The propagation rate constant, k(p), for the cationic polymerization of alpha-methylstyrene (alpha MeSt) was determined. First, the polymerization of alpha MeSt was carried out initiated by the di alpha-MeStHCl/SnBr4 initiator/coinitiator system in dichloromethane (DCM) or in DCM/methylcyclohexane (MeCHx) 1: 1 (v/v) solvent mixture at -80 degrees 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) x 10(7) L mol(-1) s(-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 alpha MeSt 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 degrees C. From the limiting conversions and molecular weights, the reactivity ratios k(p)/k(12) were determined. A comparison of the k(12) 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 oil carbocation reactivity is much larger than their effect on monomer reactivity.