Radical homopolymerization of tetrafluoroethylene initiated by perfluorodiacyl peroxide in supercritical carbon dioxide: Reaction mechanism and initiation kinetics

Radical homopolymerizations of tetrafluoroethylene (TFE) in supercritical carbon dioxide (sc-CO2) initiated by bis(perfluoro-2-n-propoxypropionyl) peroxide (BPPP) were conducted. Low molecular weight polytetrafluoroethylenes (PTFEs) which are widely used in diverse fields with stable end groups were successfully obtained. PTFEs were characterized by solid-state 19F NMR and FT-IR spectroscopy. From rational assignment of the characteristic signals, an overall reaction mechanism explaining the homopolymerization processes is proposed. The carboxyl radicals resulted from thermal decomposition of BPPP were completely decarboxylated to n-C3F7OCF(CF3) before reacting with TFE. Additionally, a small amount of n-C3F7OCF(CF3) rearranged into n-C3F7 with decreased rearrangement fraction from 0.11 to 0.04 when the reaction temperature was lowered from 35 to 5 °C. Initiation rate constants (kd) were slightly increased with elevated pressure. The initiation activation energy derived from kd is 90.3 kJ mol−1, which is much lower than those of the other systems where nonfluorinated diacyl peroxides are used. Such mechanism and kinetics insights into the homopolymerizations of TFE in sc-CO2 will be instructive for the syntheses of fluoropolymers with desired properties in future.

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► The overall mechanism for TFE homopolymerization initiated by BPPP was presented.
► The carboxyl radical completely decarboxylated before reacting with TFE.
► A small fraction of n-C3F7OCF(CF3) radicals rearranged into n-C3F7 radicals.
► The initiation activation energy for BPPP is very low.