Bifunctional silyl enol ethers, specifically 1,3-bis{p-[l-[(trimethylsilyl)oxy]vinyl]phenoxy}propane (5),1,4-diethoxy-1,4-bis[(trimethylsilyl)oxy]-1,3-butadiene(6), and 2,4-bis[(trimethylsilyl)oxy]-1,3-pentadiene (7), are highly soluble and efficient bifunctional coupling agents for the living polymers of isobutyl vinyl ether (IBVE) initiated with hydrogen chloride/zinc chloride (HCI/ZnCl2) at -15-degrees-C in methylene chloride and toluene solvents. The products are linear poly(IBVE) with molecular weights twice those of the starting living chains and with molecular weight distributions are narrow as those of the precursors. NMR structural analysis of the products shows the coupling reaction to involve an electrophilic addition of the living end onto the enol ether double bond of the coupling agent to form a ketone with the formation of trimethylsilyl chloride. In particular, the bifunctional enol ether 5, bearing a highly electron-donating substituent at the alpha-carbon of the silyl enol ether, leads to a nearly quantitative coupling reaction (yield > 95 %) for a relatively short living poly(IBVE) (DP(n)BAR approximately 10). The other two coupling agents, 6 and 7, also gave high coupling yields above 90%. By contrast, a similar but vicinal bis(enol ether), 2,3-bis[(trimethylsilyl)oxy]-1,3-butadiene(8), failed to undergo such a coupling reaction under the same conditions. The coupling efficiency thus depends on the silyl enol ether structure and also the chain length of the living poly(IBVE); i.e., more coupling occurs with shorter chains. Model quenching reactions with the corresponding monofunctional silyl enol ethers [e.g., p-methoxy-alpha-[(trimethylsilyl)oxy]styrene (1) for 5] show the importance of electron-donating a-substituents on coupling agents that increase the electron density of the enol ether double bond.