Determination of Propagation Rate Constants in Carbocationic Polymerization of Olefins. 1. Isobutylene

The propagation rate constant for ion pairs (k p ± ) in the polymerization of isobutylene in conjunction with TiCl4 in hexanes/methyl chloride 60/40 (v/v) at −80 °C has been determined using two different diffusion clock methods. The rate constant k p ± was in the range of (0.3−1.0) × 109 L mol-1 s-1, 4 orders of magnitude higher than presently accepted values. The first method involved on-line UV−vis monitoring of the addition of the π-nucleophiles 1,1-bis(4-methylphenyl)ethylene, 1,1-bis(4-tert-butylphenyl)ethylene, and 2-phenylfuran to hydrochlorinated isobutylene n-mers (n = 2, 3, 36). The apparent rate constants of capping, k c K i, and the rate constant of ionization, k i, have been determined. For a given n the k c K i values were identical independent of the nature and nucleophilicity of the π-nucleophile, which was attributed to diffusion-limited addition. Using the diffusion-limited second-order rate constant of k c ∼ 3 × 109 L mol-1 s-1, K i and k - i have been calculated. From the concentration of active chain ends (determined from K i) and the apparent rate constant of propagation for isobutylene (k p ± [active chain ends], determined separately), the absolute propagation rate constant of k p ± = 1 × 109 L mol-1 s-1 was calculated. The second simple diffusion clock method involved competition experiments, i.e., polymerization carried out in the presence of a π-nucleophile, which stops short of completion when all chain ends are capped. From the limiting conversions and number-average degrees of polymerization, k p ± = (3−6) × 108 L mol-1 s-1 have been obtained.