Block copolymers of hyperbranched polyethylene (PE) and linear polystyrene (PS) or poly(methyl methacrylate) (PMMA) were synthesized via atom transfer radical polymerization (ATRP) with hyperbranched PE macroinitiators. The PE macroinitiators were synthesized through a "living" polymerization of ethylene catalyzed with a Pd-diimine catalyst and end-capped with 4-chloromethyl styrene as a chain quenching agent in one step. The macroinitiator and block copolymer samples were characterized by gel permeation chromatography, H-1 and C-13 NMR, and differential scanning calorimetry. The hyperbranched PE chains had narrow molecular weight distribution and contained a single terminal benzyl chloride per chain. Both hyperbranched PE and linear PS or PMMA blocks had well-controlled molecular weights. Slow initiation was observed in ATRP because of steric effect of hyperbranched structures, resulting in slightly broad polydispersity index in the block copolymers. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3024-3032, 2010