An Extraordinarily Rapid Polymerization of Vinylpentafluorocyclopropane:  Highly Stereo- and Regioselective Synthesis of Unsaturated Fluoropolymers^†

Vinylpentafluorocyclopropane 1 was prepared from the reaction of 1,1,2-trifluoro-4-bromobutene and hexafluoropropylene oxide at 190 °C, following by treatment with KOH. 1 is stable at low temperature (−40 °C) for 7 years, but it rearranged readily to 2,3,3,4,4-pentafluorocyclopentene-1, 2, at above 80 °C (E_a = 28.7 kcal/mol). Under radical conditions, 1 extraordinarily rapidly polymerizes to give highly crystalline Z-fluoropolyolefin (CF₂CF₂CFCHCH₂)_n, 3, which is very useful for cross-linking and grafting but difficult to obtain by other means. The stereochemistry of 3 was further confirmed by radical addition of iodine to 1 to form Z-ICF₂CF₂CFCHCH₂I, 4, exclusively. The rapid polymerization with high stereoselectivity and regioselectivity could be rationalized by effects of a favorable polar transition state of a high ring strain and electron-deficient pentafluorocyclopropyl and a relative electron-rich double bond of 1.