Quantum-Mechanical Study of the Reaction Mechanism for 22π-27π Cycloaddition of Fluorinated Methylene Groups

Perfluorocyclobutyl polymers are thermally and chemically stable, may be produced without a catalyst via thermal 2 pi-2 pi cycloaddition, and can form block structures, making them suitable for commercialization of specialty polymers. Thermal 2 pi-2 pi cycloaddition is a rare reaction that begins in the singlet state and proceeds through a triplet intermediate to form an energetically stable four-membered ring in the singlet state. This reaction involves two changes in spin state and, thus, two spin-crossover transitions. Presented here are density functional theory calculations that evaluate the energetics and reaction mechanisms for the dimerizations of two different polyfluorinated precursors, 1,1,2-trifluoro-2(trifluoromethoxy)ethane and hexafluoropropylene. The spin crossover transition states are thoroughly investigated, revealing important kinetics steps and an activation energy for the gas phase cycloaddition of two hexafluoropropene molecules of 36.9 kcal/mol, which is in good agreement with the experimentally determined value of 34.3 kcal/mol. It is found that the first carbon carbon bond formation is the rate-limiting step, followed by a rotation about the newly formed bond in the triplet state that results in the formation of the second carbon carbon bond. Targeting the rotation of the C-C bond, a set of parameters were obtained that best produce high molecular weight polymers using this chemistry.