Trialkyl phosphites ((RO) 3P) can act as co-initiators for the diaryliodonium-induced cationic polymerization of cyclohexene oxide (CHO) or THF. A radical initiation step is also required, consistent with the essential role of a radical chain reaction of the phosphite with the iodonium salt to form polymerization-starting aryltrialkoxyphosphonium salts (ArP+ (OR) 3). We used the visible photolysis of phenylazoisobutyronitrile (PAIBN) as the radical ...