 and 6.
b
c
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The accurate structure of block copolymers cannot be speci-
fied easily, but it is thought that the structure is probably a
nonsymmetrical triblock because both the alkoxyamine and
the dithiobenzoate groups can be activated simultaneously at
120ꢀC producing propagation at different rates. The SEC
traces of triblock copolymers are shown in Figure 8(a,b). It
can be seen an entire shift in the chromatograms of the
block copolymers to higher MW without ostensible formation
of shoulders or tails.
The results for the synthesis of two diblock copolymers;
P(AA-b-BA) and P(tBA-b-S) prepared by RAFT polymerization
are summarized in Table 3. These AB copolymers exhibit a
large dispersity due to the low chain transfer constant of 1
and 2; however, the behavior in water of hydrolyzed materi-
als suggests that the block copolymer structure has been
ꢀ
formed. The extension of these diblock copolymers at 120 C
in the presence of styrene increases MW indicating the for-
mation of CABC tetrablock copolymers.
The SEC traces in Figure 9(a,b) correspond to P(ClS-b-tBA-b-
S-b-ClS) or P(S-b-AA-b-BA-b-S) and their corresponding pre-
cursors, respectively. It can be seen a shift to higher MW
without the buildup of shoulders indicating the successful
formation of CABC diblock copolymer. These examples show
how the versatility of RAFT polymerization is enhanced by
presence of the alkoxyamine group, which allows the exten-
sion with a further block afterward the preparation of a spe-
cific A or AB precursor.
FIGURE 7 (a) SEM micrograph of self-assembled P(S-b-NIPAM-
b-S) vesicles and (b) P(S-b-EOMA-b-S) micelles prepared by
RAFT (central block) and NMP/RAFT polymerization (external
blocks).
Inifers 1 and 2 were initially intended to be used in the
preparation of amphiphilic block copolymers (in situ forma-
tion of surfactants) in miniemulsion polymerization above
100ꢀC. However, the results presented here show that such
-
FIGURE 8 (a) SEC traces for the PEOMA (Mn ¼ 38,140 g/mol; D ¼ 1.2) and its corresponding block copolymer P(S-b-EOMA-b-S) in
-
-
dashed line (Mn ¼ 63,820 g/mol; D ¼ 1.4); (b) SEC traces for the PNIPAM (Mn ¼ 5480 g/mol; D ¼ 1.3) and its corresponding block
-
copolymer P(S-b-NIPAM-b-S) in dashed line (Mn ¼ 38,700 g/mol; D ¼ 1.8).
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FIGURE 9 (a) SEC traces for polymers described in Entries 1–3 in Table 3 corresponding to the synthesis of P(ClS-b-tBA-b-S); (b)
SEC traces for polymers described in Entries 4–6 in Table 3 corresponding to the synthesis of P(S-b-AA-b-BA).
compounds can be also used to prepare a wide range of new
multiblock copolymers to form vesicles or micelles in a vari-
ety of sizes. For instance, it is quite appealing to create
vesicles from water soluble block polymers and then form a
cross-linked shell using surface initiated NMP. Our procedure
is now being extended to other block copolymers that will
be reported in due time. We also continue our efforts for
developing advanced methods for the synthesis of new het-
erofunctional compounds for RDRP.
mined gravimetrically from precipitated and dried samples.
-
MW and D were determined by SEC. The SEC analysis was
conducted with a Hewlett-Packard instrument (HPLC series
1100) equipped with UV light and refractive index detectors
using a series of three PL-Gel columns (103, 105, and 106 Å)
in THF as eluent at 40ꢀC and at a flow rate of 1 mL/min.
-
The apparent MWs (Mn and Mw) and D were determined
with a calibration based on linear poly(methyl methacrylate)
or PS standards. SEM images were obtained using a JEOL
equipment JSM-7401F, Cold type of FE-SEM field emission
scanning electron microscope. The sample preparation for
SEM measurements was done using the following procedure.
The plate (copper) was cleaned with acetone and then air-
dried. The block copolymer solution in THF/H2O with a con-
centration of 1–3 mg/mL was drop-cast onto the surface of
the plate. It was then allowed to dry at room temperature.
The surface of the films was coated with gold and palladium
alloy layer before recording the SEM images.
EXPERIMENTAL
Materials
Styrene (S) (99%) was distilled under sodium at low pres-
sure, N-isopopylacrylamide (NIPAM) was recrystallized twice
in hexane before use. BA and tBA were purified under vac-
uum pressure using calcium hydride as drying agent. AA was
distilled under reduced pressure. AIBN (98%) was recrystal-
lized from methanol and stored in a refrigerator before use.
TEMPO (Aldrich, 99%) was sublimated before application.
MALDI-TOF-MS
MALDI-TOF-MS measurements were performed on a Shi-
madzu Biotech AXIMA Performance equipped with a 337 nm
nitrogen laser. Positive ion spectra were acquired in linear
mode and 20 kV acceleration voltage. The mass scale was
calibrated with Bradykinin (mass: 757.3997), P14R (mass:
1533.8582), and ACTH (mass: 2465.1989) with alpha-cyano-
4-hydroxy-cinnamic acid. Samples were prepared from THF
solution by mixing matrix (Dithranol [Fluka, >99% purity]
10 mg/mL), sodium trifluoroacetate (10 mg/mL) and sample
(10 mg/mL) in a ratio 10:80:10. One microliter of the mix-
ture was spotted on the Maldi plate. The spectrum shown in
this article represents the original data without any filtering
or background abstraction.
All other reagents, carbon tetrachloride (99.8%, ACS Rea-
gent), carbon disulfide, (anhydrous ꢄ 99%), THF, (anhy-
drous, ꢄ 99.9%, Inhibitor free), bromobenzene, (99%), mag-
nesium powder, (ꢃ50 mesh, 99 þ %), magnesium sulfate,
(anhydrous Reagent Plus), iodine, (ACS Reagent Plus, ꢄ 99%
Chips), bromide, (99.99%), CD3Cl, and solvents, were pur-
chased from Aldrich and was used as received. The polyeth-
ylene oxide methacrylate (EOMA) and 4-chlorostyrene were
used as received.
Analyses
NMR spectra were recorded on a Bruker instrument operat-
ing at 500 MHz in CDCl3. Monomer conversions were deter-
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TGA measurements were performed on a computer con-
trolled TGA model Q500 (TA Instruments). The sample was
kept at a constant 25ꢀC under a dry nitrogen atmosphere
(40 mL/min) and then the same specimen was heated from
ambient to 600ꢀC at a rate of 10ꢀC/min and from 600 to
800ꢀC in oxygen atmosphere at a rate of 20ꢀC/min.
eluent. 1 was formed in a yield of 21% and with more than
96% purity (from 1H NMR). The same procedure was
repeated with 20.
1. 1H NMR (CDCl3, d, ppm) 0.76–1.27 (m, 12H CH3ACN),
1.31–1.72 (m, 6h CH2AC(CH3)AN), 3.74 (dd, 1H, CH2AS, J
¼ 5, J ¼ 8, J ¼ 13 Hz), 4.16 (dd, 1H CH2AS, J ¼ 5, J ¼ 8, J
¼ 13 Hz), 5.09 (dd, 1H CHAO, J ¼ 5, J ¼ 8 Hz), 7.31–7.62
(m, 7H Ph-H,), 7.88 (d, 2H,CH¼¼CACS, J ¼ 8 Hz). 13C NMR
(65 MHz, CDCl3): d: 17.5–32.4 (CACH2), d: 18.7 (CACH3),
d: 44.7 (CASAC¼¼S), d: 83.2 (CAOAN), d: 122.43 (C-Ph),
d: 135.2 (CACl), d: 141.60 (CACAOAN), d: 147.2
(CAC¼¼S), d: 226.7 (SAC¼¼S), GC-MS: m/z 448 (Mþ, 1%)
156 (C9H18NO, 45%), 138 (C8H7Cl, 40%), 105 (C8H7,
10%), 77 (C6H5, 25%).
ESR Measurements
The ESR spectra were recorded by an X-band Bruker EleXsys
instrument using 100 kHz modulation frequency and 0.1 G
modulation width. The applied microwave power was 1 mW,
the scan time was 20.97 s, the number of points in the spec-
tra was 1024, and the delay time between scans was 240 s.
In the kinetical recording, the number of spectra was 36–48
for 2.5–3.5 h at temperature 120ꢀC. The spectra were simu-
lated by an automatic fitting program.20 ESR parameters for
low concentration g ¼ 2.0057, AN ¼ 15.41 G, width ¼ 0.55
G. When the concentration was higher, broader lines (width
¼ 1.5 G) were obtained and the nitrogen coupling was
increased (AN ¼ 15.25 G).
2. 1H NMR (CDCl3, d, ppm) 0.713–1.10 (m, 12H CH3ACN),
1.21–1.622 (m, 6h CH2AC(CH3) AN), 3.78 (dd, 1H, CH2-S,
J ¼ 5, J ¼ 8 Hz), 4.17 (dd, 1H CH2-S, J ¼ 5, J ¼ 8 Hz),
5.12 (dd, 1H CHAO, J ¼ 3, J¼ 5 Hz), 7.25–7.50 (m, 8H Ph-
H,), 7.865 (d, 2H,CH¼¼CACS, J ¼ 9 Hz). 13C NMR (65 MHz,
CDCl3) d: 17.5–32.4 (CACH2); d: 20.2 (CACH3); d: 44.7
(CASAC¼¼S); d: 83.6 (CAOAN); d: 122.43 (C-Ph); d:
141.60 (CACAOAN); d: 146 (CAC¼¼S); d: 226.7 (SAC¼¼S).
GC-MS: m/z 413 (Mþ, 1%), 156 (C9H18NO, 5%) 246
(C16H24NO, 20%), 105 (C8H7, 100%), 77 (C6H5, 35%).
Synthesis of 2-(4-Chlorophenyl)-2-(2,2,6,6-
tetramethylpiperidin-1-yloxy)ethyldithio benzoate (1)
The formation of chain transfer agents 1 was performed in
two steps. First, TEMPO (1,6 g, 10.24 mmol) in 6 mL of CCl4
was treated with bromide (0.81 g, 10.18 mmol) under mag-
netic stirring to form the oxoaminium bromide, a brown col-
ored solid, that precipitates soon after the contact between
both reactants. After completion of bromide addition, the so-
lution was kept under agitation for 30 min. Then, 4-chloros-
tyrene (7.62 g, 54.58 mmol) diluted with 6 mL of CCl4 was
added and the temperature was raised to 50ꢀC for 30 min
producing a gradual consumption of oxoaminium bromide.
The crude product was washed with 50 mL of water, taken
up in hexane (30 mL), dried with magnesium sulfate, fil-
trated and evaporated to obtain an oily reddish product.
This product was purified on neutral alumina oxide column
using hexane/dichloromethane (9/1) as eluent. A colorless
liquid (yield 45%), identified as 1-(4-chlorophenyl)etoxy)-
2,2,6,6-tetramethylpiperidine (10) or Br(4-chlorostyrene)-
TEMPO adduct was obtained. The same procedure was
repeated with styrene to obtain 1-(2-bromo-1-phenyle-
thoxy)-2,2,6,6-tetramethylpiperidine (20).
General Procedure for the RAFT
Polymerization of Styrene
Bulk styrene RAFT polymerization was performed at two dis-
tinct molar ratios: [S]/[1]/[AIBN] ¼ 150/1/0.5 and 650/1/1.
In typical experiment, a series of heavy-wall glass tubes were
filled with the monomer solution degassed by three freeze–
pump–thaw cycles and sealed. Then, tubes were heated at
60ꢀC in thermostated oil bath and removed at predetermined
times. All polymer samples were isolated by precipitation and
characterized by SEC. Results are summarized in Supporting
Information, Table S1 for the experiment performed with a
molar ratio [S]/[inifer]/[AIBN] ¼ 650/1/1 (see also Support-
ing Information, Table S2, for the experiment performed with
a molar ratio [S]/[1]/[AIBN] ¼ 150/1/0.5).
General Procedure for NMP of Styrene
To perform NMP experiments, the temperature and molar ra-
tio were set at 120ꢀC and S/1 ¼ 200/1, respectively. A solu-
tion containing 0.147 g of 1, (0.356 mmol) and 7.42 g (71.3
mmol) of styrene was divided and placed in several ignition
heavy-wall glass tubes. After degassing and sealing, tubes
were immersed in the thermostated oil bath and removed at
different time interval. SEC was used to determine MW and
In a second step, an ice-bathed Grignard solution prepared
from bromobenzene (0.93 g, 5.9 mmol) and magnesium
powder (0.14 g, 5.9 mmol) in 5 mL of THF was vigorously
stirred for 1 h, whereas a solution of carbon disulfide (0.45
g, 5.9mmol) in 5 mL of dry THF was dropped into the vessel
under argon. The resulting dark red mixture was warmed at
50ꢀC for 1 h before cooled to room temperature. A solution
of 10 (2 g, 59 mmol) in 10 mL of THF was then added while
raising temperature at 50ꢀC and maintaining it for 4 h. The
crude product was treated with 50 mL of 10% aqueous solu-
tion of ammonium chloride and extracted in hexane (30 mL).
The organic layer was dried on MgSO4 and collected after ro-
tary evaporation of solvent. This red oil product was purified
twice by column chromatography using neutral aluminum
oxide as stationary phase and hexane/ethyl acetate (9/1) as
-
D. A similar procedure was performed with inifer 2. Results
are summarized in Supporting Information, Table S3.
Chain Extension
RAFT First Approach
In a heavy-wall glass tube were placed 4.32 g, (85 mmol) of
styrene, 0.127 g(0.58 mmol) of 1 and 0.046 g (0.58 mmol)
of AIBN. This mixture was degassed by three freeze–thaw
pump cycles and then the tube was sealed and immersed in
a thermostated oil bath for 8 h at 60ꢀC. The polymer was
isolated by precipitation in methanol and characterized by
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-
SEC (Mn ¼ 12,960 g/mol, D ¼ 1.8). 0.222 g, (0.017 mmol) of
this polymer was placed into a heavy-wall glass tube and
dissolved with 1.16 g (11 mmol) of styrene and 1.16 g of tol-
uene. The mixture was degassed, sealed and heated for 3 h
at 120ꢀC. The polymer was isolated by precipitation in meth-
entirely by RAFT polymerization followed by the chain exten-
sion with styrene using NMP/RAFT tandem polymerization.
The P(S-b-AA-b-BA-b-S) was prepared as follows. In a heavy-
wall glass tube were placed 2.03 g (28.2 mmol) of AA,
0.0411 g (0.092 mmol) of 1, and 0.0054 g (0.033 mmol) of
AIBN along with 1.98 g of methanol as dissolvent. The tube
was degassed by three freeze–thaw pump cycles then sealed
and heated in oil bath thermostated at 60ꢀC for 3 h. After
polymerization, the PAA was diluted in methanol and precip-
itate in petroleum ether to obtain a reddish product (yield
33%). This purified polymer (0.208 g, 0.01 mmol) was
placed into a heavy-wall glass tube along with 1.06 g (8.3
mmol) of BA, 0.0019 g (0.01 mmol) of AIBN, and 0.99 g of
DMF as solvent. The tube was degassed by three freeze–
thaw pump cycles and sealed. The tube was then heated at
60ꢀC for 5 h in a thermostated oil bath. The polymer was
isolated by precipitation and characterized. The P(AA-b-BA)
(0.32 g, 0.0059 mmol) was retaken with 1.33 g (13 mmol)
of distilled S and 1.1 g of DMF in heavy-wall glass tube. The
tube was degassed using the standard technique, sealed and
heated at 120ꢀC for 2 h. Quantitative methylation of the
polyacrylic acid block was achieved using trimethylsilyldiazo-
methane21 before SEC characterization. The same procedure
was used to prepare P(ClS-b-tBA-b-S-b-ClS) by the interme-
diation of a diblock copolymer prepared by RAFT polymer-
ization from tert-butylacrylate and styrene and then chain
extended with 4-chlorostyrene (ClS) by NMP/RAFT tandem
polymerization. A summary of experimental conditions and
results is presented in the Table 3.
-
anol and characterized by SEC (Mn ¼ 26,330 g/mol, D ¼
1.3). See Entry PS2 in Supporting Information, Table S4.
NMP First Approach
The polymer prepared by NMP with 1 described in Entry 12
in Supporting Information, Table S3 (Mn ¼ 10,630 g/mol
-
and D ¼ 1.5) was retaken for this experiment. Thus, 0.105 g
(0.0098 mmol) of this polymer was placed into a heavy-wall
tube and mixed with 0.23 g (2.2 mmol) of styrene and
0.0017 g (0.0010 mmol) of AIBN. This mixture was degassed
by three freeze–thaw pump cycles and the tube was sealed.
The tube was immersed in a thermostated oil bath for 8 h at
60ꢀC. The polymer was isolated by precipitation in methanol
-
and characterized by SEC (Mn ¼ 12,240 g/mol, D ¼ 1.8). See
Entry 4, Supporting Information, Table S4 for detailed infor-
mation. The same polymer was retaken (0.1356 g, 0.0127
mmol) in 0.905 g (8 mmol) of styrene in a heavy-wall glass
tube. The tube was sealed after three freeze–thaw pump
cycles and placed in an oil bath thermostated at 120ꢀC for 2
h. After precipitation in methanol, MW was calculated by
-
SEC (Mn ¼ 27,150 g/mol and D ¼ 1.3). More detailed infor-
mation is provided in the Supporting Information Section;
see Entry PS5, Table S4.
General Procedure for the Synthesis of
Block Copolymers
Type BAB
The synthesis of BAB block copolymers was performed in
two steps. First, a RAFT polymerization a monomer A was
carried out followed by a tandem polymerization of a mono-
mer B at 120ꢀC as illustrated in the following example. For
the synthesis of P(S-b-EOMA-b-S), 3.75 g (11.7 mmol) of
EOMA (MW ¼ 321.43 g/mol) were mixed with 0.0595 g
(0.132 mmol) of 1, 0.0073 g (0.044 mmol) of AIBN, and 3.2
g of methanol and transferred in heavy-wall glass tube. The
mixture was degassed by three freeze–thaw pump cycles
and then sealed under vacuum. The tube was immersed in
oil bath thermostated at 60ꢀC for 3 h. The PEOMA obtained
was dissolved in methanol and precipitate in petroleum
ether. A reddish product was obtained with a yield of 74%.
This isolated polymer was retaken (0.359 g, 0.00094 mmol)
in a heavy-wall glass tube with 1.702 g (16 mmol) of S and
1.5 g of DMF. The mixture was degassed by freezing–pump–
thaw three cycles and sealed. The tube was heated at 120ꢀC
for 5 h in a thermostated oil bath. The product was dis-
solved in THF and precipitated in a mixture methanol–water
(yield 44%). The P(S-b-NIPAM-b-S) was prepared following
the same procedure using NIPAM as monomer A and styrene
as monomer B. Both BAB copolymers were characterized by
SEC (Fig. 8) and results are described in Table 2.
CONCLUSIONS
Inifers 1 and 2 were synthesized and used as RAFT or NMP
agents in the polymerization of styrene. Both compounds
were able to control the RAFT polymerization of styrene at
conversions up to 71% and MWs up to 56,000 g/mol (vs. PS
standards). Samples exhibit narrow molar mass distributions
-
(D ¼ 1.3) at low conversion but the control is progressively
loosed with conversion. Both inifers allow better control in
-
the combined NMP/RAFT polymerization (D ¼ 1.2–1.5) but,
as expected, polystyrenes prepared with 1 or 2 reveals large
discrepancy between the theoretical and calculated MWs due
to possible side reactions, likely the ineludible radical–radi-
cal termination of difunctional propagating species. NIPAM,
butylacrylate, EOMA, and so forth were used to prepare
homopolymers and block copolymers. The preparation of
some multiblock copolymers was also achieved.
ACKNOWLEDGMENTS
The authors acknowledge The CONACyT, for the financial sup-
port (Grant CB-2008-1/10137) and for the scholarship of one
of us (CST). The authors also thank for the financial support of
the Hungarian-Mexican MX/2007 state project. GVOP-3.2.1-
2004-04-0214/3.0. Thanks are due to J. Cabello-Romero, J. G.
Tellez-Padilla, M. L. Lopez-Quintanilla, and J. A. Cepeda-Garza
for their technical support.
Type CABC
The synthesis of CABC block copolymers involves the prepa-
ration of a diblock copolymer with monomer A and B
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10 Tong, Y. Y.; Dong, Y. D.; Du, F. S.; Li. Z. C. Macromolecules
2008, 41, 7339–7346.
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