Well-defined multiarm star block copolymers poly(glycidol)-b-poly(methyl methacrylate) (PGOHBr-b-PMMA(x)) with an average number of PMMA arms of 85, 55, and 45 have been prepared. The core-first approach has been selected as the methodology using atom transfer radical polymerization (ATRP) of MMA from an activated hyperbranched poly(glycidol) as the core. These activated hyperbranched macroinitiators were prepared by esterification of hyperbranched poly(glycidol) (PGOH) with 2-bromoisobutyryl bromide. The effect of monomer/initiator ratio, catalyst concentration, time, temperature, and solvent on the growing of the arms has been studied in detail in order to optimize the process and to diminish the radical-radical coupling. The final products and intermediates were characterized by means of size exclusion chromatography (SEC), nuclear magnetic resonance (NMR) and Fourier transform-infrared (FTIR) spectroscopy. The length of PMMA arms was determined by SEC after cleavage of ester bond linked to PGOH core. Glass transition temperature (T(g)), thermal stability and rheological properties of the multiarm star copolymers were also studied. Finally, tapping mode atomic force microscopy (TMAFM) allowed the clear visualization of nano-sized particles (80-200 nm) corresponding to individual star molecules. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 3138-3151, 2011