Quantum-Mechanical Study of the Reaction Mechanism for 2π-2π Cycloaddition of Fluorinated Methylene Groups.

Perfluorocyclobutyl polymers are thermally and chemically stable, may be produced without a catalyst via thermal 2π-2π cycloaddition, and can form block structures, making them suitable for commercialization of specialty polymers. Thermal 2π-2π 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.