AbstractIn order to investigate the machanism of the monomer transfer reaction in cationic polymerization,o‐ andp‐methoxystyrene were polymerized by BF3et2O at 30°, 0° and −20 °C. to estimate monomer transfer constant ratio, and the effects of solvents (CCl4, CHCl3, CH2Cl2and C2H4Cl2) were studied. Considering the experimental results in copolymerizations of styrene witho‐ andp‐methoxystyrene together, the electrostatic and steric effect of the substituent on the monomer transfer reaction were investigated.It was found that the nature of the solvent did not affect ktm/kpofo‐methoxystyrene, but that ktm/kpofp‐methoxystyrene decreased with increasing dielectric constant of the solvent as was found in the polymerization of styrene and chlorostyrenes. It was also found that methoxy substitution at para‐position of styrene increased the activity of styrene in the propagation reaction more than in the monomer transfer reaction, but that methoxy substitution at ortho‐position of styrene increased the activity of styrene in the monomer transfer reaction more than in the propagation reaction.From these results and by considering the results on styrerne and chlorostyrenes, it was conceivable that the monomer transfer reaction in the polymerization of styrene derivatives proceeds through an alkylation reaction between the growing ion‐pair and the phenyl group in the monomer.The results of the molecular orbital calculation on the propagation and the monomer transfer reaction of methoxystyrenes and the infrared analysis of the end group of the resultant polymers supported the mechanism.The spontaneous termination reaction in the polymerization of methoxystyr