Abstract: In the continuous catalytic polymerization of isobutylene, the exothermic heat of reaction is removed by vaporizing unreacted hydrocarbons, compressing and condensing the vaporized hydrocarbons to yield a liquid condensate at a pressure and temperature higher than that in the reaction zone, reducing the pressure on the liquid condensate to reduce its temperature to that of the reaction zone, separating any vapors thus formed and recycling the liquid condensate to the reaction zone. The process provides an energy efficient means of controlling reaction zone temperature. The process can be employed when using either a fluidized or fixed bed catalytic system.