Fluorinated ester monomer, making method, fluorinated ester polymer, and difluorohydroxycarboxylic acid

A polymer comprising recurring units having the general formula (1a):

• wherein R1 is hydrogen, methyl or trifluoromethyl, R2 and R3 are each independently hydrogen or a straight, branched or cyclic monovalent C1-C20 hydrocarbon group, or R2 and R3 taken together represent a divalent group which forms a C3-C20 hydrocarbon ring with the carbon atom to which they are attached, R4 is a straight, branched or cyclic, primary or secondary, monovalent hydrocarbon group of 1 to 20 carbon atoms in which a hydrogen atom may be replaced by a fluorine atom or in which a methylene moiety may be replaced by an oxygen atom or carbonyl radical, and k is 0 or 1.

A difluorohydroxycarboxylic acid compound having the general formula (3), obtained by treating a pentafluorohydroxyketone compound having the general formula (2) with a base,

• wherein R2 and R3 are each independently hydrogen or a straight, branched or cyclic monovalent C1-C20 hydrocarbon group, or R2 and R3 taken together represent a divalent group which forms a C3-C20 hydrocarbon ring with the carbon atom to which they are attached.