Conformational energies of perfluoroalkanes. Part 4.—Preferred conformations of isotactic polyhexafluoropropylene

Abstract

A semi-empirical energy equation has been used to calculate conformational energies of isotactic polyhexafluoroprophylene (PHFP) as a function of its internal rotation angles ϕ about the C—C bonds. These calculations predict that the most stable conformation is a helix characterized by a CCC valence angle close to 113° and a repetition of rotation angles (ϕ′,ϕ″)(ϕ′,ϕ″)…, where ϕ′ and ϕ″ are the rotation angles about the CF(CF₃)—CF₂ and CF₂—CF(CF₃) bonds respectively. The predicted values of (ϕ′,ϕ″) are in the range (40°+Δϕ,–100°–Δϕ), where Δϕ < 10°. An equivalent energy conformation, characterized by (ϕ′,ϕ″)=(100°+Δϕ, –40°–Δϕ), corresponds to a reversal in the pitch of the helix. The number of monomer units per turn of the helix is 3.87 ± 0.04 which agrees satisfactorily with the experimental value of 4.0 for crystalline isotactic PHFP.