HELICAL CONFORMATIONS OF ISOTACTIC POLYACETALDEHYDE AND OTHER ISOTACTIC POLYTRIHALOACETALDEHYDES

Minimum potential energy helical conformations for a family of four is otactic polyacetaldehydes have been determined. Our results indicate t hat all of the polymers form irrational helices. Comparisons have been made with the reported structures for two of these stereoregular poly mers based on earlier X-ray diffraction data, c-Axis values associated with the pitch of the helix for polyacetaldehyde and for polytrichlor oacetaldehyde (polychloral) were experimentally measured to be 0.48 an d 0.51 nm, respectively. Our calculated conformations afforded values for a helix pitch of 0.47 and 0.52 nm, respectively, which derive from a 3.9/1 helix for polyacetaldehyde and a 3.7/1 helix for polychloral. The structure for polytribromoacetaldehyde (polybromal) was predicted to be similar to that for polychloral. For polytrifluoroacetaldehyde (polyfluoral) and polyacetaldehyde, a number of helical conformations with similar energies were found. All of these conformations could be related to the polychloral helical structure. (C) 1998 John Wiley & So ns, Inc.