Stimuli-responsive star polymers through thiol–yne core functionalization/ crosslinking of block copolymer micelles† Niels ten Brummelhuis and Helmut Schlaad * Received 5th January 2011, Accepted 26th January 2011 DOI: 10.1039/c1py00002k Thiol–yne photochemistry was applied for simultaneous functionalization and crosslinking of the core of poly(2-(3-butinyl)-2-oxazoline)-block-poly(2-ethyl-2-oxazoline) micelles in water to yield functional star polymers. Samples with core-confined ionic groups exhibited stimuli-responsive properties and were sensitive to changes in temperature, pH, ionic strength, and nature of the salt. Salt-induced shifts of the cloud point temperatures were found to correlate with the Hofmeister series. Introduction The crosslinking of self-assembled polymer structures, e.g. micelles and vesicles in solution, gels, and films, becomes an important research topic because it produces structures with enhanced stability or robustness against environmental stimuli such as dilution. For drug delivery applications, for instance, where normally only very small amounts of the drug are administered, crosslinking prevents the drug-delivery vehicle from disintegration in the blood-stream. 1,2 Also, core-crosslinked micelles, or star polymers, with core-confined catalysts have successfully been applied in homogeneous catalysis to perform reaction cascades in one pot. 3 Thiol–yne chemistry, which is the radical addition of usually two equivalents of a thiol to a carbon–carbon triple bond (Scheme 1a), 4 has become a powerful tool for creating highly functional hyperbranched polymers, dendrimers, networks, and so on. 5,6 The thiol–alkyne double addition has the characteristics of a ‘‘click’’ reaction as it is modular, wide in scope, proceeds rapidly to completion, and is selective for a single product. 7 Here, we apply thiol–yne photochemistry in an efficient but non-‘‘click’’ fashion 8 for a simultaneous functionalization and crosslinking of poly(2-(3-butinyl)-2-oxazoline)-block-poly- (2-ethyl-2-oxazoline) (PBOX-b-PEOX) copolymer micelles in water (see structure in Scheme 1b). The star polymers were analyzed by Raman spectroscopy and light scattering for the determination of functionality and confirmation of morphology. Stimuli-responsive (pH, salt, and temperature) star polymers were obtained by the addition of thiols carrying ionic groups and were characterized by phototurbidimetry. Experimental part Materials Methyl triflate ($98%) and acetonitrile (ChromasolvÒ) were received from Sigma-Aldrich, dried over CaH 2 , and distilled. Tetrahydrofuran (THF, 99.5%), dioxane (99.5%), methanol (99.5%), methyl 3-mercaptopropionate ($98%), 3-mercapto- propionic acid ($99%), sodium sulfate ($99.0%), and sodium thiocyanate ($99.99%) were used as received from Sigma- Aldrich. 2-Mercaptoethylamine hydrochloride (98%) was received from Acros Organics. Sodium nitrate (99.99%) was received from Merck. Monomers. 2-Ethyl-2-oxazoline (EOX, $99%) was received from Sigma-Aldrich and distilled from CaH 2 . 2-(3-Butinyl)-2- oxazoline (BOX) was synthesized in three steps starting from 4-pentynoic acid (98%, Acros Organics) as described in the Scheme 1 (a) Reaction pathway of radical thiol–yne addition. (b) Structure of PBOX-b-PEOX and corresponding spherical micelles in water. Max Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany. E-mail: Helmut.Schlaad@mpikg.mpg.de; Fax: +49 331 567 9502; Tel: +49 331 567 9514 † Electronic supplementary information (ESI) available: Monomer synthesis (including intermediate reaction steps), polymer synthesis and characterization ( 1 H NMR and SEC), Raman spectra, z-potential and turbidimetric measurements. See DOI: 10.1039/c1py00002k 1180 | Polym. Chem., 2011, 2, 1180–1184 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Polymer Chemistry Cite this: Polym. Chem., 2011, 2, 1180 www.rsc.org/polymers PAPER Downloaded on 18 May 2011 Published on 15 February 2011 on http://pubs.rsc.org | doi:10.1039/C1PY00002K View Online