The addition reaction of 1,1-diphenylethylene to polymeric and dimeric isobutylene living chain ends in the presence of TiCl4 was investigated in hexane/MeCl 60/40 (v/v) at -80 to -40 degrees C using on-line W-vis spectroscopy. The apparent equilibrium constants of capping/decapping, K-e, and the apparent rate constants of capping, k(c)K(i), were directly measured, and from these values, the corresponding rate constants of decapping, k(d), were calculated. The activation energy is found to be apparently negative for the capping reaction (k(c)K(i)) and positive for the decapping reaction (ha). However, the respective activation parameters for the polyisobutylene differ significantly from those of the dimer; i.e., the chain length affects the kinetics of the capping/decapping reaction.