Degradation of perfluoropolyethers catalyzed by aluminum chloride

Abstract

Thermal degradation processes of perfluoropolyethers, Fomblin Y, Krytox, and Demnum, catalyzed by Lewis acid (AlCl3) were examined. 19F NMR analyses of degraded materials revealed that the acid catalyzed disproportionation reaction of the form R-CF2-O-:CF2R→R-F{divides}CO+Cf3-R dominates the degradation process. The reaction occurs most readily at the acetal sectors of the polymer chains, -O-CF2-O-, and at, in order of increasing difficulty, longer linear polyether sectors, -O-CF2-CF2-CF2-O-, found in Demnum, and alkoxy end-groups, CF3-O-CF2- and CF3-CF2-O-CF2-, found in Fomblin Y and Krytox. Internal sectors with pendant CF3 group as those in Krytox and alkoxy end-groups adjacent to -CF(CF3)- are spared from or much more resistant to the process. When the reaction occurs at alkoxy end-groups, the fluorine atom transfer is from the end-group to the internal sector. Thus the process, when it occurs at internal units, results in fragmentation of polymer chains, and formation of chain segments having acylfluoride end-group, R-CFO. When it occurs at end-groups, the acylflouride group is formed in the leaving monomer unit, and the chain length is decremented stepwise. It is shown that Demnum degrades by fragmentation while Krytox degrades through successive losses of end-groups. It is suggested that reaction (1) occurs when differential charges are induced at the two carbon atoms of an ether linkage, e.g. R-C+aF2-O-C+a+χF2-R, and that such differential induction occurs when two successive ether oxygens of a perfluoropolyether chain interact with Lewis site(s). © 1991.