Improvements in the production of polymer solutions

In a process for producing solutions of acrylonitrile polymers containing at least 80% by weight of acrylonitrile units by polymerizing acrylonitrile or a mixture containing it in solution in a liquid which is also a solvent for the polymer, the solvent employed is a cyclic sulphone or sulphoxide comprising tetramethylene sulphoxide, pentamethylene sulphone, pentamethylene sulphoxide, or a 5- or 6-membered ring containing only the sulphone or sulphoxide group, an oxygen atom and carbon atoms. or a substituted cyclic sulphone or sulphoxide comprising a 5- or 6-membered ring containing only the sulphone or sulphoxide group, an oxygen atom if desired and carbon atoms wherein there is a single substituent which has no adverse effect on the polymerization. The medium may contain minor proportions of another solvent or a non-solvent, e.g. water. Specified solvents are tetramethylene sulphoxide, pentamethylene sulphone, pentamethylene sulphoxide, 1:4-thioxane-4-oxide, and compounds in which the ring carries a substituent such as methyl or methoxy, e.g. b -methyl tetramethylene sulphone. Specified comonomers are methyl acrylate, itaconic acid, styrene, vinyl acetate, vinyl pyridine and methallyl sulphonic acid and its salts. Polymerization may be effected in the presence of catalysts, e.g. azobisisobutyronitrile, and the solutions used for spinning fibres.