The atom transfer radical polymerization of methyl acrylate in 1-butyl-3-methylimidazolium hexafluorophosphate was studied with a molybdenum halide catalytic system. A combination of a bromine-containing initiator (ethyl-2-bromopropionate) with a chloride-containing catalyst [MoCl3(PMe3)(3)/MoCl4(PMe3)(3)] led to a controlled polymerization, and matrix-assisted laser desorption/ionization time-of-flight analysis of the polymers indicated that the reversibly deactivated macromolecules contained both bromine and chloride end groups. (C) 2007 Wiley Periodicals, Inc.