Polymorphism of the co-crystalline forms of syndiotactic polystyrene with chloroform: Crystal structure of the δ clathrate

The crystal structure of the delta clathrate form of syndiotactic polystyrene (s-PS) containing CHCl3, a molecule having a pivotal role in respect to the co-crystalline phases formation of this polymer, has been determined through X-ray diffraction data and molecular mechanics calculations. Analogously to all the other 8 clathrate forms of s-PS, this structure presents a monoclinic unit cell (cell constants a = 1.77 nm, b = 1.32 nm, c = 0.78 nm and gamma = 121.5 degrees) in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P2(1)/a. At variance with all the other delta clathrate forms of s-PS whose crystal structure has been reported in the literature, probably due to the not planar shape of the chloroform guest molecule, in this structure guest molecules occupy each centrosymmetric cavity in a very low efficient way, giving rise to a disorder in the positioning of the guest molecules along the b + a/2 direction of the unit cell. A comparison with the e type clathrate with the same guest, for which some preliminary results have been reported too, is also presented. (C) 2009 Elsevier Ltd. All rights reserved.