Computational investigation of the kinetic and thermodynamic parameters governing polytetrafluoroethylene chain shortening.

Mechanisms of fluoropolymer pyrolysis are poorly understood. Oversimplifications in both experimental and computational studies related to high temperature decomposition of PTFE have led to the elucidation of potentially incomplete and inaccurate pathways. While most conflicts within the literature claim that individual decomposition occurs through either CF elimination or CF elimination, one mechanism that is largely ignored is the thermodynamically favored 1,2-F atom transfer. Quantum chemical calculations with M06-2X/6-311+G(d,p) level of theory were applied to a PTFE model to investigate the thermodynamic and kinetic barriers of their primary thermal decomposition mechanisms, and to a six carbon perfluoroalkyl radical to investigate the kinetics and thermodynamics of chain shortening. The present computational study aims first to provide activation energies and kinetic rate constants at a range of temperatures from 500 to 1500 K for three primary mechanisms of perfluoroalkyl radical chain shortening. These primary mechanisms include: 1) α-scission forming difluorocarbene, 2) β-scission forming CF monomer fragments, and 3) 1,2-F atom transfer followed by β-scission to form CF monomer fragments. Reaction energetics of these three pathways showed that CF elimination required the highest activation energy barrier regardless of chain length and temperature, and CF elimination was kinetically favored at all temperatures.