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Abstract A practical and direct electrophilic polymerization of hexafluoroacetone hydrate with diphenyl ether toward the preparation of semi‐fluorinated polyaryl ethers (PAE) is reported. Electrophilic aromatic substitution (EAS) polymerization under interfacial conditions with phase transfer catalyst (Aliquat 336) proceeds in trifluoromethanesulfonic anhydride by generation of trifluoromethanesulfonic acid and the protonated hexafluoroacetone (HFA) in situ affording 1,1,1,3,3,3‐hexafluoroisopropylidene (6F) PAE with high regioselectivity (4,4’‐DPE) and high molecular weight (≈60 kDa). Although first reported in a 1966 US Patent by DuPont using harsh conditions, improved synthetic methods or modern characterization has not been disclosed until now. Despite the presence of the 6F group, known to impart disordered morphology, this simple semi‐fluorinated PAE exhibits anomalous crystallinity with polymorphic melting points (T_m) ranging from 230–309 °C, high solubility in common organic solvents, a glass transition (T_g) of 163 °C, and thermo‐oxidative stability above 500 °C. Tough optically clear films prepared from solution give transmittance higher than 90% throughout the visible region. Synthesis, mechanistic aspects, and characterization including surface and dielectric properties are discussed.