1900404 (1 of 5) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de COMMUNICATION Poly(2-oxazoline)s Based on Phenolic Acids Nils Lüdecke, Steffen M. Weidner, and Helmut Schlaad* N. Lüdecke, Prof. H. Schlaad Institute of Chemistry, University of Potsdam Karl-Liebknecht Str. 24–25, 14476 Potsdam, Germany E-mail: schlaad@uni-potsdam.de Dr. S. M. Weidner Federal Institute for Materials Research and Testing – BAM 1.3 Richard-Willstätter-Straße 11, 12489 Berlin, Germany The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/marc.201900404. DOI: 10.1002/marc.201900404 can be finely tuned by variation of the alkyl/aryl substituent. [9] Another appealing aspect is that catechol-containing 2-oxa- zoline monomers can be derived from naturally occurring phenolic acids, that is, carboxylic acids with methoxy-substituted benzoic or cinnamic skeleton, such as veratric acid, eudesmic acid, caffeic acid, or O-methylsinapic acid. [10] Here, we report on the synthesis and microwave-assisted cationic polymerization of methoxy-substituted phenyl and cin- namyl 2-oxazolines based on naturally occurring phenolic acids (Scheme 1). All, yet not reported, poly(2-oxazoline)s were characterized by nuclear magnetic reso- nance (NMR) spectroscopy, matrix-assisted laser desorption/ion- ization time-of-flight (MALDI-TOF) mass spectrometry, and size exclusion chromatography (SEC). Cleavage of the methyl aryl ethers finally yields a poly(2-oxazoline) with catechol side chains. 2-Substituted 2-oxazolines can be readily obtained from the corresponding carboxylic acids, that is why the oxazoline is used as a carboxyl protecting group in organic synthesis. [11] We therefore considered first the direct synthesis of the methoxy- substituted phenyl and cinnamyl 2-oxazolines (3) from the cor- responding benzoic and cinnamic acids (1) and 2-aminoethanol (see Scheme 1). [12] Reactions were performed with titanium(IV) isopropoxide as a catalyst in either bulk or chlorobenzene solu- tion at 170—230 °C or 140 °C, respectively, to give the desired products 3 (d–f) in ≈20–40% yield (see exemplary procedure for 3d in Supporting Information). The rather poor yields and required extended reaction times (up to 5 days) prompted us to search for an alternate route, that is, the 2-oxazoline synthesis via the corresponding nitriles (2). Nitriles are either commercially available (such as the benzo- nitrile derivatives 2a–c) or can be prepared in high yields from carboxylic acids by the amidation with ethyl carbamate and sub- sequent dehydration with thionyl chloride [13] or from primary amides under catalytic Swern oxidation conditions. [14] The first method, however, failed to give the cinnamonitrile derivatives 2d–f. We therefore prepared the corresponding primary amides by treatment of 1d–f with thionyl chloride and ammonia (yields: 65–95%), followed by their dehydration with oxalyl chloride/trie- thyl amine and catalytic amounts of dimethyl sulf-oxide (DMSO) in acetonitrile solution at room temperature, to give the nitriles 2d–f in excellent yields (83–95%). All nitriles 2a–f were then converted into the corresponding 2-oxazolines 3a–f by reaction with 2-aminoethanol for 24 h in the presence of zinc(II) ace- tate as catalyst in chlorobenzene solution at 130—140 °C. The products were purified by flash chromatography and recrystal- lization and isolated in yields of 55–93% (3a–c: 81–93%, 3d–f: 55–73%). The chemical structures of 3a–f were confirmed by A series of phenolic-acid-based 2-oxazoline monomers with methoxy- substituted phenyl and cinnamyl side chains is synthesized and polymerized in a microwave reactor at 140 °C using methyl tosylate as the initiator. The obtained poly(2-oxazoline)s are characterized by NMR spectroscopy, MALDI- TOF mass spectrometry, and size-exclusion chromatography (SEC). Kinetic studies reveal that the microwave-assisted polymerization is fast and com- pleted within less than ≈10 min for low monomer-to-initiator ratios of ≤25. Polymers with number-average molar masses of up to 6500 g mol -1 and low dispersity (1.2–1.3) are produced. The aryl methyl ethers are successfully cleaved with aluminum triiodide/N,N′-diisopropylcarbodiimide to give a poly(2-oxazoline) with pendent catechol groups. Polymers containing repeating catechol units are an interesting class of materials, inspired by naturally occurring biopolymers such as mussel-glue proteins or lignin, with potentially valuable chelating and adhesive properties. Mussels are the most promi- nent example of marine organisms that make use of catechol- containing proteins to attach to many kinds of surfaces in wet marine surroundings. [1] The mussel byssus protein fibers con- tain a high number of 3,4-dihydroxy-L-phenylalanine (DOPA) as the catechol-donating system, which are able to interact with var- iable organic and inorganic materials by coordination, hydrogen bonding, π–π stacking, covalent bonding, or electrostatic attrac- tion. [2] Also, polymers containing catechol or hydroquinone side chains are used to produce organic redox active materials. [3] Yet the examples of synthetic polymers containing catechol repeat units are scarce. Polymers were made by for instance functionalization of resins, [2b] step-growth polymerization, [4] co- polymerization of vinyl catechol derivatives, [5] or ring-opening polymerization of DOPA N-thiocarboxyanhydride. [6] Poly(2- oxazoline)s with catechol side chains, however, have not been reported so far, although poly(2-oxazoline)s are regarded as bio- compatible pseudopeptides or peptide-like polymers [7] and hence could be ideal candidates to mimic mussel-glue proteins. Poly(2- alkyl/aryl-2-oxazoline)s are synthesized by a living cationic ring- opening polymerization (CROP) process [8] and their properties Macromol. Rapid Commun. 2020, 41, 1900404