The addition reaction of isobutylene (IB), styrene (St), p-methylstyrene (pMeSt), and p-chlorostyrene (pClSt) to living poly(p-methoxystyrene) (PpMeOSt) obtained using the 1-chloro-1-(p-methoxyphenyl)ethane/tin tetrabromide (pMeOStCl/SnBr4) initiating system in dichloromethane (CH2Cl2) and CH2Cl2/methylcyclohexane (MeCHx) from -40 to 0 degrees C in the presence of 2,6-di-tert-butylpyridine (DTBP) as a proton trap was studied. Quantitative crossover to IB, St, pMeSt, and pClSt was observed, and changing the temperature, solvent polarity, and concentrations could control the number of capping monomer units incorporated per chain before irreversible ion collapse. Accordingly, the polymerization of pMeOSt in the presence of IB, St, pMeSt, or pClSt stopped short of completion. These so-called competition experiments were used to determine the reactivity ratios, k(p)/k(12), where k(p) is the propagation rate constant of pMeOSt and k(12) is the cross-propagation rate constant. On the basis of the reactivity ratios calculated from the limiting conversions and limiting molecular weights, in CH2Cl2/MeCHx at -40 degrees C, pMeOSt is 42, 271, 291, and 771 times more reactive than pMeSt, IB, St, and pClSt, respectively. A similar reactivity order was established with CH2Cl2 as solvent at -40 degrees C. With increasing temperature differences in reactivities decreased. With the published k(p) values the cross-propagation rate constants were calculated. A comparison of the k(12) values and the propagation rate constants of the different monomers reported before indicated that substituents have a much larger effect on the cation reactivity than on the monomer reactivity.