Surface-initiated Kumada catalyst-transfer polycondensation of poly(9,9-dioctylfluorene) from organosilica particles: Chain-confinement promoted β-phase formation

Article abstract of DOI:10.1039/b920214e

Surface-initiated Kumada catalyst-transfer polycondensation of poly(9,9-dioctylfluorene) (PFO) was performed from organosilica microparticles that resulted in PFO brushes with densely grafted PFO chains with a significantly enhanced propensity to adopt planar and ordered conformations (β-phase). The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b920214e

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Surface-initiated Kumada catalyst-transfer polycondensation
of poly(9,9-dioctylfluorene) from organosilica particles:
chain-confinement promoted b-phase formationw
Roman Tkachov, Volodymyr Senkovskyy,* Marta Horecha,
Ulrich Oertel, Manfred Stamm and Anton Kiriy*
Received (in Cambridge, UK) 29th September 2009, Accepted 8th December 2009
First published as an Advance Article on the web 23rd December 2009
DOI: 10.1039/b920214e
Surface-initiated Kumada catalyst-transfer polycondensation of
poly(9,9-dioctylfluorene) (PFO) was performed from organosilica
microparticles that resulted in PFO brushes with densely grafted
PFO chains with a significantly enhanced propensity to adopt
planar and ordered conformations (b-phase).
and block copolymers.8–11 Recently, we have developed
a method for the initiation of KCTP by externally-added
Ar–(PPh3)2–X initiators, synthesized from Ni(PPh3)4 and aryl
halides (Ar–X), and prepared poly(3-alkylthiophenes) brushes.10a
The first generation of externally-added initiators was based
on relatively poor performing monodentate PPh3 ligands.
Very recently, we have found an efficient route to a library
of Ar–Ni(L2)–X initiators supported by bidentate phosphorous
Molecular organization of conjugated polymers is critical to
the control and tuning of their optoelectrical properties.
Poly(9,9-dioctylfluorene) (PFO), an important blue light-emitting
polymer,1 forms two distinct conformations in solid state and
solutions, an amorphous a-phase and a more planar b-phase,
and optoelectronic properties of PFO depend upon the phases
content.2,3 The b-phase frequently appears as a minority
constituent in the photoabsorption but dominates the optical
emission and provides for better PFO-based light emitting
diodes color stability.4 For many optoelectronic applications
of PFO it is necessary to spatially control the formation of
the a- and b-phases, pattern them,5 or create structurally
well-defined PFO micro- and nanoparticles.6 Self-assembly
(spontaneous organization at certain processing conditions)
is a simple and cost-efficient approach for the organization
of various conjugated polymers, however, a desired type of
arrangement is not always achievable. Another problem is that
self-assembled structures are usually not robust enough under
the conditions used and therefore a covalent immobilization of
PFO chains would be helpful for many applications. Surface-
initiated polymerization (SIP) is an alternative approach for
the directed assembly of polymers at surfaces into stable
structures, called polymer brushes.7 At sufficiently high grafting
densities, conformations and orientations of end-tethered
polymer chains are restricted and their properties are altered
compared to their untethered counterparts. Examples of SIP
of conjugated polymers remain scarce in literature because
synthesis of most of the conjugated polymers involves
step-growth polycondensations, not compatible with the
SIP technique.
ligands (L2 = dppe (Ph2P(CH2)2PPh2), or the like).11
A
two-step transformation involves reaction of Ar–X with
diethylbipyridylnickel, Et2Ni(bipy), followed by addition of
dppe to replace bipy. An attractive feature of this approach is
that the reaction proceeds cleanly even with para-substituted
Ar–X, in contrast to Ni(PPh3)4-based schemes which require
incorporation of substituents into ortho-positions of Ar–X for
stabilization of intermediate complexes.10b
Efficient engineering of materials with tailored optoelectronic
properties requires involvement of various types of monomers
into KCTP, including monomers containing several aromatic
rings. A scope of the KCTP that involves an intriguing ring-
walking mechanism was recently extended onto polymeriza-
tion of several lengthy monomers.12 Particularly, it was found
that KCTP of 2-bromo-7-chloromagnesio-9,9-dioctylfluorene
(1) into PFO involved the chain-growth mechanism, however
the reaction was not controlled due to chain-terminations.12b
It was therefore not clear whether this process would be
suitable for the engineering of complex architectures of
PFO. In this communication we report on the surface-initiated
KCTP of 1 from functionalized organosilica microparticles
and describe optical properties of the resulting spherical PFO
brushes.
Monodisperse, B880 nm in diameter, silica particles
having B10 nm thick organosilica shells were prepared by a
modified Stober method13 via sequential sol–gel hydrolysis of
¨
tetramethyl orthosilicates (TMOS) and [2-(4-bromo-phenyl)-
ethyl]-triethoxy-silane. The latter silane provides a useful
functionality on the surface of the particles, required for
catalyst immobilization. Initiating sites were developed by
the treatment of the particles consecutively by Et2Ni(bipy)
and dppe (Scheme 1). The dppe supporting ligand was chosen
because dppe-based complexes exhibit a good solubility that is
important to prevent aggregation of the particles at the
catalyst-immobilization step. Prior to the SI-KCTP, the
particles were carefully purified from physiosorbed Ni
compounds by several centrifugation/redispersion cycles using
anhydrous and degassed THF in an argon atmosphere.
Fortunately, chain-growth polycondensations, such as
Ni-catalyzed Kumada catalyst-transfer polycondensation
(KCTP), were also discovered and they became a powerful
tool in the preparation of well-defined conjugated polymers
Leibniz-Institut fur Polymerforschung Dresden e.V., Hohe Straße 6,
¨
01069 Dresden, Germany. E-mail: kiriy@ipfdd.de,
senkovskyy@ipfdd.de
w Electronic supplementary information (ESI) available: Experimental
data. See DOI: 10.1039/b920214e
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To perform SI-KCTP, monomer 1 was added to the dispersion
of the activated particles and the polymerizations were
performed at room temperature (RT) for 40 min. Afterwards,
the resulting composite particles were carefully separated from
any ungrafted polymers, catalyst and byproducts. According
to TGA, the resulting particles having PFO shells (further
designated as m-PFO particles) lost B10% of their weight in
the temperature interval from 450 to 600 1C in an oxygen
atmosphere which was typical decomposition behaviour of
PFO. For analytical purposes, grafted PFO chains were
detached from the particles by treatment with aqueous hydro-
fluoric acid, HF. The mass fraction of the released polymer
(further designated as ‘‘degrafted PFO’’) relative to the mass of
the sample before the HF-treatment was B9% which was in
good agreement with the TGA data. For the given particle size
and densities of the components, the PFO content in the
m-PFO particles corresponded to the thickness of the PFO shell,
h, of B25 nm. A control experiment was performed that revealed
that immobilization of the Ni initiator, as described above, was
essential for the successful grafting of PFO from/to the particles.
Since we aimed to maximize the amount of the grafted PFO,
the monomer was used in a large excess relative to the surface-
bound initiator. Such conditions also favour the formation of
unbound PFO because the catalytic species dissociating from
the growing surface-bound chains upon the chain termination
can initiate the polymerization of the excess monomer in the
bulk solution. A quantitative analysis of the reaction mixture
obtained after the separation of the particles revealed the
monomer conversion of about 30%. Unbound PFO with
Mn = 31 000 g molÀ1 and PDI = 2.3 was also formed in
the grafting experiment with the ratio between the bound and
unbound PFO of 1 to 4. Despite the relatively low selectivity of
the grafting process (i.e., 20%), this method can be used in
preparative purposes in cases when separation from unbound
polymers is possible. On the other hand, there is a significant
potential for improvement of the grafting selectivity via
optimization of initiator-to-monomer ratio and of the
polymerization time.
from the silica core. It is also noteworthy that GPC utilized
polystyrene calibration standards which significantly overestimate
the real molecular weight of rod-like polymers, such as PFO;
an overestimation factor of 2.7 was previously found for this
polymer.2 Hence, the real Mn and Mw can be estimated to be at
the level of B17 000 g molÀ1 and B68 000 g molÀ1, respectively,
that corresponds to quite realistic mean-average and weight-
average polymerization degrees of 45 and 176, respectively.
From these data, a grafting density, s, determined as the
number of grafted chains per surface unit, and a reduced
tethered density, S, showing to what extent the conformation
of the tethered polymer deviates from its unperturbed
conformation in solution, can be estimated.14 The calculated
values of s = 0.89 chains nmÀ2 and S = 280 correspond to
the brush regime of very densely grafted and strongly stretched
chains. As a result of the high grafting density, the hairy PFO
particles display much higher colloidal stability than the
parent unmodified particles. The m-PFO particles can be
arranged on surfaces by a dip coating at a controlled solvent
evaporation to form quasi-periodic hexagonal arrays. As
evident from a fluorescent micrograph, the m-PFO particles
display a strong emission in a dry state (Fig. 1) that can be
exploited in optoelectronic devices.
Photophysical properties of the m-PFO particles dispersed in
THF were compared with those for PFO degrafted from the
m-PFO particles. The relatively red-shifted position of lmax at
392 nm for the degrafted PFO dissolved in THF without
heating is consistent with the rather high molecular weight
of PFO (Fig. 2a, black line). The sample also exhibits a
substantial absorption at 439 nm reflecting a small fraction
of the planar conformation of PFO (b-phase).2,3,15,16 Emission
spectrum of the degrafted PFO at 382 nm excitation wave-
length shows substantial contributions from both the amorphous
a-phase (417 nm) and from the b-phase (441, 465, 500 nm,
Fig. 2b, black line). Upon heating above 50 1C the signatures
of the b-phase in the absorption and emission spectra
disappear and they do not recover upon cooling to RT; their
recovery requires cooling below À20 1C (Fig. S6, ESIw), in
accordance with earlier reported data.3,15,16 The thermochromic
behaviour of the m-PFO particles follows similar trends,
however, the tethered PFO chains demonstrate a much higher
propensity to the formation of the b-phase than the degrafted
PFO and unbound PFO (Fig. S2 and S7w). Particularly,
the absorption spectrum of the m-PFO particles dispersed in
THF and CH2Cl2 at RT exhibits a strong contribution from
the b-phase manifesting itself not only by the signal at 439 nm,
The detached PFO has a broad GPC trace with Mn
=
48 000 g molÀ1 and Mw = 180 000 g molÀ1. The observed
polydispersity of the grafted PFO is much higher than the
polydispersity of PFO obtained in a model KCTP of 1
initiated by Ph-Ni(dppe)-Br (PDI B1.65). This result may
reflect a decreased performance of the SI-KCTP compared to
the bulk KCTP due to the heterogeneous character of the
former process. Alternatively, the broad distribution may be
explained by an incomplete detachment of PFO chains
Scheme 1 Grafting of PFO from microparticles via SI-KCTP.
1426 | Chem. Commun., 2010, 46, 1425–1427
Fig. 1 Fluorescent micrograph for the m-PFO particles array.
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and ordered conformations (b-phase). Our method should be
extendable to the preparation of various micro- and nanosized
objects having conjugated polymer shells with precisely
preorganized chains and thus it would become a powerful
tool in the engineering of new materials for optoelectronic
application.
We thank DFG for financial support (KI-1094/3-1).
Notes and references
Fig. 2 UV-vis (a) and emission spectra (b) of the m-PFO particles
(red lines) and PFO detached from the particles (black line) dissolved
in THF and recorded at room temperature.
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403 nm (Fig. 2a, red line). The emission spectrum of the
m-PFO particles recorded at 382 nm excitation wavelength is
well-resolved and the peak from the a-phase at 417 nm is
strongly suppressed (Fig. 2b, red line). Instead, the one at
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but induces only a weak blue-shift of the absorption maximum
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In summary, surface-initiated polymerization of poly-
(9,9-dioctylfluorene) from microparticles was performed. The
grafting process is based on Kumada catalyst-transfer
polycondensation and leads to PFO brushes of a high grafting
density. The high grafting density is a reason for the unusually
strong propensity of the tethered PFO chains to adopt planar
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1425–1427 | 1427