Crystal Structure of Form I of Poly[tris(ethylene sulfide)- alt -tris(ethylene oxide)]

The crystal structure of form I of poly[tris(ethylene sulfide)-alt-tris(ethylene oxide)] was determined by combining X-ray and electron diffraction with molecular force field calculations. The polymer was synthesized by thiol-ene emulsion polymerization. The electron diffraction pattern from flat-on single crystals revealed a two-dimensional orthorhombic unit cell with parameters a = 8.60 Å and b = 4.95 Å. The value of the c axis of 18.7 Å was determined from the electron diffraction pattern of tilted single crystals. Molecular force field calculations optimized the chain conformation of the sequence −(CH2−S−CH2−CH2−S−CH2−CH2−S−CH2)−(CH2−O−CH2−CH2−O−CH2−CH2−O−CH2)− to a succession of torsion angles (G−G− T G+G+ T G−G−) T (TT G+ TT G+ TT) exhibiting repetition after one monomeric unit with a chain axis of 18.7 Å, in agreement with the experimental c axis determined by tilted electron diffraction. The main features of the experimental diffraction patterns are well reproduced with a model of the crystal structure characterized by the packing of chains in this low-energy conformation packed in an orthorhombic unit cell with axes a = 8.60 Å, b = 4.95 Å, and c = 18.7 Å, according to the space group Pa (β = 90°). Statistical disorder in the orientation of the chains about their axes and disorder in the conformation of the chains are present in the crystal structure. The conformational disorder has been modeled assuming a statistical succession of monomeric units having the tris(ethylene sulfide) segment always in the conformation G−G− T G+G+ T G−G− T and the tris(ethylene oxide) segment in the helical conformation (TTG+)3 or in the trans-planar conformation (TTT)3. The real crystal structure is intermediate between the limit-ordered models and the limit statistical disordered models.