Advanced supramolecular initiator for nitroxide-mediated polymerizations containing both metal-ion coordination and hydrogen-bonding sites

Article abstract of DOI:10.1039/b902636c

The synthesis of a new difunctional nitroxide initiator combining two orthogonal supramolecular entities is reported; controlled radical polymerization of styrene using this initiator is demonstrated to generate well-defined heterotelechelic polymers in a

Full text of DOI:10.1039/b902636c

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Advanced supramolecular initiator for nitroxide-mediated
polymerizations containing both metal-ion coordination
and hydrogen-bonding sitesw
Ulrich Mansfeld,ab Martin D. Hager,ab Richard Hoogenboom,b Christina Ott,b
Andreas Winterb and Ulrich S. Schubert*ab
Received (in Cambridge, UK) 9th February 2009, Accepted 25th March 2009
First published as an Advance Article on the web 24th April 2009
DOI: 10.1039/b902636c
The synthesis of a new difunctional nitroxide initiator combining
two orthogonal supramolecular entities is reported; controlled
radical polymerization of styrene using this initiator is
demonstrated to generate well-defined heterotelechelic polymers
in a one-step procedure.
be realized by utilizing a functionalized alkoxyamine as
unimolecular initiator for NMP that already mimics both
end groups of the resulting polymer.
The targeted alkoxyamine 5 is based on the structure
of 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane
(PhEt-TIPNO), which was developed by Hawker et al. as a
universal initiator for NMP (Fig. 1).6a Functionalization of
the phenyl ring of the initiating fragment,6b,7a the nitroxide
fragment6b,7b or both8 is possible without affecting the
efficiency of the alkoxyamine as initiator for the controlled
radical polymerization of styrene and tert-butylacrylate. In
fact, high end-group fidelity on each site (495%) can
Recent research interest in materials science has focused on the
modification of polymers with supramolecular binding units to
combine covalent bonding with secondary interactions such as
hydrogen bonding and metal-ion coordination.1 Such
supramolecular polymer systems exhibit novel properties
based on self-assembly and reversibility of the supramolecular
interactions, such as e.g. self-healing2 or responsiveness
towards external stimuli (e.g. temperature and pH-value).1
In addition, the incorporation of orthogonal supramolecular
moieties into a polymer could further enhance the responsive-
ness since the non-covalent interactions can be addressed
independently by external stimuli,3 which we recently demon-
strated for heterotelechelic poly(e-caprolactone) containing
both metal-ion coordination and hydrogen-bonding sites.4
Another important aspect in polymer science is the control
of soft matter on the nanometre scale, which can be achieved
by tailoring the molecular structure of polymers in terms of,
e.g., molar mass and placement of functional groups.
be assured for molar masses up to 50 000 g molꢀ1 6b
In
.
accordance with the synthetic strategy by Hawker et al. for
functional a-hydrogen nitroxides6a and the catalyzed radical
trapping of functional styrene derivatives,9 we synthesized the
heterodifunctional alkoxyamine 1 as a basic framework. This
structure contains a benzylic chloro group at the initiating
fragment as well as a benzylic THP-ether group, i.e. protected
benzyl alcohol, at the nitroxide-mediating fragment and, thus,
could be functionalized further in a stepwise fashion (Fig. 1).
Following our previously established route for the prepara-
tion of
a mono-terpyridine-functionalized TIPNO-based
alkoxyamine,10 the terpyridine unit was selectively attached to
1 via nucleophilic substitution with 2,6-bis-(20-pyridyl)-4-
pyridone (2) to yield the terpyridine-functionalized alkoxyamine
3. After deprotection of the benzylic alcohol with p-toluene
sulfonic acid (PTSA), the isocyanate-functionalized UPy-
unit11 4 was attached via an addition reaction onto the
benzylic alcohol using dibutyltin dilaurate (DBTDL) as
catalyst to yield alkoxyamine 5. The 1H NMR spectrum of
alkoxyamine 5, Fig. 2, top, shows the hydrogen bonding
signals of UPy in the region 10–13 ppm as well as the
characteristic terpyridine signals between 7.2 and 8.8 ppm.
The integral ratio of the signals of both tpy and Upy to the
alkoxyamine skeleton reveals a 1 : 1 ratio demonstrating,
besides quantitative hydrogen bonding, that both supra-
molecular units are attached to the alkoxyamine. From the
hydrogen-bonded proton signals it can be concluded that
‘‘tpy~TIPNO~UPy’’ (5) is present as a dimer in dichloro-
methane linked via the quadruple hydrogen bonding array
of the [1H]-tautomer1a of UPy. Product 5 could also be
characterized by MALDI-TOF mass spectrometry (Fig. 3).
The efficiency of ‘‘tpy~TIPNO~UPy’’ (5) as initiator for
NMP was subsequently examined for the polymerization of
styrene using anisole as solvent due to the poor solubility of
Nowadays, this control over molecular structure is easily
attained by controlled radical polymerization techniques.5 In
addition, well-defined polymers with supramolecular end
groups represent appealing building blocks for the construc-
tion of chain-extended supramolecular polymers as well as
advanced macromolecular architectures. In the present case,
nitroxide-mediated radical polymerization (NMP) is chosen
since it tolerates a wide variety of functional groups and
assures furthermore a high end-group fidelity.5a,6
Herein, we report for the first time a one-step preparation of
heterotelechelic supramolecular polymers containing both a
2,20:60,200-terpyridine1b (tpy) moiety as well as a 2-ureido-
4[1H]-pyrimidinone1a (UPy) moiety as end groups. This could
a Laboratory of Organic and Macromolecular Chemistry,
Friedrich-Schiller-Universitat Jena, Humboldtstrasse 10,
¨
07743 Jena, Germany. E-mail: ulrich.schubert@uni-jena.de;
Fax: +49 (0)3641 9482 02; Tel: +49 (0)3641 9482 01
b Laboratory of Macromolecular Chemistry and Nanoscience,
Eindhoven University of Technology, PO Box 513, Eindhoven,
5600 MB, The Netherlands
w Electronic supplementary information (ESI) available: Synthesis
and characterization details. See DOI: 10.1039/b902636c
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Fig. 1 Schematic representation of the synthesis of heterodifunctional alkoxyamine 5 containing both the terpyridine and the ureidopyrimidinone
moieties.
the alkoxyamine in the bulk monomer. The first polymerization
([M]/[I] = 35) was conducted up to 50% conversion within 6 h
at 123 1C. The polymer was characterized by size exclusion
chromatography (SEC) revealing a narrow molar mass
distribution, i.e. with a polydispersity index (PDI) below
1.10, and a number average molar mass (Mn) of 6800 g molꢀ1
(polystyrene calibration) indicating a controlled radical poly-
merization (Fig. 4). It can be assumed that hydrogen bonding
also occurs to some extent during the polymerization, which
could lead to stabilization of the radicals.7a The resulting
heterotelechelic PS 6a was characterized by 1H NMR spectro-
scopy (Fig. 2, bottom) revealing the characteristic signals for
hydrogen bonding of UPy as well as for tpy with an integral
ratio of approximately 1 : 1. It should be noted that the
integrals for the Upy signals are slightly smaller in comparison
to the tpy ones, which can be ascribed to the presence of
the [3H]-tautomer of UPy as indicated by the signal at d =
5.6 ppm.1a Nonetheless, the combined integrals of the UPy
signals at 5.6 and 5.8 ppm are equal to the tpy signals
indicating near-quantitative incorporation of both supra-
molecular units into the polymer. This high end-group fidelity
is assured for lower conversions and molar masses.6b The
molar mass calculated from 1H NMR spectroscopy (Mn
=
4800 g molꢀ1) is significantly smaller compared to the
SEC-value. The NMR-value is assumed to be more accurate,
since ‘‘tpy~PSm~UPy’’ might elute differently during SEC
in comparison to the PS calibration due to the bulkiness of
the end groups and/or the changed solubility. Further
polymerizations were performed (up to [M]/[I] = 200) yielding
additional well-defined heterotelechelic polystyrenes with
polydispersity indices below 1.2 for molar masses up to
13 000 g molꢀ1 (Fig. 4).
Fig. 2 1H NMR spectra of initiator 5 (top) and telechelic polymer 6a
(bottom) showing signals of both UPy and tpy (CD2Cl2).
Fig.
3
MALDI-TOF-MS spectrum of tpy~TIPNO~Upy (5)
Fig. 4 SEC-traces of initiator 5 and heterotelechelic polystyrenes
6a–c. Eluent: N,N-dimethyl acetamide (DMA) with LiCl (2.1 g Lꢀ1).
(with DCTB).
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Fig. 5 Schematic representation of the metal-ion complexation with
macroligand tpy~PSm~UPy leading to supramolecular metallo-
polymers 7 and 8.
Fig. 7 SEC-traces of tpy~PS60~UPy (6b) and [bis-tpy]nickel(II) complex
8. Eluent: N,N-Dimethyl formamide (DMF) with 0.005 M NH4PF6.
The prepared telechelic polymers are promising building
blocks for the stepwise construction of supramolecular archi-
tectures. The Upy gives rise to the formation of hydrogen-
bonded dimers as demonstrated by 1H NMR spectroscopy
while the tpy unit can be used for the formation of metal
complexes (Fig. 5). To investigate the potential metal coordi-
nation of the heterotelechelic polymers, UV-vis-titration
experiments were performed:3a a methanol solution of iron(II)
chloride was added stepwise to a chloroform solution of
‘‘tpy~PS120~UPy’’. A linear increase of both the metal-to-
ligand charge-transfer (MLCT) band of the iron(II) complex
at 559 nm and the ligand-centered (LC) band of terpyridine at
318 nm was observed confirming the formation of the
[bis-tpy]iron(II) complex (Fig. 6). The equivalence point is indi-
cated by a significant change in the slope at both wavelengths,
whereby a plateau was reached for the MLCT absorption. The
molar mass calculated from UV-vis (Mn = 9000 g molꢀ1) is
comparable to the SEC-value (Mn = 11400 g molꢀ1).
A complete shift to higher molar masses was observed
indicating near-quantitative complexation (Fig. 7).
However, a shoulder at higher molar masses was observed that
cannot be explained up to now. Similar shoulders were also
observed for purified polymeric nickel(II) complexes12 suggesting
that the shoulder is not necessarily caused by side products, but
might be due to different solvation or interactions with the column.
To summarize, the synthesized ‘‘tpy~TIPNO~UPy’’ alkoxy-
amine represents a powerful NMP initiator for the one-step
preparation of well-defined heterotelechelic polymers as supra-
molecular building blocks. This approach is attractive for the
creation of polymer libraries towards tailor-made architec-
tures, since the nature and length of the polymeric spacer
between the supramolecular motifs can be controlled in a
facile way. In addition, the orthogonal non-covalent binding
units in the polymers are believed to provide access to
extended stimuli responsiveness opening new pathways towards
‘‘smart’’ materials, which will be investigated in future work.
Besides a change in the optical properties upon metal
complexation, an increase in molar mass should be detectable
by SEC for the complexation of macroligand ‘‘tpy~PSm~UPy’’.
To be able to analyze the tpy metal complex system, polar
SEC-conditions were utilized that suppress the formation of
hydrogen-bonded dimers. In particular, [bis-tpy]nickel(II)
complexes are stable under the used SEC-conditions12 and,
therefore, the complexation of the macroligand with nickel(II)
acetate was performed in a mixture of methanol and chloroform.
The product was characterized by SEC without purification.
Notes and references
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Fig. 6 UV-vis titration of tpy~PS120~UPy (6c) with FeCl2 revealing
the formation of the [bis-tpy]iron(II) complex 7 in chloroform by the
appearance of the MLCT band at 559 nm and the LC band at 318 nm.
The inset shows their absorption increase with addition of FeCl2.
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This journal is The Royal Society of Chemistry 2009
3388 | Chem. Commun., 2009, 3386–3388