Process for preparing high purity monocarboxylic perfluoropolyethers

A process for preparing monocarboxylic perfluoropolyethers of formula:

• A-O—(RF)z—(CFY)t—C(O)OX (I)
• wherein:
• X=H, a linear or branched, when possible, alkyl radical R with a number of carbon atoms between 1 and 10, or a C5-C7 cyclic radical, optionally containing one or more heteroatoms;
• Y=F, CF3;
• t is an integer from 1 to 3, t=1 when Y=CF3;
• t=1, 2 or 3 when Y=F;
• A is a linear or branched, when possible, C1-C4 perfluoroalkyl end group, one fluorine atom can be substituted by one chlorine atom;
• z is an integer 0 or 1;
• RF is a perfluorooxyalkylene chain containing one or more of the following units statistically distributed along the backbone:
• (C3F6O), (CFYO) wherein Y is F or CF3;
• (CF2CF2O), (CF2CF2CF2O), (CF2CF2CF2CF2O),
• having molecular weight or number average molecular weight in the range 180-8,000;
• comprising the following steps:
• a) one or more distillations of perfluoropolyethers of formula:
• T-O—(RF)Z-T′ (II)
• wherein:
• z and RF are as above;
• T, T′, equal to or different from each other, are selected from the functional end groups —CF2COF, —CF(CF3)COF, —CF2CF2COF, —CF2CF2CF2COF, —CF2C(O)CF3, —COF, and the non functional (neutral) end groups —CF3, —CF2CF3, —C3F7, —C4F9, one fluorine atom can be substituted by one chlorine atom, with the proviso that, when present, the neutral end groups are 20% molar at most with respect to the total of the end groups T and T′;
• to obtain a PFPE fraction of formula (II) wherein the difference between the minimum and the maximum molecular weight of the components, defined as molecular weight delta, is lower than or equal to 600, by operating at temperatures between −50° C. and +300° C. and at a pressure between 10−3 mbar and 4 bar;
• b) partial fluorination of the fraction obtained in a) until obtaining a content of neutral end groups from 40% to 60% molar with respect to the total of the end groups;
• c) esterification c1) and/or hydrolysis c2) as follows:
• c1) esterification of the mixture obtained in b) with an alcohol of formula R—OH wherein R is as above, in stoichiometric amount with the functional acylfluoride end groups of the mixture obtained in b) and distillation of the formed HF;
• or
• c2) hydrolysis of the mixture obtained in b) with water used in stoichiometric ratio with the functional acylfluoride end groups of the mixture obtained in b), by distilling the formed HF;
• d) distillation of the product obtained in c1) or in c2) by operating at temperatures from +20° C. to +300° C. and under a pressure between 10−3 mbar and 4 bar, so as to obtain a mixture of compounds of formula (I) wherein the molecular weight delta of the components of the mixture is the same of the fraction of the perfluoropolyethers of formula (II) obtained in step a).