Three electrophilic olefins with various beta-leaving groups, namely, 1-cyano-1-carbomethoxy-vinyl 2-chloride (I), 1-cyano-1-carbomethoxyvinyl 2-iodide (II), and 2-cyano-2-carbomethoxyvinyl p-toluenesulfonate (III), reacted under the photochemical conditions with three moderately electron-rich aromatic olefins, styrene, p-methylstyrene, and p-methoxystyrene, in 1,2-dichloroethane solution at 0 degrees C. Reaction conditions were adjusted to minimize both the thermal dark polymerization and the photo-self-initiation of the styrenes. The reactivity of these olefins depends on the nature of the leaving group, tosylate > chloride > iodide. The electron-rich character of the styrene monomers also controls the tendency toward cationic homopolymerization : p-methoxystyrene > p-methylstyrene > styrene. Also the stability of the leaving group anion (tosylate > iodide > chloride) controls the distribution between cationic homopolymerization and free-radical copolymerization. A mechanism was proposed to show the interaction between the excited electron-rich styrene derivative and the electron-poor olefins which leads to the formation of tetramethylene polar 1,4-diradicals. These either initiate the free-radical copolymerization or eject the leaving group anion and create carbocations which initiate the homopolymerization of styrenes.