Organic solvents which are inert under the polymerization conditions prevailing may be used as solvents, in particular aromatic hydrocarbons, for example benzene, toluene or xylene; chlorinated aromatic hydrocarbons, for example chlorobenzene; aliphatic ketones and ethers, for example acetone, methyl ethyl ketone, diisopropyl ether or diethylether; and cyclic ethers, for example 1,4-dioxane.