The addition of 1,1-diphenylethylene (DPE) and 1,1-bis(4-methylphenyl)ethylene (DTE) to hydrochlorinated isobutylene (IB) n-mers, H-[IB](n)-Cl (n = 2, 3, 4, 36), in the presence of TiCl4, was investigated in hexane (Hex)/methyl chloride (MeCl) 60/40 (v/v) at -80 degrees C using on-line visible spectroscopy. The apparent equilibrium constants of capping/decapping, K-e, and the apparent rate constants of capping, k(c)K(i) have been determined. The K-e value was found to be similar for the capping reaction of all investigated IB n-mers with DPE, and they were about 10 times higher for DTE but independent of the chain length n. The value of k(c)K(i) increased with increasing n for the capping with both DPE and DTE: For n = 3, 4, and 36, it was approximately three, four, and five times higher, respectively, than for n = 2. The capping reaction of IB n-mers was approximately fifteen times faster with DTE than with DPE, which could be ascribed to a higher reactivity of DTE compared to DPE. The capping reaction was found to be first order in [diarylethylene] at [DPE] < 2 x 10(-2) M and at [DTE] < 3 x 10(-3) M. At higher than 3 x 10(-3) M DTE concentrations, the order in [DTE] was lower than unity, and the apparent initial rate of addition became independent of DTE concentration at [DTE] > 10(-1) M. The change in the reaction order from unity to zero was attributed to a change in the rate-determining step from addition of DTE to the H-[IB](n)(+) cation to ionization of H-[IB](n)-Cl. The rate constant of ionization, k(i) = 6, 11, and 15 M-2 s(-1), could then be calculated for the hydrochlorinated IB 2-, 3-, and 36-mers, respectively. Comparison of these k(i) values with the corresponding k(c)K(i) values indicated that the observed increase of the apparent rate constant of capping with increasing chain length can be mainly ascribed to a similar increase in k(i) and K-i.