Polymer Communication Preparation of hydrogel nanoparticles by atom transfer radical polymerization of N-isopropylacrylamide in aqueous media using PEG macro-initiator Keon Hyeong Kim a , Jungahn Kim b , Won Ho Jo a, * a Hyperstructured Organic Materials Research Center, School of Materials Science and Engineering, Seoul National University, Seoul 151-742, South Korea b Polymer Hybrids Center, Korea Institute of Science and Technology, Seoul, South Korea Received 9 August 2004; received in revised form 2 February 2005; accepted 8 February 2005 Available online 14 March 2005 Abstract PEG-b-PNIPAM block copolymers are synthesized by the atom transfer radical polymerization of NIPAM using PEG macro-initiator. When the polymerization temperature is 25 8C, the block copolymer is soluble in water, whereas the block copolymer is phase-separated to form micelles during polymerization as the polymerization temperature is raised to 50 8C, the temperature above the LCST of PEG-b- PNIPAM. To prepare stable hydrogel nanoparticles in water at room temperature, a small amount of N,N 0 -ethylenebisacrylamide is added as a cross-linker to the reaction system, where the size of nanoparticles is controlled by the composition of mixed solvent. q 2005 Elsevier Ltd. All rights reserved. Keywords: Atom transfer radical polymerization; N-isopropylacrylamide; Nanoparticles 1. Introduction Recently, stimuli-responsive hydrogel nanoparticles have attracted much attention, because they can respond quickly to their environmental stimuli in contrast to macroscopic gels [1]. Poly(N-isopropylacrylamide) (PNI- PAM) is the most prominent temperature-responsive polymer because PNIPAM-based gels exhibit a sharp volume change at thier lower critical solution temperature (LCST) [2]. But, the synthesis of PNIPAM with controlled molecular weight with a narrow molecular weight distri- bution has not been satisfactory until recently. Only a few works on the living polymerization of N-isopropylacryla- mide (NIPAM) have been reported by the reversible addition-fragmentation chain transfer (RAFT) [3–5] and the atom transfer radical polymerization (ATRP) [6]. The standard method used for preparation of polymeric particles is heterogeneous polymerization such as emulsion, dispersion, and suspension polymerization. Dispersion polymerization usually differs from suspension or emulsion polymerization in the initial state of polymerization system and the solubility of synthesized polymer, i.e., the monomer for dispersion polymerization is soluble in an organic solvent, which becomes nonsolvent for the resulting polymer [7–10]. In this paper, we report the dispersion polymerization of N-isopropylacrylamide (NIPAM) in aqueous media by ATRP using hydrophilic poly(ethylene glycol) methyl ether (PEG) as a macro-initiator in order to synthesize PEG-PNIPAM block copolymers. Although homopolymers of NIPAM by precipitation polymerization, block copoly- mers from PEG and PNIPAM (PEG-b-PNIPAM) by a ceric ion redox system, and their corresponding cross-linked micelles have been successfully prepared [11–15], the synthesis of PEG-b-PNIPAM by dispersion polymerization of NIPAM via ATRP and the preparation of hydrogel nanoparticle there from have not been reported yet to the best of our knowledge. The aim of this work is to prepare hydrogel nanoparticles by dispersion polymerization of NIPAM in aqueous media via ATRP and to control the size of nanoparticles by changing the reaction media. Polymer 46 (2005) 2836–2840 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2005.02.009 * Corresponding author. Tel.: C82 2 880 7192; fax: C82 2 885 1748. E-mail address: whjpoly@plaza.snu.ac.kr (W.H. Jo).