We have investigated the effect of the concentration and molecular weight on the activity of polymeric silver(I)-NHC (NHC = N-heterocyclic carbene) catalyst complexes in ultrasound-induced mechanochemical catalyst activation. A strong dependence of the turnover number (TON) on initial catalyst concentration was observed in the transesterification of vinyl acetate with benzyl alcohol. The main findings of this study are that the concentration and molecular weight effects on TON are caused by competition between mechanochemical catalyst activation and deactivation, most likely by reactive species produced during the sonication process. Performing the transesterification reaction under radical-suppressing conditions resulted in a significant increase of TON. This result clearly demonstrates the increased catalyst lifetime when reducing the amount of sonochemical impurities, and it highlights the importance of controlling and suppressing secondary, sonochemical processes when using ultrasound-induced mechanochemical generation of reactive species such as catalysts. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 4929-4935, 2012