Perfluoro ethers Rf-O-CX1X2-CX3X4-F are prepd. by addn. reaction of perfluorinated hypofluorites RfOF with perfluoroolefins CX1X2:CX3X4. The process is characterized by a molar ratio of hypofluorite to olefin in the reaction medium which is greater than unity. In the above formulas, Rf is linear or branched C1-5 perfluoroalkyl or perfluoro(oxy)alkylene contg. at least one O atom and optionally an SO2F or COF group; X1-4 are independently F or ORf. For example, a flow of 3.3 N L/h CF3OF dild. with 6.6 N L/h He and a flow of 3.0 N L/h CF3OCF:CF2 were directed into CFC 316 (100 g) at -60 for 7 h, followed by flushing of excess CF3OF with 3.3 N L/h He for 30 min, to show a 99.6% yield of addn. products, these being the desired CF3OCF2CF2OCF3 and the undesired (CF3O)2CFCF3, with a selectivity of 99.9%. In a comparison example, where the flow of both reactants was 3.3 N L/h, selectivity for the desired isomer dropped to 71.2%. In a similar example using CF2:CF2 as the olefin, selectivity for CF3OCF2CF3 over dimerization and polymn. products was likewise improved, from 48.3% with a slight excess of olefin, to 99.3% using an excess of hypofluorite according to the invention.
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