A container for thermal conditioning of a fluid under pressure, use of the container for thermally conditioning a fluid under pressure, a chemical reactor including the container and a polymerization process using the reactor, and a method of producing the container and the reactor. The container is bounded over at least part of its surface facing the outside by at least one external plate and facing the inside by at least one internal plate, the external plate being thicker than the internal plate, the external plate and the internal plate being joined together in places to define a space between joints for circulation of a heat-transfer fluid and the container including a mechanism to adjust the pressure of the heat-transfer fluid to that of the fluid under pressure.