omer with at least one radical polymerizable organic peroxides having the formulae (a) or (b) ##STR4## wherein R1 is a hydrogen atom or an alkyl group having 1 or 2 carbon atoms, each of R2 and R7 is a hydrogen atom or a methyl group, R6 is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each of R3, R4, R8 and R9 is an alkyl group having 1 to 4 carbon atoms, each of R5 and R10 is an