Inverse electron demand Diels-Alder (iEDDA) functionalisation of macroporous poly(dicyclopentadiene) foams

Article abstract of DOI:10.1039/c3cc42925c

Inverse electron demand Diels-Alder reactions performed on the double bonds in open cellular macroporous poly(dicyclopentadiene) monoliths yield a high degree of functionalisation (up to 2 mmol pyridazines per g or 8 mmol N per g) with grafted di(pyridyl)pyridazines in a single step.

Full text of DOI:10.1039/c3cc42925c

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Inverse electron demand Diels–Alder (iEDDA)
functionalisation of macroporous
poly(dicyclopentadiene) foams†
Cite this: Chem. Commun., 2013,
49, 7325
Received 19th April 2013,
Accepted 22nd June 2013
ˇ ˇ
Astrid-Caroline Knall,* Sebastijan Kovacic, Manuel Hollauf, David Reishofer,
Robert Saf and Christian Slugovc*
DOI: 10.1039/c3cc42925c
www.rsc.org/chemcomm
Inverse electron demand Diels–Alder reactions performed on the which is one of the first reports on using this promising conjugation
double bonds in open cellular macroporous poly(dicyclopentadiene) scheme in materials science.11 The iEDDA reaction of tetrazines and
monoliths yield a high degree of functionalisation (up to 2 mmol olefins has emerged as a high potential click chemistry scheme in
pyridazines per g or 8 mmol N per g) with grafted di(pyridyl)- life- and materials sciences in the last few years.12 In these applica-
pyridazines in a single step.
tions mostly highly strained olefins such as trans-cyclooctenes or
norbornenes and hydrolytically stable, yet less reactive, tetrazine
Curing of monomers constituting the minority phase of a species are used for conjugation purposes.13 However, using more
stabilised High Internal Phase Emulsion (HIPE) is a frequently reactive dienes or applying higher reaction temperatures14 allows
used method to prepare macroporous polymeric foams charac- conversion of less reactive dienophiles15 such as double bonds with
terised by a microcellular structure which is desirably interconnected a large degree of steric hindrance. For example, the iEDDA reaction
with smaller pores.1 Such foams find manifold applications in such has been used for the synthesis of pyridazine-based ligands16 which
diverse fields as tissue engineering,2 water purification3 or as were used, amongst other applications in organometallic chemistry,
separator membranes in lithium-ion batteries.4 Common pre- as building blocks in metal–organic frameworks.17
requisites for most applications are good mechanical proper-
ties and easy methods to post-functionalise the foams.5
A pDCPD monolith with 80% porosity was prepared according
to a previously developed procedure.10 To ensure that no oligo-
A particularly promising class of HIPE templated foam is meric DCPD units or surfactant molecules remain entrapped in
made of dicyclopentadiene (DCPD) via Ring-Opening Metathesis the pDCPD framework, the foams were purified using sequential
Polymerisation (ROMP).6 These self-crosslinked poly-(dicyclo- Soxleth extractions in dichloromethane, THF and acetone (24 h
pentadiene) (pDCPD) foams combine an interconnected micro- each) and dried in vacuo. For the immobilisation of pyridazines
cellular open porous structure with the highest mechanical resilience using iEDDA chemistry, pieces of this monolith were immersed in
reported amongst HIPE templated materials.6,7 Furthermore, scintillation vials containing 0.5 and 0.1 equiv. (with respect to
pDCPD bears a high degree of unsaturation (as a consequence of repeating units) of pyTz (3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine,
the metathesis polymerisation) offering ways for further function- Fig. 1) in THF or MeOH, respectively, for 48 h. The monoliths,
alisation (e.g. epoxidation8 or bromination9 followed by conver- which were initially white, exhibited a yellow colour while the
sion with nucleophiles or radical initiated thiol–ene chemistry10). supernatant remained virtually colourless.
Accordingly, pDCPD foams ideally fulfil the prerequisites for the
preparation of highly functionalised porous materials.
While the experiments in THF were completed after this
time (as indicated by the discolouration of the initially pink
Herein we disclose a straightforward and versatile single- tetrazine solution), the reactions in MeOH could not be brought
step method for the post-functionalisation of pDCPD foams via to completion, even after 9 d, which is due to the more
the inverse electron demand Diels–Alder (iEDDA) reaction of pronounced swelling of pDCPD18 in THF in comparison to
tetrazines with the residual double bonds in the foam skeleton, MeOH. This facilitates diffusion of pyTz into the swollen
pDCPD scaffold making more double bonds accessible in
THF in spite of a higher reaction rate expected for MeOH.12,19
Institute for Chemistry and Technology of Materials, Stremayrgasse 9/5,
A-8010 Graz, Austria. E-mail: a.knall@TUGraz.at, slugovc@TUGraz.at;
Fax: +43 316 873 1032284; Tel: +43 316 873 32284
Then, the samples were purified by immersion in acetone
and, after drying in vacuo, subsequently subjected to elemental
analysis to determine the nitrogen content of the modified
samples. Due to the high degree of unsaturation and their large
surface area, pDCPD foams are known to be rapidly oxidised
when stored under ambient conditions.8 Fully oxidised pDCPD
† Electronic supplementary information (ESI) available: Experimental procedures,
NMR spectra of conversion products of pyTz and norbornadiene, cyclopentene
or hexene, respectively, FTIR, elemental analysis data, additional SEM micro-
graphs, EDX, porosity and synthesis of the europium reference compound. See
DOI: 10.1039/c3cc42925c
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Fig. 1 Representative chemical structures, SEM micrographs and photographs of (A) pDCPD foam, (B) after modification with 0.5 equiv. of pyTz (3,6-di(pyridin-2-yl)-
1,2,4,5-tetrazine) and (C) after impregnation with Eu(NO3)3 (photograph taken under UV light (l = 365 nm)).
has an oxygen content of about 36%.10 This was also inevitable approach (with approximately every 8th or 9th repeating unit
during the purification procedure and the time allowed being derivatised).22 Following our post-modification approach,
between sample preparation and elemental analysis. Therefore, we were able to derivatise every second repeating unit in the
varying oxygen contents were observed for our samples (ESI†). pDCPD network, or, in other words, every 4th double bond in a
In Fig. 2, the pyridazine contents of reactively modified foams single step under mild conditions.
(calculated from elemental analysis) are compared with the
The morphology of the pDCPD foam before and after
respective theoretically expected ones. For samples immersed functionalisation was found to be broadly similar according
in THF, values close to the theoretical compositions (2.2 mmol to evaluation of the scanning electron micrographs of repre-
grafted pyridazines per g monolith vs. 2.1 calculated) were sentative samples (cf. Fig. 1). Mercury porosimetry and helium
observed which correspond to a value of 8.8 mmol gÀ1 nitrogen. pycnometry revealed a somewhat higher porosity for the monolith
Raising the reaction temperature to 60 1C resulted in shorter functionalised in THF (85 Æ 1%) compared to the unmodified
conversion times (3 h) while the same composition was specimens (80 Æ 1%) and the sample functionalised in methanol
obtained. In methanol, only a fraction of the applied tetrazine (70 Æ 1%). An increase in skeletal density from 1.20 g cmÀ3 (initial
amount (0.37 mmol gÀ1 vs. 2.1 calculated) was grafted.
sample) to 1.52 g cmÀ3 (MeOH) and 1.75 g cmÀ3 (THF) reflected
About 1–2 mmol of functional units per gram of the poly- the increasing loading with pyridazines.
meric scaffold are typical for similar, commercially available
To confirm the presence of pyridazine moieties in the foams
systems,20,21 such as (bipyridyl)-functionalised polystyrene, which a model reaction was elaborated. Soluble low molecular weight
is obtained from polystyrene in a three-step postmodification oligo-DCPD (n = about 10) was prepared using a 1st generation
Grubbs initiator at 0 1C and subsequently reacted with pyTz.
The reaction was followed by 1H NMR spectroscopy (Fig. 3)
which revealed complete consumption of pyTz after 24 h at
room temperature. Furthermore, no characteristic dihydro-
pyridazine signals (which should appear at 9–9.5 ppm) were
found, and characteristic signals of the respective polymer-
grafted pyridazine products peaking up at 7.1, 7.4, 7.5 and
8.5 ppm confirmed conversion of pyTz. Indeed, in a model
experiment using pyTz and cyclopentene we also observed fast
oxidation of the intermediate dihydropyridazine product with-
out additional oxidants and found similar chemical shifts for
the corresponding pyridazine (ESI†). The sharp peaks found in
the NMR spectrum of fully converted oligo-DCPD (Fig. 3C) can
be attributed to terminal vinyl groups transformed in an iEDDA
Fig. 2 Di(pyridyl)pyridazine content of modified monoliths.
7326 Chem. Commun., 2013, 49, 7325--7327
reaction, which was confirmed by comparison with the iEDDA
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involving mild reaction conditions and maintaining the characteri-
stic open-porous morphology of emulsion-templated pDCPD foams.
A high loading of up to 2 mmol gÀ1 of di(pyridyl)pyridazines
(8 mmol gÀ1 of nitrogen) could be achieved. Thereby, a modular
way of introducing functionalities on porous materials with a
high degree of functionalisation (50% with respect to repeating
units) has been disclosed and can be further exploited using
other tetrazine derivatives.
Financial support from the Austrian Science Fund (FWF):
[T 578-N19] is gratefully acknowledged. Karel Jerabek is thanked
for mercury and helium porosimetry measurements.
Notes and references
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 7325--7327 7327