Full Paper
1382
Summary: The synthesis of polyacrylonitrile-block-polystyrene (PAN-b-PS) copolymers by atom transfer radical
polymerization (ATRP) is reported. Chain extension of
bromine terminated PAN macroinitiators with styrene was
performed using a CuBr/N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine catalyst system and 2-cyanopyridine as a
solvent. The first-order kinetic plots of styrene consumption
showed a significant curvature, indicating a progressive decrease in the concentration of active species during copolymerization. The loss of the bromide end group was mainly
ascribed to the elimination of HBr, as shown by 1H NMR
spectroscopy. By varying the molar ratio of either the catalyst
or the monomer to the initiator, a series of PAN-b-PS copolymers were prepared, with polydispersities as low as 1.3,
and molar compositions ranging from 8.6/91.4 to 35.5/64.5.
1
H NMR spectra of PAN-b-PS in DMF-d7 at 80 8C.
Synthesis of Polyacrylonitrile-block-Polystyrene
Copolymers by Atom Transfer Radical Polymerizationa
Massimo Lazzari,*1,2 Oscar Chiantore,2 Raniero Mendichi,3 M. Arturo López-Quintela1
1
Department of Physical Chemistry, Faculty of Chemistry, University of Santiago de Compostela,
E-15782 Santiago de Compostela, Spain
2
Department of Chemistry IPM, Nanostructured Interfaces and Surfaces – Centre of Excellence, University of Torino,
Via P. Giuria 7, I-10125 Torino, Italy
E-mail: massimo.lazzari@unito.it
3
Istituto per lo Studio delle Macromolecole (CNR), Via E. Bassini 15, I-20133 Milano, Italy
Received: April 22, 2005; Accepted: May 12, 2005; DOI: 10.1002/macp.200500159
Keywords: atom transfer radical polymerization (ATRP); block copolymers; 2-cyanopyridine; PAN-b-PS; polyacrylonitrile
Introduction
Random copolymers of styrene and acrylonitrile (SAN)
are a class of commercially important thermoplastics with
unique and well-known properties, such as excellent
chemical resistance, dimensional stability, impact strength
and ease of processing.[1] SAN copolymers are prepared
industrially by free radical polymerization which has a
major drawback (limited control over molecular parameters
a
: Supporting information for this article is available at the
bottom of the article’s abstract page, which can be accessed from
the journal’s homepage at http://www.mcp-journal.de, or from
the author.
Macromol. Chem. Phys. 2005, 206, 1382–1388
and particularly molecular weight and its distribution)
which strongly conditions processing. The copolymerization of styrene and acrylonitrile can also be carried out by
controlled/living radical polymerization (CRP) methods,[2]
including reversible addition fragmentation chain transfer
processes (RAFT),[3] nitroxide-mediated radical polymerization (NMP)[4] and atom transfer radical polymerization
(ATRP),[5] which yield well-defined materials. NMP and
ATRP also permit the preparation of block copolymers
comprising random SAN sequences and styrene, ethylene
oxide, propylene oxide, e-caprolactone, butadiene, n-butyl
acrylate, tert-butyl acrylate, glycydyl acrylate or methyl
methacrylate blocks.[4,5]
DOI: 10.1002/macp.200500159
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Synthesis of PAN-b-PS Copolymers
On the other hand, notwithstanding the fact that polyacrylonitrile (PAN) can be formed by different controlled
polymerization processes,[4b,6–8] with an excellent degree
of control reached essentially by ATRP, only a few studies
on the synthesis of block copolymers containing PAN
blocks have been reported.[7d,8] Such copolymers are challenging to produce, mainly due to the limited solubility of
PAN in most common organic solvents used in CRP and in
typical monomers, as well as to the difficulty of controlling
the initiation efficiency of the macroinitiator. The latter
depends not only on the limited solubility of the other block
in the diluents for PAN, but also on the stability of the active
radical species in such solvents. As a matter of fact, only
polyacrylonitrile-block-poly(n-butyl acrylate)s have been
successfully prepared, the best results being obtained
by ATRP,[7d] showing at the same time the potentiality of
PAN-based block copolymers for the preparation of
nanostructured carbon materials by controlled pyrolysis
of their well-organized morphologies.[9] In this sense, the
availability of other polymers containing PAN segments in
the backbone would be of great use for further exploration
of the potential of such a family of block copolymers in the
fabrication of nanomaterials.[10] In particular, the synthesis
of polyacrylonitrile-block-polystyrene (PAN-b-PS) is also
a worthwhile challenge because of the different polarities
and solubilities of the blocks, and the expected mechanical
properties, which could be compared with those of SAN.
In this contribution we report for the first time the
synthesis of PAN-b-PS copolymers by chain extension of
PAN macroinitiator through copper-mediated ATRP. The
novelty of the process described is mainly due to the use of
2-cyanopyridine (2CNP) as the polymerization solvent.
The final goal of this work is to pave the way to the routine
preparation of PAN-containing block copolymers.
Experimental Part
Materials
Acrylonitrile (Aldrich, 99þ%) (AN) was passed through
a macroreticular ion exchange resin (De-Hibit-200, Polysciences Inc.) to remove inhibitor. All other reagents, unless
otherwise specified, were purchased from Aldrich. Styrene
(Sty, 99%), 2-bromopropionitrile (BPN, 97%), CuIBr (98%),
2,20 -bipyridine (bpy, 99þ%), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA, 99%), ethylene carbonate
(EC, Fluka, >99%), N,N-dimethylformamide (DMF, 99.8%),
2-cyanopyridine (2CNP, 99%), anisole anhydrous (99.7%),
methanol (Fluka), 2-propanol (Prolabo) and tetrahydrofuran
(THF, Fluka) were used as received.
Polymerization Procedures
All the syntheses were carried out by copper-mediated ATRP
in EC, DMF or 2CNP, using BPN or bromine-terminated PAN
Macromol. Chem. Phys. 2005, 206, 1382–1388
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as initiators for homopolymerization and copolymerization,
respectively. Polymerization of Sty was carried out as follows.
First, 12.6 mL of Sty (1.10 101 mol), 9.5 102 mL
(1.10 103 mol) of BPN and 14.0 g of 2CNP were mixed in
a 50 mL Schlenk flask, maintaining an overpressure of dry
nitrogen. The flask was subjected to four freeze-pump-thaw
(FPT) cycles, then 2.30 101 mL (1.10 103 mol) of
PMDETA and 1.58 101 g (1.10 103 mol) of CuBr were
added to the frozen mixture. The flask was then placed in an
oil bath at 100 8C for 15 h. At the end of this time, the reaction mixture was dissolved in 20 mL of CH2Cl2 and the
polymer was precipitated by adding it into a methanol/2propanol (3/1 v/v) mixture. The polymer was filtered, washed
several times with methanol and water, and finally dried under
vacuum to constant weight.
Typical Polymerization of AN (Table 2, Entry 1)
First, 10 mL of AN (1.52 101 mol), 3.86 101 mL of BPN
(4.47 103 mol), 2.10 101 g of bpy (1.34 103 mol)
and 23.3 g of 2CNP were added to a 50 mL Schlenk flask,
maintaining an overpressure of dry nitrogen. The flask was
subjected to four FPT cycles, then 6.41 102 g of CuBr
(4.47 104 mol) was added to the frozen mixture. The
flask was finally placed in an oil bath at 55 8C for 24 h. At the
end, 20 mL of DMF were added to dissolve the reaction
mixture and the polymer was precipitated into a large excess of
a 50% aqueous methanol solution. The polymer was filtered,
washed several times with methanol and water, dissolved in
DMF and precipitated again, repeating the washing procedure.
The polymer was finally dried under vacuum to constant
weight.
Typical Chain Extension to Prepare PAN-b-PS
Block Copolymer (Table 2, Entry 6)
First, 2.01 101 g of the PAN macroinitiator of Entry 1
(M n;NMR ¼ 1.18 103; 1.70 104 mol), 8.8 mL of Sty
(7.68 102 mol) and 11.9 g of 2CNP were added to a
50 mL Schlenk flask, maintaining an overpressure of dry
nitrogen. The flask was subjected to four FPT cycles, then
3.55 102 mL of PMDETA (1.70 104 mol) and 2.44 102 g (1.70 104 mol) of CuBr were added to the frozen
mixture. The flask was finally placed in an oil bath at 100 8C for
4.2 h. The purification procedure was the same as that reported
for PAN.
Analysis
The conversion of monomers was measured using a HP-5890
series II gas chromatograph with a HP-5972 mass detector
equipped with a HP5 MS 30 m column. The injector and
detector were kept at 280 8C. Periodically, controlled amounts
of samples were removed from the reaction by a syringe and
diluted into THF. Only in the case of Sty was conversion
measured by using an internal standard (anisole).
Molecular weight averages and molecular weight distributions were determined by size exclusion chromatography
ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Lazzari, O. Chiantore, R. Mendichi, M. A. López-Quintela
(SEC) in a THF or DMF mobile phase, also depending on the
polymer solubility. SEC of PSs and block copolymers with
higher PS contents were performed in a THF mobile phase at
room temperature on 0.3% (w/v) sample solutions in distilled
solvent. Solutions were filtered using 0.45 mm membrane
syringe filters and 200 mL were then injected into the chromatograph with the eluent flow rate set at 1 mL min1. The
system was equipped with 3 PL-Gel columns with different
nominal porosities (500, 103 and 104), a Waters 515 HPLC
pump and an Erma 7510 differential refractometer. Column
calibrations were performed with PS narrow distribution
polymer standards and a third-order polynomial equation
was obtained from regression analysis. Molecular weight evaluations were performed with internal standard corrections for
flow rate fluctuations. In the comparison of the chromatograms
of polymerization products at different reaction times, the
peak areas were normalized relative to the monomer conversion. The average molecular weight and molecular weight
distribution of some block copolymers were also determined
by a multi-angle laser light scattering (MALS) photometer
on line to a SEC system, which has been described in
detail elsewhere.[11] The experimental conditions and the
results of these characterizations are reported in the Supporting
information.
The molecular weights of PAN macroinitiators and block
copolymers with lower PS contents were also determined by
MALDI-TOF MS (Bruker Autoflex) with delayed extraction
using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix. Composition and end group
analyses were carried out using 1H NMR in DMF-d7 at 80 8C
on a Bruker DRX-500 MHz spectrometer.
Results and Discussion
The synthesis of block copolymers by ATRP requires the
presence of an activated alkyl halide as a polymer chain end
group which can act as a macroinitiator for the subsequent
block. Hence, the large variety of possible monomer combinations is theoretically limited only by the relative
reactivities of the chain ends,[2c,d] i.e., in order to attain
the appropriate initiation efficiency of the macroinitiator,
the constant rate of cross-propagation should be at least
comparable to that of propagation of the subsequent
monomer. According to homopolymerization and model
studies,[12] the chain extension of a bromine-terminated
PAN macroinitiator (PAN-Br) with styrene should be
highly efficient, with the main practical obstacle being a
pronounced solvent effect upon the activity of styryl
bromide chain end groups. The limited stability of
such end groups was proposed following the observation
of significant decomposition of a model compound, i.e., 1phenylethyl bromide, in polar solvents such as acetonitrile
and nitromethane,[13] and was confirmed by our group.
Indeed, chain extension of PAN-Br with styrene by coppermediated ATRP in ethylene carbonate (EC) or in N,Ndimethylformamide (DMF) with CuBr/2,20 -bipyridine
(bpy) as a catalytic system failed, leading to the formation
Macromol. Chem. Phys. 2005, 206, 1382–1388
www.mcp-journal.de
of PAN chains bearing a single phenylethyl unit as the
terminal group.b This suggests a quantitative addition of
styrene to the macroinitiator, immediately followed by side
reactions with loss of the halogen from the styryl end.
Notwithstanding the oxidation of active species to cations
that was postulated in previous studies as the reaction of
termination in the ATRP of styrene,[13] the observation of
negligible amounts of unsaturated chain ends by 1H NMR
has driven the proposal of a completely different process
(Scheme 1). The reduction to anionic intermediates yielding
dead polymer chains through reaction with a proton source
(reported as HS in the scheme), i.e., either EC or DMF, or
even the acidic H-atoms in PAN, is likely to be favored by
the backbone which gives an electrophilic character to the
radical. Also, the quick change of the aspect of the solution,
from a brown color due to the CuI/bpy complex to a deep
green, possibly indicating the formation of CuII species,
supports this mechanism.
The need for a solvent that does not contain easily
extractable hydrogens but at the same time is polar enough
to solubilize PAN rules out the large variety of solvents
already used in ATRP. Also taking into account the
solubility parameter for PAN, dH ¼ 28.4,c we looked for
substances with aromatic or heteroaromatic structures bearing substituents which increase the polarity up to the
expected values. Cyanopyridines and particularly 2CNP
(m.p. ¼ 24 8C, b.p. ¼ 212 8C, dH ¼ 27.2)[14] appeared to be
a good candidate. Its suitability as a solvent for ATRP was
first tested for the homopolymerization of acrylonitrile and
styrene.
The polymerization of AN in 2CNP initiated with 2bromopropionitrile (BPN) using CuBr/bpy as the catalyst
showed results comparable with those already obtained in
EC,[7b] at least in terms of end group functionality, with a
polymerization rate intermediate between those produced in
EC and propylene carbonate, respectively.d The polymerization of styrene with the same catalytic system only
permitted the preparation of polymers with broad molecular
weight distributions, whereas in the presence of a more
active coordinating ligand, such as N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA), the synthesis
b
c
d
The possibilities of chain extension of PAN-Br in traditional
solvents were explored at 100 8C, with the molar ratio of the
catalyst to the macroinitiator in the range 0.1/1–1/1.
This value of Hildebrand’s solubility parameter, expressed in
MPa1/2, may be considered as an average as values also vary
with the method of determination and test conditions.
As an example, the polymer of Entry 7 in Table 2 (synthesis in
2CNP at 65 8C with [AN]0 ca. 5 M and [AN]0/[BPN]0[CuBr]0/
[bpy]0 ¼ 100/1/0.1/0.3) showed a conversion of 28.0 and 34.2%
after 10 and 24 h, respectively, which may be compared with
the values of around 35 and 50% after the same reaction times
obtained by K. Matyjaszewski et al. in ref.[7b] for a PAN
prepared in EC at 65 8C ([AN]0 ¼ 5.25 M and [AN]0/
[BPN]0[CuBr]0/[bpy]0 ¼ 95/1/0.1/0.3). The fraction of Br end
groups was close to 90% in both cases.
ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Synthesis of PAN-b-PS Copolymers
Scheme 1.
occurred with good control over molecular weight and its
distribution (Table 1), at least at low conversion. Even
though the polydispersity increased with conversion and the
molecular weights overestimated the theoretical values,
indicating a partially inefficient initiation, this preliminary
result showed the feasibility of obtaining PS chains by
ATRP in such a solvent.
Two PAN macroinitiators with low molecular weights
of 1 180 and 3 200 g mol1, respectively, were used for
the preparation of block copolymers in 2CNP at 100 8C
(Table 2, Entry 2–6 and 8). The macroinitiators had high
chain end functionality in order to ensure that nearly all
chains were capable of initiating the second block. The
conditions for chain extension are reported in Table 2. In all
the cases, the polymerization systems remained homogeneous throughout the reaction. Reaction in the presence
of CuBr/bpy (Table 2, Entry 2) occurred with a very low
conversion, resulting in a copolymer with a short styrenic
block, which indicates both low efficiency of the catalytic
system and quick loss of the bromine end groups. The latter
was confirmed by 1H NMR spectroscopy, as no evidence
of the methine proton of the –CH(Ph)–Br group could be
found.
Replacement of bpy by PMDETA permitted higher conversions to be reached in a reasonable time, leading to welldefined block copolymers, with the optimal [PMDETA]/
[CuBr] ratio of 1/1[15] maintained for all the series. The
copolymerization reactions were performed by varying the
molar ratio of the catalyst to the initiator from 0.2/1 to 1/1,
using ratios of monomer to initiator of either 250 or 450.
Figure 1 shows the kinetics of polymerization of some of
these diblock copolymers. The significant curvature of the
first-order kinetic plots of monomer consumption indicated
a progressive decrease of the concentration of active species
since the initial phases of polymerization, and was more
evident in the case of the lowest catalyst to initiator molar
ratio. The apparent number-average molecular weights,
experimentally determined by SEC in THF, displayed an
almost linear dependence on conversion (Figure 2). The
values for the higher conversions were, in general, higher
than those predicted by the ratios of starting monomer and
macroinitiator concentration, that is [Equation (1)],
M n;th ¼ M n;PAN-Br þ MSty ½Sty0 =½PAN-Br0
conversion
Size exclusion chromatograms showed an initial monomodal and symmetrical distribution, indicating that the
initiation was efficient and at least as fast as the propagation,
while for higher conversions a tail at longer retention
times was visible. Such asymmetry of the molecular
weight distribution towards lower molecular weights is
due to termination processes. The polymer peak continuously shifted to the higher molar mass region with
monomer conversion, while the polydispersity decreased
with the progress of polymerization down to values as low
Table 1. Experimental parameters for the ATRP of styrene at 100 8C in 2CNP. [Sty]0 ¼ 8.5
[PMDETA]0 ¼ 100/1/1/1.
Time
h
M;
[Sty]0/[BPN]0/[CuBr]0/
M n;SEC-RI 103
M n;th 103 a)
M w =M n;SEC-RI
2.15
3.36
6.96
10.2
1.89
3.33
5.76
8.68
1.15
1.19
1.26
1.66
%
1.2
3.3
6.0
15
a)
Conversion
ð1Þ
16.8
30.8
54.1
82.2
M n;th ¼ M BPN þ M Sty conversion ½Sty0 =½BPN0 ¼ 134 þ 104 conversion 100.
Macromol. Chem. Phys. 2005, 206, 1382–1388
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M w =M n;SEC-RI
(1.03)b)
(1.03)b)
1.94
1.50
1.54
1.32
(1.06)b)
1.59
M n;SEC-RI ð103 Þ
(1.10)b)
(1.30)b)
10.9
7.55
26.3
19.4
(2.81)b)
17.9
–
80.5/19.5
21.7/78.3
24.4/75.6
8.6/91.4
10.0/90.0
–
35.5/64.5
as 1.3–1.4. As an example, a typical evolution of the SEC
traces (RI detector), relative to the copolymer of Entry 5 in
Table 2, is shown in Figure 3. Block copolymers were also
characterized by SEC equipped with a MALS on-line
detector (results reported in the Supporting information).
1
H NMR analysis allowed the determination of molar
composition and molecular weights, as well as enlightening
us on the termination reactions. Figure 4 shows as an
example the 1H NMR spectrum of PAN-Br of Entry 1 compared with that of PAN-b-PS of Entry 3. Molar compositions were determined from the relative intensity of the
resonance at 3.3 ppm (–CH– proton) for acrylonitrile and
those between 6.3 and 7.4 ppm (aromatic protons of the
phenyl group) for styrene. On the basis of such a molar
fraction composition, the molecular weights of the
48.4
<5
32.8
29.1
41.0
40.5
34.2
45.3
24
24
7.8
3.3
6.5
3.5
24
3.3
Determined by 1H NMR.
Determined by MALDI.
a)
Macromol. Chem. Phys. 2005, 206, 1382–1388
b)
PAN
PAN-b-PS
PAN-b-PS
PAN-b-PS
PAN-b-PS
PAN-b-PS
PAN
PAN-b-PS
1
2
3
4
5
6
7
8
34/1/0.1/0.3
250/1/0.1/0.3
250/1/0.2/0.2
250/1/0.5/0.5
450/1/0.5/0.5
450/1/1/1
100/1/0.5/1.5
250/1/0.5/0.5
%
h
bpy
bpy
PMDETA
PMDETA
PMDETA
PMDETA
bpy
PMDETA
Convn
Mon/I/C/L
Polymer
Ligand
Time
1.18
1.67
8.59
7.54
23.0
20.0
3.20
14.3
M n;NMR ð103 Þ
Molar
composition
AN/STYa)
1.01
–
9.72
8.76
20.4
20.2
1.95
15.3
M n;th ð103 Þ
Figure 1. Percentage monomer conversion (filled symbols) and
first-order kinetic plot (open symbols) as a function of time for
the block copolymerization of styrene in 2CNP at 100 8C with
various [Sty]0/[PAN-Br]0/[CuBr]0/[PMDETA]0 (Table 2: (*)
Entry 3, (~) Entry 5 and (&) Entry 6).
No.
Table 2.
Experimental parameters for the block copolymerization from PAN to Sty at 65 8C in 2CNP (bold entries refer to PAN macroinitiators).
M. Lazzari, O. Chiantore, R. Mendichi, M. A. López-Quintela
www.mcp-journal.de
Figure 2. Number average molecular weights as measured by
SEC-RI (filled symbols) and polydispersity (open symbols) as a
function of conversion for block copolymerization of styrene
in 2CNP at 100 8C with various [Sty]0/[PAN-Br]0/[CuBr]0/
[PMDETA]0: 250/1/0.2/0.2 (*), 450/1/0.5/0.5 (~) and 450/1/1/
1 (&), which correspond to Entry 3, 5 and 6 of Table 2. Solid
and dashed lines indicate the theoretical values of molecular
weights predicted by the ratios of starting monomer and
initiator concentrations 450 and 250, respectively.
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Synthesis of PAN-b-PS Copolymers
Figure 3. SEC-RI traces in THF of the chain extension products
of styrene prepared in 2CNP at 100 8C using [Sty]0/[PAN-Br]0/
[CuBr]0/[PMDETA]0 ¼ 450/1/0.5/0.5 (PAN-b-PS of Entry 5 in
Table 2).
copolymers were calculated using Equation (2):
M n;NMR ¼ M n;PAN-Br þ ðFSty =1 FSty ÞM n;PAN-Br
MSty =MAn
ð2Þ
where FSty is the molar fraction of styrene in the copolymer.
The signal of the o-end of the macroinitiator, with a
Br atom adjacent to the proton, was clearly visible at
5.1–5.3 ppm, while in the case of the block copolymers
the presence of a less intense signal at 4.5–4.8 ppm due to
the –CH(Ph)–Br proton of the corresponding o-end confirmed that a significant percentage of active centers were
removed during the chain extension. For the copolymer of
Figure 4(b), the area of the peak corresponded to 33% of Br
end groups, and comparable results were obtained for all
of the series. On the other hand, in addition to the signals
of the PS backbone, two signals at ca. 6.1 and 6.2 ppm
characteristic of vinylic protons, tentatively in the structure
type –CH CH–Ph,[16] were distinguishable in the copolymers. These terminals may be formed through two main
processes, the elimination of HBr from the growing
chains[12] and a disproportionation reaction. The latter also
leads to equimolar amounts of saturated ends whose signals
overlap with the other CH and CH2 signals of the PS chains.
Even though polystyryl radicals from o-bromopolystyrene
in a ligand/CuBr system undergo termination essentially by
coupling and disproportionation,[16,17] from the area of the
signals relative to vinylic protons we finally deduced that in
our system elimination of HBr was the main termination
process.e
It has been reported previously that the high concentration of catalyst in the ATRP homopolymerization of
AN,[7b] and also in the case of chain extension of PAN
macroinitiator in EC,[7d] can cause a loss of chain end
functionality, essentially through radical reduction, with a
consequent increase in polydispersity. In contrast, the use of
an aprotic and polar solvent such as 2CNP for the block
polymerization of PAN with styrene promoted termination
through a process that experimentally appeared to be almost
independent of the radical concentration and reagent ratios.
For these reasons, high amounts of catalyst and therefore
higher polymerization rates were necessary to reach appropriate values of conversion and molecular weight before the
unavoidable terminations limited further chain growth in a
controlled fashion. Also, the narrower molecular weight
distributions reported in Table 2 for higher ratios of the
catalyst to initiator may be considered to be a direct consequence of this behavior.
Conclusion
The use of 2CNP as the solvent in copper-mediated ATRP
allowed the synthesis of challenging PAN-containing block
e
Figure 4. 1H NMR spectra in DMF-d7 at 80 8C of PAN-Br (a)
and PAN-b-PS (b) of Table 2, Entry 1 and 3, respectively.
Macromol. Chem. Phys. 2005, 206, 1382–1388
www.mcp-journal.de
The fraction of double bond terminated chains was in the range
33–57% for the copolymers of Entry 3 to 6 and 8 in Table 2.
Hence, taking into account: i) the amount of residual Brterminated chains; ii) the absence of coupling byproducts (by
recombination) as confirmed by SEC; iii) the extent of
disproportionation evaluated as the difference from the amount
of saturated chain ends deduced from the total number of chain
ends, the extent of HBr elimination was estimated to be between
29 and 47%. These determinations have to be considered as
approximate, as for the higher molecular weight polymers the
quantification of chain end groups loses precision and accuracy.
ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Lazzari, O. Chiantore, R. Mendichi, M. A. López-Quintela
copolymers through a pathway of general applicability.
PAN with a narrow polydispersity and high chain end
functionality could be prepared in such a solvent, and its
subsequent use as a macroinitiator for chain extension with
styrene may form a block copolymer. Even though extension to prepare PAN-b-PS using CuBr/PMDETA as the
catalyst system did not allow us to obtain block copolymers
with such a high degree of control of molecular weight and
molecular weight distribution control as in the case of more
affordable comonomer pairs (i.e., methacrylates, acrylates,
styrenes),[12] the procedure proposed in this work still
maintained the usual advantages of ATRP leading to good
block efficiency and polydispersities as low as 1.3. 1H NMR
results showed that the partial loss of control was mainly
due to the progressive termination of polystyryl propagating chains through the elimination of HBr. However, it
is possible that either optimization or further changes in
the catalyst system will permit better control over the
‘‘livingness’’ of the polymerization.f
The series of PAN-b-PS copolymers prepared so far
covers a range of compositions varying from 8.6/91.4 to
35.5/64.5. These compositions correspond to morphologies
with spherical or cylindrical nanophases.[10] Controlled
pyrolysis will allow further investigation of the transformation of the PAN component into well-defined nanostructured carbons.[9]
Acknowledgements: Funding for this work was provided by the
Ministerio de Ciencia y Tecnologı́a (MAT2002-00824: Synthesis
and properties of 1D, 2D and 3D magnetic nanomaterials).
M. L. also acknowledges financial support from the European
Union for his stay at the University of Santiago, the Ministerio de
Ciencia y Tecnologı́a for support through the program Ramon y
Cajal and the University of Torino for allowing research leave. We
also thank Dr. E. Guitián Fernández for his support with MALDI
measurements.
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An optimization of the catalyst system which is in progress has
already permitted the synthesis of PAN-b-PMMA copolymers
in 2CNP, as well as PMMAs, with very narrow polydispersities.
Macromol. Chem. Phys. 2005, 206, 1382–1388
www.mcp-journal.de
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