the co 2 is introduced under an absolute pressure of 6 bars the beginning crystallization temperature of 15 u00b0 c is gradually brought to 12 u00b0 c during 5 h the duration of crystallization is 5 h the precipitate thus obtained is filtered and dried 9 g of dry product with 73 purity of u03b1 cd are collected the weight yield is 8 and the u03b1 cd yield is 15 str1 137 g of a mixture containing 35 by weight of u03b1 cd or 48 g of u03b1 cd are placed in solution in 500 g of exchanged water the stirred solution is thereafter solubilized by heating t u00b0 40 u00b0 c the solution thus obtained is not filtered the solution previously obtained is introduced into the reactor and a gas pressure is obtained as previously described the absolute pressure of co 2 is 6 bars the medium temperature of 20 u00b0 c is gradually brought to 8 u00b0 c during 2 h 30 min and is kept at 8 u00b0 c during an additional 3 h 10 min the precipitate is then dried there are obtained 24 g of dry product the weight yield is equal to 17 the content of u03b1 cd of the precipitate obtained is 80 or an u03b1 cd yield of 40 a solution containing 278 g of a mixture containing 49 weight of u03b1 cd is introduced into the reactor previously described the reactor is then flushed by reducing the pressure and introduction of nitrogen then ethylene is introduced under an absolute pressure of 2 bars after 1 h 50 min the pressure of ethylene is increased to 2 5 bars and maintained constant.