AbstractParallel atom transfer radical polymerization (ATRP) of styrene and t‐butyl acrylate (t‐BA) was investigated. A series of ATRP polymerizations were carried out in parallel with varying targeted molecular weight ($\overline M _{\rm n}$) and showed excellent reproducibility of $\overline M _{\rm n}$, polydispersity, and conversion. In addition, polymerizations were done to determine the effect of inhibitor on styrene polymerization, yielding similar results. In synthesizing a library of polymers having varying $\overline M _{\rm n}$ using ATRP, polymers having lower $\overline M _{\rm n}$ reach their target values earlier and the others continue to react under heating, resulting in peak broadening for the low $\overline M _{\rm n}$ polymers. Reinitiation experiments indicated that termination reactions were taking place resulting in “dead” polymer chains which were unable to reinitiate polymerization.magnified image