The capping reaction of living polyisobutylene cation (PIB+) with 1-butene, cis-2-butene, trans-2-butene, or 1,3-butadiene in hexanes (Hex)/methyl chloride (MeCl) 60/40 (v/v) solvent mixtures at -80 degrees C was studied. The reaction products were characterized by elemental analysis, gel permeation chromatography, and NMR spectroscopy. Monoaddition (capping) was observed with 1-butene and cis-2-butene but not with trans-2-butene, which did not react with PIB+. Monoaddition of 1,3-butadiene followed by instantaneous halide transfer from the counteranion and selective formation of the 1,4-adduct were observed in Hex/MeCl 60/40 (v/v) solvent mixtures at -80 degrees C at [1,3-butadiene] <= 0.05 mol L-1. The polymerization of isobutylene (IB) was studied in the presence of 1-butene, cis-2-butene, trans-2-butene, or 1,3-butadiene as capping agent in Hex/MeCl 80/20 to 40/60 (v/v) in the temperature range of -40 to -80 C. From the limiting conversion and limiting number-average degrees of polymerization the reactivity ratio, k(p)(+/-)/k(c)+/- (where k(p)(+/-) is the absolute rate constant of propagation for IB and k(c)(+/-) (is the cross-propagation (capping) rate constant from PIB+ cation to the C4 olefins), was calculated. Since the propagation of IB (addition of IB to PIB+) does not have an enthalpic barrier, the activation energy for cross-propagation was calculated to be 5.1-6.0 kJ mol(-1). By extrapolating the plot of k(p)(+/-)/k(c)(+/-) vs hexanes (vol %), the reactivity ratios were calculated for 100% hexanes at - 80 degrees C. From the cross-propagation activation energy and the calculated reactivity ratios at - 80 C in 100% hexanes, the reactivity ratios for 0 degrees C have also been calculated. According to these values at 0 degrees C, isobutylene is similar to 17 times more reactive than 1,3-butadiene, similar to 543 times more reactive than 1-butene, and similar to 294 times more reactive than cis-2-butene.