The crystal structures of two cyclic polyurethane oligomers, cyclobisurethane and cyclotetraurethane

Abstract

The crystal structures of cyclobisurethane, C12H22N204, and cyclotetraurethane, C24H44N408, have been determined by single-crystal X-ray diffraction analysis. Intensities were collected with an automated XRD-5 diffractometer using Cu K~ radiation. Cyclobisurethane crystallizes in the monoclinic space group C2/c, with a= 17-330 (2), b=8-866 (1), c= 17.944 (3)~, ,8=91.77 (1) °, and Z=8. Cyclotetraurethane crystallizes in monoclinic space group P21/c, with a= 16-463 (4), b = 9.929 (2), c = 9"464 (2) A, fl= 110-19 (3) °, and Z=2. The structures were solved by direct methods and refined by full-matrix least squares to final R values of 0.056 and 0.055 respectively. The molecules of both structures are linked two-dimensionally into sheets by C=O...H-N hydrogen bonds. However, in cyclobisurethane all urethane groups participate in hydrogen bonding, whereas in cyclotetraurethane one carbonyl oxygen accepts two hydrogens, one from each of the crystallographically nonequivalent nitrogens. The carbonyl oxygen not participating in hydrogen bonding appears to interact weakly with a methylene hydrogen of a molecule in an adjoining layer. Cyclobisurethane furnishes conformational models for regular adjacent reentry chain folding in the corresponding polyurethane system. The hydrogen-bonding patterns from both structures provide possible models for hydrogen bonding within hard segment domains of segmented urethane elastomers. The structures of these two polyurethane oligomers are compared with those of the analogous cyclic polyhexamethylene adipamide oligomers.

Structural Studies on Linear Polyurethanes. II. Crystal and Molecular Structures of Aliphatic Polyurethanes from Trimethylene Diisocyanate, and Tetra- and Hexa-Methylene Glycols

“…In the study of polymer structure, we usually try to calculate intra-(and/or inter-) molecular potential energy to find one or more molecular models which will minimize the potential energy and satisfy other conditions. In this study, a more conventional o.o876 (6.5 A 2 ) 0.0022 (4.5 A 2 ) 0.0012 (3.5 A 2 ) 0.0734 (7.0A 2 ) 0.0627 (6.0A 2 ) 0.0062 (6.0A 2 ) 0.0028 (5.0 A 2 ) 0.0062 (6.0 A 2 ) o.oo28 (5. method was used to find the most suitable model for polyurethanes, i.e., the molecular structure that would form full hydrogen bonding between neighbouring chains. Hydrogen bonds are stronger than van der Waals bonds, but much weaker than covalent bonds, and play an important role in the structure and function of polypeptides, 9 • 10 polyamides, 12 polyurethanes 1 • 2 and numerous other polymers.…”

“…The assumed bond lengths and bond angles are shown in Figure 3 and are those of cyclic urethane oligomers determined by single-crystal X-ray diffraction analysis by Beres and Alexander. 2 The fiber periods for the planar zig-zag conformations of 4,3-and 6,3-polyurethanes were calculated to be 31.46A and 36.50A, respectively, assuming two chemical repeating units per fiber period. Comparison of the measured fiber identity periods with those calculated indicates that the polymer chains in the crystal should be distorted from the planar zig-zag.…”