Countercurrent fractionation of methylol-terminated perfluoropolyoxyalkylene oligomers by supercritical carbon dioxide

Abstract

A methylol-terminated perfluoropolyether having a broad molecular weight distribution has been countercurrent fractionated with supercritical carbon dioxide using an isothermal increasing pressure profile. Depending on the selected physical conditions, a fractionation as a function of the molecular weight has been observed while the influence of different end-groups, present as admixtures, on the solubility was negligible. As a consequence, the different –CF2H, –CF2Cl, –CF2CH2OH and –COOX (X = CH2CH3 or H) end-groups content in the fractions reflected their non-statistical distribution in the starting mixture. This last finding opens the way to a deeper investigation of the reason for this structural diversification. Based on a full evaluation of the multi-step process for the synthesis of the methylol-terminated perfluoropolyether it appears that these end-groups originate from different stability/reactivity of PFPE-reagents and PFPE-intermediates involved in the whole process. These differences are not only function of the specific reaction and experimental conditions considered, but also depend on the PFPE chain length (i.e. molecular weight of the species).