alpha, omega-Hybrid (hetero-telechelic) poly(ethylene oxide) (PEO) macromonomers carrying both cationic and radical or anionic polymerizable vinyl end groups were newly synthesized by the living anionic polymerization of ethylene oxide (EO) initiated with partially K-alkoxidated vinylic alcohols such as the monovinyl ether of tetramethylene glycol or di(ethylene glycol) and p-vinylbenzyl alcohol (VBA), followed by reaction with methacryloyl chloride (MAC). They possess a couple of alpha- and omega-end groups that can polymerize concurrently or selectively by radical and/or cationic or anionic mechanism. The reactivity of the radical and cationic species formed upon photolysis of benzophenone and triphenylsulfonium tetrafluoroborate towards these end groups were studied by means of H-1 NMR analysis following the disappearance of the respective olefinic groups. Studies with macromonomers and model compounds revealed that photoexcited benzophenone abstracts hydrogen atoms from the PEO backbone to form radicals without added hydrogen donors. In the case of the sulfonium salt, both radical and cationic species are formed, which react with the respective functional groups. It was also shown that vinyl ether moieties react more readily than the methacrylate in both radical and cationic processes.