Chain-conformation and chain-folding in 'PK99': evidence from single-crystal X-ray studies of linear and cyclic oligomers

The structure of a seven-ring model for the crystalline polyetherketone known as 'PK99', has been determined by single crystal X-ray methods. The central benzene-1,3-dicarbonyl group in this oligomer adopts an 'open' conformation, approximately in agreement with that proposed earlier for PK99 on the basis of X-ray powder and fibre diffraction data. The present oligomer structure land a polymer structure simulated by extrapolation from the oligomer) do not, however, require the benzene-1,3-dicarbonyl group to adopt the previously assumed coplanar conformation. A single crystal X-ray study of a cyclic ether-ketone oligomer, also based on the benzene-1,3-dicarbonyl unit, demonstrates the potential of this group to act as a site for sharply re-entrant chain-folding. (C) 1998 Elsevier Science Ltd. All rights reserved.