Atom transfer radical polymerization (ATRP) was used to prepare thermosensitive cationic block copolymers of (3-acrylamidopropyl)-trimethylammonium chloride (AMPTMA) and N-isopropylacrylamide (NIPAAM) with different block lengths. By using ethyl-2-chloropropionate (ECP) as initiator and CuCl/CuCl2/tris(2-dimethylaminoethyl)amine (Me6TREN) catalytic system in DMF:water 50:50 (v/v) mixtures at 20 degrees C the polymerization was controlled. The association properties in NaCl aqueous solution were studied as a function of temperature and. polymer concentration by dynamic light scattering, NMR spectroscopy, fluorescence spectroscopy and energy filtered-transmission electron microscopy. The block copolymers formed micellar aggregates above the lower critical solution temperature (LCST) of pNIPAAM. The LCST is strongly influenced by the relative length of the two blocks and is significantly higher than that of pure pNIPAAM. The size of core and shell of the micelles is discussed in terms of block copolymer composition. (C) 2008 Elsevier Ltd. All rights reserved.