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

摘要

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 deg C using on-line visible spectroscopy. The apparent equilibrium constants of capping/decapping, K_e, and the apparent rate constants of capping, k_cK_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_oK_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_oK_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.