ISSN:
0025-116X
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
The stereostructures of linear chloral oligomers were determined by X-ray single crystal analysis. The chloral oligomers from dimer to pentamer were isolated by distillation and by GPC fractionation from the oligomer mixture prepared with t-BuOLi initiation and acetate endcapping. The dimer fraction was a diastereomeric mixture of the racemo (1) and meso (2) isomers, while the trimer, tetramer and pentamer fractions each contained a single diastereomer (3, 4 and 5). The single crystals of 2, 3, 4 and 5 grown from methanol solutions belong to the space groups P1-, P21/n, P21/a and Pca21, respectively. X-ray analyses showed that the acetal backbones of 2, 3 and 4 consist exclusively of meso dyads. The backbone arrangements of 2, 3 and 4, are essentially identical and approximate the repeat gauche(+)-skew(-) sequences from the t-BuO group to the acetyl terminal group (for S-configurations). The conformation of (S,S,S,S)-4 is very similar to a unit sequence of a 4/1-helical structure of isotactic polychloral with left-handed helicity. The axis of the helix is parallel to the c-axis of the crystal, and the repeat distance is 4,81 Å. The conformations of the acetal backbones of 4 and 5 in solution were also investigated by NMR spectroscopy, on the basis of dihedral angle dependence of 3JC—O—C—H coupling constants.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/macp.1990.021910311
