A COMPUTER MODELING STUDY ON THE INTERACTION OF -(CF2CF2O)-POLYPERFLUORINATED ETHERS WITH LEWIS-ACID SITES - PERFLUORODIETHYL ETHER

Lewis acid catalysis significantly enhances the thermally induced degr adation of polyperfluorinated ether lubricants. We have used CF3CF2OCF 2CF3 and AlF3 as models to investigate via ab initio theory, the Lewis acid-base interactions between polyperfluorinated ethers and Lewis ac id sites. The results of these studies indicate that perfluorodiethyl ether, or PFDEE, may form a Lewis acid-base pair between the ether oxy gen atom in PFDEE and the aluminum atom in AlF3 provided that there is no steric hindrance limiting access to the ether oxygen atom. This ma y be achieved, for example, by PFDEE adopting a trans-gauche instead o f a trans-trans conformation. In the trans-trans conformation, a termi nal perfluoromethyl group sterically interferes with the accessibility of the ether oxygen atom to the AlF3 surface. These results indicate that in lubricant molecules such as the 'Z' family of Fomblin fluids, composed of a copolymer of perfluoromethylene and perfluoroethylene ox ide units, steric hindrance will limit AlF3 access to the ether oxygen atom and would depend upon the ratio of-CF2CF2O and -CF2O- structural units and their spatial distribution. Even when the -CF2CF2OCF2CF2- P FDEE backbone adopts a local trans-gauche conformation such as to spat ially allow a Lewis acid-base interaction between the ether oxygen ato m and aluminum, the interatomic distance is much greater, 2.5 Angstrom compared to 1.8 Angstrom for perfluoromethylene oxide. The binding en ergy is correspondingly smaller, - 3 compared to - 9 kcal/mol, respect ively. Several degradation paths for the decomposition of PFDEE, origi nating within the -CF2OCF2- structural group, were computed. Thus, the decomposition of PFDEE to CF3CF3 and CF3CFO via an initial cleavage o f a C-O bond, and, alternatively, via a transition state, were investi gated. In both cases, the activation energy to initiate the decomposit ion is rather high, near 80-100 kcal/mol. However, in the presence of an AlF3 catalyst, the activation energy for the decomposition reaction via the transition structure is significantly reduced to 50 kcal/mol. Alternatively, a C-O bond scission leading to the formation of radica l sites adjacent to -CF2O- structural units may lead to mass loss via elimination of COF2. Such a reaction proceeds with an activation energ y of only 24 kcal/mol. (C) 1998 Elsevier Science S.A. All rights reser ved.