Construction of functional porphyrin polystyrene nano-architectures by ATRP

Article abstract of DOI:10.1039/b412067a

A series of mono-functionalized metallo-porphyrin polystyrenes have been synthesized using atom transfer radical polymerization and their self-assembling behavior was studied by electron microscopy showing that the polystyrene tail-length influences the a

Full text of DOI:10.1039/b412067a

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Construction of functional porphyrin polystyrene nano-architectures by
ATRP{
Femke de Loos, Irene C. Reynhout, Jeroen J. L. M. Cornelissen,* Alan E. Rowan* and Roeland J. M. Nolte
Received (in Cambridge, UK) 5th August 2004, Accepted 3rd November 2004
First published as an Advance Article on the web 1st December 2004
DOI: 10.1039/b412067a
equivalents of copper, polymerization did take place (Fig. 1).
Using 2 equivalents of copper bromide the polymerization
progressed in a controlled fashion resulting in polystyrene-
functionalized Cu(II)–porphyrin with low polydispersity, as was
confirmed by GPC (Table 1 and Fig. 2a) and MALDI-TOF mass
spectrometry. When 10 equivalents of copper bromide were used
GPC showed that the formed polystyrene had a much higher
molecular weight than expected. Also, a low molecular weight
peak was observed, which had a strong absorption at 420 nm
pointing to the presence of a high concentration of free porphyrin.
Probably, the large amount of copper leads to the cleavage of the
ester bond between the porphyrin and the growing polymer chain.
This alters the initiator : monomer ratio in favour of high
molecular weight polymer. Another possible explanation is that
A series of mono-functionalized metallo-porphyrin polystyr-
enes have been synthesized using atom transfer radical
polymerization and their self-assembling behavior was studied
by electron microscopy showing that the polystyrene tail-length
influences the aggregate architecture.
The construction of well-defined functional nano-objects is nowa-
days one of the main topics in supramolecular chemistry. The use
of porphyrins as components of the nano-system is of interest since
they may introduce functional properties such as energy transfer
and catalysis. To construct such porphyrin-functionalized nano-
structures we decided to prepare amphiphilic porphyrins, since
these can be expected to self-assemble into a large variety of
structures. Small molecular weight amphiphilic porphyrins have
already been shown to assemble into interesting architectures.1 The
need to increase the stability of the aggregates prompted us to
develop polystyrene-based porphyrin amphiphiles. Polymeric
amphiphiles, for example block copolymers, are known to
assemble into various (kinetically) stable structures.2 Our goal is
to control the aggregate morphology by adjusting the ratio
between the polar and apolar part of the amphiphile and thereby
influence the functionality of the aggregated system. To this end,
we have utilized atom transfer radical polymerization (ATRP) to
construct porphyrin-functionalized polystyrenes. ATRP is a rather
well established polymerization process, allowing good control
over the degree of polymerization and chain length distribution.3,4
Recently, Beil and Zimmerman have used ATRP in combination
with a porphyrin complex to perform polymer imprinting.5
We prepared a series of ATRP initiators, all based on a
mono initiator-functionalized tritolylporphyrin, which differ in
porphyrin inner-core functionalization. Scheme 1 depicts the
synthesis of the free-base porphyrin initiator 2a, the copper
porphyrin initiator 2b, the manganese porphyrin initiator 2c
and the zinc porphyrin initiator 2d. With these initiators we
performed bulk-polymerization reactions of styrene under ATRP
conditions (Scheme 1).{
Scheme 1 Synthesis of porphyrin-functionalized polystyrene.
Polymerization of styrene with free-base initiator 2a did not take
place with one equivalent of copper, due to preferential insertion of
copper(II) into the porphyrin core. UV-vis spectroscopy showed
that this insertion was already complete after the first 15 min of the
reaction.6 As a result, no Cu(II) was left to be converted into Cu(I)
to enter the catalytic cycle. However, both with two and ten
{ Electronic supplementary information (ESI) available: ATRP procedure
and characterization data. See http://www.rsc.org/suppdata/cc/b4/
b412067a/
*j.cornelissen@science.ru.nl (Jeroen J. L. M. Cornelissen)
a.rowan@science.ru.nl (Alan E. Rowan)
Fig. 1 First order log plots of the conversion of styrene as a function of
time for the polymerization of styrene with porphyrin initiators 2a–2d.
60 | Chem. Commun., 2005, 60–62
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Table 1 Polymerization conditions and characterisation of polystyrene compounds
Initiator
Polystyrene product
Polymerisation conditions
PDIa
Mn/kg mol21a
Mw/kg mol21a
2a
2a
2b
2c
2d
a
3a (2 equiv.)
3a (10 equiv.)
3b
3c
3d
b
2 equiv. CuBr, 2 equiv. PMDETAb
10 equiv. CuBr, 20 equiv. PMDETAb
1 equiv. CuBr, 2 equiv. PMDETAb
1 equiv. CuBr, 2 equiv. PMDETAb
1 equiv. CuBr, 2 equiv. PMDETAb
1.39
1.28
1.22
1.57
1.14
5.8
16.0
4.2
3.9
3.6
8.0
20.0
5.2
6.2
4.2
Measured by GPC. Pentamethyl diethylene triamine.
Fig. 2 (A) GPC-traces of Cu–porphyrin polystyrene measured at two
different wavelengths, 254 nm (polystyrene), 420 nm (Cu–porphyrin). (B)
GPC-traces (254 nm) of Mn–porphyrin polystyrene fractions collected
after size exclusion column.
the increase in Cu(I) concentration results in an increased number
of radicals at the start of the polymerisation and hence to an
increase in termination reactions. When Cu–porphyrin 2b was
applied as the initiator, 1 equivalent of CuBr proved to be
sufficient to perform a controlled polymerization, as expected.
In order to construct amphiphilic polystyrenes with catalytic
properties polymer 3c was prepared from initiator 2c having a
positively charged manganese headgroup. The Mn(III)–porphyrin–
polystyrene 3c showed a relatively high polydispersity of 1.5.
Only a limited number of metals inside the porphyrin catalyst
can be used, since these metals may interact with the copper
catalyst. The use of free-base initiator followed by insertion of the
metal after polymerization is not an option due to insertion of
copper during the polymerization of styrene. In addition, the
copper is difficult to remove from the porphyrin core and will
result in decomposition of the macromolecule. We decided
therefore to synthesize Zn–porphyrin–polystyrene 3d. Zinc is easily
removed from the porphyrin in this compound under mild acidic
conditions. This will give the possibility to insert any metal after
the polymerization reaction. The polymerization of styrene from
the Zn–porphyrin initiator 2d was performed in a similar
controlled fashion yielding a Zn–porphyrin–polystyrene with low
polydispersity (Table 1).
Fig. 3 (A) and (B) Transmission electron micrographs of the aggregation
behaviour of amphiphilic Cu–porphyrin–polystyrene (A) and Mn–
porphyrin–polystyrene Mn 5 9973 (B). (C) and (D) Scanning electron
micrographs of the aggregation behaviour of Mn–porphyrin–polystyrene
with Mn 5 1495.
was fractionated in different samples (f1–f5) with Mn’s ranging
from 1495 to 9973 and polydispersities ranging from 1.09 to
1.27 (Fig. 2b). With two of these fractions, f1 (Mn 5 9973,
PDI 5 1.24) and f5 (Mn 5 1495, PDI 5 1.11), aggregation studies
were performed. For the more hydrophilic Mn(III)–porphyrin–
polystyrene, sonication was not required to obtain well defined
architectures. The polymer was dissolved in THF and water was
slowly added until the solution turned cloudy. TEM studies
showed the formation of small spherical micellar aggregates for
f1 (Fig. 3b). The Mn(III)–porphyrin–polystyrene with the shortest
apolar tail (f5), showed the formation of large spherical
architectures 0.5–2 mm in size, which are vesicular-like in nature.
SEM studies showed that these structures are porous, suggesting
they are hollow spheres (Fig. 3c and 3d). In the case of f5 the
average length of the apolar tail is comparable to the diameter of
the porphyrin head (estimated from calculations). However, for f1,
the length of the polystyrene tail is ten times larger. The difference
in the ratio head : tail results in a different aggregation behavior.
The structures proved to be stable over time for at least one
month.
For the free-base porphyrin initiator with 2 equivalents of CuBr
as well as for all three metallo-porphyrin initiators the conversion
of styrene in time showed first-order kinetics (Fig. 1). The rate
constants varied from 5.1 6 1026 s21 to 8.9 6 1026 s21, depend-
ing on the initiator that was used.
The aggregation behavior of the Cu–porphyrin–polystyrene
prepared with 2 equivalents of CuBr (3a) was studied in water.
When the porphyrin-functionalized polystyrene was injected from
a THF-solution into water while sonicating at 60 uC, highly
monodisperse (DI 5 1.02) micellar structures, 20–35 nm in size,
were formed (Fig. 3a). In subsequent experiments, the aggregation
behavior of the Mn–porphyrin–polystyrene was investigated. 3c
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achieved by refluxing this compound in a mixture of methanol–chloroform
In summary, it is shown that the polymerization of styrene
by ATRP using a series of metallo-porphyrin initiators proceeds
(1 : 1, v : v) with the corresponding metal(II) bromide. The conversion of
styrene during the polymerization was followed by GC. Anisole was used
as a standard.
in
a
controlled fashion and yields well-functionalized
metallo-porphyrin polystyrenes with low polydispersities. The
Cu–porphyrin–polystyrene (3a) formed well defined highly mono-
dispersed spheres in water. Aggregation studies of the Mn–
porphyrin–polystyrene (3c) showed that the morphology of the
architectures formed in water was controlled by the polystyrene
tail length. Currently we are investigating the catalytic properties of
the different Mn(III)–porphyrin–polystyrene architectures.
The Council for the Chemical Sciences of the Netherlands
Organization for Scientific Research is acknowledged for financial
support to R. J. M. Nolte (TOP grant), J. J. L. M. Cornelissen
(Veni grant) and A. E. Rowan (Vidi grant).
1 A. P. H. J. Schenning, M. C. Feiters and R. J. M. Nolte, Tetrahedron
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Chem. Soc., 1993, 115, 11036.
2 (a) N. S. Cameron, M. K. Corbierre and A. Eisenberg, Can. J. Chem.,
1999, 77, 1311; (b) M. Antonietti and S. Fo¨rster, Adv. Mater., 2003, 15,
1323.
3 (a) V. Coesens, T. Pintauer and K. Matyjaszewski, Prog. Polym. Sci.,
2001, 26, 337; (b) K. Matyjaszewski and J. Xia, Chem. Rev., 2001, 101,
2921.
4 (a) G. Deng and Y. Chen, Macromolecules, 2004, 37, 18; (b) R. Narain
and S. P. Armes, Macromolecules, 2003, 36, 4675; (c) D. M. Haddleton
and K. Ohno, Biomacromolecules, 2000, 1, 152; (d) L. Ayres,
M. R. J. Vos, P. J. H. M. Adams, I. O. Shklyarevskiy and J. C. M. van
Hest, Macromolecules, 2003, 36, 5967.
5 J. B. Beil and S. C. Zimmerman, Macromolecules, 2004, 37, 778. It
should be noted that in the work of Beil and Zimmerman, 10 equiv. of
Cu(I)Br are used for 8 initiator sites. Even if one copper is inserted into
the porphyrin (a possibility not studied in the paper), there would still be
an excess of copper per initiator (1.1 equiv.) and hence catalysis occurs.
In addition, the basicity of the ligand may also play a role in the
efficiency of copper insertion, when a stronger ligand is used the copper
may not be extracted by the porphyrin.
6 The radical formed upon homolytic cleavage of the carbon–bromine
bond must be immediately trapped since no conversion of styrene is
observed. From entry 3, Table 1 it appears that a copper–porphyrin
does not act as the radical trap; its precise nature is still under
investigation.
Femke de Loos, Irene C. Reynhout, Jeroen J. L. M. Cornelissen,*
Alan E. Rowan* and Roeland J. M. Nolte
Department of Organic Chemistry, Institute for Molecules and
Materials (IMM), Radboud University Nijmegen, Toernooiveld 1, 6525
ED Nijmegen, The Netherlands. E-mail: j.cornelissen@science.ru.nl;
a.rowan@science.ru.nl; Fax: (+31) 24 365 2929; Tel: (+31) 24 365 2323
Notes and references
{ Mono hydroxy-functionalized tritolylporphyrin 1 was functionalized
with the initiator group by stirring it for 30 min in dry dichloromethane
with a small excess of 2-bromoisobutyryl bromide in the presence of an
equimolar amount of triethylamine. Insertion of the desired metal was
62 | Chem. Commun., 2005, 60–62
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