A low temperature polymerization reactor for the production of butyl rubber by catalytic polymerization of isobutylene with small amount of a conjugated diolefin such as isoprene includes: an extended upper head with fluid deflector fixed to the top tube-sheet, the shapes of the head and deflector being optimized in order to reach a very homogeneous slurry velocity in the rows of tubes and to minimize the pressure drop due to the inversion of flow from up-flow to down-flow and to the entrance of the slurry in the tubes; an hemispheric bottom head with fluid deflectors placed between the impeller and the bottom itself and designed to minimize the pressure drop due to the inversion of flow from down-flow to up-flow; and straightening baffles inside the draft tube, which shape and dimensions are optimized to turn the radial velocity components produced by the rotation of the impeller into axial velocity components.