The invention relates to an arrangement for continuous production of polymer including a polymerisation reactor. The reactor includes a first pulley and a second pulley arranged at a distance from each other, which distance, defining a longitudinal first direction, and a flexible endless belt having a first edge section and a second edge section, which belt, is arranged to travel over the first pulley and the second pulley as a flat belt. The reactor further includes a supporting structure, which is arranged to support the flexible endless belt between the first pulley and the second pulley, whereby the supporting structure is arranged to form the flexible endless belt into a trough section by deviating the first edge section and the second edge section of the flexible endless belt away from the first direction. The trough section has a width-to-height ratio