AbstractThe free cationic copolymerization behavior of styrene derivatives in methylene dichloride solution was examined. The copolymerization behavior of all monomer pairs was found to be strongly ideal. The relationships between reactivity ratios of the monomer pairs can be accurately correlated by a number of simple models. Hammett plots give very similar values for reaction constants for the different cations, and the copolymerization behavior can also be correlated by a generalized Hammett relation using an overall reaction constant. This suggests that under the particular reaction conditions of this work, the cations do not differ greatly in their absolute reactivity. This idea is briefly discussed in light of other work that has been done concerning the nature of the propagating cations in the polymerization of styrene derivatives.