A single pass shell and tubes type polymerization reactor, used to produce butyl rubber in slurry, copolymerizing at least 95 wt% of isobutylene with not more than 5 wt% of isoprene, in concentration of 35 to 45% by vol. in a solvent, preferably methyl chloride, in presence of 0.015% to 0.15% by weight of catalyst, preferably aluminium chloride, and at a temperature within the range of - 104°C to -90°C, wherein the velocity in the tubes is the range from 2.4 to 4.5 m/sec, with the reaction in tube side and a cooling media in the shell side to remove the heat of reaction, wherein: - the slurry circulates inside the tubes and a central draft tube pushed by a rotating impeller in the lower part of the draft tube to permit high velocity of the rubber slurry in the central draft tube and in downflow smaller tubes, and - the reaction mixture is introduced from the bottom side near the impeller shaft and the outflow is in the top, characterized in that the reactor comprises: a - an extended upper head containing a fluid deflector, according to figure 2, wherein the shape of the head and of the deflector are sized 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; the vortex at the entrance of the tubes being also minimized, while is eliminated any bad slurry distribution; b - an hemispheric bottom head with a first fluid deflectors and a second fluid deflector, both according to figure 3 and, sized to minimize the pressure drop due to the inversion of flow from down-flow to up-flow; c - three straightening baffles in the draft tube, having a periodicity of 120° and designed as surfaces with a lower curved part and an upper straight radial part and the curved part is constructed as a sequence of radial segments starting from the bottom and going upward each one having an increased angular coordinate together with an increasing axial coordinates in order to turn the radial velocity components produced by the rotation of the impeller into axial velocity components, allowing an increased hydraulic efficiency, thereby, improving the homogeneity of the velocity and reducing the overall pressure drop, and allowing an increased circulation in the bottom pump and longer reactor cycles. - the slurry circulates inside the tubes and a central draft tube pushed by a rotating impeller in the lower part of the draft tube to permit high velocity of the rubber slurry in the central draft tube and in downflow smaller tubes, and - the reaction mixture is introduced from the bottom side near the impeller shaft and the outflow is in the top, a - an extended upper head containing a fluid deflector, according to figure 2, wherein the shape of the head and of the deflector are sized 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; the vortex at the entrance of the tubes being also minimized, while is eliminated any bad slurry distribution; b - an hemispheric bottom head with a first fluid deflectors and a second fluid deflector, both according to figure 3 and, sized to minimize the pressure drop due to the inversion of flow from down-flow to up-flow; c - three straightening baffles in the draft tube, having a periodicity of 120° and designed as surfaces with a lower curved part and an upper straight radial part and the curved part is constructed as a sequence of radial segments starting from the bottom and going upward each one having an increased angular coordinate together with an increasing axial coordinates in order to turn the radial velocity components produced by the rotation of the impeller into axial velocity components, allowing an increased hydraulic efficiency, thereby, improving the homogeneity of the velocity and reducing the overall pressure drop, and allowing an increased circulation in the bottom pump and longer reactor cycles.