The polymerization of p-methylstyrene (pMeSt) in the presence of isobutylene (113), styrene (St), p-chlorostyrene)(pClSt), and 1,3-butadiene (BD) was carried out at -40 degrees C. By selecting CH2Cl2/methylcyclohexane (MeCHx) 50/50 (v/v) solvent mixture and the appropriate weak Lewis acid, SnBrCl2 monoaddition of IB, St, pClSt, and BD followed by instantaneous termination was achieved, and the polymerizations stopped short of completion. The reactivity ratios, k(p)/k(12), where k(p) is propagation rate constant and k(12) is the cross-propagation rate constant, were calculated on the basis of limiting conversions and limiting molecular weights. According to these values, pMeSt is 3.8 +/- 0.5, 4.8 +/- 1.1, 7.2 +/- 1.6, and 100 +/- 6 times more reactive than IB, St, pClSt, and BD respectively. Similar competition reactions between IB/BD, St/BD, and pClSt/BD in conjunction with TiCl4 as Lewis acid yielded k(p)/k(12) = 77 +/- 5, 7.1 +/- 1.0, and 3.2 +/- 0.7, respectively. Using k(p)/k(12) and k(p) for pMeSt, St, and pClSt, the cross-propagation k(12) values were calculated to establish structure-reactivity scales. Similarly to our earlier observations, comparison of these values indicated that structural differences have a much larger effect on cation reactivity than on monomer reactivity.