Dimethyl cross-bridged cyclam (DMCBCy) forms a very reducing Cu-I complex that possesses the highest known catalytic activity to date in atom transfer radical polymerization (ATRP) reactions. The value of the ATRP equilibrium constant in a reaction with methyl chloroacetate for (CuCl)-Cl-I/DMCBCy is 30-fold larger than for the very active catalyst (CuCl)-Cl-I/Me6TREN. The activation rate constant for the DMCBCy-based complex was also determined and is about 1.5 times larger than for (CuCl)-Cl-1/Me6TREN. The ATRP of n-butyl acrylate mediated by (CuBr)-Br-I/DMCBCy was fast even at 30 degrees C and yielded well-defined polymers when a sufficient amount (about 10 mol% of the total catalyst) of deactivator ((CuBr2)-Br-II/DMCBCy) was added to the reaction mixture. Side reactions such as electron transfer from the very reducing (CuBr)-Br-I/DMCBCy to the electrophilic acrylate radicals most likely account for the observed limited conversions. (c) 2006 Elsevier B.V. All rights reserved.