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 °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, kp/k12, where kp is the propagation rate constant of pMeOSt and k12 is the cross-propagation rate constant. On the basis of the reactivity ratios calculated f...