DOI: 10.1002/cssc.201500847
Communications
Microwave-Assisted Syntheses in Recyclable Ionic Liquids:
Photoresists Based on Renewable Resources
The copoly(2-oxazoline) pNonOx80-stat-pDc=Ox20 can be syn-
thesized from the cationic ring-opening copolymerization of 2-
nonyl-2-oxazoline NonOx and 2-dec-9’-enyl-2-oxazoline Dc=Ox
in the ionic liquid n-hexyl methylimidazolium tetrafluoroborate
under microwave irradiation in 250 g/batch quantities. The
polymer precipitates upon cooling, enabling easy recovery of
the polymer and the ionic liquid. Both monomers can be ob-
tained from fatty acids from renewable resources. pNonOx80-
stat-pDc=Ox20 can be used as polymer in a photoresist (resolu-
tion of 1 mm) based on UV-induced thiol–ene reactions.
In earlier studies, we reported negative photoresists based
on copoly(2-oxazoline)s that can be crosslinked upon illumina-
tion with UV light.[4,5] As crosslinking reaction, the thiol–ene re-
action, a prominent congener of the click reactions, was
chosen.[6,7] Copoly(2-oxazoline)s bearing unsaturated double
bonds in their side chains can be crosslinked with bisfunctional
thiols, such as butane dithiol and 3,4-dimercapto toluene, or
tetrafunctional thiols, such as pentaerythritol-tetra-(3-mercap-
toproprionate), yielding structured polymer films with a resolu-
tion of 2 mm.
In order to further advance this toolbox of copoly(2-oxazo-
line)-based photoresists, we have redesigned our synthetic
strategy meeting the current demand for more environmental-
ly benign syntheses, aiming at the elimination of volatile or-
ganic compounds as reaction medium [i.e., using ionic liquids
(ILs) as substitute][8–11] and the employment of reactants from
renewable resources (i.e., fatty acids as resource).
Photolithography is a standard procedure for the production
of 2.5-dimensional polymer structures.[1–3] A polymer film is illu-
minated through a mask, subjecting the illuminated areas of
the photoresist film to photochemical reactions that change
the solubility of the polymer. In the case of so-called negative
photoresists, the illuminated areas become insoluble while the
non-illuminated areas stay soluble and, consequently, can be
dissolved in a subsequent development step, yielding a struc-
tured polymer film that reproduces the geometric pattern
preset by the mask in negative fashion.
Polymerizations in ILs have previously focused on radical
polymerizations as prominent examples.[12] In the area of
poly(2-oxazoline)s, the polymerization of 2-ethyl-2-oxazoline in
ILs has been investigated in a detailed kinetic study.[13] Notably,
for the polymerization of 2-oxazolines, which are commonly
performed under microwave irradiation nowadays,[14–16] ILs
offer the additional advantage that, due to their ionic charac-
ter, they are prime absorbers of microwave irradiation, which
paves the way for a highly energy-efficient process.[17]
[a] C. Petit,+ Prof. B. Grassl, Dr. S. Reynaud
IPREM, UMR 5254 UPPA/CNRS
HØlioparc, 2 Avenue du PrØsident Angot
64053 Pau CEDEX 09 (France)
In this study, the polymerization of 2-oxazolines in ILs is ex-
panded to the preparation of functional materials. Based on
earlier studies on the application of copoly(2-oxazoline)s as
photoresist,[4,5] two different 2-oxazoline monomers for the
synthesis of the copoly(2-oxazoline) should be chosen: one
with an unsaturated double bond in its side chain and another
with a nonfunctionalized side chain. In order to meet the crite-
ria for green synthesis as comprehensively as possible, several
considerations were taken into account. Among them, organic
solvents should not be involved in the monomer syntheses.
This prerequisite eliminated double-bond-bearing 2-oxazoline
monomers with short side chains such as 2-but-3’-enyl-2-oxa-
zoline, the synthesis of which requires vast amounts of halo-
genated solvents.[18] 2-Oxazoline monomers with longer side
chains, on the other hand, can be synthesized from the reac-
tion of ethanol amine with either the corresponding nitriles[19]
or the corresponding carboxylates or carboxylic acids.[20] As the
reaction involving nitriles requires catalysis by toxic cadmium
compounds, the reaction involving carboxylates or carboxylic
acids (which is catalyzed by titanium compounds) was favored.
[b] K. P. Luef,+ Prof. F. Wiesbrock
Polymer Competence Center Leoben (PCCL)
Roseggerstrasse 12, 8700 Leoben (Austria)
[c] K. P. Luef+
Institute for Chemistry and Technology of Materials
Graz University of Technology, NAWI Graz
Stremayrgasse 9, 8010 Graz (Austria)
[d] Dr. M. Edler, Prof. T. Griesser
Chair of Chemistry of Polymeric Materials
University of Leoben
Otto-Gloeckel-Strasse 2, 8700 Leoben (Austria)
[e] Dr. J. M. Kremsner, Dr. A. Stadler
Anton Paar GmbH (Ltd.)
Anton-Paar-Strasse 20, 8054 Graz (Austria)
[+] These authors contributed equally to this work.
Supporting Information and ORCIDs from the authors of this article are
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
any medium, provided the original work is properly cited.
ChemSusChem 2015, 8, 3401 – 3404
3401ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim