Kinetic Study on the Capping Reaction of Living Polyisobutylene with 1,1-Diarylethylenes. 2. Effect of Chain Length

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 TiCl 4, was investigated in hexane (Hex)/methyl chloride (MeCl) 60/40 (v/v) at −80 °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 × 10 -2 M and at [DTE] < 3 × 10 -3 M. At higher than 3 ×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 .