The propagation rate constant, k_p, for the cationic polymerization of α-methylstyrene (αMeSt) was determined. First, the polymerization of αMeSt was carried out initiated by the diαMeStHCl/SnBr₄ initiator/coinitiator system in dichloromethane (DCM) or in DCM/methylcyclohexane (MeCHx) 1:1 (v/v) solvent mixture at −80 °C in the presence of allyltrimethylsilane (ATMS). Clean and quantitative addition of ATMS to the propagating end terminates the polymerization, and from the limiting conversion and molecular weights, the k_p/k_ATMS ratio was calculated. The k_p = (5.2 and 7.7) × 10⁷ L mol⁻¹s⁻¹ in DCM and DCM/MeCHx 1:1 (v/v), respectively, were calculated from the k_ATMS values determined by the diffusion clock method. Competition polymerizations between αMeSt and each of the following monomers p-methylstyrene (pMeSt), isobutylene (IB), styrene (St), and p-chlorostyrene (pClSt) were performed in DCM/MeCHx 1:1 (v/v) at −80 °C. From the limiting conversions and molecular weights, the reactivity ratios k_p/k₁₂ were determined. A comparison of the k₁₂ values for different monomers against a standard polymer cation and that for different polymer cations against a standard monomer indicated that the effect of substituents on carbocation reactivity is much larger than their effect on monomer reactivity.