The kinetic and thermodynamic parameters of the ionization-ion collapse of HCl, tent-butyl chloride (t-BuCl), and 2-chloro-2,4,4-trimethylpentane (TMPCl) in conjunction with ethyl-aluminum dichloride in CH3Cl or CH2Cl2 in the temperature range of -60 to -90 degreesC were determined under high-purity conditions by following the kinetics of the capping reaction of the corresponding cations with 1,1-ditolylethylene. The apparent rate (k(i)(app)) and equilibrium constant of ionization (K-i(app)) increased in the HCl < t-BuCl < TMPCl order. Both k(i)(app) and the rate of ion-collapse (k(-i)) increased with increasing temperature: however, because of the higher increase in k(-i), the apparent equilibrium constant of ionization increased with decreasing temperature. On the basis of the kinetic results, a new mechanism was proposed for the precipitation polymerization of IB in polar solvents. This mechanism involves slow ionization followed by diffusion-limited propagation until ion collapse and polymer precipitation. The calculated molecular weight and composite activation energy for the degree of polymerization based on this mechanism were in excellent agreement with experimental findings.