SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDIES OF AROMATIC OLIGOMERS - RESOLUTION OF THE BOND-ANGLE ANOMALY IN POLY(ARYLETHERKETONE)S

To resolve long-standing uncertainties surrounding the bridge-bond angles of crystalline poly(aryletherketone)s, the structure of an oligomeric ether-ketone, ClArCOArOArCOArCl (Ar = 1,4-phenylene), has been determined by single crystal X-ray methods. The average torsion angle phi of the two 'ether-ketone' rings, relative to a plane defined by the bridging oxygen and carbonyl-carbon atoms is 31-degrees. The bond angles at carbonyl-carbon and ether-oxygen are 121.9(3)-degrees and 121.2(3)-degrees, respectively, values which are substantially lower than those consistently reported for poly(aryletherketone)s on the basis of X-ray powder- and fibre-diffraction studies. [An average bridge-bond angle of 126.5(6)-degrees has, for example, been proposed to account for the c-axis length of 10.09 (2) angstrom in the prototype poly (etherketone) (PEK) (-OArCOAr-)n.] Despite this apparent difference in bridge-bond angles, a PEK c-axis distance of 10.11 (1) angstrom, entirely consistent with that of the polymer, is found in the present oligomer structure. More detailed analysis of the oligomer structure reveals a pattern of sterically induced bond angle distortions at the aromatic carbon atom linked to an ether or ketone bridge, a pattern that is repeated in the X-ray structure of a second oligomer ArOArCOArArCOArOAr (Ar = phenyl or 1,4-phenylene). These findings strongly suggest that bridge-bond angles in poly(aryletherketone)s actually lie in the region of 121-122-degrees, and that the range 125-127-degrees consistently reported in the literature is an artifact deriving from the assumption of ideal geometry elsewhere in the polymer chain.