Abstract: In a process for polymerizing a monomer capable of being formed, upon polymerization, into a solution having an upper cloud point of the corresponding polymer in a reaction medium being liquid under the reaction conditions, the polymerization being carried out under such conditions that the resulting polymer dissolves in the reaction medium; the improvement wherein(i) the polymerization is carried out in a polymerization zone at a temperature above the upper cloud point of said polymer solution and under conditions which enable the polymer solution to be separated into two phases,(ii) the polymerization is carried out under stirring conditions which maintain the two phases under said phase-separating conditions in a dispersed and mixed state, and(iii) the resulting polymer solution is sent to a separating zone located independently of said polymerization zone, thereby separating it into two phases in the separating zone at a temperature above the upper cloud point, and thereafter, the polymer-rich liquid phas