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 polyetherketon e 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 ear lier 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 conforma tion. A single crystal X-ray study of a cyclic ether-ketone oligomer, also based on the benzene-1,3-dicarbonyl unit, demonstrates the potent ial of this group to act as a site for sharply re-entrant chain-foldin g. (C) 1998 Elsevier Science Ltd. All rights reserved.