Cyclic oligomers of poly(ether ketone) (PEK): synthesis, extraction from polymer, fractionation, and characterisation of the cyclic trimer, tetramer and pentamer

Abstract

Exhaustive extraction of poly(ether ketone) (PEK) with hot chloroform affords a novel range of cyclic oligo(ether ketone)s. The proportion of cyclic material extracted in this way ranged from as high as 13% for laboratory-synthesised PEK to as low as 2% for an industrial development-grade polymer. Extracts were characterised by 1H NMR spectroscopy, GPC, and MALDI-TOF mass spectrometry, which indicated the presence of ring sizes from the cyclic trimer up to at least the cyclic decamer. These macrocyclic oligomers were also prepared (in ca. 80% total yield) by self-polycondensation of 4-fluoro-4′-hydroxybenzophenone under pseudo-high-dilution conditions. Column chromatography on silica gel enabled isolation of the cyclic trimer, cyclic tetramer and cyclic pentamer as pure compounds, characterised by NMR spectroscopy, mass spectrometry, elemental analysis and (for the cyclic trimer and cyclic tetramer) X-ray crystallography. Ring-opening polymerisation of mixed cyclic oligomers was achieved in the melt at 385 °C, using the potassium salt of 4-hydroxybenzophenone as initiator.