The activation processes for the organotellurium-mediated living radical polymerizations (TERPs) of styrene (St), methyl methacrylate (MMA), methyl acrylate (MA), and vinyl acetate (VAc) were systematically studied. For the St, MMA, and MA homopolymerizations, both thermal dissociation and degenerative chain transfer (DT) were involved in the activation process with the main mechanism being DT at the examined temperatures (40-100 degrees C). The degenerative (exchange) chain transfer constant C-ex increased in the order of MMA < St similar to MA. The temperature dependence of C-ex was weak and negative for these monomers. The VAc homopolymerization also included DT as the main activation mechanism. For the VAc polymerization, head-to-head monomer addition is significant on propagation, forming a primary alkyl chain-end (-CH2-TeCH3) adduct. The activation of this adduct was too slow to yield low-polydispersity polymers, explaining why the polydispersity control is not satisfactory for VAc at high degrees of polymerization. The Cex for a poly(methyl methacrylate) (PMMA) radical to PMMA-TeCH3 (homopolymerization) and polystyrene-TeCH3 (block copolymerization) adducts were similar, suggesting that the DT in TERP is a (nearly) single-step reaction without forming a kinetically important intermediate.