The polymerization reactivity of isobutylene/SnCl 4 mixtures in the absence of polar solvent, was investigated in a temperature interval from Ϫ78 to 60 °C. The mixture is nonreactive below Ϫ20 °C but slow polymerization proceeds from Ϫ20 to 20 °C with the initial rate r 0 of the order 10 Ϫ5 mol ⅐ l Ϫ1. s Ϫ1 . The rate of the process increases with increasing temperature up to ϳ10 Ϫ2 mol ⅐ l Ϫ1 ⅐ s Ϫ1 at 60 °C. Logarithmic plots of r 0 and M n versus 1/T exhibit a break in the range from 20 to 35 °C. Activation energy is positive with values E ϭ 21.7 Ϯ 4.2 kJ/mol in the temperature interval from Ϫ20 to 35 °C and E ϭ 159.5 Ϯ 4.2 kJ/mol in the interval from 35 to 60 °C. The values of activation enthalpy difference of molecular weights in these temperature intervals are ⌬H Mn ϭ Ϫ12.7 Ϯ 4.2 kJ/mol and Ϫ38.3 Ϯ 4.2 kJ/mol, respectively. The polymerization proceeds quantitatively, the molecular weights of products are relatively high, M n ϭ 1500 -2500 at 35 °C and about 600 at 60 °C. It is assumed that initiation proceeds via [isobutylene ⅐ SnCl 4 ] charge transfer complex which is thermally excited and gives isobutylene radical-cations. Oxygen inhibits the polymerization from Ϫ20 to 20 °C. Possible role of traces of water at temperatures above 20 °C is discussed. It was verified by NMR analysis that only low molecular weight polyisobutylenes are formed with high contents of exo-terminal unsaturated structures. In addition to standard unsaturated groups, new structures were detected in the products.