AbstractCationic polymerization of the geometrical isomers of β‐methylstyrene and anethole (p‐β‐methoxy‐β‐methylstyrene) was studied in order to clarify the relation between the geometrical structure of the α,β‐disubstituted olefins and their monomer reactivities, subsequent to previous work. Polymerization was carried out in toluene or ethylene dichloride with stannic chloride at 0°C.trans‐β‐Methylstyrene was 1.3 to 1.5 times more reactive thancis‐β‐methylstyrene to the styrene carbonium ion; copolymerization between thetransand thecisisomers of β‐methylstyrene showed little difference in monomer reactivity. By contrast, thecisanethole was 1.5–2.0 times more reactive than thetrans, according to copolymerization between the two isomers together. The lower reactivity of thecis‐β‐methylstyrene could be explained by steric hindrance in theci