Photooxidation of functionalized perfluorinated polyethers—II

Abstract

Three functionalized perfluorinated polyethers (PFPE) were exposed to UV light ( λ >300 nm) in air, and chemical changes recorded by FTIR and 1H, 13C, 19F NMR spectrometry. One of the derivatives (ZTET), end-capped by an hydrogenated ether segment and containing four hydroxyl groups per molecule, was found to readily undergo photooxidation as shown by rapid development of carbonyl oxidation species and a corresponding decrease in C–H character. The carbonyl products were assigned to formate and ester groups respectively, which formation can be described by hydroperoxidation of the functionalized chain ends followed by their photolysis. The formate group is produced as a result of beta-scission of the secondary hydroperoxide γ to the perfluorinated chain and is further transformed to a hydroxymethyl chain end group (ZDOL) stabilised by the shielding effect of the adjacent perfluoromethylene group. The ester formation can be accounted for by oxidation of the methylene group α to the perfluorinated chain giving α carboxylic group which then reacts with ZDOL. The final oxidation product of ZTET is thus a polymer made of fluorinated polyether segments linked by an ester bond. The modification of ZTET by urethane or ester end-caps, in order to simulate crosslinks that are present in an eventual isocyanate-cured polyurethane or polyester coating, strongly increased the photostability of the compound. This marked increase in stability was attributed to both chemical and physical modification of the structure.