Novel .beta.-carbonylpolyfluoroalkyl sulfonate esters, e.g., ##STR1## are prepared by reacting SO.sub.3 with the corresponding acyclic fluorovinyl esters, e.g., CF.sub.3 CF.dbd.CFOCH.sub.3, at a temperature of about -40.degree. to 110.degree. C. These esters are useful as very active cationic initiators, e.g., for the polymerization of tetrahydrofuran.