1001 Research Article Received: 16 June 2010 Revised: 12 January 2011 Accepted: 11 March 2011 Published online in Wiley Online Library: 2 May 2011 (wileyonlinelibrary.com) DOI 10.1002/pi.3103 Star poly(2-ethyl-2-oxazoline)s – synthesis and thermosensitivity Agnieszka Kowalczuk, a Juraj Kronek, b Kornelia Bosowska, c Barbara Trzebicka a and Andrzej Dworak a,c ∗ Abstract A series of star-shaped poly(2-ethyl-2-oxazoline)s was prepared by cationic polymerization. The polymerization was initiated by dipentaerythrityl hexakis(4-nitrobenzene sulfonate) and a tosylated hyperbranched polymer of glycidol. The polymerization proceeded in a controlled manner. The star structure of the products was determined by nuclear magnetic resonance. The molar mass distributions that were measured by gel permeation chromatography with multiangle laser light scattering were narrow, and the experimental values of the molar masses were close to those predicted. The very compact structure of the polymers obtained (compared with the linear counterparts) confirmed the star formation. The star poly(2-ethyl-2-oxazoline)s show a phase transition temperature in the range 62 – 75 ◦ C. Comparison of this phase transition temperature with that of the linear poly(2-ethyl-2-oxazoline)s with the same molar masses indicates the influence of molar mass and topological structure of the macromolecule on temperature behavior. The prepared copolymers are spherical, which might be useful for the controlled transport and release of active compounds. c  2011 Society of Chemical Industry Keywords: cationic polymerization; star polymers; stimuli-sensitive polymers; structure-property relations INTRODUCTION Macromolecules with well-defined branched structures are of substantial interest because of their unique properties and possible applications. Polymers with star-shaped structures are essentially covalently stabilized nanoparticles and therefore they can be used as chemically stable systems in solution or in the solid state. Using either an ‘arm first’ method or a ‘core first’ method, star polymers with different core and arm compositions were prepared. 1–3 The core of the star polymer can be composed of low molar mass compounds, dendrimers or hyperbranched polymers with reactive functional groups. Recently, multifunctional cyclic compounds such as porphyrins, cyclodextrins or calixarenes have been used in the synthesis of star polymers. 4–6 In particular, multifunctional hyperbranched polymers with well-defined structures and reactive end-groups are convenient core molecules for the synthesis of star-shaped polymers with higher numbers of arms. 7–11 2-Substituted-2-oxazolines are frequently used in both organic synthesis and polymer chemistry. 12–14 The cationic polymerization of these compounds has been studied extensively and is frequently used for the synthesis of macromolecules with controlled structures and properties. A broad range of cationic initiators (e.g. esters of sulfonic acids or alkyl halides) can be used in the polymerization of 2-oxazolines. Poly(2-ethyl-2-oxazoline) is a water-soluble polymer with a lower critical solution temperature of 60–90 ◦ C, depending on the molar mass. 15 In the case of poly (2-substituted-2-oxazoline)s, the cloud point can be decreased to the physiological range (35–40 ◦ C) by choosing a monomer or co-monomer with appropriate substituents and by optimizing the amount of co-monomer present. 16–18 For example, the lower critical solution temperature of poly(2-isopropyl-2-oxazoline) is approximately 37 ◦ C. 19 The early syntheses of star polyoxazolines relied upon the formation of the arms by living cationic polymeriza- tions of 2-methyl-2-oxazoline and 2-phenyl-2-oxazoline; the polymerization was initiated with 2,2-bis(hydroxymethyl)-1- butanol-trichloroformate. 20 Four-arm star polymers with poly (2-methyl-2-oxazoline) and poly(2-ethyl-2-oxazoline) arms were prepared using tetrakis(bromomethyl)ethylene as the initiator. 21 For the preparation of poly(2-methyl-2-oxazoline) stars with well-defined but fewer arms, either halomethylated silsesquiox- ane with eight initiating species in one molecule 22,23 or hexakis[p-(bromomethyl) phenoxycyclotriphosphazene] can be used as initiators. 24 Iron and ruthenium tris(bipyridine) complexes have also been employed as the macroinitiators for the polymeriza- tion of 2-R-2-oxazoline monomers, where R = ethyl, methyl, phenyl or undecyl. 25–28 The resulting stars can be easily fragmented to their arms by chemical cleavage of the metal – bipyridine complex in aqueous potassium carbonate solution. Four-arm stars were prepared by polymerization that was initiated by tetra-meso-[4-(chloromethyl)phenyl]porphyrin. 29,30 The arms of the stars were made of amphiphilic block copolymers; ∗ Correspondence to: Andrzej Dworak, Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland. E-mail: andrzej.dworak@cmpw-pan.edu.pl a Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie- Sklodowskiej 34, 41-819 Zabrze, Poland b Polymer Institute, Slovak Academy of Sciences, Centre of Excellence GLYCOMED, Dubravska cesta 9, 84236 Bratislava, Slovakia c University of Opole, Faculty of Chemistry, Oleska 48, 45-052 Opole, Poland Polym Int 2011; 60: 1001–1009 www.soci.org c  2011 Society of Chemical Industry