The effect of electron-donating and -withdrawing groups on the ligands of Pd(II) alpha-diimine olefin polymerization catalysis on catalyst stability, activity, and polymer molecular weight is investigated. The polyethylene molecular weight and the productivity of catalysts bearing substituted his(aryl)dimethyldiazabutadiene (Me(2)DAB) and bis(aryl)acenaphthenequinonediimine (BIAN) ligands were analyzed over time at room temperature and 40 degrees C to monitor catalyst stability and chain transfer processes. The introduction of electron-donating groups led to a dramatic increase in polymer molecular weight, with polymer chains still growing after 24 h of polymerization. The amino-substituted Me(2)DAB analogue afforded polymer of more than twice the molecular weight compared to the polymer made with the unsubstituted analogue after 24 h of polymerization. The unsubstituted catalysts and those bearing electron-withdrawing groups, however, reached a maximal molecular weight, generally lower, after a comparatively short time, which was presumably due to higher chain transfer rates. Electron-donating groups also provided increased stability to the catalysts leading to longer catalyst lifetimes. Both of these effects are likely due to stabilization of the reactive, electron-deficient, and coordinatively unsaturated alkyl agostic intermediate, the reactivity of which is key to both chain transfer and decomposition processes.