Wavelength selective polymer network formation of end-functional star polymers

Article abstract of DOI:10.1039/c5cc09444e

A wavelength selective technique for light-induced network formation based on two photo-active moieties, namely ortho-methylbenzaldehyde and tetrazole is introduced. The network forming species are photo-reactive star polymers generated via reversible activation fragmentation chain transfer (RAFT) polymerization, allowing the network to be based on almost any vinylic monomer. Direct laser writing (DLW) allows to form any complex three-dimensional structure based on the photo-reactive star polymers.

Full text of DOI:10.1039/c5cc09444e

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Hiltebrandt, V. Trouillet, P. Mueller, A. S. Quick, M. Wegener and C. Barner-Kowollik, Chem. Commun.,
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DOI: 10.1039/C5CC09444E
ChemComm
COMMUNICATION
Wavelength Selective Polymer Network Formation of End-
Functional Star-Polymers
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
c
a, c
Michael Kaupp, Kai Hiltebrandt, Vanessa Trouillet, Patrick Mueller, Alexander S. Quick,
c
a
Martin Wegener and Christopher Barner-Kowollik*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A wavelength selective technique for light-induced network
formation based on two photo-active moieties, namely ortho-
methylbenzaldehyde and tetrazole is introduced. The network
forming species are photo-reactive star polymers generated via
reversible activation fragmentation chain transfer (RAFT)
polymerization, allowing the network to be based on almost any
vinylic monomer. Direct laser writing (DLW) allows to form any
complex three-dimensional structure based on the photo-reactive
star-polymers.
1
Polymer networks find widespread applications in areas such
as biomedicine, drug delivery, separation processes, hydrogels,
and gas storage.2 Light induced reactions benefit from
environmentally friendly conditions, spatial and temporal
control as well as overcoming large activation barriers in
3
relatively short periods of time. Photo-chemical processes
have already been shown to be a highly efficient tool in
general4 and in particular for the formation of complex Figure 1 Schematic overview of the employed combination of wavelength selective
macromolecular structures such as block copolymers5 or photo-induced reactions with RAFT-polymerization for network formation. DLW allows
for the formation of close to arbitrary three-dimensional structures.
6
telechelic precursors for gels. The facile spatial control over
(DLW). DLW is a highly versatile lithographic technique
the reaction enables the synthesis of structured materials for
example by employing photo-lithographic processes.4c, 7
employing two-photon absorption and allows for the
fabrication of micrometer scale structures with a tightly
In principle, networks are formed via common polymerization
techniques leading to crosslinking of multifunctional monomer
units.8 The current contribution describes the first light-
induced network formation which is selective towards the
employed wavelength and can further be employed to
fabricate three-dimensional structures via direct laser writing
9
focused laser. Figure 1 shows a schematic overview of the
synthetic steps leading to a macroscopic and microscopic
network formation. To the best of the authors’ knowledge, the
current work also entails the first description of photo-reactive
reversible activation fragmentation chain transfer (RAFT) star
polymers. Kuriara and coworkers described RAFT star polymers
able to photo-isomerize based on azo-benzene groups.10 Other
reports describe photo-isomerizing (azo-benzene groups)1
and photo-degradable (nitrobenzene groups) star polymers
that were synthesized via atom transfer radical polymerization
(ATRP) as well as photo-linkable star polymers (due to
1
a.Preparative Macromolecular Chemistry, Institut für Technische Chemie und
12
Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstaße 18, 76128
Karlsruhe, Germany and Institut für Biologische Grenzflächen, Karlsruhe Institute
of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany.
E-mail: christopher.barner-kowollik@kit.edu
Institute for Applied Materials (IAM) and Karlsruhe Nano Micro Facility (KMNF),
Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany.
13
anthracene groups) fabricated via anionic polymerization.
b.
In the current publication the wavelength required for the
network formation is only dependent on the photo-active
species attached to the RAFT-agent and can thereby be
c.
Institut für Angewandte Physik, Institut für Nanotechnologie, Karlsruhe Institute
of Technology (KIT), Wolfgang-Gaede-Straße 1, 76131 Karlsruhe, Germany.
Electronic Supplementary Information (ESI) available: Comprehensive matched to other light sensitive groups in the system or the
experimental and analytical information. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2015
Chem. Commun., 2015, 00, 1-3 | 1
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DOI: 10.1039/C5CC09444E
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Core
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Figure 2 Structure of the 4-arm RAFT agent TetraPE-RAFT and formation of the ortho-
Core
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quinodimethane structure upon irradiation.
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envisaged application, for example in biological systems,
where certain wavelengths may be harmful. RAFT has been
employed numerous times for the formation of star
polymers with up to twelve arms15 and there also exist
several RAFT agents containing photo-active groups, such as
the here employed ortho-methylbenzaldehyde (photo-enol)16
S
N
O
N
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and tetrazole (nitrile imine mediated tetrazole ene coupling; Figure 3 Structure of the 4-arm RAFT agent TetraTet-RAFT and formation of the nitrile
1
7
18
imine upon irradiation.
NITEC) as well as phenacyl sulfide moiety. Previous studies
revealed that RAFT polymers can be utilized under DLW
high end-group functionalization (see Figure S 13 to S 16 in the
ESI).
As a suitable counterpart for both photo-functional groups
maleimide derivatives were selected, based on their high
reactivity towards ortho-quinodimethane and nitrile imine
intermediates, enabling a rapid network formation. For the
18
conditions. The previously published photo-active RAFT
agents have two properties in common: First, they are based
on trithiocarbonates which are known to be stable under
irradiation with UV-light; second, the photo-active group is
tethered to the so-called R-group of the RAFT agent.
The two herein employed photo-active RAFT agents share the
aforementioned properties. Thereby, the star conformation
does not undergo cleavage reactions while being polymerized
19
macroscopic network formation
a
small trifunctional
maleimide was employed (refer to the ESI). In the DLW
process, a polymer containing maleimide side groups was
used, as it leads to crosslinking at lower conversions and
20
(
Z-group approach), and the photo-active groups remains on
the more accessible periphery of the stars. The design of the
RAFT agents is based on four symmetric arms attached to a
pentaerythritol center (see Figure 2 and Figure 3).
9e
therefore allows for a faster writing speed.
Network formation between the functional star polymers and
the small trifunctional maleimide is facile and non-demanding.
The two components are dissolved in a stoichiometric ratio
with regard to the reactive end-group concentrations in a
small volume of solvent (refer to Table S 1 in the ESI). For the
macroscopic networks dichloromethane (DCM) was employed,
yet the only demand the solvent has to fulfil is to dissolve the
components and not absorb in the employed UV-light
The syntheses of the 4-arm RAFT agents as well as the
conditions for the RAFT polymerization are described in the
ESI. As a proof-of-concept, two styrene derivatives containing
different heteroatoms were selected (4-chloromethyl-styrene
and 4-bromostyrene), so they can readily be differentiated in
X-ray photoelectron spectroscopy (XPS) measurements. In
principal, a large range of monomers can be employed.
Trithiocarbonate-based RAFT agents are able to mediate the
(implying that the network formation is independent of the
polymerization of different monomer classes such as acrylate, solvent). Only in the case of the photo-enol functional star the
14b
acrylamide and styrene derivatives. As the main part of the solution has to be subsequently freed from oxygen via purging
network consists of the polymeric chains, the chemical with nitrogen to ensure a rapid reaction. Network formation
(functionality, polarity…) and physical (toughness, rigidity…) proceeds via irradiation with a suitable light source, in the
properties of the network can be easily adjusted via the choice
of monomer and chain length. Since polymerization and
network formation are independent, the free choice of
monomer and arm length leads to further versatility of the
presented system.
The conversion was kept relatively low (DP around 6 per arm,
approximately 30 % conversion) to avoid side reactions such as
arm-arm coupling and loss of end-group functionality. Low
dispersities, lack of multimodalities in the gel permeation
chromatogram (GPC) traces as well as the signals in the NMR
spectra indicate an excellent control over polymerization with
current case simple low pressure compact lamps and LEDs,
until solidification can be observed (2.5 h - 23.5 h; see Figure 4
and Figure 5 top). Subsequently, the network is freed from
unreacted starting material via washing several times with
DCM. Since the tetrazole moiety is profluorescent, a
fluorescent network (see Figure S 22) is formed from non-
fluorescent starting material, giving a simple indication of a
successful reaction.
The formed networks were analyzed via XPS in order to gain
specific chemical information and to detect bromine or
2
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Figure 6 NMR (in CDCl ) and GPC comparison of the maleimide trilinker, the 4-arm
tetrazole RAFT star polymer and the reaction mixture irradiated for 5 h with the LEDs at
75 nm. No reaction occurs.
3
trace at higher retention times can be attributed to the
addition of the small maleimide-trilinker prior to the
irradiation. The selectivity towards a specific wavelength is
very useful for systems or applications where only a small
wavelength range can be employed due to other photo-
sensible groups or possible side reactions.
To evidence the versatility of the photo-functional RAFT stars,
they further were employed in DLW. The many possibilities of
DLW are demonstrated by writing three-dimensional logos of
the Karlsruhe Institute of Technology (KIT) with dimensions of
Figure 4 Network formation employing the photo-enol functional RAFT star polymer.
Top: Via a fluorescent lamp (λmax = 355 nm). Bottom: Via LEDs (λmax = 375 nm).
chlorine signals from the polymer corresponding to one
network species, respectively. The results can be found in the
ESI (Figure S 24 to S 31) and can be summarized as follows: The
UV-induced network formation does not change the chemical
constitution of the precursor materials, as they are
incorporated into the network. Only in the case of the
tetrazole based network one side reaction can be identified,
namely the oxidation of the trithiocarbonate, which does not
alter the integrity of the network. The oxidation leads to a new
peak at around 168.6 eV binding energy, which has been found
in XP spectra of RAFT polymers before.21
4
0×15×5 µm. The structures were written at a fundamental
center wavelength of 640 nm in order to excite the molecules
via two-photon absorption. Scanning electron microscopy
(SEM) images of both structures can be found in Figure 7. For
both formulations, the fabricated structures are well-defined
After establishing the network forming ability of the individual
networks it is important to show the wavelength selectivity.
Therefore, the behaviour of the tetrazole-functional star
(mixed with the maleimide linker) when irradiated with the
wavelength corresponding to the photo-enol activation (LEDs
at 375 nm) is investigated. Hereby, the network formation as
well as the photo-triggered NITEC reaction is suppressed,
evidenced by unaltered GPC traces and NMR spectra (see
Figure 6). Thus, the formation of photo-enol and tetrazole
networks is wavelength selective. The difference in the GPC
Figure 7 Top: SEM image of three-dimensional structure generated via DLW based on
photo-enol functional star polymers; Middle: Structure stemming from tetrazole
functional star polymers. The spherical impurities most are caused by so called
microexplosions. Bottom: Fluorescence of the three-dimensional structure associated
with the tetrazole functional star polymers irradiated at 405 nm. Bottom right:
Emission spectrum of the fluorescence. Scale bars: 2 µm.
Figure 5 Top: Network formation employing the tetrazole functional RAFT star polymer
(
(λmax = 320 nm).
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and only show moderate post-polymerization shrinkage. The
lamellar edges are an artefact from the discretization of the
structure. The structure stemming from photo-enol functional
star polymers (Figure 7 top) is very clean, whereas the
structure employing tetrazole functional star polymers
Goldmann, M. Bastmeyer and C. Barner-KowollikV,ieAwdAvr.ticMleaOtnelirn.e,
013, 25, 6123; (b) T. Pauloehrl, A. WellDe,OKI:.1K0..1O03e9h/lCe5nCscCh0l9a4e4g4eEr
and C. Barner-Kowollik, Chem. Sci., 2013, 4, 3503.
2
8
9
(a) J. E. Elliott and C. N. Bowman, Macromolecules, 2001, 34,
4
642; (b) M.-A. Tehfe, F. Dumur, N. Vilà, B. Graff, C. R. Mayer, J.
P. Fouassier, D. Gigmes and J. Lalevée, Macromol. Rapid
Commun., 2013, 34, 1104.
(Figure 7 middle) shows some spherical impurities most
probably stemming from microexplosions or contaminations.
Similar to the macroscopic networks, the DLW structure
prepared via NITEC also displays fluorescence, which can be
seen in Figure 7 bottom. The fluorescence can only be
observed for the three-dimensional NITEC structure indicating
that the impurities observed in SEM are neither stemming
from structure formation nor structure degradation.
(a) C. N. LaFratta, J. T. Fourkas, T. Baldacchini and R. A. Farrer,
Angew. Chem. Int. Ed., 2007, 46, 6238; (b) S. Maruo and J. T.
Fourkas, Laser & Photon. Rev., 2008, 2, 100; (c) T. G. Leong, A.
M. Zarafshar and D. H. Gracias, Small, 2010, 6, 792; (d) M.
Malinauskas, M. Farsari, A. Piskarskas and S. Juodkazis, Phys.
Rep., 2013, 533, 1; (e) A. S. Quick, H. Rothfuss, A. Welle, B.
Richter, J. Fischer, M. Wegener and C. Barner-Kowollik, Adv.
Funct. Mater., 2014, 3571.
1
1
1
1
0 M. Z. Alam, A. Shibahara, T. Ogata and S. Kurihara, Polymer,
2011, 52, 3696.
1 E. Blasco, B. V. K. J. Schmidt, C. Barner-Kowollik, M. Piñol and L.
Oriol, Macromolecules, 2014, 47, 3693.
2 J. A. Johnson, J. M. Baskin, C. R. Bertozzi, J. T. Koberstein and N.
J. Turro, Chem. Commun., 2008, 3064.
In summary, a novel concept for the light-induced network
formation is described as a platform technology. Key feature is
the wavelength selective network formation, as it is based on
two different photo-active moieties, i.e. photo-enol and
tetrazole functionalities which are activated at different
wavelength. The latter group allows for the synthesis
3 C. Mengel, W. H. Meyer and G. Wegner, Macromol. Chem.
Phys., 2001, 202, 1138.
offluorescent networks. The polymeric precursors are 14 (a) C. Barner-Kowollik, T. P. Davis and M. H. Stenzel, Aust. J.
Chem., 2006, 59, 719; (b) G. Moad, E. Rizzardo and S. H. Thang,
Aust. J. Chem., 2012, 65, 985.
5 X. Hao, C. Nilsson, M. Jesberger, M. H. Stenzel, E. Malmström, T.
P. Davis, E. Östmark and C. Barner-Kowollik, J. Polym. Sci., Part
A: Polym. Chem., 2004, 42, 5877.
6 M. Kaupp, T. Tischer, A. F. Hirschbiel, A. P. Vogt, U. Geckle, V.
Trouillet, T. Hofe, M. H. Stenzel and C. Barner-Kowollik,
Macromolecules, 2013, 46, 6858.
7 C. J. Dürr, P. Lederhose, L. Hlalele, D. Abt, A. Kaiser, S. Brandau
and C. Barner-Kowollik, Macromolecules, 2013, 46, 5915.
8 M. Kaupp, A. S. Quick, C. Rodriguez-Emmenegger, A. Welle, V.
Trouillet, O. Pop-Georgievski, M. Wegener and C. Barner-
Kowollik, Adv. Funct. Mater., 2014, 24, 5649.
produced with the versatile RAFT polymerization process,
which allows the network to be based on almost any material
from vinylic monomer, and is hence an important lever for
adjusting the material properties of the respective networks.
The versatility of the entire concept is further demonstrated
via the combination with DLW, which allows for the network
to be fabricated into any desired three-dimensional structure
with micrometer scale resolution.
1
1
1
1
Acknowledgements
1
2
2
9 T. Gruendling, M. Kaupp, J. P. Blinco and C. Barner-Kowollik,
Macromolecules, 2011, 44, 166.
0 H. Chaffey-Millar, M. H. Stenzel, T. P. Davis, M. L. Coote and C.
Barner-Kowollik, Macromolecules, 2006, 39, 6406.
1 A. S. Goldmann, T. Tischer, L. Barner, M. Bruns and C. Barner-
Kowollik, Biomacromolecules, 2011, 12, 1137.
C. B.-K. acknowledges support from the Karlsruhe Institute of
Technology (KIT) via the STN program of the Helmholtz
association.
References
1
2
E. S. Dragan, Chem. Eng. J., 2014, 243, 572.
D. Yuan, W. Lu, D. Zhao and H.-C. Zhou, Adv. Mater., 2011, 23,
3723.
3
4
(a) T. Wu, H. Tang, C. Bohne and N. R. Branda, Angew. Chem.
Int. Ed., 2012, 51, 2741; (b) T. Muraoka, C.-Y. Koh, H. Cui and S.
I. Stupp, Angew. Chem., 2009, 121, 6060.
(a) M. A. Tasdelen and Y. Yagci, Angew. Chem. Int. Ed., 2013, 52,
5
930; (b) A. Emeline, A. Salinaro, V. K. Ryabchuk and N.
Serpone, Int. J. Photoenergy, 2001, 3, 1; (c) G. Delaittre, A. S.
Goldmann, J. O. Mueller and C. Barner-Kowollik, Angew. Chem.
Int. Ed., 2015, 54, 11388.
5
K. K. Oehlenschlaeger, J. O. Mueller, N. B. Heine, M. Glassner, N.
K. Guimard, G. Delaittre, F. G. Schmidt and C. Barner-Kowollik,
Angew. Chem. Int. Ed., 2013, 52, 762.
6
7
H. Zhou and J. A. Johnson, Angew. Chem., 2013, 125, 2291.
(a) C. Rodriguez-Emmenegger, C. M. Preuss, B. Yameen, O. Pop-
Georgievski, M. Bachmann, J. O. Mueller, M. Bruns, A. S.
4
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