Multiarm star polymers containing thiol-reactive maleimide groups at their core have been synthesized by utilization of atom transfer radical polymerization (ATRP) of various methacrylates using a masked maleimide containing multiarm initiator One end of the initiator contains multiple halogen groups that produce the star architecture upon polymerization and the other end contains a masked maleimide functional group. Unmasking of the maleimide group after the polymerization provides the thiol reactive maleimide core that is widely used in bioconjugation. Functionalization of the core maleimide group with a thiol containing tripeptide was used to demonstrate facile reactivity of the core of these multiarm polymers under reagent-free conditions (C) 2010 Wiley Periodicals, Inc J Polym Sci Part A. Polym Chem 48 2546-2556, 2010