Various silyl enol ethers were employed as quenchers for the living radical polymerization of methyl methacrylate with the R-Cl/RuCl2(PPh3)(3)/Al(Oi-Pr)(3) initiating system. The most effective quencher was a silyl enol ether with an electron-donating phenyl group conjugated with its double bond [CH2=C(OSiMe3)(4-MeOPh) (2a)] that afforded a halogen-fi ee polymer with a ketone terminal at a high end functionality [(F) over bar (n) similar to 1]. Such silyl compounds reacted with the growing radical generated from the dormant chloride terminal and the ruthenium complex to give the ketone terminal via the release of the silyl group along with the chlorine that originated from the dormant terminal. In contrast, less conjugated silyl enol ethers such as CH2=C(OSiMe3)Me were less effective in quenching the polymerization. The reactivity of the silyl compounds to the poly(methyl methacrylate) radical can be explained by the reactivity of their double bonds, namely, the monomer reactivity ratios of their model vinyl monomers without the silyloxyl groups. The lifetime of the living polymer terminal was also estimated by the quenching reaction mediated with 2a.