AbstractAcrylamide (AAm), methacrylamide (MAm),N‐methylacrylamide (NMAAm) andN‐methylmethacrylamide (NMMAm) were found to yield long‐lived propagating polymer radicals in the photo‐sensitized polymerizations in 1,4‐dioxane or benzene. The concentration of poly(NMAAm) radicals reached 1.10−3mol/l. Some post‐effect was observed at room temperature in the photo‐sensitized polymerization of AAm with di‐tert‐butyl peroxide in 1,4‐dioxane, while no post‐polymerization proceeded at room temperature in the polymerization of NMMAm in benzene. The reactions of poly(NMAAm) and poly(NMMAm) radicals with various vinyl monomers were found to produce long‐lived propagating polymer radicals of the second monomers at room temperature. Polymer radicals of non‐homopolymerizable monomers such as α‐methylstyrene and 1,1‐diphenylethylene were easily formed in such a block‐copolymerization matrix. The formation of the propagating polymer radicals of the vinyl monomers was investi