Single crystal X-ray diffraction studies of aromatic oligomers: resolution of the bond-angle anomaly in poly(aryletherketone)s

• Published in: Polymer (Guilford) 1993, Vol.34 (1), p.218-221
• Main Authors: Colquhoun, Howard M., O'Mahoney, Caroline A., Williams, David J.
• Format: Article
• Language: English
• Subjects: Applied sciences; crystal structure; Exact sciences and technology; oligomer; Organic polymers; Physicochemistry of polymers; poly(aryletherketone); Properties and characterization; Structure, morphology and analysis; X-ray
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/0032-3861(93)90310-7

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 ø of the two ‘ether-ketone’ rings, relative to a plane defined by the bridging oxygen and carbonyl-carbon atoms is 31°. The bond angles at carbonyl-carbon and ether-oxygen are 121.9(3)° and 121.2(3)°, 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)° has, for example, been proposed to account for the c-axis length of 10.09(2) Å 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) Ǎ, 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°, and that the range 125–127° consistently reported in the literature is an artifact deriving from the assumption of ideal geometry elsewhere in the polymer chain.