Preference is also given to solvents which have a boiling point at atmospheric pressure in the range from about 40 to 100??? C. Aprotic polar solvents, e.g. tetrahydrofuran, ethyl acetate, N-methylpyrrolidone, dimethylformamide, dimethylacetamide and, preferably, ketones, such as acetone and methyl ethyl ketone, are suitable.